Influence of the Density Functional and Basis Set on the Relative Stabilities of Oxygenated Isomers of Diiron Models for the Active Site of [FeFe]-Hydrogenase

A series of different density functional theory (DFT) methodologies (24 functionals) in conjunction with a variety of six different basis sets (BSs) was employed to investigate the relative stabilities in the oxygenated isomers of diiron complexes that mimic the active site of [FeFe]-hydrogenase: (μ-pdt)­[Fe­(CO)₂L]­[Fe­(CO)₂L′] (pdt = propane-1,3-dithiolate; L = L′ = CO (<b>1</b>); L = PPh₃, L′ = CO (<b>2</b>); L = PMe₃, L′ = CO (<b>3</b>); L = L′ = PMe₃ (<b>4</b>). Although the enzyme may have a variety of possible sites for oxygenation, the model complexes would necessarily be oxygenated at either the diiron bridging site (<i><b>μ</b></i>-<b>O</b>) or at a sulfur (<b>SO</b>). Previous DFT studies with both B3LYP and TPSS functionals predicted a more stable <i><b>μ</b></i>-<b>O</b> isomer, whereas only the <b>SO</b> isomer was observed experimentally (<i>J. Am. Chem. Soc</i>. <b>2009</b>, 131, 8296–8307). Here, further calculations reveal that the relative stabilities of the <b>SO</b> and <i><b>μ</b></i>-<b>O</b> isomers are extremely sensitive to the choice of the functional, moderately sensitive to the S basis set, but not to the Fe basis set. The relative free energies [<i>G</i>_solv(<i><b>μ</b></i>-<b>O</b>) – <i>G</i>_solv(<b>SO</b>)] range from +10 to −60 kcal/mol, a range much larger than what would have been expected on the basis of the previous DFT results. Benchmarking of these results against coupled cluster with single and double excitation calculations, which predict that the <b>SO</b> isomer is favored, shows that the best performing functionals are BP86 and PBE0, while B97-D, M05, and SVWN overestimate and B2PLYP, BH&HLYP, BMK, M06-HF, and M06-2X underestimate the energy differences. Most of the variation occurs with the <i><b>μ</b></i>-<b>O</b> isomer and appears to be associated with a functional’s ability to predict the strength of the Fe–Fe bond in the reactant. With respect to the S basis set, it appears that the SO bond is sensitive to the nature of the d polarization functions available on the S atom. The S seems to need a d function more diffuse than the d orbital optimized to provide polarization for the S atom alone; that is, S seems to need a d orbital that has strong overlap with the O atom’s valence 2p. Other basis functions and the relative position of the PR₃ (R = Ph and Me) substituent groups have smaller influences on the free energy differences

Synthesis, Characterization, and Reactivity of Fe Complexes Containing Cyclic Diazadiphosphine Ligands: The Role of the Pendant Base in Heterolytic Cleavage of H₂

The iron complexes CpFe­(P^Ph₂N^Bn₂)Cl (<b>1-Cl</b>), CpFe­(P^Ph₂N^Ph₂)Cl (<b>2-Cl</b>), and CpFe­(P^Ph₂C₅)Cl (<b>3-Cl</b>) (where P^Ph₂N^Bn₂ is 1,5-dibenzyl-1,5-diaza-3,7-diphenyl-3,7-diphosphacyclooctane, P^Ph₂N^Ph₂ is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane, and P^Ph₂C₅ is 1,4-diphenyl-1,4-diphosphacycloheptane) have been synthesized and characterized by NMR spectroscopy, electrochemical studies, and X-ray diffraction. These chloride derivatives are readily converted to the corresponding hydride complexes [CpFe­(P^Ph₂N^Bn₂)H (<b>1-H</b>), CpFe­(P^Ph₂N^Ph₂)H (<b>2-H</b>), CpFe­(P^Ph₂C₅)H (<b>3-H</b>)] and H₂ complexes [CpFe­(P^Ph₂N^Bn₂)­(H₂)]­BAr^F₄, <b>[1-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, (where BAr^F₄ is B­[(3,5-(CF₃)₂C₆H₃)₄]⁻), [CpFe­(P^Ph₂N^Ph₂)­(H₂)]­BAr^F₄, <b>[2-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, and [CpFe­(P^Ph₂C₅)­(H₂)]­BAr^F₄, <b>[3-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, as well as [CpFe­(P^Ph₂N^Bn₂)­(CO)]­BAr^F₄, <b>[1-CO]­Cl</b>. Structural studies are reported for <b>[1-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, <b>1-H</b>, <b>2-H</b>, and <b>[1-CO]­Cl</b>. The conformations adopted by the chelate rings of the P^Ph₂N^Bn₂ ligand in the different complexes are determined by attractive or repulsive interactions between the sixth ligand of these pseudo-octahedral complexes and the pendant N atom of the ring adjacent to the sixth ligand. An example of an attractive interaction is the observation that the distance between the N atom of the pendant amine and the C atom of the coordinated CO ligand for <b>[1-CO]­BAr</b>^<b>F</b>_<b>4</b> is 2.848 Å, considerably shorter than the sum of the van der Waals radii of N and C atoms. Studies of H/D exchange by the complexes <b>[1-H</b>_<b>2</b><b>]</b>^<b>+</b>, <b>[2-H</b>_<b>2</b><b>]</b>^<b>+</b>, and <b>[3-H</b>_<b>2</b><b>]</b>^<b>+</b> carried out using H₂ and D₂ indicate that the relatively rapid H/D exchange observed for <b>[1-H</b>_<b>2</b><b>]</b>^<b>+</b> and <b>[2-H</b>_<b>2</b><b>]</b>^<b>+</b> compared to <b>[3-H</b>_<b>2</b><b>]</b>^<b>+</b> is consistent with intramolecular heterolytic cleavage of H₂ mediated by the pendant amine. Computational studies indicate a low barrier for heterolytic cleavage of H₂. These mononuclear Fe^II dihydrogen complexes containing pendant amines in the ligands mimic crucial features of the distal Fe site of the active site of the [FeFe]-hydrogenase required for H–H bond formation and cleavage

Facile Thermal W–W Bond Homolysis in the N-Heterocyclic Carbene Containing Tungsten Dimer [CpW(CO)₂(IMe)]₂

The thermal W–W bond homolysis in [CpW­(CO)₂(IMe)]₂ (IMe = 1,3-dimethylimidazol-2-ylidene) was investigated and was found to occur to a large extent in comparison to other tungsten dimers such as [CpW­(CO)₃]₂. CpW­(CO)₂(IMe)H was prepared by heating a solution of [IMeH]⁺[CpW­(CO)₂(PMe₃)]⁻, and it exists in solution as a mixture of interconverting cis and trans isomers. The carbene rotation in CpW­(CO)₂(IMe)H was explored by DFT calculations, and low enthalpic barriers (<3.5 kcal mol^–1) are predicted. CpW­(CO)₂(IMe)H has p<i>K</i>_a^MeCN = 31.5(3), and deprotonation with KH gives K⁺[CpW­(CO)₂(IMe)]⁻ (·MeCN). Hydride abstraction from CpW­(CO)₂(IMe)H with Ph₃C⁺PF₆^– in the presence of a coordinating ligand L (MeCN or THF) gives [CpW­(CO)₂(IMe)­(L)]⁺PF₆^–. Electrochemical measurements on the anion [CpW­(CO)₂(IMe)]⁻ in MeCN, together with digital simulations, give an <i>E</i>_1/2 value of −1.54(2) V vs Cp₂Fe^+/0 for the [CpW­(CO)₂(IMe)]^•/– couple. A thermochemical cycle provides the solution bond dissociation free energy of the W–H bond of CpW­(CO)₂(IMe)H as 61.3(6) kcal mol^–1. In the electrochemical oxidation of [CpW­(CO)₂(IMe)]⁻, reversible dimerization of the electrogenerated radical CpW­(CO)₂(IMe)^• occurs, and digital simulation provides kinetic and thermodynamic parameters for the monomer–dimer equilibrium: <i>k</i>_dimerization ≈ 2.5 × 10⁴ M^–1 s^–1, <i>k</i>_homolysis ≈ 0.5 s^–1 (i.e., <i>K</i>_dim ≈ 5 × 10⁴ M^–1). Reduction of [CpW­(CO)₂(IMe)­(MeCN)]⁺PF₆^– with cobaltocene gives the dimer [CpW­(CO)₂(IMe)]₂, which in solution exists as a mixture of anti and gauche rotamers. As expected from the electrochemical experiments, the dimer is in equilibrium with detectable amounts of CpW­(CO)₂(IMe)^•. This species was observed by IR spectroscopy, and its presence in solution is also in accordance with the observed reactivity toward 2,6-di-<i>tert</i>-butyl-1,4-benzoquinone, chloroform, and dihydrogen

Facile Thermal W–W Bond Homolysis in the N-Heterocyclic Carbene Containing Tungsten Dimer [CpW(CO)₂(IMe)]₂

The thermal W–W bond homolysis in [CpW­(CO)₂(IMe)]₂ (IMe = 1,3-dimethylimidazol-2-ylidene) was investigated and was found to occur to a large extent in comparison to other tungsten dimers such as [CpW­(CO)₃]₂. CpW­(CO)₂(IMe)H was prepared by heating a solution of [IMeH]⁺[CpW­(CO)₂(PMe₃)]⁻, and it exists in solution as a mixture of interconverting cis and trans isomers. The carbene rotation in CpW­(CO)₂(IMe)H was explored by DFT calculations, and low enthalpic barriers (<3.5 kcal mol^–1) are predicted. CpW­(CO)₂(IMe)H has p<i>K</i>_a^MeCN = 31.5(3), and deprotonation with KH gives K⁺[CpW­(CO)₂(IMe)]⁻ (·MeCN). Hydride abstraction from CpW­(CO)₂(IMe)H with Ph₃C⁺PF₆^– in the presence of a coordinating ligand L (MeCN or THF) gives [CpW­(CO)₂(IMe)­(L)]⁺PF₆^–. Electrochemical measurements on the anion [CpW­(CO)₂(IMe)]⁻ in MeCN, together with digital simulations, give an <i>E</i>_1/2 value of −1.54(2) V vs Cp₂Fe^+/0 for the [CpW­(CO)₂(IMe)]^•/– couple. A thermochemical cycle provides the solution bond dissociation free energy of the W–H bond of CpW­(CO)₂(IMe)H as 61.3(6) kcal mol^–1. In the electrochemical oxidation of [CpW­(CO)₂(IMe)]⁻, reversible dimerization of the electrogenerated radical CpW­(CO)₂(IMe)^• occurs, and digital simulation provides kinetic and thermodynamic parameters for the monomer–dimer equilibrium: <i>k</i>_dimerization ≈ 2.5 × 10⁴ M^–1 s^–1, <i>k</i>_homolysis ≈ 0.5 s^–1 (i.e., <i>K</i>_dim ≈ 5 × 10⁴ M^–1). Reduction of [CpW­(CO)₂(IMe)­(MeCN)]⁺PF₆^– with cobaltocene gives the dimer [CpW­(CO)₂(IMe)]₂, which in solution exists as a mixture of anti and gauche rotamers. As expected from the electrochemical experiments, the dimer is in equilibrium with detectable amounts of CpW­(CO)₂(IMe)^•. This species was observed by IR spectroscopy, and its presence in solution is also in accordance with the observed reactivity toward 2,6-di-<i>tert</i>-butyl-1,4-benzoquinone, chloroform, and dihydrogen

Iron Complexes Bearing Diphosphine Ligands with Positioned Pendant Amines as Electrocatalysts for the Oxidation of H₂

The synthesis and spectroscopic characterization of Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^Bn₂)­Cl, <b>[3-Cl]</b> (where C₅F₄N is a tetrafluoropyridyl substituent and P^<i>t</i>Bu₂N^Bn₂ = 1,5-dibenzyl-3,7-di­(<i>tert</i>-butyl)-1,5-diaza-3,7-diphosphacyclooctane), are reported. Complex <b>3-Cl</b> and [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^<i>t</i>Bu₂)­Cl], <b>4-Cl</b>, are precursors to intermediates in the catalytic oxidation of H₂, including Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^Bn₂)H <b>(3-H)</b>, Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^<i>t</i>Bu₂)H (<b>4-H)</b>, [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^Bn₂)]­BAr^F₄ (<b>[3]­(BAr</b>^<b>F</b>_<b>4</b>), [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^<i>t</i>Bu₂)]­BAr^F₄ (<b>[4]­(BAr</b>^<b>F</b>_<b>4</b>), [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^Bn₂)­(H₂)]­BAr^F₄ (<b>[3-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>), and [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^<i>t</i>Bu₂<i>H</i>)<i>H</i>]­BAr^F₄ (<b>[4-Fe</b><i><b>H</b></i><b>(N</b><i><b>H</b></i><b>)]­BAr</b>^<b>F</b>_<b>4</b>). All of these complexes were characterized by spectroscopic and electrochemical studies; <b>3-Cl</b>, <b>3-H</b>, and <b>4-Cl</b> were also characterized by single crystal diffraction studies. <b>3-H</b> and <b>4-H</b> are electrocatalysts for H₂ (1.0 atm) oxidation in the presence of an excess of the amine bases <i>N</i>-methylpyrrolidine, Et₃N or ^<i>i</i>Pr₂EtN. Turnover frequencies at 22 °C for <b>3-H</b> and <b>4-H</b> with <i>N</i>-methylpyrrolidine as the base are 2.5 and 0.5 s^–1, and overpotentials at <i>E</i>_cat/2 are 235 and 95 mV, respectively. Studies of individual chemical and electrochemical reactions of the various intermediates provide important insights into the factors governing the overall catalytic activity for H₂ oxidation

Synthesis, Characterization, and Reactivity of Fe Complexes Containing Cyclic Diazadiphosphine Ligands: The Role of the Pendant Base in Heterolytic Cleavage of H₂

The iron complexes CpFe­(P^Ph₂N^Bn₂)Cl (<b>1-Cl</b>), CpFe­(P^Ph₂N^Ph₂)Cl (<b>2-Cl</b>), and CpFe­(P^Ph₂C₅)Cl (<b>3-Cl</b>) (where P^Ph₂N^Bn₂ is 1,5-dibenzyl-1,5-diaza-3,7-diphenyl-3,7-diphosphacyclooctane, P^Ph₂N^Ph₂ is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane, and P^Ph₂C₅ is 1,4-diphenyl-1,4-diphosphacycloheptane) have been synthesized and characterized by NMR spectroscopy, electrochemical studies, and X-ray diffraction. These chloride derivatives are readily converted to the corresponding hydride complexes [CpFe­(P^Ph₂N^Bn₂)H (<b>1-H</b>), CpFe­(P^Ph₂N^Ph₂)H (<b>2-H</b>), CpFe­(P^Ph₂C₅)H (<b>3-H</b>)] and H₂ complexes [CpFe­(P^Ph₂N^Bn₂)­(H₂)]­BAr^F₄, <b>[1-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, (where BAr^F₄ is B­[(3,5-(CF₃)₂C₆H₃)₄]⁻), [CpFe­(P^Ph₂N^Ph₂)­(H₂)]­BAr^F₄, <b>[2-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, and [CpFe­(P^Ph₂C₅)­(H₂)]­BAr^F₄, <b>[3-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, as well as [CpFe­(P^Ph₂N^Bn₂)­(CO)]­BAr^F₄, <b>[1-CO]­Cl</b>. Structural studies are reported for <b>[1-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, <b>1-H</b>, <b>2-H</b>, and <b>[1-CO]­Cl</b>. The conformations adopted by the chelate rings of the P^Ph₂N^Bn₂ ligand in the different complexes are determined by attractive or repulsive interactions between the sixth ligand of these pseudo-octahedral complexes and the pendant N atom of the ring adjacent to the sixth ligand. An example of an attractive interaction is the observation that the distance between the N atom of the pendant amine and the C atom of the coordinated CO ligand for <b>[1-CO]­BAr</b>^<b>F</b>_<b>4</b> is 2.848 Å, considerably shorter than the sum of the van der Waals radii of N and C atoms. Studies of H/D exchange by the complexes <b>[1-H</b>_<b>2</b><b>]</b>^<b>+</b>, <b>[2-H</b>_<b>2</b><b>]</b>^<b>+</b>, and <b>[3-H</b>_<b>2</b><b>]</b>^<b>+</b> carried out using H₂ and D₂ indicate that the relatively rapid H/D exchange observed for <b>[1-H</b>_<b>2</b><b>]</b>^<b>+</b> and <b>[2-H</b>_<b>2</b><b>]</b>^<b>+</b> compared to <b>[3-H</b>_<b>2</b><b>]</b>^<b>+</b> is consistent with intramolecular heterolytic cleavage of H₂ mediated by the pendant amine. Computational studies indicate a low barrier for heterolytic cleavage of H₂. These mononuclear Fe^II dihydrogen complexes containing pendant amines in the ligands mimic crucial features of the distal Fe site of the active site of the [FeFe]-hydrogenase required for H–H bond formation and cleavage

Iron Complexes for the Electrocatalytic Oxidation of Hydrogen: Tuning Primary and Secondary Coordination Spheres

A series of iron hydride complexes featuring P^RN^{R^′}P^R (P^RN^{R^′}P^R = R₂PCH₂N­(R′)­CH₂PR₂ where R = Ph, R′ = Me; R = Et, R′ = Ph, Bn, Me, ^<i>t</i>Bu) and cyclopentadienide (Cp^X = C₅H₄X where X = H, C₅F₄N) ligands has been synthesized; characterized by NMR spectroscopy, X-ray diffraction, and cyclic voltammetry; and examined by quantum chemistry calculations. Each compound was tested for the electrocatalytic oxidation of H₂, and the most active complex, (Cp^C₅F₄N)­Fe­(P^EtN^MeP^Et)­(H), exhibited a turnover frequency of 8.6 s^–1 at 1 atm of H₂ with an overpotential of 0.41 V, as measured at the potential at half of the catalytic current and using N-methylpyrrolidine as the exogenous base to remove protons. Control complexes that do not contain pendant amine groups were also prepared and characterized, but no catalysis was observed. The rate-limiting steps during catalysis are identified through combined experimental and computational studies as the intramolecular deprotonation of the Fe^III hydride by the pendant amine and the subsequent deprotonation by an exogenous base

Cobalt Complexes Containing Pendant Amines in the Second Coordination Sphere as Electrocatalysts for H₂ Production

A series of heteroleptic 17e cobalt complexes, [CpCo^II(P^<i>t</i>Bu₂N^Ph₂)]­(BF₄), [Cp^C6F5Co^II(P^<i>t</i>Bu₂N^Ph₂)]­(BF₄), and [Cp^C5F4NCo^II(P^<i>t</i>Bu₂N^Ph₂)]­(BF₄) (where P^<i>t</i>Bu₂N^Ph₂ = 1,5-diphenyl-3,7-di-<i>tert</i>-butyl-1,5-diaza-3,7-diphosphacyclooctane, Cp^C6F5 = C₅H₄(C₆F₅), and Cp^C5F4N = C₅H₄(C₅F₄N)) were synthesized, and the structures of all three were determined by X-ray crystallography. Electrochemical studies showed that the Co^III/II couple of [Cp^C5F4NCo^II(P^<i>t</i>Bu₂N^Ph₂)]⁺ appears 250 mV positive of the Co^III/II couple of [CpCo^II(P^<i>t</i>Bu₂N^Ph₂)]⁺ as a result of the strongly electron withdrawing perfluoropyridyl substituent on the Cp ring. Reduction of these paramagnetic Co^II complexes by KC₈ led to the diamagnetic 18e complexes CpCo^I(P^<i>t</i>Bu₂N^Ph₂), Cp^C6F5Co^I(P^<i>t</i>Bu₂N^Ph₂), and Cp^C5F4NCo^I(P^<i>t</i>Bu₂N^Ph₂), which were also characterized by crystallography. Protonation of these neutral Co^I complexes led to the Co^III hydrides [CpCo^III(P^<i>t</i>Bu₂N^Ph₂)­H]­(BF₄), [Cp^C6F5Co^III(P^<i>t</i>Bu₂N^Ph₂)­H]­(BF₄), and [Cp^C5F4NCo^III(P^<i>t</i>Bu₂N^Ph₂)­H]­(BF₄), and crystal structures of each of these cobalt hydrides were determined. The cobalt complex with the most electron withdrawing Cp ligand, [Cp^C5F4NCo^II(P^<i>t</i>Bu₂N^Ph₂)]⁺, is an electrocatalyst for production of H₂ using [<i>p</i>-MeOC₆H₄NH₃]­[BF₄] (p<i>K</i>_a^MeCN = 11.86), with a turnover frequency of 350 s^–1 and an overpotential of 0.86 V at <i>E</i>_cat/2. A p<i>K</i>_a value of 15.6 was measured in CH₃CN for [Cp^C5F4NCo^III(P^<i>t</i>Bu₂N^Ph₂)­H], which was used in conjunction with electrochemical measurements to obtain thermodynamic data for cleavage of the Co–H bond

Highly Reversible Mg Insertion in Nanostructured Bi for Mg Ion Batteries

Rechargeable magnesium batteries have attracted wide attention for energy storage. Currently, most studies focus on Mg metal as the anode, but this approach is still limited by the properties of the electrolyte and poor control of the Mg plating/stripping processes. This paper reports the synthesis and application of Bi nanotubes as a high-performance anode material for rechargeable Mg ion batteries. The nanostructured Bi anode delivers a high reversible specific capacity (350 mAh/g_Bi or 3430 mAh/cm³_Bi), excellent stability, and high Coulombic efficiency (95% initial and very close to 100% afterward). The good performance is attributed to the unique properties of in situ formed, interconnected nanoporous bismuth. Such nanostructures can effectively accommodate the large volume change without losing electric contact and significantly reduce diffusion length for Mg²⁺. Significantly, the nanostructured Bi anode can be used with conventional electrolytes which will open new opportunities to study Mg ion battery chemistry and further improve its properties

Two Pathways for Electrocatalytic Oxidation of Hydrogen by a Nickel Bis(diphosphine) Complex with Pendant Amines in the Second Coordination Sphere

A nickel bis­(diphosphine) complex containing pendant amines in the second coordination sphere, [Ni­(P^Cy₂N^{<i>t‑</i>Bu}₂)₂]­(BF₄)₂ (P^Cy₂N^{<i>t‑</i>Bu}₂ = 1,5-di­(<i>tert</i>-butyl)-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane), is an electrocatalyst for hydrogen oxidation. The addition of hydrogen to the Ni^II complex gives three isomers of the doubly protonated Ni⁰ complex [Ni­(P^Cy₂N^{<i>t‑</i>Bu}₂H)₂]­(BF₄)₂. Using the p<i>K</i>_a values and Ni^II/I and Ni^I/0 redox potentials in a thermochemical cycle, the free energy of hydrogen addition to [Ni­(P^Cy₂N^{<i>t</i>‑Bu}₂)₂]²⁺ was determined to be −7.9 kcal mol^–1. The catalytic rate observed in dry acetonitrile for the oxidation of H₂ depends on base size, with larger bases (NEt₃, <i>t</i>-BuNH₂) resulting in much slower catalysis than <i>n</i>-BuNH₂. The addition of water accelerates the rate of catalysis by facilitating deprotonation of the hydrogen addition product before oxidation, especially for the larger bases NEt₃ and <i>t</i>-BuNH₂. This catalytic pathway, where deprotonation occurs prior to oxidation, leads to an overpotential that is 0.38 V lower compared to the pathway where oxidation precedes proton movement. Under the optimal conditions of 1.0 atm H₂ using <i>n</i>-BuNH₂ as a base and with added water, a turnover frequency of 58 s^–1 is observed at 23 °C