Reactivities of the coordinated organonitriles in molybdenum(0) and tungsten(0) phosphine complexes: protonation of the nitrile carbon and cleavage of the CN triple bond

Abstract

The molybdenum and tungsten dinitrogen-organonitrile complexes trans-[M(N 2)(NCR)(dppe) 2] ( 2, MMo; 4, MW; RPh, C 6H 4Me- p, C 6H 4OMe- p, Me; dppePh 2PCH 2CH 2PPh 2) underwent double protonation at the nitrile carbon atom with loss of N 2 and a change in oxidation state to +4 on treatment with hydrochloric acid to afford the cationic imido complexes trans-[MCl(NCH 2R)(dppe) 2] +. The solid-state structure of trans-[WCl(NCH 2CH 3)(dppe) 2][PF 6]·CH 2Cl 2 was determined by single-crystal X-ray analysis. Protonation of complexes 2 by fluoroboric acid or hydrobromic acid also formed the similar imido complexes trans-[MoX(NCH 2R)(dppe) 2] + (XF, Br). In contrast, the dinitrogen complex trans-[Mo(N 2) 2(dppe) 2] reacted with two equiv. of benzoylacetonitrile, a nitrile with acidic CH hydrogen atoms, to give the nitrido complex trans-[Mo(N)(NKCCHCOPh)(dppe) 2] ( 12), which was accompanied by evolution of dinitrogen and the formation of 1-phenyl-2-propen-1-one in high yields. For complex 12, the zwitterionic structure, where the anionic enolate ligand PhC(O +)CHCN coordinates to the cationic Mo(IV) center through its nitrogen atom, was confirmed by spectroscopic measurements and single-crystal X-ray analysis. A unique intermolecular aromatic CH⋯O hydrogen bonding was observed in that crystal structure. Complex 12 is considered to be formed via the cleavage of the CN triple bond of benzoylacetonitrile on the metal. A reaction mechanism is proposed, which includes the double protonation of the nitrile carbon atom of the ligating benzoylacetonitrile on a low-valent molybdenum center.