Molybdenum Hydride and Dihydride Complexes Bearing Diphosphine Ligands with a Pendant Amine: Formation of Complexes with Bound Amines

CpMo­(CO)­(PNP)H complexes (PNP = (R₂PCH₂)₂NMe, R = Et or Ph) were synthesized by displacement of two CO ligands of CpMo­(CO)₃H by the PNP ligand; these complexes were characterized by IR and variable temperature ¹H and ³¹P NMR spectroscopy. CpMo­(CO)­(PNP)­H complexes are formed as mixture of <i>cis</i>- and <i>trans</i>-isomers. The structures of both <i>cis-</i>CpMo­(CO)­(P^EtN^MeP^Et)H and <i>trans-</i>CpMo­(CO)­(P^PhN^MeP^Ph)H were determined by single crystal X-ray diffraction. Electrochemical oxidation of CpMo­(CO)­(P^EtN^MeP^Et)­H and CpMo­(CO)­(P^PhN^MeP^Ph)H in CH₃CN are both irreversible at slow scan rates and quasireversible at higher scan rates, with <i>E</i>_1/2 = −0.36 V (vs Cp₂Fe^+/0) for CpMo­(CO)­(P^EtN^MeP^Et)H and <i>E</i>_1/2 = −0.18 V for CpMo­(CO)­(P^PhN^MeP^Ph)­H. Hydride abstraction from CpMo­(CO)­(PNP)H with [Ph₃C]⁺[A]⁻ (A = B­(C₆F₅)₄ or BAr^F₄; [Ar^F = 3,5-bis­(trifluoromethyl)­phenyl]) afforded “tuck-in” [CpMo­(CO)­(κ³-PNP)]⁺ complexes that feature the amine bound to the metal. Displacement of the κ³ Mo–N bond by CD₃CN gives [CpMo­(CO)­(PNP)­(CD₃CN)]⁺. The kinetics of this reaction were studied by ³¹P­{¹H} NMR spectroscopy for [CpMo­(CO)­(κ³-P^EtN^MeP^Et)]⁺, providing the activation parameters Δ<i>H</i>^⧧ = 21.6 ± 2.8 kcal/mol, Δ<i>S</i>^⧧ = −0.3 ± 9.8 cal/(mol K), <i>E</i>_a = 22.1 ± 2.8 kcal/mol. Protonation of CpMo­(CO)­(P^EtN^MeP^Et)H affords the Mo dihydride complex [CpMo­(CO)­(κ²-P^EtN^MeP^Et)­(H)₂]⁺, which loses H₂ to generate [CpMo­(CO)­(κ³-P^EtN^MeP^Et)]⁺ at room temperature. Our results show that the pendant amine has a strong driving force to form stable “tuck-in” [CpMo­(CO)­(κ³-PNP)]⁺ complexes, and also promotes hydrogen elimination from [CpMo­(CO)­(PNP)­(H)₂]⁺ complexes by formation of a Mo–N dative bond. CpMo­(CO)­(dppp)H (dppp = 1,3-bis­(diphenylphosphino)­propane) was studied as a Mo diphosphine analogue without a pendant amine, and the product of protonation of this complex gives [CpMo­(CO)­(dppp)­(H)₂]⁺

Homogeneous Catalysis with Inexpensive Metals: Ionic Hydrogenation of Ketones with Molybdenum and Tungsten Catalysts

Homogeneous Catalysis with Inexpensive Metals:  Ionic Hydrogenation of Ketones with Molybdenum and Tungsten Catalyst

Molybdenum Hydride and Dihydride Complexes Bearing Diphosphine Ligands with a Pendant Amine: Formation of Complexes with Bound Amines

CpMo­(CO)­(PNP)H complexes (PNP = (R₂PCH₂)₂NMe, R = Et or Ph) were synthesized by displacement of two CO ligands of CpMo­(CO)₃H by the PNP ligand; these complexes were characterized by IR and variable temperature ¹H and ³¹P NMR spectroscopy. CpMo­(CO)­(PNP)­H complexes are formed as mixture of <i>cis</i>- and <i>trans</i>-isomers. The structures of both <i>cis-</i>CpMo­(CO)­(P^EtN^MeP^Et)H and <i>trans-</i>CpMo­(CO)­(P^PhN^MeP^Ph)H were determined by single crystal X-ray diffraction. Electrochemical oxidation of CpMo­(CO)­(P^EtN^MeP^Et)­H and CpMo­(CO)­(P^PhN^MeP^Ph)H in CH₃CN are both irreversible at slow scan rates and quasireversible at higher scan rates, with <i>E</i>_1/2 = −0.36 V (vs Cp₂Fe^+/0) for CpMo­(CO)­(P^EtN^MeP^Et)H and <i>E</i>_1/2 = −0.18 V for CpMo­(CO)­(P^PhN^MeP^Ph)­H. Hydride abstraction from CpMo­(CO)­(PNP)H with [Ph₃C]⁺[A]⁻ (A = B­(C₆F₅)₄ or BAr^F₄; [Ar^F = 3,5-bis­(trifluoromethyl)­phenyl]) afforded “tuck-in” [CpMo­(CO)­(κ³-PNP)]⁺ complexes that feature the amine bound to the metal. Displacement of the κ³ Mo–N bond by CD₃CN gives [CpMo­(CO)­(PNP)­(CD₃CN)]⁺. The kinetics of this reaction were studied by ³¹P­{¹H} NMR spectroscopy for [CpMo­(CO)­(κ³-P^EtN^MeP^Et)]⁺, providing the activation parameters Δ<i>H</i>^⧧ = 21.6 ± 2.8 kcal/mol, Δ<i>S</i>^⧧ = −0.3 ± 9.8 cal/(mol K), <i>E</i>_a = 22.1 ± 2.8 kcal/mol. Protonation of CpMo­(CO)­(P^EtN^MeP^Et)H affords the Mo dihydride complex [CpMo­(CO)­(κ²-P^EtN^MeP^Et)­(H)₂]⁺, which loses H₂ to generate [CpMo­(CO)­(κ³-P^EtN^MeP^Et)]⁺ at room temperature. Our results show that the pendant amine has a strong driving force to form stable “tuck-in” [CpMo­(CO)­(κ³-PNP)]⁺ complexes, and also promotes hydrogen elimination from [CpMo­(CO)­(PNP)­(H)₂]⁺ complexes by formation of a Mo–N dative bond. CpMo­(CO)­(dppp)H (dppp = 1,3-bis­(diphenylphosphino)­propane) was studied as a Mo diphosphine analogue without a pendant amine, and the product of protonation of this complex gives [CpMo­(CO)­(dppp)­(H)₂]⁺

Electrochemical Detection of Transient Cobalt Hydride Intermediates of Electrocatalytic Hydrogen Production

A large variety of molecular cobalt complexes are used as electrocatalysts for H₂ production, but the key cobalt hydride intermediates are frequently difficult to detect and characterize due to their high reactivity. We report that a combination of variable scan rate cyclic voltammetry and foot-of-the-wave analysis (FOWA) can be used to detect transient Co^IIIH and Co^IIH intermediates of electrocatalytic H₂ production by [Co^II(P^<i>t</i>Bu₂N^Ph₂)­(CH₃CN)₃]²⁺ and Co^II(dmgBF₂)₂(CH₃CN)₂. In both cases, reduction of a transient catalytic intermediate occurs at a potential that coincides with the Co^II/I couple. Each reduction displays quasireversible electron-transfer kinetics, consistent with reduction of a Co^IIIH intermediate to Co^IIH, which is then protonated by acid to generate H₂. A bridge-protonated Co^I species was ruled out as a catalytic intermediate for Co^II(dmgBF₂)₂(CH₃CN)₂ from voltammograms recorded at 1000 psi of H₂. Density functional theory was used to calculate Co^III–H and Co^II–H bond strengths for both catalysts. Despite having very different ligands, the cobalt hydrides of both catalysts possess nearly identical heterolytic and homolytic Co–H bond strengths for the Co^IIIH and Co^IIH intermediates

Synthesis and Reactivity of Fe(II) Complexes Containing Cis Ammonia Ligands

The development of earth-abundant transition-metal complexes for electrocatalytic ammonia oxidation is needed to facilitate a renewable energy economy. Important to this goal is a fundamental understanding of how ammonia binds to complexes as a function of ligand geometry and electronic effects. We report the synthesis and characterization of a series of Fe­(II)–NH3 complexes supported by tetradentate, facially binding ligands with a combination of pyridine and N-heterocyclic carbene donors. Electronic modification of the supporting ligand led to significant shifts in the FeIII/II potential and variations in NH bond acidities. Finally, investigations of ammonia oxidation by cyclic voltammetry, controlled potential bulk electrolysis, and through addition of stoichiometric organic radicals, TEMPO and tBu3ArO• are reported. No catalytic oxidation of NH3 to N2 was observed, and 15N2 was detected only in reactions with tBu3ArO•

Direct Determination of Equilibrium Potentials for Hydrogen Oxidation/Production by Open Circuit Potential Measurements in Acetonitrile

Open circuit potentials were measured for acetonitrile solutions of a variety of acids and their conjugate bases under 1 atm H₂. Acids examined were triethylammonium, dimethylformamidium, 2,6-dichloroanilinium, 4-cyanoanilinium, 4-bromoanilinium, and 4-anisidinium salts. These potentials, along with the p<i>K</i>_a values of the acids, establish the value of the standard hydrogen electrode (SHE) potential in acetonitrile as −0.028(4) V vs the ferrocenium/ferrocene couple. Dimethylformamidium forms homoconjugates and other aggregates with dimethylformamide; open circuit potentials (OCPs) were used to quantify the extent of these reactions. Overpotentials for electrocatalytic hydrogen production and oxidation were determined from open circuit potentials and voltammograms of acidic or basic catalyst solutions under H₂. For these solutions, agreement between OCP values and potentials calculated using the Nernst equation is within 12 mV. Use of the measured equilibrium potential allows direct comparison of catalytic systems in different media; it requires neither p<i>K</i>_a values, homoconjugation constants, nor the SHE potential

Direct Determination of Equilibrium Potentials for Hydrogen Oxidation/Production by Open Circuit Potential Measurements in Acetonitrile

Open circuit potentials were measured for acetonitrile solutions of a variety of acids and their conjugate bases under 1 atm H₂. Acids examined were triethylammonium, dimethylformamidium, 2,6-dichloroanilinium, 4-cyanoanilinium, 4-bromoanilinium, and 4-anisidinium salts. These potentials, along with the p<i>K</i>_a values of the acids, establish the value of the standard hydrogen electrode (SHE) potential in acetonitrile as −0.028(4) V vs the ferrocenium/ferrocene couple. Dimethylformamidium forms homoconjugates and other aggregates with dimethylformamide; open circuit potentials (OCPs) were used to quantify the extent of these reactions. Overpotentials for electrocatalytic hydrogen production and oxidation were determined from open circuit potentials and voltammograms of acidic or basic catalyst solutions under H₂. For these solutions, agreement between OCP values and potentials calculated using the Nernst equation is within 12 mV. Use of the measured equilibrium potential allows direct comparison of catalytic systems in different media; it requires neither p<i>K</i>_a values, homoconjugation constants, nor the SHE potential

Hydride Transfer Reactions of Transition Metal Hydrides: Kinetic Hydricity of Metal Carbonyl Hydrides

Hydride transfer from neutral transition metal hydrides (MH) to Ph3C+BF4- gives M-FBF3 and Ph3CH. The rate law −d[Ph3C+BF4-]/dt = k[Ph3C+BF4-][MH] was established from kinetic measurements using stopped-flow methods. Second-order rate constants determined in CH2Cl2 solution at 25 °C range from kH− = 7.2 × 10-1 M-1 s-1 to kH− = 4.6 × 106 M-1 s-1. The order of increasing kinetic hydricity is (C5H4CO2Me)(CO)3WH 5MnH 3CrH 3WH 3 cis-(CO)4(PCy3)MnH cis-(CO)4(PPh3)MnH 5H4Me)(CO)3WH 3MoH 3WH 3WH 5ReH 3MoH cis-(CO)4(PPh3)ReH 2WH trans-Cp(CO)2(PCy3)MoH trans-Cp(CO)2(PPh3)MoH trans-Cp(CO)2(PMe3)MoH (Cp = η5-C5H5, Cp* = η5-C5Me5, Cy = cyclohexyl). Ranges of activation parameters for hydride transfer from trans-Cp(CO)2(PMe3)MoH, trans-Cp(CO)2(PCy3)MoH, cis-(CO)4(PPh3)ReH, and Cp*(CO)3MoH are ΔH⧧ = 3.0−5.9 kcal mol-1 and ΔS⧧ = −18 to −24 cal K-1 mol-1. The rate constant for hydride transfer (kH−) from cis-Cp(CO)2(PCy3)MoH at −55 °C is 3 orders of magnitude lower than that for trans-Cp(CO)2(PCy3)MoH. Phosphine substitution for CO generally enhances the kinetic hydricity, with trans-Cp(CO)2(PMe3)MoH being 104 times as reactive as Cp(CO)3MoH. The electronic effect of phosphine substitution is attenuated by steric factors when the phosphine is cis to the metal hydride. The hydride transfer kinetics reported here are interpreted to be single-step hydride transfers, rather than a multiple-step mechanism involving an initial electron transfer followed by hydrogen atom transfer. A distinction is made between hydricity and nucleophilicity of metal hydrides

Alcohol Complexes of Tungsten Prepared by Ionic Hydrogenations of Ketones

Ionic hydrogenation of acetone by Cp(CO)3WH and HOTf (OTf = OSO2CF3) gives the 2-propanol complex [Cp(CO)3W(HOiPr)]+OTf-. 1H NMR data suggest O−H···O hydrogen bonding between the alcohol OH and an oxygen of the triflate anion in solution, and a crystal structure of this complex shows that hydrogen bonding also exists in the solid state. The short O···O distance of 2.63(1) Å indicates a strong hydrogen bond. Hydrogenation of other ketones and aldehydes gives related [Cp(CO)3W(alcohol)]+OTf- complexes. Aldehydes are selectively hydrogenated over ketones, and alkyl ketones are selectively hydrogenated over aromatic ketones. Hydrogenation of acetophenone gives ethylbenzene, with no intermediate tungsten complexes being observed. Reaction of 1-phenyl-1,3-butanedione with Cp(CO)3WH and HOTf gave {Cp(CO)3W[CH3CH(OH)CH2C(O)Ph]}+OTf -, the structure of which was determined by X-ray diffraction. The alcohol complexes [Cp(CO)3W(alcohol)]+OTf- decompose in solution to give free alcohols and Cp(CO)3WOTf. The cationic dihydride [Cp(CO)2(PMe3)W(H)2]+OTf- hydrogenates aldehydes and ketones; in these reactions a metal hydride serves as both the proton and hydride donor

Catalytic Ionic Hydrogenation of Ketones by {[Cp*Ru(CO)₂]₂(μ-H)}⁺

{[Cp*Ru(CO)2]2(μ-H)}+OTf − functions as a homogeneous catalyst precursor for hydrogenation of ketones to alcohols, with hydrogenations at 1 mol % catalyst loading at 90 °C under H2 (820 psi) proceeding to completion and providing >90% yields. Hydrogenation of methyl levulinate generates γ-valerolactone, presumably by ring-closing of the initially formed alcohol with the methyl ester. Experiments in neat Et2CO show that the catalyst loading can be 2(η2-H2)]+OTf− being formed under the reaction conditions from reaction of H2 with {[Cp*Ru(CO)2]2(μ-H)}+OTf −