Jenny Y. Yang

Moving Protons with Pendant Amines: Proton Mobility in a Nickel Catalyst for Oxidation of Hydrogen

Proton transport is ubiquitous in chemical and biological processes, including the reduction of dioxygen to water, the reduction of CO(2) to formate, and the production/oxidation of hydrogen. In this work we describe intramolecular proton transfer between Ni and positioned pendant amines for the hydrogen oxidation electrocatalyst [Ni(P(Cy)(2)N(Bn)(2)H)(2)](2+) (P(Cy)(2)N(Bn)(2) = 1,5-dibenzyl-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane). Rate constants are determined by variable-temperature one-dimensional NMR techniques and two-dimensional EXSY experiments. Computational studies provide insight into the details of the proton movement and energetics of these complexes. Intramolecular proton exchange processes are observed for two of the three experimentally observable isomers of the doubly protonated Ni(0) complex, [Ni(P(Cy)(2)N(Bn)(2)H)(2)](2+), which have N-H bonds but no Ni-H bonds. For these two isomers, with pendant amines positioned endo to the Ni, the rate constants for proton exchange range from 10(4) to 10(5) s(-1) at 25 °C, depending on isomer and solvent. No exchange is observed for protons on pendant amines positioned exo to the Ni. Analysis of the exchange as a function of temperature provides a barrier for proton exchange of ΔG(‡) = 11-12 kcal/mol for both isomers, with little dependence on solvent. Density functional theory calculations and molecular dynamics simulations support the experimental observations, suggesting metal-mediated intramolecular proton transfers between nitrogen atoms, with chair-to-boat isomerizations as the rate-limiting steps. Because of the fast rate of proton movement, this catalyst may be considered a metal center surrounded by a cloud of exchanging protons. The high intramolecular proton mobility provides information directly pertinent to the ability of pendant amines to accelerate proton transfers during catalysis of hydrogen oxidation. These results may also have broader implications for proton movement in homogeneous catalysts and enzymes in general, with specific implications for the proton channel in the Ni-Fe hydrogenase enzyme.

Mechanistic Insights into Catalytic H2 Oxidation by Ni Complexes Containing a Diphosphine Ligand with a Positioned Amine Base

The mixed-ligand complex [Ni(dppp)(P(Ph)(2)N(Bz)(2))](BF(4))(2), 3, (where P(Ph)(2)N(Bz)(2) is 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane and dppp is 1,3-bis(diphenylphosphino)propane) has been synthesized. Treatment of this complex with H(2) and triethylamine results in the formation of the Ni(0) complex, Ni(dppp)(P(Ph)(2)N(Bz)(2)), 4, whose structure has been determined by a single-crystal X-ray diffraction study. Heterolytic cleavage of H(2) by 3 at room temperature forms [HNi(dppp)(P(Ph)(2)N(Bz)(mu-H)N(Bz))](BF(4))(2), 5a, in which one proton interacts with two nitrogen atoms of the cyclic diphosphine ligand and a hydride ligand is bound to nickel. Two intermediates are observed for this reaction using low-temperature NMR spectroscopy. One species is a dihydride, [(H)(2)Ni(dppp)(P(Ph)(2)N(Bz)(2))](BF(4))(2), 5b, and the other is [Ni(dppp)(P(Ph)(2)N(Bz)(2)H(2))](BF(4))(2), 5c, in which both protons are bound to the N atoms in an endo geometry with respect to nickel. These two species interconvert via a rapid and reversible intramolecular proton exchange between nickel and the nitrogen atoms of the diphosphine ligand. Complex 3 is a catalyst for the electrochemical oxidation of H(2) in the presence of base, and new insights into the mechanism derived from low-temperature NMR and thermodynamic studies are presented. A comparison of the rate and thermodynamics of H(2) addition for this complex to related catalysts studied previously indicates that for Ni(II) complexes containing two diphosphine ligands, the activation of H(2) is favored by the presence of two positioned pendant bases.

Reversible Electrocatalytic Production and Oxidation of Hydrogen at Low Overpotentials by a Functional Hydrogenase Mimic

An efficient ligand combination: A new bis(diphosphine) nickel(II) complex (see picture) is described. A ΔG° value of 0.84 kcal mol^(−1) for hydrogen addition for this complex was calculated from the experimentally determined equilibrium constant. This complex displayed reversible electrocatalytic activity for hydrogen production and oxidation at low overpotentials, which are characteristic for hydrogenase enzymes.

Hydrogen oxidation catalysis by a nickel diphosphine complex with pendant tert-butyl amines

A bis-diphosphine nickel complex with tert-butyl functionalized pendant amines [Ni(P(Cy)(2)N(t-Bu)(2))(2)](2+) has been synthesized. It is a highly active electrocatalyst for the oxidation of hydrogen in the presence of base. The turnover rate of 50 s(-1) under 1.0 atm H(2) at a potential of -0.77 V vs. the ferrocene couple is 5 times faster than the rate reported heretofore for any other synthetic molecular H(2) oxidation catalyst.

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(Cy)2N(t-Bu)2)2](BF4)2 (P(Cy)2N(t-Bu)2 = 1,5-di(tert-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 Ni(0) complex [Ni(P(Cy)2N(t-Bu)2H)2](BF4)2. Using the pKa values and Ni(II/I) and Ni(I/0) redox potentials in a thermochemical cycle, the free energy of hydrogen addition to [Ni(P(Cy)2N(t-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, t-BuNH2) resulting in much slower catalysis than n-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 t-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 n-BuNH2 as a base and with added water, a turnover frequency of 58 s(-1) is observed at 23 °C.

Reduction of oxygen catalyzed by nickel diphosphine complexes with positioned pendant amines

Nickel(II) bis(diphosphine) complexes that contain positioned bases in the second coordination sphere have been found to catalyze the reduction of O(2) with H(2) to selectively form water. The complexes also serve as electrocatalysts for the reduction of O(2) with the addition of a weak acid. In contrast, a closely related nickel diphosphine complex without the positioned bases is catalytically inactive for O(2) reduction. These results indicate that pendant bases in synthetic catalysts for O(2) reduction can play a similar role to proton relays in enzymes, and that such relays should be considered in the design of catalysts for multi-electron and multi-proton reactions.

Distant protonated pyridine groups in water-soluble iron porphyrin electrocatalysts promote selective oxygen reduction to water

Fe(III)-meso-tetra(pyridyl)porphyrins are electrocatalysts for the reduction of dioxygen in aqueous acidic solution. The 2-pyridyl derivatives, both the triflate and chloride salts, are more selective for the desired 4e(-) reduction than the isomeric 4-pyridyl complexes. The inward-pointing pyridinium groups influence proton delivery despite their distance from the iron centre.

Solvation Effects on Transition Metal Hydricity

The free energy of hydride donation (hydricity) for [HNi(DHMPE)2][BF4] (DHMPE = 1,2-bis(dihydroxymethylphosphino)ethane was experimentally determined versus the heterolytic cleavage energy of hydrogen in acetonitrile, dimethyl sulfoxide, and water to be 57.4, 55.5, and 30.0 kcal/mol, respectively. This work represents the first reported hydricity values for a transition metal hydride donor in three different solvents. A comparison between our values and the hydricity of hydrogen and formate reveals a narrowing in the range of values with increasing solvent polarity. The thermochemical values also reveal solvation effects that impact the overall thermodynamic favorability of hydride generation from hydrogen and transfer to carbon dioxide. The quantitative solvation effects described herein have important consequences to the design and reactivity of catalysts for transformations that have hydride transfer steps throughout synthetic chemistry.

Reactivity of a Series of Isostructural Cobalt Pincer Complexes with CO2, CO, and H+

The preparation and characterization of a series of isostructural cobalt complexes [Co(t-Bu)2P(E)Py(E)P(t-Bu)2(CH3CN)2][BF4]2 (Py = pyridine, E = CH2, NH, O, and X = BF4 (1a-c)) and the corresponding one-electron reduced analogues [Co(t-Bu)2P(E)Py(E)P(t-Bu)2(CH3CN)2][BF4]2 (2a-c) are reported. The reactivity of the reduced cobalt complexes with CO2, CO, and H(+) to generate intermediates in a CO2 to CO and H2O reduction cycle are described. The reduction of 1a-c and subsequent reactivity with CO2 was investigated by cyclic voltammetry, and for 1a also by infrared spectroelectrochemistry. The corresponding CO complexes of (2a-c) were prepared, and the Co-CO bond strengths were characterized by IR spectroscopy. Quantum mechanical methods (B3LYP-d3 with solvation) were used to characterize the competitive reactivity of the reduced cobalt centers with H(+) versus CO2. By investigating a series of isostructural complexes, correlations in reactivity with ligand electron withdrawing effects are made.

Spin-state diversity in a series of Co(ii) PNP pincer bromide complexes.

We describe the structural and electronic impacts of modifying the bridging atom in a family of Co(ii) pincer complexes with the formula Co(t-Bu)2PEPyEP(t-Bu)2Br2 (Py = pyridine, E = CH2, NH, and O for compounds 1-3, respectively). Structural characterization by single crystal X-ray diffraction indicates that compounds 1 and 3 are 5-coordinate complexes with both bromides bound to the Co(ii) ion, while compound 2 is square planar with one bromide in the outer coordination sphere. The reduction potentials of 1-3, characterized by cyclic voltammetry, are consistent with the increasing electron-withdrawing character of the pincer ligand as the linker (E) between the pyridine and phosphine arms becomes more electronegative. Magnetic property studies of compounds 1 and 2 confirm high- and low-spin behavior, respectively, through a broad temperature range. However, complex 3 features an unusual combination of high spin S = 3/2 Co(ii) and temperature dependent spin-crossover between S = 3/2 and S = 1/2 states. The different magnetic behaviors observed among the three CoBr2 pincer complexes reflects the importance of small ligand perturbations on overall coordination geometry and resulting spin state properties.