Synthesis and Characterization of a Family of POCOP Pincer Complexes with Nickel: Reactivity Towards CO 2 and Phenylacetylene

A cyclohexyl‐based POC   sp   3OP pincer ligand (POC   sp   3OP= cis ‐1,3‐bis(di‐ tert ‐butylphosphinito)cyclohexyl) cyclometalates with nickel to generate a series of new POC   sp   3OP‐supported Ni II complexes, including the halide, hydride, methyl, and phenyl species. trans ‐[NiCl{ cis ‐1,3‐bis(di‐ tert ‐butylphosphinito)cyclohexane}], [(POC   sp   3OP)NiCl] ( 1 a ) and the analogous bromide complex ( 1 b ) were synthesized and fully characterized by NMR spectroscopy and X‐ray crystallography. Cyclic voltammetry measurements of 1 a and 1 b alongside their bis(phosphine) analogues [(PC   sp   3P)NiCl] ( 2 a ) and [(PC   sp   3P)NiCl] ( 2 a ) (PC   sp   3P= cis ‐1,3‐bis(di‐ tert ‐butylphosphino)cyclohexyl) indicate a reduced electron density at the metal center upon introducing electron‐withdrawing oxygen atoms in the pincer arms. The methyl [(POC   sp   3OP)NiMe] ( 3 ) and phenyl [(POC   sp   3OP)NiPh] ( 4 ) complexes were formed from 1 a by reaction with the corresponding organolithium reagents. 1 a also reacts with LiAlH 4 to give the hydride complex [(POC   sp   3OP)NiH] ( 5 ). The methyl complex 3 reacts with phenyl acetylene to give the acetylide complex [(POC   sp   3OP)NiCCPh] ( 6 ). The reactivity of compounds 3 – 5 towards CO 2 was studied. The hydride complex 5 and the methyl complex 3 both underwent CO 2 insertion to form the formate species [(POC   sp   3OP)NiOCOH] ( 7 ) and acetate species [(POC   sp   3OP)NiOCOCH 3 ] ( 8 ), respectively, although with a higher barrier of insertion in the latter case. Compound 4 was unreactive towards CO 2 even at elevated temperatures. Complexes 3 – 8 were all characterized by NMR spectroscopy and X‐ray crystallography.