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Mono and oligonuclear vanadium complexes as catalysts for alkane oxidation : synthesis, molecular structure, and catalytic potential
Auteur(s)
Gonzalez Cuervo, Laura
Shul’pin, Georgiy B.
Date de parution
2004-01-30
In
Inorganica Chimica Acta, 2004/357/475-484
Résumé
A series of mono- and oligonuclear vanadium(V) and vanadium(IV) complexes containing various chelating <i>N,O-, N</i><sub>3</sub>-, and <i>O</i><sub>2</sub>-ligands have been prepared. The biphasic reaction of an aqueous solution of ammonium vanadate and a dichloromethane solution of hexamethylphosphoramide (hmpa) and pyrazine-2-carboxylic acid (pcaH) or pyrazine-2,5-dicarboxylic acid (pdcaH<sub>2</sub>) or pyridine-2,5-dicarboxylic acid (pycaH<sub>2</sub>) yields yellow crystals of [VO<sub>2</sub> (pca)(hmpa)] (<b>1</b>), [(VO<sub>2</sub>)<sub>2</sub>(pdca)(hmpa)<sub>2</sub>] (<b>2</b>), and [VO<sub>2</sub>(pycaH)(hmpa)] (<b>3</b>), respectively. The single-crystal X-ray structure analyses reveal <b>1</b> and <b>3</b> to be mononuclear vanadium(V) complexes, in which a VO<sub>2</sub> unit coordinates to one nitrogen and one oxygen atom of a pca or pycaH chelating ligand, and 2 to be a dinuclear vanadium(V) complex, in which two VO<sub>2</sub> units are coordinated through one nitrogen and one oxygen atom of a pdca bridging ligand; in the three complexes the vanadium atoms also coordinate to the oxygen atom of a hmpa ligand. The reaction of <i>N,N,N′,N′</i>-tetrakis(2-benzimidazolylmethyl)-2-hydroxo-1,3-diaminopropane (hptbH) and VOSO<sub>4</sub> in methanol gives the cationic complex [(VO)<sub>4</sub>(hptb) <sub>2</sub>(μ-O)]<sup>4+</sup> (<b>4</b>), which can be crystallized as the perchlorate salt. In this tetranuclear complex, two dinuclear vanadium(IV) units are held together by a μ-oxo bridge. The known complex [VOCl<sub>2</sub> (tmtacn)] (<b>5</b>) was synthesized from the reaction of 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn) and VCl<sub>3</sub> in acetonitrile; the reaction of tetrabutylammonium vanadate with <i>pyro</i>-cathecol (catH<sub>2</sub>) in acetonitrile gives the known anionic complex [V(cat)<sub>3</sub>]<sup>−</sup> (<b>6</b>), in which the vanadium(V) center is bonded to three cat chelating ligands through the oxygen atoms, obtained as the tetrabutylammonium salt. All compounds synthesized are highly efficient oxidation catalysts for the reaction of cyclohexane with air and hydrogen peroxide in the presence of four equivalents of pcaH per vanadium, although the catalytic activity of the complexes containing bulky chelating ligands <b>4</b> and <b>5</b> is somewhat lower in the initial period of the reaction. During this period the active species are formed from the complexes and final turnover numbers are high. The catecholate ligands of complex <b>6</b> may reduce from V(V) to V(IV) in the beginning of the process, thus providing very high initial oxidation rates.
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Type de publication
journal article
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