Multiply-bonded dinuclear complexes of early-transition metals as minimum entities of metal cluster catalysts

Dinuclear complexes of early-transition metals of group 5 (Nb and Ta), 6 (Cr, Mo, and W), and 7 (Re) tend to form multiple bonds between two metal centers, i.e., double, triple, quadruple, and quintuple bonds. These complexes have intrinsic features such as (1) redox properties caused by the multiple metal-metal bond responses for mediating the oxidation and reduction of an organic substrate, and (2) unoccupied dσ∗-orbitals at each axial position of metal-metal multiple bonds for accepting donor molecules as inimitable Lewis acid catalysts. In addition, the metal-metal multiple bonds in the dinuclear complexes participate in metallacycle formation in combination with alkynes. In this review, we summarize the unique reactivity of multiply-bonded dinuclear complexes as reagents and catalysts for some organic transformations such as radical transformation of haloalkanes including radical addition, hydrodehalogenation, and radical polymerization; alkyne cyclotrimerization; ring-opening metathesis polymerization (ROMP) of cyclic alkenes or alkynes; metathesis reaction; hydrogenation of alkenes; Lewis-acid site mediated transformation namely regiospecific ring-opening of α,β-epoxysilanes, Aza-Diels-Alder, Mukaiyama-Aldol, Strecker, and cyclopropanation reaction.