Synthesis and Crystal Structure of (μ-Acetato)bis(μ-Alkoxo)dicobalt(II, III) Complex with an Unsymmetric Dinucleating Ligand

The bimetallic active centers of metalloproteins exhibit versatile roles; model studies using simple dinuclear metal complexes are important in order to gain insight into the biological functions of such bimetallic cores.1–5 The end-off type dinucleating ligands, having a phenolic and alcoholic oxygen as an endogenous bridge, have been used for modeling carboxylate-bridged bimetallic active sites of metalloproteins.1–5 Previously, we reported on the synthesis and characterization of a peroxo complex, [Co2(tpdb)(O2)(CH3CO2)], and a mixed-valence complex, [FeFe(tpdb)(C6H5CO2)2], having an unsymmetric dinucleating ligand with pyridyl groups (tpdb) as models for hemerythrin and purple acid phosphatase. In this paper, we report on the crystal structure of a dicobalt(II, III) mixed-valence complex, [CoII Co(tpdb)(CH3CO2)(CH3O)](ClO4)2·0.75CH3OH· 0.25CH3CN (1) (Fig. 1), which was obtained by an oxidative degradation of [Co2(tpdb)(CH3CO2)] in methanol/acetonitrile at ambient temperature under air. A mixture of Co(ClO4)2·6H2O (0.366 g, 1.0 mmol) and Htpdb (0.235 g, 0.5 mmol) in methanol/acetonitrile (3:1, 12 mL) was added to a methanol solution (5 mL) containing CH3CO2Na·3H2O (0.07 g, 0.5 mmol) and triethylamine (69 μL, 0.5 mmol) to give a dark-brown solution. Oxygen gas was bubbled for 5 min into the solution. The resulting dark-brown solution was allowed to stand for several weeks at ambient temperature under air to give single crystals of [CoCo(tpdb)(CH3CO2)(CH3O)](ClO4)2· 0.75CH3OH· 0.25CH3CN (1) suitable for X-ray crystallography. Yield: 0.28 g, 63 % (Anal. Found: C, 41.20; H, 4.37; N, 9.28%. Calcd for [CoCo(tpdb)(CH3CO2)(CH3O)](ClO4)2·1.5H2O, C31H40Cl2Co2N6O13.5: C, 41.30; H, 4.47; N, 9.32%.). It was picked up from the solution by a loop on a hand-made cold copper plate mounted inside a liquid N2 Dewar vessel at –80°C; the loop was mounted on the diffractometer. X-ray diffraction measurements were made on a Bruker D8 VENTURE diffractometer (Cu Kα radiation) at 90 K. The structure was solved by a direct method (SHELXT)8 and expanded using a Fourier technique. The structure was refined by a full-matrix least-squares method by using SHELXL 20149 (Yadokari-XG).10 All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were included using a riding model. For the disordered anion and solvent molecules, occupancy factors were refined using distance/ADP restraints. The crystal data are summarized in Table 1. The X-ray crystallography of 1 reveals that the asymmetric unit contains a complex cation [CoCo(tpdb)(CH3CO2) (CH3O)], two ClO4, and 0.75 methanol and 0.25 acetonitrile molecule (Fig. S1). An ORTEP drawing of the complex cation, [CoCo(tpdb)(CH3CO2)(CH3O)] of 1, is shown in Fig. 2. Selected bond distances (Å) and angle (°) are given in Table 2. The complex cation of 1 has distinct cobalt centers that are triply bridged by alkoxo oxygens of tpdp and methoxo, and acetate group as found for closely related dicobalt(II, III) mixedvalence complex [CoCo(L)2(CH3CO2)(CH3O)2] (2) (L = N-methyl-N,N-bis(2-pyridylmethyl)amine).11 The Co1···Co2 separation is 2.9918 Å, which is slightly longer than that of 2 (2.956 Å). Both of the cobalt atoms in 1 have a distorted octahedral structure with an N3O3 donor set, where three 2021 © The Japan Society for Analytical Chemistry