Synthesis and structure of hexaaquacobalt bis(2-methyl-1H-imidazol-3-ium) tetraaquabis(benzene-1,3,5-tricarboxylato-κO)cobalt

An hexaaquacobalt-based complex was synthesized and its structure determined by single-crystal X-ray diffraction. The observed Co—Ocarboxylate bond length is 2.0835 (9) Å and the Co—Owater bond lengths are in the range 2.0576 (9)-2.1196 (9) Å.


Chemical context
Effective bifunctional electrocatalysts for oxygen reduction/ evolution reactions (ORR/OER) are indispensable for the development of energy storage and conversion systems, such as fuel cells and rechargeable metal-air batteries (Cai et al., 2017;Wang et al., 2014). Currently, platinum-based materials are considered the most effective due to their superior catalytic activity and stability. However, their high cost, caused by the scarcity of the metal, rules them out for scale-up development. Therefore, a great deal of effort has been devoted to the development of cost-effective and earth-abundant replacements for platinum-based catalysts. Among the different substitute materials, a hexaaquacobalt bis(1H-imidazol-3-ium) tetraaquabis(benzene-1,3,5-tricarboxylato)cobalt complex, 2, has shown excellent bifunctional catalytic activity and durability for both the oxygen-reduction reaction and oxygenevolution reaction in alkaline media (Wang et al., 2020). Unfortunately, the solvothermal synthesis required to produce the material hinders its implementation on a large scale.
Herein, we present the synthesis and structure of a hexaaquacobalt bis(1H-2-methyl-imidazol-3-ium) tetraaquabis(-benzene-1,3,5-tricarboxylato)cobalt complex, a related material with the imidazolium cations replaced by 2-methylimidazolium, which can be obtained under ambient conditions. The introduction of the methyl substituent to the C 2 position of the imidazolium ring induces only small structural changes, when compared to 2, and therefore, the title compound could be a promising material for ORR and OER.
plane formed by all ions. Each ion interacts with others via hydrogen bonds of the O-HÁ Á ÁO or N-HÁ Á ÁO type. A summary of the hydrogen-bonding interactions is given in Table 1. The table demonstrates that all possible donor and acceptor groups are involved in moderate hydrogen bonds. The presence of various hydrogen bonds in 1 results in characteristic arrays that may be described by graph-set analysis (Etter et al., 1990;Bernstein et al., 1995). In the structure of 1, there are 27 possible motifs involved in discrete D (types a-f and k-l) and intermolecular S (type h) motifs, as well as rings R (types g, i and j) and chains C (types a-f). It is worth noting that while hydrogen bonds b, c and i hold the aforementioned layers together through C 2 2 (20) and D arrays, other hydrogen bonds, such as type a and e, form C 2 2 (20) arrays, which generates a three-dimensional network with channels along the a and c axes in which the imidazolium ions are located (Fig. 3).

Database survey
A search of the Cambridge Structural Database (CSD version 5.41, update of August 2020; Groom et al., 2016) for hexaaquacobalt and the ditrimesate tetraaquacobalt moiety revealed only one hit, namely refcode: (Wang et al., 2020). Compounds 1 and 2 crystallize in the triclinic system, space group P1. The Co-O carboxylate and C-O water bond lengths are similar in both complexes. The coordination polyhedra of compound 2 are slightly more distorted. The calculated values of AE/Â for compound 2 are equal to 21 /63 for Co1 and 10 / 39 for Co2 -that is, the trigonal distortion (Â) in 2 is higher by 1 and 8 for Co1 and Co2, respectively. The slightly different distortion of the metal centres in 2 and the introduction of the imidazolium allow for shorter hydrogen bonds with distances between 1.73 and 2.00 Å . Other complexes with a low degree of similarity to the title compound were also found, for example refcodes DOWFUS (Clegg & Holcroft, 2014), IQOZUK (Li et al., 2011) and SETQOX (Wolodkiewicz et al., 1996). However, these compounds are polymeric and/or incorporate a different organic ligand than btc. Additionally, none of them contain the imidazolium anion. These changes in chemical composition may provide them with totally different properties than those desired for ORR and OER, and therefore, they will not be discussed further.
816 Velazquez-Garcia and Techert (C 4 The three-dimensional supramolecular network with one-dimensional channels along the (a) a and (b) c axes showing O-HÁ Á ÁO hydrogen bonds of type a and e in the magnified area. Imidazolium ions are drawn in green for clarity.

Synthesis and crystallization
In a typical synthesis, H-2mIm (160 mg, 1.96 mmol), Hbtc (412, 1.96 mmol) and cobalt chloride (127 mg, 0.95mmol) were dissolved in 160 ml of a 1:1:1 mixture of deionized water, ethanol and dimethylformamide by stirring for 10 min at room temperature. After 5 minutes, light-pink crystals of 1 were obtained. The product was collected by filtration and washed three times with ethanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Positions of remaining non-H atoms were found from the electron density difference maps. The positions of hydrogen atoms were refined with U iso (H) = 1.5U eq (C or N) for CH and NH groups and U iso (H) = 1.5U eq (C or O) for others. The O-H and HÁ Á ÁH distances in the water molecules as well as the N-H distances were restrained to be approximately equal within each type (SHELXL instruction SADI). The protons of the methyl group were refined as disordered over two geometrically idealized positions.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (