M. Rakowski DuBois

A Synthetic Nickel Electrocatalyst with a Turnover Frequency Above 100,000 s−1 for H2 Production

Precisely shaped basic ligands protect highly reactive protons and electrons to help accelerate catalytic hydrogen formation. Reduction of acids to molecular hydrogen as a means of storing energy is catalyzed by platinum, but its low abundance and high cost are problematic. Precisely controlled delivery of protons is critical in hydrogenase enzymes in nature that catalyze hydrogen (H2) production using earth-abundant metals (iron and nickel). Here, we report that a synthetic nickel complex, [Ni(PPh2NPh)2](BF4)2, (PPh2NPh = 1,3,6-triphenyl-1-aza-3,6-diphosphacycloheptane), catalyzes the production of H2 using protonated dimethylformamide as the proton source, with turnover frequencies of 33,000 per second (s−1) in dry acetonitrile and 106,000 s−1 in the presence of 1.2 M of water, at a potential of –1.13 volt (versus the ferrocenium/ferrocene couple). The mechanistic implications of these remarkably fast catalysts point to a key role of pendant amines that function as proton relays.

Development of Molecular Electrocatalysts for CO2 Reduction and H2 Production/Oxidation

The conversion of solar energy to fuels in both natural and artificial photosynthesis requires components for both light-harvesting and catalysis. The light-harvesting component generates the electrochemical potentials required to drive fuel-generating reactions that would otherwise be thermodynamically uphill. This Account focuses on work from our laboratories on developing molecular electrocatalysts for CO(2) reduction and for hydrogen production. A true analog of natural photosynthesis will require the ability to capture CO(2) from the atmosphere and reduce it to a useful fuel. Work in our laboratories has focused on both aspects of this problem. Organic compounds such as quinones and inorganic metal complexes can serve as redox-active CO(2) carriers for concentrating CO(2). We have developed catalysts for CO(2) reduction to form CO based on a [Pd(triphosphine)(solvent)](2+) platform. Catalytic activity requires the presence of a weakly coordinating solvent molecule that can dissociate during the catalytic cycle and provide a vacant coordination site for binding water and assisting C-O bond cleavage. Structures of [NiFe] CO dehydrogenase enzymes and the results of studies on complexes containing two [Pd(triphosphine)(solvent)](2+) units suggest that participation of a second metal in CO(2) binding may also be required for achieving very active catalysts. We also describe molecular electrocatalysts for H(2) production and oxidation based on [Ni(diphosphine)(2)](2+) complexes. Similar to palladium CO(2) reduction catalysts, these species require the optimization of both first and second coordination spheres. In this case, we use structural features of the first coordination sphere to optimize the hydride acceptor ability of nickel needed to achieve heterolytic cleavage of H(2). We use the second coordination sphere to incorporate pendant bases that assist in a number of important functions including H(2) binding, H(2) cleavage, and the transfer of protons between nickel and solution. These pendant bases, or proton relays, are likely to be important in the design of catalysts for a wide range of fuel production and fuel utilization reactions involving multiple electron and proton transfer steps. The generation of fuels from abundant substrates such as CO(2) and water remains a daunting research challenge, requiring significant advances in new inexpensive materials for light harvesting and the development of fast, stable, and efficient electrocatalysts. Although we describe progress in the development of redox-active carriers capable of concentrating CO(2) and molecular electrocatalysts for CO(2) reduction, hydrogen production, and hydrogen oxidation, much more remains to be done.

[Ni(PPh2NC6H4X2)2]2+ Complexes as Electrocatalysts for H2 Production: Effect of Substituents, Acids, and Water on Catalytic Rates

A series of mononuclear nickel(II) bis(diphosphine) complexes [Ni(P(Ph)(2)N(C6H4X)(2))(2)](BF(4))(2) (P(Ph)(2)N(C6H4X)(2) = 1,5-di(para-X-phenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane; X = OMe, Me, CH(2)P(O)(OEt)(2), Br, and CF(3)) have been synthesized and characterized. X-ray diffraction studies reveal that [Ni(P(Ph)(2)N(C6H4Me)(2))(2)](BF(4))(2) and [Ni(P(Ph)(2)N(C6H4OMe)(2))(2)](BF(4))(2) are tetracoordinate with distorted square planar geometries. The Ni(II/I) and Ni(I/0) redox couples of each complex are electrochemically reversible in acetonitrile with potentials that are increasingly cathodic as the electron-donating character of X is increased. Each of these complexes is an efficient electrocatalyst for hydrogen production at the potential of the Ni(II/I) couple. The catalytic rates generally increase as the electron-donating character of X is decreased, and this electronic effect results in the favorable but unusual situation of obtaining higher catalytic rates as overpotentials are decreased. Catalytic studies using acids with a range of pK(a) values reveal that turnover frequencies do not correlate with substrate acid pK(a) values but are highly dependent on the acid structure, with this effect being related to substrate size. Addition of water is shown to dramatically increase catalytic rates for all catalysts. With [Ni(P(Ph)(2)N(C6H4CH2P(O)(OEt)2)(2))(2)](BF(4))(2) using [(DMF)H](+)OTf(-) as the acid and with added water, a turnover frequency of 1850 s(-1) was obtained.

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.

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.

Free Energy Landscapes for S−H Bonds in Cp*2Mo2S4 Complexes

An extensive family of thermochemical data is presented for a series of complexes derived from Cp*Mo(mu-S)(2)(mu-SMe)(mu-SH)MoCp* and Cp*Mo(mu-S)(2)(mu-SH)(2)MoCp*. These data include electrochemical potentials, pK(a) values, homolytic solution bond dissociation free energies (SBDFEs), and hydride donor abilities in acetonitrile. Thermochemical data ranged from +0.6 to -2.0 V vs FeCp(2)(+/o) for electrochemical potentials, 5 to 31 for pK(a) values, 43 to 68 kcal/mol for homolytic SBDFEs, and 44 to 84 kcal/mol for hydride donor abilities. The observed values for these thermodynamic parameters are comparable to those of many transition metal hydrides, which is consistent with the many parallels in the chemistry of these two classes of compounds. The extensive set of thermochemical data is presented in free energy landscapes as a useful approach to visualizing and understanding the relative stabilities of all of the species under varying conditions of pH and H(2) overpressure. In addition to the previously studied homogeneous reactivity and catalysis, Mo(2)S(4) complexes are also models for heterogeneous molybdenum sulfide catalysts, and therefore, the present results demonstrate the dramatic range of S-H bond strengths available in both homogeneous and heterogeneous reaction pathways.

Formation and Reactivity of a Persistent Radical in a Dinuclear Molybdenum Complex

The reactivity of the S-H bond in Cp*Mo(mu-S) 2(mu-SMe)(mu-SH)MoCp* ( S 4 MeH) has been explored by determination of kinetics of hydrogen atom abstraction to form the radical Cp*Mo(mu-S) 3(mu-SMe)MoCp* ( S 4 Me*), as well as reaction of hydrogen with the radical-dimer equilibrium to reform the S-H complex. From the temperature dependent rate data for the abstraction of hydrogen atom by benzyl radical, Delta H (double dagger) and Delta S (double dagger) were determined to be 1.54 +/- 0.25 kcal/mol and -25.5 +/- 0.8 cal/mol K, respectively, giving k abs = 1.3 x 10 (6) M (-1) s (-1) at 25 degrees C. In steady state abstraction kinetic experiments, the exclusive radical termination product of the Mo 2S 4 core was found to be the benzyl cross-termination product, Cp*Mo(mu-S) 2(mu-SMe)(mu-SBz)MoCp* ( S 4 MeBz), consistent with the Fischer-Ingold persistent radical effect. S 4 Me* was found to reversibly dimerize by formation of a weak bridging disulfide bond to form the tetranuclear complex (Cp*Mo(mu-S) 2(mu-SMe)MoCp*) 2(mu-S 2) ( ( S 4 Me) 2 ). The radical-dimer equilibrium constant has been determined to be 5.7 x 10 (4) +/- 2.1 x 10 (4) M (-1) from EPR data. The rate constant for dissociation of the dimer was found to be 1.1 x 10 (3) s (-1) at 25 degrees C, based on variable temperature (1)H NMR data. The rate constant for dimerization of the radical has been estimated to be 6.5 x 10 (7) M (-1) s (-1) in toluene at room temperature, based on the dimer dissociation rate constant and the equilibrium constant for dimerization. Structures are presented for ( S 4 Me) 2 , S 4 MeBz, and the cationic Cp*Mo(mu-S 2)(mu-S)(mu-SMe)MoCp*(OTf) ( S 4 Me ( + )), a precursor of the radical and the alkylated derivatives. Evidence for a radical addition/elimination pathway at an Mo 2S 4 core is presented.