Hyperfine interactions and metal atom dynamics in a number of t-butyl ferrocene derivatives: The counter intuitive effects of ring substitution

Abstract

Using temperature-dependent Mössbauer effect spectroscopy, the hyperfine interactions and metal atom dynamics of three structurally related 1,1′ bis tert. butyl ferrocenes have been examined in detail over the temperature range 90 < T < 300° K. It is remarkable that while ferrocene is a solid at room temperature (448° K), the replacement by two tert. butyl groups for the Cp ring protons lowers the melting point close to RT. Further ring substitution again raises the melting point above RT. As noted previously, the substitution of a tert. butyl group for a ring proton has little effect on the IS and QS parameters at 90° K. In contrast, the temperature-dependence of the IS reflects the covalency of the metal atom-ring interaction. The evaluation of the k2<x2> parameter, and its comparison to its value in the parent ferrocene shows that the vibrational amplitude of the metal atom is significantly smaller in the butyl compounds than in ferrocene. The metal atom motions are isotropic in the phenyl phosphine and bromine complexes, indicative of the absence of a Goldanskii-Karayagin effect in these two molecular solids.