Insertion of unsaturated organic electrophiles into molybdenumbond-alkoxide and rheniumbond-alkoxide bonds of neutral,stable carbonyl complexes

Alkoxo complexes Re(OR)(CO)(3)(N-N)] (R=Me, Et, tBu; N-N=2,2'-bipyridine (bipy), 4,4'-dimethyl-2,2'bipyridine (bipy'), 1,10-phenanthroline (phen)) and M(OMe)(eta(3)-allyl)(CO)(2)(phen)] (M=Mo, W) have been synthesized in good yields and using mild conditions by the reaction of sodium alkoxides with Re(OTf)(CO)(3)(N-N)] and MCl(eta(3)-allyl)(CO)(2)(phen)] precursors. These have been characterized by IR and NMR spectroscopy as well as by X-ray diffraction for W(OMe)(eta(3)-allyl)(CO)(2)(phen)] (10). The reactions of the molybdenum and rhenium alkoxo complexes with isocyanates, R'NCO, yield L(n)MN(R')C(O)OR]] complexes; the carbamate ligand, which results from an R'NCO insertion into the Mbond;OR bond, is monodentate through the nitrogen atom. The solid-state structures of Mo and Re examples have been determined by X-ray diffraction. The geometry around the carbamate nitrogen of these compounds is planar, and the distances indicate delocalization of the nitrogen lone pair involving mainly the carbonyl groups. Experiments carried out with the Re complexes showed that aryl isocyanates are more reactive than their alkyl counterparts, and that bulky R' groups led to slow rates of insertion. Insertion reactions were also observed with isothiocyanates, although here it is the Sbond;C bond that inserts into the Mbond;OR bond, and the resulting ligand is bound to the metal by sulfur. Competition experiments with the Re compounds indicate that isocyanates are more reactive than isothiocyanates towards the Rebond;OR bonds. Tetracyanoethylene inserts into the Rebond;OMe bond of Re(OMe)(CO)(3)(bipy')], forming a complex with a 2-methoxytetracyanoethyl ligand; the structure of which was determined by X-ray diffraction. The formation of the xanthato complex Re(SC(S)OtBu)(CO)(3)(bipy)] (20) by reaction of Re(OTf)(CO)(3)(bipy)] with CS(2) and NaOtBu, but not by the reaction of CS(2) and Re(OtBu)(CO)(3)(bipy)] (5 a), suggests that the insertion reactions do not take place by ionization of the alkoxo complexes to give the free alkoxide ion.