Concerted conformational exchange processes in molecular adducts of antimony pentachloride and N,N′-alkylureas

Abstract

A variable temperature proton magnetic resonance (PMR) study of 1 : 1 molecular adducts between antimony pentachloride and sym-di-, tri- and tetramethylurea, 1-ethyl-, 1-isopropyl-and 1-isobutyl-3-methylurea and N,N′- dimethyl-2-imidazolidone has been carried out. Separate signals ascribed to free and complexed donors were observed at room temperature. In systems of di-, tri- and tetramethylurea the donor-donor exchange became rapid on the NMR time scale at 130°. At low temperatures the signals from the 1,3-dialkyl- and tetramethylurea adducts separated into two sets revealing an exchange between conformational isomers. The activation parameters for a two-site exchange of each alkylamino group in the ethylmethylurea adduct were determined from a total lineshape treatment of the PMR signals. The two groups gave the same values within the combined uncertainties of the measurementsof ΔG‡ (16·0±0·1 kcal mo1 −1and ΔH‡ (15·0±0-6 kcal mol −1). From observed population ratios and the results of the total lineshape treatment, the conformational exchange of SbCl 5-1-R-3-R′-alkylurea adducts is judged to be a concerted rearrangement of the adduct molecule, consisting of inversion of SbCl 5 at the oxygen atom coupled to an interlocked rotation of the alkylamino groups.