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PubMed1999

Purification and molecular characterization of the tungsten-containing formaldehyde ferredoxin oxidoreductase from the hyperthermophilic archaeon Pyrococcus furiosus: the third of a putative five-member tungstoenzyme family.

Pyrococcus furiosus is a hyperthermophilic archaeon which grows optimally near 100 degreesC by fermenting peptides and sugars to produce organic acids, CO2, and H2. Its growth requires tungsten, and two different tungsten-containing enzymes, aldehyde ferredoxin oxidoreductase (AOR) and glyceraldehyde-3-phosphate ferredoxin oxidoreductase (GAPOR), have been previously purified from P. furiosus. These two enzymes are thought to function in the metabolism of peptides and carbohydrates, respectively. A third type of tungsten-containing enzyme, formaldehyde ferredoxin oxidoreductase (FOR), has now been characterized. FOR is a homotetramer with a mass of 280 kDa and contains approximately 1 W atom, 4 Fe atoms, and 1 Ca atom per subunit, together with a pterin cofactor. The low recovery of FOR activity during purification was attributed to loss of sulfide, since the purified enzyme was activated up to fivefold by treatment with sulfide (HS-) under reducing conditions. FOR uses P. furiosus ferredoxin as an electron acceptor (Km = 100 microM) and oxidizes a range of aldehydes. Formaldehyde (Km = 15 mM for the sulfide-activated enzyme) was used in routine assays, but the physiological substrate is thought to be an aliphatic C5 semi- or dialdehyde, e.g., glutaric dialdehyde (Km = 1 mM). Based on its amino-terminal sequence, the gene encoding FOR (for) was identified in the genomic database, together with those encoding AOR and GAPOR. The amino acid sequence of FOR corresponded to a mass of 68.7 kDa and is highly similar to those of the subunits of AOR (61% similarity and 40% identity) and GAPOR (50% similarity and 23% identity). The three genes are not linked on the P. furiosus chromosome. Two additional (and nonlinked) genes (termed wor4 and wor5) that encode putative tungstoenzymes with 57% (WOR4) and 56% (WOR5) sequence similarity to FOR were also identified. Based on sequence motif similarities with FOR, both WOR4 and WOR5 are also proposed to contain a tungstobispterin site and one [4Fe-4S] cluster per subunit.

Aldehyde Oxidoreductases↗
PubMed2002

Characterization of a fourth tungsten-containing enzyme from the hyperthermophilic archaeon Pyrococcus furiosus.

Pyrococcus furiosus grows optimally near 100 degrees C using peptides and carbohydrates as carbon sources, and it reduces elemental sulfur (S(0)), if present, to H(2)S. Tungsten (W), an element rarely used in biology, is required for optimal growth, and three different tungsten-containing enzymes have been previously purified from this organism. They all oxidize aldehydes of various types and are thought to play primary roles in the catabolism of sugars or amino acids. Here, the purification of a fourth tungsten-containing enzyme, termed WOR 4, from cell extracts of P. furiosus grown with S(0) is described. This was achieved by monitoring through multiple chromatography steps the W that is not associated with the three characterized tungstoenzymes. The N-terminal sequence of WOR 4 and the approximate molecular weight of its subunit determined electrophoretically (69,000) correspond to the product of an ORF (PF1961, wor4) present in the complete genome sequence of P. furiosus. WOR 4 is a homodimer and contains approximately one W, three Fe, three or four acid-labile sulfide, and one Ca atom per subunit. The visible and electron paramagnetic resonance spectra of the oxidized and reduced enzyme indicate the presence of an unusual iron-sulfur chromophore. WOR 4 does not oxidize aliphatic or aromatic aldehydes or hydroxy acids, nor does it reduce keto acids. Consistent with prior microarray data, the protein could not be purified from P. furiosus cells grown in the absence of S(0), suggesting that it may have a role in S(0) metabolism.

Aldehyde Oxidoreductases↗
PubMed1975

A comparison of x-ray spectra and outputs from molybdenum and tungsten targets.

The relative merits of tungsten and molybdenum targets for mammography have been the subject of much discussion. Therefore the spectra and outputs (at constant potential) from molybdenum and tungsten targets, interchangeable in the same tube, have been measured with a Ge(Li) detector and ion chamber respectively. All conditions apart from the target material were unaltered. The spectra have been corrected for the distortions produced by the detector. The effects of filtration on spectra and exposure rates have been calculated and are in agreement with measured values. The spectra and outputs from molybdenum and tungsten targets filtered by aluminium and molybdenum have been investigated and the results are discussed with reference to mammography and the radiography of specimens.

Electrons↗
PubMed2005

Embedded weapons-grade tungsten alloy shrapnel rapidly induces metastatic high-grade rhabdomyosarcomas in F344 rats.

Continuing concern regarding the potential health and environmental effects of depleted uranium and lead has resulted in many countries adding tungsten alloy (WA)-based munitions to their battlefield arsenals as replacements for these metals. Because the alloys used in many munitions are relatively recent additions to the list of militarily relevant metals, very little is known about the health effects of these metals after internalization as embedded shrapnel. Previous work in this laboratory developed a rodent model system that mimicked shrapnel loads seen in wounded personnel from the 1991 Persian Gulf War. In the present study, we used that system and male F344 rats, implanted intramuscularly with pellets (1 mm times 2 mm cylinders) of weapons-grade WA, to simulate shrapnel wounds. Rats were implanted with 4 (low dose) or 20 pellets (high dose) of WA. Tantalum (20 pellets) and nickel (20 pellets) served as negative and positive controls, respectively. The high-dose WA-implanted rats (n = 46) developed extremely aggressive tumors surrounding the pellets within 4-5 months after implantation. The low-dose WA-implanted rats (n = 46) and nickel-implanted rats (n = 36) also developed tumors surrounding the pellets but at a slower rate. Rats implanted with tantalum (n = 46), an inert control metal, did not develop tumors. Tumor yield was 100% in both the low- and high-dose WA groups. The tumors, characterized as high-grade pleomorphic rhabdomyosarcomas by histopathology and immunohistochemical examination, rapidly metastasized to the lung and necessitated euthanasia of the animal. Significant hematologic changes, indicative of polycythemia, were also observed in the high-dose WA-implanted rats. These changes were apparent as early as 1 month postimplantation in the high-dose WA rats, well before any overt signs of tumor development. These results point out the need for further studies investigating the health effects of tungsten and tungsten-based alloys.

Alloys↗
PubMed2004

Durability of tungsten carbide burs for the fabrication of titanium crowns using dental CAD/CAM.

The purpose of this study was to evaluate the durability of tungsten carbide burs for the fabrication of titanium crowns using two dental CAD/CAM systems (DECSY, Digital Process, Japan and Cadim, Advance, Japan). A tungsten carbide bur in each system was examined and used without fracture to fabricate 51 titanium crowns. For both systems tiny chips were found on the bur blade at the 11th machining. These chips gradually enlarged as the number of machining times increased. At the first machining no significant difference in the average surface roughness was found on the crown between the two systems (1.6 microm for DECSY and 1.2 microm for Cadim). The cutting grooves became dull and the average surface roughness increased as the number of machining times increased. It is concluded that the tungsten carbide burs for both systems can be used to fabricate up to 51 titanium crowns.

Computer-Aided Design↗
PubMed1991

The extremely thermophilic eubacterium, Thermotoga maritima, contains a novel iron-hydrogenase whose cellular activity is dependent upon tungsten.

Thermotoga maritima is the most thermophilic eubacterium currently known and grows up to 90 degrees C by a fermentative metabolism in which H2, CO2, and organic acids are end products. It was shown that the production of H2 is catalyzed by a single hydrogenase located in the cytoplasm. The addition of tungsten to the growth medium was found to increase both the cellular concentration of the hydrogenase and its in vitro catalytic activity by up to 10-fold, but the purified enzyme did not contain tungsten. It is a homotetramer of Mr 280,000 and contains approximately 20 atoms of Fe and 18 atoms of acid-labile sulfide/monomer. Other transition metals, including nickel (and also selenium), were present in only trace amounts (less than 0.1 atoms/monomer). The hydrogenase was unstable at both 4 and 23 degrees C, even under anaerobic conditions, but no activity was lost in anaerobic buffer containing glycerol and dithiothreitol. Under these conditions the enzyme was also quite thermostable (t50% approximately 1 h at 90 degrees C) but extremely sensitive to irreversible inactivation by O2 (t50% approximately 10 s in air). The optimum pH ranges for H2 evolution and H2 oxidation were 8.6-9.5 and greater than or equal to 10.4, respectively, and the optimum temperature for catalytic activity was above 95 degrees C. In contrast to mesophilic Fe hydrogenases, the T. maritima enzyme had very low H2 evolution activity, did not use T. maritima ferredoxin as an electron donor for H2 evolution, was inhibited by acetylene but not by nitrite, and exhibited EPR signals typical of [2Fe-2S]1+ clusters. Moreover, the oxidized enzyme did not exhibit the rhombic EPR signal that is characteristic of the catalytic iron-sulfur cluster of mesophilic Fe hydrogenases. These data suggest that T. maritima hydrogenase has a different FeS site and/or mechanism for catalyzing H2 production. The potential role of tungsten in regulating the activity of this enzyme is discussed.

Amino Acids↗
PubMed1990

Characterization of a tungsten-iron-sulfur protein exhibiting novel spectroscopic and redox properties from the hyperthermophilic archaebacterium Pyrococcus furiosus.

The archaebacterium, Pyrococcus furiosus, is a strict anaerobe that grows optimally at 100 degrees C by a fermentative-type metabolism in which H2 and CO2 are the only detectable products. Tungsten is known to stimulate the growth of this organism. A red-colored tungsten-containing protein (abbreviated RTP) that is redox-active and extremely thermostable has been purified. RTP is a monomer of Mr = 85,000 and contains approximately 6 iron, 1 tungsten, and 4 acid-labile sulfide atoms/molecule. Titrations using visible spectroscopy were consistent with the oxidation and reduction of the protein each requiring two electrons/molecule, suggesting that these metals and the sulfide are arranged in two redox active centers. P. furiosus ferredoxin served as an electron acceptor for the protein. Dithionite-reduced RTP exhibited a remarkable and complex EPR spectrum at 6 K with g values ranging from 1.3 to 10.0. This was shown to arise from the spin-coupling interaction of two paramagnetic centers. One (center A) has a S = 3/2 spin system (effective g values: gx = 3.33, gy = 4.75, and gz = 1.92, where D = 4.3 cm-1 and lambda = 0.135), whereas the EPR properties of the other (center B) could not be deduced. Nevertheless, theoretical analyses show how the redox properties of both centers may be determined using EPR spectroscopy. Their midpoint potentials (Em) at 20 degrees C and pH 8.0 are -410 mV (center A) and -500 mV (center B) with an effective potential for the spin coupled system (Em, A + B) of -505 mV. The Em values are dependent on temperature (delta Em/delta T = -2 mV/degrees C between 20 and 70 degrees C) and pH with pK alpha values of 8.0 (A) and approximately 8.5 (B). The Em values at 100 degrees C, the growth temperature, were estimated at -590, -650, and -660 mV for centers A, B, and A + B, respectively. These data indicate that RTP catalyzes a dehydrogenase-type reaction of extremely low potential, which involves the transfer of two protons and of two electrons, to and from two adjacent and interacting but nonidentical metal centers.

Archaea↗
PubMed1983

Purification and properties of NADP-dependent formate dehydrogenase from Clostridium thermoaceticum, a tungsten-selenium-iron protein.

NADP-dependent formate dehydrogenase (NADP+) (EC 1.2.1.43) from Clostridium thermoaceticum has been purified to a specific activity of about 1100 mumol min-1 mg-1 when assayed at 55 degrees C and pH 7.5. The enzyme is extremely oxygen-sensitive and 7.6 microM of O2 causes 50% inhibition of initial velocity under assay conditions. Purification was done in an atmosphere at 95% N2 and 5% H2 and by including azide, dithionite, and glycerol as stabilizing agents in all buffer solutions. The enzyme contains, in molar ratios, 2 tungsten, 2 selenium, 36 iron, and about 50 inorganic sulfur. It has a molecular weight of about 340,000 and consist of two each of two different subunits giving the composition alpha 2 beta 2. The molecular weight of the alpha-subunit is 96,000 and that of the beta-subunit is 76,000. The selenium resides in the two alpha-subunits. Tungsten is released from the protein on denaturation and may exist as a tungsten cofactor. The enzyme catalyzes a reduction of CO2 with NADPH at pH 7.5 and 55 degrees C and Keq at these conditions is (2.35 +/- 0.49) x 10(-2) if CO2 is considered the active species and (1.48 +/- 0.31) x 10(-3) if HCO3- is the active species.

Aerobiosis↗
PubMedDate not supplied

Podocyte architecture in puromycin aminonucleoside-treated rats administered tungsten or allopurinol.

The role of xanthine oxidase as a source of reactive oxygen species in puromycin aminonucleoside nephrosis was examined. The effects of allopurinol (a xanthine oxidase inhibitor as well as a reactive oxygen species scavenging enzyme) and tungsten (a specific xanthine oxidase inhibitor) on glomerular epithelial cell ultrastructure, renal xanthine oxidase and xanthine dehydrogenase activity, and urinary protein excretion were examined in puromycin aminonucleoside-treated rats. Co-administration of allopurinol to such rats reduced proteinuria by approximately 70% over the 10 days studied, and reduced the degree of glomerular epithelial cell foot process effacement at both 5 and 10 days, compared to rats that received puromycin aminonucleoside alone. Unexpectedly, co-administration of allopurinol to puromycin aminonucleoside-treated rats did not reduce xanthine oxidase activity; however, the combined activity of xanthine oxidase and xanthine dehydrogenase in such animals was reduced on day 5. Co-administration of tungsten to puromycin aminonucleoside-treated rats did not reduce proteinuria or alter the number of filtration slits. Rats co-administered tungsten and puromycin aminonucleoside had significantly reduced renal xanthine oxidase and combined xanthine oxidase and xanthine dehydrogenase activities on days 5 and 10, compared to rats treated with puromycin aminonucleoside alone. These results provide evidence that the protection provided by allopurinol in puromycin aminonucleoside-treated rats is due to the antioxidant properties of allopurinol, rather than to its activities as a xanthine oxidase inhibitor.

Allopurinol↗
PubMed1997

Long-term angiographic and histopathologic findings in experimental aneurysms of the carotid bifurcation embolized with platinum and tungsten coils.

PURPOSE: To evaluate the long-term outcome of endovascular occlusion of arterial aneurysms effected with metal coils. METHODS: Microsurgical methods were used to produce carotid bifurcation aneurysms in 20 rabbits and the radiologic and histologic changes were examined. Eight of these aneurysms were occluded with electrically detachable platinum coils (Guglielmi detachable coils [GDCs] and nine were treated with mechanically detachable tungsten coils (mechanical detachable system [MDS]). Three aneurysms remained untreated and served as controls. One animal died of embolic complications 12 hours after endovascular treatment. After observation periods of 3 to 6 months, the remaining animals were examined by intraarterial digital subtraction angiography and subsequent fixation and light and electron microscopy. RESULTS: Large open spaces without signs of thrombosis were found between the loops of the coil baskets in 12 aneurysms (six treated with GDCs and six treated with MDS) regardless of the observation period. In very densely packed aneurysms (four cases with complete occlusion as determined by angiographic criteria), the coil surfaces were for the most part covered by thin cell layers; however, complete endothelialization was never seen. In aneurysms with an initial partial occlusion of 70% to 90%, coil compaction and/or recanalization was a consistent finding. A comparison of the radiologic findings with the histologic aspect revealed that the degree of occlusion was often overrated on the radiographs (in eight of 17 cases). In general, the fibrous tissue reaction appeared to be slightly more pronounced in aneurysms occluded with tungsten coils. CONCLUSIONS: Platinum and tungsten coils were not always effective in causing endoluminal thrombosis leading to long-term occlusion by organized thrombus.

Angiography, Digital Subtraction↗
PubMed1996

Endovascular embolization of intracranial aneurysms with self-made tungsten coils in a dog model.

OBJECTIVE: To evaluate the embolization effects and the long-term histologic changes, including the ultrastructure of the neoendothelium, in experimental canine aneurysms obliterated with self-made tungsten microcoil. METHODS: Twenty-eight experimental aneurysms were microsurgically created in 14 mongrel dogs by using end-to-side jungular carotid anastomosis. The aneurysms were obliterated with either self-made tungsten microcoil alone or the microcoil plus micro direct current electrocoagulation, with preservation of the parent vessel. The animals were kept in observation. 48 hours, 4 weeks, and 4 months after the embolization of the aneurysms, repeated carotid arteriography was performed to assess the potential recanalization of the aneurysms. The animals were then respectively killed and submitted for autopsy. The carotid artery and the embolized aneurysms were resected and studied with light and electron microscopy. RESULTS: A total of 14 obliterated aneurysms were completely excluded from the parent circulation by an endothelialized layer of connective tissue. The fundus of the aneurysm was completely obliterated by heavy reactive fibrous tissue surrounding the microcoils with very minimal inflammatory reaction. CONCLUSIONS: Tungsten microcoils are a relatively ideal embolic material at present for endovascular treatment of aneurysms. Microcoils plus micro direct current electrocoagulation yield more reliable, accurate, and safer embolic effects than microcoils alone for the obliteration of aneurysms.

Animals↗
PubMed1998

The formylmethanofuran dehydrogenase isoenzymes in Methanobacterium wolfei and Methanobacterium thermoautotrophicum: induction of the molybdenum isoenzyme by molybdate and constitutive synthesis of the tungsten isoenzyme.

Formylmethanofuran dehydrogenase catalyzes the first step in methane formation from CO2 in methanogenic archaea. Methanobacterium wolfei and Methanobacterium thermoautotrophicum have been shown to contain two isoenzymes, a tungsten-containing isoenzyme (Fwd) and a molybdenum-containing isoenzyme (Fmd). We report here that in both thermophilic organisms the encoding genes are organized in a highly conserved fwdHFGDACB tungsten operon and in an fmdECB molybdenum operon. In both organisms, the tungsten isoenzyme was found to be constitutively transcribed, whereas the transcription of the molybdenum operon was induced by molybdate. Induction by molybdate was not significantly affected by tungstate.

Aldehyde Oxidoreductases↗
PubMed2000

Tungsten atomizer--theory of atomization mechanism of some volatile analytes.

Several types of chemical reactions may participate in the evolution of free atoms in a tungsten furnace. Reactions may take place either in the homogeneous or heterogeneous phase. The assumed reactions may be classified into four types according to the phases in which they take place. Reactions occurring in the gaseous phase, i.e. in the inner volume of the furnace, are kinetically more significant. However, for atomization of easily volatile analytes heterogeneous reaction between gaseous compounds and between condensed salts of analytes and the solid surface of the furnace become significant. With regards to the reaction mechanisms during drying, pyrolysis and atomization of nitrates of volatile analytes, three basic types of chemical reactions may be assumed. Free atoms of analytes arise by evaporation of the elementary form of analytes at atomization temperature, where the particular analyte in its elementary form is produced by direct reduction of analyte nitrate by tungsten or by hydrogen at higher temperatures. Precursory reactions of atom formation are reduction reactions which occur between analyte nitrates and tungsten, between analyte nitrates and hydrogen, as well as reactions of thermal dissociation of relevant nitrates. The importance of particular types of precursory reactions for formation of metallic analytes or their oxides is documented by dependence of Gibbs energy values of particular reactions on the temperature.

Journal Article↗
PubMed2000

Formation of ammonia in the reactions of a tungsten dinitrogen with ruthenium dihydrogen complexes under mild reaction conditions.

Treatment of cis-[W(N2)2(PMe2Ph)4] (5) with an equilibrium mixture of trans-[RuCl(eta 2-H2)(dppp)2]X (3) with pKa = 4.4 and [RuCl(dppp)2]X (4) [X = PF6, BF4, or OTf; dppp = 1,3-bis(diphenylphosphino)propane] containing 10 equiv of the Ru atom based on tungsten in benzene-dichloroethane at 55 degrees C for 24 h under 1 atm of H2 gave NH3 in 45-55% total yields based on tungsten, together with the formation of trans-[RuHCl(dppp)2] (6). Free NH3 in 9-16% yields was observed in the reaction mixture, and further NH3 in 36-45% yields was released after base distillation. Detailed studies on the reaction of 5 with numerous Ru(eta 2-H2) complexes showed that the yield of NH3 produced critically depended upon the pKa value of the employed Ru(eta 2-H2) complexes. When 5 was treated with 10 equiv of trans-[RuCl(eta 2-H2)(dppe)2]X (8) with pKa = 6.0 [X = PF6, BF4, or OTf; dppe = 1,2-bis(diphenylphosphino)ethane] under 1 atm of H2, NH3 was formed in higher yields (up to 79% total yield) compared with the reaction with an equilibrium mixture of 3 and 4. If the pKa value of a Ru(eta 2-H2) complex was increased up to about 10, the yield of NH3 was remarkably decreased. In these reactions, heterolytic cleavage of H2 seems to occur at the Ru center via nucleophilic attack of the coordinated N2 on the coordinated H2 where a proton (H+) is used for the protonation of the coordinated N2 and a hydride (H-) remains at the Ru atom. Treatment of 5, trans-[W(N2)2(PMePh2)4] (14), or trans-[M(N2)2(dppe)2] [M = Mo (1), W (2)] with Ru(eta 2-H2) complexes at room temperature led to isolation of intermediate hydrazido(2-) complexes such as trans-[W(OTf)(NNH2)(PMe2Ph)4]OTf (19), trans-[W(OTf)(NNH2)(PMePh2)4]OTf (20), and trans-[WX(NNH2)(dppe)2]+ [X = OTf (15), F (16)]. The molecular structure of 19 was determined by X-ray analysis. Further ruthenium-assisted protonation of hydrazido(2-) intermediates such as 19 with H2 at 55 degrees C was considered to result in the formation of NH3, concurrent with the generation of W(VI) species. All of the electrons required for the reduction of N2 are provided by the zerovalent tungsten.

Journal Article↗
PubMed2003

Stabilization of the P(CF3)(2)(-) and P(C6F5)(2)(-) ions by coordination to pentacarbonyl tungsten: structures of [18-crown-6-K]P(CF3)2, [18-crown-6-K][W[P(CF3)2](CO)5], and [18-crown-6-K][[W(CO)5]2[mu-P(C6F5)2]].THF.

The stabilization of the P(CF(3))(2)(-) ion by intermediary coordination to the very weak Lewis acid acetone gives access to single crystals of [18-crown-6-K]P(CF(3))(2). The X-ray single crystal analysis exhibits nearly isolated P(CF(3))(2)(-) ions with an unusually short P-C distance of 184(1) pm, which can be explained by negative hyperconjugation and is also found by quantum chemical hybrid DFT calculation. Coordination of the P(CF(3))(2)(-) ion to pentacarbonyl tungsten has only a minor effect on electronic and geometric properties of the P(CF(3))(2) moiety, while a strong increase in thermal stability of the dissolved species is achieved. The hitherto unknown P(C(6)F(5))(2)(-) ion is stabilized by coordination to pentacarbonyl tungsten and isolated as a stable 18-crown-6 potassium salt, [18-crown-6-K][W[P(C(6)F(5))(2)](CO)(5)], which is fully characterized. The tungstate, [W[P(C(6)F(5))(2)](CO)(5)](-), decomposes slowly in solution, while coordination of the phosphorus atom to a second pentacarbonyl tungsten moiety results in an enhanced thermal stability in solution. The single-crystal X-ray analysis of [18-crown-6-K][[W(CO)(5)](2)[mu-P(C(6)F(5))(2)]].THF exhibits a very tight arrangement of the two C(6)F(5) and two W(CO)(5) groups around the central phosphorus atom. NMR spectroscopic investigations of the [[W(CO)(5)](2)[mu-P(C(6)F(5))(2)]](-) ion exhibit a hindered rotation of both the C(6)F(5) and W(CO)(5) groups in solution.

Journal Article↗
PubMed2003

Bis(trifluoromethyl)phosphane and bis(pentafluorophenyl)phosphane and their pentacarbonyl tungsten complexes: improved synthesis and an experimental and density functional study.

The use of Bu(3)SnH and Me(3)SnH in the synthesis of HP(CF(3))(2) and HP(C(6)F(5))(2) from the corresponding bromides leads to a high-yield synthesis, which additionally provides these compounds in large quantities. The pentacarbonyl tungsten complexes [W(CO)(5)PH(CF(3))(2)] and [W(CO)(5)PH(C(6)F(5))(2)] were synthesized reacting the corresponding phosphanes with [W(CO)(5)THF] and characterized by X-ray and elemental analysis as well as multinuclear NMR and mass spectroscopy. The vibrational analyses of HP(CF(3))(2) and HP(C(6)F(5))(2) and their tungsten pentacarbonyl complexes were achieved in combination with hybrid DFT calculations. The optimized structures of [W(CO)(5)PH(CF(3))(2)] and [W(CO)(5)PH(C(6)F(5))(2)] at the B3PW91 level of theory using a LanL2DZ basis and ECP at the tungsten atom and a 6-311G(3d,p) and 6-311G(d,p) basis set for the nonmetal atoms, respectively, yield an impressively good agreement between experimental and theoretical geometric parameters. An increased pi-acidity of HP(CF(3))(2) in comparison with HP(C(6)F(5))(2) and HPPh(2) is discussed in the context of vibrational analysis, X-ray structural investigations, and theoretical calculations.

Journal Article↗
PubMed2003

Tungsten and molybdenum nitrosyl cations in fluorosulfonic acid.

The tungsten and molybdenum hexacarbonyls, M(CO)(6) (M = W, Mo), dissolve in fluorosulfonic acid, HSO(3)F, to generate the tungsten and molybdenum carbonyl cations, [M(CO)(4)](2+)(solv), which are transformed, by exposure to an NO atmosphere, into the tungsten and molybdenum carbonyldinitrosyl cations, [M(CO)(NO)(2)](2+)(solv), respectively. These complexes have been characterized by NMR ((183)W, (13)C, and (15)N), IR, and Raman spectroscopy, and they are the first well-characterized metal nitrosyl cations in strong acids or superacids although the spectroscopic techniques do not address the number or coordination mode of the solvent molecules. Their formation suggests that strong acids and superacids can hopefully be used to generate a number of metal nitrosyl cations as they have been successfully used for preparing a series of metal carbonyl cations.

Journal Article↗
PubMed2006

Mixed guanidinato/alkylimido/azido tungsten(VI) complexes: synthesis and structural characterization.

A series of structurally characterized new examples of pentacoordinated heteroleptic tungsten(VI)-guanidinates complexes are described. Starting out from [WCl(2)(Nt-Bu)(2)py(2)] (1) (py = pyridine) and the guanidinato transfer reagents (TMEDA)Li[(Ni-Pr)(2)CNi-Pr(2)] (2a) (TMEDA = N,N,N',N'-tetramethylethylendiamine) and [Li(NC(NMe(2))(2))](x) (2b), the title compounds [WCl(Nt-Bu)(2)[(Ni-Pr)(2)CNi-Pr(2)]] (3) and [W(Nt-Bu)(2)Cl{NC(NMe(2))(2)]](2) (6) were selectively formed by the elimination of one mole equivalent of lithium chloride. The isopropyl-substituted guanidinato ligand [(Ni-Pr)(2)CNi-Pr(2)} of monomeric 3 is N(1),N(3)-bonded to the tungsten center. The introduction of the sterically less-demanding tetramethyl guanidinato ligand [NC(NMe(2))(2)] expectedly leads to dimeric 6 exhibiting a planar W(2)N(2) ring with the guanidinato group bridging the two tungsten centers via the deprotonated imino N-atom. The remaining chloro ligand of 3 is labile and can be substituted by sterically less-crowded groups such as dimethylamido or azido that yield the presumably monomeric compounds 4 and 5, respectively. A similar treatment of 6 with sodium azide yields the dimeric azido derivative 7. Reacting [WCl(2)(Nt-Bu)(2)py(2)] directly with an excess of sodium azide leads to the dimeric bis-azide species [[W(Nt-Bu)(2)(N(3))(mu(2)-N(3))py](2)]. The new compounds were fully characterized by single-crystal X-ray diffractometry (except 2, 4, and 5), NMR, IR, and mass-spectroscopy as well as elemental analysis. Compound 5, [W(N(3))(Nt-Bu)(2)[(Ni-Pr)(2)CNi-Pr(2)]], can be sublimed at 80 degrees C, 1 Pa.

Journal Article↗