Selective Complexation of Uranium(III) over Cerium(III) and Neodymium(III) by 2,2′:6′,2′′-Terpyridine − X-ray Crystallographic Evidence for Uranium-to-Ligand π Back-Bonding

The addition of 2,2′:6′,2′′-terpyridine (terpy) to MI3 (M = Ce, Nd, U) in anhydrous pyridine led to the quantitative formation of the bis-terpy complexes [MI2(terpy)2]I [M = Ce (1) and Nd (2)] and [MI2(terpy)2L]I [M = U and L = pyridine (3); M = Ce and L = H2O (4)]. Conductometry measurements indicated that these complexes are 1:1 electrolytes in pyridine solution. 1H NMR competition experiments showed that the terdentate ligand has a better affinity for UIII than for CeIII or NdIII. The X-ray crystal structures of solvates of 1−4 have been determined. The average U−N(terpy) bond lengths in 3 are 0.05 A shorter than the mean Ce−N bond length in 4; the U−N(central pyridine) distances are shorter than the U−N(distal pyridines) distances in 3, while the reverse order is found in the early lanthanide compounds. These differences could reflect the presence of a π back-bonding interaction between the uranium atom and the terpy ligand. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)