Hydride Reactivity of NiII—X—NiII Entities:Mixed-Valent Hydrido Complexes and Reversible Metal Reduction

After the lithiation of PYRH2 (PYR2− =[{NC(Me)C(H)C(Me)NC6H3(iPr)2}2(C5H3N)]2−), which is the precursor of an expanded β-diketiminato ligand system with two binding pockets, its reaction with [NiBr2(dme)] led to a dinuclear nickel(II)–bromide complex, [(PYR)Ni(µ-Br)NiBr]. The bridging bromide ligand could be selectively exchanged for a thiolate ligand to yield [(PYR)Ni(µ-SEt)NiBr] (3). In an attempt to introduce hydride ligands, both compounds were treated with KHBEt3. This treatment afforded [(PYR)Ni(µ-H)Ni], which is a mixed valent NiI—µ-H—NiII complex, and [(PYR-H)Ni(µ-SEt)Ni] (4), in which two tricoordinated NiI moieties are strongly antiferromagnetically coupled. Compound 4 is the product of an initial salt metathesis, followed by an intramolecular redox process that separates the original hydride ligand into two electrons, which reduce the metal centres, and a proton, which is trapped by one of the binding pockets, thereby converting it into an olefin ligand on one of the NiI centres. The addition of a mild acid to complex 4 leads to the elimination of H2 and the formation of a NiIINiII compound, [(PYR)Ni(µ-SEt)NiOTf], so that the original NiII(µ-SEt)NiIIX core of compound 3 is restored. All of these compounds were fully characterized, including by X-ray diffraction, and their molecular structures, as well as their formation processes, are discussed.