Experimental and density functional theory studies of ethyltrichlorotitanium, EtTiCl3 and of the ethyl group in EtTiCl3 complexes: Vibrational properties of a β-agostic ethylligand

Abstract

Infrared spectra are reported for the range 4000–400 cm⁻¹ for CH₃CH₂TiCl₃, CH₂DCH₂TiCl₃ , CH₃CD₂TiCl₃, CHD₂CD₂TiCl₃ and CD₃CD₂TiCl₃, and the dmpe adducts of some of these species (dmpe = Me₂PCH₂CH₂PMe₂). Density functional theory is used to calculate force fields for EtTiCl₃, EtCl, EtTiCl₃(dmpe) and the model complex EtTiCl₃(dhpe) (dhpe = H₂PCH₂CH₂PH₂). Scaling of the free EtTiCl₃ force field allows many features of the spectra to be assigned. Comparison of the scaled force constants with similar ones for EtCl enables changes in the ethyl group force constants to be identified. The development of β-agostic behaviour on passing from EtTiCl₃ to the complex is characterised in the geometry by a switch from a staggered to an eclipsed structure for the ethyl group, a small TiCC angle and a short Ti···H distance. In the vibrational spectra it is characterised by the appearance of the lowest isolated stretching frequency, ν^isCH, observed to date, 2585 cm⁻¹, an unusually high methyl bending force constant, and marked changes in the magnitude and direction of the dipole derivative ∂μ/∂r for the agostic C–H bond. A small Fermi resonance affecting the above ν^isCH value is diagnosed and an r₀ value of 1.131 Å is predicted for the agostic C–H bond. Force constants and electrical properties associated with the other ethyl C–H bonds and with the Ti–Cl bonds are compared between the base-free and complexed species.