REACTIVITIES OF THE COORDINATED ORGANONITRILES IN MOLYBDENUM(0) AND TUNGSTEN(0) PHOSPHINE COMPLEXES - PROTONATION OF THE NITRILE CARBON ANDCLEAVAGE OF THE C-N TRIPLE BOND

The molybdenum and tungsten dinitrogen-organonitrile complexes trans-[ M(N-2)(NCR)(dppe)(2)] (2, M=Mo; 4, M=W; R=Ph, C6H4Me-p, C6H4OMe-p, Me; dppe = Ph2PCH2CH2PPh2) underwent double protonation at the nitrile ca rbon atom with loss of N-2 and a change in oxidation state to +4 on tr eatment with hydrochloric acid to afford the cationic imido complexes trans-[MCl(NCH2R) (dppe)(2)](+). The solid-state structure of trans-[W Cl(NCH2CH3) (dppe)(2)] [PF6]. CH2Cl2 was determined by single-crystal X-ray analysis. Protonation of complexes 2 by fluoroboric acid or hydr obromic acid also formed the similar imido complexes trans- [MoX(NCH2R ) (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) (NCCHCOPh) (dppe)(2)] (12), which was accompanied by evol ution of dinitrogen and the formation of 1-phenyl-2-propen-1-one in hi gh yields. For complex 12, the zwitterionic structure, where the anion ic enolate ligand PhC(O-)=CHCN coordinates to the cationic Mo(IV) cent er through its nitrogen atom, was confirmed by spectroscopic measureme nts and single-crystal X-ray analysis. A unique intermolecular aromati c C-H ... O hydrogen bonding was observed in that crystal structure. C omplex 12 is considered to be formed via the cleavage of the C=N tripl e bond of benzoylacetonitrile on the metal. A reaction mechanism is pr oposed, which includes the double protonation of the nitrile carbon at om of the ligating benzoylacetonitrile on a low-valent molybdenum cent er. (C) 1998 Elsevier Science S.A. All rights reserved.