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(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 NiIV complex ligated by tris(pyrazolyl)borate and a cationic octahedral NiIII 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 NiIII complex reacts up to 300-fold faster than the NiIV , 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 NiIV complex exclusively undergoes C(sp3 )-OAc bond formation, while the NiIII 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 SN 2-type bond-forming reactions.