Dielectric, electro-optic and infrared and 1H NMR spectral properties of methyl- and t-butyl-substituted (η 6-benzene)tricarbonylchromium(0) complexes: a study of solvent effects

Abstract

Dipole moment, microwave dielectric absorption, electric birefringence and infrared and 1H NMR spectral data are presented for [Cr(CO) 3(η 6-C 6H 6)] and for some of its methyl- and t-butyl-substituted derivatives in the non-dipolar solvents, cyclohexane, benzene and dioxane. Atomic polarisations for these complexes are shown to be negligibly small. Specific interactions are shown to occur between dioxane or benzene solvent molecules and the coordinated arene which acts as an electrophile. The effects on such interactions of progressive methyl or t-butyl group insertion on the arene nucleus are studied. In 1,3,5-tri-t-butylbenzenetricarbonylchromium(0), “face-to-face” associations between the arene ligand and solvating benzene are sterically inhibited. Direct complexation with chromium is shown to be improbable with these solvents. The solute polarisability tensor of [Cr(CO) 3(η 6-C 6H 6)], unlike the dipole moment vector, is not changed by interaction with dioxane. With methyl or t-butyl substitution, the solute polarisability anisotropy in dioxane tends to decrease, relative to that in cyclohexane. Electric birefringences and 1H NMR chemical shifts are interpreted in terms of transient stereospecific solute-benzene associations additional to “face-to-face” charge-transfer interactions. Benzene-induced upfield shifts or aryl protons (ASIS) decrease in magnitude with progressive methyl or t-butyl substitution.