Mercury Lα1 High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy: A Versatile Speciation Probe for Mercury.

Mercury is in some sense an enigmatic element. The element and some of its compounds are a natural part of the biogeochemical cycle; while many of these can be deadly poisons at higher levels, environmental levels in the absence of anthropogenic contributions would generally be below the threshold for concern. However, mercury pollution, particularly from burning fossil fuels such as coal, is providing dramatic and increasing emissions into the environment. Because of this, the environmental chemistry and toxicology of mercury are of growing importance, with the fate of mercury being vitally dependent upon its speciation. X-ray absorption spectroscopy (XAS) provides a powerful tool for in situ chemical speciation, but is severely limited by poor spectroscopic energy resolution. Here, we provide a systematic examination of mercury Lα1 high energy resolution fluorescence detected XAS (HERFD-XAS) as an approach for chemical speciation of mercury, in quantitative comparison with conventional Hg LIII-edge XAS. We show that, unlike some lighter elements, chemical shifts in the Lα1 X-ray fluorescence energy can be safely neglected, so that mercury Lα1 HERFD-XAS can be treated simply as a high-resolution version of conventional XAS. We present spectra of a range of mercury compounds that may be relevant to the environmental and life science research and show that density functional theory can produce adequate simulations of the spectra. We discuss strengths and limitations of the method and quantitatively demonstrate improvements both in speciation for complex mixtures and in background rejection for low concentrations.

Tungsten Ligand-Based Sulfur-Atom-Transfer Catalysts: Synthesis, Characterization, Sustained Anaerobic Catalysis, and Mode of Aerial Deactivation.

The synthesis, properties, X-ray structures, and catalytic sulfur-atom-transfer (SAT) reactions of W2(µ-S)(µ-S2)(dtc)2(dped)2 [1; dtc = S2CNR2-, where R = Me, Et, iBu, and Bn; dped = S2C2Ph22-] and W2(µ-S)2(dtc)2(dped)2 (2) are reported. These complexes represent the oxidized (1) and reduced (2) forms of anaerobic SAT catalysts operating through the bidirectional, ligand-based half-reaction (µ-S)(µ-S2) ↔ (µ-S)2 + S0. The catalysts are deactivated in air through the formation of catalytically inactive oxo complexes, (dtc)WO(µ-S)(µ-dped)W(dtc)(dped) (3), prompting us to recommend that group 6 SAT activity be assessed under strictly anaerobic conditions.

Mononuclear Sulfido-Tungsten(V) Complexes: Completing the Tp*MEXY (M = Mo, W; E = O, S) Series.

Orange Tp*WSCl2 has been synthesized from the reactions of Tp*WOCl2 with boron sulfide in refluxing toluene or Tp*WS2Cl with PPh3 in dichloromethane at room temperature. Mononuclear sulfido-tungsten(V) complexes, Tp*WSXY {X = Y = Cl, OPh, SPh, SePh; X = Cl, Y = OPh; XY = toluene-3,4-dithiolate (tdt), quinoxaline-2,3-dithiolate (qdt); and Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate} were prepared by metathesis of Tp*WSCl2 with the respective alkali metal salt of X-/XY2-, or [NHEt3]2(qdt). The complexes were characterized by microanalysis, mass spectrometry, electrochemistry, and infrared (IR), electron paramagnetic resonance (EPR) and electronic absorption spectroscopies. The molecular structures of Tp*WS(OPh)2, Tp*WS(SePh)2, and Tp*WS(tdt) have been determined by X-ray crystallography. The six-coordinate, distorted-octahedral W centers are coordinated by terminal sulfido (W≡S = 2.128(2) - 2.161(1) Å), terdentate facial Tp*, and monodentate/bidentate O/S/Se-donor ligands. The sulfido-W(V) complexes are characterized by lower energy electronic transitions, smaller giso, and larger Aiso(183W) values, and more positive reduction potentials compared with their oxo-W(V) counterparts. This series has been probed by sulfur K-edge X-ray absorption spectroscopy (XAS), the spectra being assigned by comparison to Tp*WOXY (X = Y = SPh; XY = tdt, qdt) and time-dependent density functional theoretical (TD-DFT) calculations. This study provides insight into the electronic nature and chemistry of the catalytically and biologically important sulfido-W unit.

d(1) Oxosulfido-Mo(V) Compounds: First Isolation and Unambiguous Characterization of an Extended Series.

Reaction of Tp(iPr)Mo(VI)OS(OAr) with cobaltocene in toluene results in the precipitation of brown, microcrystalline oxosulfido-Mo(V) compounds, [CoCp2][Tp(iPr)Mo(V)OS(OAr)] (Cp(-) = η(5)-C5H5(-), Tp(iPr)(-) = hydrotris(3-isopropylpyrazol-1-yl)borate, OAr(-) = phenolate or 2-(s)Bu, 2-(t)Bu, 3-(t)Bu, 4-(s)Bu, 4-Ph, 3,5-(s)Bu2, 2-CO2Me, 2-CO2Et or 2-CO2Ph derivative thereof). The compounds are air- and water-sensitive and display ν(Mo═O) and ν(Mo[Formula: see text]S) IR absorption bands at ca. 890 and 435 cm(-1), respectively, 20-40 cm(-1) lower in energy than the corresponding bands in Tp(iPr)MoOS(OAr). They are electrochemically active and exhibit three reversible cyclovoltammetric waves (E(Mo(VI)/Mo(V)) = -0.40 to -0.66 V, E([CoCp2](+)/CoCp2) = -0.94 V and E(CoCp2/[CoCp2](-)) = -1.88 V vs SCE). Structural characterization of [CoCp2][Tp(iPr)MoOS(OC6H4CO2Et-2)]·2CH2Cl2 revealed a distorted octahedral Mo(V) anion with Mo═O and Mo[Formula: see text]S distances of 1.761(5) and 2.215(2) Å, respectively, longer than corresponding distances in related Tp(iPr)MoOS(OAr) compounds. The observation of strong S(1s) → (S(3p) + Mo(4d)) S K-preedge transitions indicative of a d(1) sulfido-Mo(V) moiety and the presence of short Mo═O (ca. 1.72 Å) and Mo[Formula: see text]S (ca. 2.25 Å) backscattering contributions in the Mo K-edge EXAFS further support the oxosulfido-Mo(V) formulation. The compounds are EPR-active, exhibiting highly anisotropic (Δg 0.124-0.150), rhombic, frozen-glass spectra with g1 close to the value observed for the free electron (ge = 2.0023). Spectroscopic studies are consistent with the presence of a highly covalent Mo[Formula: see text]S π* singly occupied molecular orbital. The compounds are highly reactive, with reactions localized at the terminal sulfido ligand. For example, the compounds react with cyanide and PPh3 to produce thiocyanate and SPPh3, respectively, and various (depending on solvent) oxo-Mo(V) species. Reactions with copper reagents also generally lead to desulfurization and the formation of oxo-Mo(V) or -Mo(IV) complexes.

Structural characterization and unusual reactivity of oxosulfido-Mo(V) compounds: implications for the structure and electronic description of the very rapid form of xanthine oxidase.

The first structural characterization of a mononuclear, EPR-active, oxosulfido-Mo(V) compound related to the very rapid form of xanthine oxidase (VR-XnO) is reported. The compound, [CoCp2][Tp(iPr)Mo(V)OS(2-OC6H4CO2Et)] [Cp = cyclopentadienyl; Tp(iPr) = hydrotris(3-isopropylpyrazol-1-yl)borate], exhibits a distorted octahedral geometry with Mo═O and Mo=/--S distances of 1.761(5) and 2.215(2) Å, respectively, and an O═Mo=/--S angle of 107.33(14)°. Significantly, the Mo(V)=/--S distance is much shorter than the value of 2.36 Å reported for oxosulfido-Mo(V) compounds (Singh, R.; et al. Inorg. Chem. 1989, 28, 8) but close to the range established for VR-XnO by protein crystallography. The methyl and phenyl esters were also prepared but the latter is highly reactive and undergoes an intramolecular, radical-based cyclization/elimination reaction to form [CoCp2][Tp(iPr)Mo(IV)O{2-OC6H4C(O)S-κO,κS}]. This study provides the first definitive measurement of the Mo(V)=/--S bond distance in an unambiguously characterized oxosulfido-Mo(V) compound and supports the presence of a short (ca. 2.22 Å) Mo=/--S bond in VR-XnO. It also demonstrates that the Mo(V)=/--S moiety participates in radical-based reactions that are facilitated by the facile redox interplay of Mo and S and by substrates susceptible to radical eliminations.

Reactivity studies of oxo-Mo(IV) complexes containing potential hydrogen-bond acceptor/donor phenolate ligands.

Reactivity studies of oxo-Mo(IV) complexes, Tp(iPr)MoO{2-OC(6)H(4)C(O)R-κ(2)O,O'} (R = Me, Et, OMe, OEt, OPh, NHPh), containing chelated hydrogen-bond donor/acceptor phenolate ligands are reported. Hydrolysis/oxidation of Tp(iPr)MoO(2-OC(6)H(4)CO(2)Ph-κ(2)O,O') in the presence of methanol yields tetranuclear [Tp(iPr)MoO(µ-O)(2)MoO](2)(µ-OMe)(2) (1), while condensation of Tp(iPr)MoO{2-OC(6)H(4)C(O)Me-κ(2)O,O'} and methylamine gives the chelated iminophenolate complex, Tp(iPr)MoO{2-OC(6)H(4)C(Me)NMe-κ(2)O,N} (2), rather than the aqua complex, Tp(iPr)MoO{2-OC(6)H(4)C(Me)NMe-κO}(OH(2)). The oxo-Mo(IV) complexes are readily oxidized by dioxygen or hydrogen peroxide to the corresponding cis-dioxo-Mo(VI) complexes, Tp(iPr)MoO(2){2-OC(6)H(4)C(O)R}; in addition, suitable one-electron oxidants, e.g., [FeCp(2)]BF(4) and [N(C(6)H(4)Br)(3)][SbCl(6)], oxidize the complexes to their EPR-active (g(iso) ≈ 1.942) molybdenyl counterparts (3, 4). Molybdenyl complexes such as Tp(iPr)MoOCl{2-OC(6)H(4)C(O)R} (5) and Tp(iPr)MoOCl(2) also form when the complexes react with chlorinated solvents. The ester derivatives (R = OMe, OEt, OPh) react with propylene sulfide to form cis-oxosulfido-Mo(VI) complexes, Tp(iPr)MoOS{2-OC(6)H(4)C(O)R}, that crystallize as dimeric µ-disulfido-Mo(V) species, [Tp(iPr)MoO{2-OC(6)H(4)C(O)R}](2)(µ-S(2)) (6-8). The crystal structures of [Tp(iPr)MoO(µ-O)(2)MoO](2)(µ-OMe)(2), Tp(iPr)MoO{2-OC(6)H(4)C(Me)NMe}, Tp(iPr)MoOCl{2-OC(6)H(4)C(O)NHPh}·{2-HOC(6)H(4)C(O)NHPh}, and [Tp(iPr)MoO{2-OC(6)H(4)C(O)R}](2)(µ-S(2)) (R = OMe, OEt) are reported.

Paramagnetic oxotungsten(V) complexes containing the hydrotris(3,5-dimethylpyrazol-1-yl)borate ligand.

Sky-blue Tp*WOCl(2) has been synthesized from the high-yielding reaction of Tp*WO(2)Cl with boron trichloride in refluxing toluene. Dark-red Tp*WOI(2) was prepared via thermal decarbonylation followed by aerial oxidation of Tp*WI(CO)(3) in acetonitrile. From these precursors, an extensive series of mononuclear tungstenyl complexes, Tp*WOXY [X = Cl(-), Y = OPh(-), SPh(-); X = Y = OPh(-), 2-(n-propyl)phenolate (PP(-)), SPh(-), SePh(-); XY = toluene-3,4-dithiolate (tdt(2-)), quinoxaline-2,3-dithiolate (qdt(2-)), benzene-1,2-diselenolate (bds(2-)); Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate], was prepared by metathesis with the respective alkali-metal salt of X(-)/XY(2-) or (NHEt(3))(2)(qdt). The complexes were characterized by microanalysis, mass spectrometry, electrochemistry, IR, electron paramagnetic resonance (EPR), and electronic absorption spectroscopies, and X-ray crystallography (for X = Y = OPh(-), PP(-), SPh(-); XY = bds(2-)). The six-coordinate, distorted-octahedral tungsten centers are coordinated by terminal oxo [W≡O = 1.689(6)-1.704(3) Å], tridentate Tp*, and monodentate or bidentate O/S/Se-donor ligands. Spin Hamiltonian parameters derived from the simulation of fluid-solution X-band EPR spectra revealed that the soft-donor S/Se ligand complexes had larger g values and smaller (183)W hyperfine coupling constants than the less covalent hard-donor O/Cl species. The former showed low-energy ligand-to-metal charge-transfer bands in the near-IR region of their electronic absorption spectra. These oxotungsten(V) complexes display lower reduction potentials than their molybdenum counterparts, underscoring the preference of tungsten for higher oxidation states. Furthermore, the protonation of the pyrazine nitrogen atoms of the qdt(2-) ligand has been examined by spectroelectrochemistry; the product of the one-electron reduction of [Tp*WO(qdtH)](+) revealed usually intense low-energy bands.

Influence of the oxygen atom acceptor on the reaction coordinate and mechanism of oxygen atom transfer from the dioxo-Mo(VI) complex, Tp(iPr)MoO(2)(OPh), to tertiary phosphines.

The oxygen atom transfer reactivity of the dioxo-Mo(VI) complex, Tp(iPr)MoO(2)(OPh) (Tp(iPr) = hydrotris(3-isopropylpyrazol-1-yl)borate), with a range of tertiary phosphines (PMe(3), PMe(2)Ph, PEt(3), PBu(n)(3), PEt(2)Ph, PEtPh(2), and PMePh(2)) has been investigated. The first step in all the reactions follows a second-order rate law indicative of an associative transition state, consistent with nucleophilic attack by the phosphine on an oxo ligand, namely, Tp(iPr)MoO(2)(OPh) + PR(3) --> Tp(iPr)MoO(OPh)(OPR(3)). The calculated free energy of activation for the formation of the OPMe(3) intermediate (Chem. Eur. J. 2006, 12, 7501) is in excellent agreement with the experimental DeltaG() value reported here. The second step of the reaction, that is, the exchange of the coordinated phosphine oxide by acetonitrile, Tp(iPr)MoO(OPh)(OPR(3)) + MeCN --> Tp(iPr)MoO(OPh)(MeCN) + OPR(3), is first-order in starting complex in acetonitrile. The reaction occurs via a dissociative interchange (I(d)) or associative interchange (I(a)) mechanism, depending on the nature of the phosphine oxide. The activation parameters for the solvolysis of Tp(iPr)MoO(OPh)(OPMe(3)) (DeltaH(++) = 56.3 kJ mol(-1); DeltaS(++) = -125.9 J mol(-1) K(-1); DeltaG(++) = 93.8 kJ mol(-1)) and Tp(iPr)MoO(OPh)(OPEtPh(2)) (DeltaH(++) = 66.5 kJ mol(-1); DeltaS(++) = -67.6 J mol(-1) K(-1); DeltaG(++) = 86.7 kJ mol(-1)) by acetonitrile are indicative of I(a) mechanisms. In contrast, the corresponding parameters for the solvolysis reaction of Tp(iPr)MoO(OPh)(OPEt(3)) (DeltaH(++) = 95.8 kJ mol(-1); DeltaS(++) = 26.0 J mol(-1) K(-1); DeltaG(++) = 88.1 kJ mol(-1)) and the remaining complexes by the same solvent are indicative of an I(d) mechanism. The equilibrium constant for the solvolysis of the oxo-Mo(V) phosphoryl complex, [Tp(iPr)MoO(OPh)(OPMe(3))](+), by acetonitrile was calculated to be 1.9 x 10(-6). The oxo-Mo(V) phosphoryl complex is more stable than the acetonitrile analogue, whereas the oxo-Mo(IV) acetonitrile complex is more stable than the phosphoryl analogue. The higher stability of the Mo(V) phosphoryl complex may explain the phosphate inhibition of sulfite oxidase.

cis-Dioxo- and cis-(hydroxo)oxo-Mo(V) complexes stabilized by intramolecular hydrogen-bonding.

The reactions of Tp(iPr)Mo(VI)O(2)Cl with salicylanilides and NEt(3) produce cis-Tp(iPr)Mo(VI)O(2)(2-OC(6)H(4)CONHR) (Tp(iPr) = hydrotris(3-isopropylpyrazol-1-yl)borate, R = Ph, 4-C(6)H(4)Cl, 4-C(6)H(4)OMe). The N-methyl complex, Tp(iPr)MoO(2){2-OC(6)H(4)CON(Me)Ph}, is similarly prepared. Reduction of the amido complexes by cobaltocene produces green, EPR-active compounds, [CoCp(2)][Tp(iPr)Mo(V)O(2)(2-OC(6)H(4)CONHR)], that exhibit strong, low energy, ν(MoO(2)) IR bands at ∼ 895 and 790 cm(-1) (cf. ∼ 935 and 900 cm(-1) for the Mo(VI) analogues). The X-ray structures of all seven complexes have been determined. In each case, the Mo center exhibits a distorted octahedral coordination geometry defined by mutually cis oxo and phenolate ligands and a tridentate fac-Tp(iPr) ligand. The Mo(V) anions exhibit greater Mo═O distances (av. 1.738 Å vs 1.695 Å) and O═Mo═O angles (av. 112.4° vs 102.9°) than their Mo(VI) counterparts, indicative of the presence of a three-center (MoO(2)), π* semioccupied molecular orbital in these d(1) complexes. The amido Mo(VI) and Mo(V) complexes exhibit an intramolecular hydrogen-bond between the NH and O(phenolate) atoms. Protonation of [CoCp(2)][Tp(iPr)Mo(V)O(2)(2-OC(6)H(4)CONHR)] by lutidinium tetrafluoroborate is quantitative and produces EPR-active, cis-(hydroxo)oxo-Mo(V) complexes, Tp(iPr)Mo(V)O(OH)(2-OC(6)H(4)CONHR), related to the low pH Mo(V) forms of sulfite oxidase.

Synthesis and characterisation of fifteen-electron dihalo(carbonyl)tungsten(iii) complexes, Tp*WX2(CO) (X = Br, I).

The oxidation of Tp*WIII(NCMe)(CO) [Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate] by iodine in refluxing acetonitrile produces paramagnetic Tp*WIIII2(CO), with µeff = 1.70 µB and ν(CO) = 1881 cm-1. The analogous bromo complex, Tp*WIIIBr2(CO) (µeff = 1.72 µB, ν(CO) = 1905 cm-1), is produced by the oxidative decarbonylation of Tp*WI(CO)3 with bromoacetonitrile in refluxing tetrahydrofuran. The crystal structures of both complexes reveal mononuclear, six-coordinate, distorted octahedral metal complexes bearing facial Tp* and mutually cis halide and carbonyl ligands. The structures, magnetic moments and spectroscopic data (including isotropically shifted NMR spectra) are consistent with a low-spin, doublet ground state in these, the first reported, 15-electron, d3 dihalo(carbonyl)-W(iii) complexes.

Synthesis and characterization of TpiPrMoO(S2PR2) (R = Pri, Ph, OEt, OPri, (-)-mentholate) and {HB(OMe)(Pripz)2}MoO(S2PPri2), including isomers of known 1,2-borotropically-shifted complexes.

Green/blue TpiPrMoO(S2PR2) (TpiPr = hydrotris(3-isopropylpyrazolyl)borate; R = Pri, Ph, OEt, OPri, (-)-mentholate) complexes were synthesized and characterized by elemental analysis, mass spectrometry, IR and NMR spectroscopy, and X-ray crystallography. The diamagnetic, six-coordinate, oxo-Mo(IV) complexes possess distorted octahedral geometries defined by terminal oxo, bidentate dithio acid, and tridentate TpiPr ligands. The R = Pri and Ph derivatives are isomers of previously reported 1,2-borotropically shifted complexes, TpiPr*MoO(S2PR2) (TpiPr* = hydrobis(3-isopropylpyrazolyl)(5-isopropylpyrazolyl)borate; ref: Inorg. Chem. 1996, 35, 5368). Conversion of TpiPrMoO(S2PPh2) into TpiPr*MoO(S2PPh2) at elevated temperatures (>80 degrees C) showed that the borotropically shifted isomer was thermodynamically more stable than the unshifted species. Reaction with methanol converts TpiPrMoO(S2PPri2) into {HB(OMe)(Pripz)2}MoO(S2PPri2) (Pripz = 3-isopropylpyrazolyl), which was characterized by spectroscopic and crystallographic methods.

Insights into the Nature of Mo(V) Species in Solution: Modeling Catalytic Cycles for Molybdenum Enzymes.

The tris(pyrazolyl)borate and related tripodal N-donor ligands originally developed by Trofimenko stabilize mononuclear compounds containing Mo(VI)O(2), Mo(VI)O, Mo(V)O, and Mo(IV)O units and effectively inhibit their polynucleation in organic solvents. Dioxo-Mo(VI) complexes of the type LMoO(2)(SPh), where L = hydrotris(3,5-dimethylpyrazol-1-yl)borate (Tp*), hydrotris(3-isopropylpyrazol-1-yl)borate (Tp(i) (Pr)), and hydrotris(3,5-dimethyl-1,2,4-triazol-1-yl)borate (Tz) and related derivatives are the only model systems that mimic the complete reaction sequence of sulfite oxidase, in which oxygen from water is ultimately incorporated into product. The quasi-reversible, one-electron reduction of Tp*MoO(2)(SPh) in acetonitrile exhibits a positive potential shift upon addition of a hydroxylic proton donor, and the magnitude of the shift correlates with the acidity of the proton donor. These reductions produce two Mo(V) species, [Tp*Mo(V)O(2)(SPh)](-) and Tp*Mo(V)O(OH)(SPh), that are related by protonation. Measurement of the relative amounts of these two Mo(V) species by EPR spectroscopy enabled the pK(a) of the Mo(V)(OH) unit in acetonitrile to be determined and showed it to be several pK(a) units smaller than that for water in acetonitrile. Similar electrochemical-EPR experiments for Tp(i) (Pr)MoO(2)(SPh) indicated that the pK(a) for its Mo(V)(OH) unit was ∼1.7 units smaller than that for Tp*Mo(V)O(OH)(SPh). Density functional theory calculations also predict a smaller pK(a) for (iPr)Mo(V)O(OH)(SPh) compared to Tp*Mo(V)O(OH)(SPh). Analysis of these results indicates that coupled electron-proton transfer (CEPT) is thermodynamically favored over the indirect process of metal reduction followed by protonation. The crystal structure of Tp(i) (Pr)MoO(2)(SPh) is also presented.

Electronic structure description of the cis-MoOS unit in models for molybdenum hydroxylases.

The molybdenum hydroxylases catalyze the oxidation of numerous aromatic heterocycles and simple organics and, unlike other hydroxylases, utilize water as the source of oxygen incorporated into the product. The electronic structures of the cis-MoOS units in CoCp2[TpiPrMoVOS(OPh)] and TpiPrMoVIOS(OPh) (TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate), new models for molybdenum hydroxylases, have been studied in detail using S K-edge X-ray absorption spectroscopy, vibrational spectroscopy, and detailed bonding calculations. The results show a highly delocalized Mo=S pi* LUMO redox orbital that is formally Mo(dxy) with approximately 35% sulfido ligand character. Vibrational spectroscopy has been used to quantitate Mo-Ssulfido bond order changes in the cis-MoOS units as a function of redox state. Results support a redox active molecular orbital that has a profound influence on MoOS bonding through changes to the relative electro/nucleophilicity of the terminal sulfido ligand accompanying oxidation state changes. The bonding description for these model cis-MoOS systems supports enzyme mechanisms that are under orbital control and dominantly influenced by the unique electronic structure of the cis-MoOS site. The electronic structure of the oxidized enzyme site is postulated to play a role in polarizing a substrate carbon center for nucleophilic attack by metal activated water and acting as an electron sink in the two-electron oxidation of substrates.

A novel ligand modification and diamond-core molybdenum(IV) 2,6-bis(2,2-diphenyl-2-thioethyl)pyridinate(2-) complex.

The reaction of Mo(VI)O(2)(L-NS(2)) [L-NS(2) = 2,6-bis(2,2-diphenyl-2-thioethyl)pyridinate(2-)] or Mo(V)(2)O(3)(L-NS(2))(2) with excess PPh(3) in N,N-dimethylformamide at 70 degrees C results in the formation of gray-green (L-NOS)Mo(IV)(mu-O)(mu-S)Mo(IV)(L-NS(2)) [L-NOS = 2-(2,2-diphenyl-2-thioethyl)-6-(2,2-diphenyl-2-oxoethyl)pyridinate(2-)] (1). The crystal structure of 1 revealed a dinuclear complex comprised of two trigonal bipyramidal Mo centers bridged along an axial-equatorial edge (the mu-O-mu-S vector) such that the Mo-N bonds are trans to the bridging atoms and are anti with respect to the Mo-Mo bond (d(Mo-Mo) = 2.5535(5) A); the remaining coordination sites are occupied by the S- and O-donor atoms of the L-NOS and L-NS(2) ligands. The diamond core is asymmetric, with Mo(1/2)-O(1) distances of 1.845(2) and 2.009(2) A and Mo(1/2)-S(1) distances of 2.374(1) and 2.230(1) A. Compound 1 is unique in possessing a planar, diamond-core unit devoid of terminal oxo ligation and a new tridentate L-NOS ligand formed via a novel intramolecular modification of the original L-NS(2) ligand.

Models for aerobic carbon monoxide dehydrogenase: synthesis, characterization and reactivity of paramagnetic MoVO(µ-S)CuI complexes.

Reaction of [CoCp2][TpiPrMoOS(OAr)] [Cp = η5-cyclopentadienyl; TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate; OAr = phenolate or derivative thereof] with [Cu(NCMe)(Me3tcn)]BF4 (Me3tcn = 1,4,7-trimethyl-1,4,7-triazacyclononane) in MeCN at -30 °C results in the formation of red-brown/black, paramagnetic, µ-sulfido-Mo(v)/Cu(i) complexes, TpiPrMoO(OAr)(µ-S)Cu(Me3tcn). The complexes possess the MoO(µ-S)Cu core found in aerobic carbon monoxide dehydrogenases (CODHs) and exhibit X-band EPR spectra closely related to those of semi-reduced CODH, with g iso ∼ 1.937, hyperfine coupling to 95,97Mo (a iso = 39-42 × 10-4 cm-1) and strong superhyperfine coupling to 63,65Cu (a iso = 34-63 × 10-4 cm-1). Anisotropic spectra exhibit monoclinic symmetry with g 1 ∼ 1.996, g 2 ∼ 1.944 and g 3 ∼ 1.882, and nearly isotropic A Cu values (75-90 × 10-4 cm-1). The X-ray structures of four derivatives (Ar = Ph, C6H4 t Bu-2, C6H4 sBu-2, C6H4Ph-4) are reported and discussed along with that of the Ar = C6H3 t Bu2-3,5 derivative (communicated in C. Gourlay, D. J. Nielsen, J. M. White, S. Z. Knottenbelt, M. L. Kirk and C. G. Young, J. Am. Chem. Soc., 2006, 128, 2164). The complexes exhibit distorted octahedral oxo-Mo(v) and distorted tetrahedral Cu(i) centres bridged by a single bent µ-sulfido ligand, with Mo-S and Cu-S distances and Mo-S-Cu angles in the ranges 2.262-2.300 Å, 2.111-2.134 Å and 115.87-134.27°, respectively. The 2 t-butyl derivative adopts a unique phenolate conformation with O[double bond, length as m-dash]Mo-O-Cα and O[double bond, length as m-dash]Mo-S-Cu torsion angles of 92.7 and 21.1°, respectively, very different from those of the other structurally characterized derivatives (31-47 and 33-45°, respectively) and exhibits a relatively short Mo···Cu distance [3.752(2) Å vs. 3.806(7)-4.040(2) Å]. As well, the a Cu value of this complex (34.3 × 10-4 cm-1) is much lower than the values observed for other members of the series (55-63 × 10-4 cm-1), supporting the hypothesis that the electronic structure of the MoO(µ-S)Cu core unit and the degree of intermetallic communication are strongly dependent on the geometry of the MoO(OR)(µ-S)Cu unit. The complexes participate in an electrochemically reversible Mo(vi)/Mo(v) redox couple and react with cyanide undergoing decupration and desulfurization reactions of the type observed for CODH.

Facets of early transition metal-sulfur chemistry: metal-sulfur ligand redox, induced internal electron transfer, and the reactions of metal-sulfur complexes with alkynes.

Metal-sulfur ligand redox interplay, induced internal electron transfer reactions, and the generation of dithiolene and organosulfur ligands in the reactions of metal-sulfur compounds with alkynes are important and useful facets of early transition metal-sulfur chemistry. This review focuses on developments in these areas over the past 30 years.

Highly-oxidised, sulfur-rich, mixed-valence vanadium(IV/V) complexes.

The reactions of [V(2)(micro-S(2))(2)(S(2)CNR(2))(4)] (R = alkyl) with NOBF(4) produce highly-oxidised, sulfur-rich, V(iv/v) complexes, [V(2)(micro-S(2))(2)(S(2)CNR(2))(4)]BF(4), that exhibit 15-line EPR spectra and structures consistent with Class III mixed-valence behaviour.

Chemical systems modeling the d1 Mo(V) states of molybdenum enzymes.

This review focuses on the synthesis, properties and electron paramagnetic resonance (EPR), electron spin echo envelope modulation (ESEEM) and electron-nuclear double resonance (ENDOR) spectroscopy of mononuclear d1 oxo- and sulfido-Mo(V) complexes relevant to the understanding of the EPR-active Mo(V) forms of pterin-containing molybdenum enzymes.

Tocilizumab in refractory rheumatoid arthritis: long-term efficacy, safety, and tolerability beyond 2 years.

OBJECTIVES: To evaluate the long-term efficacy and safety of tocilizumab (TCZ) in clinical patients with rheumatoid arthritis (RA) refractory to synthetic disease-modifying antirheumatic drugs, anti-tumor necrosis factor agents, and B-cell depletion therapy with rituximab (RTX). METHODS: We conducted a single-center retrospective study of 22 patients with RA treated with TCZ. We collected data including demographics and medication histories. We recorded clinical parameters including tender joint counts and swollen joint counts, and laboratory parameters including inflammatory makers and lipid profiles over regular intervals of TCZ treatment. RESULTS: In all, 22 patients with RA were included, 20 of whom were female. The median age at the first dose of TCZ was 62 years (range: 35-75 years). The mean duration of the disease from diagnosis with RA to May 2015 was 15.7 years (range: 6-30 years). A total of 15 out of 22 patients remained on TCZ at the end of the study, and in all, there was an improvement in markers of disease activity following initiating TCZ. The effect was sustained for a mean of 35 months (SD±15.5 months, range: 9-72 months). Of the 17 patients who failed to respond to RTX previously, 12 patients remained on TCZ. In all, eight out of 22 patients developed adverse events, five of whom discontinued TCZ. In contrast to previously documented short-term data, TCZ did not result in a statistically significant (P<0.05) long-term deterioration in lipid profile for any of the lipid parameters measured in our cohort (mean ± SD at initiation of TCZ to most recent follow-up: total cholesterol 5.25±1.05 to 5.28±0.77 mmol/L, high-density lipoprotein 1.72±0.54 to 1.67±0.43 mmol/L, low-density lipoprotein 3.05±0.98 to 2.98±0.81 mmol/L, and cholesterol to high-density lipoprotein ratio 3.41±1.23 to 3.40±1.22). CONCLUSION: The efficacy of TCZ in patients with RA refractory to disease-modifying drugs, including anti-tumor necrosis factor blockade and RTX, is sustained over 3 years. TCZ confers a good safety profile in the long term even in patients who previously developed adverse events to other rheumatic drugs. In the long run, there is no statistically significant deterioration in lipid profile during treatment with TCZ.

Oxygen Atom Transfer, Coupled Electron-Proton Transfer, and Correlated Electron-Nucleophile Transfer Reactions of Oxomolybdenum(IV) and Dioxomolybdenum(VI) Complexes.

The oxo-Mo(IV) complexes LMoO(S(2)PR(2)-S,S') [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate; R = Me, Et, Pr(i)(), Ph] were prepared by reacting MoO(S(2)PR(2))(2) and KL in refluxing toluene. The dioxo-Mo(VI) complexes cis-LMoO(2)(S(2)PR(2)-S) (R = Pr(i)(), Ph) were prepared by oxidation of the oxo-Mo(IV) complexes or by reaction of LMoO(2)Cl with NaS(2)PR(2). Oxygen atom transfers from Me(2)SO to LMoO(S(2)PR(2)) were first-order with respect to Me(2)SO and complex; the overall second-order rate constants at 40 degrees C range from 9.0(1) x 10(-)(5) M(-)(1).s(-)(1) for LMoO(S(2)PMe(2)) to 2.08(5) x 10(-)(4) M(-)(1).s(-)(1) for LMoO(S(2)PPr(2)); activation parameters were in the ranges DeltaH() = 63(1) to 73(1) kJ.mol(-)(1), DeltaS() = -88(1) to -111(1) J.K(-)(1).mol(-)(1), and DeltaG() = 100(2) kJ.mol(-)(1) for LMoO(S(2)PMe(2)) to 98(2) kJ.mol(-)(1) for LMoO(S(2)PPr(2)). Oxygen atom transfer from pyridine N-oxide to LMoO(S(2)PPr(2)) was also second-order with a rate constant of 1.54(5) x 10(-)(3) M(-)(1).s(-)(1) at 40 degrees C, DeltaH() = 62(1) kJ.mol(-)(1), DeltaS() = -90(1) J.K(-)(1).mol(-)(1), and DeltaG() = 90(1) kJ.mol(-)(1). The second-order rate laws and large negative entropies of activation are consistent with associative mechanisms for the above reactions. Oxygen atom transfer from LMoO(2)(S(2)PPr(2)) to PPh(3) was first-order with respect to reactants, with an overall second-order rate constant of 2.5(3) x 10(-)(4) M(-)(1).s(-)(1) at 30 degrees C. In toluene at 40 degrees C, all the above complexes catalyzed the oxidation of PPh(3) by Me(2)SO, with turnover rates of ca. 0.9 mol of PPh(3)/(mol of catalyst/h). Reduction of LMoO(2)(S(2)PR(2)) by SH(-) led to the generation of the dioxo-Mo(V) anions [LMoO(2)(S(2)PR(2)-S)](-), which were slowly converted to the analogous oxothio-Mo(V) complexes [LMoOS(S(2)PR(2)-S)](-). Dioxygen reacted with [LMoOS(S(2)PPr(2))](-) to produce the oxothio-Mo(VI) complex LMoOS(S(2)PPr(2)-S). The (hydroxo)oxo-Mo(V) complexes LMoO(OH)(S(2)PR(2)-S) were formed upon reduction of LMoO(2)(S(2)PR(2)) with PPh(3) in wet (3-5 M H(2)O) tetrahydrofuran or upon ferrocenium oxidation of LMoO(S(2)PR(2)) in wet tetrahydrofuran. In dry solvents, LMoO(S(2)PR(2)) were oxidized to the corresponding cations, [LMoO(S(2)PR(2)-S,S')](+), which reacted with water to form LMoO(OH)(S(2)PR(2)). The Mo(V) complexes have been characterized by EPR spectroscopy.