Single-molecule magnet behaviour and catalytic properties of tetrahedral Co(II) complexes bearing chloride and 1,2-disubstituted benzimidazole as ligands.

Abstract

Five cobalt(II) complexes of formula [CoCl2(Ln)2] [1 with L1 = 1-benzyl-2-phenyl-1H-benzimidazole, 2 with L2 = 2-(furan-2-yl)-1-(furan-2-ylmethyl)-1H-benzimidazole, 3 with L3 = 1-(4-chlorobenzyl)-2-(4-chlorophenyl)-1H-benzimidazole, 4 with L4 = 1-(2-methoxybenzyl)-2-(2-methoxyphenyl)-1H-benzimidazole and 5 with L5 = 2-(thiophen-2-yl)-1-(thiophen-2-ylmethyl)-1H-benzimidazole] have been synthesised, spectroscopically characterised and cryomagnetically investigated. The crystal structures of 1, 3, 4 and 5 have been determined by X-ray diffraction on single crystals. Each cobalt(II) ion is four-coordinate in a distorted tetrahedral environment built by two chloride anions and two benzimidazole ligands. The neutral molecules are well separated from each other, shortest intermolecular cobalt⋯cobalt distances being greater than 9.0 Å. Static (dc) magnetic susceptibility measurements in the temperature range 2.0-300 K of 1-5 reveal the occurrence of a Curie law behaviour of magnetically non-interacting spin quadruplets in the high-temperature domain with a downturn at low temperatures due to magnetic anisotropy. The values of the D and E/D parameters for these compounds vary in the ranges -8.75 to +8.96 cm-1 and 0.00140 to 0.23, respectively. Dynamic (ac) magnetic susceptibility measurements of 1-5 show slow magnetic relaxation in the lack (1) or under the presence (1-5) of applied dc magnetic fields, a feature which is typical of single-molecule magnet behaviour (SMM). The analysis of the ac data shows that a thermally activated Orbach relaxation mechanism dominates this behaviour. Complexes 1-5 also act as efficient and highly selective eco-friendly catalysts in the coupling reaction between CO2 and epoxides to produce cyclic carbonates under solvent-free conditions. Under optimized reaction conditions, different epoxides were converted to the respective cyclic carbonate, with excellent conversions, using catalyst 4.