Experimental and computational studies of high-valent nickel and palladium complexes

This article describes a detailed comparison of the organometallic chemistry of high-valent nickel and palladium model complexes supported by tris(pyrazolyl)borate and cycloneophyl ligands. The accessibility of the M^III and M^IV oxidation states with each metal is investigated through electrochemical and chemical oxidation of the M^II precursors. These studies show that the Ni^II precursor readily undergoes both one- and two-electron oxidations to generate stable Ni^III and Ni^IV products. In contrast, under the conditions examined, the Pd^II analogue undergoes exclusively two-electron-oxidation reactions to form Pd^IV. Reactivity studies of isolated Ni^IV and Pd^IV complexes show that both participate in C(sp³)–heteroatom coupling reactions and that the reactions at Ni^IV are approximately 2 orders of magnitude faster than those at Pd^IV. Experimental and computational mechanistic studies implicate outer-sphere S_N2-type pathways for these processes. With most nucleophiles (e.g., phenoxide, acetate, thiophenoxide), the C(sp³)–heteroatom coupling reaction yields a TpM^II(σ-aryl) product. However, with azide as the nucleophile, the Ni^II product of initial C(sp³)–N₃ coupling undergoes a subsequent C(sp²)–N insertion reaction. Computations implicate an anionic Ni^III–nitrene intermediate in this process and show that the Pd analogue of this species is a much higher energy species. Overall, the combined experimental and computational studies demonstrate remarkable similarities in the chemistry of Ni^IV and Pd^IV but an enhanced role for Ni^III in enabling reactivity which is distinct from that of palladium.