Structure and single crystal EPR study of Cu(II)(L-threonine)2·H2O

Abstract

We report the structure and single crystal EPR studies at 9.8 and 33.3 GHz of the title compound, [Cu(C4H8NO3)2]·H2O. The Cu(II) ion is in an elongated octahedral environment equatorially trans-coordinated by two oxygen atoms and two nitrogen atoms of threonine molecules which act as bidentate ligands (mean d(Cu-O) = 1.95(1) and d(Cu-N) = 1.98(1) Å). It is axially coordinated by carboxylic oxygen atoms belonging to a pair of molecules translated in ±b (Cu-O distances of 2.478(8) and 2.972(3) Å). This axial O···Cu···O interaction gives rise to infinite copper ion chains along the b crystal axis. Neighboring chains are coupled through complex chemical paths including H-bonds and the amino acid side chain. Single crystal EPR spectra show a single, exchange-collapsed resonance at both microwave frequencies for any magnetic field orientation. We evaluated the crystal and the molecular g-tensors from the angular variation of the EPR line position. The results (g is perpendicular to = 2.060(2) and g is parallel with = 2.240(3)) indicate that the electronic ground orbital of the Cu ions is mainly of the 3d(x2-y2) type. The analysis of the angular variation of the EPR linewidth allows us to estimate a mean exchange coupling constant |J/k| = 1.45(5) K for the inter-chain chemical paths.