Homogeneous Hydrogenation of Nitriles Catalyzed by Molybdenum and Tungsten Amides

Low-valent molybdenum and tungsten amides M­(NO)­(CO)­(PNP) {M = Mo, <b>1a</b>; W, <b>1b</b>; PNP = N­(CH₂CH₂P^<i>i</i>Pr₂)₂} were found to be active catalysts for the hydrogenation of various nitriles to the corresponding imines, primary amines, and N-substituted imines with high selectivity for the latter type of product. A wide range of p- and m-substituted aromatic nitriles<i>p</i>-methyl, <i>p</i>-methoxy, <i>p</i>-bromobenzonitriles; 3-trifluoromethylbenzonitrile, m- and p-disubstituted benzonitrile; the heterocyclic 2-thiophencarbonitrile; and the aliphatic nitriles cyclohexylcarbonitrile and benzylcyanidecould be hydrogenated at 140 °C and 60 bar H₂ in THF with high yields. TOFs were found to be between 0.4 and 36 h^–1

Ethylene Reactions of a [ReH(η²-BH₄)(NO)(PPh₃)₂] Complex: Reductive Elimination of Ethane and Oxidative Coupling to Butadiene

The borohydride complex [Re^+IH­(η²-BH₄)­(NO)­(PPh₃)₂] (<b>1ph</b>) reacts with ethylene to yield [Re^+IH₂(η²-C₂H₄)­(NO)­(PPh₃)₂] (<b>2ph</b>) and triethylborane formed by ethylene hydroboration. Subsequent ethylene insertion into the Re–H bond of <b>2ph</b> and uptake of another 1 equiv of ethylene led to the kinetically stable <i>cis</i>-hydrido–ethyl complex [Re^+IH­(Et)­(η²-C₂H₄)­(NO)­(PPh₃)₂] (<b>3ph</b>). <b>3ph</b> was found to slowly reductively eliminate ethane. The rate of this process was determined by quantitative NMR spectroscopy in the temperature range from 293 to 338 K, enabling calculation of the activation parameters (Δ<i>H</i>^⧧ = 68.7 kJ mol^–1, Δ<i>S</i>^⧧ = −94 J mol^–1 K^–1; half-life time 1.8 h at 303 K). The reaction was found to follow first-order kinetics in <i>c</i>(<b>3ph</b>) and is zeroth order in <i>c</i>(C₂H₄) and <i>c</i>(PPh₃), ruling out preceding ligand dissociation. The presumptive intermediate [Re^–I(η²-C₂H₄)­(NO)­(PPh₃)₂] could not be traced, since it rapidly reacted further with ethylene, furnishing the stable butadiene complex [Re^–I(η²-C₂H₄)­(η⁴-C₄H₆)­(NO)­(PPh₃)] (<b>4ph</b>) in 88% yield. This transformation of dehydrogenative ethylene coupling is suggested to involve the elementary steps of rhenacyclopentane formation from two coordinated ethylene ligands and then double C–H activation via β-hydride shifts to generate the butadiene unit and formal H₂ elimination from the rhenium dihydride with concomitant triphenylphosphine elimination. An X-ray crystallographic study confirmed the spectroscopically derived pentacoordinate structure of <b>4ph</b>

Highly Efficient Large Bite Angle Diphosphine Substituted Molybdenum Catalyst for Hydrosilylation

Treatment of the complex Mo­(NO)­Cl₃(NCMe)₂ with the large bite angle diphosphine, 2,2′-bis­(di­phenyl­phos­phino)­diphenylether (DPEphos) afforded the dinuclear species [Mo­(NO)­(P∩P)­Cl₂]₂[μCl]₂ (P∩P = DPEphos = (Ph₂PC₆H₄)₂O (<b>1</b>). <b>1</b> could be reduced in the presence of Zn and MeCN to the cationic complex [Mo­(NO)­(P∩P)­(NCMe)₃]⁺[Zn₂Cl₆]^2–_1/2 (<b>2</b>). In a metathetical reaction the [Zn₂Cl₆]^2–_1/2 counteranion was replaced with NaBAr^F₄ (BAr^F₄ = [B­{3,5-(CF₃)₂C₆H₃}₄]) to obtain the [BAr^F₄]⁻ salt [Mo­(NO)­(P∩P)­(NCMe)₃]⁺[BAr^F₄]⁻ (<b>3</b>). <b>3</b> was found to catalyze hydrosilylations of various <i>para</i> substituted benzaldehydes, cyclohexanecarboxaldehyde, 2-thiophenecarboxaldehyde, and 2-furfural at 120 °C. A screening of silanes revealed primary and secondary aromatic silanes to be most effective in the catalytic hydrosilylation with <b>3</b>. Also ketones could be hydrosilylated at room temperature using <b>3</b> and PhMeSiH₂. A maximum turnover frequency (TOF) of 3.2 × 10⁴ h^–1 at 120 °C and a TOF of 4400 h^–1 was obtained at room temperature for the hydrosilylation of 4-methoxyacetophenone using PhMeSiH₂ in the presence of <b>3</b>. Kinetic studies revealed the reaction rate to be first order with respect to the catalyst and silane concentrations and zero order with respect to the substrate concentrations. A Hammett study for various <i>para</i> substituted acetophenones showed linear correlations with negative ρ values of −1.14 at 120 °C and −3.18 at room temperature

Catalytic CO₂ Activation Assisted by Rhenium Hydride/B(C₆F₅)₃ Frustrated Lewis PairsMetal Hydrides Functioning as FLP Bases

Reaction of <b>1</b> with B­(C₆F₅)₃ under 1 bar of CO₂ led to the instantaneous formation of the frustrated Lewis pair (FLP)-type species [ReHBr­(NO)­(PR₃)₂(η²-OCO–B­(C₆F₅)₃)] (<b>2</b>, R = <i>i</i>Pr <b>a</b>, Cy <b>b</b>) possessing two <i>cis</i>-phosphines and O_CO₂-coordinated B­(C₆F₅)₃ groups as verified by NMR spectroscopy and supported by DFT calculations. The attachment of B­(C₆F₅)₃ in <b>2a</b>,<b>b</b> establishes cooperative CO₂ activation via the Re–H/B­(C₆F₅)₃ Lewis pair, with the Re–H bond playing the role of a Lewis base. The Re­(I) η¹-formato dimer [{Re­(μ-Br)­(NO)­(η¹-OCHO–B­(C₆F₅)₃)­(P<i>i</i>Pr₃)₂}₂] (<b>3a</b>) was generated from <b>2a</b> and represents the first example of a stable rhenium complex bearing two <i>cis</i>-aligned, sterically bulky P<i>i</i>Pr₃ ligands. Reaction of <b>3a</b> with H₂ cleaved the μ-Br bridges, producing the stable and fully characterized formato dihydrogen complex [ReBrH₂(NO)­(η¹-OCHO–B­(C₆F₅)₃)­(P<i>i</i>Pr₃)₂] (<b>4a</b>) bearing <i>trans</i>-phosphines. Stoichiometric CO₂ reduction of <b>4a</b> with Et₃SiH led to heterolytic splitting of H₂ along with formation of bis­(triethylsilyl)­acetal ((Et₃SiO)₂CH₂, <b>7</b>). Catalytic reduction of CO₂ with Et₃SiH was also accomplished with the catalysts <b>1a</b>,<b>b</b>/B­(C₆F₅)₃, <b>3a</b>, and <b>4a</b>, showing turnover frequencies (TOFs) between 4 and 9 h^–1. The stoichiometric reaction of <b>4a</b> with the sterically hindered base 2,2,6,6-tetramethylpiperidine (TMP) furnished H₂ ligand deprotonation. Hydrogenations of CO₂ using <b>1a</b>,<b>b</b>/B­(C₆F₅)₃, <b>3a</b>, and <b>4a</b> as catalysts gave in the presence of TMP TOFs of up to 7.5 h^–1, producing [TMPH]­[formate] (<b>11</b>). The influence of various bases (R₂NH, R = <i>i</i>Pr, Cy, SiMe₃, 2,4,6-tri-<i>tert</i>-butylpyridine, NEt₃, P<i>t</i>Bu₃) was studied in greater detail, pointing to two crucial factors of the CO₂ hydrogenations: the steric bulk and the basicity of the base

Structural and Electronic Variations of sp/sp² Carbon-Based Bridges in Di- and Trinuclear Redox-Active Iron Complexes Bearing Fe(diphosphine)₂X (X = I, NCS) Moieties

Starting from the mononuclear precursor <i>trans</i>-Fe­(depe)₂I₂ (depe = 1,2-bis­(diethylphosphino)­ethane), four dinuclear complexes IFe­(depe)₂–R–Fe­(depe)₂I, with R = 1,4-(−CC–C₆H₄–CC−) <b>1</b>, 1,3-(−CC–C₆H₄–CC−) <b>2</b>, 4,4′-(−CC–C₆H₄–C₆H₄–CC−) <b>3</b>, and 2,5-(−CC–thiophene–CC−) <b>4</b>, as well as a trinuclear complex, {I–Fe­(depe)₂(CC−)}₃(1,3,5-C₆H₃)} <b>5</b>, were prepared in a facile way by transmetalation from stannylated precursors. Substitution of the terminal iodides applying an excess of NaSCN yielded the corresponding isothiocyanate complexes <b>6</b>–<b>10</b> in very good yields. All complexes <b>1</b>–<b>10</b> are intrinsically functional due to the redox-active Fe centers embedded in a structurally rigid and covalent sp/sp² framework. <b>1</b>–<b>10</b> were characterized by NMR, IR, and Raman spectroscopy, as well as elemental analyses. X-ray diffraction studies were carried out for <b>1</b>, <b>2</b>, <b>4</b>, <b>5</b>, <b>6</b>, <b>8</b>, and <b>9</b>. Cyclic voltammetry was employed to explore the redox behavior of <b>1</b>–<b>10</b>. The 1,4-(−CC–C₆H₄–CC−) and the 2,5-(−CC–thiophene–CC−) bridged compounds <b>1</b>, <b>4</b>, <b>6</b>, and <b>9</b> exhibit two fully reversible oxidation waves, while the 1,3-(−CC–C₆H₄–CC−) and 4,4′-(−CC–C₆H₄–C₆H₄–CC−) bridged dinuclear complexes and the trinuclear complexes show only one reversible oxidation wave corresponding to 2 e^– and 3 e^– processes, respectively. Calculations were carried out for truncated model complexes to determine the HOMO/LUMO energies. The DFT results confirmed that by changing the sp/sp² bridging ligand, tuning of the energies of the molecular orbitals and modifying of the HOMO–LUMO gap Δ<i>E</i>_(H‑L) and the chemical hardness are possible

The “Catalytic Nitrosyl Effect”: NO Bending Boosting the Efficiency of Rhenium Based Alkene Hydrogenations

Diiodo Re­(I) complexes [ReI₂(NO)­(PR₃)₂(L)] (<b>3</b>, L = H₂O; <b>4</b> , L = H₂; R = <i>i</i>Pr <b>a</b>, Cy <b>b</b>) were prepared and found to exhibit in the presence of “hydrosilane/B­(C₆F₅)₃” co-catalytic systems excellent activities and longevities in the hydrogenation of terminal and internal alkenes. Comprehensive mechanistic studies showed an inverse kinetic isotope effect, fast H₂/D₂ scrambling and slow alkene isomerizations pointing to an Osborn type hydrogenation cycle with rate determining reductive elimination of the alkane. In the catalysts’ activation stage phosphonium borates [R₃PH]­[HB­(C₆F₅)₃] (<b>6</b>, R = <i>i</i>Pr <b>a</b>, Cy <b>b</b>) are formed. VT ²⁹Si- and ¹⁵N NMR experiments, and dispersion corrected DFT calculations verified the following facts: (1) Coordination of the silylium cation to the O_NO atom facilitates nitrosyl bending; (2) The bent nitrosyl promotes the heterolytic cleavage of the H–H bond and protonation of a phosphine ligand; (3) H₂ adds in a bifunctional manner across the Re–N bond. Nitrosyl bending and phosphine loss help to create two vacant sites, thus triggering the high hydrogenation activities of the formed “superelectrophilic” rhenium centers

Efficient Lewis Acid Promoted Alkene Hydrogenations Using Dinitrosyl Rhenium(−I) Hydride Catalysts

Highly efficient alkene hydrogenations were developed using NO-functionalized hydrido dinitrosyl rhenium catalysts of the type [ReH­(PR₃)₂(NO)­(NO­(LA))]­[Z] (<b>2</b>, LA = B­(C₆F₅)₃; <b>3</b>, LA = [Et]⁺, Z = [B­(C₆F₅)₄]⁻; <b>4</b>, LA = [SiEt₃]⁺, Z = [HB­(C₆F₅)₃]⁻; R = <i>i</i>Pr <b>a</b>, Cy <b>b</b>). Lewis acid attachment to the NO ligand was found to facilitate bending at the N_OLA atom and concomitantly to open up a vacant site at the rhenium center. According to DFT calculations, the ability to bend follows the order <b>4</b> > <b>3</b> > <b>2</b>, which did not match with the order of increasing hydrogenation activities: <b>3</b> > <b>4</b> > <b>2</b>. The main factor spoiling catalytic performance was catalyst deactivation by detachment of the LA group occurring during the catalytic reaction course, which was found to go along with the decrease in order of DFT-calculated strengths of the O_NO–LA bonds. LA detachment from the O_NO atom could at least partly be prevented by LA addition as cocatalysts, which led to an extraordinary boost of the hydrogenation activities. For instance the “<b>1</b>/hydrosilane/B­(C₆F₅)₃” (1:5:5) system exhibited the highest performance, with TOFs up to 1.2 × 10⁵ h^–1 (1-hexene, 1-octene, cyclooctene, cyclohexene). The cocatalyst [Et₃O]­[B­(C₆F₅)₄] showed the smallest effect, presumably due to the strong Lewis acidic character of the reagent causing side-reactions before reacting with <b>1a</b>,<b>b</b>. The catalytic reaction course crucially involves not only reversible bending at the N_OLA atom but also loss of a PR₃ ligand, forming 16<i>e</i> or 14<i>e</i> monohydride reactive intermediates, which drive an Osborn-type hydrogenation cycle with olefin before H₂ addition

Efficient Lewis Acid Promoted Alkene Hydrogenations Using Dinitrosyl Rhenium(−I) Hydride Catalysts

Highly efficient alkene hydrogenations were developed using NO-functionalized hydrido dinitrosyl rhenium catalysts of the type [ReH­(PR₃)₂(NO)­(NO­(LA))]­[Z] (<b>2</b>, LA = B­(C₆F₅)₃; <b>3</b>, LA = [Et]⁺, Z = [B­(C₆F₅)₄]⁻; <b>4</b>, LA = [SiEt₃]⁺, Z = [HB­(C₆F₅)₃]⁻; R = <i>i</i>Pr <b>a</b>, Cy <b>b</b>). Lewis acid attachment to the NO ligand was found to facilitate bending at the N_OLA atom and concomitantly to open up a vacant site at the rhenium center. According to DFT calculations, the ability to bend follows the order <b>4</b> > <b>3</b> > <b>2</b>, which did not match with the order of increasing hydrogenation activities: <b>3</b> > <b>4</b> > <b>2</b>. The main factor spoiling catalytic performance was catalyst deactivation by detachment of the LA group occurring during the catalytic reaction course, which was found to go along with the decrease in order of DFT-calculated strengths of the O_NO–LA bonds. LA detachment from the O_NO atom could at least partly be prevented by LA addition as cocatalysts, which led to an extraordinary boost of the hydrogenation activities. For instance the “<b>1</b>/hydrosilane/B­(C₆F₅)₃” (1:5:5) system exhibited the highest performance, with TOFs up to 1.2 × 10⁵ h^–1 (1-hexene, 1-octene, cyclooctene, cyclohexene). The cocatalyst [Et₃O]­[B­(C₆F₅)₄] showed the smallest effect, presumably due to the strong Lewis acidic character of the reagent causing side-reactions before reacting with <b>1a</b>,<b>b</b>. The catalytic reaction course crucially involves not only reversible bending at the N_OLA atom but also loss of a PR₃ ligand, forming 16<i>e</i> or 14<i>e</i> monohydride reactive intermediates, which drive an Osborn-type hydrogenation cycle with olefin before H₂ addition

Efficient Lewis Acid Promoted Alkene Hydrogenations Using Dinitrosyl Rhenium(−I) Hydride Catalysts

Highly efficient alkene hydrogenations were developed using NO-functionalized hydrido dinitrosyl rhenium catalysts of the type [ReH­(PR₃)₂(NO)­(NO­(LA))]­[Z] (<b>2</b>, LA = B­(C₆F₅)₃; <b>3</b>, LA = [Et]⁺, Z = [B­(C₆F₅)₄]⁻; <b>4</b>, LA = [SiEt₃]⁺, Z = [HB­(C₆F₅)₃]⁻; R = <i>i</i>Pr <b>a</b>, Cy <b>b</b>). Lewis acid attachment to the NO ligand was found to facilitate bending at the N_OLA atom and concomitantly to open up a vacant site at the rhenium center. According to DFT calculations, the ability to bend follows the order <b>4</b> > <b>3</b> > <b>2</b>, which did not match with the order of increasing hydrogenation activities: <b>3</b> > <b>4</b> > <b>2</b>. The main factor spoiling catalytic performance was catalyst deactivation by detachment of the LA group occurring during the catalytic reaction course, which was found to go along with the decrease in order of DFT-calculated strengths of the O_NO–LA bonds. LA detachment from the O_NO atom could at least partly be prevented by LA addition as cocatalysts, which led to an extraordinary boost of the hydrogenation activities. For instance the “<b>1</b>/hydrosilane/B­(C₆F₅)₃” (1:5:5) system exhibited the highest performance, with TOFs up to 1.2 × 10⁵ h^–1 (1-hexene, 1-octene, cyclooctene, cyclohexene). The cocatalyst [Et₃O]­[B­(C₆F₅)₄] showed the smallest effect, presumably due to the strong Lewis acidic character of the reagent causing side-reactions before reacting with <b>1a</b>,<b>b</b>. The catalytic reaction course crucially involves not only reversible bending at the N_OLA atom but also loss of a PR₃ ligand, forming 16<i>e</i> or 14<i>e</i> monohydride reactive intermediates, which drive an Osborn-type hydrogenation cycle with olefin before H₂ addition

Efficient Lewis Acid Promoted Alkene Hydrogenations Using Dinitrosyl Rhenium(−I) Hydride Catalysts

Highly efficient alkene hydrogenations were developed using NO-functionalized hydrido dinitrosyl rhenium catalysts of the type [ReH­(PR₃)₂(NO)­(NO­(LA))]­[Z] (<b>2</b>, LA = B­(C₆F₅)₃; <b>3</b>, LA = [Et]⁺, Z = [B­(C₆F₅)₄]⁻; <b>4</b>, LA = [SiEt₃]⁺, Z = [HB­(C₆F₅)₃]⁻; R = <i>i</i>Pr <b>a</b>, Cy <b>b</b>). Lewis acid attachment to the NO ligand was found to facilitate bending at the N_OLA atom and concomitantly to open up a vacant site at the rhenium center. According to DFT calculations, the ability to bend follows the order <b>4</b> > <b>3</b> > <b>2</b>, which did not match with the order of increasing hydrogenation activities: <b>3</b> > <b>4</b> > <b>2</b>. The main factor spoiling catalytic performance was catalyst deactivation by detachment of the LA group occurring during the catalytic reaction course, which was found to go along with the decrease in order of DFT-calculated strengths of the O_NO–LA bonds. LA detachment from the O_NO atom could at least partly be prevented by LA addition as cocatalysts, which led to an extraordinary boost of the hydrogenation activities. For instance the “<b>1</b>/hydrosilane/B­(C₆F₅)₃” (1:5:5) system exhibited the highest performance, with TOFs up to 1.2 × 10⁵ h^–1 (1-hexene, 1-octene, cyclooctene, cyclohexene). The cocatalyst [Et₃O]­[B­(C₆F₅)₄] showed the smallest effect, presumably due to the strong Lewis acidic character of the reagent causing side-reactions before reacting with <b>1a</b>,<b>b</b>. The catalytic reaction course crucially involves not only reversible bending at the N_OLA atom but also loss of a PR₃ ligand, forming 16<i>e</i> or 14<i>e</i> monohydride reactive intermediates, which drive an Osborn-type hydrogenation cycle with olefin before H₂ addition