[FeFe]-Hydrogenase Mimetic Metallopolymers with Enhanced Catalytic Activity for Hydrogen Production in Water.

Electrocatalytic [FeFe]-hydrogenase mimics for the hydrogen evolution reaction (HER) generally suffer from low activity, high overpotential, aggregation, oxygen sensitivity, and low solubility in water. By using atom-transfer radical polymerization (ATRP), a new class of [FeFe]-metallopolymers with precise molar mass, defined composition, and low polydispersity, has been prepared. The synthetic methodology introduced here allows facile variation of polymer composition to optimize the [FeFe] solubility, activity, and long-term chemical and aerobic stability. Water soluble functional metallopolymers facilitate electrocatalytic hydrogen production in neutral water with loadings as low as 2 ppm and operate at rates an order of magnitude faster than hydrogenases (2.5×105  s-1 ), and with low overpotential requirement. Furthermore, unlike the hydrogenases, these systems are insensitive to oxygen during catalysis, with turnover numbers on the order of 40 000 under both anaerobic and aerobic conditions.

Intramolecular electron transfer in bipyridinium disulfides.

Reductive cleavage of disulfide bonds is an important step in many biological and chemical processes. Whether cleavage occurs stepwise or concertedly with electron transfer is of interest. Also of interest is whether the disulfide bond is reduced directly by intermolecular electron transfer from an external reducing agent or mediated intramolecularly by internal electron transfer from another redox-active moiety elsewhere within the molecule. The electrochemical reductions of 4,4'-bipyridyl-3,3'-disulfide (1) and the di-N-methylated derivative (2(2+)) have been studied in acetonitrile. Simulations of the cyclic voltammograms in combination with DFT (density functional theory) computations provide a consistent model of the reductive processes. Compound 1 undergoes reduction directly at the disulfide moiety with a substantially more negative potential for the first electron than for the second electron, resulting in an overall two-electron reduction and rapid cleavage of the S-S bond to form the dithiolate. In contrast, compound 2(2+) is reduced at less negative potential than 1 and at the dimethyl bipyridinium moiety rather than at the disulfide moiety. Most interesting, the second reduction of the bipyridinium moiety results in a fast and reversible intramolecular two-electron transfer to reduce the disulfide moiety and form the dithiolate. Thus, the redox-active bipyridinium moiety provides a low energy pathway for reductive cleavage of the S-S bond that avoids the highly negative potential for the first direct electron reduction. Following the intramolecular two-electron transfer and cleavage of the S-S bond the bipyridinium undergoes two additional reversible reductions at more negative potentials.

Synthesis and electrochemical reactivity of molybdenum dicarbonyl supported by a redox-active α-diimine ligand.

Low-valent molybdenum dicarbonyl complexes with a diazabutadiene [(mes)DAB(R); [ArN═C(R)C(R)═NAr]; Ar = 2,4,6-trimethylphenyl (mes), R = H or CH3] ligand have been synthesized and fully characterized. The title complexes exhibit elongated DAB C-N and shortened C-C bond lengths over the free ligand and other zerovalent molybdenum complexes of DAB. Compared to known examples theoretically described as iminato π-radicals (L(•-)), the oxidation state assignment fits a molybdenum(II) description. However, Mo K-edge X-ray absorption spectroscopy indicates that the complexes are best described as molybdenum(0). This example demonstrates that caution should be exercised in assigning the oxidation state based on structural parameters alone. Cyclic voltammetry studies reveal an electrochemical-chemical process that has been identified by in situ Fourier transform infrared spectroelectrochemistry as cis-to-trans isomerization.

Reinvestigation of a former concerted proton-electron transfer (CPET), the reduction of a hydrogen-bonded complex between a proton donor and the anion radical of 3,5-di-tert-butyl-1,2-benzoquinone.

In 2001, Lehmann and Evans (J. Phys. Chem. B, 2001, 105, 8877-8884) reported that the electrochemical reduction of a hydrogen-bonded complex between a proton donor and the anion radical of 3,5-di-tert-butyl-1,2-benzoquinone in acetonitrile proceeded by a concerted proton-electron transfer (CPET) reaction in which electron transfer from the electrode and proton transfer from proton donor to the quinone moiety occurred concertedly. Support for this conclusion was based upon ruling out both of the competing two-step processes, electron transfer followed by proton transfer (EP) and proton transfer followed by electron transfer (PE). In the course of studies of related compounds it was decided to reinvestigate the reduction of 3,5-di-tert-butyl-1,2-benzoquinone. It was discovered that the earlier conclusion that a CPET reaction was occurring was tenable only for the particular electrolyte that was used, tetrabutylammonium hexafluorophosphate and for lower concentrations of the quinone. Even the small change of carrying out the reduction of the quinone in the presence of water with tetramethylammonium hexafluorophosphate as electrolyte, produced voltammograms with clear signatures that the process was EP rather than CPET. Even more dramatic effects were seen with cesium, potassium or sodium ions in the electrolyte. A general reaction scheme to explain results with all electrolytes will be presented.

On the causes of potential inversion in 1,2,4,5-tetrakis(amino)benzenes.

Three 3,6-difluoro-1,2,4,5-tetrakis(amino)benzene compounds, bearing dimethylamino (1), piperidin-1-yl (3), or morpholin-1-yl (5) substituents, have been synthesized and subsequently defluorinated to give the corresponding 1,2,4,5-tetrakis(amino)benzene compounds 2, 4, and 6; the crystal structures of compounds 1, 4, and 6 have been obtained. Cyclic voltammetry shows that all six compounds will lose two electrons to form dications, and the use of suitable oxidizing agents has allowed isolation and crystallographic characterization of the dications 2(2+) and 6(2+) (as [PF(6)](2) salts) and 4(2+) (as a [I(5)][I(3)] salt). The separation DeltaE between the loss of the first electron and the second varies between compounds, from 0.23 V in 1 to 0.01 V in 6. Electrochemical studies involving the use of the noncoordinating electrolyte [Bu(4)N][B{C(6)H(3)(CF(3))(2)}(4)] show that it is possible to increase this separation, stabilizing the intermediate monocationic phase, and this has allowed the isolation and crystallographic characterization of the radical salts 2[B{C(6)H(3)(CF(3))(2)}(4)] and 4[B{C(6)H(3)(CF(3))(2)}(4)], the first radical cations of this family to be isolated. DFT studies of the ion pairing between oxidized forms of 1 and 2 and anions imply that the location of the ion pairing is different in the two species.

Electrochemical and chemical oxidation of dithia-, diselena-, ditellura-, selenathia-, and tellurathiamesocycles and stability of the oxidized species.

The diverse electrochemical and chemical oxidations of dichalcogena-mesocycles are analyzed, broadening our understanding of the chemistry of the corresponding radical cations and dications. 1,5-Diselenocane and 1,5-ditellurocane undergo reversible two-electron oxidation with inverted potentials analogous to 1,5-dithiocane. On the other hand, 1,5-selenathiocane and 1,5-tellurathiocane undergo one-electron oxidative dimerization. The X-ray crystal structures of the Se-Se dimer of the 1,5-selenathiocane one-electron oxidized product and the monomeric two-electron oxidized product (dication) of 1,5-tellurathiocane are reported. 1,5-Dithiocanes and 1,5-diselenocanes with group 14 atoms as ring members undergo irreversible oxidation, unlike the reversible two-electron oxidation of the corresponding silicon-containing 1,5-ditellurocanes. These results demonstrate the chemical consequences of the dication stabilities Te(+)-Te(+) > Se(+)-Se(+) > S(+)-S(+), as well as Se(+)-Se(+) > Se(+)-S(+) and Te(+)-Te(+) > Te(+)-S(+).

One- to two-electron reduction of an [FeFe]-hydrogenase active site mimic: the critical role of fluxionality of the [2Fe2S] core.

The one- to two-electron reduction of mu-(1,2-ethanedithiolato)diironhexacarbonyl that has been observed under electrochemical conditions is dependent on scan rate and temperature, suggesting activation of a structural rearrangement. This structural rearrangement is attributed to fluxionality of the [2Fe2S] core in the initially formed anion. Computations support this assessment. Upon an initial one-electron reduction, the inherent fluxionality of the [2Fe2S] complex anion allows for a second one-electron reduction at a less negative potential to form a dianionic species. The structure of this dianion is characterized by a rotated iron center, a bridging carbonyl ligand, and, most significantly, a dissociated Fe-S bond. This fluxionality of the [2Fe2S] core upon reduction has direct implications for the chemistry of [FeFe]-hydrogenase mimics and for iron-sulfur cluster chemistry in general.

Insights into the Nature of Mo(V) Species in Solution: Modeling Catalytic Cycles for Molybdenum Enzymes.

The tris(pyrazolyl)borate and related tripodal N-donor ligands originally developed by Trofimenko stabilize mononuclear compounds containing Mo(VI)O(2), Mo(VI)O, Mo(V)O, and Mo(IV)O units and effectively inhibit their polynucleation in organic solvents. Dioxo-Mo(VI) complexes of the type LMoO(2)(SPh), where L = hydrotris(3,5-dimethylpyrazol-1-yl)borate (Tp*), hydrotris(3-isopropylpyrazol-1-yl)borate (Tp(i) (Pr)), and hydrotris(3,5-dimethyl-1,2,4-triazol-1-yl)borate (Tz) and related derivatives are the only model systems that mimic the complete reaction sequence of sulfite oxidase, in which oxygen from water is ultimately incorporated into product. The quasi-reversible, one-electron reduction of Tp*MoO(2)(SPh) in acetonitrile exhibits a positive potential shift upon addition of a hydroxylic proton donor, and the magnitude of the shift correlates with the acidity of the proton donor. These reductions produce two Mo(V) species, [Tp*Mo(V)O(2)(SPh)](-) and Tp*Mo(V)O(OH)(SPh), that are related by protonation. Measurement of the relative amounts of these two Mo(V) species by EPR spectroscopy enabled the pK(a) of the Mo(V)(OH) unit in acetonitrile to be determined and showed it to be several pK(a) units smaller than that for water in acetonitrile. Similar electrochemical-EPR experiments for Tp(i) (Pr)MoO(2)(SPh) indicated that the pK(a) for its Mo(V)(OH) unit was ∼1.7 units smaller than that for Tp*Mo(V)O(OH)(SPh). Density functional theory calculations also predict a smaller pK(a) for (iPr)Mo(V)O(OH)(SPh) compared to Tp*Mo(V)O(OH)(SPh). Analysis of these results indicates that coupled electron-proton transfer (CEPT) is thermodynamically favored over the indirect process of metal reduction followed by protonation. The crystal structure of Tp(i) (Pr)MoO(2)(SPh) is also presented.

Alkylated trimethylene-bridged bis(p-phenylenediamines).

One and two electron oxidation of N,N',N'',N'''-tetramethyl-1,5,12,16-tetraaza[5,5]paracyclophane (Me3C), a bis-trimethylene bridged bis-p-phenylene diamine (PD), and its ethyl and isopropyl analogues are discussed. The monocation and dication are both stable, as demonstrated by optical studies that show they are in equilibrium in solution, with an especially small difference in first and second oxidation potentials for Me3C in MeCN (+23 to -20 mV, measured by simulation of the optical spectrum and of the cyclic voltammogram, respectively). The monocations have charge localized in one PD unit and show a Hush-type mixed valence transition between their PD0 and PD.+ groups. The dications Me3C2+ and Et3C2+ have optical spectra that appear to show large splittings between their PD.+ groups and have a weak ESR spectrum, and 1H NMR data show that the former is a ground-state singlet. iPr3C2+ has a very different optical spectrum and exhibits a triplet ESR spectrum at 120 K. X-ray crystal structures show that for Me3C0 the N(CH2)3N units on each side are in doubly anti (aa) conformations that put the aryl rings as far apart as possible, but Me3C2+ has doubly gg N(CH2)3N trimethylene bridges and both N,N and C,C distances between the PD.+ groups that are significantly below van der Walls contact. In contrast, iPr3C0 is in a doubly ag conformation, and its diradical dication is suggested to be a triplet because it does not attain the doubly gg conformation.

Macromolecular Engineering of the Outer Coordination Sphere of [2Fe-2S] Metallopolymers to Enhance Catalytic Activity for H2 Production.

Small-molecule catalysts inspired by the active sites of [FeFe]-hydrogenase enzymes have long struggled to achieve fast rates of hydrogen evolution, long-term stability, water solubility, and oxygen compatibility. We profoundly improved on these deficiencies by grafting polymers from a metalloinitiator containing a [2Fe-2S] moiety to form water-soluble poly(2-dimethylamino)ethyl methacrylate metallopolymers (PDMAEMA-g-[2Fe-2S]) using atom transfer radical polymerization (ATRP). This study illustrates the critical role of the polymer composition in enhancing hydrogen evolution and aerobic stability by comparing the catalytic activity of PDMAEMA-g-[2Fe-2S] with a nonionic water-soluble metallopolymer based on poly(oligo(ethylene glycol) methacrylate) prepared via ATRP (POEGMA-g-[2Fe-2S]) with the same [2Fe-2S] metalloinitiator. Additionally, the tunability of catalyst activity is demonstrated by the synthesis of metallocopolymers incorporating the 2-(dimethylamino)ethyl methacrylate (DMAEMA) and oligo(ethylene glycol) methacrylate (OEGMA) monomers. Electrochemical investigations into these metallo(co)polymers show that PDMAEMA-g-[2Fe-2S] retains complete aerobic stability with catalytic current densities in excess of 20 mA·cm-2, while POEGMA-g-[2Fe-2S] fails to reach 1 mA·cm-2 current density even with the application of high overpotentials (η > 0.8 V) and loses all activity in the presence of oxygen. Random copolymers of the two monomers polymerized with the same [2Fe-2S] initiator showed intermediate activity in terms of current density, overpotential, and aerobic stability.

Nickel Complexes of C-Substituted Cyclams and Their Activity for CO2 and H+ Reduction.

Several nickel(II) complexes of cyclams bearing aryl groups on the carbon backbone were prepared and evaluated for their propensity to catalyze the electrochemical reduction of CO2 to CO and/or H+ to H2, representing the first catalytic analysis to be performed on an aryl-cyclam metal complex. Cyclic voltammetry (CV) revealed the attenuation of catalytic activity when the aryl group bears the strong electron-withdrawing trifluoromethyl substituent, whereas the phenyl, p-tolyl, and aryl-free derivatives displayed a range of catalytic activities. The gaseous-product distribution for the active complexes was determined by means of controlled-potential electrolysis (CPE) and revealed that the phenyl derivative is the most active as well as the most selective for CO2 reduction over proton reduction. Stark differences in the activity of the complexes studied are rationalized through comparison of their X-ray structures, absorption spectra, and CPE profiles. Further CV studies on the phenyl derivative were undertaken to provide a kinetic insight.

Study of the electrochemical reduction of dioxygen in acetonitrile in the presence of weak acids.

The electrochemical reduction of dioxygen has been studied in acetonitrile at glassy-carbon electrodes. The initial step is the reversible one-electron reduction to form superoxide. In the presence of hydrogen-bond donors (water, methanol, 2-propanol), the superoxide forms a complex with the donor resulting in a positive shift in the potential that can be analyzed to obtain formation constants for these complexes. Stronger acids result in protonation of the superoxide followed by reduction to produce HO2-. In the absence of hydrogen-bond donors, the reduction of superoxide occurs at very negative potentials, and this second reduction peak is very much drawn-out along the potential axis, indicating a small value of the transfer coefficient, alpha. The addition of hydrogen-bond donors, HA, brings about a positive shift in this peak, without a noticeable change in shape. The reaction occurring at the second peak is a concerted proton and electron transfer (CPET) in which the electron is transferred to superoxide and a proton is transferred from HA to the superoxide, forming HO2- and A- in a concerted process. An estimation of the standard potential for this reaction shows that the second reduction always occurs at a high driving force, which explains the small value of alpha that is observed. Consistent with a CPET, a kinetic isotope effect, HA versus DA, was detected for the three hydrogen-bond donors. The increasing positive shift of the second peak with increasing water concentration has been interpreted as being a consequence of the change in the formal potential, as water is both a reactant in the process and a participant through the hydrogen-bond stabilization of the anions.

Change in reaction pathway in the reduction of 3,5-di-tert-butyl-1,2-benzoquinone with increasing concentrations of 2,2,2-trifluoroethanol.

The electrochemical reduction of 3,5-di-tert-butyl-1,2-benzoquinone, 1, has been studied in acetonitrile with added 2,2,2-trifluoroethanol, 2. At low concentrations of 2 the reaction proceeds by the following pathway: reduction of the quinone (Q) to its anion radical (Q*-) followed by complexation of the anion radical with 2 (HA) and the further reduction of the hydrogen-bonded complex (Q*- (HA)) to form HQ- and A-. The latter reaction is a concerted proton and electron- transfer reaction (CPET). At higher concentrations of 2, the pathway changes. The first steps remain the same, but now Q*- (HA) is reduced to HQ- via a disproportionation reaction with Q*- along with proton transfer from HA to Q*- to form HQ* which is reduced to HQ-. The only mechanism that could be found which would account for all of the data involves proton transfer to Q*- occurring within a higher complex, Q*-(HA)3.

Electron-transfer reactions with significant changes in structure. Unsymmetrical crowded ethylenes.

The electrochemical reduction mechanisms of xanthylideneanthrone, 6, thioxanthylideneanthrone, 7, 10-(diphenylmethylene)anthrone, 8, and 9-(diphenylmethylene)-9H-fluorene, 9, have been studied in dimethylformamide. The reduction of the first two compounds proceeds from folded forms of the neutral to twisted forms of the anion radical according to a square scheme. The data for reduction of 8 can be well accounted for by the same square scheme. However, one-step reduction with concerted electron transfer and structural change cannot be ruled out. Compound 9, whose fluorene ring system cannot fold, exists only in twisted forms in the neutral, anion radical, and dianion. Consequently, there are no major changes in structure upon reduction, and the compound is reduced in two reversible steps with the second complicated by rapid loss of the dianion that is probably due to protonation by components of the medium.

Studies of potential inversion in the electrochemical reduction of 11,11,12,12-tetracyano-9,10-anthraquinodimethane and 2,3,5,6-tetramethyl-7,7,8,8-tetracyano-1,4-benzoquinodimethane.

The electrochemical reduction of the title compounds, 2a and 3a, and 7,7,8,8-tetracyanoquinodimethane, 1a, was studied in acetonitrile. The reduction of 1a shows normal ordering of potentials, i.e., the potential for insertion of the first electron, E degrees1, is more positive than the potential for the second step of reduction, E degrees2. Thus, E degrees1 - E degrees2 > 0. By contrast, 2a and 3a show inversion of potentials where introduction of the second electron occurs with greater ease than the first (E degrees1 - E degrees2 < 0). The extent of inversion has been determined by simulation of the cyclic voltammograms obtained at 298 and 257 K. Electron paramagnetic resonance measurements at room temperature of solutions containing equimolar mixtures of the neutral and dianion allow determination of the concentration of anion radicals from which the disproportionation equilibrium constant and E degrees1 - E degrees2 can be calculated. The results were in good agreement with the voltammetric determinations. Calculations were conducted to characterize the structural changes accompanying reduction to the anion radical and dianion forms. Fast scan experiments at low temperatures (up to 10 000 V/s at 257 K; 500 V/s at 233 K) were conducted in an attempt to detect intermediates in the reduction, but none was found. Thus, it is not possible to state whether structural change and electron transfer are concerted or occur in discrete steps.

Study of the effects of ion pairing and activity coefficients on the separation in standard potentials for two-step reduction of dinitroaromatics.

The electrochemical reduction of 9,10-dinitroanthracene, 1, and 3,6-dinitrodurene, 2, occurs with potential inversion. That is, the standard potential for formation of the anion radical is shifted in the negative direction from the standard potential for the anion radical/dianion couple. This behavior has been attributed to significant structural changes accompanying the reduction steps. In this work, an assessment was made of the magnitude of the effects of activity coefficients and ion pairing, two effects which contribute to potential inversion. 1,4-Dinitrobenzene, 3, and 2,5-dimethyl-1,4-dinitrobenzene, 4, were studied in acetonitrile and N,N-dimethylformamide with R(4)N(+) salts as electrolytes (R = CH(3)-, CH(3)CH(2)-, CH(3)(CH(2))(3)-, and CH(3)(CH(2))(7)-) at concentrations from 0.010 to 0.100 M. Significant ion pairing between the dianion, A(2-), and R(4)N(+) was found for (CH(3))(4)N(+) with both 3 and 4 while the effects of the other electrolytes were smaller. The data were successfully interpreted without recourse to other ion pairs, e.g., ion pairing between the anion radical and the electrolyte cation. Ion pair formation constants are reported along with the infinite-dilution values of the difference in the two standard potentials. The effects of activity coefficients and ion pairing at 0.10 M electrolyte do not exceed 100 mV for (CH(3)N(+)) and are only 20 to 60 mV for (CH(3))(4)N(+), a cation commonly used in studies of potential inversion. It is concluded that structural changes accompanying the reduction, rather than activity and ion pairing effects, are the dominant factors underlying potential inversion.

One-step synthesis of substituted 6-amino-5-cyanospiro-4-(piperidine-4')- 2H,4H-dihydropyrazolo[3,4-b]pyrans.

Three-component condensation of 4-piperidinones (7), 5-pyrazolones (8), and malononitrile (4) proceeds chemically and electrochemically and is a convenient one-step means of synthesis of substituted 6-amino-5-cyanospiro-4-(piperidine-4')-2H,4H-dihydropyrazolo[3,4-b]pyrans (12). The electrochemical reactions proceed under milder conditions and with yields 12-15% greater than those of the reactions catalyzed by chemical bases.

Comparative Studies of Cathodically-Promoted and Base-Catalyzed Michael Addition Reactions of Levoglucosenone.

Regioselective Michael addition of nitro and heterocyclic compounds to levoglucosenone, 1, is effectively catalyzed by amines and also by cathodic electrolysis. In comparison to the base-catalyzed reaction, it was found that under electrochemical conditions the reaction proceeds under milder conditions and with higher yields. Cathodically-initiated Michael addition of thiols to levoglucosenone using small currents produces the previously unknown threo addition product in several instances. The normal erythro isomer, identified as the kinetic product, tends to be formed when large currents are used. In contrast, slow, low current electrolyses promote equilibration of the two forms so that erythro can be converted to threo by the retro reaction and readdition. Addition of 2-naphthalenethiol to (R)-(+)-apoverbenone is also reported.

Electronic and geometric effects of phosphatriazaadamantane ligands on the catalytic activity of an [FeFe] hydrogenase inspired complex.

The [FeFe] hydrogenase enzyme active site inspired complexes [Fe(2)(mu-C(6)H(4)S(2))(CO)(5)PTA] (1PTA) and [Fe(2)(mu-C(6)H(4)S(2))(CO)(4)PTA(2)] (1PTA(2)) (PTA = 1,3,5-triaza-7-phosphaadamantane) were synthesized and characterized. The ability of 1PTA and 1PTA(2) to catalytically produce molecular hydrogen in solution from the weak acid acetic acid was examined electrochemically and compared to previous studies on the all carbonyl containing analogue [Fe(2)(mu-C(6)H(4)S(2))(CO)(6)] (1). Computational methods and cyclic voltammograms indicated that the substitution of CO ligands by PTA in 1 resulted in markedly different reduction chemistry. Both 1PTA and 1PTA(2) catalytically produce molecular hydrogen from acetic acid, however, the mechanism by which and 1PTA and 1PTA(2) catalyze hydrogen differ in the initial reductive processes.