Combined NMR, DFT and X-ray studies highlight structural and hydration changes of [Ln(AAZTA)]− complexes across the series

We report a detailed structural study of [Ln(AAZTA)]− complexes by using a combination of experimental and theoretical tools. The analysis of the 1H NMR paramagnetic shift of the methyl peak across the series suggests that a structural change occurs between Ho and Er. Chemical exchange saturation transfer experiments were subsequently used to determine the number of coordinated water molecules and their corresponding exchange rates kex at 278 K. The Z-spectra recorded for the Ho(III) complex present two signals that confirm the presence of two coordinated water molecules, which are endowed with rather different exchange rates: 5.8(±3.0) × 103 and 8.1(±0.5) × 104 s−1. On the contrary, the Er(III) and Tm(III) complexes present a single signal in the Z-spectra. The exchange rate of the coordinated water molecule(s) decreases markedly across the series from Gd(III) to Yb(III). DFT calculations support the change in hydration number by the end of the lanthanide series, which is the result of an increased steric compression around one of the coordinated water molecules on decreasing the size of the metal ion. X-ray diffraction studies on the Er(III) complex confirm the presence of a single inner-sphere water molecule and the dodecahedral coordination environment of the metal ion.