Equilibria between α- and β-Agostic Stabilized Rotamers of Secondary Alkyl Niobium Complexes

• Published in: Journal of the American Chemical Society 2001-06, Vol.123 (25), p.6000-6013
• Main Authors: Jaffart, Joëlle, Etienne, Michel, Maseras, Feliu, McGrady, John E, Eisenstein, Odile
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0038169

The isopropyl chloro complex TpMe2NbCl(i-Pr)(PhC⋮CMe) (2) [TpMe2 = hydrotris(3,5-dimethylpyrazolyl)borate] exhibits a β-agostic structure in the crystal. The conformation of the alkyl group is such that the agostic methyl group lies in the Cα−Nb−Cl plane and the nonagostic one, in a wedge formed by two pyrazole rings. As observed by solution NMR spectroscopy, restricted rotation about the Nb−C bond allows the observation of an equilibrium between this species, 2 β, and a minor α-agostic rotamer 2 α. A putative third rotamer which would have the secondary hydrogen in the wedge is not observed. Similar behavior is observed for related Tp‘NbCl(i-Pr)(R2C⋮CMe) [Tp‘ = TpMe2, R2 = Me (3); Tp‘ = TpMe2,4Cl, R2 = Ph (4)]. The two diastereomers of the sec-butyl complex TpMe2NbCl(sec-Bu)(MeC⋮CMe) (5) have been separated. In the crystal, 5CR-AS has a β-agostic methyl group with the ethyl group located in the wedge formed by two pyrazole rings. The same single β-agostic species is observed in solution. The other diastereomer, 5AR - CS has a β-agostic methylene group in the solid state, and the methyl group sits in the wedge. In solution, an equilibrium between this β-agostic methylene complex 5AR - CSβ and a minor α-agostic species 5AR - CS α, where the ethyl substituent of the sec-Bu group is located in the wedge between two pyrazole rings, is observed. NMR techniques have provided thermodynamic parameters for these equilibria (K = 2β/2 α = 4.0 ± 0.1 at 193 K, ΔG o 193 = −2.2 ± 0.1, ΔH o = −7.4 ± 0.1 kJ mol-1, and ΔS o = −27 ± 1 J K-1 mol-1), as well as kinetic parameters for the rotation about the Nb−C bond (at 193 K, ΔG ‡(2) = 47.5 ± 2.5, ΔH‡ = 58.8 ± 2.5 kJ mol-1, and ΔS ‡ = 59.0 ± 10 J K-1 mol-1). Upon selective deuteration of the β-methyl protons in TpMe2NbCl[CH(CD3)2](PhC⋮CMe) (2-d 6), an expected isotope effect that displaces the equilibrium toward the α-agostic rotamer is observed (K = 2-d 6 β/2-d 6 α = 3.1 ± 0.1 at 193 K, ΔG o 193 = −1.8 ± 0.1, ΔH o = −8.3 ± 0.4 kJ mol-1 and ΔS o = −34 ± 2 J K-1 mol-1). The anomalous values for ΔH o and ΔS o are discussed. Hybrid quantum mechanics/molecular mechanics calculations (IMOMM (B3LYP:MM3)) on the realistic model TpMe2NbCl(i-Pr)(HC⋮CMe) have reproduced the energy differences between the α- and β-agostic species with remarkable accuracy. Similar calculations show that TpMe2NbCl(CH2Me)(HC⋮CMe) is α-agostic only and that Tp5-MeNbCl(CH2Me)(HC⋮CMe), which has no methyl groups at the 3-positions of the pyrazole rings, is β-