Sequential One-electron Reductions Research Articles

Reductive Dynamic and Static Excited State Quenching of a Homoleptic Ruthenium Complex Bearing Aldehyde Groups.

A new homoleptic Ru polypyridyl complex bearing two aldehyde groups on each bipyridine ligand, [Ru(dab)3](PF6)2, where dab is 4,4'-dicarbaldehyde-2,2'-bipyridine, was synthesized, characterized, and utilized for iodide photo-oxidation studies. In acetonitrile (CH3CN) solution, the complex displayed an intense metal-to-ligand charge transfer (MLCT) absorbance maximum at 475 nm (ε = 22,000 M-1 cm-1) and an infrared (IR) band at 1712 cm-1 assigned to the pendent aldehyde groups. Visible light excitation in air-saturated solution resulted in room temperature photoluminescence (PL) with a maximum at 675 nm, a quantum yield, ϕPL = 0.048, and an excited state lifetime, το = 440 ns, from which radiative and nonradiative relaxation rate constants were extracted, kr = 9.1 × 104 s-1 and knr = 1.8 × 106 s-1. Pulsed visible light excitation yielded transient UV-vis and IR absorption spectra consistent with an MLCT excited state; relaxation occurred with the maintenance of two isosbestic points in the visible region, and a lifetime that agreed with that measured by time-resolved PL. Cyclic voltammetry studies in a CH3CN solution with 0.1 M TBAPF6 electrolyte revealed a quasi-reversible oxidation, E°(RuIII/II) = +1.25 V vs. Fc+/0, and three sequential one-electron reductions at -1.10, -1.25, and -1.54 V vs. Fc+/0. An excited state reduction potential of E°(Ru*2+/+) = +0.89 V vs. Fc+/0 was estimated with the Rehm-Weller expression. Titration of tetrabutylammonium iodide, TBAI, into a CD3CN solution of [Ru(dab)3](PF6)2 resulted in significant shifts in the aldehyde H atom and 3,3'-biypridyl resonances that were analyzed with a 1:1 equilibrium model, from which Keq = 460 M-1 was extracted, increasing to 5800 M-1 when the solvent was changed to acetone-d6. Iodide titrations resulted in a significant quenching of the [Ru(dab)3]*2+ lifetime and quantum yield in both CH3CN and acetone solvents. In CH3CN, the quenching was mainly dynamic and well described by the Stern-Volmer model, from which a quenching rate constant, kq, of 4.5 × 1010 M-1 s-1 and an equilibrium constant, Keq, of 8.3 × 103 M-1 were obtained. In acetone, the static quenching pathway by iodide was greatly enhanced, with a Keq of 1.2 × 104 M-1 and a higher kq of 9.2 × 1010 M-1 s-1.

Exploring the Impact of Successive Redox Events in Thin Films of Metal-Organic Frameworks: An Absorptiometric Approach.

Metal-organic frameworks (MOFs) featuring redox activity are highly appealing for electrocatalytic or charge accumulation applications. An important aspect in this field is the ability to address as many redox centers as possible in the material by an efficient diffusion of charges. Herein, we investigate for the first time the charge diffusion processes occurring upon two sequential one-electron reductions in an MOF thin film. Two pyrazolate-zinc(II)-based MOFs including highly electro-deficient perylene diimide (PDI) ligands were grown on conducting substrates, affording thin films with double n-type electrochromic properties as characterized by spectroelectrochemical analysis. In depth electrochemical and chronoabsorptiometric investigations were carried out to probe the charge diffusion in the MOF layers and highlighted significant differences in terms of diffusion kinetics and material stability between the first and second successive reduction of the redox-active PDI linkers. Our results show that MOFs based on multiredox centers are more sensitive to encumbrance-related issues than their monoredox analogues in the context of electrochemical applications, an observation that further underlines the fundamental aspect of careful pore dimensions toward efficient and fast ion diffusion.

Effect of Redox Active Ligands on the Electrochemical Properties of Manganese Tricarbonyl Complexes.

The synthesis, structural characterization, and electrochemical behavior of the neutral Mn(azpy)(CO)3(Br) 4 (azpy = 2-phenylazopyridine) complex is reported and compared with its structural analogue Mn(bipy)(CO)3(Br) 1 (bipy = 2,2'-bipyridine). 4 exhibits reversible two-electron reduction at a mild potential (-0.93 V vs Fc+/0 in acetonitrile) in contrast to 1, which exhibits two sequential one-electron reductions at -1.68 V and -1.89 V vs Fc+/0 in acetonitrile. The key electronic structure differences between 1 and 4 that lead to disparate electrochemical properties are investigated using a combination of Mn-K-edge X-ray absorption spectroscopy (XAS), Mn-Kβ X-ray emission spectroscopy (XES), and density functional theory (DFT) on 1, 4, their debrominated analogues, [Mn(L)(CO)3(CH3CN)][CF3SO3] (L = bipy 2, azpy 5), and two-electron reduced counterparts [Mn(bipy)(CO)3][K(18-crown-6)] 3 and [Mn(azpy)(CO)3][Cp2Co] 6. The results reveal differences in the distribution of electrons about the CO and bidentate ligands (bipy and azpy), particularly upon formation of the highly reduced, formally Mn(-1) species. The data show that the degree of ligand noninnocence and resulting redox-activity in Mn(L)(CO)3 type complexes impacts not only the reducing power of such systems, but the speciation of the reduced complexes via perturbation of the monomer-dimer equilibrium in the singly reduced Mn(0) state. This study highlights the role of redox-active ligands in tuning the reactivity of metal centers involved in electrocatalytic transformations.

Spectroscopic, thermodynamic and computational evidence of the locations of the FADs in the nitrogen fixation-associated electron transfer flavoprotein.

Flavin-based electron bifurcation allows enzymes to redistribute energy among electrons by coupling endergonic and exergonic electron transfer reactions. Diverse bifurcating enzymes employ a two-flavin electron transfer flavoprotein (ETF) that accepts hydride from NADH at a flavin (the so-called bifurcating FAD, Bf-FAD). The Bf-FAD passes one electron exergonically to a second flavin thereby assuming a reactive semiquinone state able to reduce ferredoxin or flavodoxin semiquinone. The flavin that accepts one electron and passes it on via exergonic electron transfer is known as the electron transfer FAD (ET-FAD) and is believed to correspond to the single FAD present in canonical ETFs, in domain II. The Bf-FAD is believed to be the one that is unique to bifurcating ETFs, bound between domains I and III. This very reasonable model has yet to be challenged experimentally. Herein we used site-directed mutagenesis to disrupt FAD binding to the presumed Bf site between domains I and III, in the Bf-ETF from Rhodopseudomonas palustris (RpaETF). The resulting protein contained only 0.80 ± 0.05 FAD, plus 1.21 ± 0.04 bound AMP as in canonical ETFs. The flavin was not subject to reduction by NADH, confirming absence of Bf-FAD. The retained FAD displayed visible circular dichroism (CD) similar to that of the ET-FAD of RpaETF. Likewise, the mutant underwent two sequential one-electron reductions forming and then consuming anionic semiquinone, reproducing the reactivity of the ET-FAD. These data confirm that the retained FAD in domain II corresponds the ET-FAD. Quantum chemical calculations of the absorbance and CD spectra of each of WT RpaETF's two flavins reproduced the observed differences between their CD and absorbance signatures. The calculations for the flavin bound in domain II agreed better with the spectra of the ET-flavin, and those calculated based on the flavin between domains I and III agreed better with spectra of the Bf-flavin. Thus calculations independently confirm the locations of each flavin. We conclude that the site in domain II harbours the ET-FAD whereas the mutated site between domains I and III is the Bf-FAD site, confirming the accepted model by two different tests.

Chemical and Electrochemical Properties of [Cp*Rh] Complexes Supported by a Hybrid Phosphine-Imine Ligand

A series of [Cp*Rh] complexes (Cp* = η5-pentamethylcyclopentadienyl) bearing the κ2-[P,N]-8-(diphenylphosphino)quinoline (PQN) ligand have been prepared and characterized. Chemical or electrochemical reduction of the rhodium(III) form generates an isolable rhodium(I) complex; this rhodium(I) complex reacts with a range of organic acids to yield a rhodium(III) hydride bearing [Cp*] in the η5 mode and [PQN] in the expected κ2 mode. Solid-state structures of these three compounds from X-ray diffraction studies reveal only small changes in the intraligand bond distances across the series, suggesting the redox events associated with interconversion of these compounds are primarily metal centered. Cyclic voltammetry data show that the rhodium(III) chloride complex undergoes a two-electron reduction at −1.19 V vs ferrocenium/ferrocene, whereas the analogous solvento rhodium(III) acetonitrile complex undergoes two sequential one-electron reductions. The rhodium(III) hydride undergoes an irreversible, ligand-centered reduction near −1.75 V vs ferrocenium/ferrocene. Carrying out this reduction alone or in the presence of added of triethylammonium as a source of protons results in only modest yields of H2, as shown by bulk electrolyses and chemical reduction experiments. These results are discussed in the context of recent work with [Cp*Rh] complexes bearing more symmetric 2,2′-bipyridyl and diphosphine ligands.

Distinct properties underlie flavin-based electron bifurcation in a novel electron transfer flavoprotein FixAB from Rhodopseudomonas palustris

A newly recognized third fundamental mechanism of energy conservation in biology, electron bifurcation, uses free energy from exergonic redox reactions to drive endergonic redox reactions. Flavin-based electron bifurcation furnishes low-potential electrons to demanding chemical reactions, such as reduction of dinitrogen to ammonia. We employed the heterodimeric flavoenzyme FixAB from the diazotrophic bacterium Rhodopseudomonas palustris to elucidate unique properties that underpin flavin-based electron bifurcation. FixAB is distinguished from canonical electron transfer flavoproteins (ETFs) by a second FAD that replaces the AMP of canonical ETF. We exploited near-UV-visible CD spectroscopy to resolve signals from the different flavin sites in FixAB and to interrogate the putative bifurcating FAD. CD aided in assigning the measured reduction midpoint potentials (E° values) to individual flavins, and the E° values tested the accepted model regarding the redox properties required for bifurcation. We found that the higher-E° flavin displays sequential one-electron (1-e-) reductions to anionic semiquinone and then to hydroquinone, consistent with the reactivity seen in canonical ETFs. In contrast, the lower-E° flavin displayed a single two-electron (2-e-) reduction without detectable accumulation of semiquinone, consistent with unstable semiquinone states, as required for bifurcation. This is the first demonstration that a FixAB protein possesses the thermodynamic prerequisites for bifurcating activity, and the separation of distinct optical signatures for the two flavins lays a foundation for mechanistic studies to learn how electron flow can be directed in a protein environment. We propose that a novel optical signal observed at long wavelength may reflect electron delocalization between the two flavins.

Development of a UiO-Type Thin Film Electrocatalysis Platform with Redox-Active Linkers.

Metal-organic frameworks (MOFs) as electrocatalysis scaffolds are appealing due to the large concentration of catalytic units that can be assembled in three dimensions. To harness the full potential of these materials, charge transport to the redox catalysts within the MOF has to be ensured. Herein, we report the first electroactive MOF with the UiO/PIZOF topology (Zr(dcphOH-NDI)), i.e., one of the most widely used MOFs for catalyst incorporation, by using redox-active naphthalene diimide-based linkers (dcphOH-NDI). Hydroxyl groups were included on the dcphOH-NDI linker to facilitate proton transport through the material. Potentiometric titrations of Zr(dcphOH-NDI) show the proton-responsive behavior via the -OH groups on the linkers and the bridging Zr-μ3-OH of the secondary building units with pKa values of 6.10 and 3.45, respectively. When grown directly onto transparent conductive fluorine-doped tin oxide (FTO), 1 μm thin films of Zr(dcphOH-NDI)@FTO could be achieved. Zr(dcphOH-NDI)@FTO displays reversible electrochromic behavior as a result of the sequential one-electron reductions of the redox-active NDI linkers. Importantly, 97% of the NDI sites are electrochemically active at applied potentials. Charge propagation through the thin film proceeds through a linker-to-linker hopping mechanism that is charge-balanced by electrolyte transport, giving rise to cyclic voltammograms of the thin films that show characteristics of a diffusion-controlled process. The equivalent diffusion coefficient, De, that contains contributions from both phenomena was measured directly by UV/vis spectroelectrochemistry. Using KPF6 as electrolyte, De was determined to be De(KPF6) = (5.4 ± 1.1) × 10-11 cm2 s-1, while an increase in countercation size to n-Bu4N+ led to a significant decrease of De by about 1 order of magnitude (De(n-Bu4NPF6) = (4.0 ± 2.5) × 10-12 cm2 s-1).

Multiple binding modes of an unconjugated bis(pyridine) ligand stabilize low-valent [Cp*Rh] complexes.

The ligand 2,2'-bipyridine (bpy) can support metal centers in low formal oxidation states by delocalization of electron density into its π-system. We show that, in a model rhodium complex supported by the pentamethylcyclopentadienyl ligand (Cp*), the analogous dimethyldipyridylmethane ligand (Me2dpma) enforces a bpy-like coordination environment but disrupts the inter-ring conjugation responsible for charge delocalization upon metal reduction. As a result, reduction proceeds in discrete one-electron steps (Rh(iii) to Rh(ii) to Rh(i)), contrasting with the 2e- chemistry engendered by bpy. Upon reduction to Rh(i), the Me2dpma ligand rearranges to activate strong π-backbonding via facial coordination of one pyridine motif. Structural and spectroscopic studies confirm stabilization of the Rh(i) center in this compound, revealing a mode of metal-ligand cooperation that represents a useful counterpoint to charge delocalization in conjugated poly(pyridyl) ligands.

Phosphate Shifted Oxygen Reduction Pathway on Fe@Fe2O3 Core-Shell Nanowires for Enhanced Reactive Oxygen Species Generation and Aerobic 4-Chlorophenol Degradation.

Phosphate ions widely exist in the environment. Previous studies revealed that the adsorption of phosphate ions on nanoscale zerovalent iron would generate a passivating oxide shell to block reactive sites and thus decrease the direct pollutant reduction reactivity of zerovalent iron. Given that molecular oxygen activation process is different from direct pollutant reduction with nanoscale zerovalent iron, it is still unclear how phosphate ions will affect molecular oxygen activation and reactive oxygen species generation with nanoscale zerovalent iron. In this study, we systematically studied the effect of phosphate ions on molecular oxygen activation with Fe@Fe2O3 nanowires, a special nanoscale zerovalent iron, taking advantages of rotating ring disk electrochemical analysis. It was interesting to find that the oxygen reduction pathway on Fe@Fe2O3 nanowires was gradually shifted from a four-electron reduction pathway to a sequential one-electron reduction one, along with increasing the phosphate ions concentration from 0 to 10 mmol·L-1. This oxygen reduction pathway change greatly enhanced the molecular oxygen activation and reactive oxygen species generation performances of Fe@Fe2O3 nanowires, and thus increased their aerobic 4-chlorophenol degradation rate by 10 times. These findings shed insight into the possible roles of widely existed phosphate ions in molecular oxygen activation and organic pollutants degradation with nanoscale zerovalent iron.

Multielectron Transfer at Cobalt: Influence of the Phenylazopyridine Ligand

The dicationic complex [CpCo(azpy)(CH3CN)](ClO4)2 1 (azpy = phenylazopyridine) exhibits a reversible two-electron reduction at a very mild potential (-0.16 V versus Fc0/+) in acetonitrile. This behavior is not observed with the analogous bipyridine and pyrazolylpyridine complexes (3 and 4), which display an electrochemical signature typical of CoIII systems: two sequential one-electron reductions to CoII at -0.4 V and CoI at -1.0 to -1.3 V versus Fc0/+. The doubly reduced, neutral complex [CpCo(azpy)] 2 is isolated as an air-stable, diamagnetic solid via chemical reduction with cobaltocene. Crystallographic and spectroscopic characterization together with experimentally calibrated density functional theory calculations illuminate the key structural and electronic changes that occur upon reduction of 1 to 2. The electrochemical potential inversion observed with 1 is attributed to effective overlap between the metal d and the low-energy azo π* orbitals in the intermediary redox state and additional stabilization of 2 from structural reorganization, leading to a two-electron reduction. This result serves as a key milestone in the quest for two-electron transformations with mononuclear first-row transition metal complexes at mild potentials.

Coordination Chemistry and Reactivity of Bis(aldimino)pyridine Nickel Complexes in Four Different Oxidation States

A series of nickel complexes with potentially redox active bis(aldimino)pyridine ligands [NNN] ([NNN] = 1,1′-(pyridine-2,6-diyl)bis(N-arylmethanimine), where aryl = 2,6-diisopropylphenyl, mesityl, 4-methoxyphenyl, 4-trifluoromethylphenyl, and 3,5-bis(trifluoromethyl)phenyl) were synthesized, and their properties and reactivities were investigated as a function of the overall oxidation state of the system. (Ni[NNN])2+ complexes of ligands featuring bulky electron-rich substituents (1a-Br2 and 1b-Br2, [NNN] = 1,1′-(pyridine-2,6-diyl)bis(N-(2,6-diisopropylphenyl)methanimine) and 1,1′-(pyridine-2,6-diyl)bis(N-mesitylmethanimine), respectively) demonstrated five electrochemical reduction events, the first three of which were quasi-reversible. In contrast, only two quasi-reversible reductions were observed for the less bulky and electron-deficient N-aryl substituents 4-(trifluoromethyl)phenyl and 3,5-bis(trifluoromethyl)phenyl. Chemical reduction of 1a-Br2 and 1b-Br2 with 1 equiv of KC8 or CoCp*2 forms (Ni[NNN])+ complexes of the general formula Ni[NNN]Br (2a-Br and 2b-Br). Structural, spectroscopic, and theoretical studies reveal that these complexes feature significant unpaired spin density on the metal, consistent with “nickel(I)” character. This behavior is in contrast with previously reported bis(ketimino)pyridine systems, in which at the (Ni[NNN])+ state the unpaired electron resided exclusively in the ligand. Further reduction forms a series of (Ni[NNN])0 complexes, in which all of the potentially tridentate [NNN] ligands bind via only one iminopyridine unit; the second arm is left unbound in most complexes. Variable temperature NMR spectroscopy demonstrates that bound and unbound arms exchange via a postulated tridentate intermediate. Electrochemical reduction, via three sequential one-electron reductions, of 1a-Br2 and 1b-Br2 in the presence of CO2/H+ forms an active catalyst for H2 evolution at a glassy-carbon electrode surface, again emphasizing the unique redox chemistry of the bulky bis(aldimino)pyridine nickel complexes.

Electrocatalytic reduction of CO2 to CO in the presence of a mononuclear polypyridyl ruthenium(II) complex

A new polypyridyl Ru(II) complex, [Ru(tptz)(CH3CN)Cl2] (tptz=2,4,6-tri(2-pyridyl)-1,3,5-triazine) has been prepared and characterized. It crystallizes with one molecule of acetonitrile and a partly occupied water solvent molecule. X-ray analysis of the complex revealed that the central Ru(II) atom has a distorted octahedral Cl2N3N' coordination environment. The complex was used as an electrocatalyst for the reduction of CO2 to CO in an acetonitrile solution as monitored by cyclic voltammetry. The electrocatalytic activity of the complex was investigated for CO2 reduction under different reaction conditions. The results show that the CO2 reduction peak current increases with increasing concentration of the electrocatalyst from 0.5 to 2mM, increasing purging time of CO2 from 5 to 30min, and increasing scan rate from 0.1 to 0.5Vs−1. The catalytic activity of the Ru(II) complex is decreased at lower temperatures because of the relationship between the activation energy and temperature. The suggested electrocatalytic mechanism was confirmed by computational studies. The calculations indicate that the electrocatalytic mechanism includes six steps. The electrocatalyst undergoes two sequential one-electron tptz-based reductions followed by substitution of the CH3CN by CO2 to yield a metallocarboxylate intermediate. This intermediate (RuII(η1-CO22–)) is involved in further tptz-based reductions and an oxide (O2–) transfer to the free CO2 to yield a monocarbonyl Ru(II) complex ([RuII(tptz2–)(CO)Cl2]2–) and the carbonate anion. The monocarbonyl intermediate undergoes a rapid loss of the coordinating CO molecule at room temperature. Finally, the dianionic intermediate reacts with a new molecule CO2 restarting the electrocatalytic cycle.

The role of metals in production and scavenging of reactive oxygen species in photosystem II.

Metal ions play a crucial role in enzymatic reactions in all photosynthetic organisms such as cyanobacteria, algae and plants. It well known that metal ions maintain the binding of substrate in the active site of the metalloenzymes and control the redox activity of the metalloenzyme in the enzymatic reaction. A large pigment-protein complex, PSII, known to serve as a water-plastoquinone oxidoreductase, contains three metal centers comprising non-heme iron, heme iron of Cyt b559 and the water-splitting manganese complex. Metal ions bound to PSII proteins maintain the electron transport from water to plastoquinone and regulate the pro-oxidant and antioxidant activity in PSII. In this review, attention is focused on the role of PSII metal centers in (i) the formation of superoxide anion and hydroxyl radicals by sequential one-electron reduction of molecular oxygen and the formation of hydrogen peroxide by incomplete two-electron oxidation of water; and (ii) the elimination of superoxide anion radical by one-electron oxidation and reduction (superoxide dismutase activity) and of hydrogen peroxide by two-electron oxidation and reduction (catalase activity). The balance between the formation and elimination of reactive oxygen species by PSII metal centers is discussed as an important aspect in the prevention of photo-oxidative damage of PSII proteins and lipids.

Direct Evidence of Active-Site Reduction and Photodriven Catalysis in Sensitized Hydrogenase Assemblies

We report photocatalytic H(2) production by hydrogenase (H(2)ase)-quantum dot (QD) hybrid assemblies. Quenching of the CdTe exciton emission was observed, consistent with electron transfer from the quantum dot to H(2)ase. GC analysis showed light-driven H(2) production in the presence of a sacrificial electron donor with an efficiency of 4%, which is likely a lower limit for these hybrid systems. FTIR spectroscopy was employed for direct observation of active-site reduction in unprecedented detail for photodriven H(2)ase catalysis with sensitivity toward both H(2)ase and the sacrificial electron donor. Photosensitization with Ru(bpy)(3)(2+) showed distinct FTIR photoreduction properties, generating all of the states along the steady-state catalytic cycle with minimal H(2) production, indicating slow, sequential one-electron reduction steps. Comparing the H(2)ase activity and FTIR results for the two systems showed that QDs bind more efficiently for electron transfer and that the final enzyme state is different for the two sensitizers. The possible origins of these differences and their implications for the enzymatic mechanism are discussed.

Synthesis and Molecular and Electronic Structures of Reduced Bis(imino)pyridine Cobalt Dinitrogen Complexes: Ligand versus Metal Reduction

Sodium amalgam reduction of the aryl-substituted bis(imino)pyridine cobalt dihalide complexes ((Ar)PDI)CoCl(2) and ((iPr)BPDI)CoCl(2) ((Ar)PDI = 2,6-(2,6-R(2)-C(6)H(3)N=CMe)(2)C(5)H(3)N (R = (i)Pr, Et, Me); (iPr)BPDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)N=CPh)(2)C(5)H(3)N) in the presence of an N(2) atmosphere furnished the corresponding neutral cobalt dinitrogen complexes ((Ar)PDI)CoN(2) and ((iPr)BPDI)CoN(2). Magnetic measurements on these compounds establish doublet ground states. Two examples, ((iPr)PDI)CoN(2) and ((iPr)BPDI)CoN(2), were characterized by X-ray diffraction and exhibit metrical parameters consistent with one-electron chelate reduction and a Co(I) oxidation state. Accordingly, the toluene solution EPR spectrum of ((iPr)PDI)CoN(2) at 23 degrees C exhibits an isotropic signal with a g value of 2.003 and hyperfine coupling constant of 8 x 10(-4) cm(-1) to the I = 7/2 (59)Co center, suggesting a principally bis(imino)pyridine-based SOMO. Additional one-electron reduction of ((iPr)PDI)CoN(2) was accomplished by treatment with Na[C(10)H(8)] in THF and yielded the cobalt dinitrogen anion [((iPr)PDI)CoN(2)](-). DFT calculations on the series of cationic, neutral, and anionic bis(imino)pyridine cobalt dinitrogen compounds establish Co(I) centers in each case and a chelate-centered reduction in each of the sequential one-electron reduction steps. Frequency calculations successfully reproduce the experimentally determined N[triple bond]N infrared stretching frequencies and validate the computational methods. The electronic structures of the reduced cobalt dinitrogen complexes are evaluated in the broader context of bis(imino)pyridine base metal chemistry and the influence of the metal d electron configuration on the preference for closed-shell versus triplet diradical dianions.

Redox Properties of the Prosthetic Groups of Na+-Translocating NADH:Quinone Oxidoreductase. 2. Study of the Enzyme by Optical Spectroscopy

Redox titration of the electronic spectra of the prosthetic groups of the Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) from Vibrio harveyi at different pH values showed five redox transitions corresponding to the four flavin cofactors of the enzyme and one additional transition reflecting oxidoreduction of the [2Fe-2S] cluster. The pH dependence of the measured midpoint redox potentials showed that the two-electron reduction of the FAD located in the NqrF subunit was coupled with the uptake of only one H(+). The one-electron reduction of neutral semiquinone of riboflavin and the formation of anion flavosemiquinone from the oxidized FMN bound to the NqrB subunit were not coupled to any proton uptake. The two sequential one-electron reductions of the FMN residue bound to the NqrC subunit showed pH-independent formation of anion radical in the first step and the formation of fully reduced flavin coupled to the uptake of one H(+) in the second step. All four flavins stayed in the anionic form in the fully reduced enzyme. None of the six redox transitions in Na(+)-NQR showed dependence of its midpoint redox potential on the concentration of sodium ions. A model of the sequence of electron transfer steps in the enzyme is suggested.

Ferrate(VI) and ferrate(V) oxidation of cyanide, thiocyanate, and copper(I) cyanide

Cyanide (CN −), thiocyanate (SCN −), and copper(I) cyanide (Cu(CN) 4 3−) are common constituents in the wastes of many industrial processes such as metal finishing and gold mining, and their treatment is required before the safe discharge of effluent. The oxidation of CN −, SCN −, and Cu(CN) 4 3− by ferrate(VI) (Fe VIO 4 2−; Fe(VI)) and ferrate(V) (Fe VO 4 3−; Fe(V)) has been studied using stopped-flow and premix pulse radiolysis techniques. The rate laws for the oxidation of cyanides were found to be first-order with respect to each reactant. The second-order rate constants decreased with increasing pH because the deprotonated species, FeO 4 2−, is less reactive than the protonated Fe(VI) species, HFeO 4 −. Cyanides react 10 3–10 5 times faster with Fe(V) than with Fe(VI). The Fe(V) reaction with CN − proceeds by sequential one-electron reductions from Fe(V) to Fe(IV) to Fe(III). However, a two-electron transfer process from Fe(V) to Fe(III) occurs in the reaction of Fe(V) with SCN − and Cu(CN) 4 3−. The toxic CN − species of cyanide wastes is converted into relatively non-toxic cyanate (NCO −). Results indicate that Fe(VI) is highly efficient in removing cyanides from electroplating rinse water and gold mill effluent.

Synthesis and Characterization of Ruthenacarborane Complexes Incorporating ChelatingN-Donor Ligands: Unexpected Luminescence from the Complex [3-CO-3,3-{κ2-Me2N(CH2)2NMe2}-closo-3,1,2-RuC2B9H11

A synthetic methodology using double carbonyl substitution of the starting tricarbonyl complex [3,3,3-(CO)(3)-closo-3,1,2-RuC(2)B(9)H(11)] (1) with 2 mol equiv of the reagent Me(3)NO has been employed to afford ruthenacarborane complexes with chelating N-donor ligands. Three of these complexes, [3-CO-3,3-[kappa(2)-4,4'-R(2)-2,2'-(NC(5)H(3))(2)]-closo-3,1,2-RuC(2)B(9)H(11)] (3a, R = H; 3b, R = (CH(2))(8)Me; 3c, R = Bu(t)), comprise 2,2'-bipyridyl ligands with hydrogen, n-nonyl, or t-butyl groups in the 4,4'-positions of the rings, respectively. Photophysical analysis revealed no substantial luminescent activity, but the complexes are electrochemically active, undergoing sequential (reversible and quasi-reversible) one-electron reductions, the second of which likely precipitating a ligand displacement. Cyclic voltammetry (CV) experiments revealed an irreversible one-electron oxidation (E(pa) approximately 0.9 V) in MeCN, on the other hand, followed by rapid CO substitution by the solvent and reversible secondary reduction (E(1/2) approximately 0.1 V). The primary redox couple became quasi-reversible in CH(2)Cl(2), and spectroelectrochemical analysis of complex 3c provided evidence of a closo --> isocloso structural modification upon oxidation. An analogue of these complexes employing the TMEDA (N,N,N',N'-tetramethylethylenediamine) ligand, [3-CO-3,3-[kappa(2)-Me(2)N(CH(2))(2)NMe(2)]-closo-3,1,2-RuC(2)B(9)H(11)] (4), was synthesized using the same methodology. Cyclic voltammetric measurements displayed a reversible metal-based one-electron oxidation whether in CH(2)Cl(2) or MeCN, with no indication of subsequent CO substitution or a similar closo --> isocloso adjustment. Complex 4 was unexpectedly weakly luminescent (lambda(em) = 360 nm) in THF (tetrahydrofuran) at ambient temperatures, demonstrating a more intense phosphorescent emission in MeTHF (2-methyltetrahydrofuran) glass at 77 K (lambda(em) = 450 nm, tau(450) = 0.77 ms). The X-ray crystallographic structures of complexes 3a and 4 are reported along with spectroscopic IR, NMR ((1)H, (13)C, (11)B), UV-vis absorption, EPR, and CV data.

Pyridine-Substituted Oligopeptides as Scaffolds for the Assembly of Multimetallic Complexes: Variation of Chain Length

This paper presents the synthesis and characterization of pyridine-substituted artificial oligopeptides with an aminoethylglycine backbone of varying length, which are designed to act as scaffolds for the self-assembly of multimetallic structures. The identities and purities of the oligopeptides are confirmed with mass spectrometry, (1)H NMR, HPLC, and pH titrations. The acid dissociation constants for the oligopeptides were determined and were found to decrease with increasing pyridine units. Titrations of the oligopeptides with Cu(II) and Pt(II) complexes containing the tridentate ligands 2,2':6',2''-terpyridine and pyridine 2,6-dicarboxylic acid were monitored using UV-visible absorption spectroscopy and showed stoichiometric binding based on the number of pyridines on the peptide strand. Metal titrations performed using an analogous oligopeptide with methyl substituents (in place of the pyridine ligands) showed very weak or no binding. In the case of the oligopeptides containing bound Pt(terpyridine)(2+) complexes, cyclic voltammetry reveals two sequential one-electron reductions at formal potentials that do not vary as a function of oligopeptide length. The measured diffusion coefficients were measured with chronoamperometry and were found to decrease with increasing oliopeptide length.

Redox modulation of benzene triimides and diimides via noncovalent interactions.

[reaction: see text] Mellitic triimides undergo three sequential one-electron reduction processes whose potentials are significantly lowered in the presence of alkyl thioureas. The two sequential reductions of benzene diimides are similarly stabilized. Calculation of the relative free energy change between the different electronic states of the imide acceptors and their corresponding alkyl thiourea complexes indicates dramatic increases in hydrogen bond strength with increasing acceptor charge density.