Dynamic solution behaviour of metal complexes of the hexaamine cage ligand Me8tricosane

Abstract

The solid state structures of the hexaamine cage complexes [M(Me8tricosane)]2+ (M = Zn, Cd, Hg) were previously shown to feature highly symmetric octahedral or trigonal prismatic coordination geometries. Here we show that the solution NMR spectra of each of these three complexes are far more complex than what would be expected if the solid state structures were retained after dissolution. The large numbers of resonances observed in the solution NMR spectra reflects establishment of equilibria involving multiple diastereoisomers with different nitrogen atom configurations. Support for this explanation was provided by the 15N HSQC spectrum of the Zn complex, and by variable temperature 1H and 13C NMR spectra of each of the three metal complexes. In addition, the results of calculations performed using DFT methods showed that the differences in energy between diastereoisomers were sometimes very small, and therefore unlikely to present a significant barrier to isomerisation processes.