NIOBOCENE ALKYNE COMPLEXES - SYNTHESIS AND CHARACTERIZATION OF NEUTRAL AND CATIONIC D(2) METAL ALKYNE DERIVATIVES - X-RAY CRYSTAL-STRUCTUREOF B(ETA(5)-C5H4SIME3)(2)(ETA(2)(C,C)-HC-CPH)(CH2PH)]

The paramagnetic niobocene alkyne complexes Nb(eta(5)-C5H4SiMe3)(2)(et a(2)(C,C)-RC=CR') (2) have been synthesized from Nb(eta(5)-C5H4SiMe3)( 2)(eta(2)(C,C)-RC=CR')(Cl) (1) and subsequently oxidized in the presen ce of different ligands to give stable cationic d(2) derivatives, )C(5 )H(4)SiMe(3))(2)(eta(2)(C,C)-RC=CR')(L)][BPh4] (L = CH3CN, R = R' = Ph (3a); L = CH3CN, R = R' = CO2Me (3b); L = CH3CN, R = R' = Me (3c); L = CH3CN, R = Me, R' = CO2Me (3d); L = tBuCN, R = R' = Ph (4a); L = tBu CN, R = R' = CO2Me (4b); L = tBuCN, R = R' = Me (4c); L = tBuCN, R = M e, R' = CO2Me (4d); L = tBuNC, R = R' = Ph (5a); L = tBuNC, R = R' = C O2Me (5b); L = tBuNC, R = R' = Me (5c); L = THF, R = R' = Me (6c); L = THF, R = Me, R' = CO2Me (6d); L = Py, R = R' = Ph (7a); L = Py, R = R ' = Me (7c)). Oxidation of the complex Nb(eta(5)-C5H4SiMe3)(2)(eta(2)( C,C)-MeC=CCO2Me) (2d) under different experimental conditions gave ris e to the divinylidene binuclear d(2) niobocene complex CH3)(CH3)C=C=Nb (CO)(eta(5)-C5H4SiMe3)(2)][BPh4](2) (8a). A mechanism involving the in termediacy of a sigma-acetylide species is proposed for the formation of this compound. Thus, the divinylidene complex (Ph)C=C=Nb(CO)(eta(5) C(5)H(4)SiMe(3))(2)][BPh4](2) (8c) was easily synthesized by oxidation of the corresponding Nb(III) alkynyl derivative Nb(eta(5)-C5H4SiMe3)( 2)(CO)(C=CPh) (13), obtained from Nb(eta(5)-C5H4SiMe3)(2)(CO)Cl and Mg (C=CPh)(2). Furthermore, the paramagnetic Nb(IV) complexes (2) undergo clean reactions with alkyl halides to give a mixture of both alkyl-al kyne and halo-alkyne compounds. The former complexes, Nb(eta(5)-C5H4Si Me3)(2)(eta(2)(C,C)-RC=CR')(R '') (R = R' = Ph, R '' = Me (9a); R = R' = R '' = Me (9c); R = H, R' = Ph, R '' = Me (9e); R = R' = Me, R '' = Et (10c); R = R' = Me, R '' = CH2Ph (11c); R = H, R' = Ph, R '' = CH2 Ph (11e); R = H, R' = Ph, R '' = CH2CH=CH2 (12e)), were also prepared by an alternative procedure in which the corresponding chloro derivati ves 1 were reacted with the appropriate dialkylmagnesium reagents. The structure of 11e was determined by single-crystal diffractometry.