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

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.