Anomalous solution behaviour of isomorphous complexes of zirconium- and hafnium-(IV)

The synthesis of [Zr(IV)(egta)] and [Hf(IV)(egta)] in aqueous solution, where H-4egta = ethylenedioxydiethyl-enedinitrilotetraacetic acid, is reported. The eight-co-ordinate complexes have been characterised by X-ray crystallography, H-1 NMR spectroscopy, mass spectrometry and elemental analysis. The two compounds are isostructural in the solid state. Solution studies by H-1 NMR spectroscopy show that the hafnium analogue has a higher degree of symmetry. In particular, the spectrum of the zirconium complex reveals all of the methylene groups to be inequivalent whereas in the hafnium complex the -N (CH2CO2)2- and the -NCH2CH2O- methylene groups show increased magnetic equivalence.