Volume 81, Issue 7

Knowledge of the chemistry and biochemistry of vanadium has increased enormously since the early 1980s, particularly that of vanadium(III), (IV), and (V). This led to remarkable developments in the understanding and recognition of the properties of vanadium compounds. A consequence of these new insights has been numerous applications of vanadium complexes ranging from therapeutics to catalysis and from new materials to green chemistry. Many of the more recent advances were presented at the 6th International Vanadium Symposium held 17‚Äì19 July 2008 in Lisbon, Portugal. The conference included 56 oral communications and 52 poster presentations. The following 12 papers are a selection from the oral communications presented. This conference attracted over 100 participants from 27 countries and 4 continents. The inorganic chemistry of vanadium, application of vanadium chemistry in catalysis and organic synthesis, and biological aspects of vanadium chemistry were discussed in invited lectures as well as in poster communications. The Vanadium Award, a prize first introduced at the 4th International Vanadium Symposium held in 2004 in Szeged, Hungary to recognize an outstanding contributor to the advancement of vanadium science, was awarded in 2008 to Prof. Toshikazu Hirao of the Osaka University, Japan, and his contribution is the first paper presented herein. The additional contributions that embody this issue are mostly papers covering aspects related to catalytic applications of vanadium compounds, including their use as functional models of vanadiumdependent haloperoxidases. Electron transfer in oxo and non-oxo vanadium complexes, the use of NMR to characterize the complexes, and synthesis of new polyoxovanadates are the subjects of other papers included. We look forward to the 7th International Vanadium Symposium to be held in Japan in 2010, where additional studies on the role of vanadium in life, the vanadium nutritional essentiality, vanadium toxicity, and vanadium therapy, as well as new contributions to the use of vanadium compounds in catalysis, including "green chemical" industrial applications, will certainly be presented. João Costa Pessoa Conference Chair

Structural tuning and self-association of (arylimido)vanadium(V) compounds are demonstrated to be controlled through π-conjugation by the para -substituents of the aryl moieties.

With bidentate alkoxy alkoxide and alkoxy alcohol ligands, respectively, a series of oxovanadium complexes in the oxidation state +4 is synthesized starting from oxovanadium(V) compounds. The reaction of two or more equivalents of 2-methoxyethanol with VOCl 3 in n -hexane yields a mixture of the monomeric oxovanadium(IV) complex cis -[VOCl 2 (HOCH 2 CH 2 OMe-κ 2 O )(HOCH 2 CH 2 OMe-κ O 1 )] and the alkoxide-bridged oxovanadium(IV) dimer syn -[VOCl(µ-OCH 2 CH 2 OMe-κ 2 O )] 2 , which are separated by fractionated crystallization. The same reaction with 2-ethoxy- and 2- iso -propoxyethanol gives only the alkoxide-bridged oxovanadium(IV) dimers anti -[VOCl(µ-OCH 2 CH 2 OR-κ 2 O )] 2 (R = Et, i Pr). All alkoxide bridged oxovanadium(IV) dimers are furthermore obtained as decomposition products of the chloride-bridged oxovanadium(V) complexes [VO(µ-Cl)Cl(OCH 2 CH 2 OR-κ 2 O )] 2 (R = Me, Et, i Pr) by Cl 2 elimination and react inversely with Cl 2 to the vanadium(V) compounds.

The synthesis and solution and solid-state structural characterization of a family of amine bis(phenolate) [ONNO]-vanadium complexes is reviewed. These compounds have oxidation states ranging from vanadium(II) to vanadium(V), and were evaluated as olefin polymerization catalysts. In association with EtAlCl 2 cocatalyst, we studied the homopolymerization of ethylene, propene, and 1-hexene, as well as the copolymerization of ethylene with α-olefins (1-hexene, 1-octene) and cycloolefins (norbornene, cyclopentene). Some of these catalysts were shown to produce copolymers with a good activity and comonomer content.

The V-scorpionate vanadium complexes [VCl 3 {HC(pz) 3 }] (pz = pyrazolyl) and [VCl 3 {SO 3 C(pz) 3 }] catalyze cyclohexane oxidation with dioxygen, to cyclohexanol (the main product) and cyclohexanone, under solvent-free conditions. [VCl 3 {HC(pz) 3 }] provides the best activity (13 % conversion into the ketone and alcohol, with high selectivity, at the O 2 pressure of 15 atm, at 140 ºC, 18 h reaction time). The reaction is further promoted (to 15 % conversion) by pyrazinecarboxylic acid (PCA). The use of C- or O-radical traps supports the involvement of a free-radical reaction mechanism. Several reaction parameters have been varied in a systematic study, directed toward optimization of the process.

In the active-site cavity of vanadium haloperoxidases vanadate as the prosthetic group is solely fixed by one covalent bond to a histidine residue and embedded in a supramolecular environment of extensive hydrogen bonds. Structural aspects of relevant vanadium complexes with supramolecular interactions, including assemblies with chiral hosts, are presented. The importance of hydrogen-bonding relays is presented together with relevant examples. The reactivity of related functional mimics containing vanadium and molybdenum toward the oxidation of thioethers is described. Computational modeling based on density functional theory (DFT) is used for the investigation of model systems. The resulting implications for structure and function of vanadium haloperoxidases, including their substrate and cofactor specificity, are discussed.

Electron-transfer reactions of the eight-coordinate vanadium complex, bis-( N -hydroxyiminodiacetate)vanadium(IV) [V(HIDA) 2 ] 2– , a synthetic analog of amavadin with ascorbic acid and hexachloroiridate(IV), have been studied. The self-exchange rate constant for this analog has been calculated from oxidation and reduction cross-reactions using Marcus theory and directly measured using 51 V NMR paramagnetic line-broadening techniques. The average self-exchange rate constant for the bis-HIDA vanadium(IV/V) couple equals 1.5 × 10 5 M –1 s –1 . The observed rate enhancements are proposed to be due to the small structural differences between the oxidized and reduced forms of the HIDA complex and inner-sphere reorganizational energies. The electron-transfer reaction of this synthetic analog is experimentally indistinguishable from amavadin itself, although significant differences exist in the reduction potential of these compounds. This suggests that ligand modification effects the thermodynamic driving force and not the self-exchange process.

Peroxidase (PO) activity of vanadate(V)-dependent bromoperoxidase (BPO) I ( Ascophyllum nodosum ) [V Br PO( An I)] was retained with a half-life time of ~60 days, if stored in H 2 O 2 -incubated, morpholin-4-ethane sulfonic acid (MES)-buffered aqueous alcoholic solutions. These conditions were applied for converting bromide and, e.g., methyl pyrrole-2-carboxylate into bromopyrroles with an almost quantitative peroxide yield. δ,ε-unsaturated alcohols furnished β-bromohydrins and products of bromocyclization, i.e., tetrahydrofurans and tetrahydropyrans (70–84 % mass balance), if treated with H 2 O 2 , KBr, and V Br PO( An I) in phosphate-buffered, CH 3 CN-diluted media.

The application of vanadium(V) catalysts in selective oxidation with peroxides offers an efficient procedure that is compatible with different functional groups and leads to good yields and selectivities. However, the search for more efficient and sustainable procedures that employ H 2 O 2 as oxidant remains an important topic. In the last few years, striking results have been obtained by applying microwave (MW) activation in metal-catalyzed reactions carried out in ionic liquids (ILs). In the present study, results achieved with vanadium-based catalysts in oxidations of various substrates with H 2 O 2 are presented; in particular, epoxidation of alkenes and sulfoxidation of thioethers have been investigated. Notably, in the latter oxidation, a significant improvement in the rate of reaction and an increase in selectivity have been observed in the case of hydrophobic ILs in combination with MW activation.

Salen complexes are a versatile and standard system in oxidation catalysis. Their reduced derivatives, called salan, share their versatility but are still widely unexplored. We report the synthesis of a group of new vanadium-salen and -salan complexes, their characterization and application in the oxidation of simple organic molecules with H 2 O 2 . The ligands are derived from pyridoxal and chiral diamines (1,2-diaminocyclohexane and 1,2-diphenylethylenediamine) and were easily obtained in high yields. The V IV complexes were prepared and characterized in the solid state (Fourier transform infrared, FTIR, and magnetic properties) and in solution by spectroscopic techniques: UV–vis, circular dichroism (CD), electron paramagnetic resonance (EPR), and 51 V NMR, which provide information on the coordination geometry. Single crystals suitable for X-ray diffraction studies were obtained from solutions containing the V IV -pyr( S , S -chan) complex: [V V O{pyr( S , S -chen)}] 2 (μ-O) 2 ·2(CH 3 ) 2 NCHO, where the ligand is the “half” Schiff base formed by pyridoxal and 1 S ,2 S -diaminocyclohexane. The dinuclear species shows a OV V (μ-O) 2 V V O unit with tridentate ligands and two μ-oxo bridges. The V IV complexes of the salan-type ligands oxidize in organic solvents to a V V species, and the process was studied by spectroscopic techniques. The complexes were tested as catalysts in the oxidation of styrene, cyclohexene, and cumene with H 2 O 2 as oxidant. Overall, the V-salan complexes show higher activity than the parent V-salen complexes and are an alternative ligand system for oxidation catalysis.

A review discussing general structural features of oxygen-bridged dinuclear vanadium(IV and/or V) complexes is presented, covering those that have been characterized by single-crystal X-ray diffraction. Many of these compounds contain functional Schiff bases or amines as ligands, this work illustrating the high propensity of the V center to increase its coordination number via dimerization of two tetra- or penta-coordinate monomers, if the steric and electronic control exerted by the ligands allows it. We also report the synthesis and characterization by single-crystal X-ray diffraction of two new dinuclear complexes: [{V V O[sal( R , R -chen)]} 2 (μ 2 -O) 2 ] and [{V V O[mvan( S , S -chen)]} 2 (μ 2 -O) 2 ]. The complexes contain tridentate ligands with O-phenolate, N-imine, and N-amine coordination to the V V center. The molecular structures of these compounds demonstrate that they form dinuclear species in the solid state with a {OV V (μ 2 -O) 2 V V O} core.

Reaction of NH 4 VO 3 with (phenolate/hydroquinonate) iminodiacetate tripod ligands in aqueous solution in the pH range 7–8 results in the isolation of three new dioxido vanadium(V) complexes, one dinuclear (NH 4 ) 4 {(V V O) 2 [μ-bicah(6-)-N,O,O,O]}·8H 2 O, (H 6 bicah, 2,5-bis[ N , N '-bis(carboxymethyl)aminomethyl]-hydroquinone), and two mononuclear (NH 4 ) 2 {(V V O) 2 [Hcah(3-)-N,O,O,O]·2H 2 O, (H 4 cah, 2-[ N , N '-bis(carboxymethyl)aminomethyl]-hydroquinone) and (NH 4 ) 2 {(V V O)[Hcacp(3-)-N,O,O,O]}·4H 2 O, (H 4 cacp, 2-[ N , N '-bis(carboxymethyl)aminomethyl]-4-carboxyphenol). The 51 V, 1 H, and 13 C NMR spectra in D 2 O show the coordination environment of vanadium in the above complexes to be octahedral, with four out of six positions to be occupied by the two carboxylate oxygen, one hydroquinonate oxygen, and one amine nitrogen atoms of the tripod ligand. The two acetate arms are in cis position to each other. Variable-temperature 1 H and 13 C NMR measurements show that the complexes are kinetically labile. An intramolecular exchange that proceeds through the opening of the phenolate/hydroquinonate chelate ring has been revealed by 2D{ 1 H} NMR EXSY (exchange spectroscopy).

A new route to an alkoxohexavanadate species leads to the isolation of a bowl-type dodecavanadate without a guest molecule in the cavity. The 1D tetravanadate, [V 4 O 11 ] 2- , is a good precursor for the alkoxohexavanadate, [V 6 O 13 (OCH 3 ) 6 ] 2- , which is then utilized for the [V6 + V6] coupling reaction to form the dodecavanadate, [V 12 O 42 ] 4- , with a capped dichloromethane molecule at the cavity entrance. We also obtained structural information on a newly discovered octadecavanadate,  [V 18 O 46 (NO 3 )] 5- ,in both the solid and solution states through extended X-ray absorption fine structure (EXAFS) studies. The existence of a chiral inorganic species in acetonitrile with double-stranded V8 chains was confirmed through EXAFS oscillations.

The IUPAC/IUPAP Joint Working Party (JWP) on the priority of claims to the discovery of new elements has reviewed the relevant literature pertaining to several claims. In accordance with the criteria for the discovery of elements previously established by the 1992 IUPAC/IUPAP Transfermium Working Group (TWG), and reiterated by the 1999 and 2003 IUPAC/IUPAP JWPs, it was determined that the 1996 and 2002 claims by the Hofmann et al. research collaborations for the discovery of the element with atomic number 112 at Gesellschaft für Schwerionenforschung (GSI) share in the fulfillment of those criteria. A synopsis of Z = 112 experiments and related efforts is presented. A subsequent report will address identification of higher-Z elements including those of odd atomic number.