Ligand-based reduction of CO(2) to CO mediated by an anionic niobium nitride complex.

The terminal nitride anion complex [Na][N[triple bond]Nb(N[(t)Bu]Ar)(3)] ([Na][1], Ar = 3,5-Me(2)C(6)H(3)) reacts quantitatively with CO(2) to give the carbamate complex [Na][O(2)CN[triple bond]Nb(N[(t)Bu]Ar)(3)] ([Na][O(2)C-1]). The structure of [Na][O(2)C-1] as the bis-12-crown-4 solvate, as determined by X-ray crystallography, displays a unique N-bound carbamate ligand without any metal-oxygen interactions. When treated with organic acid anhydrides or acid chlorides, complex [Na][O(2)C-1] reacts via salt elimination to give the five-coordinate complexes (RC(O)O)(OCN)Nb(N[(t)Bu]Ar)(3) (R-2, R = Me, (t)Bu, F(3)C). We show that complexes R-2 yield the starting complex [Na][1] with concomitant release of CO upon two-electron reduction. This series of reactions constitutes a closed cycle for the conversion of CO(2) to CO mediated by a terminal nitride anion complex.