Comparison of [Ni(P^Ph₂N^Ph₂)₂(CH₃CN)]²⁺ and [Pd(P^Ph₂N^Ph₂)₂]²⁺ as Electrocatalysts for H₂ Production

The complexes [Ni­(P^Ph₂N^Ph₂)₂(CH₃CN)]²⁺ and [Pd­(P^Ph₂N^Ph₂)₂]²⁺, where P^Ph₂N^Ph₂ is 1,5-diphenyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane, are compared as electrocatalysts for H₂ production under identical experimental conditions. With [(DMF)­H]⁺ as the acid in acetonitrile solution, [Pd­(P^Ph₂N^Ph₂)₂]²⁺ afforded a turnover frequency (TOF) of 230 s^–1 for formation of H₂ under dry conditions and a TOF of 640 s^–1 when H₂O was added. These rates are similar to the TOFs of 590 s^–1 (dry) and 720 s^–1 (wet) that were previously measured for [Ni­(P^Ph₂N^Ph₂)₂(CH₃CN)]²⁺ using [(DMF)­H]⁺. The [Ni­(P^Ph₂N^Ph₂)₂(CH₃CN)]²⁺ and [Pd­(P^Ph₂N^Ph₂)₂]²⁺ complexes both exhibited large current enhancements when treated with trifluoroacetic acid (TFA). At a TFA concentration of 1.8 M, TOF values of 5670 and 2060 s^–1 were measured for [Ni­(P^Ph₂N^Ph₂)₂(CH₃CN)]²⁺ and [Pd­(P^Ph₂N^Ph₂)₂]²⁺, respectively. The fast rates observed using TFA are, in part, attributed to homoconjugation of TFA in acetonitrile solutions, which decreases the effective p<i>K</i>_a^MeCN of the acid. In support of this hypothesis, dramatically lower rates of H₂ production were observed using <i>p</i>-anisidinium, which has a p<i>K</i>_a^MeCN value comparable to that of TFA but does not homoconjugate significantly in acetonitrile solutions

Kinetic Analysis of Competitive Electrocatalytic Pathways: New Insights into Hydrogen Production with Nickel Electrocatalysts

The hydrogen production electrocatalyst Ni­(P^Ph₂N^Ph₂)₂²⁺ (<b>1</b>) is capable of traversing multiple electrocatalytic pathways. When using dimethylformamidium, DMF­(H)⁺, the mechanism of H₂ formation by <b>1</b> changes from an ECEC to an EECC mechanism as the potential approaches the Ni­(I/0) couple. Two electrochemical methods, current–potential analysis and foot-of-the-wave analysis (FOWA), were performed on <b>1</b> to measure detailed kinetics of the competing ECEC and EECC pathways. A sensitivity analysis was performed on the methods using digital simulations to understand their strengths and limitations. Chemical rate constants were significantly underestimated when not accounting for electron-transfer kinetics, even when electron transfer was fast enough to afford a reversible noncatalytic wave. The EECC pathway of <b>1</b> was faster than the ECEC pathway under all conditions studied. Buffered DMF:DMF­(H)⁺ mixtures afforded an increase in the catalytic rate constant (<i>k</i>_obs) of the EECC pathway, but <i>k</i>_obs for the ECEC pathway did not change when using buffered acid. Further kinetic analysis of the ECEC path revealed that base increases the rate of isomerization from exo-protonated Ni(0) isomers to the catalytically active endo-isomers, but decreases the rate of protonation of Ni­(I). FOWA did not provide accurate rate constants, but FOWA was used to estimate the reduction potential of the previously undetected exo-protonated Ni­(I) intermediate. Comparison of catalytic Tafel plots for <b>1</b> under different conditions reveals substantial inaccuracies in the turnover frequency at zero overpotential when the kinetic and thermodynamic effects of the conjugate base are not accounted for properly

Nickel Bis-Diphosphine Complexes: Controlling the Binding and Heterolysis of H₂

Two new Ni­(II) complexes, the homoleptic [Ni­(8P^Cy₂N^H)₂]²⁺ complex (8P^Cy₂N^H = 3,7-cyclohexyl-1-amino-3,7-diphosphacyclooctane) containing two pendant amines on adjacent ligands, and the heteroleptic [Ni­(8P^Cy₂N^H)­(dppe)]²⁺ complex (dppe = bis­(diphenylphosphino)­ethane) containing only a single pendant amine, have been synthesized, and their electrochemical properties are reported. The [Ni­(8P^Cy₂N^H)­(dppe)]²⁺ complex is capable of heterolytically cleaving hydrogen, allowing for the first observation of an endo-protonated nickel hydride related to the [Ni­(P^R₂N^{R^′}₂)₂]²⁺ family of complexes. The [Ni­(8P^Cy₂N^H)­(dppe)]²⁺ complex did not exhibit electrocatalytic H₂ oxidation activity; however, [Ni­(8P^Cy₂N^H)₂]²⁺ is an active electrocatalyst for H₂ oxidation with a maximum turnover frequency (<i>k</i>_obs) of 18 s^–1 under 1 atm H₂ at <i>E</i>_cat/2 = −0.71 V versus the ferrocenium/ferrocene (Cp₂Fe^+/0) couple. In addition to an analysis of the effect of the number of pendant amines on the rates of electrocatalytic H₂ oxidation of compounds related to the [Ni­(P^R₂N^{R^′}₂)₂]²⁺ family, the effect of the secondary pendant amine on intermolecular deprotonation is discussed

Nickel Bis-Diphosphine Complexes: Controlling the Binding and Heterolysis of H₂

Two new Ni­(II) complexes, the homoleptic [Ni­(8P^Cy₂N^H)₂]²⁺ complex (8P^Cy₂N^H = 3,7-cyclohexyl-1-amino-3,7-diphosphacyclooctane) containing two pendant amines on adjacent ligands, and the heteroleptic [Ni­(8P^Cy₂N^H)­(dppe)]²⁺ complex (dppe = bis­(diphenylphosphino)­ethane) containing only a single pendant amine, have been synthesized, and their electrochemical properties are reported. The [Ni­(8P^Cy₂N^H)­(dppe)]²⁺ complex is capable of heterolytically cleaving hydrogen, allowing for the first observation of an endo-protonated nickel hydride related to the [Ni­(P^R₂N^{R^′}₂)₂]²⁺ family of complexes. The [Ni­(8P^Cy₂N^H)­(dppe)]²⁺ complex did not exhibit electrocatalytic H₂ oxidation activity; however, [Ni­(8P^Cy₂N^H)₂]²⁺ is an active electrocatalyst for H₂ oxidation with a maximum turnover frequency (<i>k</i>_obs) of 18 s^–1 under 1 atm H₂ at <i>E</i>_cat/2 = −0.71 V versus the ferrocenium/ferrocene (Cp₂Fe^+/0) couple. In addition to an analysis of the effect of the number of pendant amines on the rates of electrocatalytic H₂ oxidation of compounds related to the [Ni­(P^R₂N^{R^′}₂)₂]²⁺ family, the effect of the secondary pendant amine on intermolecular deprotonation is discussed

Catalytic Oxidation of Alcohol via Nickel Phosphine Complexes with Pendant Amines

Nickel complexes were prepared with diphosphine ligands that contain pendant amines, and these complexes catalytically oxidize primary and secondary alcohols to their respective aldehydes and ketones. Kinetic and mechanistic studies of these prospective electrocatalysts were performed to understand what influences the catalytic activity. For the oxidation of diphenylmethanol, the catalytic rates were determined to be dependent on the concentration of both the catalyst and the alcohol and independent of the concentration of base and oxidant. The incorporation of pendant amines to the phosphine ligand results in substantial increases in the rate of alcohol oxidation with more electron-donating substituents on the pendant amine exhibiting the fastest rates

Catalytic Oxidation of Alcohol via Nickel Phosphine Complexes with Pendant Amines

Nickel complexes were prepared with diphosphine ligands that contain pendant amines, and these complexes catalytically oxidize primary and secondary alcohols to their respective aldehydes and ketones. Kinetic and mechanistic studies of these prospective electrocatalysts were performed to understand what influences the catalytic activity. For the oxidation of diphenylmethanol, the catalytic rates were determined to be dependent on the concentration of both the catalyst and the alcohol and independent of the concentration of base and oxidant. The incorporation of pendant amines to the phosphine ligand results in substantial increases in the rate of alcohol oxidation with more electron-donating substituents on the pendant amine exhibiting the fastest rates

Structural and Spectroscopic Characterization of 17- and 18-Electron Piano-Stool Complexes of Chromium. Thermochemical Analyses of Weak Cr–H Bonds

The 17-electron radical CpCr­(CO)₂(IMe)^• (IMe = 1,3-dimethylimidazol-2-ylidene) was synthesized by the reaction of IMe with [CpCr­(CO)₃]₂, and characterized by single crystal X-ray diffraction and by electron paramagnetic resonance (EPR), IR, and variable temperature ¹H NMR spectroscopy. The metal-centered radical is monomeric under all conditions and exhibits Curie paramagnetic behavior in solution. An electrochemically reversible reduction to 18-electron CpCr­(CO)₂(IMe)⁻ takes place at <i>E</i>_1/2 = −1.89(1) V vs Cp₂Fe^+•/0 in MeCN, and was accomplished chemically with KC₈ in tetrahydrofuran (THF). The salts K⁺(18-crown-6)­[CpCr­(CO)₂(IMe)]⁻·¹/₂THF and K⁺[CpCr­(CO)₂(IMe)]⁻·³/₄THF were crystallographically characterized. Monomeric ion pairs are found in the former, whereas the latter has a polymeric structure because of a network of K···O_(CO) interactions. Protonation of K⁺(18-crown-6)­[CpCr­(CO)₂(IMe)]⁻·¹/₂THF gives the hydride CpCr­(CO)₂(IMe)­H, which could not be isolated, but was characterized in solution; a p<i>K</i>_a of 27.2(4) was determined in MeCN. A thermochemical analysis provides the Cr–H bond dissociation free energy (BDFE) for CpCr­(CO)₂(IMe)H in MeCN solution as 47.3(6) kcal mol^–1. This value is exceptionally low for a transition metal hydride, and implies that the reaction 2 [Cr–H] → 2 [Cr^•] + H₂ is exergonic (Δ<i>G</i> = −9.0(8) kcal mol^–1). This analysis explains the experimental observation that generated solutions of the hydride produce CpCr­(CO)₂(IMe)^• (typically on the time scale of days). By contrast, CpCr­(CO)₂(PCy₃)H has a higher Cr–H BDFE (52.9(4) kcal mol^–1), is more stable with respect to H₂ loss, and is isolable

Electrocatalytic Hydrogen Production by [Ni(7P^Ph₂N^H)₂]²⁺: Removing the Distinction Between Endo- and Exo-Protonation Sites

A new Ni­(II) complex, [Ni­(7P^Ph₂N^H)₂H]³⁺ (7P^Ph₂N^H = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane), has been synthesized, and its electrochemical properties have been reported. The 7P^Ph₂N^H ligand features an NH, ensuring properly positioned protonated amine groups (N–H⁺) for electrocatalysis, regardless of whether protonation occurs exo or endo to the metal center. The compound is an electrocatalyst for H₂ production in the presence of organic acids (p<i>K</i>_a range 10–13 in CH₃CN), with turnover frequencies ranging from 160 to 780 s^–1 at overpotentials between 320 and 470 mV, as measured at the potential of the catalytic wave. In stark contrast to [Ni­(P^Ph₂N^{R^′}₂)₂]²⁺ (P^Ph₂N^{R^′}₂ = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) and other [Ni­(7P^Ph₂N^{R^′})₂]²⁺ complexes, catalytic turnover frequencies for H₂ production by [Ni­(7P^Ph₂N^H)₂]²⁺ do not show catalytic rate enhancement upon the addition of H₂O. This finding supports the assertion that [Ni­(7P^Ph₂N^H)₂]²⁺ eliminates the distinction between the endo- and exo-protonation isomers

Effects of Phosphine–Carbene Substitutions on the Electrochemical and Thermodynamic Properties of Nickel Complexes

Nickel­(II) complexes containing chelating N-heterocyclic carbene–phosphine ligands ([NiL₂]­(BPh₄)₂, for which L = [MeIm­(CH₂)₂PR₂], MeIm = 1-methylimidizolylidene, and R = Ph or Et) have been synthesized for the purpose of studying how this class of ligand affects the electrochemical and thermodynamic properties compared to the nickel bis-diphosphine analogues. The nickel complexes were synthesized and then characterized by X-ray crystallography, electrochemical methods, and thermodynamic studies, including DFT calculations. On the basis of the reduction potentials (<i>E°</i>), substitution of an NHC for one of the phosphines in a diphoshine ligand resulted in negative shifts in potential by 0.6 to 1.2 V relative to the corresponding nickel bis-diphosphine complexes. From computational studies of the nickel hydride complex of the phenyl-substituted phosphine–carbene ligand, the hydride donor ability was determined to improve by 32 kcal/mol relative to the estimated hydride donor ability for the analogous nickel complex of the chelating diphosphine ligand 1,3-bis­(diphenylphosphino)­propane. The free energy for addition of H₂ is presented, and the implications for catalysis are discussed. These quantitative results highlight the substantial effect that NHC ligands can have on the electronic properties of the metal complexes

Electrocatalytic Hydrogen Production by [Ni(7P^Ph₂N^H)₂]²⁺: Removing the Distinction Between Endo- and Exo-Protonation Sites

A new Ni­(II) complex, [Ni­(7P^Ph₂N^H)₂H]³⁺ (7P^Ph₂N^H = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane), has been synthesized, and its electrochemical properties have been reported. The 7P^Ph₂N^H ligand features an NH, ensuring properly positioned protonated amine groups (N–H⁺) for electrocatalysis, regardless of whether protonation occurs exo or endo to the metal center. The compound is an electrocatalyst for H₂ production in the presence of organic acids (p<i>K</i>_a range 10–13 in CH₃CN), with turnover frequencies ranging from 160 to 780 s^–1 at overpotentials between 320 and 470 mV, as measured at the potential of the catalytic wave. In stark contrast to [Ni­(P^Ph₂N^{R^′}₂)₂]²⁺ (P^Ph₂N^{R^′}₂ = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) and other [Ni­(7P^Ph₂N^{R^′})₂]²⁺ complexes, catalytic turnover frequencies for H₂ production by [Ni­(7P^Ph₂N^H)₂]²⁺ do not show catalytic rate enhancement upon the addition of H₂O. This finding supports the assertion that [Ni­(7P^Ph₂N^H)₂]²⁺ eliminates the distinction between the endo- and exo-protonation isomers