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

Five cobalt(II) complexes of formula [Cocl(2)(L-n)(2)] [1 with L-1 = 1-benzyl-2-phenyl-1H-benzimidazole, 2 with L-2 = 2-(furan-2-yl)-1-(furan-2-ylmethyl)-1H-benzimidazole, 3 with L-3 = 1-(4-chlorobenzyl)-2-(4-chlorophenyl)-1H-benzimidazole, 4 with L-4 = 1-(2-methoxybenzyl)- 2-(2-methoxyphenyl)-1H- benzimidazole and 5 with L-5 = 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(u) 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 center dot center dot center dot cobalt distances being greater than 9.0 angstrom. 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.

Automated summary

This article reports the synthesis and magnetic properties of five cobalt(II) complexes. These cobalt complexes exhibit characteristics similar to single-molecule magnets and act as efficient catalysts in the coupling reaction between CO2 and epoxides.