NMR and Theoretical Investigations on the Structures and Dynamics of Octahedral Bis(chelate)dichloro VIII Compounds Isolated by an Unusual Reduction of Non-Oxo VIV Species

Reaction of the non-oxo VIV species [VIVCl2(LOO)2] [LOO = acetylacetonate (acac-) or benzoylacetonate (bzac-)] with a chelate nitrogen-donor ligand LNN in acetonitrile leads to the reduction of VIV to VIII and the formation of the mononuclear VIII compounds of the general formula [VIIICl2(LOO)(LNN)] (LOO and LNN are acac- and bipy for 1; acac- and 5,5ʹ-me2bipy for 2; acac- and 4,4ʹ-tb2bipy for 3; acac- and phen for 4; bzac- and bipy for 5; bzac- and phen for 6). The reduction of the VIV complexes was monitored by GC-MS and 1H NMR spectroscopy. Both one- and two-dimensional (2D COSY and 2D EXSY) 1H NMR techniques were used to assign the observed 1H NMR resonances of 1-6 in CD2Cl2 or CDCl3 solution. It appeared that in solution these VIII complexes form two isomers which are in equilibrium: cis-[VIIICl2(LOO)(LNN)] (rightwards harpoon over leftwards harpoon) trans-[VIIICl2(LOO)(LNN)]. 2D EXSY cross-peaks were clearly observed between bipy- and acac-hydrogen atoms of the two geometrical isomers of 1-3 as well as between bipy and acac- protons of the cis isomer, indicating a dynamic process that corresponds to cis-trans isomerization and a cis-cis racemization. The thermodynamic and kinetic parameters of the equilibrium between these two isomers were calculated for compounds 1 and 2 by using variable temperature (VT) NMR data. Both cis-trans isomerization and cis-cis racemization processes probably proceed with an intramolecular twist mechanism involving a trigonal prismatic transition state. Density functional calculations (DFT) also indicated such a rearrangement mechanism.