Homogeneous catalysis by transition metal oxygen anion clusters

Publisher Summary This chapter describes the essence of the catalytic chemistry of heteropoly compounds in solution and in the solid state. The catalytic properties of heteropoly compounds have drawn wide attention, owing to the versatility of these compounds as catalysts, which has been demonstrated both by successhl large-scale applications and by promising laboratory results. Heteropolyanions are polymeric oxoanions formed by condensation of more than two different mononuclear oxoanions. Heteropolyanions formed from one kind of polyanion are called isopolyanions. Acidic elements such as Mo, W, V, Nb and Ta, which are present as oxoanions in aqueous solution, tend to polymerize by dehydration at low pH, forming polyanions and water.Heteropoly catalysts can be applied in various ways. They are used as acid as well as oxidation catalysts. They are used in various phases, as homogeneous liquids, in two-phase liquids (in phase-transfer catalysis), and in liquid-solid and in gas-solid combinations, etc.

Catalytic Chemistry of Heteropoly Compounds

Present General Status of Understanding of Heteropoly Electrolytes and a Tracing of Some Major Highlights in the History of Their Elucidation

Vanadium is an essential element. It is found widely distributed in minor and trace amounts throughout the lithosphere and biosphere. Vanadium is present in all mammalian tissues at concentrations of about 10 µM or less. Numerous physiological effects of this element are known at both the organismic and cellular levels, and in some instances these effects can be explained in terms of its biochemistry. Vanadate(V) and/or vanadyl(IV) ions are very potent inhibitors (K1 ≲ 10−6 M) of certain phosphatases, ATPases, phosphotransferases, nucleases, and kinases, among others. Vanadate(V) activates adenylate kinase. These ions have been quite useful as kinetic and spectroscopic (EPR) probes of complex cellular processes. Vanadium is an insulin mimetic agent. The ability of vanadium at low concentrations to influence enzymes is probably important to its function in vivo. Much of the biochemistry of vanadium is intricately related to the redox and coordination chemistry of this element. In particular, the parallel between the chemistries of vanadates and phosphates can account for the observed inhibition of various phosphohydrolases. However, there is ample evidence that vanadium in the body exists primarily as the vanadyl(IV) ion, VO2+, complexed to proteins and other cellular components; several biological reducing agents can convert vanadium(V) to this form. Serum transferrin appears to be involved in the metabolism of vanadium, perhaps as the transport protein for VO2+. Despite recent major advances in our understanding of the biochemistry of vanadium, the delineation of the physiological function(s) of this important element has remained an elusive goal.

The biochemistry of vanadium

Polyoxometalate Complexes in Organic Oxidation Chemistry

Vanadium-51 n.m.r. spectra of a number of iso- and hetero-polyvanadate anions are reported. Spectra of solutions containing [V10O28]6–(pH 6.5–3.5) are consistent with the known solid-state structure of this anion. The [VO2]+ cation in aqueous solution almost certainly has an octahedral cis-dioxo-structure as its n.m.r. linewidth is comparable with those for the cis compounds [VO2(04C2)2]3– and [VO2(edta)]3–(edta = ethylenediaminetetra-acetate), which are broader than those for pseudo-tetrahedral [VCl2O2]– and [VF2O2]–. Chemical shifts of pseudo-octahedral vanadium atoms in [VxW6–xO19]n–. [PVxW12–xO40]n–. [PMo12–xVxO40]n–, and [VVxW12–xO40]n– anions range from 506 to 545 p.p.m. (VCl3O reference) and linewidthsfrom 60 to 200 Hz (2.6–9 p.p.m.). Multiple lines in the specta of the 1 : 12 (Keggin) heteropolyanions confirm the existence of geometrical isomers distinguished by the relative positions of two or more vanadium atoms in the polyanion structure. Phosphorus-31 n.m.r. spectra confirm that most if not all of the possible isomers of this type areformed. The chemical shifts of 31P in [PMo12040]3– and [PW12040]3– are ca. 11 and 21 p.p.m. upfield from the unprotonated [PO4]3– ion (–6 with respect to 85% H,PO,). Introduction of V atoms in the heteropoly structures causes a progressive decrease in the chemical shift, enabling mixtures of such Keggin anions to be analyzed by n.m.r. A parallel variation in 51V chemical shift is found for [VO4]3–(536) and the tetrahedral vanadium in [VV3W9040]6–(553) and [VV2W10O40]5–(556 p.p.m.).

Applications of vanadium-51 and phosphorus-31 nuclear magnetic resonance spectroscopy to the study of iso- and hetero-polyvanadates

The existence of several isomers of the Keggin anions PVxW12 –xO40(3 +x)– and PVxMo12 –xO40(3 +x)–, (x= 1–4), is demonstrated by 31P n.m.r., e.s.r., and i.r. spectroscopy.

Stephen E. O'Donnell

Papers

Applications of vanadium-51 and phosphorus-31 nuclear magnetic resonance spectroscopy to the study of iso- and hetero-polyvanadates

Identification of stereoisomers of mixed heteropoly anions. Mixed valence and triplet state electron spin resonance spectra of vanadium(IV)

Electron Exchange between Pairs of Vanadium Atoms in Novel Geometric Isomers of Heteropoly Tungstates

Applications of vanadium-51 and phosphorus-31 nuclear magnetic resonance spectroscopy to the study of iso- and hetero-polyvanadates