Vanadium(V) complexes of a chelating dianionic [ONNO]-type amine bis(phenolate) ligand: synthesis and solid state and solution structures.

The reaction between VO(OR)(3) (R = (i)()Pr, (t)()Bu, CH(2)CF(3)) and the chelating dianionic bis(phenoxy)amine ligand [ONNO]H(2) affords a mixture of two isomers (A and B in a ratio A:B approximately 3:1) formulated as VO(OR)[ONNO] (1a-c) (R = (i)()Pr (1a), (t)Bu (1b), CH(2)CF(3) (1c)). Multinuclear and NOESY NMR spectroscopy experiments were able to determine the structure in solution of the complexes. Both isomers have the symmetry-related phenolate groups in a trans configuration, the difference arising from the different configuration of the oxo and alkoxo ligands being located either cis (in isomer A) or trans (in isomer B) to the tripodal amino nitrogen donor atom and the (dimethylamino)ethyl sidearm respectively for the oxo and the alkoxo ligands. Crystals of isomer A (cis-1a) were obtained, and the structure determination confirms the arrangement of the ligands around the vanadium center. Analogue complexes VO(X)[ONNO] (X = Cl (2); X = N(3) (3)) were prepared by reacting equimolar amount of [ONNO]H(2) and VO(X)(n)(OR)(3-n) (X = Cl, R = Et, n = 1; X = N(3), R = (i)Pr, n = 2) at ambient temperature. Compounds 2 and 3 were further characterized by NMR spectroscopy experiments and X-ray structure determination. For both 2 and 3, a single isomer is obtained, having a trans-(O,O) configuration for the phenolate groups and a trans configuration of the oxo ligand in respect to the tripodal amino nitrogen donor atom. Finally, complex 2 could also be obtained by chlorination of 1a or 3 using a large excess of ClSiMe(3) in refluxing toluene.