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<sup>III</sup> and M<sup>IV</sup> oxidation states with each metal is investigated through electrochemical and chemical oxidation of the M<sup>II</sup> precursors. These studies show that the Ni<sup>II</sup> precursor readily undergoes both one- and two-electron oxidations to generate stable Ni<sup>III</sup> and Ni<sup>IV</sup> products. In contrast, under the conditions examined, the Pd<sup>II</sup> analogue undergoes exclusively two-electron-oxidation reactions to form Pd<sup>IV</sup>. Reactivity studies of isolated Ni<sup>IV</sup> and Pd<sup>IV</sup> complexes show that both participate in C­(sp<sup>3</sup>)–heteroatom coupling reactions and that the reactions at Ni<sup>IV</sup> are approximately 2 orders of magnitude faster than those at Pd<sup>IV</sup>. Experimental and computational mechanistic studies implicate outer-sphere S<sub>N</sub>2-type pathways for these processes. With most nucleophiles (e.g., phenoxide, acetate, thiophenoxide), the C­(sp<sup>3</sup>)–heteroatom coupling reaction yields a TpM<sup>II</sup>(σ-aryl) product. However, with azide as the nucleophile, the Ni<sup>II</sup> product of initial C­(sp<sup>3</sup>)–N<sub>3</sub> coupling undergoes a subsequent C­(sp<sup>2</sup>)–N insertion reaction. Computations implicate an anionic Ni<sup>III</sup>–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<sup>IV</sup> and Pd<sup>IV</sup> but an enhanced role for Ni<sup>III</sup> in enabling reactivity which is distinct from that of palladium.