Stepwise synthesis and catalysis in C–S cross-coupling of pyridine-functionalized N-heterocyclic carbene nickel(ii) complexes by mechanochemistry

Abstract

The synthesis of three N-heterocyclic carbene complexes by stepwise grinding is described in this paper. The benzimidazolium salts ([H₂L]Br₂ and [H₂L](PF₆)₂ ([H₂L] = 1,1′-di(2-picolyl)-3,3′-methylenedibenzoimidazolium)) were initially prepared. Their reactions with Ni(OAc)₂·4H₂O by grinding afforded three nickel complexes, [NiL]Br₂·CH₃OH (1), [NiL]Br₂·2H₂O (1′) and [NiL](PF₆)₂·0.5CH₃CN (2), respectively. A five-coordinated complex [NiLBr]PF₆ (3) was further obtained by grinding NH₄PF₆ with complex 1 or 1′, or grinding KBr with complex 2. Complex 3 can also be obtained by direct grinding of complexes 1/1′ and 2. Complex 3 was subsequently used as a catalyst in the C–S bond cross-coupling of 2-bromoacetophenone and 2-mercaptobenzothiazole. The entire procedure from the synthesis to the catalytic reaction was performed by mechanochemistry. The green metrics E-factor and EcoScale close to ideal values showed the eco-friendly nature of the entire procedure.