Reduction of ((t)BuN═)NbCl3(py)2 in a Salt-Free Manner for Generating Nb(IV) Dinuclear Complexes and Their Reactivity toward Benzo[c]cinnoline.

The organosilicon reducing reagent 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (1a) was used for the one-electron, salt-free reduction of ((t)BuN═)NbCl3(py)2 (2), resulting in the formation of a neutral, triply chloride-bridged dinuclear niobium(IV) complex, [((t)BuN═)ClNb(py)](μ-Cl)3[((t)BuN═)Nb(py)2] (3) in moderately high yield. Heating 3 in toluene at 80 °C caused a unique intramolecular rearrangement of 3 to another neutral dinuclear complex, [Cl2Nb(py)](μ-Cl)(μ-N(t)Bu)2[ClNb(py)2] (4), in which two niobium(IV) atoms were bridged by one chloride atom and two imido ligands. Reaction of complex 3 with benzo[c]cinnoline produced a benzo[c]cinnoline-bridged dinuclear niobium(V) complex 7 by an overall two-electron reduction of benzo[c]cinnoline through a disproportionation of 3 into a mixture of a niobium(V) complex 2 and a niobium(III) complex, the latter of which efficiently reduced benzo[c]cinnoline.