Evaluation of attractive interactions in the second coordination sphere of iron complexes containing pendant amines.

The interactions between pendant amines in the second coordination sphere and ligands in the first coordination sphere are important for understanding the structures and reactivity of complexes containing PR2NR'2 ligands, which have been shown to be highly active H2 oxidation/production catalysts. A series of [Fe(PPh2NBn2)2(X)(Y)]n+ complexes have been prepared and structurally characterized. These complexes have two different ligands with which the pendant amines of the diphosphine ligand can interact. The solid state structure of cis-Fe(PPh2NBn2)2Cl2 reveals that the six-membered rings adjacent to the P atoms are in a boat confirmation, resulting in close NP distances that suggests the P atoms have a greater affinity for the lone pair of electrons on the N atom than chloride ligands. Similarly, boat conformations are observed for both rings adjacent to the hydride ligands of trans-[HFe(PPh2NBn2)2(CH3CN)]+ and trans-HFe(PPh2NBn2)2Cl, resulting in short NH distances. Spectroscopic and computational studies of trans-[HFe(PPh2NBn2)2(CO)]+, trans-[HFe(PPh2NBn2)(PPh2NBn2H)(CO)]2+, and trans-[HFe(PPh2NBn2)2(H2)]+ indicate the complexes are more stable when the pendant amines in boat conformations are adjacent to the hydride ligand. These data suggest an attractor ordering of H- > CO > H2 > PR3 > Cl- ∼ CH3CN.

Water-assisted proton delivery and removal in bio-inspired hydrogen production catalysts.

Electrocatalysts for H2 production are envisioned to play an important role in renewable energy utilization systems. Nickel-based catalysts featuring pendant amines functioning as proton relays in the second coordination sphere of the metal center have led to catalysts achieving turnover frequencies as high as 10(7) s(-1) for H2 production. The fastest rates are observed when water is present in solution, with rates up to 10(3) times faster than those found in dry solvent. The focus of this paper is to provide mechanistic insight into the unexpected enhancement due to water. Addition of H2 to [Ni(P(Cy)2N(R')2)2](2+) was previously shown to give three isomers of a Ni(0) product with two protonated amines, where the N-H can be endo or exo to the Ni. By investigating the deprotonation of these two N-protonated Ni(0) intermediates resulting from the addition of H2 to [Ni(P(Cy)2N(R')2)2](2+), we observe by NMR spectroscopy studies an enhancement in the rate of deprotonation for protons positioned on the pendant amine next to the metal (endo) vs. protons that are positioned away from the metal (exo). Computational studies suggest that for smaller bases, the desolvation energy of the exogenous base is the primary contribution limiting the rate of endo deprotonation, while steric accessibility and facile proton movement also contribute. For more bulky bases, steric accessibility can play the dominant role. The significant reduction in these barriers observed in the presence of water has important implications for disfavoring less productive catalytic pathways and increasing catalytic rates.

Toward molecular catalysts by computer.

CONSPECTUS: Rational design of molecular catalysts requires a systematic approach to designing ligands with specific functionality and precisely tailored electronic and steric properties. It then becomes possible to devise computer protocols to design catalysts by computer. In this Account, we first review how thermodynamic properties such as redox potentials (E°), acidity constants (pKa), and hydride donor abilities (ΔGH(-)) form the basis for a framework for the systematic design of molecular catalysts for reactions that are critical for a secure energy future. We illustrate this for hydrogen evolution and oxidation, oxygen reduction, and CO conversion, and we give references to other instances where it has been successfully applied. The framework is amenable to quantum-chemical calculations and conducive to predictions by computer. We review how density functional theory allows the determination and prediction of these thermodynamic properties within an accuracy relevant to experimentalists (∼0.06 eV for redox potentials, ∼1 pKa unit for pKa values, and 1-2 kcal/mol for hydricities). Computation yielded correlations among thermodynamic properties as they reflect the electron population in the d shell of the metal center, thus substantiating empirical correlations used by experimentalists. These correlations point to the key role of redox potentials and other properties (pKa of the parent aminium for the proton-relay-based catalysts designed in our laboratory) that are easily accessible experimentally or computationally in reducing the parameter space for design. These properties suffice to fully determine free energies maps and profiles associated with catalytic cycles, i.e., the relative energies of intermediates. Their prediction puts us in a position to distinguish a priori between desirable and undesirable pathways and mechanisms. Efficient catalysts have flat free energy profiles that avoid high activation barriers due to low- and high-energy intermediates. The criterion of a flat energy profile can be mathematically resolved in a functional in the reduced parameter space that can be efficaciously calculated by means of the correlation expressions. Optimization of the functional permits the prediction by computer of design points for optimum catalysts. Specifically, the optimization yields the values of the thermodynamic properties for efficient (high rate and low overpotential) catalysts. We are on the verge of design of molecular electrocatalysts by computer. Future efforts must focus on identifying actual ligands that possess these properties. We believe that this can also be achieved through computation, using Taft-like relationships linking molecular composition and structure with electron-donating ability and steric effects. We note also that the approach adopted here of using free energy maps to decipher catalytic pathways and mechanisms does not account for kinetic barriers associated with elementary steps along the catalytic pathway, which may make thermodynamically accessible intermediates kinetically inaccessible. Such an extension of the approach will require further computations that, however, can take advantage of Polanyi-like linear free energy relationships linking activation barriers and reaction free energies.

Amino acid modified Ni catalyst exhibits reversible H2 oxidation/production over a broad pH range at elevated temperatures.

Hydrogenases interconvert H2 and protons at high rates and with high energy efficiencies, providing inspiration for the development of molecular catalysts. Studies designed to determine how the protein scaffold can influence a catalytically active site have led to the synthesis of amino acid derivatives of [Ni(P2(R)N2(R'))2](2+) complexes, [Ni(P2(Cy)N2(Amino acid))2](2+) (CyAA). It is shown that these CyAA derivatives can catalyze fully reversible H2 production/oxidation at rates approaching those of hydrogenase enzymes. The reversibility is achieved in acidic aqueous solutions (pH = 0-6), 1 atm 25% H2/Ar, and elevated temperatures (tested from 298 to 348 K) for the glycine (CyGly), arginine (CyArg), and arginine methyl ester (CyArgOMe) derivatives. As expected for a reversible process, the catalytic activity is dependent upon H2 and proton concentrations. CyArg is significantly faster in both directions (∼300 s(-1) H2 production and 20 s(-1) H2 oxidation; pH = 1, 348 K, 1 atm 25% H2/Ar) than the other two derivatives. The slower turnover frequencies for CyArgOMe (35 s(-1) production and 7 s(-1) oxidation under the same conditions) compared with CyArg suggests an important role for the COOH group during catalysis. That CyArg is faster than CyGly (3 s(-1) production and 4 s(-1) oxidation) suggests that the additional structural features imparted by the guanidinium groups facilitate fast and reversible H2 addition/release. These observations demonstrate that outer coordination sphere amino acids work in synergy with the active site and can play an important role for synthetic molecular electrocatalysts, as has been observed for the protein scaffold of redox active enzymes.

Heterolytic cleavage of hydrogen by an iron hydrogenase model: an Fe-H⋠⋠⋠H-N dihydrogen bond characterized by neutron diffraction.

Hydrogenase enzymes in nature use hydrogen as a fuel, but the heterolytic cleavage of H-H bonds cannot be readily observed in enzymes. Here we show that an iron complex with pendant amines in the diphosphine ligand cleaves hydrogen heterolytically. The product has a strong Fe-H⋅⋅⋅H-N dihydrogen bond. The structure was determined by single-crystal neutron diffraction, and has a remarkably short H⋅⋅⋅H distance of 1.489(10) Šbetween the protic N-H(δ+) and hydridic Fe-H(δ-) part. The structural data for [Cp(C5F4N)FeH(P(tBu)2N(tBu)2H)](+) provide a glimpse of how the H-H bond is oxidized or generated in hydrogenase enzymes. These results now provide a full picture for the first time, illustrating structures and reactivity of the dihydrogen complex and the product of the heterolytic cleavage of H2 in a functional model of the active site of the [FeFe] hydrogenase enzyme.

Development of molecular electrocatalysts for energy storage.

Molecular electrocatalysts can play an important role in energy storage and utilization reactions needed for intermittent renewable energy sources. This manuscript describes three general themes that our laboratories have found useful in the development of molecular electrocatalysts for reduction of CO2 to CO and for H2 oxidation and production. The first theme involves a conceptual partitioning of catalysts into first, second, and outer coordination spheres. This is illustrated with the design of electrocatalysts for CO2 reduction to CO using first and second coordination spheres and for H2 production catalysts using all three coordination spheres. The second theme focuses on the development of thermodynamic models that can be used to design catalysts to avoid high- and low-energy intermediates. In this research, new approaches to the measurement of thermodynamic hydride donor and acceptor abilities of transition-metal complexes were developed. Combining this information with other thermodynamic information such as pKa values and redox potentials led to more complete thermodynamic descriptions of transition-metal hydride, dihydride, and related species. Relationships extracted from this information were then used to develop models that are powerful tools for predicting and understanding the relative free energies of intermediates in catalytic reactions. The third theme is control of proton movement during electrochemical fuel generation and utilization reactions. This research involves the incorporation of pendant amines in the second coordination sphere that can facilitate H-H bond heterolysis and heteroformation, intra- and intermolecular proton-transfer steps, and coupling of proton- and electron-transfer steps. Studies also indicate an important role for the outer coordination sphere in the delivery of protons to the second coordination sphere. Understanding these proton-transfer reactions and their associated energy barriers is key to the design of faster and more efficient molecular electrocatalysts for energy storage.

Thermochemical and mechanistic studies of electrocatalytic hydrogen production by cobalt complexes containing pendant amines.

Two cobalt(tetraphosphine) complexes [Co(P(nC-PPh2)2N(Ph)2)(CH3CN)](BF4)2 with a tetradentate phosphine ligand (P(nC-PPh2)2N(Ph)2 = 1,5-diphenyl-3,7-bis((diphenylphosphino)alkyl)-1,5-diaza-3,7-diphosphacyclooctane; alkyl = (CH2)2, n = 2 (L2); (CH2)3, n = 3 (L3)) have been studied for electrocatalytic hydrogen production using 1:1 [(DMF)H](+):DMF. A turnover frequency (TOF) of 980 s(-1) with an overpotential at Ecat/2 of 1210 mV was measured for [Co(II)(L2)(CH3CN)](2+), and a TOF of 980 s(-1) with an overpotential at Ecat/2 of 930 mV was measured for [Co(II)(L3)(CH3CN)](2+). Addition of water increases the TOF of [Co(II)(L2)(CH3CN)](2+) to 18,000 s(-1). The catalytic wave for each of these complexes occurs at the reduction potential of the corresponding HCo(III) complex. Comprehensive thermochemical studies of [Co(II)(L2)(CH3CN)](2+) and [Co(II)(L3)(CH3CN)](2+) and species derived from them by addition/removal of protons/electrons were carried out using values measured experimentally and calculated using density functional theory (DFT). Notably, HCo(I)(L2) and HCo(I)(L3) were found to be remarkably strong hydride donors, with HCo(I)(L2) being a better hydride donor than BH4(-). Mechanistic studies of these catalysts reveal that H2 formation can occur by protonation of a HCo(II) intermediate, and that the pendant amines of these complexes facilitate proton delivery to the cobalt center. The rate-limiting step for catalysis is a net intramolecular isomerization of the protonated pendant amine from the nonproductive exoisomer to the productive endo isomer.

Synthesis and electrochemical studies of cobalt(III) monohydride complexes containing pendant amines.

Two new tetraphosphine ligands, P(nC-PPh2)2N(Ph)2 (1,5-diphenyl-3,7-bis((diphenylphosphino)alkyl)-1,5-diaza-3,7-diphosphacyclooctane; alkyl = (CH2)2, n = 2 (L2); (CH2)3, n = 3 (L3)), have been synthesized. Coordination of these ligands to cobalt affords the complexes [Co(II)(L2)(CH3CN)](2+) and [Co(II)(L3)(CH3CN)](2+), which are reduced by KC8 to afford [Co(I)(L2)(CH3CN)](+) and [Co(I)(L3)(CH3CN)](+). Protonation of the Co(I) complexes affords [HCo(III)(L2)(CH3CN)](2+) and [HCo(III)(L3)(CH3CN)](2+). The cyclic voltammetry of [HCo(III)(L2)(CH3CN)](2+), analyzed using digital simulation, is consistent with an ErCrEr reduction mechanism involving reversible acetonitrile dissociation from [HCo(II)(L2)(CH3CN)](+) and resulting in formation of HCo(I)(L2). Reduction of HCo(III) also results in cleavage of the H-Co bond from HCo(II) or HCo(I), leading to formation of the Co(I) complex [Co(I)(L2)(CH3CN)](+). Under voltammetric conditions, the reduced cobalt hydride reacts with a protic solvent impurity to generate H2 in a monometallic process involving two electrons per cobalt. In contrast, under bulk electrolysis conditions, H2 formation requires only one reducing equivalent per [HCo(III)(L2)(CH3CN)](2+), indicating a bimetallic route wherein two cobalt hydride complexes react to form 2 equiv of [Co(I)(L2)(CH3CN)](+) and 1 equiv of H2. These results indicate that both HCo(II) and HCo(I) can be formed under electrocatalytic conditions and should be considered as potential catalytic intermediates.

Rapid, reversible heterolytic cleavage of bound H2.

Heterolytic cleavage of dihydrogen into a proton and a hydride ion is a fundamentally important step in many reactions, including the oxidation of hydrogen by hydrogenase enzymes and ionic hydrogenation of organic compounds. We report the facile, reversible heterolytic cleavage of H2 in a manganese complex bearing a pendant amine, leading to the formation of a manganese hydride and a protonated amine that undergo H(+)/H(-) exchange at an estimated rate of >10(7) s(-1) at 25 °C.

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.

An iron complex with pendent amines as a molecular electrocatalyst for oxidation of hydrogen.

The increasing energy needs of society have led to a search for technologies that can tap carbon-neutral and sustainable energy sources, such as solar and wind. Using properly designed catalysts, such sources can also be used to create fuels such as hydrogen; however, a significant barrier to the use of hydrogen as an energy carrier is the need for an inexpensive and efficient catalyst for its oxidation. The oxidation of hydrogen is the process by which electricity is produced in low-temperature fuel cells, and the best catalyst for this is platinum-a precious metal of low abundance. Here we report a molecular complex of iron (an abundant and inexpensive metal) as a rationally designed electrocatalyst for the oxidation of H(2) at room temperature, with turnover frequencies of 0.66-2.0 s(-1) and low overpotentials of 160-220 mV. This iron complex, Cp(C(6)F(5))Fe(P((t)Bu)(2)N(Bn)(2))(H), has pendent amines in the diphosphine ligand that function as proton relays.

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.

Proton delivery and removal in [Ni(P(R)2N(R')2)2]2+ hydrogen production and oxidation catalysts.

To examine the role of proton delivery and removal in the electrocatalytic oxidation and production of hydrogen by [Ni(P(R)(2)N(R')(2))(2)](2+) (where P(R)(2)N(R')(2) is 1,5-R'-3,7-R-1,5-diaza-3,7-diphosphacyclooctane), we report experimental and theoretical studies of the intermolecular proton exchange reactions underlying the isomerization of [Ni(P(Cy)(2)N(Bn)(2)H)(2)](2+) (Cy = cyclohexyl, Bn = benzyl) species formed during the oxidation of H(2) by [Ni(II)(P(Cy)(2)N(Bn)(2))(2)](2+) or the protonation of [Ni(0)(P(Cy)(2)N(Bn)(2))(2)]. Three protonated isomers are formed (endo/endo, endo/exo, or exo/exo), which differ in the position of the N-H bond's with respect to nickel. The endo/endo isomer is the most productive isomer due to the two protons being sufficiently close to the nickel to proceed readily to the transition state to form/cleave H(2). Therefore, the rate of isomerization of the endo/exo or exo/exo isomers to generate the endo/endo isomer can have an important impact on catalytic rates. We have found that the rate of isomerization is limited by proton removal from, or delivery to, the complex. In particular, the endo position is more sterically hindered than the exo position; therefore, protonation exo to the metal is kinetically favored over endo protonation, which leads to less catalytically productive pathways. In hydrogen oxidation, deprotonation of the sterically hindered endo position by an external base may lead to slow catalytic turnover. For hydrogen production catalysts, the limited accessibility of the endo position can result in the preferential formation of the exo protonated isomers, which must undergo one or more isomerization steps to generate the catalytically productive endo protonated isomer. The results of these studies highlight the importance of precise proton delivery, and the mechanistic details described herein will be used to guide future catalyst design.

Acidic ionic liquid/water solution as both medium and proton source for electrocatalytic H2 evolution by [Ni(P2N2)2]2+ complexes.

The electrocatalytic reduction of protons to H(2) by [Ni((P(Ph)(2)N(C6H4-hex))(2)(2)]((BF(4))(2) (where P(Ph)(2)N(C6H4-hex)(2) = 1,5-di(4-n-hexylphenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) in the highly acidic ionic liquid dibutylformamidium bis(trifluoromethanesulfonyl)amide shows a strong dependence on added water. A turnover frequency of 43,000-53,000 s(-1) has been measured for hydrogen production at 25 °C when the mole fraction of water (χ(H(2)O)) is 0.72. The same catalyst in acetonitrile with added dimethylformamidium trifluoromethanesulfonate and water has a turnover frequency of 720 s(-1). Thus, the use of an ionic liquid/aqueous solution enhances the observed catalytic rate by more than a factor of 50, compared to a similar acid in a traditional organic solvent. Complexes [Ni((P(Ph)(2)N(C6H4X))(2)(2)]((BF(4))(2) (X = H, OMe,CH(2)P(O)(OEt)(2), Br) are also catalysts in the ionic liquid/water mixture, and the observed catalytic rates correlate with the hydrophobicity of X.

The role of pendant amines in the breaking and forming of molecular hydrogen catalyzed by nickel complexes.

We present the results of a comprehensive theoretical investigation of the role of pendant amine ligands in the oxidation of H(2) and formation of H(2) by [Ni(P(R)(2)N(R')(2))(2)](2+) electrocatalysts (P(R)(2)N(R')(2) is the 1,5-R'-3,7-R derivative of 1,5-diaza-3,7-diphosphacyclooctane, in which R and R' are aryl or alkyl groups). We focus our analysis on the thermal steps of the catalytic cycle, as they are known to be rate-determining for both H(2) oxidation and production. We find that the presence of pendant amine functional groups greatly facilitates the heterolytic H(2) bond cleavage, resulting in a protonated amine and a Ni hydride. Only one single positioned pendant amine is required to serve this function. The pendant amine can also effectively shuttle protons to the active site, making the redistribution of protons and the H(2) evolution a very facile process. An important requirement for the overall catalytic process is the positioning of at least one amine in close proximity to the metal center. Indeed, only protonation of the pendant amines on the metal center side (endo position) leads to catalytically active intermediates, whereas protonation on the opposite side of the metal center (exo position) leads to a variety of isomers, which are detrimental to catalysis.

Synthesis, characterization, and reactivity of Fe complexes containing cyclic diazadiphosphine ligands: the role of the pendant base in heterolytic cleavage of H2.

The iron complexes CpFe(P(Ph)(2)N(Bn)(2))Cl (1-Cl), CpFe(P(Ph)(2)N(Ph)(2))Cl (2-Cl), and CpFe(P(Ph)(2)C(5))Cl (3-Cl)(where P(Ph)(2)N(Bn)(2) is 1,5-dibenzyl-1,5-diaza-3,7-diphenyl-3,7-diphosphacyclooctane, P(Ph)(2)N(Ph)(2) is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane, and P(Ph)(2)C(5) is 1,4-diphenyl-1,4-diphosphacycloheptane) have been synthesized and characterized by NMR spectroscopy, electrochemical studies, and X-ray diffraction. These chloride derivatives are readily converted to the corresponding hydride complexes [CpFe(P(Ph)(2)N(Bn)(2))H (1-H), CpFe(P(Ph)(2)N(Ph)(2))H (2-H), CpFe(P(Ph)(2)C(5))H (3-H)] and H(2) complexes [CpFe(P(Ph)(2)N(Bn)(2))(H(2))]BAr(F)(4), [1-H(2)]BAr(F)(4), (where BAr(F)(4) is B[(3,5-(CF(3))(2)C(6)H(3))(4)](-)), [CpFe(P(Ph)(2)N(Ph)(2))(H(2))]BAr(F)(4), [2-H(2)]BAr(F)(4), and [CpFe(P(Ph)(2)C(5))(H(2))]BAr(F)(4), [3-H(2)]BAr(F)(4), as well as [CpFe(P(Ph)(2)N(Bn)(2))(CO)]BAr(F)(4), [1-CO]Cl. Structural studies are reported for [1-H(2)]BAr(F)(4), 1-H, 2-H, and [1-CO]Cl. The conformations adopted by the chelate rings of the P(Ph)(2)N(Bn)(2) ligand in the different complexes are determined by attractive or repulsive interactions between the sixth ligand of these pseudo-octahedral complexes and the pendant N atom of the ring adjacent to the sixth ligand. An example of an attractive interaction is the observation that the distance between the N atom of the pendant amine and the C atom of the coordinated CO ligand for [1-CO]BAr(F)(4) is 2.848 Å, considerably shorter than the sum of the van der Waals radii of N and C atoms. Studies of H/D exchange by the complexes [1-H(2)](+), [2-H(2)](+), and [3-H(2)](+) carried out using H(2) and D(2) indicate that the relatively rapid H/D exchange observed for [1-H(2)](+) and [2-H(2)](+) compared to [3-H(2)](+) is consistent with intramolecular heterolytic cleavage of H(2) mediated by the pendant amine. Computational studies indicate a low barrier for heterolytic cleavage of H(2). These mononuclear Fe(II) dihydrogen complexes containing pendant amines in the ligands mimic crucial features of the distal Fe site of the active site of the [FeFe]-hydrogenase required for H-H bond formation and cleavage.

Synthesis and reactivity of molybdenum and tungsten bis(dinitrogen) complexes supported by diphosphine chelates containing pendant amines.

Molybdenum and tungsten bis(dinitrogen) complexes of the formula M(N(2))(2)(PNP)(2) (M = Mo and W) and W(N(2))(2)(dppe)(PNP), supported by diphosphine ligands containing a pendant amine of the formula (CH(2)PR(2))(2)NR' = P(R)N(R')P(R) (R = Et, Ph; R' = Me, Bn), have been prepared by Mg reduction of metal halides under an N(2) atmosphere. The complexes have been characterized by NMR and IR spectroscopy, X-ray crystallography, and cyclic voltammetry. Reactivity of the target Mo and W bis(dinitrogen) compounds with CO results in the formation of dicarbonyl complexes.

Synthesis and hydride transfer reactions of cobalt and nickel hydride complexes to BX3 compounds.

Hydrides of numerous transition metal complexes can be generated by the heterolytic cleavage of H(2) gas such that they offer alternatives to using main group hydrides in the regeneration of ammonia borane, a compound that has been intensely studied for hydrogen storage applications. Previously, we reported that HRh(dmpe)(2) (dmpe = 1,2-bis(dimethylphosphinoethane)) was capable of reducing a variety of BX(3) compounds having a hydride affinity (HA) greater than or equal to the HA of BEt(3). This study examines the reactivity of less expensive cobalt and nickel hydride complexes, HCo(dmpe)(2) and [HNi(dmpe)(2)](+), to form B-H bonds. The hydride donor abilities (ΔG(H(-))°) of HCo(dmpe)(2) and [HNi(dmpe)(2)](+) were positioned on a previously established scale in acetonitrile that is cross-referenced with calculated HAs of BX(3) compounds. The collective data guided our selection of BX(3) compounds to investigate and aided our analysis of factors that determine favorability of hydride transfer. HCo(dmpe)(2) was observed to transfer H(-) to BX(3) compounds with X = H, OC(6)F(5), and SPh. The reaction with B(SPh)(3) is accompanied by the formation of dmpe-(BH(3))(2) and dmpe-(BH(2)(SPh))(2) products that follow from a reduction of multiple B-SPh bonds and a loss of dmpe ligands from cobalt. Reactions between HCo(dmpe)(2) and B(SPh)(3) in the presence of triethylamine result in the formation of Et(3)N-BH(2)SPh and Et(3)N-BH(3) with no loss of a dmpe ligand. Reactions of the cationic complex [HNi(dmpe)(2)](+) with B(SPh)(3) under analogous conditions give Et(3)N-BH(2)SPh as the final product along with the nickel-thiolate complex [Ni(dmpe)(2)(SPh)](+). The synthesis and characterization of HCo(dedpe)(2) (dedpe = Et(2)PCH(2)CH(2)PPh(2)) from H(2) and a base is also discussed, including the formation of an uncommon trans dihydride species, trans-[(H)(2)Co(dedpe)(2)][BF(4)].