Vanadium(III) and Vanadium(V) Amine Tris(Phenolate) Complexes

The coordination chemistry of amine tris(phenolate) ligands around V(III) and V(V) is described for the first time.
Three amine tris(phenolate) ligands were employed featuring different steric and electronic influence exerted by
the phenolate substituents in the ortho and para positions being either t-Bu, Me, or Cl. V(III) complexes of all
ligands (1−3) were readily obtained by reaction between the ligand precursors and VCl3(THF)3 in the presence of
triethylamine. The complexes obtained were pentacoordinate, a THF ligand completing the coordination sphere of
the metal, which was found to be of almost perfect TBP geometry, as revealed by crystallography. V(V) oxo
complexes of all the ligands (4−6) were readily obtained by a reaction between the ligand precursors and
VO(OPr)3. The oxo complexes of the alkyl-bearing ligands (4 and 5) could also be synthesized by the air oxidation
of the corresponding V(III) complexes (1 and 2); however, the attempted air oxidation of the V(III) complex bound
to the electron-poor ligand (3) did not yield the corresponding oxo complex 6. 1H NMR and crystallographic analysis
of complexes 4 and 5 supported their TBP structures. Complex 6, on the other hand, was found to be composed
of a TBP complex (6a) and an octahedral complex (6b) in equilibrium, the octahedral complex being more stable
at lower temperatures. An X-ray structure of 6b revealed a mononuclear oxo complex, the sixth coordination site
being occupied by an aqua ligand to which two THF molecules are H-bonded. Complexes 4−6 catalyze the
epoxidation of olefins by t-BuOOH, albeit slowly. These complexes may thus be considered as structural and
functional models of vanadium-dependent haloperoxidase enzymes.