Equilibria between α- and β-agostic stabilized rotamers of secondary alkyl niobium complexes

The isopropyl chloro complex Tp^Me2NbCl(i-Pr)(PhC≡CMe) (2) [Tp^Me2 = 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, 2β, and a minor α-agostic rotamer 2α. 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′ = Tp^Me2, R2 = Me (3); Tp′ = Tp^Me2,4Cl, R2 = Ph (4)]. The two diastereomers of the sec-butyl complex Tp^Me2NbCl(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-CSα, 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/2α = 4.0 ± 0.1 at 193 K, ΔG°193 = -2.2 ± 0.1, ΔH° = -7.4 ± 0.1 kJ mol^-1, and ΔS° = -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 Tp^Me2NbCl[CH(CD3)2](PhC≡CMe) (2-d6), an expected isotope effect that displaces the equilibrium toward the α-agostic rotamer is observed (K = 2-d6β/2-d6α = 3.1 ± 0.1 at 193 K, ΔG°193 = -1.8 ± 0.1, ΔH° = -8.3 ± 0.4 kJ mol^-1 and ΔS° = -34 ± 2 J K^-1 mol^-1). The anomalous values for ΔH° and ΔS° are discussed. Hybrid quantum mechanics/molecular mechanics calculations (IMOMM (B3LYP:MM3)) on the realistic model Tp^Me2NbCl(i-Pr)(HC≡CMe) have reproduced the energy differences between the α- and β-agostic species with remarkable accuracy. Similar calculations show that Tp^Me2NbCl(CH2Me)(HC≡CMe) is α-agostic only and that Tp^5-MeNbCl(CH2Me)(HC≡CMe), which has no methyl groups at the 3-positions of the pyrazole rings, is β-agostic only. Analysis and discussion of the computational and experimental data indicate that the unique behavior observed for the secondary alkyl complexes stems from competition between electronic effects favoring a β-agostic structure and steric effects directing a bulky substituent in the wedge between two pyrazole rings of Tp^Me2. All of the secondary alkyl complexes thermally rearrange to the corresponding linear alkyl complexes via a first-order reaction.