Theory-Guided Experiments on the Mechanistic Elucidation of the Reduction of Dinuclear Zinc, Manganese, and Cadmium Complexes

摘要

Since the recognition of the first Zn~I-Zn~I bond in the dinuclear sandwich decamethyldizincocene ( η~5-C5Me5)Zn— Zn( η~5-C5Me5) , the chemistry of Zn-Zn-honded species has grown so rapidly that many complexes of the type LZn—ZnL have been characterized and studied. Regardless of the denticity of the supporting ligands L, they all coordinate to Zn in a terminally chelating mode. However, formation of these dinuclear compounds has not been mechanistically examined. We recently described the characterization of dinuclear Zn~I— Zn~I-bonded species {k~2-Me2Si(NDipp)2}Zn-Zn{k~2-Me2Si-(NDipp)2}~(2-) (2) (Dipp = 2,6-iPr2C6H3) from KC8 reduction of dinuclear zinc complex Zn2(μ-k~2-Me2Si(NDipp)2)2 (1), whereby the coordination mode of the diamido ligands dramatically changes from bridging to chelating. We thus became interested in the structural preference and the formation mechanism of Zn~I—Zn~I-bonded complexes. Elaborate calculations were performed to understand the reduction of 1, and a plausible mechanism was then proposed (Scheme 1). On two-electron reduction of 1, two intermediates, 1a and 1b, are generated, and the energy difference between them is only 0.3 kcalmol~(-1). The Zn~(II)—Zn~I-bonded mixed-valent intermediate 1a is produced by one-electron reduction of 1, and subsequently undergoes a dramatic structural rearrangement to give 1b, in which one three-coordinate and one one-coordinate Zn atoms are proposed. The exact valence of the Zn atoms in 1b is still not clear.