A Monometallic Bis(cyaphido) Complex.

The first example of a bis(cyaphido) complex, trans-[Ru(dppe)2(C≡P)2], is described, unequivocally demonstrating the synthetic accessibility and stability of complexes that feature more than one cyaphido ligand. Synthesis is achieved from the precedent cation [Ru(dppe)2(C≡P)]+ via sequential coordination and desilylation of the phosphaalkyne Me3SiC≡P. The heteroleptic analogue trans-[Ru(dppe)2(C≡N)(C≡P)] is also prepared from the same cation and NaCN; both cyaphido complexes are structurally characterized, enabling the first direct comparison of cyaphide with cyanide, its isoelectronic and isolobal counterpart. This demonstrates an enhanced π-acidity for -C≡P over -C≡N, while computational studies reveal also a higher π-donor character for the cyaphido ligand.

Formation of Reversible Solid Electrolyte Interface on Graphite Surface from Concentrated Electrolytes.

Li-ion batteries (LIB) have been successfully commercialized after the identification of ethylene-carbonate (EC)-containing electrolyte that can form a stable solid electrolyte interphase (SEI) on carbon anode surface to passivate further side reactions but still enable the transportation of the Li+ cation. These electrolytes are still utilized, with only minor changes, after three decades. However, the long-term cycling of LIB leads to continuous consumption of electrolyte and growth of SEI layer on the electrode surface, which limits the battery's life and performance. Herein, a new anode protection mechanism is reported in which, upon changing of the cell potential, the electrolyte components at the electrode-electrolyte interface reorganize reversibly to form a transient protective surface layers on the anode. This layer will disappear after the applied potential is removed so that no permanent SEI layer is required to protect the carbon anode. This phenomenon minimizes the need for a permanent SEI layer and prevents its continuous growth and therefore may lead to largely improved performance for LIBs.

Controlling Proton Delivery through Catalyst Structural Dynamics.

The fastest synthetic molecular catalysts for H2 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 H2 production rates for [Ni(PPh2 NC6H4R2 )2 ]2+ catalysts (R=n-hexyl, n-decyl, n-tetradecyl, n-octadecyl, phenyl, or cyclohexyl). The turnover frequencies correlate inversely with the rates of chair-boat ring inversion of the ligand, since this dynamic process governs protonation at either catalytically productive or non-productive sites. These results demonstrate that the dynamic processes involved in proton delivery can be controlled through modification of the outer coordination sphere, in a manner similar to the role of the protein architecture in many enzymes. As a design parameter, controlling structural dynamics can increase H2 production rates by three orders of magnitude with a minimal increase in overpotential.

Experimental and Computational Mechanistic Studies Guiding the Rational Design of Molecular Electrocatalysts for Production and Oxidation of Hydrogen.

Understanding how to control the movement of protons and electrons is crucial to the design of fast, efficient electrocatalysts for H2 production and oxidation based on earth-abundant metals. Our work seeks to address fundamental questions about proton movement. We have demonstrated that incorporating a pendant amine functioning as a proton relay in the second coordination sphere of a metal complex helps proton mobility, resulting in faster and more energy-efficient catalysts. Proton-transfer reactions can be rate-limiting and are influenced by several factors, such as pKa values, steric effects, hydrogen bonding, and solvation/desolvation of the exogenous base and acid employed. The presence of multiple protonation sites introduces branching points along the catalytic cycle, making less productive pathways accessible or leading to the formation of stable off-cycle species. Using ligands with only one pendant amine mitigates this problem and results in catalysts with high rates for production of H2, although generally at higher overpotentials. For H2 oxidation catalysts, iron complexes with a high H2 binding affinity were developed. However, these iron complexes had a pKa mismatch between the protonated metal center and the protonated pendant amine, and consequently intramolecular proton movement was slow. Taken altogether, our results demonstrate the necessity of optimizing the entire catalytic cycle because optimization of a specific catalytic step can negatively influence another step and not necessarily lead to a better catalytic performance. We discuss a general procedure, based on thermodynamic arguments, which allows the simultaneous minimization of the free-energy change of each catalytic step, yielding a nearly flat free-energy surface, with no large barriers due to energy mismatches from either high- or low-energy intermediates.

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

The hydrogen production electrocatalyst Ni(P(Ph)2N(Ph)2)2(2+) (1) is capable of traversing multiple electrocatalytic pathways. When using dimethylformamidium, DMF(H)(+), the mechanism of H2 formation by 1 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 1 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 1 was faster than the ECEC pathway under all conditions studied. Buffered DMF:DMF(H)(+) mixtures afforded an increase in the catalytic rate constant (k(obs)) of the EECC pathway, but k(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 1 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.

Investigating the role of chain and linker length on the catalytic activity of an H2 production catalyst containing a ß-hairpin peptide.

Building on our recent report of an active H2 production catalyst [Ni(PPh 2NProp-peptide)2]2+ (Prop = para-phenylpropionic acid, peptide (R10) = WIpPRWTGPR-NH2, p = D-proline and P2N = 1-aza-3,6-diphosphacycloheptane) that contains structured ß-hairpin peptides, here we investigate how H2 production is effected by: (1) the length of the hairpin (eight or ten residues) and (2) limiting the flexibility between the peptide and the core complex by altering the length of the linker: para-phenylpropionic acid (three carbons) or para-benzoic acid (one carbon). Reduction of the peptide chain length from ten to eight residues increases or maintains the catalytic current for H2 production for all complexes, suggesting a non-productive steric interaction at longer peptide lengths. While the structure of the hairpin appears largely intact for the complexes, NMR data are consistent with differences in dynamic behavior which may contribute to the observed differences in catalytic activity. Molecular dynamics simulations demonstrate that complexes with a one-carbon linker have the desired effect of restricting the motion of the hairpin relative to the complex; however, the catalytic currents are significantly reduced compared to complexes containing a three-carbon linker as a result of the electron withdrawing nature of the -COOH group. These results demonstrate the complexity and interrelated nature of the outer coordination sphere on catalysis.

Anion-Tunable Properties and Electrochemical Performance of Functionalized Ferrocene Compounds.

We report a series of ionically modified ferrocene compounds for hybrid lithium-organic non-aqueous redox flow batteries, based on the ferrocene/ferrocenium redox couple as the active catholyte material. Tetraalkylammonium ionic moieties were incorporated into the ferrocene structure, in order to enhance the solubility of the otherwise relatively insoluble ferrocene. The effect of various counter anions of the tetraalkylammonium ionized species appended to the ferrocene, such as bis(trifluoromethanesulfonyl)imide, hexafluorophosphate, perchlorate, tetrafluoroborate, and dicyanamide on the solubility of the ferrocene was investigated. The solution chemistry of the ferrocene species was studied, in order to understand the mechanism of solubility enhancement. Finally, the electrochemical performance of these ionized ferrocene species was evaluated and shown to have excellent cell efficiency and superior cycling stability.

Molecular electrocatalysts for oxidation of hydrogen using earth-abundant metals: shoving protons around with proton relays.

Sustainable, carbon-neutral energy is needed to supplant the worldwide reliance on fossil fuels in order to address the persistent problem of increasing emissions of CO2. Solar and wind energy are intermittent, highlighting the need to develop energy storage on a huge scale. Electrocatalysts provide a way to convert between electrical energy generated by renewable energy sources and chemical energy in the form of chemical bonds. Oxidation of hydrogen to give two electrons and two protons is carried out in fuel cells, but the typical catalyst is platinum, a precious metal of low earth abundance and high cost. In nature, hydrogenases based on iron or iron/nickel reversibly oxidize hydrogen with remarkable efficiencies and rates. Functional models of these enzymes have been synthesized with the goal of achieving electrocatalytic H2 oxidation using inexpensive, earth-abundant metals along with a key feature identified in the [FeFe]-hydrogenase: an amine base positioned near the metal. The diphosphine ligands P(R)2N(R')2 (1,5-diaza-3,7-diphosphacyclooctane with alkyl or aryl groups on the P and N atoms) are used as ligands in Ni, Fe, and Mn complexes. The pendant amines facilitate binding and heterolytic cleavage of H2, placing the hydride on the metal and the proton on the amine. The pendant amines also serve as proton relays, accelerating intramolecular and intermolecular proton transfers. Electrochemical oxidations and deprotonations by an exogeneous amine base lead to catalytic cycles for oxidation of H2 (1 atm) at room temperature for catalysts derived from [Ni(P(Cy)2N(R')2)2](2+), Cp(C6F5)Fe(P(tBu)2N(Bn)2)H, and MnH(P(Ph)2N(Bn)2)(bppm)(CO) [bppm = (PAr(F)2)2CH2]. In the oxidation of H2 catalyzed by [Ni(P(Cy)2N(R')2)2](2+), the initial product observed experimentally is a Ni(0) complex in which two of the pendant amines are protonated. Two different pathways can occur from this intermediate; deprotonation followed by oxidation occurs with a lower overpotential than the alternate pathway involving oxidation followed by deprotonation. The Mn cation [Mn(P(Ph)2N(Bn)2)(bppm)(CO)](+) mediates the rapid (>10(4) s(-1) at -95 °C), reversible heterolytic cleavage of H2. Obtaining the optimal benefit of pendant amines incorporated into the ligand requires that the pendant amine be properly positioned to interact with a M-H or M(H2) bond. In addition, ligands are ideally selected such that the hydride-acceptor ability of the metal and the basicity of a pendant are tuned to give low barriers for heterolytic cleavage of the H-H bond and subsequent proton transfer reactions. Using these principles allows the rational design of electrocatalysts for H2 oxidation using earth-abundant metals.

Highly active electrolytes for rechargeable Mg batteries based on a [Mg2(µ-Cl)2](2+) cation complex in dimethoxyethane.

A novel [Mg2(µ-Cl)2](2+) cation complex, which is highly active for reversible Mg electrodeposition, was identified for the first time in this work. This complex was found to be present in electrolytes formulated in dimethoxyethane (DME) through dehalodimerization of non-nucleophilic MgCl2 by reacting with either Mg salts (such as Mg(TFSI)2, TFSI = bis(trifluoromethane)sulfonylimide) or Lewis acid salts (such as AlEtCl2 or AlCl3). The molecular structure of the cation complex was characterized by single crystal X-ray diffraction, Raman spectroscopy and NMR. The electrolyte synthesis process was studied and rational approaches for formulating highly active electrolytes were proposed. Through control of the anions, electrolytes with an efficiency close to 100%, a wide electrochemical window (up to 3.5 V) and a high ionic conductivity (>6 mS cm(-1)) were obtained. The understanding of electrolyte synthesis in DME developed in this work could bring significant opportunities for the rational formulation of electrolytes of the general formula [Mg2(µ-Cl)2][anion]x for practical Mg batteries.

Increasing the rate of hydrogen oxidation without increasing the overpotential: a bio-inspired iron molecular electrocatalyst with an outer coordination sphere proton relay.

Oxidation of hydrogen (H2) to protons and electrons for energy production in fuel cells is currently catalyzed by platinum, but its low abundance and high cost present drawbacks to widespread adoption. Precisely controlled proton removal from the active site is critical in hydrogenase enzymes in nature that catalyze H2 oxidation using earth-abundant metals (iron and nickel). Here we report a synthetic iron complex, (CpC5F4N)Fe(PEtN(CH2)3NMe2 PEt)(Cl), that serves as a precatalyst for the oxidation of H2, with turnover frequencies of 290 s-1 in fluorobenzene, under 1 atm of H2 using 1,4-diazabicyclo[2.2.2]octane (DABCO) as the exogenous base. The inclusion of a properly tuned outer coordination sphere proton relay results in a cooperative effect between the primary, secondary and outer coordination spheres for moving protons, increasing the rate of H2 oxidation without increasing the overpotential when compared with the analogous complex featuring a single pendant base. This finding emphasizes the key role of pendant amines in mimicking the functionality of the proton pathway in the hydrogenase enzymes.

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. The solid-state structures were determined for this heterobimetallic complex and the nickel-bromide precursor, and the two complexes were characterized electrochemically to determine the influence of coordinating the second metal.

Solvent and electrolyte effects on Ni(P(R)(2)N(R')2)(2)-catalyzed electrochemical oxidation of hydrogen.

We report solvent and electrolyte effects on the electrocatalytic oxidation of H2 using Ni(P(Cy)2N(R')2)2 (R = Bn, (t)Bu) complexes. A turnover frequency of 46 s(-1) for Ni(P(Cy)2N(Bn)2)2 was obtained using 0.2 M [(n)Bu4N][BF4] in THF. A turnover frequency of 51 s(-1) was observed for Ni(P(Cy)2N(tBu)2)2 using 0.2 M [(n)Bu4N][B(C6F5)4] in fluorobenzene. These observations, in conjunction with previous studies, indicate nitrile binding inhibits catalysis supported by Ni(P(Cy)2N(Bn)2)2.

Production of hydrogen by electrocatalysis: making the H-H bond by combining protons and hydrides.

Generation of hydrogen by reduction of two protons by two electrons can be catalysed by molecular electrocatalysts. Determination of the thermodynamic driving force for elimination of H2 from molecular complexes is important for the rational design of molecular electrocatalysts, and allows the design of metal complexes of abundant, inexpensive metals rather than precious metals ("Cheap Metals for Noble Tasks"). The rate of H2 evolution can be dramatically accelerated by incorporating pendant amines into diphosphine ligands. These pendant amines in the second coordination sphere function as protons relays, accelerating intramolecular and intermolecular proton transfer reactions. The thermodynamics of hydride transfer from metal hydrides and the acidity of protonated pendant amines (pK(a) of N-H) contribute to the thermodynamics of elimination of H2; both of the hydricity and acidity can be systematically varied by changing the substituents on the ligands. A series of Ni(II) electrocatalysts with pendant amines have been developed. In addition to the thermochemical considerations, the catalytic rate is strongly influenced by the ability to deliver protons to the correct location of the pendant amine. Protonation of the amine endo to the metal leads to the N-H being positioned appropriately to favor rapid heterocoupling with the M-H. Designing ligands that include proton relays that are properly positioned and thermodynamically tuned is a key principle for molecular electrocatalysts for H2 production as well as for other multi-proton, multi-electron reactions important for energy conversions.

Controlling proton movement: electrocatalytic oxidation of hydrogen by a nickel(II) complex containing proton relays in the second and outer coordination spheres.

A nickel bis(diphosphine) complex containing proton relays in the second and outer coordination spheres, Ni(P(Cy)2N((CH2)2OMe))2, (P(Cy)2N((CH2)2OMe) = 1,5-di(methoxyethyl)-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane), is an electrocatalyst for hydrogen oxidation. The addition of hydrogen to the Ni(II) complex results in rapid formation of three isomers of the doubly protonated Ni(0) complex, [Ni(P(Cy)2N((CH2)2OMe)2H)2](2+). The three isomers show fast interconversion at 40 °C, unique to this complex in this class of catalysts. Under conditions of 1.0 atm H2 using H2O as a base, catalytic oxidation proceeds at a turnover frequency of 5 s(-1) and an overpotential of 720 mV, as determined from the potential at half of the catalytic current. Compared to the previously reported Ni(P(Cy)2N(Bn))2 complex, the new complex operates at a faster rate and at a lower overpotential.

Production of H2 at fast rates using a nickel electrocatalyst in water-acetonitrile solutions.

We report a synthetic nickel complex containing proton relays, [Ni(P(Ph)2N(C6H4OH)2)2](BF4)2 (P(Ph)2N(C6H4OH)2 = 1,5-bis(p-hydroxyphenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclo-octane), that catalyzes the production of H2 in aqueous acetonitrile with turnover frequencies of 750-170,000 s(-1) at experimentally determined overpotentials of 310-470 mV.

High catalytic rates for hydrogen production using nickel electrocatalysts with seven-membered cyclic diphosphine ligands containing one pendant amine.

A series of Ni-based electrocatalysts, [Ni(7P(Ph)2N(C6H4X))2](BF4)2, featuring seven-membered cyclic diphosphine ligands incorporating a single amine base, 1-para-X-phenyl-3,6-triphenyl-1-aza-3,6-diphosphacycloheptane (7P(Ph)2N(C6H4X), where X = OMe, Me, Br, Cl, or CF3), have been synthesized and characterized. X-ray diffraction studies have established that the [Ni(7P(Ph)2N(C6H4X))2](2+) complexes have a square planar geometry, with bonds to four phosphorus atoms of the two bidentate diphosphine ligands. Each of the complexes is an efficient electrocatalyst for hydrogen production at the potential of the Ni(II/I) couple, with turnover frequencies ranging from 2400 to 27,000 s(-1) with [(DMF)H](+) in acetonitrile. Addition of water (up to 1.0 M) accelerates the catalysis, giving turnover frequencies ranging from 4100 to 96,000 s(-1). Computational studies carried out on the [Ni(7P(Ph)2N(C6H4X))2](2+) family indicate the catalytic rates reach a maximum when the electron-donating character of X results in the pKa of the Ni(I) protonated pendant amine matching that of the acid used for proton delivery. Additionally, the fast catalytic rates for hydrogen production by the [Ni(7P(Ph)2N(C6H4X))2](2+) family relative to the analogous [Ni(P(Ph)2N(C6H4X)2)2](2+) family are attributed to preferred formation of endo protonated isomers with respect to the metal center in the former, which is essential to attain suitable proximity to the reduced metal center to generate H2. The results of this work highlight the importance of precise pKa matching with the acid for proton delivery to obtain optimal rates of catalysis.

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 (1)H 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 E(1/2) = -1.89(1) V vs Cp(2)Fe(+•/0) in MeCN, and was accomplished chemically with KC(8) in tetrahydrofuran (THF). The salts K(+)(18-crown-6)[CpCr(CO)(2)(IMe)](-)·½THF and K(+)[CpCr(CO)(2)(IMe)](-)·(3)/(4)THF 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)](-)·½THF gives the hydride CpCr(CO)(2)(IMe)H, which could not be isolated, but was characterized in solution; a pK(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(•)] + H(2) is exergonic (ΔG = -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)(PCy(3))H has a higher Cr-H BDFE (52.9(4) kcal mol(-1)), is more stable with respect to H(2) loss, and is isolable.

A modular, energy-based approach to the development of nickel containing molecular electrocatalysts for hydrogen production and oxidation.

This review discusses the development of molecular electrocatalysts for H2 production and oxidation based on nickel. A modular approach is used in which the structure of the catalyst is divided into first, second, and outer coordination spheres. The first coordination sphere consists of the ligands bound directly to the metal center, and this coordination sphere can be used to control such factors as the presence or absence of vacant coordination sites, redox potentials, hydride donor abilities and other important thermodynamic parameters. The second coordination sphere includes functional groups such as pendent acids or bases that can interact with bound substrates such as H2 molecules and hydride ligands, but that do not form strong bonds with the metal center. These functional groups can play diverse roles such as assisting the heterolytic cleavage of H2, controlling intra- and intermolecular proton transfer reactions, and providing a physical pathway for coupling proton and electron transfer reactions. By controlling both the hydride donor ability of the catalysts using the first coordination sphere and the proton donor abilities of the functional groups in the second coordination sphere, catalysts can be designed that are biased toward H2 production, oxidation, or bidirectional (catalyzing both H2 oxidation and production). The outer coordination sphere is defined as that portion of the catalytic system that is beyond the second coordination sphere. This coordination sphere can assist in the delivery of protons and electrons to and from the catalytically active site, thereby adding another important avenue for controlling catalytic activity. Many features of these simple catalytic systems are good models for enzymes, and these simple systems provide insights into enzyme function and reactivity that may be difficult to probe in enzymes. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Investigating the role of the outer-coordination sphere in [Ni(P(Ph)2N(Ph­R)2)2]2+ hydrogenase mimics.

A series of dipeptide substituted nickel complexes with the general formula, [Ni(P(Ph)(2)N(NNA-amino acid/ester)(2))(2)](BF(4))(2), have been synthesized and characterized (P(2)N(2) = 1,5-diaza-3,7-diphosphacyclooctane, and the dipeptide consists of the non-natural amino acid, 3-(4-aminophenyl)propionic acid (NNA), coupled to amino acid/esters = glutamic acid, alanine, lysine, and aspartic acid). Each of these complexes is an active electrocatalyst for H(2) production. The effects of the outer-coordination sphere on the catalytic activity for the production of H(2) were investigated; specifically, the impact of sterics, the ability of the side chain or backbone to protonate and the pK(a) values of the amino acid side chains were studied by varying the amino acids in the dipeptide. The catalytic rates of the different dipeptide substituted nickel complexes varied by over an order of magnitude. The amino acid derivatives display the fastest rates, while esterification of the terminal carboxylic acids and side chains resulted in a decrease in the catalytic rate by 50-70%, implicating a significant role of protonated sites in the outer-coordination sphere on catalytic activity. For both the amino acid and ester derivatives, the complexes with the largest substituents display the fastest rates, indicating that catalytic activity is not hindered by steric bulk. These studies demonstrate the significant contribution that the outer-coordination sphere can have in tuning the catalytic activity of small molecule hydrogenase mimics.