Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Tungsten Compounds”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,009 records · Page 56Linked to original sources
PubMed2000

Purity testing of organometallic catalysts by packed capillary supercritical fluid chromatography.

A packed capillary column supercritical fluid chromatography system with flame ionization detection has been used for purity testing of candidates for homogeneous catalysis such as methyl tricarbonyl pentamethylcyclopentadienyl tungsten [Cp*W(CO)3Me], methyl tricarbonyl cyclopentadienyl tungsten [CpW(CO)3Me], tetramethyl pentamethylcyclopentadienyl iridium (Cp*IrMe4), trimethyl (1,4,7-trimethyl-1,4,7-triazocyclononane) rhodium (CnRhMe3), trimethylphosphine hydride dicarbonyl cyclopentadienyl molybdenum [eta5-CpMoH(CO)2PMe3] and triphenylphosphine hydride dicarbonyl cyclopentadienyl molybdenum [eta5-CpMoH(CO)2PPh3]. A mass limit of detection of 240 pg was found for eta5-CpMoH(CO)2PMe3 when using a 60-nl injection volume and pure CO2 as mobile phase on a 5 microm Kromasil C18 column. The stability of the catalysts in solution has been examined. After 24 h more than 70% of eta5-CpMoH(CO)2PMe3 and 50% of eta5-CpMoH(CO)2PPh3 had decomposed. Due to the instability of the compounds the purity testing had to take place rapidly after sample dissolution.

Catalysis↗
PubMed1996

Synthesis of Tungsten Carbonyl and Nitrosyl Complexes of Monodentate and Chelating Aryl-N-sulfonylphosphoramides, the First Members of a New Class of Electron-Withdrawing Phosphine Ligands. Comparative IR and (13)C and (31)P NMR Study of Related Phosphorus Complexes.

Reaction of N,N'-bis(tolylsulfonyl)-1,2-diaminoethane with PhPCl(2) gives in 62% yield the phosphonous diamide 2-phenyl-1,3-bis(p-tolylsulfonyl)-1,3,2-diazaphospholidine (4, "TosL") and with Ph(2)PCl in 43% yield the diphosphinous amide N,N'-bis(diphenylphosphino)-N,N'-bis(p-tolylsulfonyl)-1,2-ethanediamine (5, "diTosL"). Reaction of 4 with (THF)W(CO)(5) gives (TosL)W(CO)(5) (6) in 77% yield, and reaction of 5 with trans-BrW(CO)(4)NO gives cis, cis, trans-(diTosL)W(CO)(2)(NO)Br (8) in 86% yield. The IR, (13)C NMR, and (31)P NMR spectra of 4, 5, 6, and 8 are compared to those of a variety of compounds including LW(CO)(5) (L = PMe(3), PPh(3), PPh(NEt(2))(2), P(OMe)(3), P(CF(3))(3)), L(2)W(CO)(2)(NO)Br (L(2) = Ar(2)PCH(2)CH(2)PAr(2) (Ar = Ph (diphos), C(6)F(5) (diphos-F(20))), (CH(3)CN)(2)), and the free ligands as appropriate. The IR data are interpreted to suggest a relative ordering of ligand acceptor ability of P(CF(3))(3) > 4 approximately P(OMe)(3) > PPh(3) approximately PPh(NEt(2))(2) and a relative ordering of ligand donor ability of PPh(NEt(2))(2) >/= P(OMe)(3) > PPh(3) > 4 > P(CF(3))(3). The chelating ligand diTosL is about as electron-withdrawing as diphos-F(20), on the basis of the IR data. The (31)P NMR data qualitatively support the conclusion that TosL and diTosL are highly electron-withdrawing ligands, on the basis of (1)J(PW). The (13)C data do not permit any such generalizations, although the spectra of the diphosphine ligands and adducts are of interest due to the observation of "virtual coupling" that surprisingly can be simulated only as ABX rather than AA'X spin-systems.

Journal Article↗
PubMed1997

Microiontophoretic application of serotonin (5HT)1B/1D agonists inhibits trigeminal cell firing in the cat.

Migraine is a common and debilitating condition. Its treatment has received considerable attention in recent times with the introduction into clinical use of the 5HT1B/1D agonist sumatriptan. It is known from human studies that the intracranial blood vessels and dura mater are important pain-producing structures since mechanical or electrical stimulation of these vessels, such as the superior sagittal sinus, causes pain. We have developed a model of craniovascular pain by stimulating the superior sagittal sinus and monitoring trigeminal neuronal activity using electrophysiological techniques. Cats were anaesthetized with alpha-chloralose (60 mg/kg, intraperitoneally), paralysed (gallamine 6 mg/kg, intravenously) and ventilated. The superior sagittal sinus was accessed and isolated for electrical stimulation by a mid-line circular craniotomy. The region of the dorsal surface of C2 spinal cord was exposed by a laminectomy and an electrode placed for recording evoked activity from sinus stimulation and spontaneous activity of the same cells. The electrode was a custom-made seven barrel glass microelectrode with the central barrel containing a tungsten recording wire. Signals were amplified and monitored on-line using a custom-written sampling program. Cells were recorded that were activated by electrical stimulation of the sinus and were also spontaneously activated. Cells fired with latencies consistent with A delta and C fibres, generally firing three or four times per stimulus (0.3 Hz, 250 microseconds duration, 100 V) delivered to the sinus. Both evoked and spontaneous firing could be inhibited by iontophoresis of ergometrine (9/10 cells), sumatriptan (2/3 cells) and zolmitriptan (9/15 cells) but not by saline (3/10 cells). These data are the first demonstration of inhibition of second order trigeminal neurons by direct local application of 5HT1B/1D agonists. Although intravenous administration of these compounds has demonstrated inhibition of sinus evoked firing in previous studies, it is not possible using the intravenous route to be clear at which anatomical site inhibition is taking place, whereas microiontophoresis offers a clear locus of action. These data demonstrate that the second order trigeminal nucleus synapse in the brainstem and upper cervical cord is the most likely site of action for brain penetrant anti-migraine drugs of the 5HT1B/1D class.

Afferent Pathways↗
PubMed1992

Alternative staining methods for Lowicryl sections.

A number of stains and stain combinations have been identified that, when used with the hydrophilic resin Lowicryl K11M, produce marked improvements over aqueous uranyl and lead salts (UA-Pb) in terms of low granularity, specificity, and range of components contrasted. Three test specimens, tobacco mosaic virus (TMV), starfish sperm, and cultured mouse fibroblasts, were used to evaluate stain characteristics. UA-Pb showed a preference for nuclei acids, which were stained specifically by osmium ammine-B at pH 1.5. A number of stain combinations in which UA was followed or preceded by salts containing barium, manganese, tungsten, molybdenum, and vanadium provided excellent staining of protein-containing components, each stain combination being unique in terms of the degree to which specific components were discriminated. These stains were particularly effective for visualizing internal components of the nucleus where a number of fibrillar and particulate structures not seen with UA-Pb were well contrasted.

Acrylic Resins↗
PubMed2004

Synthons for coordinatively unsaturated complexes of tungsten, and their use for the synthesis of high oxidation-state silylene complexes.

Reduction of Cp*WCl4 afforded the metalated complex (eta6-C5Me4CH2)(dmpe)W(H)Cl (1) (Cp* = C5Me5, dmpe = 1,2-bis(dimethylphosphino)ethane). Reactions with CO and H(2) suggested that 1 is in equilibrium with the 16-electron species [Cp(dmpe)WCl], and 1 was also shown to react with silanes R2SiH2 (R2 = Ph2 and PhMe) to give the tungsten(IV) silyl complexes Cp*(dmpe)(H)(Cl)W(SiHR2) (6a, R2 = Ph2; 6b, R2 = PhMe). Abstraction of the chloride ligand in 1 with LiB(C6F5)4 gave a reactive species that features a doubly metalated Cp ligand, [(eta7-C5Me3(CH2)2)(dmpe)W(H)2][B(C6F5)4] (4). In its reaction with dinitrogen, 4 behaves as a synthon for the 14-electron fragment [Cp*(dmpe)W]+, to give the dinuclear dinitrogen complex ([Cp*(dmpe)W]2(micro-N2)) [B(C6F5)4]2 (5). Hydrosilanes R2SiH2 (R2 = Ph2, PhMe, Me2, Dipp(H); Dipp = 2,6-diisopropylphenyl) were shown to react with 4 in double Si-H bond activation reactions to give the silylene complexes [Cp*(dmpe)H2W = SiR2][B(C6F5)4] (8a-d). Compounds 8a,b (R2 = Ph2 and PhMe, respectively) were also synthesized by abstraction of the chloride ligands from silyl complexes 6a,b. Dimethylsilylene complex 8c was found to react with chloroalkanes RCl (R = Me, Et) to liberate trialkylchlorosilanes RMe2SiCl. This reaction is discussed in the context of its relevance to the mechanism of the direct synthesis for the industrial production of alkylchlorosilanes.

Journal Article↗
PubMed1998

New Metallacarborane Chemistry: Molybdenum and Tungsten Carbonyl Multidecker Sandwiches. Cluster Dimers Linked via Metal-Metal Bonds(1).

This article reports the synthesis, characterization, and chemistry of the first molybdenum and tungsten complexes bearing small carborane ligands, including novel M-M-linked dicluster species. The reaction of the nido-2,3-Et(2)C(2)B(4)H(4)(2)(-) dianion with (RCN)(3)M(CO)(3) reagents (M = Mo, W; R = Me, Et) gave the (Et(2)C(2)B(4)H(4))M(CO)(3)(2)(-) anions, which were isolated as lithium (12-crown-4) salts 1b (M = Mo) and 2b (M = W). Treatment of 1b and 2b with Ph(4)PX (X = Cl, Br, I) followed by triflic acid afforded the dimeric products [(Et(2)C(2)B(4)H(4))Mo(CO)(2)](2)(&mgr;-X)(2) (X = Cl (3a), Br (3b), I (3c)) and the corresponding tungsten dimers 4a-c, all of which are red or orange air-stable crystalline solids. X-ray crystallography on 3b revealed a geometry without precedent in small metallacarborane dimers, in which two MC(2)B(4) pentagonal pyramidal clusters are linked via an intercluster metal-metal bond. From NMR and other spectroscopic evidence, the remaining dimeric products 3a, 3c, and 4a-c are proposed to have structures similar to 3b. Decapitation of 3b with HCl/EtOAc in THF gave red solid [(Et(2)C(2)B(3)H(5))Mo(CO)(2)](2)(&mgr;-Br)(2) (5), which is proposed to have two open planar C(2)B(3) end rings and, hence, is a potential building block for linear polymers via deprotonation and metal-ion-stacking reactions. Compound 3b was also generated in a different reaction, involving the nido-Et(2)C(2)B(4)H(5)(-) anion and [Mo(CO)(4)Br](2)[&mgr;-Br](2) which also produced the bis(carboranyl)molybdenum complex (Et(2)C(2)B(4)H(4))(2)Mo(CO)(2) (6), a nearly colorless solid for which a bent sandwich structure is postulated. The reaction of the nido-cobaltacarborane anion CpCo(Et(2)C(2)B(3)H(4))(-) with [Mo(CO)(4)Cl](2)[&mgr;-Cl](2) in toluene gave two isolable products, red CpCo(Et(2)C(2)B(3)H(3))Mo(CO)(4) (7) and dark red [CpCo(Et(2)C(2)B(3)H(3))](2)Mo(CO)(2) (8), which have, respectively, triple-decker and bent-tetradecker sandwich structures based on X-ray crystallographic structure determinations. The reaction of CpCo(Et(2)C(2)B(3)H(4))(-) with [Mo(CO)(4)Br](2)[&mgr;-Br](2) in toluene gave 8 and a known fused-cluster complex Cp(2)Co(2)(Et(4)C(4)B(6)H(6)) (9). Treatment of the CpCo(Et(2)C(2)B(3)H(4))(-) anion with [W(CO)(4)Br](2)[&mgr;-Br](2) afforded red crystalline CpCo(Et(2)C(2)B(3)H(3))W(CO)(4) (10), which on reaction with PhLi followed by Me(3)OBF(4) gave 11, a B(5)-benzyl derivative of 10. X-ray diffraction analyses established triple-decker sandwich structures for both compounds. The new products were characterized by multinuclear NMR, IR, UV-vis, and mass spectroscopy, and elemental analysis, supplemented by electrochemical data on 3b, 8, and 11.

Journal Article↗
PubMed1992

Nitric oxide synthesis couples cerebral blood flow and metabolism.

The most fundamental aspect of the cerebral circulation is the well-described coupling of cerebral metabolic activity and cerebral blood flow. A number of substances have been proposed to link flow and metabolism, including K+, pH and adenosine. In the alpha-chloralose anaesthetised cat we studied simultaneously cerebral neuronal activity and local blood flow to attempt to dissociate the two and thus determine the coupling substance. Neuronal activity was determined by monitoring unit firing in the parietal cortex with tungsten in glass microelectrodes while local cerebral blood flow in the same area was monitored continuously using laser Doppler flowmetry. To initiate an increase in metabolic activity and, pari passu, blood flow spreading depression was elicited by needle stick injury. Spreading depression when initiated causes a wave of depolarization, measured as an increased firing rate and associated marked (400 +/- 95%) increase in local cerebral blood flow. Intravenous administration of NG-nitro-L-arginine methyl ester (1-NAME), a potent nitric oxide synthase inhibitor, produced a complete blockade of the hyperemia associated with spreading depression but no change in either resting cell firing or spreading depression-evoked increases in firing rate. These data demonstrate at least for spreading depression-elicited increases in metabolic activity, that nitric oxide (NO) is a key coupling compound that links changes in cerebral blood flow and metabolism. These data imply that NO may have a more general role in flow/metabolism coupling and further studies in other situations are required to determine the extent to which NO is responsible for this fundamental cerebrovascular phenomenon.

Animals↗
PubMed2006

Synthesis and coordination behavior of planar-chiral ferrocene alkenylphosphines.

A series of planar-chiral ferrocene alkenylphosphines, (S(p))-2-(diphenylphosphino)-1-vinylferrocene (2), (S(p))-2-(diphenylphosphino)-1-(prop-1-en-1-yl)ferrocene (3; as a mixture of Z and E isomers in ca. 5:1 ratio), and (E,S(p))-2-(diphenylphosphino)-1-(2-phenylethen-1-yl)ferrocene ((E)-4), was obtained by Wittig and Horner-Wadsworth-Emmons reactions from the common precursor, (S(p))-2-(diphenylphosphino)ferrocene-1-carboxaldehyde (1). Coordination properties of these novel ferrocene donors were studied in their palladium(II) and tungsten(0)-carbonyl complexes. The reaction between 2 and [{Pd(mu-Cl)(L(NC))}2] (5, L(NC) = 2-{(dimethylamino)methyl-kappaN}phenyl-kappaC(1)) gave the bridge-cleavage product [PdCl(L(NC))(2-kappaP)] (6) while the reaction with [Pd(L(NC))(MeCN)2]ClO4 (7) yielded the cationic bis(chelate) [Pd(L(NC))(2-eta2:kappaP)]ClO4 (8). Chelate complexes of the type [W(CO)4(L-eta2:kappaP)] (9 with L = 2; (Z/E)-10 with L = (Z/E)-3) were obtained by reacting [W(CO)4(cod)] (cod = eta2:eta2-cycloocta-1,5-diene) with the appropriate phosphinoalkene in refluxing toluene while a similar reaction with (E)-4 yielded mixtures of [W(CO)5(4-kappaP)] ((E)-11) and [W(CO)4(4-eta2:kappaP)] ((E)-12). All compounds were characterized by spectral methods (multinuclear NMR, IR, MS, and CD), and the structures of 1, 2, 8, 9, (Z/E)-10, and (E)-11 were corroborated by X-ray diffraction analysis. Ligands 2 and (E)-4 as well as their complexes 6, 8, 9, (E)-11, and (E)-12 were further studied by electrochemical methods.

Journal Article↗
PubMed2000

Electrochemical sensor detecting free sulfhydryl groups: evaluation of milk heat treatment.

We describe a new and rapid method for the evaluation of reactive sulfhydryl groups in whey proteins obtained after precipitation of casein by acetic acid at pH 4.6. The procedure is based on the use of a wire tungsten electrode operating at -0.2 V versus saturated calomel electrode in flow injection analysis. The method was applied to raw milks and to commercial pasteurized and UHT milks. Results showed that the tungsten electrode constituted a robust amperometric sensor that could be used to differentiate milks that underwent different heat treatments. The decrease of thiol content in the whey proteins from samples was in agreement with the whey protein content found by HPLC. The procedure is suitable for on-line quality control of heat-treated milks.

Animals↗
PubMed2001

Synthesis and reactivity of the monocarbon molybdenacarborane anion [1,2-mu-NHBu(t)-2,2,2-(CO)(3)-closo-2,1-MoCB(10)H(10)](-).

The reagent Li[7-NHBu(t)()-nido-7-CB(10)H(12)] reacts with [Mo(CO)(6)] in NCMe at reflux temperatures, followed by addition of [N(PPh(3))(2)]Cl, to give [N(PPh(3))(2)][1,2-mu-NHBu(t)-2,2,2-(CO)(3)-closo-2,1-MoCB(10)H(10)] (1). The tungsten (2) and chromium (3) analogues were similarly obtained, but the latter is unstable and was isolated in low yield. An X-ray diffraction study of 2 confirmed that the exo-polyhedral NHBu(t) group forms a bridge between the cage-carbon atom and the tungsten. For 1, this intramolecular donor bond is lifted on protonation in the presence of donor molecules L (CO, PPh(3), PMe(3), PEt(3), PMe(2)Ph) when zwitterionic complexes [1-NH(2)Bu(t)()-2,2,2-(CO)(3)-2-L-closo-2,1-MoCB(10)H(10)] (4) are formed. In contrast, protonation with HCl gives a salt [N(PPh(3))(2)][1-NH(2)Bu(t)()-2,2,2-(CO)(3)-2-Cl-closo-2,1-MoCB(10)H(10)] (5). Complex 1 in CH(2)Cl(2) with CNBu(t) is oxidized by iodine, affording [1,2-mu-NHBu(t)()-2,2,2-(CNBu(t)())(3)-2-I-closo-2,1-MoCB(10)H(10)] (6a). Treatment of 1 with [CuCl(PPh(3))](4) in the presence of Tl[PF(6)] yields the bimetallic compound [exo-[Cu(PPh(3))]-1,2-mu-NHBu(t)-2,2,2-(CO)(3)-closo-2,1-MoCB(10)H(10)] (8), whereas reaction with [AuCl(PPh(3))] and Tl[PF(6)] affords a mixture of [1,2-mu-NHBu(t)-2-[Au(PPh(3))]-2,2,2-(CO)(3)-closo-2,1-MoCB(10)H(10)] (9) and [Au(PPh(3))(2)][2,2'-mu-Au-[1,2-mu-NHBu(t)-2,2,2-(CO)(3)-closo-2,1-MoCB(1)(0)H(10)](2)] (10a). In solution, 9 disproportionates, giving 10a. The [N(PPh(3))(2)](+) salt (10b) is readily prepared by treating 1 with [AuCl(THT)] (THT = tetrahydrothiophene) and Tl[PF(6)], and its structure was determined by X-ray diffraction.

Journal Article↗
PubMed2006

Cytotoxic evaluation of cubic boron nitride in human origin cultured cells.

The aim of this study was to evaluate the cytotoxicity of cubic boron nitride (cBN), a component of surgical cutting tools. The small quantities of cBN that typically remain on implants as a result of the manufacturing process may act as abrasives, injuring tissues surrounding the implant. To determine how cBN affects cells, we treated human neuroblastoma cells (NB-1) and human articular chondrocytes (nHAC-kn) with different concentrations of cBN powder and assessed cell growth and cell survival using the methyl-thiazol-tetrazolium (MTT) assay and a fluorescence probe assay. We also assessed the effects of tungsten carbide (WC) and cobalt (Co), two common components of joint implants, on cell growth and cell survival. Both cBN and WC moderately inhibited NB-1 and nHAC-kn cell growth. However, cBN and WC did not affect cell survival, even at high concentrations (40 microg/ml). By contrast, Co affected cell survival, inducing cell death in both cell types at increasing concentrations. These results suggest that cBN may be less toxic than WC alloys containing Co.

3T3 Cells↗
PubMed1994

A technique for assessing the effects of pH and photodegradation on the stability of the iron chelate of ethyl N-methyl-4-hydroxy-5-oxo-3-pyrroline-3-carboxylate.

Ethyl N-methyl-4-hydroxy-5-oxo-3-pyrroline-3-carboxylate forms a deep red chelate with iron salts. The color intensity is directly related to the iron concentration. The photostability of the red color was determined at pH 1.2 and 5 by spectrophotometric assay at 484 nm at intervals during irradiation by tungsten light at 1020 microW/cm2. After 528 hr of continuous irradiation in deionized water, 90.9% of the iron chelate had decomposed. The reaction followed zero order kinetics. Maximal stability was observed at pH 5 at both 10(-1) and 10(-2) molar concentrations of the iron chelate; no detectable decomposition occurred after 192 hr of continuous irradiation. The iron chelate in biological tissues is stable for 18 months. The staining technique is superior to other histological methods for estimating low concentrations of iron in tissue.

Chlorides↗
PubMed2001

Effect of light-enhanced bleaching on in vitro surface and intrapulpal temperature rise.

PURPOSE: This study investigated the effect of the presence, absence, and aging of a heat-enhancing compound (colorant) added to bleaching gel on the temperature rise of the gel itself, as well as the temperature rise within the pulp chamber, when a tooth was exposed to a variety of light-curing units in vitro. MATERIALS AND METHODS: An extracted human upper central incisor was fitted with thermocouples placed in the pulp chamber as well as on the facial tooth surface. A temperature-controlled simulated intrapulpal fluid flow was provided to the tooth, and bleaching agent (Opalesence XTRA, Ultradent) containing heat-enhancing colorant, aged colorant, or no colorant was applied to the facial surface. The tooth and light-curing unit were placed in a thermostatically controlled oven at 37 degrees C, and real-time gel and intrapulpal temperature values were recorded digitally. Light-curing units used were a plasma arc light (PAC) (PowerPac, ADT), a conventional quartz tungsten halogen source (QTH) (Optilux 501, Demetron/Kerr), the QTH light used in high-power (bleaching) mode, and an argon ion laser (AccuCure 3000, LaserMed). An exposure scenario simulating light-enhanced bleaching of 10 upper teeth was developed. Temperature rise over the pre-exposure, baseline value associated with the last light exposure in the bleaching sequence was calculated for each curing and bleaching combination. Five replications for each test condition were made. Temperature rise values were compared using analysis of variance (ANOVA) at a preset alpha of 0.05. RESULTS: When fresh colorant-containing bleach was used, the PAC light increased bleach temperature 39.3 degrees C above baseline. With no added colorant, temperature rise was 37.1 degrees C. The QTH light in bleach mode resulted in gel temperature 24.8 degrees C above baseline, whereas the temperature increase was only 11.5 degrees C when no colorant was used. Conventional QTH light use increased fresh bleach temperature by 17.7 degrees C, whereas an increase of only 11.1 degrees C was measured without colorant. The argon ion laser produced equivalent temperature rise regardless of the presence or freshness of the colorant, approximately 9.4 degrees C. Intrapulpal temperatures were all significantly lower than those recorded in the bleaching gel and ranged from 5 degrees to 8 degrees C. As a rule, the presence of fresh heat-enhancing colorant in the bleaching gel resulted in a significant intrapulpal temperature increase (approximately 1 degrees C) over that reached using other lights. The PAC and the QTH light used in bleach mode induced greater intrapulpal temperature rise than the laser. CLINICAL SIGNIFICANCE: Freshness of bleaching agent incorporating light-activated, heat-enhancing colorant influences temperature rise of bleaching gel and also may increase intrapulpal temperature values. Use of intense lights does elevate bleach temperature and also results in increased intrapulpal temperature that may further impact on patient sensitivity and pulpal health resulting from this treatment.

Analysis of Variance↗
PubMed2000

Three- and four-center trans effects in triply bonded ditungsten complexes: an ab initio molecular dynamics study of compounds with stoichiometry W2Cl4(NHEt)2(PMe3)2.

We have performed ab initio molecular dynamics simulations based on density functional theory to characterize the structural, electronic, and dynamic properties of the three major isomeric forms of the title compound. In agreement with experimental results, calculations with two different parametrizations of the exchange-correlation functional (BLYP and BP) both indicate the cis-C2 form as the most stable isomer. The relative energies of the different forms are, however, small (less than approximately 1-2 kcal/mol), and the three compounds show overall very similar ground-state properties. Larger differences exist in their finite temperature behavior, which is dominated by the facile dissociation of one or both phosphine ligands. The calculated activation energies for phosphine dissociation differ clearly for the trans and the cis isomers and vary in the order trans << cis-C2 less than approximately cis-Ci. Analysis of the electronic structure of the transition states shows that the difference in activation energy between cis and trans isomers can be rationalized in terms of a classic trans effect caused by a molecular orbital spanning the three atomic centers N-W-P. The subtle difference between the two cis isomers, on the other hand, is likely due to an analogous four-center trans effect N-W-W-P which is mediated via metal-metal orbitals and involves ligands on both tungsten atoms.

Journal Article↗
PubMed2001

Synthesis, structure, and optical-limiting properties of heterobimetallic [M(3)CuS(4)] cuboidal clusters (M = Mo or W) with terminal phosphine ligands.

Cubane-type clusters of formula [Mo(3)CuS(4)Cl(4)(dmpe)(3)](PF(6)) (4), [Mo(3)CuS(4)Br(4)(dmpe)(3)](PF(6)) (5), and [W(3)CuS(4)Br(4)(dmpe)(3)](PF(6)) (6) have been prepared by reacting the incomplete cuboidal trimers [Mo(3)S(4)Cl(3)(dmpe)(3)](PF(6)) (1), [Mo(3)S(4)Br(3)(dmpe)(3)](PF(6)) (2), and [W(3)S(4)Br(3)(dmpe)(3)](PF(6)) (3), respectively, with CuX (X = Cl or Br) or the mononuclear copper complex [Cu(CH(3)CN)(4)](+) in THF. The reaction takes place without global changes in the metal oxidation states, and compounds 4-6 with a [M(3)CuS(4)](5+) core possess 16 e(-) for metal-metal bonding. X-ray structural analysis of 4-6 revealed an effective C(3v) symmetry for the M(3)Cu unit with the M-M distances being statistically the same for M = Mo or W. However, the M-Cu distance is 0.04 and 0.1 A longer than the M-M bond length for Mo and W, respectively. There is no significant structural rearrangement of the ligand-metal bonding in proceeding from [M(3)S(4)X(3)(dmpe)(3)](+) to [M(3)CuS(4)X(4)(dmpe)(3)](+). The cyclic voltammograms of the [Mo(3)CuS(4)] cubane clusters show one quasi-reversible reduction process at E(1/2) = -0.31 V for 4 and at E(1/2) = -0.23 V for 5 and one irreversible reduction at -0.69 and -0.58 V for 4 and 5, respectively. The tungsten cluster 6 shows a unique quasi-reversible reduction wave at E(1/2) = -0.71 V. The incorporation of copper into the incomplete [M(3)S(4)] cuboidal complexes produces a decrease of the reduction potential for both molybdenum and tungsten. Absorption spectra of 1-6 are broadly similar; replacing Mo by W in proceeding from 2 to 3 or from 5 to 6 and replacing Br by Cl in proceeding from 2 to 1 or from 5 to 4 results in a blue shift of the three UV-visible absorption bands. All six clusters exhibit optical limiting, as measured by the Z-scan technique at 523 nm using 40 ns pulses. The power-limiting mechanism remains obscure, but under the conditions employed, threshold-limiting fluence decreases on replacing W by Mo on proceeding from 3 to 2 or 6 to 5 and on proceeding from tetranuclear cluster (4-6) to trinuclear precursor (1-3, respectively). For all six clusters, values of the excited-state cross section sigma(eff) are larger than those of the corresponding ground-state cross section sigma(0); i.e., all clusters are efficient optical limiters.

Journal Article↗
PubMed2006

Chromene chromium carbene complexes in the syntheses of naphthopyran and naphthopyrandione units present in photochromic materials and biologically active natural products.

The carbene complex 5-(2,2-dimethyl-2H-chromene)methoxylmethylene chromium pentacarbonyl will undergo a benzannulation reaction with phenylacetylene, 1-pentyne, 3-hexyne, and trimethylsilylacetylene to give 7-hydroxy-10-methoxy-3H-naphtho[2.1-b]pyrans as the primary product. These compounds are difficult to obtain pure due to their sensitivity to air. If the benzannulation reaction is performed in conjunction with protection of the phenol function at C-7, then good to excellent yields of 7-alkoxy-10-methoxy-3H-naphtho[2.1-b]pyrans are afforded. If the 7-hydroxy products are captured by triflic anhydride, then the resulting aryl triflate can be used to access 3H-naphtho[2.1-b]pyrans bearing C-7 carbon substituents. The 7-hydroxy products can be oxidized to 3H-naphtho[2,1-b]pyran-7,10-diones which are stable. The chromenyl carbene complex reacts with 1,6-bis(triisopropylsilyl)-1,3,5-hexatriyne to give a 2,3-dihydro-2,2-dimethylbenzo[de]chromene, a product type that has not been seen before in the reaction of Fischer carbene complexes with alkynes. A mechanism is proposed for this process that involves alpha,beta-hydride elimination from a chromacyclobutane intermediate. Chromenyl tungsten complexes react with alkynes to give products that result from cyclization without CO insertion.

Benzopyrans↗
PubMed2002

Xanthamide fluorescent dyes.

Two derivatives of fluorescein, termed "xanthamides," were prepared from fluorescein, an inexpensive dye. Relative to fluorescein, which contains a 6-phenolic OH and a 2'-carboxyl, the first derivative (5) contains a carboxymethyl ether at the 6-position and a secondary amide of dimethylamine at the 2'-position. The second derivative (8) contains a corresponding 6-methyl ether and a secondary amide of isonipecotic acid at the 2'-position. Thus, both derivatives contain a single carboxyl group, making them monofunctional. Especially 8 is much more photostable (about 10 times) than either fluorescein or BODIPY FL dye when exposed to ordinary light (tungsten light bulb). When tested for relative response in a capillary electrophoresis instrument fitted with an argon ion laser detector (488 nm) and a broad band emission filter, 8 was found to be 4-fold less bright than fluorescein. Both xanthamides, consistent with prior literature on 6-O-alkylated fluorescein, exhibit relatively pH-independent fluorescence (pH 4-10 was tested here). Because they possess two absorbance maximums, the xanthamides can be excited over a broader wavelength range than fluorescein or BODIPY. These collective properties of the xanthamides will make them advantageous over fluorescein and BODIPY dyes for some applications.

Boron Compounds↗
PubMed1978

Functional characteristics of unmyelinated fibres in the hippocampal cortex.

(1) In transverse hippocampal slices (350 micrometer thick), taken from guinea pigs initially anaesthetized with ether, intracortical afferent fibres were activated by small current pulses delivered through tungsten microelectrodes. Extracellular potentials were recorded from the zone of activated fibres in dendritic layers while intracellular recordings were made from the soma of CA1 pyramidal cells. (2) When recording was made from the same level as the stimulating cathode, the extracellular potential consisted of a diphasic deflection followed by a larger negative wave with a superimposed population spike. The negative wave corresponded to an intracellularly recorded EPSP, and is called an extracellular EPSP, whereas the initial diphasic deflection had no intracellular counterpart. (3) The initial diphasic deflection was linearly related to the size of both the intracellular and extracellular EPSP. It was not changed by removal of calcium ions from the bathing fluid, whereas all postsynaptic activity disappeared. The diphasic deflection was propagated along fibres lying parallel to the pyramidal layer with a velocity of 0.3 m/sec. It could follow short bursts of stimulation at 300 Hz. The absolute refractory period was 2.0 msec. (4) The initial diphasic deflection is interpreted as the compound action potential of the largely unmyelinated afferent fibres to the CA1 neurones.

Afferent Pathways↗