Harnessing Ferrocene for Hydrogen and Carbon Dioxide Transformations: From Electrocatalysis to Capture

Abstract

Ferrocene (Fc) is a redox-active organometallic scaffold whose unique electronic properties, stability, and modularity have enabled a broad range of catalytic and sensing applications. This review critically examines recent advances in Fc-based systems for hydrogen evolution and carbon dioxide (CO₂) conversion, encompassing electrochemical, photochemical, and thermochemical strategies. Fc serves diverse functions: it operates as a reversible redox mediator, an electron reservoir, a ligand framework, and a structural modulator. Each role contributes differently to enhancing catalytic performance, improving selectivity, or increasing operational stability. We highlight how Fc integration facilitates proton-coupled electron transfer in hydrogen evolution, supports selective CO₂ reduction in molecular and hybrid catalysts, and promotes efficient CO₂ fixation and capture within functionalised frameworks. Emerging applications in electrosynthetic organic transformations are also discussed. Together, these findings position Fc as a foundational motif for designing future electrocatalytic and carbon management platforms.

1. Introduction

Ferrocene, Fc, discovered in the early 1950s, has evolved from an organometallic curiosity into a foundational building block in redox-active catalysis [1]. Its sandwich-like bis(η⁵-cyclopentadienyl)iron(II) structure confers exceptional thermal and oxidative stability, allowing for broad chemical functionalisation while maintaining electrochemical reversibility. This redox versatility underpins diverse applications, including materials chemistry, sensing, and drug design [2,3,4,5,6,7]. Ferrocene has long been used as an internal reference in non-aqueous electrochemistry due to its reversible and well-defined Fc^0/+ redox couple. However, its potential is known to vary depending on solvent and electrolyte composition, making it a conditionally reliable standard rather than a universally fixed one [8,9]. In contrast, the Fe²⁺/Fe³⁺ redox couple in aqueous environments suffers from pH-dependent speciation, solubility issues, and sluggish kinetics, limiting its reproducibility in voltammetric applications [10].

Efficient electron transfer is fundamental to advanced catalytic processes, and the structural versatility of Fc positions it ideally for these applications. Its capacity to incorporate various substituents significantly affects solubility and electron transfer characteristics, broadening its applications beyond traditional metallocene roles [11,12,13,14,15,16,17,18,19]. Ferrocene-based materials now encompass molecular switches, electrosensors, therapeutic delivery systems, and energy storage solutions [4,20,21,22,23]. These diverse applications underscore Fc’s extensive potential within responsive materials and catalytic systems.

This review focuses on the use of Fc in electrocatalytic HER, CO₂RR, and CO₂ capture and fixation. These applications extend beyond the traditional role of Fc as a redox probe or mediator, reflecting its growing utility as a structural motif, electron relay, or iron source in multimetallic and heterogeneous catalytic systems [18,24,25,26,27,28,29,30]. Ferrocene contributes through electronic activation, photothermal properties, or structural integration into porous materials. These developments are crucial for sustainable energy management, emphasising clean hydrogen generation and advanced CO₂ handling strategies. Incorporating Fc into extended frameworks such as MOFs, COFs, and porous polymers expands its functional scope by enabling roles such as photothermal enhancement, structural flexibility, and improved guest–host interactions for gas separation and catalysis [18,27,29,31].

These benefits are further amplified by Fc’s compatibility with high-surface-area materials, where it may act as a redox mediator or structural motif depending on the system. Such Fc-modified systems have demonstrated enhanced electron transfer, optimised adsorption energies in HER, stabilisation of reaction intermediates, and modified CO₂RR selectivity via altered electron distribution at active sites [24,26,27,32,33]. Ferrocene units embedded within MOFs and COFs function concurrently as structural supports and electron-rich centres, enhancing charge transfer and intermediate stabilisation [25,28,33]. Additionally, Fc-linked POPs demonstrate significant capacity for CO₂ physisorption, opening new avenues for solid-state CO₂ capture.

Although several reviews have examined Fc in various research contexts [2,6,7,34,35,36,37,38,39,40,41,42,43,44,45], none have thoroughly consolidated Fc’s contributions specifically across HER, CO₂RR, CO₂ fixation, and CO₂ capture. Considering the growing significance of Fc-mediated catalytic enhancements, this review critically evaluates Fc’s role across these interconnected catalytic domains, emphasising structure-property relationships, redox mediation, and mechanistic insights. The review synthesises existing literature spanning molecular frameworks, hybrid designs, and emerging solar-driven and porous systems. Finally, it highlights current challenges and outlines future opportunities for sustainable catalytic advancements.

This review is thematic rather than exhaustive, with a focus on recent and representative advances. The majority of the literature cited spans the past 5–10 years, though earlier foundational studies are included where relevant. The scope is therefore deliberately selective, prioritising mechanistic insight and redox functionality over comprehensive coverage.

2. Redox Properties and Structural Features of Ferrocene in Catalysis

This section outlines the fundamental redox characteristics and structural properties of Fc that are relevant to its catalytic applications. Understanding these intrinsic features is essential for designing effective Fc-based catalysts across various reaction systems.

2.1. Electronic Structure and Redox Characteristics

Ferrocene undergoes a distinct, reversible one-electron redox transition between Fe(II) and Fe(III), typically observed near +0.40 V vs. SCE in MeCN [11]. Despite its solvent- and electrolyte-dependent potential, Fc remains a widely used internal reference compound in non-aqueous electrochemistry due to its reversible redox behaviour and relative stability [8,9,11]. The oxidation is characteristically diffusion-controlled and electrochemically reversible, with narrow peak separations and stable voltammetric behaviour even over repeated cycling [11,46].

Introducing electron-donating groups such as methyl or amino substituents shifts the oxidation potential to more negative values, sometimes by more than 0.6 V. For example, decamethylferrocene displays a redox potential of −0.505 V vs. Fc^0/+ internal reference system in MeCN containing tetrabutylammonium perchlorate as the supporting electrolyte [8,9,11], reflecting increased electron density stabilising the oxidised ferrocenium form. In contrast, electron-withdrawing groups such as carboxyl, nitro, or cyano substituents shift the redox potential to more positive values. For example, acetylferrocene oxidises near 0.237 V vs. Fc^0/+ under similar conditions [11]. These substituent-induced shifts alter HOMO energy levels, significantly influencing redox properties crucial for HER, CO₂RR, and charge-transfer dynamics in Fc-based CO₂ capture materials. Furthermore, solvent polarity and ion-pairing effects further modulate Fc’s redox potentials, affecting its electrochemical properties [8,9,18,46].

The electronic structure of Fc, characterised by a d⁶ low-spin iron(II) centre, resides within a highly symmetrical ligand field that promotes electron occupancy in non-bonding and weakly antibonding orbitals, enhancing its stability [1,46,47]. Interactions between Fe(II) 3d orbitals and Cp π-orbitals generate molecular orbitals, with the HOMOs exhibiting e₂g and a₁g symmetries. These frontier orbitals facilitate rapid and reversible electron transfer. Adjusting Cp substituents influences HOMO–LUMO energy levels, impacting redox potential and electron transfer kinetics, vital for HER, CO₂RR, and photochemical applications [1,18,47,48]. The intrinsic electronic symmetry of Fc, arising from delocalised π-conjugation between the Cp rings and the Fe(II) centre, facilitates further conjugation with extended π-systems and supports coordination through functionalised substituents (e.g., phosphines, carboxylates) to other metal complexes. These features extend Fc’s catalytic functionality, especially in multimetallic and electron-relay systems [1,46,47].

2.2. Ligand Design and Functionalisation Strategies

Ferrocene modifications often begin with monosubstitution, which proceeds without regioselectivity due to the equivalence of all Cp carbon atoms. However, subsequent substitutions are typically regioselective, enabling controlled synthesis of 1,1′- or 1,2-disubstituted derivatives depending on the reaction conditions and directing groups [49]. These synthetic approaches effectively introduce substituents that modulate redox potentials, solubility, and ligand coordination geometry, essential for optimised catalytic performance [3,11,12,24,28,49,50]. In electrocatalysis, anchoring Fc derivatives such as phosphonate or amide groups significantly enhances their integration into MOFs and conductive polymers, improving electron transfer efficiency and catalyst site stability [5,27]. Ligands incorporating bidentate or tridentate Fc structures provide precise spatial and electronic control, critical for managing catalytic activity and intermediate stabilisation, notably within CO₂RR systems [26,27]. These targeted ligand strategies enable Fc units to mediate electron transfers, facilitate proton relays, or stabilise catalytic intermediates effectively within various catalytic environments [18,28].

2.3. Framework Integration and Stability Considerations

Integrating Fc into frameworks like MOFs, COFs, and polymeric structures considerably enhances catalytic effectiveness. Ferrocene incorporation improves catalyst dispersion, moderates local redox environments, and facilitates cooperative electron transfer between catalytic and redox-active sites. For example, Fc-functionalised Zr-MOFs exhibit improved CO₂RR performance due to enhanced conductivity and optimised spatial arrangements between catalytic nodes and Fc-based redox mediators [25]. Within COFs, incorporating Fc into conjugated frameworks enhances electronic pathways and chemical stability under reductive conditions [28]. Polymers modified with Fc demonstrate increased structural resilience and improved electronic communication, crucial for sustained catalytic operation [18,24].

Generally, Fc maintains electrochemical and thermal stability within these frameworks, although local environmental factors can affect redox accessibility and operational efficacy [18,24].

3. General Functional Roles of Ferrocene in Catalysis and Functional Materials

This section discusses the broad functional roles Fc plays in catalysis and functional materials. Beyond its traditional redox activity, Fc also functions as an electron reservoir, structural scaffold, photothermal enhancer, and electronic modulator. These diverse roles underpin its effectiveness across various molecular and material contexts, setting the foundation for specific applications explored in subsequent sections.

3.1. Redox Mediation and Electron Transfer

Ferrocene’s reversible Fe²⁺/Fe³⁺ couple makes it an effective redox mediator in numerous electrochemical and photoelectrochemical systems [1,18]. It facilitates efficient charge transport between catalytic centres and electrodes, reduces overpotentials, and stabilises charged intermediates through inner- or outer-sphere electron transfer mechanisms [18,25]. The ability of Fc to fine-tune redox potentials via substituent modifications or linker environments further allows it to meet the specific thermodynamic needs of reactions, critical in multistep electron transfer processes relevant to HER, CO₂RR, and light-driven CO₂ fixation [18,30].

3.2. Structural and Electronic Modulation

In addition to its redox capabilities, Fc plays a crucial role in influencing electronic density, spatial arrangements, and coordination environments within composite materials and molecular catalysts [18,46,47]. Incorporating Fc into multidentate ligand frameworks can modulate electron density at adjacent metal centres (e.g., Ni, Cu, or Fe), thereby tuning catalytic activity in CO₂ reduction and hydrogen evolution. This modulation arises from the electronic communication between Fc and the catalytic metal, particularly in bidentate and tridentate ligands that adjust redox potential and orbital overlap [24,26,27]. Ferrocene’s electron-donating characteristics and orbital alignment control also support selective stabilisation of reaction intermediates and pathway control in catalytic processes [18,46].

3.3. Framework Integration and Hybrid Material Design

Ferrocene’s structural and functional versatility promotes its incorporation into extended architectures, such as MOFs, COFs, and porous organic polymers [24,25,28]. In MOFs, Fc serves as both a redox-active component and a structural element, strengthening frameworks and improving electron mobility [25]. Within COFs and conjugated POPs, Fc enhances electronic delocalisation, photothermal properties, and operational robustness, critical for solar-driven CO₂ fixation and physical CO₂ capture processes [25,28,33]. Furthermore, polymeric matrices integrated with Fc exhibit enhanced conductivity, structural resilience, and durability under cyclic electrochemical conditions [18,24].

Ferrocene-functionalised frameworks also offer value in applications beyond redox activity, such as reversible CO₂ physisorption [51]. In these systems, Fc incorporation contributes to enhanced framework rigidity and thermal stability, as evidenced by elevated decomposition temperatures and structural robustness in Fc-containing MOFs and porous polymers [30,52]. This increased backbone rigidity also supports well-defined pore structures that facilitate selective CO₂ uptake over other gases such as N₂, when tested under controlled gas environments [53]. Additionally, many of these materials exhibit mild-condition recyclability, meaning they can undergo repeated CO₂ adsorption–desorption cycles at near-ambient temperatures and pressures without significant structural degradation [30,52]. While most studies employ pure CO₂ or simple mixtures, emerging systems demonstrate functionality in the presence of air or O₂, pointing to progress toward oxygen-tolerant CO₂ capture [53]. However, direct atmospheric CO₂ capture using Fc-based materials remains a significant challenge and a promising direction for future research [30,52,53]. These diverse functional roles highlight ferrocene’s multifaceted utility in both active and passive carbon management strategies.

4. Catalytic Applications of Ferrocene in Energy and CO₂ Conversion

Ferrocene’s versatile redox properties, structural adaptability, and integration into hybrid systems position it as a key component for catalytic processes, addressing global energy and environmental challenges. Specifically, Fc has been extensively studied across diverse electrochemical, photochemical, and thermochemical reactions, including HER, CO₂RR, and both light- and heat-driven CO₂ fixation methods. Within these catalytic contexts, Fc acts as a redox mediator, ligand framework, electron storage site, or structural stabiliser, significantly influencing reaction kinetics, product selectivity, and catalyst stability.

This section summarises recent advancements in Fc-based catalytic technologies, structured according to reaction type. It begins with HER, a critical process for sustainable hydrogen production, and subsequently explores strategies related to CO₂ conversion.

4.1. Hydrogen Evolution Reaction

Palladium(II) and copper(II) meso-tetraferrocenylporphyrins (1 and 2 in Figure 1) have been shown to act as efficient electrocatalysts for proton reduction in DMF using TFA or TEAHCl as proton sources [54]. These complexes, in which four ferrocenyl groups are directly appended to the porphyrin core, demonstrate superior performance relative to their non-ferrocenyl analogues, highlighting the synergistic effect of the electron-donating ferrocenyl moieties. The presence of these groups lowers the overpotential for hydrogen evolution by approximately 0.2 V and increases the maximum TOF to approximately 6100 s⁻¹ for 1 in DMF/TFA. Gas chromatography experiments confirmed molecular hydrogen as the only product, with Faradaic efficiencies of 68% and 70% for 2 and 1, respectively, when using TFA, and up to 73% for 2 with TEAHCl. Notably, while 1 was inactive with the weaker acid TEAHCl, 2 remained catalytically competent, underscoring the influence of the central metal ion on the catalytic profile. Spectroelectrochemical studies and DFT calculations reveal that the HER proceeds through porphyrin-centred reductions rather than metal-centred pathways. The proposed mechanism involves a sequential two-electron reduction process to form the porphyrin dianion (1²⁻), which is then protonated at a meso carbon to generate a stable phlorin intermediate (1-H⁻). A second protonation leads to hydrogen gas evolution and regeneration of the catalyst. Computational results support this mechanistic framework, showing enhanced electron density at meso and β-pyrrole positions in 1 compared to its non-ferrocenyl analogue, and suggest that the structural distortion introduced by the bulky ferrocenyl groups facilitates re-hybridisation necessary for protonation. The strong electronic and steric contributions of the ferrocenyl substituents are thus critical to the enhanced catalytic activity observed in these systems [54].

The oxygen-tolerant molecular hydrogen evolution catalyst 3 (Figure 1) was developed by appending four Fc units to the second coordination sphere of an iron porphyrin via triazole linkers [52]. This architecture leverages the redox capacity of Fc not at the catalytic metal centre but in the surrounding ligand environment, enabling cooperative electron delivery and suppression of oxidative degradation under aerobic conditions. Hydrogen evolution is mediated by the Fe(0) state of the porphyrin core, which exhibits a preferential reactivity toward protons even in the presence of atmospheric oxygen, despite the thermodynamic favourability of O₂ reduction. Electrochemical measurements, including rotating ring–disc experiments and bulk electrolysis, confirmed that the system supports proton reduction while limiting the formation of reactive oxygen species. In an aqueous solution at pH 4 and under 100% O₂ atmosphere, the catalyst achieved a Faradaic hydrogen yield of 53%, with only 5.1% converted into oxygen reduction by-products, indicating adequate selectivity and catalyst stability. A structurally analogous control compound lacking the Fc groups generated approximately threefold higher levels of reactive oxygen species under identical conditions. Mechanistic studies revealed that the Fe(0) porphyrin intermediate undergoes protonation more rapidly than it reacts with oxygen. At the same time, the Fc moieties can donate up to three electrons to support the complete four-electron, four-proton reduction of O₂ to water, thereby circumventing the formation of superoxide or hydrogen peroxide. Spectroscopic analysis of the reaction between the reduced complex and phenol confirmed that weak acids oxidise the Fe(0) centre more readily than molecular oxygen [52]. This system illustrates a second-sphere redox approach in which peripheral Fc groups extend the functional capacity of a central porphyrin core, enabling stable and selective hydrogen evolution in the presence of air.

A novel class of donor-acceptor-donor dyes, 4–7 (Figure 1), incorporating Fc as electron-donating units and benzothiadiazole as an electron acceptor, has been developed as photosensitisers for photocatalytic HER within carbon nanotube-based nanohybrids [55]. The authors synthesised two dyes differing in the linker between Fc and benzothiadiazole (ketone in 4 and thione in 5). They encapsulated them inside semiconducting SWCNTs to form coaxial dye@SWCNT/fullerodendron heterojunctions (where the ‘@’ symbol denotes dye anchored onto or integrated with single-walled carbon nanotubes) upon photoirradiation in the presence of 1-benzyl-1,4-dihydronicotinamide, methyl viologen dichloride, and platinum nanoparticles, which, respectively, serve as the sacrificial donor, electron relay, and hydrogen-evolving co-catalyst. These hybrids produced hydrogen from water. Notably, the system using 5 achieved H₂ evolution rates of 1.3 µmol h⁻¹ at 650 nm, with a quantum yield of 7.5% at 550 nm, and significantly outperformed SWCNT/fullerodendron hybrids lacking Fc dyes at wavelengths above 500 nm. The improved performance was attributed to the efficient intra-molecular charge separation in the dye, preventing excitation energy transfer to the nanotube and enabling productive photoinduced electron transfer toward C₆₀ and the Pt co-catalyst. The study further explored analogues 6 and 7, finding 7@SWCNT/fullerodendron reached a quantum yield of 22% at 600 nm, the highest reported in this series. Mechanistically, HER proceeds via a cascade of photophysical and electron transfer steps. Upon excitation, the dye undergoes charge separation, with the photoexcited electron transferred to C₆₀ and the oxidised hole delocalised into the s-SWCNT scaffold, stabilising the charge-separated state. The reduced C₆₀ subsequently donates an electron to methyl viologen, which relays it to the Pt co-catalyst, where protons are reduced to H₂. The oxidised dye is regenerated by electron donation from 1-benzyl-1,4-dihydronicotinamide, thus completing the catalytic cycle [55]. Incorporating Fc units enhances intramolecular charge separation and visible-light absorption, enabling efficient hydrogen evolution via directional electron flow and prolonged charge separation lifetimes [55].

To model the activity of [NiFeSe]-hydrogenases, a series of nickel(II) complexes (8–14 in Figure 1) were synthesised bearing Fc-containing diphosphine ligands and diselenolate derivatives as bidentate chalcogen donors [51]. The highest-performing catalyst, designated as complex 12, incorporates a 4,5-dimethyl-1,2-benzenediselenolate ligand combined with dppf. Under electrochemical conditions in MeCN with TFA as a proton source, complex 12 exhibited an impressive TOF of 20,359 s⁻¹, significantly surpassing the parent dppf-supported system (TOF = 7838 s⁻¹). Comparative studies revealed that electron-donating substituents on the diselenolate ring enhanced catalytic activity by raising the electron density at the nickel centre, thereby facilitating the Ni(II)/Ni(I) reduction essential for HER. Complexes with modified dppf ligands (13 and 14) showed substantially lower TOFs, indicating the importance of π-conjugated aryl systems for electronic communication. Electrochemical analysis revealed reversible Ni(II)/Ni(I) couples at approx. −1.53 V vs. Fc^0/+ internal reference system, and irreversible Fe(II)/Fe(III) oxidation of the Fc backbone at approx. 0.11–0.18 V, confirming the retention of a redox-active Fc unit. Cyclic voltammetry in the presence of increasing TFA concentrations revealed typical catalytic wave behaviour, including cathodic peak shifts, disappearance of oxidation signals, and gas bubble formation at the electrode [51]. Gas chromatography confirmed the generation of hydrogen. Mechanistically, HER proceeds via protonation of the reduced Ni(I) species, enabled by electronic support from the Fc-diphosphine ligand, which stabilises intermediate states and enhances electron delivery to the catalytic metal centre. The catalytic current saturates at TFA concentrations above 75 mmol L⁻¹, indicating efficient turnover. These results support using Fc-functionalised diphosphine ligands to fine-tune molecular HER catalysts and reinforce the role of second-sphere Fc modification in enhancing both activity and stability.

A well-defined Ni(II) electrocatalyst featuring a redox-inactive dithiolate ligand and the Fc-containing diphosphine dppf has been synthesised and evaluated for hydrogen evolution activity in MeCN using TFA as a proton source [53]. The resulting complex, denoted as 15 (Figure 1), exhibits a distorted square planar NiP₂S₂ geometry, as confirmed by X-ray crystallography, and retains the Fc backbone intact, with characteristic UV-Vis and NMR signatures. Electrochemical studies revealed a quasi-reversible Ni(II)/Ni(I) redox couple near −1.32 V vs. Fc^0/+, followed by catalytic waves under acid addition. In the presence of TFA, complex 15 displays a catalytic onset potential of −0.65 V and a catalytic half-wave potential of −1.44 V, with a derived overpotential of 0.64 V. The TOF calculated via foot-of-the-wave analysis reaches 1.05 × 10³ s⁻¹, with a Tafel slope of 0.15 V dec⁻¹, confirming efficient proton reduction. Comparatively, when a phenyl-based scaffold replaced the Fc backbone in the diphosphine ligand, the resulting complex exhibited higher catalytic activity (TOF = 3.28 × 10³ s⁻¹, overpotential = 0.45 V), which was attributed to the increased flexibility of the ligand framework in the absence of the rigid metallocene unit. Mechanistic studies confirmed that both complexes follow an ECEC pathway (consisting of sequential Electrochemical (E) and Chemical (C) steps), where the first protonation of the Ni(I) intermediate results in the formation of a high-valent Ni(III)-H species via oxidative protonation, a key rate-determining step. The dppf unit in 15, although not redox-active during catalysis, contributes to electronic and steric rigidity that influences the entropic and geometric parameters of the transition state [53]. The suppression of bimetallic hydrogen formation pathways and the preservation of catalytic integrity under acidic conditions further support its molecular robustness. This study underscores the utility of dppf as an Fc-derived ligand scaffold that, even in the absence of Fc-centred redox mediation, modulates catalytic activity through second-sphere effects on geometry and electronic structure.

Building on this, a subsequent study explored the related Ni(II) electrocatalyst 16 (Figure 1), bearing the same dppf ligand in combination with a redox-active non-innocent anionic ligand [56]. This system retained the distorted square-planar NiP₂S₂ geometry and Fc backbone, as confirmed by crystallographic and spectroscopic analysis. Electrochemical measurements revealed a quasi-reversible Ni(II)/Ni(I) couple at −1.28 V vs. Fc^0/+ internal reference system and catalytic waves upon acid addition. Under catalytic conditions in MeCN with TFA, the complex displayed an onset potential of −0.66 V, an overpotential of 0.53 V, and a TOF of 1.6 × 10³ s⁻¹. Mechanistically, this catalyst also proceeded through an ECEC pathway involving oxidative protonation to yield a Ni(III) hydride intermediate, followed by hydrogen evolution and regeneration of the Ni(II) resting state. Despite not engaging in direct redox mediation, the Fc unit enhanced catalytic performance via steric and electronic effects on the diphosphine coordination environment, stabilising reactive intermediates and transition states. This work reinforced the broader design principle that dppf, even when redox-silent, can meaningfully support HER catalysis through second-sphere structural control.

A Fc-based nickel metal–organic framework (NiFc-MOF) was synthesised using 1,1′-ferrocenedicarboxylic acid as the organic ligand and served both as a redox-active linker and in situ reducing agent for ruthenium nanoparticle deposition [57]. The resulting catalyst, denoted Ru_0.3@NiFc-MOF, features a nanosheet morphology decorated with ultrasmall Ru particles (~2.23 nm) and retains the Fc backbone throughout. The HER performance in 1.0 M KOH showed exceptional activity, with an overpotential of only 0.03 V at 10 mA cm⁻², outperforming even commercial Pt/C (0.05 V), and a Tafel slope of 0.05 V dec⁻¹, indicative of Volmer–Heyrovsky kinetics. Electrochemical impedance spectroscopy confirmed a low charge transfer resistance of 2.11 Ω, and the electrochemical double-layer capacitance of 163.14 mF cm⁻² reflected a high density of active sites. Notably, the material retained structural and electrochemical integrity over 120 h of continuous HER operation, demonstrating outstanding stability. The Fc unit played a dual role: it enabled Ru³⁺ reduction via its favourable redox potential (E° = 0.64 V vs. SHE) and enhanced the electronic conductivity of the MOF framework, improving charge transport and catalytic turnover [57]. While the primary active HER sites are the Ru nanoparticles, the 1,1′-ferrocenedicarboxylic acid scaffold significantly improves the catalyst’s morphology, charge mobility, and durability, underscoring the value of Fc-based ligand engineering in constructing robust HER electrocatalysts.

4.2. Ferrocene-Based Catalysts for Carbon Dioxide Reduction Reaction

The electrochemical reduction of CO₂ represents a critical pathway toward sustainable chemical production and carbon mitigation. Effective CO₂RR catalysts must overcome the thermodynamic and kinetic barriers associated with CO₂ activation, multiple electron-proton transfer steps, and competing hydrogen evolution. Recent studies have demonstrated that Fc’s unique electrochemical reversibility and redox potential tunability make it a compelling candidate for integration into CO₂ reduction catalysts, especially when structural or electronic precision is required [18,24,25].

Building upon the principles established for HER, Fc-based systems in CO₂RR catalysis exploit the redox-active Fc core to shuttle electrons, modulate local electron density, and influence the stability of key reaction intermediates. These effects depend on the surrounding molecular architecture and proximity to catalytic metal sites. The ability to mediate charge at the active site can alter selectivity or activation barriers in CO₂ binding and conversion processes (examples are provided below). Recent developments include discrete molecular Fc complexes tailored for CO₂ activation, Fc-linked polymeric scaffolds facilitating charge transport, and extended frameworks incorporating Fc as a structural and electronic relay unit.

Ferrocene-based catalysts for CO₂RR have been explored across a range of structural motifs, from discrete molecular complexes to polymer-supported systems and extended frameworks. These diverse platforms demonstrate how Fc’s redox adaptability can be harnessed to overcome the kinetic and thermodynamic limitations of CO₂ activation.

Table 1. Summary of ferrocene-based catalysts for electrochemical and photochemical CO₂RR.

Fc Catalyst (Structure/Complex)	Support/Matrix	Product(s)	FE (%)	TOF (h−1)	Conditions	Key Notes	Ref.
17	Homogeneous (MeCN/H2O 9:1)	CO, H2, HCOOH	CO: 11.9, H2: 19.4, HCOOH: 10.8	CO: 8.5, H2: 14, HCOOH: 4.61	EC, −1.6 V vs. SCE, TBAP electrolyte	Homogeneous; moderate overpotential (0.74 V); multielectron PCET pathway	[58]
3	Homogeneous (MeCN + PhOH)	CO	>92	—	EC, −2.50 V vs. Fc0/+, TBAP, 3 M PhOH, 1 atm CO2	Fc units enhance oxygen tolerance; CO2 reduced selectively even under aerobic conditions; Fc redox-inactive for CO2RR	[59]
Ag9Cu6-Fc	Atomically precise AgCu cluster with Fc	CO	99.4	jCO = ~680 mA cm−2	EC, MEA cell, −4.0 V, CO2 flow	Fc improves electron transfer, modulates active site (Ag-Cu); boosts jCO and selectivity vs. non-Fc analogue	[60]
27	Conjugated porous polymer with Cu-porphyrin	CH4, C2H4	CH4: 75.9, C2H4: 18.1	—	EC, −0.9 V vs. RHE, 1 M KOH, flow cell	Fc enhances electron density on Cu, boosts PCET for CH4; scalable copolymerisation	[61]
28	Covalent organic framework (COF) with Ni-porphyrin and Fc post-modification	CO	93.1	—	EC, H-type cell, 0.9 V vs. RHE, 1 M KOH	Fc improves conductivity and interfacial charge transfer; enhanced anodic OER activity supports cathodic CO2RR; CO2 source confirmed by 13CO isotope tracing	[26]
18, 19	Polyoxo-titanium molecular clusters in H2O suspension	HCOO−	—	19: 350.0 μmol g−1 h−1; 18: lower	PC, visible light, no photosensitiser, 10 vol% TIPA in H2O	Fc serves as both light-harvesting antenna and electronic linker; facilitates LMCT; Ti4+ acts as an active site for CO2 reduction to formate	[4]
20	Biomimetic Co-based MOF with adenine and FA ligands	HCOOH	—	225.8 μmol g−1 h−1	PC, MeCN + TEA, visible light	Fc enhances charge transfer; light absorber = HAD; Fc improves photocurrent and quantum efficiency; the active site is adenine N, not Co	[20]
21, 22	MOF: NH2-MIL-125(Ti) with Fc via grafting or encapsulation	HCOOH	~95.7–98.6	21: 293.40 (UV), 266.33 (Vis) mmol g−1 h−1	PC, UV and visible light, H2O + TEOA	Fc improves visible light absorption and charge separation; grafted Fc gives higher performance than loaded Fc; Ti4+ active centre via Ti3+-Ti4+ cycling	[24]
26	Homogeneous (MeCN/TEA 19:1)	HCOOH	86 (HCOOH vs. CO: 215:1)	1180 (HCOOH)	PC, λ = 405 nm, organic PS, BIH donor	First Cr-based Fc system for photo-CO2RR; Fc enhances electron density; DFT-supported	[27]
Fc-COOH on Zr-MOF	Zr-based MOF (UiO-66)	CO	—	90.65 μmol g−1 h−1	PC, Visible light, Xe lamp, 400–780 nm, 5:1 MeCN/H2O + TEOA	Fc enhances LMCT; dual-channel electron transfer; optimal trade-off with Lewis acidity	[33]
Fe-Tc (Ti-MOF with Fc)	Titanium cluster-ferrocene MOF	HCOO−	—	39.5 μmol g−1 h−1	PC, 400–800 nm, TEOA, CH3CN (30:1)	Fc improves charge separation; better photoreduction than classical Ti-MOFs	[62]
FCA-grafted CsPbBr3 QDs	CsPbBr3 perovskite QD	CO	96.5	132.8 μmol g−1 h−1	PC, simulated solar light (AM 1.5 G), gas–solid setup	FCA grafting enhances exciton dissociation and charge transfer through dielectric screening and surface potential modulation. CsPbBr3-FCA outperforms pristine CsPbBr3 QDs by 9 times in CO yield. Cs site facilitates CO2 adsorption and activation.	[63]
FcMeOH	CsPbBr3 perovskite nanocrystals	CO	—	772.79 μmol g−1	PC, under AM 1.5 G light, CO2-saturated H2O vapour, 5 h	FcMeOH clusters improve charge transfer and exciton dissociation via dipole interactions; enhanced CO yield, and lower ΔG for *COOH formation.	[64]
FCA-functionalised Cs3Sb2Br9 nanocrystals	Cs3Sb2Br9 nanocrystals	CO	97.9%	45.56 μmol g−1 h−1	PC, visible light, benzyl alcohol oxidation	Fc acts as a dynamic redox centre (Fe2+/Fe3+), enabling CO2 reduction coupled to alcohol oxidation; supported by isotope and DFT studies.	[65]
29	Homogeneous (DMF + DBU)	HCOO−	—	up to 2218	TC, CO2:H2 = 1:1 (1 atm), 25–80 °C, 12–48 h	TON up to 1.07 × 105; Fc not redox-active	[66]

summarises key Fc-based electrocatalysts for CO₂RR, including their configuration, support matrix, and performance metrics to support this discussion. The following sections explore representative advances in these domains, beginning with molecular complexes directly incorporating Fc into their coordination or electronic architecture.

A Fc-based polypyridyl ligand, 17 (1,1′-bis [1,8-naphthyrid-2-yl]ferrocene) (Figure 2), has been successfully employed in homogeneous electrocatalytic CO₂ reduction under 9:1 MeCN-water conditions using TBAP as the supporting electrolyte [58]. At −1.6 V vs. SCE, 17 catalyses the formation of syngas (CO and H₂) and formic acid with moderate selectivity. After 3 h of electrolysis, the Faradaic efficiencies were 11.9% (CO), 19.4% (H₂), and 10.8% (HCOOH), with corresponding TOF of 8.5, 14, and 4.61 h⁻¹, respectively. The catalyst remained stable under operating conditions and showed minimal Fe deposition, confirming homogeneity. Mechanistically, the CO₂RR proceeds via proton-coupled two-electron reduction at the naphthyridine π* orbital, forming a reactive di-anionic intermediate that either delivers hydride to CO₂ or undergoes insertion pathways leading to CO or HCOOH, respectively. Although the Fc unit is redox-inactive in the catalytic cycle, it plays a vital electronic and structural role by stabilising charge-transfer states and facilitating π-backdonation through the conjugated framework.

The ferrocene-appended iron porphyrin catalyst previously described as structure 3 for HER (Figure 1) also demonstrates outstanding performance in CO₂ electroreduction. This bifunctional molecular complex integrates four covalently tethered Fc units into the periphery of an iron porphyrin macrocycle via triazole linkers. While initially developed to enhance oxygen tolerance in HER via outer-sphere redox buffering, 3 is also capable of selective CO₂-to-CO conversion in the presence of molecular oxygen, a rare and highly valuable property for practical CO₂RR applications [59]. Under anhydrous conditions in MeCN with 3 M phenol as a proton source, 3 exhibits a catalytic onset at −1.90 V vs. Fc^+/0 and achieves Faradaic efficiencies exceeding 92% for CO production under 100% CO₂. The catalyst maintains 84, 60, and 43% CO Faradaic yield when operating under CO₂:O₂ mixtures of 3:1, 1:1, and 1:3, respectively, demonstrating its capacity to preserve function under increasingly aerobic environments. Mechanistically, the Fe(0) state of the porphyrin core serves as the active site for CO₂ binding and reduction, while the Fe(II) state, supported by electron transfer from the four Fc units, facilitates complete 4e⁻/4H⁺ reduction of O₂ to H₂O, thereby eliminating reactive oxygen species that would otherwise degrade the catalyst. Kinetic studies confirm that CO₂ reacts with the Fe(0) intermediate approximately 500 times faster than O₂, affording both activity and selectivity advantages [59]. Thus, the dual application of 3 in HER and CO₂RR highlights the unique value of Fc-decorated coordination environments in promoting multielectron redox processes while safeguarding catalyst integrity in oxidative media.

In a distinct application of Fc in photocatalytic CO₂ reduction, two polyoxo-titanium cluster systems, 18 and 19 (Figure 2), functionalised with FcDC, exhibited outstanding visible-light-driven reduction of CO₂ to formate in water using triisopropanolamine as a sacrificial donor [4]. The incorporation of FcDC notably extended the light absorption range of the clusters into the visible region (500–600 nm) and significantly reduced the bandgap from 3.14 eV (19) to ~1.83 eV. Mechanistic studies confirmed that FcDC ligands acted as effective charge-transfer mediators, transferring photogenerated electrons to Ti⁴⁺ centres, forming catalytically active Ti³⁺ species. Among the two systems, 19 outperformed 18 with a formate yield of 35.0 μmol (350 μmol g⁻¹ h⁻¹) and selectivity of 97.5%. This enhancement was attributed to the presence of the sulphate bridges in 19, which further facilitated electronic coupling and improved charge mobility. The study provides a compelling case of non-redox-active Fc units serving as light-harvesting and charge-delocalised motifs, significantly enhancing the molecular photocatalyst’s efficiency.

A crystalline biomimetic coordination framework, 20, incorporating FcDC as an organic linker (Figure 2), was developed for photocatalytic CO₂ reduction to formic acid under visible light [20]. The structure features adenine-based N⁶-(hydroxymethyl)adenine ligands that provide multiple nitrogen donor sites for CO₂ activation, while the Fc moiety facilitates photoinduced charge separation and electron transport. 20 exhibits a paddle-wheel Co₂ node and a three-dimensional porous structure similar to its non-Fc analogue (Co₂-AW), but outperforms it in catalytic performance due to enhanced electronic conductivity. Under visible-light irradiation in MeCN with TEA as the sacrificial donor, 20 achieved a formic acid production rate of 225.8 μmol g⁻¹ h⁻¹ with high selectivity, as confirmed by ¹³CO₂ labelling and NMR analysis. The catalyst remained stable over multiple cycles, with post-reaction PXRD and FT-IR analyses confirming structural integrity.

A titanium-based MOF incorporating Fc moieties was synthesised via imine condensation between 1,1′-ferrocenedicarboxaldehyde and amino-functionalised titanium clusters [62]. The resulting material displayed crystallinity and broad absorption across the visible-light region, attributed to the extended conjugation and electron-donating properties of the Fc linkers. Photocatalytic experiments in MeCN with TEA as a sacrificial electron donor demonstrated efficient CO₂ reduction to formate under visible light, achieving a production rate of 39.5 μmol g⁻¹ h⁻¹. This activity significantly surpassed that of benchmark titanium MOFs under comparable conditions. Electrochemical studies revealed enhanced photocurrent responses and reduced interfacial charge-transfer resistance in the Fc-Ti cluster, indicating improved electron mobility. The spectroscopic analysis confirmed the preservation of Ti⁴⁺ and Fe²⁺ oxidation states, suggesting the Fc units retained redox integrity. The low-energy LUMO level of the Fc-Ti cluster (−0.60 V vs. NHE, pH 7) further supports its suitability for CO₂ reduction. Overall, Fc acts as a structural linker and an electronic promoter, enhancing charge separation and light utilisation in this photocatalytic CO₂RR framework.

Two titanium-based MOFs functionalised with Fc, 21 and 22 (Figure 2), were developed to explore the influence of grafting mode on CO₂ photoreduction efficiency [24]. In 21, Fc was covalently attached to the MOF via Schiff base condensation, forming stable imine linkages, while in 22, Fc was introduced by non-covalent encapsulation within the MOF pores. Both materials exhibited broadened visible-light absorption due to the presence of Fc; however, 21 showed superior optical properties, including a reduced band gap (1.82 eV) and enhanced charge-transfer performance. Under visible-light irradiation and in the presence of TEA as the sacrificial donor, 21 achieved a CO₂-to-formate conversion rate of 266.3 μmol g⁻¹ h⁻¹ with near-complete selectivity, outperforming 22, which exhibited lower activity and minor Fc leaching over repeated cycles. Electron paramagnetic resonance and X-ray photoelectron spectroscopic studies confirmed the generation of Ti³⁺ and Fe³⁺ in both systems under illumination, supporting a mechanism in which Fc donates photoexcited electrons to reduce Ti⁴⁺ centres, initiating CO₂ activation. The covalent anchoring in 21 ensured improved stability, redox cycling, and reusability.

Surface engineering of perovskite quantum dots, QDs, with FCA ligands has emerged as a promising strategy to overcome exciton confinement and inefficient charge transfer in photocatalytic CO₂ reduction systems [63]. In this study, CsPbBr₃ QDs were modified via ligand exchange with FCA to form nanostructures in which the ligand functions as a dielectric screening agent and an electronic mediator. The introduction of the Fc moiety dramatically improved exciton dissociation and interfacial electron transfer, reducing Coulombic interactions and surface energy barriers that typically hinder carrier mobility. Mechanistically, the FCA ligand forms a charge transfer bridge by coordinating with Cs atoms on the surface of CsPbBr₃ QDs, enabling electron injection from the Fc LUMO into the conduction band of CsPbBr₃. This resulted in a significant red shift in photoinduced absorption and stronger ground-state bleach signals. The system achieved a CO production rate of 132.8 μmol g⁻¹ h⁻¹ with 96.5% selectivity and a quantum efficiency of 0.072%, outperforming the unmodified quantum dots (14.4 μmol g⁻¹ h⁻¹, 0.006%). Stability was maintained over 72 h, and in situ diffuse reflectance FT-IR coupled with DFT, confirmed the formation of key intermediates (COOH*, *CO) and preferential CO₂ binding at the Cs site with an adsorption energy of −2.53 eV. The Fc-induced multiexciton dissociation and enhanced interfacial charge delivery position Fc-modified QDs as a high-performance photocatalyst for CO₂-to-CO conversion under simulated solar light [63].

Ferrocenemethanol-functionalised CsPbBr₃ nanocrystals were developed via ligand exchange, forming hydrogen-bonded FcMeOH clusters on the perovskite surface [64]. The introduction of FcMeOH significantly enhanced photocatalytic CO₂ reduction by promoting exciton dissociation and interfacial charge transfer. This enhancement stemmed from the strong electric dipole moment of FcMeOH, which facilitated robust electronic coupling with the CsPbBr₃ surface, reduced surface barrier energy, and stabilised charge carriers. The resulting system exhibited optimised photogenerated charge behaviour, including shortened photoluminescence lifetimes (44.45 ns), increased photocurrent density, and reduced recombination times. Under simulated sunlight irradiation using the Air Mass 1.5 Global (AM 1.5 G) standard spectrum for five hours, FcMeOH-functionalised CsPbBr₃ nanocrystals achieved a carbon monoxide yield of 772.79 µmol g⁻¹, outperforming Fc-functionalised CsPbBr₃ nanocrystals (338.70 µmol g⁻¹) and pristine caesium lead bromide (156.13 µmol g⁻¹). In situ, diffuse reflectance FT-IR spectroscopy confirmed a greater accumulation of key intermediates (such as COOH*) on the FcMeOH-functionalised catalyst. At the same time, DFT calculations revealed a reduction in the energy barrier for COOH* formation from 2.55 to 2.02 eV. These findings establish the FcMeOH-ligand interaction as a potent strategy to regulate exciton dynamics and improve quantum-confined perovskite photocatalysts for CO₂ reduction applications.

A hybrid photocatalyst system based on Fc-functionalised Cs₃Sb₂Br₉ nanocrystals has been developed to drive simultaneous CO₂ photoreduction and benzyl alcohol oxidation under visible light [65]. The catalyst structure incorporates FCA ligands onto the perovskite surface via ligand exchange, introducing dynamic Fe²⁺/Fe³⁺ redox centres directly into the charge transfer interface. These sites serve as dual electron and proton donors, mediating efficient proton-coupled electron transfer during photocatalysis. Structural analysis confirmed retention of the trigonal perovskite lattice after Fc modification, while spectroscopic characterisation (Raman, XPS, UV-Vis DRS, EPR) revealed active redox cycling between Fe²⁺ and Fe³⁺ and enhanced photoinduced charge separation. Operando attenuated total reflection-surface-enhanced infrared absorption spectroscopy and isotope labelling confirmed that protons from benzyl alcohol oxidation participate in *COOH intermediate formation. The system achieved a CO production rate of 45.56 µmol g⁻¹ h⁻¹ with a selectivity of 97.9% and an apparent quantum efficiency of 0.535% at 450 nm. Density functional theory simulations revealed that Fc-functionalised Cs₃Sb₂Br₉ nanocrystals exhibit reduced energy barriers for *CO₂ adsorption and *COOH intermediate formation compared to their unmodified counterparts. The results highlight the ability of Fc redox centres to synchronise redox half-reactions, offering an integrated strategy for solar-driven CO₂ utilisation coupled with organic oxidation.

An atomically precise bimetallic cluster catalyst, Ag₉Cu₆, has been functionalised with twelve ethynylferrocene ligands to produce a hybrid nanocatalyst (Ag₉Cu₆–Fc) exhibiting exceptional electrocatalytic CO₂ reduction performance [60]. The alkynyl Fc units bridge electronically to the AgCu cluster core, establishing a continuous charge transfer pathway and a modified local environment at the active site. The structural analysis confirmed that the core consists of a body-centred Ag₉ cube capped with Cu atoms on each face, with each Cu atom coordinated to two Fc ligands. This precise ligand architecture enhances electron delivery, stabilises intermediates, and increases surface hydrophobicity, contributing to a pronounced suppression of HER. Electrochemical testing in both H-cell and membrane electrode assembly configurations showed superior CO production selectivity and kinetics: the Ag₉Cu₆-Fc system achieved a Faradaic efficiency of 99.4% for CO and a partial current density of −680 mA cm⁻² under direct CO₂ gas feed, a threefold improvement over its non-Fc counterpart. Operando attenuated total reflection-surface-enhanced infrared absorption spectroscopy, and DFT calculations revealed that Fc ligands weaken *CO adsorption and accelerate intermediate desorption while enabling favourable C=O cleavage and CO formation. These findings establish the pivotal role of Fc-derived organometallic interfaces in modulating electronic properties and elevating catalytic performance in CO₂RR.

The photocatalytic systems 23–26, employing Cr(III) complexes bearing Fc-functionalised tetradentate ligands (Figure 2), have recently been developed for selective CO₂ reduction to formic acid [27]. The ligands combine two redox-active Fc units and hemilabile phosphine donors, enabling photoinduced electron transfer and dynamic proton handling during catalysis. Among the four tested complexes, 26 achieved the highest performance, with a TON of 1180 and formic acid selectivity of 86% after 48 h of violet light irradiation (λ = 405 nm). Mechanistically, the catalyst undergoes multielectron photoreduction and intramolecular proton transfer facilitated by phosphine lability, forming Cr-H species that subsequently insert CO₂ to yield formate. Density functional theory calculations and spectroscopic studies support a synergistic role of Fc units in mediating charge transfer and enhancing the electron density at the Cr centre. This work represents the first example of a homogeneous Cr-based molecular photocatalyst for CO₂RR, using only earth-abundant elements.

A Fc-modified Zr-based MOF was synthesised via post-synthetic ligand exchange, incorporating FCA into the UiO-66 framework. UiO-66 is a highly stable zirconium-based MOF constructed from Zr₆-oxo clusters and terephthalate linkers, widely used in catalysis due to its chemical robustness and tuneable porosity [33]. The resulting MOF exhibited a BET surface area of 693.72 m² g⁻¹ and enhanced visible-light absorption due to improved ligand-to-metal charge transfer. Under visible-light irradiation (λ > 400 nm), in the presence of TEA as a sacrificial donor, the catalyst reduced CO₂ to CO with a rate of 90.65 μmol g⁻¹ h⁻¹. Although Fc did not function as a redox centre, its inclusion improved photoinduced charge separation and electronic coupling within the MOF, supporting its role as an efficient scaffold for photocatalytic CO₂ reduction.

A conjugated porous polymer incorporating Fc and Cu–porphyrin units 27 (Figure 2) was developed for the electrocatalytic reduction of CO₂ to CH₄ in alkaline media [61]. In this material, Fc is covalently linked to the polymer backbone alongside Cu-porphyrin centres, enabling direct electronic communication between the redox-active Fc units and catalytic Cu–N₄ sites. Under flow-cell conditions in 1 M KOH and at −0.9 V vs. RHE, the system achieved a Faradaic efficiency of 75.9% for CH₄ and 18.1% for C₂H₄ (combined Faradaic efficiency = 94.0%), with a CH₄ partial current density of −185.8 mA cm⁻². Density functional theory calculations revealed that the Fc units enhance the electron density around the Cu active sites and lower the energy barrier for the CO₂-to-COOH proton-coupled electron transfer step. Additionally, the oxophilicity of Fc was shown to facilitate OH adsorption, further promoting the CH₄ pathway. In situ, attenuated total reflectance-FT-IR detected key intermediates, including *CO and *CH₂O, supporting the proposed mechanism [61]. The polymer displayed high stability under electrolysis and retained its CH₄ selectivity over prolonged operation, establishing 27 as a high-performing and scalable CO₂RR platform with Fc as a functional electronic promoter.

A Fc-modified COF was synthesised by incorporating monomer 28, which features a nickel(II)–porphyrin core and pendant Fc units, into a two-dimensional porous network (Figure 2) [26]. The Fc moieties were introduced via post-synthetic modification of hydroxyl-functionalised COFs using 1-bromoferrocene, forming stable C-O linkages without disrupting crystallinity. The resulting material exhibited a well-defined nanosheet morphology and high structural integrity, with uniform elemental distribution, including Fe from the Fc units. The presence of Fc contributed to improved charge transport across the framework, as confirmed by decreased charge transfer resistance and increased electrochemical surface area. In a two-electrode system, pairing the Fc-modified COF with an unmodified porphyrin COF at the cathode, the platform achieved a Faradaic efficiency for CO formation of 93.1% at −0.9 V vs. RHE, significantly higher than that of the control pairing. The formation of CO was confirmed as the sole reduction product by isotope-labelled ¹³CO₂ experiments and ¹H-NMR, with no detectable liquid-phase species. The structural arrangement of monomer 28 enables extended π-conjugation and facilitates electron donation from the Fc units, which serve as intramolecular electron reservoirs during catalysis. These features collectively establish monomer 28-based frameworks as efficient Fc-integrated electrocatalysts for selective CO₂-to-CO conversion under mild aqueous conditions.

Ferrocene-containing diphosphine ligands, such as dtbpf, have emerged as effective scaffolds in homogeneous hydrogenation catalysis [66]. In a notable example, the copper(I) complex 29 (Figure 2) catalyses the thermochemical hydrogenation of CO₂ to formate in the presence of a Brønsted base, specifically DBU, which facilitates H₂ splitting and stabilises the formate ion formed [66]. Though the Fc moiety is not redox-active in this context, it contributes significantly to the coordination geometry at copper by imposing a wide bite angle and steric bulk that help stabilise hydride and formate intermediates. Under 1 atm of CO₂/H₂ and in the presence of DBU, the system achieves formate yields of up to 98% and turnover numbers exceeding 10⁵, with corresponding TOF surpassing 2200 h⁻¹ at 80 °C. This system represents one of the most efficient first-row metal-based homogeneous catalysts for thermal CO₂ reduction reported to date.

The systems summarised in Table 1 reveal the broad range of strategies used to integrate Fc into catalytic platforms for CO₂ reduction, spanning homogeneous molecules, porous frameworks, nanocrystals, and atomically precise clusters. Several designs demonstrated high selectivity and notable current densities under mild conditions within electrocatalytic systems. The Ag₉Cu₆-Fc cluster remains the top performer, achieving a CO partial current density of 680 mA cm⁻² with 99.4% selectivity. Other effective systems include 28 (93.1% CO selectivity) and 27, which showed significant methane generation (75.9% CH₄ FE), highlighting Fc’s versatility in influencing which single-carbon products are formed during CO₂ reduction. Complementing these, a high-performing thermocatalytic system based on 29 achieved turnover numbers exceeding 10⁵ and formate yields up to 98% under 1 atm CO₂/H₂, demonstrating the utility of Fc-based ligands beyond redox mediation [66].

In contrast, the photocatalytic systems displayed lower overall CO₂ conversion rates typically reported in the 100–770 μmol g⁻¹ h⁻¹ range. Among these, the most promising results were obtained from FcMeOH-modified CsPbBr₃ nanocrystals, followed by 19 and MOF 21, each benefiting from Fc-enhanced charge separation and photostability. Although the photocatalytic outputs remain modest relative to electro- and thermocatalytic benchmarks, these systems underline Fc’s capacity to improve light-driven processes. Overall, electrocatalytic and thermocatalytic platforms leveraging Fc’s structural and electronic contributions currently offer the highest efficiencies in CO₂ reduction. At the same time, further optimisation of Fc-photocatalyst integration may enhance the competitiveness of photo-driven systems.

4.3. Ferrocene-Based Materials for CO₂ Fixation: Ligand Effects, Structural Advantages, and Hybrid Integration

The intensifying urgency to mitigate rising atmospheric CO₂ concentrations has driven the exploration of catalytic systems capable of transforming CO₂ into value-added products under mild, sustainable conditions. While Fc is widely recognised for its redox activity, its role in CO₂ fixation extends beyond direct electron transfer. Ferrocene acts as a structural or electronic modulator in many systems, tuning ligand geometry, enhancing thermal or chemical stability, or enabling cooperative interactions within hybrid architectures. This section examines representative systems where Fc facilitates CO₂ hydrogenation, carboxylation, or cycloaddition through these non-redox contributions, highlighting the diverse ways it supports catalytic activity in fixation pathways.

The influence of Fc-derived ligands on CO₂ activation is further illustrated in nickel-catalysed carboxylation reactions employing dppf. In this system, arylzinc reagents undergo carboxylation with CO₂ under Ni(0) catalysis, forming aryl carboxylic acids such as benzoic acid [19]. While the Fc unit within dppf does not transfer electrons, it significantly affects the electronic potential surface and CO₂ coordination geometry at the metal centre. Density functional theory calculations reveal a more diffuse electrostatic distribution in the dppf-Ni complex than tricyclohexylphosphine analogues, resulting in weaker CO₂ binding and a lower experimental yield (62 vs. 88%). The carboxylation proceeds via a stepwise oxidative cycloaddition mechanism, with the Fc scaffold influencing the conformational energy landscape.

Table 2. Summary of representative Fc-functionalised catalytic systems for CO₂ fixation, including fixation type, products formed, Fc role, and selected performance metrics.

Catalyst or Ligand	Fc Role	Fixation Type	Substrate(s)	Product(s) Formed	Yield/Notes	Ref.
Ni(dppf) + ArZnBr	Ligand scaffold	Carboxylation	Arylzinc + CO2	Aryl carboxylic acids	62% yield; Fc tunes electronic distribution	[19]
Rh2(OAc)4 + NHC ligand	Substrate backbone	C–H carboxylation	2-ferrocenylphenols	Ferrocenyl lactones	Up to 78% yield, up to 47% ee	[67]
34	Structural ligand	Carbonate formation	N-phenyl benzamide, CO2	μ-CO32− bridged Cu complex	Thermally stable, no catalytic test	[68]
29, 35–37	Ligand scaffold	Alkyne carboxylation	Terminal alkynes + CO2	Propiolic acids	80–96% yield at 2 mol% loading	[69]
Au8Pd1(dppf)42+ cluster	Structural ligand enhancing stability and electron transfer	N-formylation)	CO2, H2, o-phenylenediamine	Benzimidazole	96.4% yield; Fc improves cluster stability and charge transport; catalyst reusable for 5 cycles without degradation	[70]
Fc (electrolyte additive)	Electrolyte additive; stabilises intermediates	Electrochemical CO2 fixation in Li–CO2 battery	CO2	Li2C2O4 (main) and Li2CO3/C (minor)	Favours 2e− pathway via stabilised intermediates; 15,000 mAh g−1 capacity, 88% efficiency, 137-cycle stability at 500 mAh g−1	[71]
Fe-FcDC MOF	Ligand enabling MLMCT and photothermal effect	Cycloaddition	Epichlorohydrin + CO2	Epichlorohydrin carbonate	1135 mmol g−1 in 4 h; TON = 474.5 under full-spectrum irradiation	[72]
32	Ligand scaffold	Hydrocarboxylation	Alkenes or alkynes + CO2	Carboxylic acids	84% yield; Fc improved air-stability	[73]
PMo12@Zr–Fc MOF	Photothermal scaffold + structural support	Cycloaddition	Styrene oxide + CO2	Styrene carbonate	88.05% yield under solar heating; TON = 601; TOF = 75.1 h−1; enhanced light-to-heat conversion via Fc-containing MOF	[25]
38	Photothermal agent and Lewis acid site	Cycloaddition	Styrene oxide + CO2	Styrene carbonate	94.7% yield under 0.3–0.4 W cm−2 simulated sunlight; TOF = 71.6 h−1; retains >93% yield under 15 vol% CO2; thermal stability >130 °C	[5]
30, 31	Ligand scaffold	Reductive carboxylation	CO2 + C2H4 + Et3SiH	Triethylsilyl acrylate, propionate	TON = 10 for acrylate; Fc tunes bite angle and hydride stability	[14]
41	Redox-inert, bifunctional scaffold	Cycloaddition to epoxides	2-phenyloxirane, terminal/internal epoxides	Cyclic carbonates (e.g., styrene carbonate)	Up to 95% conversion, TOF = 160 h−1 at 0.1 mol%, 150 °C, 10 bar CO2; halide-free, bifunctional system	[74]

summarises key examples of Fc-enabled CO₂ fixation to contextualise these and related systems, detailing the fixation type, target products, role of Fc, and relevant catalytic outcomes.

Ferrocene-based diphosphine ligands have proven effective in Cu-mediated CO₂ fixation. As shown in Scheme 1, the direct reaction of complex 30 with CO₂ and benzaldehyde under basic conditions yields the μ–η²:η²-carbonato-bridged dicopper(I) complex 31 [68]. The dinuclear species features a nearly planar Cu₂CO₃ core with side-on coordination of the carbonate in relation to both Cu(I) centres. X-ray diffraction reveals Cu–O distances of ~2.0 Å and a delocalised C–O bond framework, while IR spectra confirm carbonate coordination with strong asymmetric and symmetric stretching bands at 1576 and 1341 cm⁻¹, respectively [68].

In a related system, Fc-based diphosphine ligands such as dppf and 1,1′-bis(dicyclohexylphosphino)ferrocene were employed in the Ru(II)-catalysed reductive carboxylation of ethylene, using triethylsilane as a mild reductant [14]. The active species were generated in situ from [RuHCl(CO)(PPh₃)₃] via the substitution of one PPh₃ ligand to form 32 and 33 (Figure 3). These systems yielded triethylsilyl acrylate and propionate under 20 bar of equimolar CO₂/C₂H₄ at 100 °C. 33 achieved a turnover number of 10 for triethylsilyl acrylate, while 32 showed lower activity under identical conditions. Although the Fc unit is redox-inactive, its incorporation significantly influenced catalytic efficiency by tuning the bite angle and basicity of the phosphine moiety. Mechanistically, the reaction proceeds via oxidative cyclisation of CO₂ and ethylene to a ruthenalactone intermediate, which is cleaved by the silane, affording the silyl ester product. The Fc-containing ligands were shown to affect the formation and stability of active hydride species, demonstrating a non-redox role for Fc in modulating the coordination environment and reactivity.

Beyond molecular catalysis, Fc has been incorporated into MOFs to enable solar-driven CO₂ fixation. A compelling example involves the synthesis of layered Zr-Fc MOF nanosheets functionalised with polyoxometalate clusters (PMo₁₂@Zr-Fc), which catalyse the cycloaddition of CO₂ and styrene oxide to form styrene carbonate under mild photothermal conditions [25]. In this system, the Fc units are not catalytically active but contribute to enhanced solar absorption and thermal conversion, elevating the reaction temperature to ~80 °C under 0.4 W cm⁻² simulated sunlight. The hybrid catalyst achieves an 88% yield and a turnover number of 601, aided by the Lewis acidity of Zr and Mo centres and the halide nucleophilicity of TBAB [25]. The reaction proceeds via epoxide activation, nucleophilic ring-opening, and CO₂ insertion, with in situ FT-IR confirming the formation of cyclic carbonate products.

A Cu-catalysed strategy for 3,4-boracarboxylation of 1,3-dienes with CO₂ was developed using Fc-based diphosphine ligand 34 (Figure 3), achieving high regioselectivity and catalytic efficiency [73]. The optimal system employed CuCl and dppf, delivering up to 84% yield with exclusive 3,4 selectivity. Mechanistically, the key step involves a borocupration reaction, where the 34-4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl complex interacts with the diene substrate to form an organocopper intermediate, which then undergoes CO₂ insertion. Density functional theory calculations identified this borocupration as the rate-determining step, with regioselectivity governed by the degree of substrate distortion. The Fc ligand provided an optimal balance of steric confinement and electronic properties, featuring a pocket-shaped coordination environment and a bite angle close to 100 degrees that minimised distortion and lowered the interaction energy at the transition state. Further analysis via the distortion/interaction-activation strain model confirmed that the minimal substrate deformation and optimal orbital overlap with dppf contributed to the observed selectivity. These findings highlight the role of Fc ligands in tuning both reactivity and selectivity in CO₂ fixation reactions, offering a general strategy for ligand design in metal-mediated carboxylation processes.

The Fc unit remains electronically active, and its redox behaviour shifts to more negative potentials upon metal coordination, as shown by cyclic voltammetry. The oxidation of the Fc moiety occurs at approximately 0.46 V vs. Ag/AgCl, around 80 mV lower than free dppf, indicating substantial electronic communication between the Fc backbone and the Cu-carbonato centre [68]. Although catalytic turnover and Faradaic efficiency were not reported, the structural and electrochemical insights provided by this system offer a valuable precedent for the rational design of redox-active CO_2-binding motifs. The Fc moiety in the dppf ligand plays an indirect but relevant role by modulating the electron density at the Cu(I) centres through the phosphine arms. This electron donation helps stabilise the bridging carbonate and tunes the metal’s reactivity towards CO₂ coordination, even though Fc does not interact directly with the CO₂ ligand. It illustrates how Fc’s phosphine derivatives can stabilise reactive intermediates and influence metal electronic structure in CO₂ coordination complexes.

A series of Cu(I) complexes containing dtbpf were synthesised and applied in the carboxylation of terminal alkynes with CO₂ under mild conditions [69]. These complexes, 29 (Figure 2), 35–37 (Figure 3), demonstrated high catalytic efficiency, converting phenylacetylene and substituted alkynes into propiolic acids with isolated yields of up to 96% at ambient pressure and temperature. The best performance was obtained with 29, which exhibited a P-Cu-P bite angle of 120.1 degrees, the largest reported for a copper(I) complex bearing a dtbpf ligand, indicating that wider ligand bite angles may correlate with enhanced catalytic activity [69]. Electrochemical studies indicated quasi-reversible redox processes and the presence of electronically active Fc backbones. The catalytic mechanism is proposed to proceed via copper acetylide intermediates, followed by CO₂ insertion into the Cu-C bond. The role of Fc is primarily as a redox-active structural scaffold that enables electronic tuning through ligand geometry rather than direct involvement in CO₂ activation. However, the presence of Fc appears essential for stabilising the copper centre and enabling efficient turnover.

The Fc-containing porous organic polymer 38 (Figure 3) was synthesised via imine condensation between 1,1′-ferrocenedicarboxaldehyde and 2-(4-aminophenyl)-1H-benzimidazol-5-amine, forming a conjugated framework in which Fc units are integrated within the backbone [5]. The resulting structure exhibited a high surface area (701 m² g⁻¹), hierarchical porosity, and excellent thermal stability. Under simulated solar irradiation (0.3–0.4 W cm⁻²), the polymer catalysed the cycloaddition of CO₂ with styrene oxide to yield styrene carbonate in up to 94.7% yield, with a turnover frequency of 71.6 h⁻¹. The catalyst maintained over 93% yield even under 15 vol% CO₂, simulating flue gas conditions. Mechanistically, the benzimidazole units function as Lewis bases to adsorb and activate CO₂, while the Fc units act as Lewis acids to activate the epoxide. The π-conjugated Fc core also enables strong photothermal conversion, generating local temperatures exceeding 130 °C under sunlight, which sustains catalysis without the need for charge carriers. The material demonstrated excellent recyclability across multiple catalytic cycles, confirming its potential for solar-driven CO₂ fixation in realistic conditions.

A series of halide-free, neutral, and bifunctional one-component catalysts were developed using functionalised Fc derivatives to promote the cycloaddition of carbon dioxide with epoxides [74]. Four catalysts (Figure 3), [(dimethylamino)methyl]ferrocene 39, (hydroxymethyl)ferrocene 40, 1,2-bis[(dimethylamino)methyl]ferrocene 41, and 1-[(dimethylamino)methyl]-2-(hydroxymethyl)ferrocene 42, were synthesised to explore the influence of Lewis basic substituents on catalytic activity. 41 showed the highest activity, achieving 95% conversion under 0.1 mol% catalyst loading, 10 bar CO₂, and 150 °C in 6 h, with a turnover number of 950 and TOF of 160 h⁻¹. Catalytic activity correlated with the number of dimethylamino groups, enhancing epoxide activation through nucleophilic attack. Density functional theory calculations revealed a four-step mechanism: (1) Cp ring slippage creating a coordination site; (2) nucleophilic S_N2 attack of the amino group on the epoxide; (3) CO₂ insertion via a seven-membered metallacycle intermediate; and (4) cyclisation to form the carbonate and regenerate the catalyst. The rate-determining step involved the nucleophilic attack with an activation barrier of 59.15 kcal mol⁻¹. A range of terminal and internal epoxides, including those bearing alkyl, aryl, ether, and halide substituents, were efficiently converted with retention of stereochemistry, indicating two consecutive S_N2 steps.

A rhodium(II)-catalysed protocol for the direct carboxylation of C–H bonds in Fc derivatives was developed, enabling the formation of Fc-based lactones under an atmospheric pressure involving CO₂ [67]. The transformation proceeds via the selective activation of the sp² C-H bond on the Cp ring of 2-ferrocenylphenols, followed by the nucleophilic insertion of CO₂ and intramolecular lactonisation. The reaction employs Rh₂(OAc)₄ coordinated with N-heterocyclic carbene ligands (e.g., isopropyl) in mesitylene at 150 °C, with tert-butoxide as the base. Under optimised conditions, the model substrate afforded the lactone product up to 78% yield within one hour, and a variety of substituted analogues were synthesised with yields ranging from 44 to 84%. Preliminary mechanistic studies supported a redox-neutral Rh(II) catalytic cycle, involving metallacycle formation, CO₂ insertion, and lactone closure. The method represents one of the first examples of asymmetric C-H carboxylation of Fc with CO₂, achieving enantiomeric excesses up to 47% using chiral N-heterocyclic carbene ligands. These findings establish a new strategy for regioselective and potentially enantioselective CO₂ fixation onto Fc cores under mild, homogeneous catalytic conditions.

Ferrocene-based MOFs incorporating Fe₃O clusters and FcDC ligands were engineered to explore long-range charge transport effects in photocatalytic CO₂ cycloaddition [72]. The Fc-containing MOF demonstrated enhanced activity attributed to a metal-to-ligand-to-metal charge transfer pathway, where photoexcitation induces directional electron flow from the Fc unit through the ligand backbone to the Fe₃O nodes. This mechanism prolongs charge carrier lifetimes (16.00 ns) and facilitates efficient separation of photogenerated electrons and holes. Under full-spectrum irradiation, the Fe-FcDC MOF achieved a TON of 474.5 for the cycloaddition of CO₂ with epichlorohydrin, equivalent to 1135.0 mmol g⁻¹ of cyclic carbonate. Photogenerated electrons activated CO₂, while holes promoted epoxide ring-opening via a Lewis acid-assisted mechanism. Under irradiation, the catalyst suspension reached 56 °C, indicating a significant photothermal contribution. In situ, FT-IR and GC-MS analyses confirmed intermediate formation and selective CO₂ incorporation. The MOF retained its crystallinity and catalytic performance over multiple cycles, establishing it as a stable and highly efficient light-driven CO₂ fixation system.

Ferrocene has recently been employed as a functional additive in Li-CO₂ batteries to regulate the CO₂ reduction pathway and enhance product selectivity [71]. In conventional systems, CO₂ is reduced via a four-electron pathway to form Li₂CO₃ and carbon, which are electrically insulating and difficult to decompose. In contrast, the presence of Fc favours a two-electron route, yielding Li₂C₂O₄ as a predominant discharge intermediate. This intermediate is more readily decomposed during charging, improving cycle reversibility. Ferrocene molecules interact with CO₂ and modulate the solvation environment of Li⁺, thereby stabilising key intermediates such as LiCO₂ and Li₂CO₂ and promoting their transformation into Li₂C₂O₄. Density functional theory calculations and spectroscopic analyses confirmed Fc’s ability to shorten the CO₂ reduction pathway by enhancing CO₂ adsorption and altering local electron density. Batteries with Fc exhibited a fivefold increase in initial discharge capacity (up to 15,000 mAh g⁻¹), reduced overpotential, and stable cycling over 137 cycles. These findings highlight Fc’s effectiveness in tuning product selectivity and improving Li-CO₂ battery performance by stabilising intermediates and guiding uniform discharge product formation.

A recent study demonstrated the catalytic N-formylation of CO₂ using an atomically precise bimetallic cluster, Au₈Pd₁(dppf)₄²⁺, where dppf functions as a rigid, bidentate ligand that enhances cluster stability and facilitates electron transfer [70]. The system catalyses the hydrogenation of CO₂ in the presence of o-phenylenediamine to form benzimidazole with 96.4% yield under mild solvothermal conditions. Single-crystal X-ray diffraction confirmed that ligand exchange from PPh₃ to dppf preserved the core metal architecture while increasing structural rigidity and thermal robustness. The reaction proceeds via formate intermediates that condense with the amine to generate the heterocycle. Remarkably, the benzimidazole product can coordinate with Zn²⁺ in situ to form a two-dimensional MOF, which subsequently catalyses the cycloaddition of CO₂ with epoxides, achieving up to 98% yield. This dual catalytic system highlights the potential of Fc-based ligands to support multifunctional CO₂ utilisation across both molecular and materials-based platforms.

Taken together, these systems illustrate the diverse roles Fc can play in enabling CO₂ fixation across molecular, cluster-based, and framework architectures. While Fc is rarely redox-active in these reactions, its inclusion as a ligand scaffold or framework component profoundly affects catalyst performance. In hydrogenation and N-formylation systems, dppf and dtbpf ligands enhance cluster stability, widen bite angles, and support electron flow, leading to high yields and turnover numbers under moderate reaction conditions. In carboxylation reactions, Fc-containing diphosphines modulate the electronic landscape at nickel or copper centres, improving selectivity and occasionally enabling operation under milder conditions (e.g., ≤80 °C and ambient CO₂ pressure) [18]. Cycloaddition systems further showcase Fc’s versatility, although they typically require elevated CO₂ pressures and temperatures to achieve high activity. Conjugated polymers and MOFs leverage Fc’s photothermal properties to drive CO₂ conversion under solar irradiation. Notably, the Fe–FcDC MOF and Fc-POP frameworks achieved high turnover numbers (up to 474.5) and excellent recyclability [30]. Across these platforms, Fc functions not only as a structural motif but also as a mediator of charge distribution, steric control, or thermal responsiveness. This adaptability reinforces Fc’s value in constructing multifunctional, tuneable CO₂ fixation catalysts that bridge molecular design and material integration.

5. Ferrocene-Modified Materials for CO₂ Capture and Adsorption

While the preceding examples illustrate chemical transformation pathways for CO₂ fixation, Fc has also been employed in many porous materials designed for CO₂ capture via physisorption. Ferrocene is typically not redox-active in these systems but is an electronic and structural motif that enhances CO₂ affinity, thermal stability, and recyclability. Recent studies have incorporated Fc units into polymers, porous frameworks, and hybrid composites, leveraging their π-electron density, spatial rigidity, and polarisable iron centres to improve adsorption properties. These Fc-functional materials often exhibit notable CO₂ uptake and reversible binding under mild conditions, offering operational benefits compared to conventional chemisorptive approaches. This section surveys representative systems where Fc contributes to enhanced CO₂ adsorption performance, with emphasis on structural roles, gas uptake capacities, and regeneration efficiencies.

Among the earliest examples, the polymer 43, an Fc-functionalised polyamide–polydimethylsiloxane block copolymer (Figure 4) [75], demonstrates reversible CO₂ physisorption capacities up to 9.6 cm³ g⁻¹ at 313 K and 1 atm (

Table 3. Representative Fc-functionalised materials for CO₂ capture.

Material	Fc Role	CO2 Uptake (wt%) 1	Conditions	Notes/Mechanism	Ref.
43	Structural & electronic	1.88	313 K, 1 atm	Paramagnetic NMR; polar functional groups facilitate adsorption	[75]
44	Structural	6.26	273 K, 1 atm	BET = 546 m2 g−1; Qst = 27.8–24.7 kJ mol−1; hierarchical porosity with micropore/mesopore coexistence	[76]
		3.62	298 K, 1 atm
		6.26	273 K, 1 atm	Qst = 27.8–24.7 kJ mol−1; moderate surface area (546 m2 g−1)	[77]
45	Structural	10.3	273 K, 1 atm	Higher porosity (954 m2 g−1); Qst = 30.8 kJ mol−1
46	Structural	5.1	273 K, 1 atm	Qst up to 32.9 kJ mol−1; mesoporous–microporous mix (BET = 499 m2 g−1)	[32]
47	Structural	2.38	298 K, 1 atm	Lower surface area (354 m2 g−1); reduced uptake
58	Structural + Fe centres	16.9	273 K, 1 atm	Qst = 41.6 kJ mol−1; high CO2/N2 selectivity (107:1)	[23]
59	Structural & electronic	16.61	273 K, 1 atm	BET: 752.4 m2 g−1; Qst = 32.8 kJ mol−1	[15]
Fc-crosslinked polystyrene resin	Structural & selective	4.71	298 K, 1 atm	Low BET (740 m2 g−1); IAST selectivity CO2/CH4 = 4.3, CO2/CO = 20.6	[29]
48	Structural	7.39	273 K, 1 atm	π-π and quadrupole interactions; Qst = 25.21 kJ mol−1	[78]
49	Spatial organiser	Reversible binding	298 K, 1 atm	Chemisorption via B-N FLP pairs; X-ray confirmed dicarbamate adduct	[79]
52–55	Redox-active backbone	Up to 9.77 (52 at 273 K)	273–298 K, 1 atm	BET 72–341 m2 g−1; pore widths: 0.4–1.9 nm	[28]
50	Structural	6.25	298 K, 1 atm	BET = 50 m2 g−1; planar pyrene core enhances π-stacking and porosity	[80]
51		5.77		BET = 8 m2 g−1; twisted TPE unit lowers porosity; modest post-ROP uptake increase
56	Structural & electronic	6.91	273 K, 1 atm	BET = 556 m2 g−1; large pore volume (1.26 cm3 g−1); enhanced uptake from N-rich sites	[81]
		5.90	298 K, 1 atm
57	Structural & electronic	6.73	273 K, 1 atm	BET = 428 m2 g−1; slightly lower uptake; PBDT contributes N-centres for CO2 interaction
		2.25	298 K, 1 atm

¹ To ensure consistency and facilitate comparison, all CO₂ uptake values were converted to weight percent (wt%) using the molar mass of CO₂ (44.01 g mol⁻¹). The conversion formula used was as follows: wt% = (mmol g⁻¹ × 44.01)/10. For values originally reported in cm³ g⁻¹, the conversion to wt% assumed a molar gas volume of 22,414 cm³ mol⁻¹ at STP.

).

The materials operate without thermal regeneration, suggesting a physisorptive mechanism involving polar functional groups such as carbonyl, ether, and NH moieties. Solid-state NMR reveals paramagnetic effects from the Fc moiety, suggesting a secondary electronic contribution to gas interaction [75].

Similarly, an Fc-containing POP 44 synthesised via Sonogashira–Hagihara coupling of 1,1′-dibromoferrocene with tetrakis(4-ethynylphenyl)silane (Figure 4) exhibits CO₂ uptake capacities of 1.42 mmol g⁻¹ (6.26 wt%) at 273 K and 1 atm, and 0.82 mmol g⁻¹ (3.62 wt%) at 298 K and 1 atm [76]. The isosteric heat of adsorption reaches up to 27.8 kJ mol⁻¹ at low coverage, gradually decreasing to 24.7 kJ mol⁻¹ with increasing CO₂ loading, suggesting strong physisorptive interaction. Despite its moderate surface area (BET = 546 m² g⁻¹), the polymer outperforms several higher-area analogues, highlighting the beneficial role of Fc units in enhancing framework–gas interactions.

Building upon their previous work, the same research team revisited the Fc-containing POP 44 and introduced an alternative framework, 45 (Figure 4), to further explore the effects of structural variation on CO₂ capture performance [77]. Polymer 45 exhibited superior CO₂ uptake of 10.3 wt% at 273 K and 1 bar compared to 44, which reached 6.26 wt% under identical conditions. Additionally, 45 showed a higher isosteric heat of adsorption at 30.8 kJ mol⁻¹, relative to 27.8 kJ mol⁻¹ for 44. These improvements were attributed to the enhanced porosity and surface area of 45 (BET = 954 m² g⁻¹), facilitating more extensive framework–gas interactions. Despite lacking redox activity, Fc plays a structural role in both materials by contributing to backbone rigidity and electronic polarisation, ultimately supporting high CO₂ affinity and cycling durability.

Two Fc-containing POPs, 46 and 47 (Figure 4), were synthesised via Heck coupling of 1,1′-divinylferrocene with tetrahedral silicon-based monomers, namely tetrakis(4-bromophenyl)silane and tetrakis(4′-bromo-[1,1′-biphenyl]-4-yl)silane [32]. The resulting materials possess moderate porosity and high thermal stability, with BET surface areas of 499 and 354 m² g⁻¹ and pore volumes of 0.43 and 0.49 cm³ g⁻¹, respectively. Both frameworks exhibit mixed microporous–mesoporous architectures, confirmed by non-local DFT calculations based on nitrogen adsorption–desorption isotherms at 77 K. The frameworks retain structural integrity up to ~400 °C, as thermogravimetric analysis shows. The CO₂ adsorption studies revealed that 46 displayed capacities of 1.16 mmol g⁻¹ (5.10 wt%) at 273 K and 0.54 mmol g⁻¹ (2.38 wt%) at 298 K under 1.0 atm. Meanwhile, 47 showed a slightly lower uptake, consistent with its reduced surface area and micropore fraction. The isosteric heats of adsorption, calculated via the Clausius–Clapeyron equation, reached up to 32.9 kJ mol⁻¹ for 46, indicating strong physisorptive interactions between the Fc motifs and CO₂ molecules. These findings highlight the role of Fc in modulating framework affinity toward CO₂ and underscore the utility of direct polymerisation strategies using Fc-functionalised monomers for gas sorption applications.

A structurally distinct approach to Fc-based CO₂ capture involves coordination polymerisation with lanthanide centres. In a representative example, a 1D porous coordination polymer was synthesised from ligand 48 (Figure 4), an Fc derivative bearing boronate ester groups, and terephthalic acid linkers to form a robust and moderately porous framework. While lanthanum nitrate was used in the polymer formation, its coordination role is inferred rather than explicitly shown in the reported structure [78]. The Fc unit, while redox-inactive, enhances CO₂ affinity via π-π interactions and electronic polarisation. The proposed mechanism involves a combination of weak electrostatic interactions, quadrupole–dipole coupling between CO₂ and the Fc-based ligand environment, and π-π stacking interactions involving the Fc and terephthalic aromatic units [78]. These non-covalent forces collectively stabilise CO₂ within the framework and contribute to the observed selectivity and sorption capacity. The porous coordination polymer exhibited a CO₂ uptake of 37.62 cm³ g⁻¹ (1.68 mmol g⁻¹) at 273 K and 1 atm, with an isosteric heat of adsorption of 25.21 kJ mol⁻¹. Despite a modest surface area (BET = 215.98 m² g⁻¹), the polymer showed strong selectivity over N₂ and CH₄ and excellent cycling stability.

A 1,1′-ferrocene-bridged bis(boramidinate) system, 49 (Figure 4), has been developed for reversible CO₂ capture via FLP chemistry [79]. Each arm comprises a boron–nitrogen FLP, formed from the reaction of a dicarbodiimidoferrocene with 9-borabicyclo [3.3.1]nonane, generating a sterically encumbered yet reactive boramidinate. Upon exposure to 1 atm CO₂ in toluene, both FLP sites undergo chemisorption through nucleophilic attack of the nitrogen on the carbon of CO₂, accompanied by coordination of the oxygen to the tricoordinate boron, yielding a symmetric dicarbamate adduct. Structural confirmation was provided by X-ray crystallography. The CO₂ binding is reversible; under an inert atmosphere in CH₂Cl₂, partial dissociation leads to a mono-adduct, and complete regeneration of 49 is achievable. A comparable sequence occurs with CO gas, forming a dicarbonyl adduct that also dissociates in solution. The Fc unit serves as a rigid, redox-inert spacer that spatially organises the two FLPs without participating in the binding event.

While the reversible binding of CO₂ by 49 showcases the utility of Fc-tethered FLPs in molecular capture, the efficiency of this system is best interpreted in a qualitative mechanistic context. Unlike porous physisorbents, this system operates via discrete stoichiometric chemisorption, with one CO₂ molecule bound per FLP site. Consequently, no data on the surface area, CO₂ uptake per gram, or isosteric heat are reported. This limits direct comparison with porous materials engineered for high-capacity storage. Instead, its merit lies in selective reactivity, reversible small-molecule activation, and structural precision. The redox-inert Fc scaffold enables spatial control but does not confer redox functionality or improve uptake scalability. Such systems are promising as tuneable molecular receptors or building blocks for multifunctional materials, but further integration into porous or polymeric matrices would be necessary to translate this concept into practical CO₂ capture technologies.

Two Fc-containing conjugated microporous polymers, 50 and 51 (Figure 4), were synthesised via Sonogashira coupling of a 9-ferrocenylidene-2,7-dibromo-9H-fluorene monomer with tetraethynylpyrene or tetrakis(4-ethynylphenyl)ethene, respectively [80]. These conjugated microporous polymers were further modified through host–guest complexation with a benzoxazine-linked β-cyclodextrin, generating hybrid inclusion complexes. The benzoxazine moieties underwent ring-opening polymerisation upon thermal treatment, introducing phenolic and nitrogen-rich functionalities into the polymer backbone. These modifications enhanced thermal stability and surface chemistry. The CO₂ uptake was assessed at 298 K and 1 atm, with modified 50 exhibiting the highest capacity (1.42 mmol g⁻¹), followed by modified 51 (1.31 mmol g⁻¹). The increased uptake was attributed to improved porosity and the presence of CO₂-philic functional groups (OH and NR₂), which interact via hydrogen bonding or Lewis acid–base interactions. The Fc units serve as electronically active, spatially defined anchors for the conjugated framework without directly participating in CO₂ binding.

A family of redox-active Fc-containing conjugated microporous polymers was synthesised via aminative cyclisation of 1,1′-diacetylferrocene with various aryl aldehydes to yield 52–55 (Figure 4) [28]. These polymers displayed permanent porosity, with BET surface areas ranging from 72 to 341 m² g⁻¹ and average pore widths in the microporous-mesoporous regime (0.40–1.90 nm). The Fc units served as redox-active spacers embedded within the conjugated backbone, contributing to the materials’ electrochemical properties while maintaining structural integrity. The CO₂ uptake was measured at 1 atm across 273 K and 298 K. Among the series, 52 exhibited the highest adsorption capacity, reaching 2.22 mmol g⁻¹ at 273 K and 1.3 mmol g⁻¹ at 298 K, attributed to its larger surface area and favourable pore topology. The uptake followed physisorption mechanisms, as indicated by the isotherm profiles and pore size distributions. These results reinforce the role of Fc-based frameworks as tuneable platforms for CO₂ capture in porous organic materials.

Two Fc-containing porous organic polymers, 56 and 57 (Figure 4), were synthesised via Schiff base condensation between ferrocenecarboxaldehyde and either melamine or 6,6′-(1,4-phenylene)bis(1,3,5-triazine-2,4-diamine), respectively [81]. Both materials incorporate the Fc moiety as a structural and electronic motif, linking it to nitrogen-rich aromatic amines that introduce basic sites and tuneable porosity [81]. 56 exhibited a higher surface area (556 m² g⁻¹) and total pore volume (1.26 cm³ g⁻¹) compared to 57 (428 m² g⁻¹, 0.89 cm³ g⁻¹), which translated into stronger CO₂ physisorption. At 298 K and 1 atm, CO₂ uptake capacities were 1.34 mmol g⁻¹ for 56 and 0.51 mmol g⁻¹ for 57; at 273 K, values increased to 1.57 and 1.53 mmol g⁻¹, respectively. The adsorption mechanism is dominated by non-covalent interactions, with nitrogen and imine functionalities acting as Lewis base sites for CO₂ stabilisation. Pore size distributions obtained from non-local DFT analysis confirmed the presence of both micro- and mesopores, while thermogravimetric and FT-IR analyses demonstrated excellent thermal stability and structural retention after repeated cycling. These findings position 56 and 57 as promising ferrocene-based physisorbents for low-pressure CO₂ capture under ambient conditions.

A distinctive approach to CO₂ capture involves the integration of Fc moieties into microporous aromatic polymers [23]. These polymers were synthesised via a one-step Friedel-Crafts alkylation between Fc and s-triazine units, yielding networks with homogeneous Fc dispersion and exceptional chemical stability. Among the synthesised variants, 58 (Figure 4) exhibited a CO₂ uptake of 16.9 wt% at 1.0 atm and 273 K, along with a CO₂/N₂ selectivity of 107 (v/v) based on the ideal adsorbed solution theory. This impressive selectivity was attributed to the nitrogen-rich s-triazine units and Fe centres, which act as Lewis acidic sites promoting CO₂ adsorption. Importantly, the isosteric heat of CO₂ adsorption reached 41.6 kJ mol⁻¹, indicating strong physisorptive interactions while remaining within a practical range for regeneration [23]. Although the microporous aromatic polymers are not electroactive materials in the traditional sense and were not tested for CO₂ electrocatalytic reduction, the use of Fc here is structurally relevant; it enables uniform Fe distribution and contributes to microporosity and gas affinity. This highlights an alternative functional role for Fc in carbon capture beyond redox mediation, suggesting future directions in designing porous polymeric sorbents incorporating metallocene units.

A complementary approach to Fc-enabled CO₂ capture involves its use as a structural and electronic motif within nanoporous organic polymers. The Fc-based nanoporous organic polymer 59 (Figure 4) was synthesised by coupling 1,1′-ferrocenedicarboxaldehyde with melamine under solvothermal conditions [15]. The resulting material exhibited a moderate surface area of 752.4 m² g⁻¹, measured using the BET method, and an exceptionally high pore volume of 1.32 cm³ g⁻¹. At 273 K and 1 bar, 59 demonstrated a CO₂ uptake capacity of 16.61 wt%, outperforming many other nanoporous organic polymers with larger surface areas [15]. The high performance was attributed to two key factors: (i) the electron-withdrawing nature of the Fc units, which enhances CO₂ interaction via polarisation effects, and (ii) the presence of heteroatom sites from melamine, facilitating dipole–dipole interactions. The isosteric heat of adsorption reached up to 32.8 kJ mol⁻¹, indicating strong yet reversible physisorption, a desirable balance for practical CO₂ capture. While not tested for catalytic CO₂ conversion, this material underscores the structural versatility of Fc in gas-binding frameworks and supports the rationale for further exploration of Fc-based capture and conversion hybrids.

A Fc-functionalised hyper-crosslinked polystyrene-divinylbenzene resin was synthesised via a one-step Friedel-Crafts alkylation using Fc as a co-monomer [29]. This spherical adsorbent displayed a reduced BET surface area (740 m² g⁻¹) compared to the unmodified one (1340 m² g⁻¹), attributed to Fc’s role as a steric blocker during crosslinking. Despite its lower porosity, the Fc-containing resin achieved a comparable CO₂ uptake of 1.07 mmol g⁻¹ at 298 K and 1 atm and significantly higher selectivity for CO₂ over CO (ideal adsorbed solution theory selectivity = 20.6) and CH₄ (4.3), outperforming many benchmark adsorbents. The enhancement was linked to stronger physical interactions between CO₂ and the electron-rich Cp rings, as evidenced by molecular electrostatic potential mapping and a moderate heat of adsorption of 24.5 kJ mol⁻¹, which supported low-energy regeneration under vacuum. Breakthrough experiments confirmed high separation efficiency and recyclability under ambient pressure swing adsorption conditions, establishing this material as a promising ferrocene-based physisorbent for selective CO₂ capture.

6. Outlook and Critical Assessment

The preceding sections have demonstrated the remarkable breadth of Fc’s roles across electrocatalytic domains, from hydrogen evolution and carbon dioxide transformations to organic electrosynthesis. However, the complexity and fragmentation of these examples call for more integrated reflection. This final section critically assesses the opportunities and limitations inherent to Fc-based systems while identifying unifying principles that could inform future developments. Rather than evaluating each application in isolation, emphasis is placed on cross-cutting trends, emergent design strategies, and why Fc occupies a distinctive position in redox catalysis.

6.1. Cross-Cutting Advantages of Ferrocene-Based Catalysis

Ferrocene-derived systems demonstrate exceptional modularity, combining redox activity, structural stability, and synthetic versatility. Across HER, CO₂RR, and electrosynthetic applications, Fc’s reversible one-electron redox couple supports charge mediation, redox-switchable catalysis, and tuneable ligand environments (see Section 2.2 and Section 3.1 for supporting examples). Its compatibility with molecular and framework architectures (e.g., MOFs, COFs, POPs) and hybrid materials like CNTs or semiconductors underscores its utility as a multifunctional platform. In biosensing and electrosynthesis, Fc further enables mild, selective transformations under low overpotentials, often outperforming traditional mediators in stability and recyclability.

6.2. Limitations and System-Specific Challenges

Despite these strengths, Fc-based catalysts face several limitations. In HER, Fc’s redox potential is typically misaligned with the thermodynamic requirements for efficient proton reduction. In CO₂RR and fixation, while Fc-based ligands and mediators can enhance electron delivery and modulate binding modes, they rarely function as the sole active site. Their role is often auxiliary, contributing to electronic tuning, spatial configuration, or thermal responsiveness rather than directly participating in bond activation. Stability under reductive or basic conditions, leaching in homogeneous media, and limited intrinsic catalytic activity remain obstacles, especially in long-term deployment or scaled systems.

6.3. Ferrocene as a Precursor in Electrocatalyst Fabrication

While this review has focused on systems where the Fc moiety remains chemically intact and functionally active, recent developments suggest a complementary use of Fc as a precursor for iron-based catalyst fabrication. In particular, floating catalyst chemical vapour deposition methods employ Fc as an iron source to grow carbon nanotube fibres, which are subsequently integrated into lithium–CO₂ batteries as flexible cathodes. Though the Fc structure decomposes during synthesis, it yields iron nanoparticles that serve as catalytic nucleation centres within a conductive carbon framework. These carbon nanotube fibres have demonstrated reversible CO₂ reduction and evolution reactions, high mechanical strength, and stable electrochemical performance across multiple charge–discharge cycles [82].

This approach mirrors strategies in HER catalysis, where Fc-derived ligands or MOFs are sacrificed during thermal treatment to template or dope redox-active carbon architectures. For instance, in the fabrication of Co₂P-FeP@C-5 or Fe-Ni₅P₄ catalysts, Fc serves as both a carbon and iron source, enabling the formation of porous, conductive matrices that host highly active metal phosphide domains [83]. Co₂P–FeP@C-5 exhibited an overpotential of 0.17 V at 10 mA cm⁻² and a Tafel slope of 0.09 V dec⁻¹, while Fe-Ni₅P₄ maintained industrial-scale current densities of 500 mA cm⁻² at an overpotential of 0.25 V in alkaline seawater, with near-quantitative Faradaic efficiency and operational stability exceeding 1000 h. Though the Fc unit is decomposed during the synthesis, its structural and elemental contributions are essential to achieving high electrocatalytic performance. While such systems fall outside the scope of redox-active Fc catalysis, they highlight Fc’s broader utility as a scalable, dual-function (Fe and C) molecular feedstock for electrode construction.

A highly durable hydrogen evolution electrocatalyst was developed from an Fc-based MOF, in which FcDC served as both the structural linker and carbon source for the synthesis of NiFc-MOF [84]. This design exploits the rigid metallocene backbone and π-conjugated nature of FcDC to promote crystallinity, porosity, and electronic conductivity within the MOF scaffold. Subsequent phosphidation, accompanied by controlled Fe³⁺ doping, yielded the bimetallic catalyst Fe-Ni₅P₄, embedded in a conductive carbon matrix derived from the original Fc framework [84]. The resulting catalyst exhibits exceptional activity and durability under industrially relevant alkaline conditions. In 1.0 M KOH, Fe–Ni₅P₄ achieved a current density of 500 mA cm⁻² at an overpotential of just 0.25 V and maintained continuous operation for 1170 h at 1.0 A cm⁻² in alkaline seawater and 2700 h at 0.5 A cm⁻² in pure KOH, with a near-quantitative Faradaic efficiency of approximately 100%. Structural characterisation confirmed the formation of interconnected nanospheres with high electrochemically active surface area and chloride-tolerant morphology. Density functional theory calculations revealed that Fe-Ni₅P₄ possessed a significantly improved hydrogen adsorption free energy (−0.14 eV) compared to undoped Ni₅P₄ (−0.58 eV), consistent with more favourable proton-coupled electron transfer kinetics. The presence of Fe also reduced the energy barrier for water dissociation, thereby accelerating the Volmer step and facilitating HER initiation.

Furthermore, Fe doping modulated the d-band centre and increased the density of states at the Fermi level, improving electron mobility throughout the hybrid structure. Although the Fc units are chemically transformed during thermal treatment, their inclusion is essential to the hierarchical porosity, conductive carbon framework, and structural integration of active sites. This study exemplifies how ferrocene-derived MOF architectures can serve as intelligent precursors for crafting robust, high-current HER catalysts capable of operating in demanding electrochemical environments.

Further mechanistic studies are needed to clarify how precursor identity influences structural evolution, catalytic performance, and active site distribution in these pyrolysis-derived systems.

6.4. Future Directions and Emerging Opportunities

The catalytic potential of Fc-based systems extends beyond redox activity, as highlighted in the preceding section on its role as a structural and elemental precursor. This duality, where Fc can either remain chemically intact or decompose to yield functional materials, opens multiple avenues for innovation in catalyst design and deployment.

One key direction involves systematically tuning Fc’s redox properties through ligand engineering and substituent modification. Such efforts could enable precise control over electron flow, facilitating broader compatibility with diverse catalytic environments. Mechanistic studies integrating spectroscopy and electrochemistry are essential, especially for understanding charge transport, electron mediation, and proton-coupled electron transfer dynamics in hybrid systems.

Materials integration offers another promising frontier. Embedding Fc or Fc-derived components into scalable, solid-state architectures, such as porous carbon frameworks, carbon nanotube mats, or redox-active membranes, could bridge the gap between molecular control and device-scale functionality. The approach illustrated in Section 6.3, where Fc serves as a precursor for conductive, doped matrices in HER or CO₂-related systems, exemplifies how decomposition does not preclude function but may enable superior electrochemical performance.

Moreover, hybrid composites that combine Fc with photonic, magnetic, or conductive elements could yield multi-functional catalysts responsive to light, fields, or redox gradients. Finally, computational tools, especially DFT and machine learning, can accelerate the prediction of structure–function relationships, enabling the targeted synthesis of Fc-based systems with tailored activity, stability, and selectivity.

Together, these emerging strategies suggest that Fc’s utility in catalysis may increasingly lie not only in its redox behaviour but in its adaptability as a design element across molecular and materials platforms.

7. Conclusions

Ferrocene remains pivotal in electrocatalysis, not merely due to its role in reversible redox coupling but as a structurally and electronically versatile motif deployable across molecular, supramolecular, and extended material systems. This review has synthesised recent progress in hydrogen evolution, carbon dioxide reduction and fixation, electrosynthetic oxidation, and gas capture, highlighting how Fc-based platforms function as redox mediators, secondary sphere modulators, structural linkers, or sacrificial precursors in complex catalytic environments.

What emerges is not a portrait of a static scaffold but of a dynamic design element, reconfigurable to meet diverse catalytic demands, from fine-tuned electron transfer in homogeneous systems to the formation of conductive carbon architectures in high-performance heterogeneous electrodes. Fully unlocking Fc’s potential will require integrated strategies that combine ligand and materials design, real-time characterisation under working conditions, and predictive modelling. In this context, Fc remains scientifically relevant and conceptually foundational for the next generation of redox-active catalysts and functional materials.