Niobocene Alkyne Complexes:  Synthesis and Characterization of Neutral and Cationic d²Metal Alkyne Derivatives. X-ray Crystal Structure of [Nb(η⁵-C₅H₄SiMe₃)₂(η²(C,C)-HC⋮CPh)(CH₂Ph)]

The paramagnetic niobocene alkyne complexes Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (η 2 ( C, C )-RC⋮CR‘) ( 2 ) have been synthesized from Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (η 2 ( C, C )-RC⋮CR‘)(Cl) ( 1 ) and subsequently oxidized in the presence of different ligands to give stable cationic d 2 derivatives, [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (η 2 ( C, C )-RC⋮CR‘)(L)][BPh 4 ] (L = CH 3 CN, R = R‘ = Ph ( 3a ); L = CH 3 CN, R = R‘ = CO 2 Me ( 3b ); L = CH 3 CN, R = R‘ = Me ( 3c ); L = CH 3 CN, R = Me, R‘ = CO 2 Me ( 3d ); L = tBuCN, R = R‘ = Ph ( 4a ); L = tBuCN, R = R‘ = CO 2 Me ( 4b ); L = tBuCN, R = R‘ = Me ( 4c ); L = tBuCN, R = Me, R‘ = CO 2 Me ( 4d ); L = tBuNC, R = R‘ = Ph ( 5a ); L = tBuNC, R = R‘ = CO 2 Me ( 5b ); L = tBuNC, R = R‘ = Me ( 5c ); L = THF, R = R‘ = Me ( 6c ); L = THF, R = Me, R‘ = CO 2 Me ( 6d ); L = Py, R = R‘ = Ph ( 7a ); L = Py, R = R‘ = Me ( 7c )). Oxidation of the complex Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (η 2 ( C, C )-MeC⋮CCO 2 Me) ( 2d ) under different experimental conditions gave rise to the divinylidene binuclear d 2 niobocene complex [(η 5 -C 5 H 4 SiMe 3 ) 2 (CO)Nb C C(CH 3 )(CH 3 )C C Nb(CO)(η 5 -C 5 H 4 SiMe 3 ) 2 ][BPh 4 ] 2 ( 8a ). A mechanism involving the intermediacy of a σ-acetylide species is proposed for the formation of this compound. Thus, the divinylidene complex [(η 5 -C 5 H 4 SiMe 3 ) 2 (CO)Nb C C(Ph)(Ph)C C Nb(CO)(η 5 -C 5 H 4 SiMe 3 ) 2 ][BPh 4 ] 2 ( 8c ) was easily synthesized by oxidation of the corresponding Nb(III) alkynyl derivative Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (CO)(C⋮CPh) ( 13 ), obtained from Nb(η 5 -C 5 H 4 SiMe 3 ) 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−alkyne and halo−alkyne compounds. The former complexes, Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (η 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‘‘ = CH 2 Ph ( 11c ); R = H, R‘ = Ph, R‘‘ = CH 2 Ph ( 11e ); R = H, R‘ = Ph, R‘‘ = CH 2 CH CH 2 ( 12e )), were also prepared by an alternative procedure in which the corresponding chloro derivatives 1 were reacted with the appropriate dialkylmagnesium reagents. The structure of 11e was determined by single-crystal diffractometry.