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A series of metallophthalocyanine (PcM) complexes with axial N-heterocyclic carbene ligands (NHC; 1,3-diisopropylimidazol-2-ylidene (DIP) and 1,3-dimethylbenzimidazol-2-ylidene (DMB)) were prepared and structurally characterized. PcCoII(DIP), PcZnII(DIP), and PcZnII(DMB) are five-coordinate complexes with mild dome-type Pc-ring distortions, while PcFeII(DIP)2 is six-coordinate and has a very large ruffle-type ring-distortion with respect to typical PcM(L)2 systems. The distortion is induced by the highly steric axial DIP ligands. The distortions were quantified and classified by their bond lengths and torsion angles, and according to the normal-coordinate structural decomposition (NSD) analysis. Upon ligation of the NHC, the insoluble PcM materials were solublized in common organic solvents, with typical UV-visible Q-band maxima observable between 658 and 677 nm; the increased solubility is rationalized in terms of the reduced solid-state aggregation of the complexes, attributable to the axial ligation. [ABSTRACT FROM AUTHOR]

Multi-step electron transfer reactions are important to the function of many cellular systems. The ways in which such systems have evolved to direct electrons along specific pathways are largely understood, but less so are the ways in which the reduction–oxidation potentials of individual redox sites are controlled. We prepared a series of three new artificial variants of Pseudomonas aeruginosa azurin where a tyrosine (Tyr109) is situated between the native Cu ion and a Ru(II) photosensitizer tethered to a histidine (His107). Arginine, glutamine, or methionine were introduced as position 122, which is near to Tyr109. We investigated the rate of CuI oxidation by a flash-quench generated Ru(III) oxidant over pH values from 5 to 9. While the identity of the residue at position 122 affects some of the physical properties of Tyr109, the rates of CuI oxidation are only weakly dependent on the identity of the residue at 122. The results highlight that more work is still needed to understand how non-covalent interactions of redox active groups are affected in redox proteins. [ABSTRACT FROM AUTHOR]

A series of ClRe[(CO).sub.3]-containing complexes with 1-metbyl-1H-tenzo[d]imidazol-2-yl)quinoline (Me-QuBIm), 1-benzyl-1H-benzo [d]imidazol-2-yl)qvjinoline (Bn-QuBIm), and 2-(1-(4-methoxybenzyl)-1H-benzo[d]imidazol-2-yl)quinoline (OMeBn-QuBIm) ligands were prepared and characterized. Each complex was characterized using [.sup.1]H and [.sup.13]C NMR, infrared, UV-vis, and fluorescence spectroscopies, and cyclic voltammetry. The physical properties of each complex are similar to the parent ClRe[(CO).sub.3](2-(1H-benzo[d]imidazol-2-yl)quinoline) complex. However, the electrochemical behavior is distinct from the parent, showing reversible voltammograms and poor C[O.sub.2] reduction activity. Addition of [K.sup.+] or [Mg.sup.2+] as Lewis acids modestly increases catalytic currents, but at large overpotentials. The work presented here is consistent with a mechanism that involves rhenium reduction and deprotonation of imidazole and benzimidazole prior to C[O.sub.2] reduction electrocatalysis. Key words: rhenium (I), carbon dioxide reduction, Lewis acids, electrocatalysis. Nous avons synthetise et caracterise une serie de complexes de ClRe[(CO).sub.3] portant des ligands de type l-methyl-1H-benzo[d] imidazol-2-yl)quinoline (Me-QuBIm), l-benzyl-1H-benzo[d]imidazol-2-yl)quinoline (Bn-QuBIm) et 2-(1-(4-methoxybenzyl)-1H-benzo[d]imidazol-2-yl)quinoline (OMeBn-QuBIm). Nous avons caracterise chaque complexe par spectroscopic RMN [.sup.1]H, RMN [.sup.13]C, infrarouge et UV-vis, par fluoroscopie et par voltamperometrie cyclique. Les proprietes physiques de chaque complexe sont semblables a celles du complexe ClRe[(CO).sub.3](2-(1H-benzo[d]imidazol-2-yl)qumoline) d'origine. Cependant, ces complexes presentent un comportement electrochimique distinct de celui du complexe d'origine, des voltamperogrammes reversibles, de meme qu'une faible activite reductrice envers le C[O.sub.2]. L'ajout d'acides de Lewis a base d'ions [K.sup.+] et [Mg.sup.2+] augmente legerement les courants catalytiques, mais des excedents de potentiel importants sont necessaires. Les resultats des presents travaux permettent de conclure a un mecanisme faisant intervenir la reduction et la deprotonation de l'imidazole et du benzimidazole avant que la reduction du C[O.sub.2] par electrocatalyse se produise. [Traduit par la Redaction] Mots-cles : rhenium (I), reduction du dioxyde de carbone, acides de Lewis, electrocatalyse., Introduction The electrochemical reduction of C[O.sub.2] to CO, C[H.sub.3]OH, or hydrocarbons is an important target in ongoing efforts at decarbonization. An artificial, closed-loop carbon cycle may be an important advance [...]

Metallophthalocyanine (PcM) coordination complexes are well-known mediators of the electrochemical reduction of carbon dioxide (CO2). They have many properties that show promise for practical applications in the energy sector. Such properties include synthetic flexibility, a high stability, and good efficiencies for the reduction of CO2 to useful feedstocks, such as carbon monoxide (CO). One of the ongoing challenges that needs to be met is the incorporation of PcM into the heterogeneous materials that are used in a great many CO2-reduction devices. Much progress has been made in the last decade and there are now several promising approaches to incorporate PcM into a range of materials, from simple carbon-adsorbed preparations to extended polymer networks. These approaches all have important advantages and drawbacks. In addition, investigations have led to new proposals regarding CO2 reduction catalytic cycles and other operational features that are crucial to function. Here, we describe developments in the immobilization of PcM CO2 reduction catalysts in the last decade (2013 to 2023) and propose promising avenues and strategies for future research. [ABSTRACT FROM AUTHOR]

The development and implementation of technology that can capture and transform carbon dioxide (CO2) is of ongoing interest. To that end, the integration of molecular electrocatalysts into devices is appealing because of the desirable features of molecules, such as the ability to modify active sites. Here, we explore how the identity of the aliphatic group in 1,4,8,11,15,18,22,25-octaalkoxyphthalocyanine cobalt(II) affects the catalytic behavior for heterogeneous CO2 reduction electrocatalysis. The alkyl R-groups correspond to n-butoxy, sec-butoxy, and 2-ethylhexoxy. All of the catalysts are soluble in organic solvents and are readily solution-processed. However, the larger 2-ethylhexoxy group showed solution aggregation behavior at concentrations ≥1 mM, and it was, in general, an inferior catalyst. The other two catalysts show comparable maximum currents, but the octa sec-butoxy-bearing catalyst showed larger CO2 reduction rate constants based on foot-of-the-wave analyses. This behavior is hypothesized to be due to the ability of the sec-butoxy groups to eliminate the ability of the alkoxy oxygen to block Co Sites via ligation. CO2 reduction activity is rationalized based on solid-state structures. Cobalt(II) phthalocyanine and its derivatives are known to be good CO2 reduction catalysts, but the results from this work suggest that straightforward incorporation of bulky groups can improve the processability and per site activity by discouraging aggregation. [ABSTRACT FROM AUTHOR]

Appending of ferrocene (Fc) to biologically-active organic backbones can generate novel multi-functional species for targeting bacteria and cancer. In this work Fc was linked to coumarin and anthraquinone with the goal of harnessing the redox-active Fc centre to generate new compounds that exhibit cytoxicity through the generation of toxic reactive oxygen species (ROS). A Cu(I)-catalyzed azide–alkyne cycloaddition "click" reaction was used to connect the organic and Fc components via a triazole linker. Cyclic voltammetry shows that the Fc potentials are suitable for oxidation by biological hydrogen peroxide to give reactive ferrocenium (Fc+) species, which can then generate hydroxyl radicals. The ability of the compounds to generate hydroxyl radicals in the presence of hydrogen peroxide was shown directly using EPR spin-trapping experiments. Furthermore, in vitro studies in MCF-7 breast cancer cells show significant increases in ROS following incubation with the Fc-functionalized compounds. Screening for antibacterial activity produced negative results for all of the Fc compounds, consitent with low levels of hydrogen peroxide typically found in bacteria. By contrast, Fc-coumarin showed cytotoxicity against A549 lung cancer and SKOV3 ovarian cancer cell lines, whereas the parent compound was inactive. This is consistent both with the cytoxic potential of the Fc group and the elevated hydrogen peroxide levels found in many cancers. Interestingly, the anthraquinone compounds showed the opposite effect with the parent compounds showing modest activity against A549 cells, but the Fc compounds being inactive. This demonstrates other potential negative impacts of including Fc, such as significantly increased lipophilicity. [ABSTRACT FROM AUTHOR]

The amino acid residue methionine (Met) is commonly thought of as a ligand in redox metalloproteins, for example in cytochromes c and in blue copper proteins. However, the roles of Met can go beyond a simple ligand. The thioether functional group of Met allows it to be considered as a hydrophobic residue as well as one that is capable of weak dipolar interactions. In addition, the lone pairs on sulphur allow Met to interact with other groups, inluding the aforementioned metal ions. Because of its properties, Met can play diverse roles in metal coordination, fine tuning of redox reactions, or supporting protein structures. These roles are strongly influenced by the nature of the surrounding medium. Herein, we describe several common interactions between Met and surrounding aromatic amino acids and how they affect the physical properties of both copper and iron metalloproteins. While the importance of interactions between Met and other groups is established in biological systems, less is known about their roles in redox metalloproteins and our view is that this is an area that is ready for greater attention. [ABSTRACT FROM AUTHOR]

The photochemical activation of carbon–hydrogen bonds by vanadium(V)–dioxo and vanadium(V)–oxo–peroxo diimine complexes is described. Reactions were carried out using a selection of organic substrates with C–H bond dissociation free energy values between 70 and 97 kcal mol−1. The ability to activate C–H bonds using vanadium(V)–dioxo and vanadium(V)–oxo–peroxo diimine complexes varies with different bond dissociation free energy. Compounds with weaker C–H bonds are oxidized in minutes, rather than in days for thermal oxidations by the corresponding complexes. Dioxygen is necessary for substrate consumption, which suggests that the electronically excited V complexes are radical reaction initiators via H-atom abstraction from the organic substrate. [ABSTRACT FROM AUTHOR]

Porphyrins are archetypal ligands in inorganic chemistry. The last 10 years have seen important new advances in the use of metalloporphyrins as catalysts in the activation and reduction of small molecules, in particular O2 and CO2. Recent developments of new molecular designs, scaling relationships, and theoretical modeling of mechanisms have rapidly advanced the utility of porphyrins as electrocatalysts. This Minireview focuses on the summary and evaluation of recent developments of metalloporphyrin O2 and CO2 reduction electrocatalysts, with an emphasis on contrasting homogeneous and heterogeneous electrocatalysis. Comparisons for proposed reaction mechanisms are provided for both CO2 and O2 reduction, and ideas are proposed about how lessons from the last decade of research can lead to the development of practical, applied porphyrin‐derived catalysts. [ABSTRACT FROM AUTHOR]

Electrocatalytic CO2 reduction in water using a series of chlorotricarbonylrhenium(I) diimine complexes deposited on pyrolytic graphite electrodes is described. Two known CO2 reduction catalysts (with diimine = 4,4′-di-tert-butyl-2,2′-bipyridine or 2-(2′-quinolyl)benzimidazole), that are highly active in organic solvent, proved to be only weakly active in water. In contrast, Cl(CO)3Re(L) complexes with tridentate nitrogen-containing ligands (L = 4,4′,4′′-tri-tert-butyl-2,2′:6′,2′′-terpyridine or 2,6-bis(2-benzimidazolyl)pyridine) were better CO2 reduction catalysts. In those Cl(CO)3Re(L) complexes, only two N-atoms of the ligand are coordinated to the rhenium, leaving the third arm of the ligands to support activated, CO2-bound intermediates. The 2,6-bis(2-pyridyl)pyridine (terpy) complex was the most active, with substantial activity at alkaline pH. [ABSTRACT FROM AUTHOR]

The impact of the local environment and additives on the tuning of the different thermochemical and kinetic properties of ascorbate is determined. The various solution and biochemical significance of the discussed effects is also analyzed.

Electrospray ionization in combination with Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry are used to explore the gas-phase oxygen-atom transfer chemistry, characterize and reactivities of meso-tetrakis(pentafluorophenyl)porphyrin (TPFPP) chromium complexes, as well as some other tetradentate macrocyclic ligands. The TPFPP are excellent systems to examine the effects of spin-state change on oxygen-atom transfer due to doublet nature of the [d.sup.1] [Cr.sup.V] species and quartet ground states feature of [Cr.sup.III] complexes with high-lying doublet excited states.

A number of concerted proton-electron transfer (CPET) reactions of ascorbates in MeCN, which shed light on an unusual solvent dependence of the O-H bond dissociation free energy is described. The reactions are found to proceed by concerted [H.sup.+]/[e.sup.-] transfer rather than a stepwise path involving separate electron and proton transfers.

Model porphyrin-iron-bis(imidazole) complexes have reacted readily by hydrogen atom transfer (HAT). The results have shown that HAT reactions should be considered as part of the primary arsenal of reactivity of histidine-ligated heme cofactors.

The p53 protein plays a major role in cancer prevention, and over 50 % of cancer diagnoses can be attributed to p53 malfunction. The common p53 mutation Y220C causes local protein unfolding, aggregation, and can result in a loss of Zn in the DNA‐binding domain. Structural analysis has shown that this mutant creates a surface site that can be stabilized using small molecules, and herein a multifunctional approach to restore function to p53‐Y220C is reported. A series of compounds has been designed that contain iodinated phenols aimed for interaction and stabilization of the p53‐Y220C surface cavity, and Zn‐binding fragments for metallochaperone activity. Their Zn‐binding affinity was characterized using spectroscopic methods and demonstrate the ability of compounds L4 and L5 to increase intracellular levels of Zn2+ in a p53‐Y220C‐mutant cell line. The in vitro cytotoxicity of our compounds was initially screened by the National Cancer Institute (NCI‐60), followed by testing in three stomach cancer cell lines with varying p53 status', including AGS (WTp53), MKN1 (V143A), and NUGC3 (Y220C). Our most promising ligand, L5, is nearly 3‐fold more cytotoxic than cisplatin in a large number of cell lines. The impressive cytotoxicity of L5 is further maintained in a NUGC3 3D spheroid model. L5 also induces Y220C‐specific apoptosis in a cleaved caspase‐3 assay, reduces levels of unfolded mutant p53, and recovers p53 transcriptional function in the NUGC3 cell line. These results show that these multifunctional scaffolds have the potential to restore wild‐type function in mutant p53‐Y220C. In a bind: A series of compounds has been designed that contain iodinated phenols aimed for interaction and stabilization of the p53‐Y220C surface cavity, and Zn‐binding fragments for metallochaperone activity. Their Zn‐binding affinity was characterized using spectroscopic methods and demonstrate the ability of compounds L4 and L5 to increase intracellular levels of Zn2+ in a p53‐Y220C‐mutant cell line. Results from in vitro cytotoxic tests show that these multifunctional scaffolds have the potential to restore wild‐type function in mutant p53‐Y220C. [ABSTRACT FROM AUTHOR]

Lignolytic peroxidases use an electron transfer (ET) pathway that involves amino acid-mediated substrate oxidation at the surface of the protein rather than at an embedded heme site. In many of these peroxidases, redox catalysis takes place at a substrate accessible tyrosine or tryptophan (Trp) amino acid. Here, we describe new mutants of cytochrome c peroxidase (CcP) that were designed to incorporate a Trp-based “wire” that can move oxidizing equivalents from the heme to the protein surface. Three mutant CcP proteins were expressed and characterized: A193W, Y229W, and A193W/Y229W. These mutants can oxidize veratryl alcohol substrate with turnover numbers greater than wild type CcP using H2O2 as an oxidant. The A193W/Y229W mutant is the most active. However, the reactivity is still less than typical lignin peroxidases at pH 8. The redox reactivity of these proteins is analysed using semiclassical electron transfer theory. An electron hopping mechanism is possible for A193W/Y229W mutant. These data suggest that artificial chains of aromatic amino acids can support hole transfer from embedded sites to protein surfaces for catalytic redox reactions. [ABSTRACT FROM AUTHOR]

Rapid and low overpotential oxidation of water to dioxygen remains a key hurdle for storage of solar energy. Here, we address this issue by demonstrating that deprotonation of 2-(2′-pyridyl)-imidazole (pimH)-ligated copper complexes promotes water oxidation at low overpotential and low catalyst loading. This improves upon other work on homogeneous copper-based water oxidation catalysts, which are highly active, but limited by high overpotentials. EPR and UV-vis spectroscopic evaluation of catalyst speciation shows that at pH ≥ 12 coordinated pimH is deprotonated and a bis(hydroxide) Cu2+ active catalyst forms. Rapid electrochemical water oxidation (35 s−1, 0.85 V onset potential) was observed with 150 μM catalyst. These results demonstrate that catalytic water oxidation potentials can be shifted by hundreds of mV in homogeneous metal catalysts bearing an ionisable imidazole ligand. [ABSTRACT FROM AUTHOR]

Selective functionalization of unactivated C–H bonds is an ongoing chemical challenge. C–H activation requires the transfer of H+ and e−, so called proton-coupled electron transfer (PCET) reactions. Recent efforts in photochemical PCET involving C–H bonds show great promise for the synthesis of new compounds. One such example is photochemical C–H fluorination reactions. In many cases, the discrete PCET mechanisms are yet to be defined in a systematic way. Here, we investigated electron transfer (ET) and PCET reactions of electronically excited 1,2,4,5-tetracyanobenzene and anthraquinone with the components of typical fluorination reactions. Analysis using kinetic and thermodynamic models, and steady-state and time-resolved fluorescence data, suggest that C–H activation proceeds efficiently where electronically excited sensitizers accept HṖ. [ABSTRACT FROM AUTHOR]

Oxygen reduction in acidic aqueous solution mediated by a series of asymmetric iron (III)-tetra(aryl)porphyrins adsorbed to basal- and edge- plane graphite electrodes is investigated. The asymmetric iron porphyrin systems bear phenyl groups at three meso positions and either a 2-pyridyl, a 2-benzoic acid, or a 2-hydroxyphenyl group at the remaining meso position. The presence of the three unmodified phenyl groups makes the compounds insoluble in water, enabling catalyst retention during electrochemical experiments. Resonance Raman data demonstrate that catalyst layers are maintained, but can undergo modification after prolonged catalysis in the presence of O2. The introduction of a single proton relay group at the fourth meso position makes the asymmetric iron porphyrins markedly more robust catalysts; these molecules support higher sustained current densities than the parent iron tetraphenylporphyrin. Iron porphyrins bearing a 2-pyridyl group are the most active catalysts and operate at stable current densities ≥1 mA cm−2 for over 5 h. Comparative analysis of the catalysts with different proton relays also is reported. [ABSTRACT FROM AUTHOR]

An enormous variety of biological redox reactions are accompanied by changes in proton content at enzyme active sites, in their associated cofactors, in substrates and/or products, and between protein interfaces. Understanding this breadth of reactivity is an ongoing chemical challenge. A great many workers have developed and investigated biomimetic model complexes to build new ways of thinking about the mechanistic underpinnings of such complex biological proton-coupled electron transfer (PCET) reactions. Of particular importance are those model reactions that involve transfer of one proton (H+) and one electron (e- ), which is equivalent to transfer of a hydrogen atom (H•). In this Current Topic, we review key concepts in PCET reactivity and describe important advances in biomimetic PCET chemistry, with a special emphasis on research that has enhanced efforts to understand biological PCET reactions. [ABSTRACT FROM AUTHOR]