Molecular modelling studies of N-salicylideneamino acidato complexes of oxovanadium(iv). Molecular and crystal structure of a new dinuclear LOVIV–O–VVOL mixed valence complex

Abstract

The dinuclear complex Na[V₂O₃(MeOsal-L-Ile)₂]·H₂O 1 (MeOsal-L-Ile = 4-methoxysalicylidene-L-isoleucinate) has been prepared and characterised and its crystal structure determined. The molecule consists of two C-VO(MeOsal-L-Ile) units, the Schiff base ligand being approximately planar. Each unit exhibits a distorted square-pyramidal coordination geometry around the V atoms involving the O,N,O atoms of the Schiff base ligand and the bridging O(oxo) atom. The V^IV–O–V^V bridge is almost linear (angle: 170.9(3)°), indicating extensive electron delocalization, and the V–O(oxo) bond lengths are 1.811(5) and 1.836(5) Å. Molecular mechanics (MM) and density functional theory (DFT) methods are used to calculate the structures and the main factors that determine the relative energies of the CL- and AL-[V^IVO(sal-aa)(X)] diastereomeric complexes (aa = N-salicylidene-amino acidate, X = H₂O or 2,2′-bipyridine). The results obtained indicate that for X = bpy the CL-diastereomers are more stable than the AL-diastereomers, and the energy differences increase with the degree of substitution on the β-carbon atom of the amino acid. For X = H₂O the CL- and AL-diastereomers correspond to about the same energies. DFT methods are also used to calculate the IR spectrum of C-[V^IVO(sal-L-Ala)(H₂O)] (sal-L-Ala = N-salicylidene-alaninate) which compares well with the experimental, and the g_x, g_y, g_z, and A_x, A_y, A_z parameters of the EPR spectra. The structure of [V₂O₃(HOsal-L-Gly)₂]⁻ was also calculated by DFT methods and compared with the X-ray structure of 1.