Dynamic NMR study of ligand exchange reactions in U(VI)-phosphonic acid systems

The rates of hydrogen ion exchange on phosphonic acid ligands and that of p hosphonate ligand exchange on selected uranyl-phosphonate complexes have be en investigated by dynamic NMR spectroscopy. The spin-spin relaxation time (In(1/T-2)) for H+ exchange on the free ligands exhibits a parabolic depend ence on reciprocal temperature (1/T (K-1)). The empirical fit parameters ar e correlated with the activation parameters (Delta H*, Delta S*, Delta C-p* ) by adapting the statistical mechanical framework developed by Braibanti c t al. to the Eyring activated complex theory. The correlation of the appare nt activation enthalpy with temperature indicates that 6-11 water molecules participate in the transition from reactants to activated complex for liga nd proton exchange reactions: depending on the ligand. Mechanistic details of ligand exchange reactions of phosphonic acid complexants onto the corres ponding uranium(VI) complexes are not fully developed, but the negative val ues for Delta S* imply increased order in the transition to the activated c omplex. Though Delta H* for ligand exchange covers a range of 15 kJ/mol, th e free energy of activation (Delta G*) is nearly constant for the series of ligands, implying substantial entropy compensation in the activation proce ss. Application of the Marcus relationship defines a correlation between De lta G* and the thermodynamic stability (Delta G degrees) for the 1:2 uranyl complexes with methanediphosphonic acid and 1-hydroxyelhane-1,1-diphosphon ic acid.