Search

The following alkynyl niobocene imido complexes have been prepared:  [Nb(&dbd;NBu^t)(η⁵-C5H5)2(C&tbd1;CR)] (R = Me (1), SiMe3 (2), Bu^t (3), CH2Ph (4)), [Nb(&dbd;NC6H4Me-4)(η⁵-C5H5)2(C&tbd1;CR)] (R = SiMe3 (5), Bu^t (6)), [Nb(&dbd;NBu^t)(η⁵-C5H4SiMe3)2(C&tbd1;CR)] (R = SiMe3 (7), Bu^t (8)), [Nb(&dbd;NPh)(η⁵-C5H4SiMe3)2(C&tbd1;CR)] (R = Ph (9), Bu^t (10)), [Nb(&dbd;NC6H4Me-4)(η⁵-C5H4SiMe3)2(C&tbd1;CR)] (R = SiMe3 (11), Bu^t (12)), [Nb(&dbd;NC6H4OMe-4)(η⁵-C5H4SiMe3)2(C&tbd1;CR)] (R = SiMe3 (13), Bu^t (14)), and [Nb(&dbd;NBu^t){Me2Si(η⁵-C5H4)2}(C&tbd1;CR)] (R = SiMe3 (15), Bu^t (16)). The synthetic path involved the reaction of either BrMg(C&tbd1;CR) or Mg(C&tbd1;CR)2 with the metallocene imido complex Nb(&dbd;NR‘)(Cp)2Cl, Cp = η⁵-cyclopentadienyl. The molecular structure of 9 has been determined by single-crystal X-ray diffraction studies. Protonation of 2 by CF3COOH and 9 by HBF4 gave the cationic amide species [Nb(NHBu^t)(η⁵-C5H5)2(C&tbd1;CSiMe3)]⁺ (17) and [Nb(NHPh)(η⁵-C5H4SiMe3)2(C&tbd1;CPh)]⁺ (18), respectively.

Treatment of Nb(η⁵-C5H4SiMe3)2(Cl)(L) (1) with Mg(C&tbd1;CR)2 in toluene, under appropriate reaction conditions, leads to the alkynyl complexes Nb(η⁵-C5H4SiMe3)2(C&tbd1;CR)(L) (2:  L = CO, R = Ph (2a); L = CO, R = SiMe3 (2b); L = CO, R = ^tBu (2c); L = PMe2Ph, R = Ph (2d); L = P(OEt)3, R = Ph (2e)). The alkynyl-containing niobocene species 2 can be chemically or electrochemically oxidized to give the corresponding cation-radical alkynyl complexes [Nb(η⁵-C5H4SiMe3)2(C&tbd1;CR)(L)]^•+[BPh4]^- (3:  L = CO, R = Ph (3a); L = CO, R = ^tBu (3c); L = PMe2Ph, R = Ph (3d)). These complexes, under different experimental conditions, give rise to the mononuclear vinylidene d² niobocene species [Nb(η⁵-C5H4SiMe3)2(&dbd;C&dbd;CHR)(L)][BPh4] (4:  L = CO, R = Ph (4a); L = CO, R = ^tBu (4c); L = PMe2Ph, R = Ph (4d)) with a hydrogen atom by abstraction from the solvent or, for 3a, the binuclear divinylidene d² niobocene complex [(η⁵-C5H4SiMe3)2(CO)Nb&dbd;C&dbd;C(Ph)(Ph)C&dbd;C&dbd;Nb(CO)(η⁵-C5H4SiMe3)2][BPh4]2 (4a‘) from a competitive ligand−ligand coupling process. Complexes 4 were also prepared by an alternative procedure in which the corresponding complexes 2 were reacted with HBF4. Finally, in solution the CO-containing vinylidene mononuclear complexes 4a and 4c undergo an unexpected isomerization process to give the η²-alkyne derivatives [Nb(η⁵-C5H4SiMe3)2(η²(C,C)-HC&tbd1;CR)(CO)]⁺ (5:  R = Ph (5a); R = ^tBu (5c)). The structure of 5a was determined by single-crystal diffractometry. DFT calculations were carried out on [NbCp2(&dbd;C&dbd;CHCH3)(L)]⁺/[NbCp2(HC&tbd1;CCH3)(L)]⁺ (Cp = η⁵-C5H5; L = CO, PH3; exo, endo) model systems in order to explain the η¹-vinylidene−η²-alkyne rearrangement observed. Calculations have shown that in both carbonyl−niobocene and phosphine−niobocene systems the η¹-vinylidene and the η²-alkyne complexes are isoenergetic, in marked contrast with the systems previously considered in theoretical studies. The reaction takes place through an intraligand 1,2-hydrogen shift mechanism where η²(C,H)-alkyne species are involved. The energy barrier for the isomerization process in the phosphine-containing niobocene systems is almost 10 kcal mol^-1 higher than in the analogous process for the carbonyl-containing niobocene system. This increase in activation barrier indicates that the different experimental behavior between 4a, 4c, and 4d has a kinetic rather than a thermodynamic origin. Finally, the interconversion between exo and endo isomers has been studied.

The diyne-containing niobocene complexes Nb(η⁵-C5H4SiMe3)2Cl(η²(C,C)-R¹C&tbd1;CR²) (R¹ = C&tbd1;CSiMe3, R² = SiMe3 (2a); R¹ = C&tbd1;CPh, R² = Ph (2b); R¹ = CH2CH2C&tbd1;CH, R² = H (2c)) were prepared by the reaction of Nb(η⁵-C5H4SiMe3)2Cl (1) with the diyne reagents, R−C&tbd1;C−C&tbd1;C−R (R = SiMe3 or Ph) and 1,5-hexadiyne. The enyne-containing niobocene complexes Nb(η⁵-C5H4SiMe3)2Cl(η²(C,C)-R¹C&tbd1;CR²) (R¹ = CH2CH&dbd;CH2, R² = Ph (3a); R¹ = CH2CH&dbd;C(CH3)2, R² = Ph (3b); R¹ = CH2C(CH3)&dbd;CH2, R² = Ph (3c); R¹ = C(CH3)&dbd;CH2, R² = H (3d)) were also prepared in the same way, namely by the reaction of 1 with the appropriate enyne reagents. The paramagnetic niobocene complex Nb(η⁵-C5H4SiMe3)2(η²(C,C)-R¹C&tbd1;CR²) (R¹ = C&tbd1;CSiMe3, R² = SiMe3 (4a)) was also synthesized from 2a and subsequently oxidized in the presence of different neutral ligands to give the stable cationic d² derivatives [Nb(η⁵-C5H4SiMe3)2(η²(C,C)-R¹C&tbd1;CR²)(L)][BPh4] (L = CH3CN (5a); L = ^tBuNC (5a‘)). Some diyne- and enyne-containing chloroniobocenes were alkylated by reacting with the appropriate dialkylmagnesium reagent to give the corresponding alkyl−alkyne compounds Nb(η⁵-C5H4SiMe3)2(R³)(η²(C,C)-R¹C&tbd1;CR²) (R¹ = C&tbd1;CSiMe3, R² = SiMe3, R³ = Me (6a); R¹ = CH2CH&dbd;CH2, R² = Ph, R³ = CH2Ph (7a); R¹ = CH2CH&dbd;C(Me)2, R² = Ph, R³ = Me (7b)). Furthermore, complex 2b undergoes clean reaction with Co2(CO)8 to give the complex Nb(η⁵-C5H4SiMe3)2Cl(η²(C,C)-R¹C&tbd1;CR²) (R¹ = CCPh(Co(CO)3)2, R² = Ph (8b)) through the coordination of both Co(CO)3 moieties to the uncoordinated triple bond. Finally, the enyne-containing niobocene complexes 3a and 3b can be hydrogenated at the C&dbd;C bond using Pd/C as catalyst to give the alkyne complexes Nb(η⁵-C5H4SiMe3)2Cl(η²(C,C)-R¹C&tbd1;CR²) (R¹ = CH2CH2CH3, R² = Ph (9a); R¹ = CH2CH2CH(CH3)2, R² = Ph (9b)). The structures of 2b and 3b were determined by single-crystal diffractometry.

The paramagnetic niobocene alkyne complexes Nb(η⁵-C5H4SiMe3)2(η²(C,C)-RC&tbd1;CR‘) (2) have been synthesized from Nb(η⁵-C5H4SiMe3)2(η²(C,C)-RC&tbd1;CR‘)(Cl) (1) and subsequently oxidized in the presence of different ligands to give stable cationic d² derivatives, [Nb(η⁵-C5H4SiMe3)2(η²(C,C)-RC&tbd1;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)). Oxidation of the complex Nb(η⁵-C5H4SiMe3)2(η²(C,C)-MeC&tbd1;CCO2Me) (2d) under different experimental conditions gave rise to the divinylidene binuclear d² niobocene complex [(η⁵-C5H4SiMe3)2(CO)Nb&dbd;C&dbd;C(CH3)(CH3)C&dbd;C&dbd;Nb(CO)(η⁵-C5H4SiMe3)2][BPh4]2 (8a). A mechanism involving the intermediacy of a σ-acetylide species is proposed for the formation of this compound. Thus, the divinylidene complex [(η⁵-C5H4SiMe3)2(CO)Nb&dbd;C&dbd;C(Ph)(Ph)C&dbd;C&dbd;Nb(CO)(η⁵-C5H4SiMe3)2][BPh4]2 (8c) was easily synthesized by oxidation of the corresponding Nb(III) alkynyl derivative Nb(η⁵-C5H4SiMe3)2(CO)(C&tbd1;CPh) (13), obtained from Nb(η⁵-C5H4SiMe3)2(CO)Cl and Mg(C&tbd1;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(η⁵-C5H4SiMe3)2(η²(C,C)-RC&tbd1;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‘‘ = CH2CH&dbd;CH2 (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.