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

Described is a systematic comparison of factors impacting the relative rates and selectivities of C(sp³ )-C and C(sp³ )-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(sp³ )-C(sp² ) 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(sp³ )-OAc bond formation, while the Ni^III analogue forms the C(sp³ )-C(sp² ) coupled product selectively. This difference is rationalized based on the electrophilicity of the respective M-C(sp³ ) bonds, and thus their relative reactivity towards outer-sphere S_N 2-type bond-forming reactions.