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 ·4H 2 O by grinding afforded three nickel complexes, [NiL]Br 2 ·CH 3 OH (1), [NiL]Br 2 ·2H 2 O (1') and [NiL](PF 6 ) 2 ·0.5CH 3 CN (2), respectively. A five-coordinated complex [NiLBr]PF 6 (3) was further obtained by grinding NH 4 PF 6 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.