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

Abstract

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)]Cl3·4H2O (1) in acetonitrile and methanol solution has been studied by NMR spectroscopy. The spectra were assigned by means of one-dimensional 1H 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 [kobs = 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 ΔH‡=69(1)kJ mol-1 and ΔS‡ =-9(2) J mol-1 K-1 in acetonitrile and ΔH‡ = 71(2)kJ mol-1 and ΔS‡ = 4(6) J mol-1 K-1 in methanol solution.