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Insertion of unsaturated organic electrophiles into molybdenum-alkoxide and rhenium-alkoxide bonds of neutral, stable carbonyl complexes

Hevia, E and Perez, J and Riera, L and Riera, V and del Rio, I and Garcia-Granda, S and Miguel, D (2002) Insertion of unsaturated organic electrophiles into molybdenum-alkoxide and rhenium-alkoxide bonds of neutral, stable carbonyl complexes. Chemistry - A European Journal, 8 (19). pp. 4510-4521. ISSN 0947-6539

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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),(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 [LnM{N(R')C(O)-OR}] complexes; the carbamate ligand, which results from an R'NCO insertion into the M-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 S-C bond that inserts into the M-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 Re-OR bonds. Tetracyanoethylene inserts into the Re-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 CS2 and NaO-tBu, but not by the reaction of CS, and [Re(OtBU)(CO)(3)(bipy)] (5a), suggests that the insertion reactions do not take place by ionization of the alkoxo complexes to give the free alkoxide ion.