A fluxional lanthanum(III) polypyridyl complex. A nuclear magnetic resonance investigation

The lanthanum(III) complex of the potentially octadentate polypyridyl ligand 6,6'-bis[bis(2-pyridylmethyl)aminomethyl]-2,2'-bipyridine (L) has been prepared. The dynamics of [La(L)]Cl-3. 4H(2)O (1) in acetonitrile and methanol solution has been studied by NMR spectroscopy. The spectra were assigned by means of one-dimensional H-1 NMR and two-dimensional COSY spectroscopy. The assignment showed the presence of two pairwise equivalent pyridine rings and a two-fold symmetry in the cation in 1. The two-dimensional EXSY spectrum revealed a slow interconversion of the two pairs [k(obs) = 0.43(2) s(-1) in acetonitrile at 285 K]. The first-order rate constant for the interconversion was determined in the temperature range 270-315 K. This gave the thermodynamic activation parameters Delta H double dagger = 69( 1) kJ mol(-1) and Delta S double dagger. = -9(2) J mol(-1) K-1 in acetonitrile and Delta H double dagger = 71 (2) kJ mol(-1) and Delta S double dagger = 4(6) J mol(-1) K-1 in methanol solution.