Hydrogen adsorption on various transition metal (111) surfaces in water: a DFT forecast.

The hydrogen adsorption and hydrogen evolution at the M(111), (M = Ag, Au Cu, Pt, Pd, Ni & Co) surfaces of various transition metals in aqueous suspensions were studied computationally using the DFT methods. The hydrogens are adsorbed dissociatively on all surfaces except on Ag(111) and Au(111) surfaces. The results are validated by reported experimental and computational studies. Hydrogen atoms have large mobility on M(111) surfaces due to the small energy barriers for diffusion on the surface. The hydrogen evolution via the Tafel mechanism is considered at different surface coverage ratios of hydrogen atoms and is used as a descriptor for the hydrogen adsorption capacity on M(111) surfaces. All calculations are performed without considering how the hydrogen atoms are formed on the surface. The hydrogen adsorption energies decrease with the increase in the surface coverage of hydrogen atoms. The surface coverage for the H2 evolution depends on each M(111) surface. Among the considered M(111) surfaces, Au(111) has the least hydrogen adsorption capacity and Ni, Co and Pd have the highest. Furthermore, experiments proving that after the H2 evolution reaction (HER) on Au0-NPs, and Ag0-NPs surfaces some reducing capacity remains on the M0-NPs is presented.

Reaction of FeaqII with Peroxymonosulfate and Peroxydisulfate in the Presence of Bicarbonate: Formation of FeaqIV and Carbonate Radical Anions.

Many advanced oxidation processes (AOPs) use Fenton-like reactions to degrade organic pollutants by activating peroxymonosulfate (HSO5-, PMS) or peroxydisulfate (S2O82-, PDS) with Fe(H2O)62+ (FeaqII). This paper presents results on the kinetics and mechanisms of reactions between FeaqII and PMS or PDS in the absence and presence of bicarbonate (HCO3-) at different pH. In the absence of HCO3-, FeaqIV, rather than the commonly assumed SO4•-, is the dominant oxidizing species. Multianalytical methods verified the selective conversion of dimethyl sulfoxide (DMSO) and phenyl methyl sulfoxide (PMSO) to dimethyl sulfone (DMSO2) and phenyl methyl sulfone (PMSO2), respectively, confirming the generation of FeaqIV by the FeaqII-PMS/PDS systems without HCO3-. Significantly, in the presence of environmentally relevant concentrations of HCO3-, a carbonate radical anion (CO3•-) becomes the dominant reactive species as confirmed by the electron paramagnetic resonance (EPR) analysis. The new findings suggest that the mechanisms of the persulfate-based Fenton-like reactions in natural environments might differ remarkably from those obtained in ideal conditions. Using sulfonamide antibiotics (sulfamethoxazole (SMX) and sulfadimethoxine (SDM)) as model contaminants, our study further demonstrated the different reactivities of FeaqIV and CO3•- in the FeaqII-PMS/PDS systems. The results shed significant light on advancing the persulfate-based AOPs to oxidize pollutants in natural water.

Electronic Coupling and Electrocatalysis in Redox Active Fused Iron Corroles.

Conjugated arrays composed of corrole macrocycles are increasingly more common, but their chemistry still lags behind that of their porphyrin counterparts. Here, we report on the insertion of iron(III) into a ß,ß-fused corrole dimer and on the electronic effects that this redox active metal center has on the already rich coordination chemistry of [H3tpfc] COT, where COT = cyclo-octatetraene and tpfc = tris(pentafluorophenyl)corrole. Synthetic manipulations were performed for the isolation and full characterization of both the 5-coordinate [FeIIItpfc(py)]2COT and 6-coordinate [FeIIItpfc(py)2]2COT, with one and two axial pyridine ligands per metal, respectively. X-Ray crystallography reveals a dome-shaped structure for [FeIIItpfc(py)]2COT and a perfectly planar geometry which (surprisingly at first) is also characterized by shorter Fe-N (corrole) and Fe-N (pyridine) distances. Computational investigations clarify that the structural phenomena are due to a change in the iron(III) spin state from intermediate (S = 3/2) to low (S = 1/2), and that both the 5- and 6-coordinated complexes are enthalpically favored. Yet, in contrast to iron(III) porphyrins, the formation enthalpy for the coordination of the first pyridine to Fe(III) corrole is more negative than that of the second pyridine coordination. Possible interactions between the two corrole subunits and the chelated iron ions were examined by UV-Vis spectroscopy, electrochemical techniques, and density functional theory (DFT). The large differences in the electronic spectra of the dimer relative to the monomer are concluded to be due to a reduced electronic gap, owing to the extensive electron delocalization through the fusing bridge. A cathodic sweep for the dimer discloses two redox processes, separated by 230 mV. The DFT self-consistent charge density for the neutral and cationic states (1- and 2-electron oxidized) reveals that the holes are localized on the macrocycle. A different picture emerges from the reduction process, where both the electrochemistry and the calculated charge density point toward two consecutive electron transfers with similar energetics, indicative of very weak electron communication between the two redox active iron(III) sites. The binuclear complex was determined to be a much better catalyst for the electrochemical hydrogen evolution reaction (HER) than the analogous mononuclear corrole.

Hydrogen evolution catalysis by terminal molybdenum-oxo complexes.

Stable complexes with terminal triply bound metal-oxygen bonds are usually not considered as valuable catalysts for the hydrogen evolution reaction (HER). We now report the preparation of three conceptually different (oxo)molybdenum(V) corroles for testing if proton-assisted 2-electron reduction will lead to hyper-reactive molybdenum(III) capable of converting protons to hydrogen gas. The upto 670 mV differences in the [(oxo)Mo(IV)]-/[(oxo)Mo(III)]-2 redox potentials of the dissolved complexes came into effect by the catalytic onset potential for proton reduction thereby, significantly earlier than their reduction process in the absence of acids, but the two more promising complexes were not stable at practical conditions. Under heterogeneous conditions, the smallest and most electron-withdrawing catalyst did excel by all relevant criteria, including a 97% Faradaic efficiency for catalyzing HER from acidic water. This suggests complexes based on molybdenum, the only sustainable heavy transition metal, as catalysts for other yet unexplored green-energy-relevant processes.

Dimeric Corrole Analogs of Chlorophyll Special Pairs.

Chlorophyll special pairs in photosynthetic reaction centers function as both exciton acceptors and primary electron donors. Although the macrocyclic natural pigments contain Mg(II), the central metal in most synthetic analogs is Zn(II). Here we report that insertion of either Al(III) or Ga(III) into an imidazole-substituted corrole affords an exceptionally robust photoactive dimer. Notably, attractive electronic interactions between dimer subunits are relatively strong, as documented by signature changes in NMR and electronic absorption spectra, as well as by cyclic voltammetry, where two well-separated reversible redox couples were observed. EPR spectra of one-electron oxidized dimers closely mimic those of native special pairs, and strong through-space interactions between corrole subunits inferred from spectroscopic and electrochemical data are further supported by crystal structure analyses (3 Å interplanar distances, 5 Å lateral shifts, and 6 Å metal to metal distances).

Aging and pathological aging signatures of the brain: through the focusing lens of SIRT6.

Brain-specific SIRT6-KO mice present increased DNA damage, learning impairments, and neurodegenerative phenotypes, placing SIRT6 as a key protein in preventing neurodegeneration. In the aging brain, SIRT6 levels/activity decline, which is accentuated in Alzheimer's patients. To understand SIRT6 roles in transcript pattern changes, we analyzed transcriptomes of young WT, old WT and young SIRT6-KO mice brains, and found changes in gene expression related to healthy and pathological aging. In addition, we traced these differences in human and mouse samples of Alzheimer's and Parkinson's diseases, healthy aging and calorie restriction (CR). Our results define four gene expression categories that change with age in a pathological or non-pathological manner, which are either reversed or not by CR. We found that each of these gene expression categories is associated with specific transcription factors, thus serving as potential candidates for their category-specific regulation. One of these candidates is YY1, which we found to act together with SIRT6 regulating specific processes. We thus argue that SIRT6 has a pivotal role in preventing age-related transcriptional changes in brains. Therefore, reduced SIRT6 activity may drive pathological age-related gene expression signatures in the brain.

The Role of Carbonate in Catalytic Oxidations.

CO2, HCO3-, and CO32- are present in all aqueous media at pH > 4 if no major effort is made to remove them. Usually the presence of CO2/HCO3-/CO32- is either forgotten or considered only as a buffer or proton transfer catalyst. Results obtained in the last decades point out that carbonates are key participants in a variety of oxidation processes. This was first attributed to the formation of carbonate anion radicals via the reaction OH• + CO32- → CO3•- + OH-. However, recent studies point out that the involvement of carbonates in oxidation processes is more fundamental. Thus, the presence of HCO3-/CO32- changes the mechanisms of Fenton and Fenton-like reactions to yield CO3•- directly even at very low HCO3-/CO32- concentrations. CO3•- is a considerably weaker oxidizing agent than the hydroxyl radical and therefore a considerably more selective oxidizing agent. This requires reconsideration of the sources of oxidative stress in biological systems and might explain the selective damage induced during oxidative stress. The lower oxidation potential of CO3•- probably also explains why not all pollutants are eliminated in many advanced oxidation technologies and requires rethinking of the optimal choice of the technologies applied. The role of percarbonate in Fenton-like processes and in advanced oxidation processes is discussed and has to be re-evaluated. Carbonate as a ligand stabilizes transition metal complexes in uncommon high oxidation states. These high-valent complexes are intermediates in electrochemical water oxidation processes that are of importance in the development of new water splitting technologies. HCO3- and CO32- are also very good hole scavengers in photochemical processes of semiconductors and may thus become key participants in the development of new processes for solar energy conversion. In this Account, an attempt to correlate these observations with the properties of carbonates is made. Clearly, further studies are essential to fully uncover the potential of HCO3-/CO32- in desired oxidation processes.

The FeII(citrate) Fenton reaction under physiological conditions.

The Fenton reaction of FeII(citrate) in the presence and absence of bicarbonate (HCO3-) is studied. It is found that the rate constant of the Fenton reaction (kobs) increases with increasing [citrate]. kobs also increase with increasing [HCO3-]; this effect is most significant at biological citrate concentrations. Methane and ethane gases are formed from (CH3)2SO when the Fenton reaction is carried out in the presence of large [citrate] due to the reaction of the citrate radical, (-2OC)CH2C(OH)(CO2-)CH(CO2-)/(-2OC)CH2C(O)(CO2-)CH2(CO2-) with (CH3)2SO. In the absence of citrate (CH3)2SO2 is the main product of the Fenton reaction. However, in the presence of 0.10 mM citrate, no (CH3)2SO2 is formed, some (CH3)SOOH is formed, along with a low yield of beta-ketoglutaric acid. Formation of (CH3)SOOH and beta-ketoglutaric acid are due to the citrate radical and FeIV(citrate). In the presence of bicarbonate formation of abundant beta-ketoglutaric acid confirms the formation of carbonate radical anion (CO3-). Thus, bicarbonate affects the mechanism and kinetics of the reaction dramatically. Hydroxyl radicals (OH) are not formed in the presence of bicarbonate and probably also not in its absence. These results point out that hydroxyl radicals, formed by the Fenton reaction, do not initiate oxidative stress in biological systems.

Axial/Peripheral Chloride/Fluoride-Substituted Boron Subphthalocyanines as Electron Acceptors.

Chloroboron subphthalocyanines (Cl-BsubPc) are robust compounds that can be readily modified at the axial and peripheral positions. Peripherally chlorinated derivatives were recently found to be advantageous regarding integration into organic electronic devices. We now report on the effects of fluorides introduced on both the peripheral and axial positions of BsubPcs. Specific attention on the reduction of these compounds revealed that the much fewer electronegative chlorides still shift the redox potentials as much as fluorides. The main advantage of the fluorinated derivatives was deduced to be their stability, allowing for the spectroscopic characterization of mono-anionic and even bis-anionic subphthalocyanines. This study sets the precedence for further tuning of the electrochemical properties of BsubPcs through molecular design, thus increasing their applicability regarding organic electronic devices that undergo multiple redox cycles during operational lifetime.

Rhenium(i) sapphyrins: remarkable difference between the C6F5 and CF3-substituted derivatives.

Despite sapphyrins being the first ever reported extended porphyrins, their coordination chemistry remains quite limited. The newly gained access to meso-alkyl-substituted sapphyrins encouraged the study of their rhenium chelates in comparison with meso-aryl-substituted derivatives. Both the CF3- and C6F5-substituted sapphyrins act as bidentate/monoanionic ligands, with the coordination sphere of the rhenium(i) ion completed by three carbonyls. Three pyrrole moieties are not involved in metal binding and all were disclosed to be "normal" with their N atoms within the macrocycle's core of the C6F5-substituted sapphyrin, while in the CF3-substituted sapphyrin one pyrrole ring is "inverted" with its N atom on the periphery. These structural differences are demonstrated to affect both the optical and electrochemical properties of the Re(i) sapphyrins.

Cobalt Carbonate as an Electrocatalyst for Water Oxidation.

CoII salts in the presence of HCO3 - /CO3 2- in aqueous solutions act as electrocatalysts for water oxidation. It comprises of several key steps: (i) A relatively small wave at Epa ≈0.71 V (vs. Ag/AgCl) owing to the CoIII/II redox couple. (ii) A second wave is observed at Epa ≈1.10 V with a considerably larger current. In which the CoIII undergoes oxidation to form a CoIV species. The large current is attributed to catalytic oxidation of HCO3 - /CO3 2- to HCO4 - . (iii) A process with very large currents at >1.2 V owing to the formation of CoV (CO3 )3 - , which oxidizes both water and HCO3 - /CO3 2- . These processes depend on [CoII ], [NaHCO3 ], and pH. Chronoamperometry at 1.3 V gives a green deposit. It acts as a heterogeneous catalyst for water oxidation. DFT calculations point out that Con (CO3 )3 n-6 , n=4, 5 are attainable at potentials similar to those experimentally observed.

Tuning Chemical and Physical Properties of Phosphorus Corroles for Advanced Applications.

Although the affinity of metallocorroles to axial ligands is quite low, this is not the case when the chelated element is phosphorus. This work is hence focused on the mechanism of ligand exchange of six-coordinate phosphorus corroles as a tool for affecting their chemical and physical properties. These fundamental investigations allowed for the development of facile methodologies for the synthesis of a large series of complexes and the establishment of several new structure/activity profiles that may be used to understand and predict spectroscopic features and for tailor-made modification of photophysical and electrochemical properties. This is exemplified by the facile access to complexes with terminal groups that are of large potential for practical applications based on click chemistry, optical imaging, and surface science.

Carbonate-radical-anions, and not hydroxyl radicals, are the products of the Fenton reaction in neutral solutions containing bicarbonate.

The Fenton reaction, Fe(H2O)62+ + H2O2 → Oxidizing product, is of major importance in biology as the major cause of oxidative stress, and in advanced oxidation processes. It is commonly assumed that ·OH is the product of the Fenton reaction. The results presented herein point out that ·OH is indeed the oxidizing product in acidic solutions for [Fe(H2O)62+] > [H2O2]; FeIVaq is the active oxidizing product in neutral solutions; in slightly acidic solutions for [H2O2] > [Fe(H2O)62+] a mixture of ·OH and FeIVaq is formed. However CO3·- is the active oxidizing product in neutral solutions containing HCO3- even at low concentrations, i.e. under physiological conditions. The implications to our understanding of the origins of oxidative stress and of catalytic oxidations in advanced oxidation processes are discussed.

Carbonate and carbonate anion radicals in aqueous solutions exist as CO3(H2O)62- and CO3(H2O)6˙- respectively: the crucial role of the inner hydration sphere of anions in explaining their properties.

An effort to reproduce the chemical and physical properties of carbonate and carbonate anion radicals in aqueous solutions by DFT proves that one has to include an inner hydration sphere of six water molecules for both anions. Application of the SMD model to CO3(H2O)62- and CO3(H2O)6˙- enables achievement of the experimental value of the redox potential of the CO3(H2O)6˙-/2- couple. This is a result of the direct inclusion of a considerable charge transfer (CT) from CO32- to its inner hydration sphere in the calculation of the hydration effect. The HOMO of clusters is an analogue of the non-bonding σ-type a2'-HOMO of the parent CO3 moiety with a σ*(OH) contribution. This is a MO manifestation of the CT to the first hydration shell. The localization of the CT on the first hydration shell also re-produces the very strong OHO stretch peak. Furthermore, the very large difference in the hydration energies of CO32- and CO3˙- which causes the very large differences in the length of the C-OH-O hydrogen bonds suggests that the oxidations by CO3˙- proceed via the inner sphere mechanism.

Mechanistic Studies on the Role of [CuII (CO3 )n ]2-2n as a Water Oxidation Catalyst: Carbonate as a Non-Innocent Ligand.

Recently it was reported that copper bicarbonate/carbonate complexes are good electro-catalysts for water oxidation. However, the results did not enable a decision whether the active oxidant is a CuIII or a CuIV complex. Kinetic analysis of pulse radiolysis measurements coupled with DFT calculations point out that CuIII (CO3 )n3-2n complexes are the active intermediates in the electrolysis of CuII (CO3 )n2-2n solution. The results enable the evaluation of E°[(CuIII/II (CO3 )n )aq ]≈1.42 V versus NHE at pH 8.4. This redox potential is in accord with the electrochemical report. As opposed to literature suggestions for water oxidation, the present results rule out single-electron transfer from CuIII (CO3 )n3-2n to yield hydroxyl radicals. Significant charge transfer from the coordinated carbonate to CuIII results in the formation of C2 O62- by means of a second-order reaction of CuIII (CO3 )n3-2n . The results point out that carbonate stabilizes transition-metal cations at high oxidation states, not only as a good sigma donor, but also as a non-innocent ligand.

One-Pot Synthesis of Contracted and Expanded Porphyrins with meso-CF3 Groups.

Corrole and sapphyrin with the smallest meso-substituents reported so far were prepared in a one-pot synthesis that relies on a non-aldehydic precursor for introducing CF3 groups. The substantial amounts of products obtained by this facile pathway allowed for the full characterization of 5,10,15-tris(trifluoromethyl)corrole, the access to a variety of stable chelates thereof and investigations that disclose the unique structural and chemical properties induced by the CF3 substituents. The novel 5,10,15,20-tetra(trifluoromethyl)sapphyrin undergoes only single protonation, which according to its crystal structure is stabilized by favorable non-bonding F/H interaction between the meso-CF3 and the inverted pyrrolic NH.

Reactive Intermediates Involved in Cobalt Corrole Catalyzed Water Oxidation (and Oxygen Reduction).

A detailed investigation of the cobalt corrole Co(tpfc) as molecular catalyst for electrochemical water oxidation uncovered many important mechanism-of-action details that are crucial for the design of optimally performing systems. This includes the identification of the redox states that do and do not participate in catalysis and very significant axial ligand effects on the activity of the doubly oxidized complex. Specifics deduced for the electrocatalysis under homogeneous conditions include the following: the one-electron oxidation of the cobalt(III) corrole is completely unaffected by reaction conditions; catalysis coincides with the second oxidation event; two catalytic waves develop in the presence of OH-, and the one at lower overpotential is dominant under more basic conditions. Comparative spectroelectrochemical measurements performed for Co(tpfc) and Al(tpfc), the analogous corrole chelated by the nonredox active aluminum, revealed that the second oxidation process of Co(tpfc) is much more significantly metal-centered than the first one. EPR studies revealed that shift from fully corrole-centered to partially metal-centered in the singly oxidized complex [Co(tpfc)]+ is achievable with fluoride as axial ligand. The analogous experiment, but with hydroxide instead of fluoride, could not be performed because of a surprising phenomenon: formation of a cobalt-superoxide complex that is actually relevant to oxygen reduction rather than to water oxidation. Nevertheless, fluoride and hydroxide induce very similar effects in terms of the appearance of two catalytic waves, lowering of onset potentials, and increasing the catalytic activity. The main conclusions from the accumulated data are that the apparent pH effect is actually due to hydroxide binding to the cobalt center and that π-donating axial ligands play pivotal and beneficial roles regarding the main factors that are important for facilitating the oxidation of water.

Effect of Selective CF3 Substitution on the Physical and Chemical Properties of Gold Corroles.

Gold corroles are not readily accessible and they display no interesting physical or chemical properties. A facile methodology has now been developed for obtaining selectively CF3 -substituted gold(III) corroles and the introduction of these groups has been found to have an immense effect on the structures of the complexes, their photophysical and redox properties, and on their ability to participate in catalytic processes.

The Planar Cyclooctatetraene Bridge in Bis-Metallic Macrocycles: Isolating or Conjugating?

A minor modification of the reported procedure for the synthesis of a corrole dimer that is fused by the cyclooctatetraene (COT) unit, (H3tpfc)2COT, allowed for its isolation in 18% yield. Of the two redox isomers that this interesting macrocycle does form, the current focus is on the reduced form, in which each subunit resembles that of monomeric corroles with a trianionic N4 coordination core. The corresponding bis-gallium(III) complex was prepared as an entry into the potentially rich coordination chemistry of (H3tpfc)2COT. Both X-ray crystallography and DFT calculations disclosed that the COT moiety is essentially planar with very unusual nonalternating C-C bonds. The same holds true for the bis-gallium(III) complexes [(Ga-tpfc)2]COT(py)2 and [(Ga-tpfc)2]COT(py)4, obtained with one and two pyridine molecules coordinated to each metal ion, respectively. The electronic spectra of both the free base and the gallium(III) complexes display an extremely low energy band (λmax at 720-724 nm), which points toward extensive π delocalization through the COT bridge. This aspect was fully addressed by examining the interactions between the two corrole subunits in terms of electrochemistry and DFT calculations of the oxidized and reduced macrocycle. The new near-IR bands that appear upon both oxidation (λmax 1250 nm) and reduction (λmax 1780 nm) serve as additional supporting evidence for this conclusion.

Measured rates of fluoride/metal association correlate with rates of superoxide/metal reactions for Fe(III)EDTA(H2O)- and related complexes.

The effects of 10 paramagnetic metal complexes (Fe(III)EDTA(H2O)-, Fe(III)EDTA(OH)2-, Fe(III)PDTA-, Fe(III)DTPA2-, Fe(III)2O(TTHA)2-, Fe(III)(CN)6(3-), Mn(II)EDTA(H2O)2-, Mn(II)PDTA2-, Mn(II)beta-EDDADP2-, and Mn(II)PO4(-)) on F- ion 19F NMR transverse relaxation rates (R2 = 1/T2) were studied in aqueous solutions as a function of temperature. Consistent with efficient relaxation requiring formation of a metal/F- bond, only the substitution inert complexes Fe(III)(CN)6(3-) and Fe(III)EDTA(OH)2- had no measured effect on T2 relaxation of the F- 19F resonance. For the remaining eight complexes, kinetic parameters (apparent second-order rate constants and activation enthalpies) for metal/F- association were determined from the dependence of the observed relaxation enhancements on complex concentration and temperature. Apparent metal/F- association rate constants for these complexes (k(app,F-)) spanned 5 orders of magnitude. In addition, we measured the rates at which O2*- reacts with Fe(III)PDTA-, Mn(II)EDTA(H2O)2-, Mn(II)PDTA2-, and Mn(II)beta-EDDADP2- by pulse radiolysis. Although no intermediate is observed during the reduction of Fe(III)PDTA- by O2*-, each of the Mn(II) complexes reacts with formation of a transient intermediate presumed to form via ligand exchange. These reactivity patterns are consistent with literature precedents for similar complexes. With these data, both k(app,O2-) and k(app,F-) are available for each of the eight reactive complexes. A plot of log(k(app,O2-)) versus log(k(app,F-)) for these eight showed a linear correlation with a slope approximately 1. This correlation suggests that rapid metal/O2*- reactions of these complexes occur via an inner-sphere mechanism whereas formation of an intermediate coordination complex limits the overall rate. This hypothesis is also supported by the very low rates at which the substitution inert complexes (Fe(III)(CN)6(3-) and Fe(III)EDTA(OH)2-) are reduced by O2*-. These results suggest that F- 19F NMR relaxation can be used to predict the reactivities of other Fe(III) complexes toward reduction by O2*-, a key step in the biological production of reactive oxygen species.