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Article

Catalytic Application of POSS–COF-[(Co(acetate)2] for Selective Reduction of Nitriles to Amines

by
Anosha Rubab
1,
Manzar Sohail
1,*,
Riyadh H. Alshammari
2,
Ayman Nafady
2,
Md. A. Wahab
3 and
Ahmed Abdala
4,*
1
Department of Chemistry, School of Natural Sciences, National University of Sciences and Technology, Islamabad 44000, Pakistan
2
Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia
3
Energy and Process Engineering Laboratory, School of Mechanical, Medical, and Process Engineering, Faculty of Science, Queensland University of Technology (QUT), 2 George Street, Brisbane, QLD 4000, Australia
4
Chemical Engineering Program, Texas A&M University at Qatar, Doha P.O. Box 23874, Qatar
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(9), 557; https://doi.org/10.3390/catal14090557
Submission received: 12 July 2024 / Revised: 13 August 2024 / Accepted: 21 August 2024 / Published: 25 August 2024
(This article belongs to the Section Catalysis in Organic and Polymer Chemistry)
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Versions Notes

Abstract

:
We report the reticular synthesis and structural investigations through the spectroscopic analysis of a novel polyhedral oligomeric silsesquioxane (POSS)-modified framework, hereby ascribed as a catalyst for the selective reduction of aryl nitriles to amines. The integration of the unique features of the polyhedral oligomeric silsesquioxane with 2,2′-Bipyridine-4,4′-dicarboxaldehyde and subsequently coordination to cobalt acetate manifests a distinctive feature, which is a stable covalent bond between Co and the functionalized POSS, effectively preventing catalyst leaching. The cobalt acetate-modified POSS–COF, synthesized with this approach, underwent a comprehensive characterization employing various analytical techniques including FTIR, XRD, SEM, XPS, TGA, and 29Si NMR. This thorough characterization provides a detailed insight into the structural and chemical attributes of the catalyst. Our catalyst, with its exceptional catalytic efficiency in catalyzing reduction reactions compared to its homogeneous counterparts, and its distinctive three-dimensional metalated POSS system, shows outstanding catalytic performance attributed to its diverse coordination interactions with ligands. Moreover, this catalyst presents additional merits, such as facile recovery and recyclability, making it a promising candidate for sustainable and efficient catalytic processes and thus instilling hope for a greener future.

1. Introduction

Porous silica materials, both synthetic and naturally occurring, hold a significant role across diverse fields owing to their unique advantages. These materials offer inherent thermal and chemical stability, tunable surface areas, and pore sizes. Among the porous silicas, polyhedral oligomeric silsesquioxane (POSS) is an attractive class of organic–inorganic hybrid compounds, with their distinctive cubic cage-like structure, represented by the general formula (R8Si8O12) [1]. This structure features a central rigid hydrophobic inorganic core encircled by reactive organic moieties at the eight vertices. The attractiveness of these molecules lies in their ability to integrate multiple functional groups as side chains, creating a plethora of opportunities for attaching diverse organic fragments. These functional groups, known as the “organic shell”, can range from alkyl to alkylene or arylene, forming inorganic core/shell structures capable of binding organic moieties, polymers, and natural biomaterials [2].
POSS, characterized by its well-defined and symmetric structure, typically exhibits a nanoscale size ranging from 1 to 3 nm in diameter when vertex groups are incorporated. Its nano-size places it amongst the smallest silica particles achievable within this class [3]. Moreover, three additional Si-O bonds contribute to exceptional thermal, mechanical, and chemical stability, making it highly appealing from an industrial perspective [4]. The eight organic arms possess a high degree of flexibility for incorporating further organic pendant groups, including -NH2, -SH, -OH, and -COOH, which would preclude expanding its applications. These groups offer ample reactive sites for additional functionalization.
POSS, distinguished by its unique nanostructures and chemical properties, presents many favorable physicochemical characteristics. These include effortless chemical modification, pH tolerance, exceptional thermal and oxidation stability, robust mechanical properties, optical transparency, low heat transfer coefficient, fire resistance, and high hardness within silsesquioxane-based materials [5]. The nano-crosslinking properties of POSS have been utilized in synthesizing micro- and mesoporous materials characterized by high surface areas. The significance of these materials lies in their potential applications in hydrogen storage, separation techniques, tissue engineering, catalysis, and the development of electronic- and sensor-based materials. The nano-crosslinking nature of POSS positions it as a promising candidate for creating novel functional hybrid materials [6,7,8]. Beyond its physicochemical characteristics, POSS is recognized for its exceptional biocompatibility and thermo-mechanical stability. Moreover, its versatility extends to applications as diverse as an anti-thrombogenic agent, a drug carrier, optical devices, a solid electrocatalyst, and a mesoporous organic–inorganic hybrid material [9].
Substituting one or more vertex groups of POSS with various functional groups through conventional chemical conversions results in an organo-functionalized POSS species. These versatile functional groups, including methacrylate, acrylate, styrene, norbornene, amine, epoxy, alcohol, and phenol, offer opportunities for integrating POSS into a polymer chain or network through processes such as polymerization or grafting [3].
POSS also functions as a building block for immobilizing organometallic catalysts. A prevalent structure in homogeneous catalytic systems involving POSS is the metalated T7 moiety, denoted as R7Si7O12M, where M represents a transition metal, and R represents an isobutyl or cyclopentyl organic group. The T7 moiety is a cage-like structure comprising seven silicon atoms interconnected by oxygen atoms. These metalated POSS systems exhibit catalytic activity due to the presence of the metal center, which can undergo various coordination interactions with reactants or other ligands. The ligands coordinated to the metal site can modulate the reactivity and selectivity of the catalyst, allowing for the fine-tuning of its catalytic performance [10].
Wada et al. reported the synthesis of silica-supported Ti oxide catalysts, employing a Ti-containing POSS as a precursor. The resulting catalyst exhibited remarkable catalytic activity and stability in the epoxidation of cyclooctene [11]. Grela et al. also contributed to the field by introducing a POSS-tagged Grubbs–Hoveyda-type olefin catalyst that allows for the easy recovery and recyclization of catalysts through nanofiltration techniques [12]. Yan Leng et al. demonstrated the effective use of aminopropyl-POSS as a fundamental component for immobilizing oxo-molybdenum Schiff base through covalent bonding by using (tert-butyl hydroperoxide) TBHP as an oxidant. The resulting complex denoted as POSS–SB–Mo, proved to be an exceptionally efficient catalyst for alkene epoxidation [13]. These findings collectively demonstrate the potential of POSS as a carrier for establishing highly efficient heterogeneous catalytic systems. Despite the versatile applications of POSS in various fields, its unique properties have not undergone extensive exploration in catalytic applications, particularly in facilitating valuable chemical transformations. This underlines the promising future of POSS in the field of catalysis.
Similarly, covalent organic frameworks (COFs) present a relatively new class of materials that enable the reticular integration of small organic molecules into tunable porous, crystalline, and extended molecular architectures with 2D or 3D topologies [14]. COFs exhibit fascinating features, including a broad range of design possibilities and the ability for post-synthetic modifications with various functionalities while preserving their fundamental architecture [15]. The synthesis of COFs involves light elements (C, N, O, S, B, etc.), resulting in the lowest densities among the porous materials [16]. Additionally, COFs demonstrate excellent chemical and thermal stability. These exceptional characteristics make COFs highly attractive and well suited for meeting specific application needs. Recently, COFs have been reported in various applications, including gas adsorption, electrocatalysis [17], photocatalysis [9], toxic chemicals adsorption [18], carbon dioxide reduction [19], water splitting [20], and heterogeneous catalysis [21].
Metal complexes featuring organic ligands, such as porphyrins, find widespread use as molecular catalysts in significant organic transformations within the industry [22,23]. Molecular catalysts offer numerous advantages, including high selectivity, low reaction temperatures (below 250 °C), ease of modification, facile diffusion, and efficient heat transfer. Additionally, molecular catalysts boast well-defined active sites for catalysis [24], facilitating the creation of targeted and logically homogeneous catalysts tailored for specific reactions. However, despite these merits, molecular catalysts do have certain drawbacks. The separation of these catalysts from reaction products can prove to be challenging or impractical, as they often tend to be expensive, and there might be solubility issues encountered in the reaction media [25]. Conversely, heterogeneous catalysts are preferred for developing more sustainable catalytic processes due to their ease of separation, high activity, selectivity, and reduced energy consumption [26]. However, understanding the underlying mechanisms of heterogeneous catalysts remains a challenge, thereby complicating the optimization of their activities for specific applications [27]. Consequently, both molecular and heterogeneous catalysts exhibit distinct strengths and weaknesses, and the selection depends on the specific requirements of the catalytic process and the targeted application.
Amines are central intermediates and key precursors in synthesizing life science molecules, dyes, materials, and petrochemical derivatives [28,29,30,31,32,33]. Primary amines constitute crucial precursors and key intermediates to produce fine and bulk chemicals. Preparing amines from nitriles offers cost-effective, easily available, and commercially viable routes. The reduction of nitriles to primary amines under industrial conditions is traditionally carried out using state-of-the-art Raney Ni or Co [34], and copper chromite [35], which require harsh conditions. Precious-supported metal catalysts, such as Pd, Pt, Ir, and Rh have also been utilized for this purpose [36]. However, the increased scarcity and cost of the latter and the toxic nature of the former have led to the need to discover cost-effective and sustainable catalysts. Hydrogenation of aryl nitriles to valuable amines in the presence of homogeneous catalysts based on Ru [37], Rh [38], and Re [39] complexes has been proven to be efficient for reduction. Recently, the Beller, Milstein, and Tokmic groups have reported the use of non-noble metals such as Mn [40], Co [41,42,43], and Fe [44,45] with PNP Pincer ligands. Regardless of the good catalytic activity and selectivity of these metal complexes, they are relatively less stable and difficult to use. Contrarily, heterogeneous catalysts exhibit improved stability and offer the additional advantage of recyclability. To offer the selective reduction of aromatic nitriles to amines, non-noble metals, i.e., Fe, Ni, and Co-based heterogeneous catalysts have been reported [46,47,48]. Despite the successful utility of the heterogeneous catalytic systems, it is difficult to manage the reactivity and selectivity of the hydrogenation of nitriles. Hence, given the significance of amine synthesis in industrial applications, it is imperative to design novel and selective non-noble metal catalysts that can selectively reduce aromatic nitriles to amines under mild conditions.
Cobalt (II)-containing systems have been explored in recent years as an inexpensive alternative to noble metal catalysts in catalytic transformations including hydrogenation, hydroboration, and polymerization [42,49,50]. Cobalt-based catalysts are considered to be competitive catalytic systems for the transformation of readily available feedstocks, such as nitriles, esters, amides, etc. The developed protocols for the synthesis of aromatic amines from nitriles using Co-based complexes employ a tridentate pincer-ligated catalytic system [42]. The synthesis of pincer ligands and correlating catalyst is not facile, requires expensive alkyl phosphine ligands, and involves multiple steps [51]. Therefore, a general protocol for the synthesis of primary amines using cost-effective linkers with cobalt salt under moderate hydrogenation conditions is highly desirable. To address these concerns associated with reductive amination, herein, we have developed a hydrogenation catalyst for hydrogenating aromatic nitriles to aromatic amines.
In this work, we report the reticular synthesis of a novel POSS–COF (Co) framework that can be easily modified with different linkers and metal salts thus combining the advantages of both homogeneous and heterogeneous catalysts. The formation of stable covalent bonds between metal catalysts and functionalized POSS within the framework prevents the issue of catalyst leaching [52]. This method allows for the synthesis of several new catalysts with precise control over the topography, spatial arrangement of catalytic sites, simplified post-synthesis modification, and tunable pore sizes. As a benchmark, we have synthesized and evaluated a Cobalt acetate-modified POSS–COF for hydrogenating aromatic nitriles to their corresponding aromatic amines, using molecular hydrogen gas as the hydrogen source.

2. Results and Discussion

2.1. Chemical and Morphological Properties of POSS–COF (Co) Catalyst

Powder X-ray diffraction (XRD) was conducted in the 2θ range of 5 to 50° at room temperature to investigate the crystalline nature of OAS-POSS and the functionalized POSS–COF (Co). The pXRD patterns for pure OAS-POSS revealed distinct and well-defined diffraction peaks at 2θ values of 8.3, 13.4, 15.9, 17, 18.2, 19.14, 20.94, 22, 25, 27.18, 30.6, 32.9, 37.2, and 41.2. These sharp peaks indicate the excellent crystallization property of OAS-POSS [53], aligning closely with the reported pXRD patterns for OAS-POSS [2]. Contrarily, the pXRD pattern of POSS–COF (Co) indicates that the fabricated catalytic material is porous and amorphous. The incorporation of COF into POSS is evident from the broad diffraction profile in the functionalized POSS (Figure 1A). This structural characterization provides valuable insights into the crystalline properties of OAS-POSS and the amorphous nature of the functionalized POSS–COF (Co).
To investigate the presence of functional groups and vibrational modes. Fourier transform infrared spectroscopy (FT-IR) was employed to characterize OAS-POSS and POSS–COF (Co). Figure 1B shows two stretching vibrational peaks at 3432 cm−1 and 1288 cm−1, indicating the presence of a secondary amine NH2 in OAS-POSS [54]. The FT-IR spectrum of pure OAS-POSS further revealed intense absorption peaks at 1094 cm−1 and 755 cm−1, corresponding to asymmetric and symmetric stretching modes of cubic silica oxygen cage of Si-O-Si bonds [54,55]. Additionally, the characteristic peaks between 2971 and 2852 cm−1 were identified, attributing to the vibrational frequency of the C-H bond [56]. A sharp peak at 1272 cm−1 can be ascribed to the vibrational stretching frequency of the Si-C bond within the POSS cage (Table S1) [57]. The coordination of 2,2′-Bipyridine-4,4′-dicarboxaldehyde to POSS, followed by the addition of Co (CH3COO)2 to the POSS cage, is elucidated by the charge transfer steps, and the strength of the chemical bonds is revealed through the vibrational frequencies of the as-synthesized POSS–COF. Specifically, the peaks at 1615 and 1552 cm−1 correspond to the C=N stretching frequency of the POSS and 2,2′-Bipyridine-4,4′-dicarboxaldehyde linkage [58]. The shift in C=N towards a higher frequency in POSS–COF (Co) can be assigned to the conjugation, resulting in a double-bond character to the C–N bond of bipyridine. Furthermore, the shift in the band position in the POSS–COF (Co) complex, suggests that it is strongly affected by the coordination of Co metal to the 2,2′-Bipyridine-4,4′-dicarboxaldehyde ligand [59]. The literature suggests that bands corresponding to the M–N of ligands exhibit weak bands of around 614–515 cm−1, which are absent in the spectra of OAS-POSS [59,60]. The observed shifts and changes in the intensity of vibrational signatures give evidence of the successful attachment of the Co(bipyridine)-acetate complex to POSS through covalent interactions. Obtained yield of OAS-POSS: 96% (1.47 g). Elemental analysis calculated (%) for C72H80N16O12Si8 (1586.19): C 54.52, H 5.08, N 14.13. Found: C 54.57, H 5.01, N 14.25
The SEM analysis of the as-synthesized POSS–COF (Co) shows irregular spherical morphological features, indicating a uniform and homogeneous morphology of the POSS–COF hybrid. At a magnification of 2 µm, the SEM image (Figure S1) suggests a well-coalesced integration of the Co(bipyridine)–acetate complex into the POSS. Elemental mapping conducted on different areas of the POSS–COF hybrid distinctly shows the presence of Co and Si, O, N, and C within the POSS cage (Figure 2). This finding is compelling evidence, validating the successful incorporation of the Co(bipyridine)–acetate complex. Furthermore, energy dispersive spectroscopy (EDS) provides quantitative insights into the composition of the POSS–COF (Co) catalyst, revealing weight percentages of 42.71 (C), 33.59 (O), 15.11 (N), 4.67 (Si), and 3.92 (Co) (refer to Figure S2). These findings reinforce the successful synthesis of the hybrid catalyst and offer a detailed understanding of its elemental composition.
The surface composition and elemental analysis of POSS–COF (Co) were thoroughly investigated using X-ray photoelectron spectroscopy (XPS). The survey scan spectrum shows the presence of C, N, O, Si, and Co, confirming the successful coordination of Co (acetate)2 to the POSS framework, as shown in Figure S4. Further analysis of the narrow scan spectrum of C 1s reveals three characteristic peaks at 284.5 and 284.8 eV, corresponding to C-C and C-Si bonds within the POSS–COF framework. Examining the core level spectrum of N 1s reveals the presence of C=N and C-N-C bonds at 398 and 400.5 eV, indicating the coordination of POSS to 2,2′-Bipyridine. The peak at 398 eV of C=N indicates that the bipyridine group could be covalently bonded with the amino group of OAS-POSS. In the O 1s spectrum, the peak at 531.5 eV corresponds to the Si-O bonding within the inorganic core of the POSS framework, while the peak at 530.5 eV can be ascribed to the absorbed moisture from the sample surface. The Co 2p high-resolution spectrum displays two prominent peaks at 780.5 and 796 eV, representing the Co 2p3/2 and Co 2p1/2 peaks of Co+2 with a spin-orbit splitting of ~15.5 eV. Shake-up satellite peaks are also observed at 784 and 800 eV for Co 2p3/2 and Co 2p1/2, respectively. The binding energy values at 780.5 and 796 eV provide clear and compelling evidence of the coordination of Co to the N of 2,2′-Bipyridine-4,4′-dicarboxaldehyde, as shown in Figure 3.

2.2. 29Si Solid-State NMR Spectroscopy of POSS–COF (Co) Catalyst

The interpretation of the structure of [OAS-POSS-NH2]CF3SO3H and POSS–COF (Co) was accomplished through the 29Si solid-state NMR spectroscopy. The 29Si NMR of [OAS-POSS-NH2]CF3SO3H revealed the resonance associated with the cage-like POSS lies at −66.54 ppm [5]. Following the condensation with 2,2′-Bipyridine-4,4′dicarboxaldehyde and subsequent coordination of Co to form POSS–COF (Co), a single peak at −34.42 ppm is observed. This indicates that the cage-like structure of POSS remains intact in POSS–COF (Co) (Figure 4A).
The presence of a singular symmetrical peak in the 29Si NMR spectrum aligns with the cubic structure of POSS [2], implying the existence of a single type of silicon atom characterized by siloxane -Si-O-Si- moieties in the molecule. Furthermore, the absence of silanol (-Si-OH) groups further confirms the closely retained cage-like structure.

2.3. Thermal Properties of POSS–COF (Co) Catalyst

The core structural unit, the silsesquioxane cage of POSS, consisting of Si and O atoms, ensures the POSS compounds’ thermal and chemical resistance. In contrast to pure organic compounds, hybrid organic–silsesquioxanes exhibit significantly increased thermal stability [5]. The thermal stability of OAS-POSS and POSS–COF (Co) was analyzed using thermal gravimetric (TGA) in an air environment. The OAS-POSS thermogram (Figure 4B) indicates thermal stability up to 368 °C in an O2 environment due to the rigid silsesquioxane cage. Decomposition of POSS occurs in two steps, with the initial weight reduction associated with the decomposition of organic side chains, followed by a subsequent loss corresponding to the disintegration of the siloxane cage itself. The total weight loss of OAS-POSS from the TGA curve is 77.71%, corresponding to 22.29% of the remaining SiO2.
For POSS–COF (Co), the initial 8.56% weight loss in the TGA curve can be attributed to the moisture and solvent loss trapped amidst the side arms of the POSS compound. The thermal degradation pattern of POSS–COF (Co) is similar to that of OAS-POSS, with a more significant reduction of 54.94% at 644.7 °C attributed to the decomposition of organic side arms. Notably, the total weight loss of POSS–COF (Co) at 63.5% compared to that of OAS-POSS at 77.71% indicates the enhanced thermal stability of metal-coordinated POSS–COF (Figure 4B).

2.4. Catalytic Activity of POSS–COF (Co) Catalyst for the Reduction of Hetero (Aryl)nitriles

The newly synthesized POSS–COF (Co) catalyst was tested for catalyzing the hydrogenation of benzonitrile to benzylamine as a benchmark reaction. The utilization of POSS–COF (Co) resulted in complete conversion, yielding benzylamine quantitatively (Table 1; entry 1). In contrast, other tested materials, including POSS, POSS–COF, and the Co(bipyridine)–acetate complex, exhibited reduced to no catalytic activity for this transformation (Table 1, entries 2, 3, and 4). This indicates that only the POSS–COF (Co) catalyst was active in reducing nitrile-substituted substrates.
The choice of solvent is crucial in the hydrogenation process of benzonitrile under our experimental conditions. We tested various aprotic and protic solvents for the reduction reaction of benzonitrile using POSS–COF (Co) catalyst by employing 4.5 MPa H2, as detailed in Table 2. Among these, alcoholic solvents exhibited notable activity in the hydrogenation reaction. Isopropyl alcohol, which serves both as a solvent and a hydrogen donor, resulted in a nearly complete conversion of benzonitrile with close to 100% yield of benzylamine (entry 1). Ethanol demonstrated good activity with 76% conversion (entry 2). However, methanol and butanol showed modest to low activity levels (entries 3 and 4).
Having identified the remarkable catalytic potential of the POSS–COF (Co) catalyst, we explored its scope and limitations in the broader context of reducing nitrile compounds. The hydrogenation of various structurally diverse and challenging benzonitriles was highly facile, yielding the corresponding primary amines in good to excellent yields (Table 3). Notably, electron-donating groups on substituted aryl nitriles (2a, 2b, 2c, 2d) were efficiently transformed into the respective amines with favorable yields. Even more intricate substrates, such as Benzoyl acetonitrile and 3-Bromo-4-methylbenzonitrile (2e and 2f), successfully underwent conversion into the corresponding phenylethylamine, approaching almost quantitative yields. The investigation was expanded to halogenated benzonitriles (2d, 2f, 2j, and 2o), so that crucial chemical industry feedstocks could be successfully reduced without significant dehalogenation.
To enable the utilization of this procedure in advanced organic synthesis and drug discovery, it is imperative to attain a high level of chemo-selectivity. In this context, the catalyst exhibited excellent hydrogenation selectivity in the presence of esters and ketones (2g and 2h). The nitrile group was selectively reduced in other structurally varied and functionalized aromatic nitriles, leaving hydroxyl groups, esters, and C-C bonds unaffected. Gratifyingly, this catalyst exhibited comparable activity and selectivity for hydrogenating cyclic aliphatic substrates. This resulted in the synthesis of aliphatic amines bearing long-chain and (bi)cyclic varieties, with yields reaching up to 95% (2i–2m). Moreover, the POSS–COF (Co) catalyst demonstrated its effectiveness on substrates containing two or three substituted groups and fused rings with significantly greater steric hindrance, yielding corresponding products with impressive yields (2j–2o).
Furthermore, the catalyst exhibited remarkable stability in the benchmark hydrogenation reaction, as it was reused for up to five consecutive cycles without any discernible deterioration in catalytic performance. In the recyclability test, after each run, the catalyst was separated by centrifugation, thoroughly washed with isopropyl alcohol, and subsequently dried, enabling its reusability in the next run through the same method. As shown in Figure S3, the POSS–COF (Co) catalyst can be reused up to five times without any loss of activity or selectivity. A comparison of POSS–COF with benchmark catalysts in the literature for the hydrogenation of aromatic nitriles to aromatic amines is tabulated (Table 4).

3. Materials and Methods

3.1. Experimental Supplies

Cobalt acetate (Co (OAc)2·4H2O), triethylamine (Et3N), 2,2′-Bipyridine-4,4′-dicarboxaldehyde, isopropyl alcohol, methanol, tetrahydrofuran, toluene, n-butanol, and all aromatic nitriles were purchased from Sigma Aldrich and were employed in the synthesis without any further purification.

3.2. Synthesis of POSS–COF (Co) Framework

First, 2.00 g (0.961 mmol) of freshly prepared [OAS-POSS-NH3]CF3SO3 was suspended in methanol (MeOH) (50 mL; 1:2) solution at room temperature. An excess amount of triethylamine (Et3N) (10 mmol, 10 mL) was added dropwise to the above solution with continuous stirring until a clear solution was obtained. Then, 2,2′-Bipyridine-4,4′-dicarboxaldehyde (0.885 g, 4.00 mmol) in 10 mL methanol was added slowly with continuous stirring to the above solution. The precipitate was collected by filtration, washed thrice with methanol (3 × 10 mL), and dried in an oven (90 °C, 72 h) to obtain a light yellow POSS–COF powder. Subsequently, cobalt acetate (Co (OAc)2·4H2O) (0.0354 g, 4 mmol) was added to the above mixture of POSS–COF and sonicated for 15 min to obtain a homogeneous solution, which was vacuum-dried for 12 h to give the POSS–COF (Co) catalyst. The synthesis pathway of the POSS-COF (Co) catalyst is illustrated in Scheme 1.

3.3. Plausible Mechanism for the Hydrogenation of Aromatic Nitriles Using POSS–COF (Co) Catalyst

Regarding the nature of the POSS–COF catalyst with Co (II) active sites, the hydrogenation of nitrile moiety to primary amines is a stepwise process. Initially, the C≡N bond is transformed into an imine intermediate in the presence of a POSS–COF (Co) catalyst. This step is challenging, owing to the high bond order of the triple bond [64]. The imine intermediate is subsequently reduced to primary amine with molecular hydrogen. Contrary to the first step, the second step of the imine conversion to primary amine is indeed without barrier. Under the influence of H2, hydrogen gets attached to the Co-active centre of the catalyst, replacing the labile acetate groups. Nitrile is reduced to primary amine in a concerted way by the simultaneous transfer of hydrogen from the Co centre to give the corresponding imine, which undergoes a second reaction cycle to finally yield amine. POSS within the catalysts framework indirectly tunes the reactivity of the Co (II) centre to accelerate overall reaction progress. The proposed reaction mechanism of the POSS–COF (Co) catalyst is shown in Scheme 2.

3.4. General Procedure for Catalysis Experiment

Each catalytic hydrogenation experiment was performed in four separate 8 mL glass vials arranged on an alloy plate and placed inside the 300 mL autoclave (PARR Instrument Company, Moline, IL, USA) under an argon atmosphere to prevent non-specific reductions. Initially, the 8 mL glass vial containing a stirring bar was charged with POSS–COF (Co) catalyst (25 mg) and isopropyl alcohol (5 mL) inside a glove box, and the mixture was stirred at 300 rpm for 20 min, after which the substrate was added. The vial was then capped and transferred into the autoclave. Once sealed, the autoclave underwent three purges with 1 MPa of hydrogen, followed by pressurization to 4.5 MPa hydrogen before being placed into a preheated aluminum block at 130 °C. After 20 h, the autoclave was cooled in an ice bath, and the remaining gas was released carefully. Upon completion of the reaction, the product was extracted using ethanol (3 × 5 mL). The POSS–COF (Co) catalyst was separated by centrifugation, washed three times with isopropyl alcohol, dried under vacuum at 40 °C for 6 h, and analyzed. The recovered catalyst was then reused in the next run.

3.5. Catalyst Characterization

Powder X-ray diffraction of the as-synthesized samples was analyzed by a Dron-8 diffractometer (Bourevestnik, St. Petersburg, Russia), which is operated at 20 kV using Cu Kα (λ = 0.15406 nm) radiations. The surface morphological study was analyzed using a scanning electron microscope (SEM) (HITACHI S-4800, Bancroft St. Toledo, OH, USA) equipped with EDX. The OAS-POSS and POSS–COF (Co) FTIR spectra were recorded with the ATR-FTIR Alpha II, Bruker Scientific LLC (Billerica, MA 01821, USA). TGA analysis was performed to determine the thermal decomposition of OAS-POSS and POSS–COF (Co), using the TGA 5500 Discovery instrument (TA Instruments, New Castle, DE 19720, USA) under an O2 atmosphere. X-ray photoelectron spectroscopy was employed using the Thermos Scientific (Waltham, MA 02451, USA) Escalab 250Xi spectrometer fitted with an Al Kα (1486.6 eV) X-ray source. Solid-state 29Si NMR investigations were carried out using Bruker (Karlsruhe, Germany) Advance III spectrometers.

4. Conclusions

In summary, we have successfully demonstrated the synthesis of a new type of POSS–COF hybrid material by the coordination of CoII ions and 2,2′-Bipyridine-4,4′-dicarboxaldehyde to polyhedral silsesquioxane. The resulting POSS–COF (Co) catalyst has proven to be exceptionally effective for reducing aromatic nitriles using molecular hydrogen as a reducing agent. Furthermore, we could recover and reuse the catalyst for up to five consecutive cycles without losing its catalytic efficacy. The remarkable overall catalytic performance, exceptional selectivity, and good recyclability of POSS–COF (Co) as a heterogeneous catalyst can be attributed to its structural ability as it can undergo various coordination interactions with the ligands coordinated to the metal site, which result in the modulation of the reactivity of the catalyst.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14090557/s1, Figure S1: SEM image of OAS-POSS (Co) at 2 µm resolution; Figure S2: EDS spectrum of OAS-POSS (Co) catalyst; Figure S3: Recyclability of POSS–COF (Co) catalyst for the hydrogenation of benzonitrile to benzylamine over five runs; Figure S4: Full scan XPS spectrum of POSS–COF (Co) catalyst; Table S1: Vibrational signatures of OAS-POSS characterized by FTIR.

Author Contributions

A.R.: Writing—Original Draft, Methodology, Investigation, Formal Analysis, and Data Curation; M.S.: Writing—Review and Editing, Conceptualization, Supervision, Resources; R.H.A.: Resources, Formal Analysis, Funding Acquisition; A.N.: Writing—Review and Editing, Funding Acquisition; M.A.W.: Writing—Review and Editing, Data Analysis, Resources; A.A.: Writing—Review and Editing. Resources, Funding Acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

We sincerely appreciate the Researchers Supporting Program (RSP2024R442) at King Saud University, Riyadh, Saudi Arabia.

Data Availability Statement

Data and additional information are available from CAs.

Conflicts of Interest

There are no conflicts of interest to declare.

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Catalysts 14 00557 g001
Figure 1. (A) pXRD of OAS-POSS and POSS–COF (Co) catalyst. Symbols ‘*’ and ‘♦’ represent diffraction peaks of OAS-POSS and POSS-COF (Co), respectively (B) FTIR spectra of OAS-POSS and POSS–COF (Co) catalyst. Symbols ‘♦’ and ‘♣’ represent vibrational modes of OAS-POSS and POSS-COF (Co), respectively.
Figure 1. (A) pXRD of OAS-POSS and POSS–COF (Co) catalyst. Symbols ‘*’ and ‘♦’ represent diffraction peaks of OAS-POSS and POSS-COF (Co), respectively (B) FTIR spectra of OAS-POSS and POSS–COF (Co) catalyst. Symbols ‘♦’ and ‘♣’ represent vibrational modes of OAS-POSS and POSS-COF (Co), respectively.
Catalysts 14 00557 g001
Catalysts 14 00557 g002
Figure 2. Micrograph and elemental mapping of OAS-POSS (Co) catalyst. (black: carbon, yellow: oxygen, blue: silicon, purple: nitrogen, white: cobalt).
Figure 2. Micrograph and elemental mapping of OAS-POSS (Co) catalyst. (black: carbon, yellow: oxygen, blue: silicon, purple: nitrogen, white: cobalt).
Catalysts 14 00557 g002
Catalysts 14 00557 g003
Figure 3. Narrow scan XPS spectra of (A) C 1s (B) N 1s (C) O 1s (D) Co 2p of OAS-POSS (Co) catalyst.
Figure 3. Narrow scan XPS spectra of (A) C 1s (B) N 1s (C) O 1s (D) Co 2p of OAS-POSS (Co) catalyst.
Catalysts 14 00557 g003
Catalysts 14 00557 g004
Figure 4. (A) 29Si NMR of POSS–COF (Co) in CDCl3. (B) TGA of OAS-POSS and POSS–COF (Co) in an oxidative environment.
Figure 4. (A) 29Si NMR of POSS–COF (Co) in CDCl3. (B) TGA of OAS-POSS and POSS–COF (Co) in an oxidative environment.
Catalysts 14 00557 g004
Catalysts 14 00557 sch001
Scheme 1. Pathway of synthesizing the POSS–COF (Co) catalyst.
Scheme 1. Pathway of synthesizing the POSS–COF (Co) catalyst.
Catalysts 14 00557 sch001
Catalysts 14 00557 sch002
Scheme 2. Hydrogenative transformation of aryl nitriles using the POSS–COF (Co) catalyst.
Scheme 2. Hydrogenative transformation of aryl nitriles using the POSS–COF (Co) catalyst.
Catalysts 14 00557 sch002
Table 1. Catalytic efficiency using different catalysts for the Reduction of benzonitrile.
Table 1. Catalytic efficiency using different catalysts for the Reduction of benzonitrile.
Catalysts 14 00557 i001
EntryCatalyst% Conversion% Yield
1POSS–COF (Co)>9999
2POSS<1<1
3POSS–COF<1<1
4Co-2,2′ Bipyridine<2<1
Reaction conditions: Benzonitrile (0.5 mmol), POSS–COF (Co) catalyst (25 mg), Isopropyl Alcohol (5 mL), 4.5 MPa H2, 0.5 MPa NH3, 130 °C, 20 h.
Table 2. Solvent optimization for the model reaction with POSS–COF (Co) catalyst.
Table 2. Solvent optimization for the model reaction with POSS–COF (Co) catalyst.
EntrySolvent% Conversion
1Isopropyl Alcohol>99
2Ethanol76
3Methanol72
4n-Butanol54
5Toluene40
6Tetrahydrofuran10
Reaction conditions: Benzonitrile (0.5 mmol), POSS–COF (Co) catalyst (25 mg), 4.5 MPa H2, 0.5 MPa NH3, 130 °C, 20 h.
Table 3. The substrate scope of hydrogenation of aryl nitriles using POSS–COF (Co) catalyst.
Table 3. The substrate scope of hydrogenation of aryl nitriles using POSS–COF (Co) catalyst.
EntrySubstrateProductsConv.
(%)		Sel.
(%)		S.D. *
2a.Catalysts 14 00557 i002Catalysts 14 00557 i00395823
2b.Catalysts 14 00557 i004Catalysts 14 00557 i00590853
2c.Catalysts 14 00557 i006Catalysts 14 00557 i00790884
2d.Catalysts 14 00557 i008Catalysts 14 00557 i00995924
2e.Catalysts 14 00557 i010Catalysts 14 00557 i01198953
2f.Catalysts 14 00557 i012Catalysts 14 00557 i01385805
2g.Catalysts 14 00557 i014Catalysts 14 00557 i01565822
2h.Catalysts 14 00557 i016Catalysts 14 00557 i01799902
2i.Catalysts 14 00557 i018Catalysts 14 00557 i01998941.5
2j.Catalysts 14 00557 i020Catalysts 14 00557 i02165834
2k.Catalysts 14 00557 i022Catalysts 14 00557 i02392882
2l.Catalysts 14 00557 i024Catalysts 14 00557 i02595932
2m.Catalysts 14 00557 i026Catalysts 14 00557 i02795921
2n.Catalysts 14 00557 i028Catalysts 14 00557 i02995922
2o.Catalysts 14 00557 i030Catalysts 14 00557 i03195922
Reaction Conditions: POSS–COF (Co) catalyzed hydrogenation of various functionalized nitriles. Reaction conditions: 0.5 mmol benzonitrile, 4.5 MPa H2, 0.5 MPa NH3, 25 mg catalyst, 5 mL Isopropyl alcohol, 130 °C, * S.D. indicates standard deviation.
Table 4. The corresponding catalyst comparison for the hydrogenation of aromatic nitriles.
Table 4. The corresponding catalyst comparison for the hydrogenation of aromatic nitriles.
CatalystNitrileReaction Conditions% YieldRef
Ni/Al2O3-600Benzonitrile
(1 mmol)		H2, NH3·H2O, EtOH, 60 °C, 6 h95[61]
MC/NiBenzonitrile
(1 mmol)		H2 (2.5 bar), MeOH, 60 °C, 4 h99[62]
(MesCCC)-CoCl2py
(py = pyridine)		BenzonitrileNaHBEt3, KOtBu, toluene, 115 °C, 8 h, H2>99[43]
Co-NC-MgO-700PhCN
(0.5 mmol)		H2 (20 bar), NH3, iPrOH, 24 h98[63]
PdPt–Fe3O4PhCN
(0.5 mmol)		NaBH4 (1 mmol), MeOH, 25 °C, 8 h98[47]
POSS–COF (Co)BenzonitrileH2 (4.5 MPa), IPA, 130 °C, 0.5 MPa NH3, 20 h>99This work
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Rubab, A.; Sohail, M.; Alshammari, R.H.; Nafady, A.; Wahab, M.A.; Abdala, A. Catalytic Application of POSS–COF-[(Co(acetate)2] for Selective Reduction of Nitriles to Amines. Catalysts 2024, 14, 557. https://doi.org/10.3390/catal14090557

AMA Style

Rubab A, Sohail M, Alshammari RH, Nafady A, Wahab MA, Abdala A. Catalytic Application of POSS–COF-[(Co(acetate)2] for Selective Reduction of Nitriles to Amines. Catalysts. 2024; 14(9):557. https://doi.org/10.3390/catal14090557

Chicago/Turabian Style

Rubab, Anosha, Manzar Sohail, Riyadh H. Alshammari, Ayman Nafady, Md. A. Wahab, and Ahmed Abdala. 2024. "Catalytic Application of POSS–COF-[(Co(acetate)2] for Selective Reduction of Nitriles to Amines" Catalysts 14, no. 9: 557. https://doi.org/10.3390/catal14090557

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