Impact of Oxidation State on Reactivity and Selectivity Differences between Nickel(III) and Nickel(IV) Alkyl Complexes

Described is a systematic comparison of factors impacting the relative rates and selectivities of C(sp(3))-C and C(sp(3))-O bond-forming reactions at high-valent Ni as a function of oxidation state. Two Ni complexes are compared: a cationic octahedral Ni-IV complex ligated by tris(pyrazolyl)borate and a cationic octahedral Ni-III complex ligated by tris(pyrazolyl) methane. Key features of reactivity/selectivity are revealed: 1) C(sp(3))-C(sp(2)) bond-forming reductive elimination occurs from both centers, but the Ni-III complex reacts up to 300-fold faster than the Ni-IV, depending on the reaction conditions. The relative reactivity is proposed to derive from ligand dissociation kinetics, which vary as a function of oxidation state and the presence/absence of visible light. 2) Upon the addition of acetate (AcO-), the Ni-IV complex exclusively undergoes C(sp(3))-OAc bond formation, while the Ni-III analogue forms the C(sp(3))-C(sp2) coupled product selectively. This difference is rationalized based on the electro-philicity of the respective M-C(sp(3)) bonds, and thus their relative reactivity towards outer-sphere S(N)2-type bond-forming reactions.