Electrocatalyst For Proton Reduction Research Articles

Iron complexes of BIANs: Redox trends and electrocatalysis of hydrogen evolution

We report new iron(II, III) complexes in the framework of redox non-innocent BIAN ligands acting as highly active electrocatalysts for proton reduction. The iron complexes have been investigated by cyclic voltammetry, electron paramagnetic resonance and UV–Vis spectroscopy. The formation of Fe(III) complexes owing to electron transfer from metal to ligand has been discovered in acetonitrile and dichloromethane solutions. The hydrogen evolution catalysis is fast and efficient with a turnover frequency up to 460 s−1 and a very low overpotential of only 0.17–0.19 mV in MeCN. The catalysis is first order with respect to [catalyst] and second order with respect to [acid]. Steric hindrance of dpp-BIAN decreases the catalytic activity of the complexes. The best catalyst is the p-MeO-substituted BIAN iron complex.

Role of Anation on the Mechanism of Proton Reduction Involving a Pentapyridine Cobalt Complex: A Theoretical Study.

Kinetic and thermodynamic aspects of proton reduction involving pentapyridine cobalt(II) complex were investigated with the help of quantum chemical calculations. Free energy profile of all possible mechanistic routes for proton reduction was constructed with the consideration of both anation and solvent bound pathways. The computed free energy profile shows that acetate ion plays a significant role in modulating the kinetic aspects of Co(III)-hydride formation which is found to be the key intermediate for proton reduction. Upon replacing solvent by acetate ion, one electron reduction and protonation of CoI species become more rapid along with slow displacement reaction. Most favorable pathways for hydrogen evolution from Co(III)-hydride species is also investigated. Among the four possible pathways, reduction followed by protonation of Co(III)-hydride (RPP) is found to be the most feasible pathway. On the basis of QTAIM and NBO analyses, the electronic origin of most favorable pathway is explained. The basicity of cobalt center along with thermodynamic stability of putative CoIII/II-H species is essentially a prime factor in deciding the most favorable pathway for hydrogen evolution. Our computed results are in good agreement with experimental observations and also provided adequate information to design cobalt-based molecular electrocatalysts for proton reduction in future.

Mild electrochemical synthesis of metal phosphides with dibenzo-7-phosphanorbornadiene derivatives: mechanistic insights and application to proton reduction in water.

Transition metal phosphide films were synthesized using a mild electrochemical method. Dibenzo-7-phosphanorbornadiene derivatives (XPA) are introduced as versatile precursors to amorphous metal phosphide electrocatalysts for proton reduction in acidic water. Homogeneous model reactions reveal distinct reactivity between XPA and nickel in different oxidation states, with Ni(0) resulting in NixPy formation.

Trinuclear clusters containing 2-aminopyridinate/pyrimidinate ligands as electrocatalysts for proton reduction

Triiron and triruthenium clusters containing capping 2-aminopyridinate/pyrimidinate ligands are developed as functional models of the [FeFe]-hydrogenase for electrocatalytic reduction of protons to hydrogen. The 48-electron clusters Fe3(CO)9(μ3-pyNH)(μ-H) (1), Fe3(CO)9(μ3-pymNH)(μ-H) (2), Ru3(CO)9(μ3-pyNH)(μ-H) (3) and Ru3(CO)9(μ3-pymNH)(μ-H) (4) (pyNH = 2-aminopyridinate, pymNH = 2-aminopyrimidinate) are prepared from reactions of M3(CO)12 (M = Fe or Ru) with the corresponding heterocyclic amine at elevated temperatures. Each contains a hydride and a residual amino hydrogen in close proximity (ca. 2.8 Å). The triiron 2-aminopyridinate cluster 1 does not protonate by TsOH·H2O (used as the proton source during catalysis), whereas its ruthenium analogue 3 undergoes slow protonation across a ruthenium-ruthenium bond. The 2-aminopyrimidinate clusters 2 and 4 undergo rapid protonation at the ring nitrogen. In MeCN, the triiron clusters show a single irreversible reduction wave (Ep = −1.61 V for 1; Ep = −1.47 V for 2) in the cathodic region of their CVs, while the triruthenium clusters display a pair of irreversible cathodic waves (Ep = −2.01 and −2.15 V for 1; Ep = −1.93 and −2.09 V for 2). All catalyze proton reduction in the presence of TsOH·H2O but different mechanisms are proposed. The triiron clusters are more efficient toward proton reduction and operate at reduced overpotentials as compared to their triruthenium analogues. Introduction of a potential proton relay in these clusters neither increases the efficiency nor reduces the overpotential of catalysis which is in sharp contrast with the results observed for hydrogenase biomimetics.

Spectroscopic, electrochemical and X-ray crystallographic properties of a novel palladium(II) complex of thioamide deprotonated di-2-pyridyl ketone thiosemicarbazone (dpktsc-H)−

The reaction between di-2-pyridyl ketone thiosemicarbazone (dpktsc) and PdCl2(CH3CN)2, generated in situ from the reaction between PdCl2 and CH3CN, gave the unprecedented [Pd2Cl3(κ5-Npy,Nim,S,Npy,Nam-dpktsc-H)]·2CH3CN (1) complex (py = pyridine, im = imine and am = amide). The identity of 1 was confirmed via its elemental analysis and spectroscopic properties. Infrared and 1H-NMR spectra confirmed the coordination of (dpktsc-H)− to the palladium ions. The electronic absorption spectra measured in dmso and dmf and density functional theory (DFT) calculations revealed metal-to-ligand charge-transfer (MLCT), d–d and intra-ligand charge-transfer (ILCT) electronic transitions. X-ray structural analysis on a crystal of [Pd2Cl3(κ5-Npy,Nim,S,Npy,Nam-dpktsc-H)]·H2O (2) grown from dmf solution of 1 confirmed its formulation and showed the solid-state structure contains a web of molecules locked via a network of non-covalent interactions. Electrochemical measurements on 1 in dmf revealed metal- and ligand-based redox processes. In contrast to the electrochemical decomposition of uncoordinated dpktsc, coordinated (dpktsc-H)− in 1 does not undergo electrochemical decomposition. Electrochemical titrations of 1 with p-toluenesulfonic acid monohydrate (p-TSOH) revealed electro-catalytic proton reduction. Over-potential (η) of 180 mV for the H2 evolution was observed and is comparable to several molecular electro-catalysts for proton reduction. Controlled-potential electrolysis confirmed the electro-catalytic proton reduction by the Pd-complex. Electrochemical reactions of CO2 in the presence of 1 exhibited a proton dependence, and metal- and ligand-based electrochemical reaction.

Renewable Formate from C-H Bond Formation with CO2: Using Iron Carbonyl Clusters as Electrocatalysts.

As a society, we are heavily dependent on nonrenewable petroleum-derived fuels and chemical feedstocks. Rapid depletion of these resources and the increasingly evident negative effects of excess atmospheric CO2 drive our efforts to discover ways of converting excess CO2 into energy dense chemical fuels through selective C-H bond formation and using renewable energy sources to supply electrons. In this way, a carbon-neutral fuel economy might be realized. To develop a molecular or heterogeneous catalyst for C-H bond formation with CO2 requires a fundamental understanding of how to generate metal hydrides that selectively donate H- to CO2, rather than recombining with H+ to liberate H2. Our work with a unique series of water-soluble and -stable, low-valent iron electrocatalysts offers mechanistic and thermochemical insights into formate production from CO2. Of particular interest are the nitride- and carbide-containing clusters: [Fe4N(CO)12]- and its derivatives and [Fe4C(CO)12]2-. In both aqueous and mixed solvent conditions, [Fe4N(CO)12]- forms a reduced hydride intermediate, [H-Fe4N(CO)12]-, through stepwise electron and proton transfers. This hydride selectively reacts with CO2 and generates formate with >95% efficiency. The mechanism for this transformation is supported by crystallographic, cyclic voltammetry, and spectroelectrochemical (SEC) evidence. Furthermore, installation of a proton shuttle onto [Fe4N(CO)12]- facilitates proton transfer to the active site, successfully intercepting the hydride intermediate before it reacts with CO2; only H2 is observed in this case. In contrast, isoelectronic [Fe4C(CO)12]2- features a concerted proton-electron transfer mechanism to form [H-Fe4C(CO)12]2-, which is selective for H2 production even in the presence of CO2, in both aqueous and mixed solvent systems. Higher nuclearity clusters were also studied, and all are proton reduction electrocatalysts, but none promote C-H bond formation. Thermochemical insights into the disparate reactivities of these clusters were achieved through hydricity measurements using SEC. We found that only [H-Fe4N(CO)12]- and its derivative [H-Fe4N(CO)11(PPh3)]- have hydricities modest enough to avoid H2 production but strong enough to make formate. [H-Fe4C(CO)12]2- is a stronger hydride donor, theoretically capable of making formate, but due to an overwhelming thermodynamic driving force and the increased electrostatic attraction between the more negative cluster and H+, only H2 is observed experimentally. This illustrates the fundamental importance of controlling thermochemistry when designing new catalysts selective for C-H bond formation and establishes a hydricity range of 15.5-24.1 or 44-49 kcal mol-1 where C-H bond formation may be favored in water or MeCN, respectively.

Bridgehead isomer effects in bis(phosphido)-bridged diiron hexacarbonyl proton reduction electrocatalysts.

The influence of the substitution, orientation and structure of the phosphido bridges in [Fe2(CO)6(μ-PR2)2] electrocatalysts of proton reduction has been studied. The isomers e,a-[Fe2(CO)6{μ-P(Ar)H}2] (1a(Ar): Ar = Ph, 2'-methoxy-1,1'-binaphthyl (bn')), e,e-[Fe2(CO)6{μ-P(Ar)H}2] (1b(Ar): Ar = Ph, bn') were isolated from reactions of iron pentacarbonyl and the corresponding primary phosphine, syntheses that also afforded the phosphinidene-capped tri-iron clusters, [Fe3(CO)9(μ-CO)(μ3-Pbn')] (2) and [Fe3(CO)9(μ3-PAr)2] (3(Ar), Ar = Ph, bn'). A ferrocenyl (Fc)-substituted dimer [Fe2(CO)6{μ:μ'-1,2-(P(CH2Fc)CH2)2C6H4}] (4), in which the two phosphido bridges are linked by an o-xylyl group, was also prepared. The molecular structures of complexes 1a(Ph), 1b(Ph), 1b(bn'), 2 and 4 were established by X-ray crystallography. All complexes have been examined as electrocatalysts for proton reduction using p-toluene sulfonic acid in tetrahydrofuran. Cyclic voltammograms of the dimers with acid exhibit two catalysis waves for proton reduction. The first wave, which appears at the potential of the primary reduction, reaches maximum current (turnover) at moderate acid concentrations and is rapidly overtaken by the second wave, which appears at more negative potential. Both of these reductive waves show an initial first order dependence on acid. The electrochemistry and electrocatalyses of the [Fe2(CO)6(μ-PR2)2] dimers show subtle variations with the nature of the bridging phosphido group(s), including the orientation of bridgehead hydrogen atoms.

A new nitrogen heterocyclic carbene containing diiron complex as bio-inspired catalyst for proton reduction and benzene hydroxylation

A new diiron complex (μ-dmedt)[Fe2(CO)5IPr] (2, dmedt = 2,3-butanedithiol, IPr = 1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene) was synthesized as analogue of the active site of [Fe-Fe]-H2ases. The sterically bulky nitrogen heterocyclic carbene ligand of IPr coordinated to the iron center was introduced via CO/L substitution reaction in mild conditions. The reactivity of electrocatalytic reduction of proton and catalytic hydroxylation of benzene were explored for the developed diiron complex. Complex 2 undergoes two irreversible reduction events at ca. -2.17 V and −2.50 V and acts as an efficient electrocatalyst for proton reduction to hydrogen with both HOAc and H2O as proton source. The catalytic hydroxylation of benzene by complex 2 achieved phenol yield of 32.4% with almost 100% selectivity. The enhanced catalytic activity compared to the all carbonyl precursor and related derivatives was attributed to the good electron donating ability of introduced IPr ligand.

Diiron Complexes Bearing Bridging Hydrocarbyl Ligands as Electrocatalysts for Proton Reduction

Diiron complexes with bridging hydrocarbyl ligands and containing CO and Cp ligands (Cp = η5-C5H5) have been investigated as possible electrocatalysts for H2 production. In particular, studies included the vinyliminium complexes [Fe2{μ-η1:η3-C(R′)=CRC═NMe2}(μ-CO)(CO)(Cp)2][SO3CF3] (R′ = Tol (4-MeC6H4), R = H, 1a; R′ = CH2OH, R = H, 1b; R′ = CH2OH, R = SPh, 1c), the vinylalkylidene [Fe2{μ-η1:η3-C(Tol)CH═CHNMe2}(μ-CO)(CO)(Cp)2] (2), the aminoalkylidyne [Fe2{μ-CN(Me)(R)}(μ-CO)(CO)(L)(Cp)2][SO3CF3] (R = Me, L = CO, 3a; R = Me, L = NCMe, 3b; R = Xyl (2,6-Me2C6H3), L = CO, 3c), [Fe2{μ-CN(Me)2}(μ-L′)(CO)(L)(Cp)2] (L′ = CO, L = CN, 4; L′ = H, L = CO, 5), the thiocarbyne complexes [Fe2(μ-CSEt)(μ-CO)(CO)2(Cp)2][BF4], (6) and [Fe2(μ-CSMe)(μ-CO)(CO)(CN)(Cp)2] (7), and the alkylidene complexes [Fe2{μ-C(CN)(SMe)}(μ-CO)(CO)2(Cp)2][SO3CF3] (8) and [Fe2{μ-C(CN)(PMe2Ph)}(μ-CO)(CO)2(Cp)2][SO3CF3] (9). Cyclic voltammograms (CV) of the above complexes in CH3CN have been recorded in the presence of increasing amounts of acetic acid to evidence electrocatalytic proton reduction. In spite of the fact that the above diiron complexes do not resemble the typical diiron dithiolate model systems, the aminocarbyne 4 and the thiocarbyne complex 7 exhibit significant electrocatalytic properties for proton reduction (e.g., for 4, the turnover number (TON) is 15.5).

Versatile Reactivity of CH3CN‐Coordinated Nickel‐Iron Heterodimetallic Complexes with Cp* Ligand on Diazadithiolate (N2S2) or Dithiadithiolate (S4) Platforms

AbstractBy using 3,7‐dithianonane‐1,9‐dithiol (S4) and N,N′‐dimethyl‐N,N′‐bis(2‐mercaptoethyl)ethylenediamine (N2S2) as ligands, two CH3CN‐coordinated nickel‐iron heterodimetallic complexes, [NiS4Fe(CH3CN)Cp*][PF6] (1a) and [NiN2S2Fe(CH3CN)Cp*][PF6] (1b) (Cp* = η5‐C5Me5), were synthesized and characterized spectroscopically and crystallographically. Reactivities of 1a and 1b towards CO, CH2Cl2, HBF4, and NOPF6 have also been investigated. A series of complexes coordinated with small molecules [NiS4Fe(L)Cp*][PF6]n [n = 1, L = Cl (2a), CO (3a); n = 2, L = NO (4a)], [NiN2S2Fe(L)Cp*][PF6] [L = Cl (2b), CO (3b)] were obtained. We also studied the redox properties of 2a, 2b, 3a, and 3b by cyclic voltammetry in CH2Cl2 and in CH3CN solution. Complex 3a was found to be an excellent electrocatalyst for proton reduction from HBF4.

Sulfinato iron(III) complex for electrocatalytic proton reduction.

We report the first example of a sulfinato Fe(III) complex acting as a highly active electrocatalyst for proton reduction. The sulfinate binds to the metal through oxygen, resulting in a seven-membered chelate ring that is likely hemilabile during catalysis. Proton reduction occurs at -1.57 V versus Fc/Fc(+) in CH3CN with an ic/ip = 13 in CH3CN (kobs = 3300 s(-1)) and an overpotential of 800 mV. The catalysis is first order with respect to [catalyst] and second order with respect to [trifluoracetic acid]. An 11% increase in catalytic activity is observed in the presence of water, suggesting that sulfinate moieties are viable functional groups for aqueous proton reduction catalysts.

Electrochemical proton reduction catalysed by selenolato-manganese carbonyl complexes

Four manganese selenolato carbonyl complexes have been synthesized and used as electrocatalyst for proton reduction.

Catalysis of proton reduction by a [BO4]-bridged dicobalt glyoxime.

We report the preparation of a dicobalt compound with two singly proton-bridged cobaloxime units linked by a central [BO4] bridge. Reaction of a doubly proton-bridged cobaloxime complex with trimethyl borate afforded the compound in good yield. Single-crystal X-ray diffraction studies confirmed the bridging nature of the [BO4] moiety. Using electrochemical methods, the dicobalt complex was found to be an electrocatalyst for proton reduction in acetonitrile solution. Notably, the overpotential for proton reduction (954 mV) was found to be higher than in the cases of two analogous single-site cobalt glyoximes under virtually identical conditions.

Post-synthetic modification of hangman porphyrins synthesized on the gram scale.

We report a multi-gram scale synthesis of methyl 6-formyl-4-dibenzofurancarboxylate and its subsequent use in the gram scale synthesis of a dibenzofuran-functionalized hangman porphyrin containing a pendant carboxylic acid (HPD-CO2H). HPD-CO2H can be isolated as a free carboxylic acid in high purity with minimal purification. Post-synthetic modification of HPD-CO2H allows for the introduction of any desired pendant group in good yields, resulting in a practical amount of hangman porphyrin ligand with an easily customizable second coordination sphere. The cobalt complexes of these hangman porphyrins are shown to be active proton reduction electrocatalysts.

Mechanism of the electrocatalytic reduction of protons with diaryldithiolene cobalt complexes.

A series of dimeric cobalt-diaryldithiolene complexes [Co(S2C2Ar2)2]2, possessing various aryl para substituents (OMe, F, Cl, and Br), were studied as electrocatalysts for proton reduction in nonaqueous media, in an effort to correlate dithiolene donor strength with catalyst activity. Cyclic voltammetry data acquired for the cobalt-diaryldithiolene dimers guided the isolation of chemically reduced monoanionic ([Co(S2C2Ar2)2](-)) and dianionic ([Co(S2C2Ar2)2](2-)) monomers. The potassium and tetrabutylammonium salts of dianionic cobalt-diaryldithiolene complexes have been characterized by single crystal X-ray crystallography. Treatment of the dianionic species with stoichiometric quantities of a weak acid afforded H2 and the monoanionic cobalt-diaryldithiolene species. Density functional theory (BP86) suggests that hydrogen elimination proceeds through a diprotonated intermediate with a Co-H bond and a protonated S center. A transition state for transfer of the S-H proton to the metal center was located with a computed free energy of 5.9 kcal/mol, in solution (DMF via C-PCM approach).

Molecular cobalt electrocatalyst for proton reduction at low overpotential

Linear trimetallic Co(III)/Co(II)/Co(III) cobalt complexes with bridging acyl-alkoxy ligands are electrocatalysts for the reduction of tosic acid in acetonitrile. The -OCMe2CH2COMe complex appears to operate homogeneously, and at a modest onset overpotential of 175 mV. A turnover frequency of ca. 80 s(-1) was observed at an overpotential of 300 mV.

Hydrogen Evolution Catalyzed by Aluminum‐Bridged Cobalt Diglyoximate Complexes

AbstractThe syntheses of several cobalt diglyoximate complexes connected by one or two aluminum bridges are described. The aluminum centers are supported by tunable tetradentate diamine bisphenoxide ligands. Electrochemical investigations revealed that the number of aluminum bridges and the nature of the substituents on the phenoxide ligands significantly affect the cobalt reduction potentials. The present aluminum–cobalt compounds are electrocatalysts for proton reduction to hydrogen at potentials negative relative to those of the boron‐ and proton‐bridged analogs. The reported synthetic strategies allow modulation of the reduction potentials and the secondary coordination sphere interactions by tuning the ancillary ligands bound to aluminum.

Carbon- and Sulfur-Bridged Diiron Carbonyl Complexes Containing N,C,S-Tridentate Ligands Derived from Functionalized Dibenzothiophenes: Mimics of the [FeFe]-Hydrogenase Active Site

Photochemical reactions of [Fe(CO)5] with dibenzothiophene (DBT) derivatives bearing a N-donor group produced a series of C,S-bridged diiron carbonyl complexes [{Fe(μ-L′-κ3N,C,S)(CO)2}Fe(CO)3], as previously reported for 4-(2′-pyridyl)dibenzothiophene (L1), where L′ represents the N,C,S-tridentate ligands L1′–L5′, formed by C–S bond cleavage of L1–L5, respectively. The DBT derivatives used in this study have Schiff base or oxazoline moieties at the 4-position: L2 = PhCH2N═CH-DBT, L3 = 2-MeOC6H4CH2N═CH-DBT, L4 = (S)-PhC(Me)HN═CH-DBT, L5 = (R)-4-isopropyl-2-oxazolinyl-DBT. The diiron complexes were characterized by NMR, absorption, and circular dichroism spectroscopy, and the dinuclear structures bridged by thiolate S and aryl C atoms were established by X-ray crystallography. The diiron complex [{Fe(μ-L′-κ3N,C,S)(CO)2}Fe(CO)3] consists of two units, Fe(L′)(CO)2 and Fe(CO)3: the latter unit is located on a thiolate-containing metallacycle in the former one. The chiral Schiff base ligand precursor L4 gave a 55:45 mixture of two diastereomers for [{Fe(μ-L4′-κ3N,C,S)(CO)2}Fe(CO)3], while chiral L5 with an (R)-4-isopropyl-2-oxazolinyl group afforded [{Fe(μ-L5′-κ3N,C,S)(CO)2}Fe(CO)3] in a 9:1 diastereomeric ratio. The diiron carbonyl complexes of the N,C,S-tridentate ligands (L1′—L5′) showed two reversible redox couples for [Fe2(μ-L′)(CO)5]0/– and [Fe2(μ-L′)(CO)5]−/2–. The two-electron-reduced forms undergo protonation and act as electrocatalysts for proton reduction of acetic acid in acetonitrile.

A Cobalt–Dithiolene Complex for the Photocatalytic and Electrocatalytic Reduction of Protons

The complex [Co(bdt)(2)](-) (where bdt = 1,2-benzenedithiolate) is an active catalyst for the visible light driven reduction of protons from water when employed with Ru(bpy)(3)(2+) as the photosensitizer and ascorbic acid as the sacrificial electron donor. At pH 4.0, the system exhibits very high activity, achieving >2700 turnovers with respect to catalyst and an initial turnover rate of 880 mol H(2)/mol catalyst/h. The same complex is also an active electrocatalyst for proton reduction in 1:1 CH(3)CN/H(2)O in the presence of weak acids, with the onset of a catalytic wave at the reversible redox couple of -1.01 V vs Fc(+)/Fc. The cobalt-dithiolene complex [Co(bdt)(2)](-) thus represents a highly active catalyst for both the electrocatalytic and photocatalytic reduction of protons in aqueous solutions.

Sulfonated Diiron Complexes as Water-Soluble Models of the [Fe–Fe]-Hydrogenase Enzyme Active Site

A series of diiron complexes developed as fundamental models of the two-iron subsite in the [FeFe]-hydrogenase enzyme active site show water-solubility by virtue of a sulfonate group incorporated into the -SCH(2)NRCH(2)S- dithiolate unit that bridges two Fe(I)(CO)(2)L moieties. The sulfanilic acid group imparts even greater water solubility in the presence of β-cyclodextrin, β-CyD, for which NMR studies suggest aryl-sulfonate inclusion into the cyclodextrin cavity as earlier demonstrated in the X-ray crystal structure of 1Na·2 β-CyD clathrate, where 1Na = Na(+)(μ-SCH(2)N(C(6)H(4)SO(3)(-))CH(2)S-)[Fe(CO)(3)](2), (Singleton et al., J. Am. Chem. Soc.2010, 132, 8870). Electrochemical analysis of the complexes for potential as electrocatalysts for proton reduction to H(2) finds the presence of β-CyD to diminish response, possibly reflecting inhibition of structural rearrangements required of the diiron unit for a facile catalytic cycle. Advantages of the aryl sulfonate approach include entry into a variety of water-soluble derivatives from the well-known (μ-SRS)[Fe(CO)(3)](2) parent biomimetic, that are stable in O(2)-free aqueous solutions.