Extent of hydration of octadentate lanthanide complexes incorporating phosphinate donors: solution relaxometry and luminescence studies

The behaviour of luminescent (Eu, Tb) and highly paramagnetic (Gd) complexes of 1,4,7,10-tetraazacyclododecane containing one carboxamide and three phosphinate substituents has been studied in solution. Analyses of variable-temperature nuclear magnetic relaxation dispersion profiles indicate that there is no water molecule directly co-ordinated to gadolinium(III) ions. It has been found that the observed relaxation enhancement of solvent protons is determined, in addition to the contribution from water molecules diffusing in close proximity to the paramagnetic complex, by a relatively distant water molecule in the ‘second co-ordination sphere’. This is possible because the amide carbonyl oxygen can participate in hydrogen bonding (as a hydrogen-bond acceptor) to a local water molecule, which brings the water molecule close to the metal ion. The luminescence spectra of the complexes of Eu and Tb in water and D₂O are also consistent with such a hydration scheme and there is a good correlation between the non-integral q value (number of inner-sphere water molecules) determined by this method and the distance between the metal ion and the water proton estimated by relaxometric methods. It is proposed that the hydration states q= 0 and 1 may be considered to represent boundary conditions and a given complex in solution may possess intermediate values.