Oxidation of Ammonia Catalyzed by a Molecular Iron Complex: Translating Chemical Catalysis to Mediated Electrocatalysis.

Ammonia is a promising candidate in the quest for sustainable, clean energy. With its capacity to serve as an energy carrier, the oxidation of ammonia opens avenues for carbon-neutral approaches to address worldwide growing energy needs. We report the catalytic chemical oxidation of ammonia by an Earth-abundant transition metal complex, trans-[LFe(MeCN)][PF], where L is a macrocyclic ligand bearing four N-heterocyclic carbene (NHC) donors.

Molecular Catalysts with Diphosphine Ligands Containing Pendant Amines.

Pendant amines play an invaluable role in chemical reactivity, especially for molecular catalysts based on earth-abundant metals. As inspired by [FeFe]-hydrogenases, which contain a pendant amine positioned for cooperative bifunctionality, synthetic catalysts have been developed to emulate this multifunctionality through incorporation of a pendant amine in the second coordination sphere. Cyclic diphosphine ligands containing two amines serve as the basis for a class of catalysts that have been extensively studied and used to demonstrate the impact of a pendant base.

Designing Catalytic Systems Using Binary Solvent Mixtures: Impact of Mole Fraction of Water on Hydride Transfer.

The free energy for hydride transfer reactions of transition metal hydrides is known to be influenced by solvent effects. The first-row transition metal hydride [HNi(dmpe)][BF] (dmpe = 1,2-bis(dimethylphosphino)ethane) has starkly different hydride transfer reactivities with CO in different solvents. A binary mixture of water and acetonitrile was used to tune the hydride transfer reactivity of HNi(dmpe) with CO so that the free energy for this reaction approached zero.

Computational Study of Triphosphine-Ligated Cu(I) Catalysts for Hydrogenation of CO to Formate.

The catalyzed hydrogenation of CO to formate via a triphosphine-ligated Cu(I) was studied computationally at the density functional theory level in the presence of a self-consistent reaction field. Of the four functionals benchmarked, M06 was generally in the best agreement with the available experimentally estimated values. Two bases, DBU and TBD, were studied in the context of two proposed mechanisms in the MeCN solvent.

Oxidation of Ammonia with Molecular Complexes.

Oxidation of ammonia by molecular complexes is a burgeoning area of research, with critical scientific challenges that must be addressed. A fundamental understanding of individual reaction steps is needed, particularly for cleavage of N-H bonds and formation of N-N bonds. This Perspective evaluates the challenges of designing molecular catalysts for oxidation of ammonia and highlights recent key contributions to realizing the goals of viable energy storage and retrieval based on the N-H bonds of ammonia in a carbon-free energy cycle.

The H•/H Redox Couple and Absolute Hydration Energy of H.

A supermolecule-continuum approach with water clusters up to = 16 HO molecules has been used to predict the absolute hydration free energies at 298 K (Δ) of both hydrogen (H•) and hydride (H) to be 4.6 ± 1 and -78 ± 3 kcal/mol, respectively. These values are combined with a high accuracy prediction of the gas-phase electron affinity (Δ = -16.

Thermodynamic and kinetic studies of H and N binding to bimetallic nickel-group 13 complexes and neutron structure of a Ni(η-H) adduct.

Understanding H binding and activation is important in the context of designing transition metal catalysts for many processes, including hydrogenation and the interconversion of H with protons and electrons. This work reports the first thermodynamic and kinetic H binding studies for an isostructural series of first-row metal complexes: NiML, where M = Al (), Ga (), and In (), and L = [N(-(NCHPPr)CH)]. Thermodynamic free energies (Δ°) and free energies of activation (Δ ) for binding equilibria were obtained variable-temperature P NMR studies and lineshape analysis.

Changing the Mechanism for CO Hydrogenation Using Solvent-Dependent Thermodynamics.

A critical scientific challenge for utilization of CO is the development of catalyst systems that function in water and use inexpensive and environmentally friendly reagents. We have used thermodynamic insights to predict and demonstrate that the HCo (dmpe) catalyst system, previously described for use in organic solvents, can hydrogenate CO to formate in water with bicarbonate as the only added reagent. Replacing tetrahydrofuran as the solvent with water changes the mechanism for catalysis by altering the thermodynamics for hydride transfer to CO from a key dihydride intermediate.

A Bimetallic Nickel-Gallium Complex Catalyzes CO Hydrogenation via the Intermediacy of an Anionic d Nickel Hydride.

Large-scale CO hydrogenation could offer a renewable stream of industrially important C chemicals while reducing CO emissions. Critical to this opportunity is the requirement for inexpensive catalysts based on earth-abundant metals instead of precious metals. We report a nickel-gallium complex featuring a Ni(0)→Ga(III) bond that shows remarkable catalytic activity for hydrogenating CO to formate at ambient temperature (3150 turnovers, turnover frequency = 9700 h), compared with prior homogeneous Ni-centered catalysts.

Surface Immobilization of Molecular Electrocatalysts for Energy Conversion.

Electrocatalysts are critically important for a secure energy future, as they facilitate the conversion between electrical and chemical energy. Molecular catalysts offer precise control of structure that enables understanding of structure-reactivity relationships, which can be difficult to achieve with heterogeneous catalysts. Molecular electrocatalysts can be immobilized on surfaces by covalent bonds or through non-covalent interactions.

Controlling Proton Delivery through Catalyst Structural Dynamics.

The fastest synthetic molecular catalysts for H production and oxidation emulate components of the active site of hydrogenases. The critical role of controlled structural dynamics is recognized for many enzymes, including hydrogenases, but is largely neglected in designing synthetic catalysts. Our results demonstrate the impact of controlling structural dynamics on H production rates for [Ni(P N ) ] catalysts (R=n-hexyl, n-decyl, n-tetradecyl, n-octadecyl, phenyl, or cyclohexyl).

Thermodynamic Hydricity of Transition Metal Hydrides.

Transition metal hydrides play a critical role in stoichiometric and catalytic transformations. Knowledge of free energies for cleaving metal hydride bonds enables the prediction of chemical reactivity, such as for the bond-forming and bond-breaking events that occur in a catalytic reaction. Thermodynamic hydricity is the free energy required to cleave an M-H bond to generate a hydride ion (H(-)).

Triphosphine-Ligated Copper Hydrides for CO2 Hydrogenation: Structure, Reactivity, and Thermodynamic Studies.

The copper(I) triphosphine complex LCu(MeCN)PF6 (L = 1,1,1-tris(diphenylphosphinomethyl)ethane), which we recently demonstrated is an active catalyst precursor for hydrogenation of CO2 to formate, reacts with H2 in the presence of a base to form a cationic dicopper hydride, [(LCu)2H]PF6. [(LCu)2H](+) is also an active precursor for catalytic CO2 hydrogenation, with equivalent activity to that of LCu(MeCN)(+), and therefore may be a relevant catalytic intermediate. The thermodynamic hydricity of [(LCu)2H](+) was determined to be 41.

Facile P-C/C-H Bond-Cleavage Reactivity of Nickel Bis(diphosphine) Complexes.

Unusual cleavage of P-C and C-H bonds of the P2 N2 ligand, in heteroleptic [Ni(P2 N2 )(diphosphine)](2+) complexes under mild conditions, results in the formation of an iminium formyl nickelate featuring a C,P,P-tridentate coordination mode. The structures of both the heteroleptic [Ni(P2 N2 )(diphosphine)](2+) complexes and the resulting iminium formyl nickelate have been characterized by NMR spectroscopy and single-crystal X-ray diffraction analysis. Density functional theory (DFT) calculations were employed to investigate the mechanism of the P-C/C-H bond cleavage, which involves C-H bond cleavage, hydride rotation, Ni-C/P-H bond formation, and P-C bond cleavage.

Solvent influence on the thermodynamics for hydride transfer from bis(diphosphine) complexes of nickel.

The thermodynamic hydricity of a metal hydride can vary considerably between solvents. This parameter can be used to determine the favourability of a hydride-transfer reaction, such as the reaction between a metal hydride and CO2 to produce formate. Because the hydricities of these species do not vary consistently between solvents, reactions that are thermodynamically unfavourable in one solvent can be favourable in others.

Standard Reduction Potentials for Oxygen and Carbon Dioxide Couples in Acetonitrile and N,N-Dimethylformamide.

A variety of next-generation energy processes utilize the electrochemical interconversions of dioxygen and water as the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Reported here are the first estimates of the standard reduction potential of the O2 + 4e(-) + 4H(+) ⇋ 2H2O couple in organic solvents. The values are +1.

Nickel phosphine catalysts with pendant amines for electrocatalytic oxidation of alcohols.

Nickel phosphine complexes with pendant amines have been found to be electrocatalysts for the oxidation of primary and secondary alcohols, with turnover frequencies as high as 3.3 s(-1). These complexes are the first electrocatalysts for alcohol oxidation based on non-precious metals, which will be critical for use in fuel cells.

Predicting the reactivity of hydride donors in water: thermodynamic constants for hydrogen.

The chemical reactivity of hydride complexes can be predicted using bond strengths for homolytic and heterolytic cleavage of bonds to hydrogen. To determine these bond strengths, thermodynamic constants describing the stability of H(+), H˙, H(-), and H2 are essential and need to be used uniformly to enable the prediction of reactivity and equilibria. Due to discrepancies in the literature for the constants used in water, we propose the use of a set of self-consistent constants with convenient standard states.

Nickel complexes of a binucleating ligand derived from an SCS pincer.

A binucleating ligand has been prepared that contains an SCS pincer and three oxygen donor atoms in a partial crown ether loop. To enable metalation with Ni(0), a bromoarene precursor was used and resulted in the formation of a nickel-bromide complex in the SCS pincer portion of the ligand. Reaction of the nickel complex with a lithium salt yielded a heterobimetallic complex with bromide bridging the two metal centers.

Thermochemical insight into the reduction of CO to CH3OH with [Re(CO)](+) and [Mn(CO)](+) complexes.

To gain insight into thermodynamic barriers for reduction of CO into CH3OH, free energies for reduction of [CpRe(PPh3)(NO)(CO)](+) into CpRe(PPh3)(NO)(CH2OH) have been determined from experimental measurements. Using model complexes, the free energies for the transfer of H(+), H(-), and e(-) have been determined. A pKa of 10.