Synthesis and Structures of Formal Ni(I/II/III) Imido Complexes

Late transition metal imido complexes in more than two oxidation states remain rare, owing to the intrinsic instability of metals in uncommon oxidation states, which typically exhibit strong reducing or oxidizing power. Here, we report the synthesis and redox chemistry of a series of Ni(II) imido complexes stabilized by a cyclic (alkyl)(amino)carbene (CAAC)-phosphine bidentate ligand, [(L)Ni(NAr)] (L stands for a CAAC-phosphine bidentate ligand; 3a, Ar = 2,6-ⁱPr₂C₆H₃; 3b, Ar = 2,4,6-ⁱPr₃C₆H₂; 3c, Ar = 2,4,6-^tBu₃C₆H₂). One-electron reduction of 3a-3c afforded unprecedented dimeric or monomeric Ni(I) imido complexes, [(3a)K]₂, [3b][K(18-crown-6)(THF)], and [(3c)][K(cryptand)], respectively. Additionally, one-electron oxidation of 3a-3c yielded transient Ni(III) imido species [3a][SbF₆] and [3b][BAr^F₄] (Ar^F = 3,5-(CF₃)₂C₆H₃), as well as an isolable Ni(III) imido complex, [3c][SbF₆]. The isolated complexes have been thoroughly characterized by structural, spectroscopic, and computational studies, which reveal the strong Ni-N covalency in all three oxidation states. Remarkably, the Ni(I) imides stand out as rare examples of metal-imido complexes with formal d⁹ configurations. Reactivity studies indicate that the Ni(I) imido species demonstrates enhanced nucleophilicity and basicity compared with its Ni(II) precursors. The impact of oxidation states on the hydrogen atom abstraction reactivity of these Ni imides was also explored. This work describes the first series of Ni imides across three oxidation states, and provides an efficient strategy for isolating elusive transition metal imides in the + I metal oxidation state.