SYNTHESIS, CHIROPTICAL AND ELECTROCHEMICAL STUDIES OF DIOXOURANIUM(VI) COMPLEXES OF ALDIMINE DERIVATIVES OF L-/D-HISTIDINE AND CRYSTAL-STRUCTURE OF (2,2'-BIPYRIDYL)DIOXO-(N-O-VANILLYLIDENE-L-HISTIDINATO)URANIUM(VI)-WATER-METHANOL(1/1/1)

A series of new dioxouranium(VI) complexes has been synthesised using N-(salicylidene)-L-histidine (H2sal-L-his) and N-(o-vanillylidene)-L-histidine (H2van-L-his) and the corresponding D-histidines.They have been characterized by elemental analyses and physicochemical studies. The crystal and molecular structures of [UO2(van-L-his)(bipy)].MeOH.H2O (bipy = 2,2'-bipyridine) have been determined by X-ray crystallography. The aldimine ligand in this compound is tridentate. The methanol and water molecules are hydrogen bonded to each other and the water molecule to the carboxylate 0 atom. The diamagnetic UO22+ entity serves as a chiroptical probe, undergoing stereospecific complex formation with the aldimine ligands, as well as for the interpretation of their H-1 NMR spectra, which in conjunction with two-dimensional NMR spectra, reveal the different spin-spin interactions, including the long range one between the azomethine (CH=N) and H(alpha) proton of the amino acid residue. The CD and NMR spectral data of the quasi-enantiomorphous UO22+ complexes containing the L-/D-amino acid residues have helped to ascertain the conformational differences between each such pair; these differences can modulate energies of the half-filled highest occupied molecular orbitals in different ways. As these orbitals are involved in the electron-transfer process, the relevant complexes respond differently when subjected to cyclic voltammetry.