SYNTHESIS, ELECTROSYNTHESIS AND STRUCTURAL STUDIES OF BIS(SILYLCYCLOPENTADIENYL)NIOBIUM COMPLEXES WITH ACETYLENE LIGANDS

The reduction of [Nb(eta(5)-C(5)H(4)SiMe(3))(2)Cl-2] (1) with one equivalent of sodium amalgam (10%) in the presence of acetylenes yields [Nb(eta(5)-C(5)H(4)SiMe(3))(2)Cl(eta(2)-RC(1) = CR(2))], (R(1) = H, R(2) = COOMe, 3c; R(1) = Me, R(2) = COOMe, 3d; R(1)= R(2) = COOMe, 3e). The structural characterization of these compounds has been carried out by NMR spectroscopy. The reduction of the (acetylene)niobium(V) complexes [Nb(eta(5)-C(5)H(4)SiMe(3))(2)Cl(eta(2) R(1)C = CR(2))] with one equivalent of sodium amalgam (10%) gives the paramagnetic niobium(IV) complexes [Nb(eta(5)-C(5)H(4)SiMe(3))(2)(eta(2)-R(1)C = CR(2))] (R(1) = R(2) = Ph, 4a; R(1) = H, R(2) = Ph, 4b; R(1) = H, R(2) = COOMe, 4c; R(1) = Me, R(2) = COOMe, 4d; R(1) = R(2) = COOMe, 4e), which have been characterized by ESR spectroscopy. However, electroreduction of 1 in the presence of the acetyIenes gives the complexes 3 or 4 depending both on the nature of the acetylene and on the experimental conditions. In the electrolysis of 1 in the presence of acetylenes c or d a slow production of 4e and 4d, respectively, occurs to yield the alkenyl complexes [Nb(eta(5)-C(5)H(4)SiMe(3))(2)Cl(eta(1)-R(1)C = CHR(2))], 4'c and 4'd. Finally, the addition of HCl to solutions of 4a gives a mixture of 1 and cis-stilbene.