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 = iPr, tBu, CH2CF3) and the chelating dianionic bis(phenoxy)amine ligand [ONNO]H2 affords a mixture of two isomers (A and B in a ratio A:B ∼ 3:1) formulated as VO(OR)[ONNO] (1a−c) (R =
iPr (1a), tBu (1b), CH2CF3 (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 = N3 (3)) were prepared by reacting equimolar amount of [ONNO]H2 and VO(X)n(OR)3-n
(X = Cl, R = Et, n = 1; X = N3, R = iPr, 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 ClSiMe3 in refluxing toluene.