| Abstract
| - The paramagnetic niobocene alkyne complexesNb(η5-C5H4SiMe3)2(η2(C,C)-RC⋮CR‘)(2)have been synthesized fromNb(η5-C5H4SiMe3)2(η2(C,C)-RC⋮CR‘)(Cl)(1) and subsequentlyoxidized in the presence of different ligands to give stable cationicd2 derivatives, [Nb(η5-C5H4SiMe3)2(η2(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 = tBuCN, R = R‘ =CO2Me (4b); L = tBuCN, R = R‘ =Me(4c); L = tBuCN, R = Me, R‘ = CO2Me(4d); L = tBuNC, R = R‘ = Ph (5a); L =tBuNC, R= R‘ = CO2Me (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)). Oxidationof the complexNb(η5-C5H4SiMe3)2(η2(C,C)-MeC⋮CCO2Me)(2d) under different experimentalconditions gave rise to the divinylidene binuclear d2niobocene complex[(η5-C5H4SiMe3)2(CO)NbCC(CH3)(CH3)CCNb(CO)(η5-C5H4SiMe3)2][BPh4]2(8a). A mechanism involvingthe intermediacy of a σ-acetylide species is proposed for theformation of this compound.Thus, the divinylidene complex[(η5-C5H4SiMe3)2(CO)NbCC(Ph)(Ph)CCNb(CO)(η5-C5H4SiMe3)2][BPh4]2(8c) was easily synthesized by oxidation of thecorresponding Nb(III)alkynyl derivativeNb(η5-C5H4SiMe3)2(CO)(C⋮CPh)(13), obtained fromNb(η5-C5H4SiMe3)2(CO)Cl and Mg(C⋮CPh)2. Furthermore, theparamagnetic Nb(IV) complexes (2) undergoclean reactions with alkyl halides to give a mixture of bothalkyl−alkyne and halo−alkynecompounds. The former complexes,Nb(η5-C5H4SiMe3)2(η2(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‘‘ = CH2Ph(11e); R = H, R‘ =Ph, R‘‘ = CH2CHCH2 (12e)),were also prepared by an alternative procedure in whichthecorresponding chloro derivatives 1 were reacted with theappropriate dialkylmagnesiumreagents. The structure of 11e was determined bysingle-crystal diffractometry.
- Thepreparation of both neutral and cationic d2 niobocenealkynes,Nb(η5-C5H4SiMe3)2(η2(C,C)-RC⋮CR‘)(X)(X = halide, alkyl, allyl, benzyl) and[Nb(η5-C5H4SiMe3)2(η2(C,C)-RC⋮CR‘)(L)][BPh4],has been considered. The formation of divinylidene binucleard2 niobocene complexes[(η5-C5H4SiMe3)2(L)NbCC(R)(R)CCNb(L)(η5-C5H4SiMe3)2][BPh4]2was also studied.
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