Influence of the Flexibility of Nickel PCP-Pincer Complexes on C-H and P-C Bond Activation and Ethylene Reactivity: A Combined Experimental and Theoretical Investigation.

Using a pincer platform based on a bridgehead NHC donor with functional side arms, the combined effect of increased flexibility in six-membered pyrimidine-type heterocycles compared to the more often studied five-membered imidazole, and rigidity of phosphane side arms was examined. The unique features observed include: 1) the reaction of the azolium C sp2 -H bond with [Ni(cod) 2 ] affording a carbanionic ligand in [NiCl(PC sp3 H P)] (8) rather than a carbene; 2) its transformation into the NHC, hydrido complex [NiH(PC NHC P)]PF 6 (9) upon halide abstraction; 3) ethylene insertion into the Ni-H bond of the latter and ethyl migration to the N-C-N carbon atom of the heterocycle in [Ni(PC Et P)]PF 6 (10); and 4) an unprecedented C-P bond activation transforming the P-C NHC -P pincer ligand of 8 in a C-C NHC -P pincer and a terminal phosphanido ligand in [Ni(PPh 2 )(CC NHC P)] (15). The data are supported by nine crystal structure determinations and theoretical calculations provided insights into the mechanisms of these transformations, which are relevant to stoichiometric and catalytic steps of general interest.