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)

Abstract

A series of new dioxouranium(VI) complexes has been synthesised using N-(salicylidene)-L-histidine (H₂sal-Lhis) 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 O 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 ¹H NMR spectra, which in conjunction with two-dimensional NMR spectra, reveal the different spin-spin interactions, including the long range one between the azomethine (CHN) and H_α 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.