First Examples of Metal-Organic Frameworks with Pore-Encapsulated [Co(CO)₄]⁻ Anions: Facile Synthesis, Crystal Structures and Stability Studies

Abstract

Three ionic metal-organic frameworks (MOFs) with pore-capsulated Co(CO)₄⁻ anions, formulated as [Co(bix)₃][Co(CO)₄]₂ (1), [Co(bibp)₃][Co(CO)₄]₂ (2), and [Co(bmibp)₂][Co(CO)₄]₂ (3); (bix = 1,4-bis(imidazol-1-yl-methyl)-benzene); bibp = 4,4′-bis(imidazolyl)biphenyl); bmibp = 4,4′-bis(2-methyl-imidazolyl)biphenyl), have been facilely synthesized for the first time through direct reactions of Co₂(CO)₈ with the respective bis(imidazole) ligands under mild hydro(solvo)thermal conditions. Single-crystal X-ray diffraction analysis reveals distinct structural motifs among the frameworks: MOF 1 exhibits a single pcu net, MOF 2 features a 3-fold interpenetrating pcu net, both based on 6-connected Co²⁺ centers and ditopic bix or bibp ligands, while MOF 3 forms a 2-fold interpenetrating sql layer constructed by 4-connected Co²⁺ ions and bmibp linkers. The [Co(CO)₄]⁻ anions reside within the channels of the cationic frameworks. Moreover, these MOFs, characterized by periodically ordered tetracarbonylcobaltate arrays, demonstrate notable thermal stability and maintain structural integrity in air, water, and alkaline solutions for several days.

1. Introduction

The tetracarbonyl cobaltate anion [Co(CO)₄]⁻ is one of the most well-studied catalytic species for carbonylation reactions, which cover a wide range of applications in both organic synthesis and chemical industry [1,2,3,4]. Carbonylation of epoxides, for example, has been proved to be a powerful synthetic tool for the production of various commercially valuable chemicals such as lactones, succinic anhydrides, hydroxyl carbonyl compounds, and polyesters through ring expansion or opening reactions [5,6,7,8,9,10,11]. Due to its inherent instability and sensitivity to air, [Co(CO)₄]⁻ is typically stabilized by coupling with Lewis acids (LAs), such as Al(III) or Cr(III) cations, which are often coordinated by salen or porphyrin ligands to form ion pair complexes [LA][Co(CO)₄] [12,13,14,15,16]. Alternatively, [Co(CO)₄]⁻ can be paired with cationic ionic liquids (Ils) to form organometallic polymers [IL][Co(CO)₄] [17,18,19,20,21]. While these catalytic systems demonstrate high efficiency and selectivity, they are predominantly homogeneous. This homogeneity poses significant challenges, including limited recyclability and difficulties in separating products from the catalytic system, potentially restricting their practical applications [22,23].

To address the challenges of homogeneous catalytic systems, recent research has focused on developing heterogeneous catalytic systems for the carboxylation of expoxides by immobilizing [Co(CO)₄]⁻ anions within metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). This is achieved through “ion-exchange adsorption” or “post-functionalization” strategies [24,25,26,27,28]. These strategies leverage the inherent advantages of MOFs and COFs, such as large specific surface areas, tunable pore structures, high modularity, and versatile functionalities [29]. These [Co(CO)₄]⁻ incorporated MOFs and COFs exhibited comparable catalytic activity and superior long-term stability in carbonylation reactions compared to homogeneous catalysts. Despite these advantages, incorporating [Co(CO)₄]⁻ into MOFs and COFs is synthetically challenging. This process often requires multiple steps or precise host-guest combinations to securely trap [Co(CO)₄]⁻ within the pores. Additionally, achieving uniform dispersion and controlled loading of [Co(CO)₄]⁻ within these frameworks is difficult, and the crystal structures of the resulting [Co(CO)₄]⁻ incorporated frameworks have yet to be reported. In this study, we report a straightforward synthetic method to create MOFs with pore-encapsulated [Co(CO)₄]⁻ anions. This method involves direct reactions of Co₂(CO)₈ with rod-like, ditopic bis(imidazole) ligands under mild hydro(solvo)thermal conditions. This approach successfully produced three ionic MOFs, namely [Co(bix)₃][Co(CO)₄]₂ (1) (bix = 1,4-bis(imidazol-1-yl-methyl)-benzene), [Co(bibp)₃][Co(CO)₄]₂ (2) (bibp = 4,4′-bis(imidazolyl)biphenyl) and [Co(bmibp)₂][Co(CO)₄]₂ (3) (bmibp = 4,4′-bis(2-methyl-imidazolyl)biphenyl). To our knowledge, these compounds represent the first examples of MOFs incorporated with tetracarbonyl cobaltate counteranions. Moreover, we also investigated the crystal structures and stability of these MOFs.

2. Materials and Methods

2.1. Materials and Instrumentation

All syntheses were conducted in poly(tetrafluoroethylene)-lined stainless steel autoclaves under autogenous pressure. Commercially purchased chemicals were used without further purification. Thermogravimetric analysis (TGA) was performed using a NETZSCH STA449C instrument (NETZSCH, Selb, Germany). The samples were heated from room temperature to 1000 °C at a rate of 10 °C/min within a nitrogen atmosphere. Powder X-ray diffraction (XRD) measurements were obtained using a PANalytical X’Pert PRO diffractometer with Cu-Kα radiation over a 2θ range of 5–50° (Panalytical, Eindhoven, Holland). Fourier-transform infrared (FT-IR) spectra were recorded using KBr pellets on a Spectrum-One FT-IR spectrophotometer (PerkinElmer, Waltham, MA, USA), covering the spectral range of 450–4000 cm⁻¹. Elemental analysis for carbon (C), hydrogen (H), and nitrogen (N) was conducted using an Elementar Vario EL III Analyzer (Elementar, Hanau, Germany).

2.2. Synthesis of MOFs 1–3

A mixture of Co₂(CO)₈ (1 mmol, 342 mg) and bix·2H₂O (2 mmol, 548 mg) was placed in a 20 mL Teflon-lined stainless-steel vessel containing 12 mL of a solvent mixture of ethanol and deionized water in a 1:1 volume ratio. The vessel was heated to 160 °C over four hours and was maintained at this temperature for 2 days. Following the reaction, the system was slowly cooled to room temperature over another 2 days. Orange-red prismatic crystals of MOF 1 were collected and thoroughly washed with water and ethanol, yielding 31% based on Co₂(CO)₈. Anal. Calcd. for C₅₀H₄₂N₁₂O₈Co₃ (1) (MW = 1115.74 g mol⁻¹): C 53.78, H 3.76, N 15.06%; found C 53.56, H 4.01, N 14.85%. FT-IR for MOF 1: 1882 (s), 1516 (m), 1447 (w), 1353 (w), 1284 (w), 1231 (m), 1088 (s), 1033 (m), 937 (m), 826 (w), 743 (m), 663 (m), 624 (w), 553 (s), 476 (w), 460 (w) cm⁻¹.

Similarly, orange-red crystals of MOFs 2 and 3 were obtained with yields of 36% and 39% (based on Co₂(CO)₈), respectively using the same procedure as for MOF 1, but with bibp and bmibp as the ligands instead of bix. Anal. Calcd. for C₆₂H₄₂N₁₂O₈Co₃ (2) (MW = 1259.85 g mol⁻¹): C 59.06, H 3.33, N 13.34%; found C 58.73, H 3.38, N 13.10%. FT-IR for MOF 2: 1880 (s), 1516 (s), 1241(m), 1115 (w), 1064 (s), 961(w), 821m, 737 (w), 658 (w), 528 (w) cm⁻¹. Anal. Calcd. for C₄₈H₃₆N₈O₈Co₃ (3) (MW = 1029.64 g mol⁻¹): C 55.94, H 3.50, N 10.88%; found C 55.57, H 3.72, N 10.49%. FT-IR for MOF 3: 1882 (s), 1501 (s), 1418 (m), 1376 (w), 1307 (m), 1173 (w), 1144 (w), 1115 (w), 1006 (m), 647 (w), 553 (s), 492 (w) cm⁻¹.

2.3. X-ray Crystallographic Analysis

The crystal structures of MOFs 1–3 were elucidated using single-crystal X-ray diffraction data collected at room temperature. The data acquisition was performed on a Rigaku diffractometer equipped with a Mercury CCD area detector, using Mo Kα radiation (λ = 0.71073 Å). Empirical absorption corrections were applied with the program CrystalClear. The structures were initially solved by direct methods using the SHELXS-97 program [30] and subsequently refined by full-matrix least-squares on F² using the SHELXL-97 program [31]. Metal atoms in the compounds were located using electron density maps, while other non-hydrogen atoms were identified through successive difference Fourier syntheses. All non-hydrogen atoms were refined anisotropically. For MOF 1, the organic hydrogen atoms were identified from the difference Fourier synthesis and refined isotropically. In contrast, the hydrogen atoms in MOFs 2 and 3 were generated geometrically and refined isotropically using a riding model. For MOF 2, the structure contained large voids; however, no recognizable solvent molecules were found in the Fourier maps. These voids were then treated with SQUEEZE, which reported a solvent accessible volume of 210 ų, and 15 electrons within this volume. This number of electrons is consistent with the presence of 1.5 water molecules per unit cell. Since this estimation was derived from X-ray data and corroborated by TGA and microanalysis results, H₂O has not been included in the chemical formula. Detailed crystal data and structure refinement parameters for MOFs 1–3 are provided in

Table 1. Crystal data and structure refinement parameters for MOFs 1–3.

MOFs	1	2	3
CCDC number	2,368,487	2,368,488	2,368,489
Empirical formula	C50H42N12O8Co3	C62H42N12O8Co3	C48H36N8O8Co3
Formula weight (g mol−1)	1115.74	1259.85	1029.64
Temperature (K)	293	293	293
Crystal system	monoclinic	trigonal	tetragonal
Wavelength (Å)	0.71073	0.71073	0.71073
Space group	C2/c	P3	P−421c
a (Å)	23.741(12)	11.6083(6)	12.2559(6)
b (Å)	12.626(6)	11.6083(6)	12.2559(6)
c (Å)	19.300(10)	13.5342(14)	15.4132(16)
β (°)	108.642(9)	120	90
V (Å3)	5482(5)	1579.4(2)	2315.2(3)
Z	8	3	8
Dc (g cm−3)	1.352	1.325	1.477
μ (mm−1)	0.958	0.840	1.125
θmin-max (°)	5.328–55.08	5.048–54.968	4.246–55.038
Index ranges	−30 ≤ h ≤ 30; −15 ≤ k ≤ 16; −25 ≤ l ≤ 25	−14 ≤ h ≤ 15; −15 ≤ k ≤ 14; −17 ≤ l ≤ 17	−15 ≤ h ≤ 15; −15 ≤ k ≤ 15; −20 ≤ l ≤ 19
Measured reflns.	20,683	12,140	16,984
Unique reflns./I > 2σ(I)	6267/4548	4795/4497	2654/2475
Rint	0.0194	0.0168	0.0264
F(000)	2284	643	1050
R1 a	0.0439	0.0602	0.0431
wR2 b	0.1226	0.1624	0.1198
Goodness-of-fit on F2	1.046	1.081	1.002
Largest diff. peak/hole (eÅ−3)	0.38/−0.36	0.74/−1.11	0.64/−0.40

^a R₁ = ∑$\left|\left|F\mathrm{o}\right|-\left|F\mathrm{c} \right|\right|$/∑$\left|F\mathrm{o}\right|$, ^b wR₂ = [∑[w($F\mathrm{o}$²$-F\mathrm{c}$²)²]/∑[w($F\mathrm{o}$²)²]]^1/2.

. Selected bond lengths and bond angles for these MOFs are presented in Table S1 (see in Supplementary Materials).

CCDC 2,368,487–2,368,489 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Center at http://www.ccdc.cam.ac.uk/structures/ (accessed on 7 July 2024).

3. Results

3.1. Synthesis and Stability Studies

MOFs 1–3 were synthesized in moderate yields through hydro(solvo)thermal reactions of Co₂(CO)₈ with corresponding bis(imidazole) ligands (Scheme 1) in a 1:1 mixed solvent of ethanol and water. These reactions are straightforward and can be performed without the need for an inert atmosphere, which is typically required for handling air-sensitive Co₂(CO)₈. The purity of the synthesized products was confirmed using XRD analysis. The experimental XRD patterns for MOFs 1–3 matched well with the simulated patterns derived from their single crystal X-ray diffraction structures, confirming the phase purity of the as-synthesized samples (Figure S1). All three MOFs exhibited a characteristic CO stretching peak at around 1880 cm⁻¹ in their FT-IR spectra, indicative of the presence of tetracarbonyl cobaltate anions (Figure S2). Additionally, absorption bands in the 1516–460 cm⁻¹ region were consistent with the vibrations of the bis(imidazole) ligands.

Chemical stability is a crucial feature for materials used in various industrial applications. The orange-red crystals of MOFs 1–3, synthesized through the reactions described, can be stored in air for several days without decomposition and for extended periods when submerged in water. The chemical stability of these MOFs was evaluated by analyzing their FT-IR spectra and XRD patterns under different conditions. After prolonged exposure to air, immersion in water, and submersion in a 5N NaOH solution for a week, MOFs 1–3 exhibited almost no significant changes in the positions or intensities of their XRD and FT-IR peaks. These findings underscore their good stability in air, water, and alkaline environments. Thermal stability was assessed using TGA. The analyses were conducted in a nitrogen atmosphere with a heating rate of 10 °C/min, ranging from room temperature to 1000 °C. The TGA profiles of MOFs 1–3 are similar. As shown in Figure 1, thermal decomposition for all three MOFs began around 250 °C, with the mass loss of 20.66% (Calcd 20.08%) for MOF 1, 22.00% (Calcd 21.76%) for MOF 2 and 18.34% (Calcd 17.78%) for MOF 3, each corresponding to the departure of eight CO molecules per formula unit. As the temperature increased further, the imidazole ligands decomposed progressively, leading to the collapse of the MOF structures.

3.2. Structural Description of MOFs 1–3

Single-crystal X-ray structural analysis reveals that MOF 1 forms a three-dimensional cationic network composed of Co²⁺ centers and bix linkers. It crystallizes in the monoclinic system with the space group C2/c. The asymmetric unit contains half of a [Co(bix)₃]²⁺ cation and one [Co(CO)₄]⁻ anion (Figure 2a). The Co1 atom is positioned at a crystallographic inversion center and is coordinated by six nitrogen atoms from six distinct bix ligands. The Co1 atom has a nearly ideal octahedral coordination geometry. The bond lengths for Co1-N are narrowly distributed between 2.1567(18) Å and 2.1640(18) Å, and the N-Co1-N bond angles are close to 90°, confirming an almost perfect octahedral geometry. The Co2 atom is coordinated by four carbonyl ligands in a geometry close to an ideal tetrahedron with C24-Co2-C25 = 107.21(16)° being the smallest angle and C23-Co2-C25 = 112.57(14)° being the largest angle. The average distance between the Co2 atom and the carbon atoms is 1.747 Å, slightly shorter than that in Co₂(CO)₈ [32]. In the carbonyl ligands, the mean Co2-C bond length 1.150 Å and Co2-C-O bond angle 178.4° are within the normal ranges typical for terminal carbonyl ligands. In the cationic framework, each bix ligand bridges adjacent Co²⁺ centers through its imidazolyl nitrogen atoms, adopting both trans-gauche (TG) and cis-gauche (CG) conformations. Each Co²⁺ center connects to six others via six bix ligands, with Co···Co separations across the bix bridges being 11.532 Å, 12.626 Å, and 12.681 Å, respectively (Figure 2b). This arrangement creates a three-dimensional cationic framework featuring parallelogram channels with diagonal distances of 13.445 Å × 20.169 Å along the b-axis (Figure 2c). The Co(CO)₄⁻ anions reside within these channels without direct contact with the cationic framework. Topologically, considering the Co²⁺ ions as 6-connected nodes and the bix ligands as linear linkers, the structure of MOF 1 can be described as a distorted primitive cubic (pcu) net, also known as an α-polonium net (Figure 2d) [33]. The fundamental unit of this pcu net is an oblique rectangular prism, defined by eight Co²⁺ ions at its vertices and twelve bix ligands as its edges, which provides a pocket large enough to accommodate two [Co(CO)₄]⁻ anions (Figure 2e). Bis(imidazole) ligands are versatile and efficient building blocks for the construction of MOFs [34]. They typically serve as auxiliary ligands and are often combined with primary ligands, such as multicarboxylic acids, to create anionic MOFs with diverse topological structures [35,36,37,38,39,40]. However, cationic MOFs constructed solely from bis(imidazole) ligands are extremely rare [41,42,43], and those featuring [Co(CO)₄]⁻ counteranions within their pores, as seen in MOF 1, have not been previously documented.

MOF 2 crystallizes in the trigonal P3 space group, with its asymmetric unit comprising one-third of a [Co(bibp)₃]²⁺ cation and two one-thirds of independent [Co(CO)₄]⁻ anions (Figure 3a). The MOF exhibits a threefold rotation axis passing through the Co atoms and one of the carbonyl ligands in the tetracarbonyl cobalt unit. The Co1 atom is coordinated by six N atoms from six distinct bibp ligands, showing an almost perfect octahedral coordination sphere similar to that of MOF 1. The two independent Co1-Ν bond distances are nearly equivalent (2.164(4) Å and 2.170(4) Å), and all cis N-Co1-N bond angles are close to 90°. The Co2 and Co3 atoms are each coordinated by four CO ligands in a slightly distorted tetrahedral geometry, with bond lengths and angles being typical for the [Co(CO)₄]⁻ anion. Due to the high symmetry of the structure, each Co1 ion is connected to six equivalent Co²⁺ ions by six bibp ligands, resulting in a uniform Co···Co separation of 17.831 Å (Figure 3b). Similar to MOF 1, the assembly of 6-connected Co²⁺ nodes with bibp linkers forms a three-dimensional net with a primitive cubic (pcu) topology (Figure 3c). This pcu network contains regular rhombohedral pores with dimensions of 17.831 Å (defined by Co···Co distances) significantly larger than those in MOF 1 due to the longer spacer of the bibp ligand compared to bix (Figure 3d). These large voids in the structure of MOF 2 are filled by free [Co(CO)₄]⁻ anions and the interpenetration of three equivalent networks, resulting in a 3-fold interpenetrating pcu net (Figure 3e) [44].

MOF 3 crystallizes in the tetragonal system with space group P-42₁c. its asymmetric unit comprises one-fourth of a [Co(bmibp)₂]²⁺ cation and half of a [Co(CO)₄]⁻ anion (Figure 4a). In the two crystallographically unique Co²⁺ ions, the Co1 atom lies on a 4-fold rotoinversion axis and is tetrahedrally coordinated by four N atoms from four different bmibp ligands, with equivalent Co1-N bond lengths of 2.011(3) Å, significantly shorter than those in MOFs 1 and 2. The Co2 atom lies on a 2-fold screw axis and exhibits a tetrahedral coordination environment completed by four linear CO ligands, with normal bond parameters consistent with the [Co(CO)₄]⁻ anion. Unlike MOFs 1 and 2, each Co1 atom in MOF 3 is linked to its four neighboring ones by four bmibp bridges, forming a 2D cationic sheet with nanometer-sized square grids (17.332 Å × 17.332 Å) (Figure 4b). In each 2D sheet, the Co²⁺ ions are coplanar. However, the nonplanar geometry at the Co²⁺ center and the free rotation of the bmibp ligand cause slight corrugation in the sheet structure, with a sheet thickness of only 5.7 Å, facilitating interpenetration with other identical sheets [45]. The two nearest adjacent 2D sheets are tightly intertwined to form a 2D + 2D → 2D parallel interpenetrating layer. In each layer, the two interpenetrated sheets are staggered, with every grid vertex of one sheet aligned with the grid center of the other sheet, reducing the grid size to one-fourth of a single sheet (Figure 3b). The resulting parallel layers are loosely stacked (separation 11.60 Å) in an ABAB sequence along the c-axis, where every vertex of the first layer is vertically above the edge midpoint of the second layer (Figure 4c). The [Co(CO)₄]⁻ anions are sandwiched between the layers and, when viewed from the horizontal plane, are well-fitted in the 1D square channels (Figure 4d). From a topological perspective, if the bmibp ligand is considered a linear linker and the central Co²⁺ ions as 4-connected nodes, the 2D structure of MOF 3 can be simplified as a 2-fold interpenetrating layer with a square sql (square lattice net) topology (Figure 4e). Interestingly, it is somewhat unusual that in the interpenetrating layer of MOF 3 the two planes of Co²⁺ ions that belong to the separate sheets are in fact coincident and merged into one single plane, on which bmibp ligands are interpenetrated into each other in a cross-weave pattern [46].

4. Conclusions

Compared to neutral MOFs, ionic MOFs are rarely reported due to the challenges in isolating them, which heavily depend on matching pore sizes with the counteranions to be encapsulated. In our studies, we discovered that cationic Co-based MOFs composed solely of ditopic bis(imidazole) ligands can offer suitable pore sizes for the well-distribution of [Co(CO)₄]⁻ anions. The topological structures of these ionic MOFs can be effectively tuned by selecting bis(imidazole) ligands with different spacer lengths and steric configurations. In MOF 1, combining 6-connected octahedral nodes (Co²⁺ centers) with dipotic linkers (bix) results in a 3D pcu net with parallelogram channels. Transitioning from bix to bibp, the increased ligand spacer significantly expands the pore size of the channels. Consequently, MOF 2 forms a 3-fold interpenetrating pcu net to reduce the excess porosity. With the bmibp ligand, the steric hindrance of the methyl substituent in the 2-position of the imidazolyl ring reduces the coordination number of bmibp ligands around the Co²⁺ center from six to four. Thus, the resulting tetrahedrally coordinated Co²⁺ ions, in combination with lengthy bmibp ligands, tend to form a 2-fold interpenetrating (4,4) topological layer in MOF 3. Despite the unstable nature of [Co(CO)₄]⁻, MOFs 1–3 exhibit significant thermal stability up to 250 °C, and their frameworks remain intact in air, water, and a 5 N NaOH aqueous solution for at least one week. This work demonstrates an effective method for integrating [Co(CO)₄]⁻ into cationic MOF frameworks through pore encapsulation, which could be valuable in designing efficient heterogeneous catalysts for carbonylation reactions.