ENDOR on vanadium d⁵-sandwich compounds

Abstract

The temperature dependence of the proton ENDOR spectra of dibenzene vanadium diluted into (2,2)-paracyclophane and ruthenocene single crystals is reported in the range 10-200 K. The low temperature spectra are discussed in detail in the preceding paper. The observed temperature dependence is modelled by a chemical exchange formalism for the rotational reorientation of the benzene rings with respect to the matrix crystal, leading to estimates of the activation parameters of the reorientation process. In the (2,2)-paracyclophane host crystal, both rings of dibenzene vanadium show the same reorientation dynamics, whereas in the ruthenocene host crystal, the corresponding activation parameters of the two benzene rings are considerably different. Furthermore, from the temperature dependence of the proton ENDOR spectra of dibenzene vanadium and cyclopentadienylcycloheptatrienyl vanadium in frozen toluene, the effect of ring size on the rotational reorientation dynamics is demonstrated.