A π-Donor Spectrochemical Series for X in (Me5C5)2TiX, and β-Agostic Interactions in X = Et and N(Me)Ph

Abstract

The EPR and electronic spectra of d1-bent metallocene compounds of the type Cp*2TiX, where X is halide, alkoxide, amide, alkyl, or hydride and Cp* = Me5C5, have been studied. Several of these compounds are new, and those with X = N(Me)H and F were characterized by X-ray crystallography. The crystal structure of Cp*2TiN(Me)H showed that the N(Me)H ligand lies on the plane defined by Cp*(centroid)−Ti−Cp*(centroid). This is the sterically most unfavorable conformation but allows maximum Ti−N π-bonding. The anisotropic frozen solution EPR spectra were analyzed by the method used by Petersen and Dahl for the d1-metallocenes, Cp2VX2, which gives gx, gy, and gz. Although the values of gx and gz are relatively constant throughout the series, the value of gy varies with the π-donor ability of X. The π-donor series is N(Me)H ≈ NH2 ≈ OMe > OPh ≈ F > N(Me)Ph > Cl > Br > I > H. Among the known alkyls, the π-donor ability was Et > Me > n-Pr ≈ CH2CMe3 > CH2C6H5, which is rationalized, in part, by a β-agostic interaction in the case of Et. The β-agostic interaction in Cp*2TiEt and in Cp*2TiN(Me)Ph was investigated by variable-temperature EPR spectroscopy giving an enthalpy and entropy for the agostic interaction. For Cp*2TiEt the parameters for the agostic interaction are ΔH° = −1.93(3) kcal/mol and ΔS° = −6.3(2) eu, and for Cp*2TiN(Me)Ph, ΔH° = −1.5(1) kcal/mol and ΔS° = −7.9(5) eu.