Reactivity of Mo(PMe3)6 towards benzothiophene and selenophenes: new pathways relevant to hydrodesulfurization.

Mo(PMe(3))(6) cleaves a C-S bond of benzothiophene to give (kappa(2)-CHCHC(6)H(4)S)Mo(PMe(3))(4), which rapidly isomerizes to the olefin-thiophenolate and 1-metallacyclopropene-thiophenolate complexes, (kappa(1),eta(2)-CH(2)CHC(6)H(4)S)Mo(PMe(3))(3)(eta(2)-CH(2)PMe(2)) and (kappa(1),eta(2)-CH(2)CC(6)H(4)S)Mo(PMe(3))(4). The latter two molecules result from a series of hydrogen transfers and are differentiated according to whether the termini of the organic fragments coordinate as olefin or eta(2)-vinyl ligands, respectively. The reactions between Mo(PMe(3))(6) and selenophenes proceed differently from those of the corresponding thiophenes.

A normal equilibrium isotope effect for oxidative addition of H2 to (eta6-anthracene)Mo(PMe3)3.

Oxidative addition of H2 and D2 to the anthracene complex (eta6-AnH)Mo(PMe3)3 giving (eta4-AnH)Mo(PMe3)3X2 (X = H, D) is characterized by a normal equilibrium isotope effect (KH/KD > 1) at temperatures close to ambient; calculations on (eta4-AnH)Mo(PH3)3H2 indicate that this is a consequence of relatively low energy Mo-H vibrational modes.

Oxidative addition of dihydrogen to (eta6-arene)Mo(PMe3)3 complexes: origin of the naphthalene and anthracene effects.

In contrast to the benzene and naphthalene compounds (eta(6)-PhH)Mo(PMe(3))(3) and (eta(6)-NpH)Mo(PMe(3))(3), the anthracene complex (eta(6)-AnH)Mo(PMe(3))(3) reacts with H(2) to undergo a haptotropic shift and give the eta(4)-anthracene compound (eta(4)-AnH)Mo(PMe(3))(3)H(2). Density functional theory calculations indicate that the increased facility of naphthalene and anthracene to adopt eta(4)-coordination modes compared to that of benzene is a consequence of the fact that the Mo-(eta(4)-ArH) bonding interaction increases in the sequence benzene < naphthalene < anthracene, while the Mo-(eta(6)-ArH) bonding interaction follows the sequence benzene > naphthalene approximately anthracene.

Intramolecular N-H...S hydrogen bonding in the zinc thiolate complex [Tm(Ph)]ZnSCH2C(O)NHPh: a mechanistic investigation of thiolate alkylation as probed by kinetics studies and by kinetic isotope effects.

The zinc thiolate complex [Tm(Ph)]ZnSCH2C(O)N(H)Ph, which features a tetrahedral [ZnS4] motif analogous to that of the Ada DNA repair protein, may be obtained by the reaction of Zn(NO3)2 with [Tm(Ph)]Li and Li[SCH2C(O)N(H)Ph] ([Tm(Ph)] = tris(2-mercapto-1-phenylimidazolyl)hydroborato ligand). Structural characterization of [Tm(Ph)]ZnSCH2C(O)N(H)Ph by X-ray diffraction demonstrates that the molecule exhibits an intramolecular N-H..

The molecular structure of the tris(2-mercapto-1-tolylimidazolyl)hydroborato zinc(2-mercapto-1-tolylimidazole) complex, [[Tm(p-Tol)]Zn(mim(p-Tol))][ClO4]: intermolecular N-H...OClO3versus intramolecular N-H...S hydrogen bonding interactions of the mercaptoimidazole ligand.

The molecular structure of the tris(2-mercapto-1-tolylimidazolyl)hydroborato complex [[Tm(p-Tol)]Zn(mim(p-Tol))][ClO(4)].3MeCN has been determined by X-ray diffraction, thereby demonstrating that the mim(p-Tol) ligand exhibits a N-H..

Molybdenocene trihydride complexes: influence of a [Me2Si] ansa bridge on classical versus nonclassical nature, stability with respect to elimination of dihydrogen, and acidity.

Experimental and computational studies on a series of cationic molybdenocene trihydride complexes, namely [Cp(2)MoH(3)]+, [(Cp(Bu)t)(2)MoH(3)]+, [Cp(2)MoH(3)]+, and ([Me(2)Si(C(5)Me(4))(2)]MoH(3))+, demonstrate that the most stable form for the ansa molybdenocene derivative is a nonclassical dihydrogen-hydride isomer, ([Me(2)Si(C(5)Me(4))(2)]Mo(eta(2)-H(2))(H))+, whereas the stable forms for the non-ansa complexes are classical trihydrides, [Cp(2)Mo(H)(3)]+, [(Cp(Bu)t)(2)Mo(H)(3)]+, and [Cp(2)Mo(H)(3)]+. In addition to altering the classical versus nonclassical nature of [Cp(2)MoH(3)]+ and ([Me(2)Si(C(5)Me(4))(2)]Mo(eta(2)-H(2))(H))+, the [Me(2)Si] ansa bridge also markedly influences the stability of the complex with respect to elimination of H(2) and dissociation of H+. Finally, computational studies on ([H(2)Si(C(5)H(4))(2)]MoH(2)D)+ and ([H(2)Si(C(5)H(4))(2)]MoHD(2))+ establish that deuterium exhibits a greater preference than hydrogen to occupy dihydrogen versus hydride sites.

Experimental evidence for a temperature dependent transition between normal and inverse equilibrium isotope effects for oxidative addition of H2 to Ir(PMe2Ph)2(CO)Cl.

The equilibrium isotope effect (EIE) for oxidative addition of H(2) and D(2) to Ir(PMe(2)Ph)(2)(CO)Cl has been measured over a large temperature range, thereby demonstrating that the inverse (<1) EIE previously observed at ambient temperature becomes normal (>1) at high temperature (>90 degrees C). The temperature dependence of the EIE for oxidative addition of H(2) and D(2) to Ir(PH(3))(2)(CO)Cl has been calculated using the geometry and vibrational frequencies obtained from DFT (B3LYP) calculations on Ir(PH(3))(2)(CO)ClH(2) and Ir(PH(3))(2)(CO)ClD(2), and is in accord with the experimentally observed transition from an inverse to normal EIE for oxidative addition of H(2) and D(2) to Ir(PMe(2)Ph)(2)(CO)Cl: the EIE is calculated to be inverse between 0 and 510 K, reach a maximum value of 1.15 at 867 K and then slowly decrease to unity as the temperature approaches infinity.

Temperature-dependent transitions between normal and inverse equilibrium isotope effects for coordination and oxidative addition of C-H and H-H bonds to a transition metal center.

The temperature dependence of the equilibrium isotope effects (EIEs) for coordination and oxidative addition of C-H and H-H bonds to the tungstenocene species {[H2Si(C5H4)2]W} has been determined with the aid of DFT (B3LYP) calculations. The EIE for coordination of CH4 and CD4 does not exhibit typical van't Hoff type behavior in which there is a monotonic variation of EIE with temperature; rather, the temperature dependence of the EIE exhibits a maximum, with inverse values (<1) at low temperature and normal values (>1) at high temperatures. The temperature dependence of the EIE for oxidative addition of CH4 and CD4 differs significantly from that for coordination, with the EIE being normal at all temperatures and approaching infinity at 0 K.

Computational evidence that the inverse kinetic isotope effect for reductive elimination of methane from a tungstenocene methyl-hydride complex is associated with the inverse equilibrium isotope effect for formation of a sigma-complex intermediate.

Calculations on [H2Si(C5H4)2]W(Me)H demonstrate that the interconversion between [H2Si(C5H4)2]W(Me)H and the sigma-complex [H2Si(C5H4)2]W(sigma-HMe) is characterized by normal kinetic isotope effects for both reductive coupling and oxidative cleavage; the equilibrium isotope effect, however, is inverse and is the origin of the inverse kinetic isotope effect for the overall reductive elimination of methane.

Normal and inverse primary kinetic deuterium isotope effects for C-H bond reductive elimination and oxidative addition reactions of molybdenocene and tungstenocene complexes: evidence for benzene sigma-complex intermediates.

The overall reductive elimination of RH from the ansa-molybdenocene and -tungstenocene complexes [Me(2)Si(C(5)Me(4))(2)]Mo(Ph)H and [Me(2)Si(C(5)Me(4))(2)]W(R)H (R = Me, Ph) is characterized by an inverse primary kinetic isotope effect (KIE) for the tungsten system but a normal KIE for the molybdenum system. Oxidative addition of PhH to [[Me(2)Si(C(5)Me(4))(2)]M] also differs for the two systems, with the molybdenum system exhibiting a substantial intermolecular KIE, while no effect is observed for the tungsten system. These differences in KIEs indicate a significant difference in the reactivity of the hydrocarbon adducts [Me(2)Si(C(5)Me(4))(2)]M(RH) for the molybdenum and tungsten systems.

The reactivity of Mo(PMe3)(6) towards heterocyclic nitrogen compounds: transformations relevant to hydrodenitrogenation.

The reactions of Mo(PMe3)6 towards a variety of five- and six-membered heterocyclic nitrogen compounds (namely, pyrrole, indole, carbazole, pyridine, quinoline, and acridine) have been studied to provide structural models for the coordination of these heterocycles to the molybdenum centers of hydrodenitrogenation catalysts. Pyrrole reacts with Mo(PMe3)6 to yield the eta5-pyrrolyl derivative (eta5-pyr)Mo(PMe3)3H, while indole gives sequentially (eta1-indolyl)Mo(PMe3)4H, (eta5-indolyl)Mo(PMe3)3H, and (eta6-indolyl)Mo(PMe3)3H, with the latter representing the first example of a structurally characterized complex with an eta6-indolyl ligand. Likewise, carbazole reacts with Mo(PMe3)6 to give (eta6-carbazolyl)Mo(PMe3)3H with an eta6-carbazolyl ligand.

Thiophene and butadiene-thiolate complexes of molybdenum: observations relevant to the mechanism of hydrodesulfurization.

Mo(PMe3)6 reacts with thiophene to give the eta5-thiophene complex (eta5-C4H4S)Mo(PMe3)3 and the eta5-butadiene-thiolate complex (eta5-C4H5S)Mo(PMe3)2(eta2-CH2PMe2), which are the first examples of (i) eta5-thiophene coordination and (ii) C-S cleavage and hydrogenation by a molybdenum compound. Deuterium labeling studies suggest that the hydrogenation of thiophene may involve an alkylidene intermediate, an observation that has ramifications for the mechanisms of hydrodesulfurization.