Mono- and Dinuclear Oxovanadium(V)calixarene Complexes and Their Activity as Oxidation Catalysts

The background of the investigation is constituted by reactive moieties and intermediates playing relevant roles on
the surfaces of vanadiumoxide-based catalysts during the oxygenation/dehydrogenation of organic substrates. With
the aim of modeling such species, a series of mono- and dinuclear charged and uncharged vanadium oxo complexes
containing p-tert-butylated calix[4]arene and calix[8]arene ligands (denoted H4B and H8B‘ ‘, respectively, in the
protonated forms) has been synthesized and characterized:  PPh4[OVB] (PPh41), OVBOAc (2), PPh4[O2V2HB‘ ‘]
(3), and [μ-O(OV(OMe))2BMe2] (4), where superscripts OAc and Me2 indicate that one or two protons of H4B are
substituted by these residues, respectively. These compounds were analyzed both in solution and by means of
single-crystal X-ray crystallography; it turned out that the crystal structures are retained on dissolution (2 changed
only from the paco to the cone structure). In the case of 4, it could be shown that the bulk product consists of a
mixture of two isomers (4t and 4c) differing in the relative positions of the vanadium-bound methoxy groups.
Subsequently, all compounds were tested as catalysts for the oxidation of alcohols with O2. It turned out that the
two dinuclear complexes efficiently catalyze the oxidation of 1-phenyl-1-propargyl alcohol and fluorenol; in addition,
they even show some activity with respect to the oxidation of dihydroanthracene. This may hint to a higher activity
of dinuclear sites on the surfaces of heterogeneous catalysts as well.