Photodriven Oxidation of Water by Plastoquinone Analogs with a Nonheme Iron Catalyst

A Method Probing High-Temperature Oxidation Behavior of Crystalline Materials

To date, the oxidation behavior of crystal materials is not fully understood; additional research is needed to understand the oxidation of materials. Herein, density functional theory (DFT) calculations and a 3D kinetic Monte Carlo (KMC) model are used to investigate the infiltration and diffusion behaviors of oxygen atoms within the crystal. Oxygen molecules readily adsorbes on crystal surfaces of the material and rapidly dissociates, verified by both first-principles calculations and energy-dispersive spectrometer (EDS) results. The infiltration ability of oxygen atoms into the inner crystal layers is affected by the surrounding oxygen atom, lattice compactness, and other factors. Energy-barrier calculations show that crystal thin/dense layers have significant effects on the crystal oxidation process, so high-pressure technology is used to investigate this correlation experimentally. KMC calculations and thermogravimetric analyses (TGA) show the infiltration behavior of oxygen atoms in the main crystal plane (211) toward the inner layers has the highest proportion to the actual high-temperature oxidation behavior of the title material. The results of both the KMC calculations and thermal experiments show the material peeled off upon further oxidation, which accelerates oxidation. At the same time, high-pressure treatment increases the oxidation resistance of materials at lower temperatures (<600 °C).

Artificial Photosynthesis for Regioselective Reduction of NAD(P)+ to NAD(P)H Using Water as an Electron and Proton Source

In photosynthesis, four electrons and four protons taken from water in photosystem II (PSII) are used to reduce NAD(P)+ to produce NAD(P)H in photosystem I (PSI), which is the most important reductant to reduce CO2. Despite extensive efforts to mimic photosynthesis, artificial photosynthesis to produce NAD(P)H using water electron and proton sources has yet to be achieved. Herein, we report the photocatalytic reduction of NAD(P)+ to NAD(P)H and its analogues in a molecular model of PSI, which is combined with water oxidation in a molecular model of PSII. Photoirradiation of a toluene/trifluoroethanol (TFE)/borate buffer aqueous solution of hydroquinone derivatives (X-QH2), 9-mesityl-10-methylacridinium ion, cobaloxime, and NAD(P)+ (PSI model) resulted in the quantitative and regioselective formation of NAD(P)H and p-benzoquinone derivatives (X-Q). X-Q was reduced to X-QH2, accompanied by the oxidation of water to dioxygen under the photoirradiation of a toluene/TFE/borate buffer aqueous solution of [(N4Py)FeII]2+ (PSII model). The PSI and PSII models were combined using two glass membranes and two liquid membranes to produce NAD(P)H using water as an electron and proton source with the turnover number (TON) of 54. To the best of our knowledge, this is the first time to achieve the stoichiometry of photosynthesis, photocatalytic reduction of NAD(P)+ by water to produce NAD(P)H and O2.

Arene C-H Amination with N-Heteroarenes by Catalytic DDQ Photocatalysis

Arene C-H aminations using catalytic amounts of a 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) photocatalyst are described. Benzene, which has an oxidation potential of 2.48 V (vs SCE), was functionalized by pyrazoles, triazoles, tetrazoles, purines, and tert-butoxycarbonyl amine. Arenes underwent amination via a combination of ultraviolet (UV) light and a DDQ photocatalyst without a typical co-oxidant. Although the mechanism remains an open question, DDQH₂, which is generated from DDQ after oxidation, is reactivated to DDQ under UV light irradiation conditions, possibly with the assistance of adventitious O₂ and/or a solvent as the terminal oxidant(s) in this system.

Use of Singlet Oxygen in the Generation of a Mononuclear Nonheme Iron(IV)-Oxo Complex

Nonheme iron(III)-superoxo intermediates are generated in the activation of dioxygen (O₂) by nonheme iron(II) complexes and then converted to iron(IV)-oxo species by reacting with hydrogen donor substrates with relatively weak C-H bonds. If singlet oxygen (¹O₂) with ca. 1 eV higher energy than the ground state triplet oxygen (³O₂) is employed, iron(IV)-oxo complexes can be synthesized using hydrogen donor substrates with much stronger C-H bonds. However, ¹O₂ has never been used in generating iron(IV)-oxo complexes. Herein, we report that a nonheme iron(IV)-oxo species, [Fe^IV(O)(TMC)]²⁺ (TMC = tetramethylcyclam), is generated using ¹O₂, which is produced with boron subphthalocyanine chloride (SubPc) as a photosensitizer, and hydrogen donor substrates with relatively strong C-H bonds, such as toluene (BDE = 89.5 kcal mol^-1), via electron transfer from [Fe^II(TMC)]²⁺ to ¹O₂, which is energetically more favorable by 0.98 eV, as compared with electron transfer from [Fe^II(TMC)]²⁺ to ³O₂. Electron transfer from [Fe^II(TMC)]²⁺ to ¹O₂ produces an iron(III)-superoxo complex, [Fe^III(O₂)(TMC)]²⁺, followed by abstracting a hydrogen atom from toluene by [Fe^III(O₂)(TMC)]²⁺ to form an iron(III)-hydroperoxo complex, [Fe^III(OOH)(TMC)]²⁺, that is further converted to the [Fe^IV(O)(TMC)]²⁺ species. Thus, the present study reports the first example of generating a mononuclear nonheme iron(IV)-oxo complex with the use of singlet oxygen, instead of triplet oxygen, and a hydrogen atom donor with relatively strong C-H bonds. Detailed mechanistic aspects, such as the detection of ¹O₂ emission, the quenching by [Fe^II(TMC)]²⁺, and the quantum yields, have also been discussed to provide valuable mechanistic insights into understanding nonheme iron-oxo chemistry.

Competitive interaction of Mn(II) and Fe(II) cations with the high-affinity Mn-binding site of the photosystem II: evolutionary aspect

The evolutionary origin of the oxygen-evolving complex (OEC) in the photosystem II (PSII) is still unclear, as is the nature of electron source for the photosystem before the OEC had appeared. Johnson et al. (in PNAS 110:11238, 2013) speculated that Mn(II) cations were the source of electrons for transitional photosystems. However, Archean oceans also contained Fe(II) cations at concentrations comparable or higher than that of Mn(II). Fe(II) cations can bind to the high-affinity (НА) Mn-binding site in the OEC (Semin et al. in Biochemistry 41:5854, 2002). Now we have investigated the competitive interaction of Mn(II) and Fe(II) cations with the HA site in the Mn-depleted PSII membranes (PSII[-Mn]). Fe cations, oxidized under illumination, bind strongly to the HA site and, thus, prevent the interaction of Mn(II) with this site. If the Mn(II) and Fe(II) cations, at relatively equal concentration, are simultaneously present in the buffer, together with PSII(-Mn) membranes, there is competition between these two cations for the binding site, which manifests itself in partial inhibition of the Mn(II) oxidation and the blocking of the HA site by Fe(II) cations. If the concentration of Fe(II) cations is several times higher than the concentration of Mn(II), the HA site is completely blocked and the oxidation of Mn(II) cations is inhibited; under saturating light, the effectiveness of this inhibitory effect increases. This may be due to the generation of H₂O₂ on the acceptor side of the photosystem, which significantly accelerates the rate of the turnover reaction of Mn(II) on the HA site.

Water Oxidation by Pentapyridyl Base Metal Complexes? A Case Study

The design of molecular water oxidation catalysts (WOCs) requires a rational approach that considers the intermediate steps of the catalytic cycle, including water binding, deprotonation, storage of oxidizing equivalents, O–O bond formation, and O₂ release. We investigated several of these properties for a series of base metal complexes (M = Mn, Fe, Co, Ni) bearing two variants of a pentapyridyl ligand framework, of which some were reported previously to be active WOCs. We found that only [Fe(Py5OMe)Cl]⁺ (Py5OMe = pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane]) showed an appreciable catalytic activity with a turnover number (TON) = 130 in light-driven experiments using the [Ru(bpy)₃]²⁺/S₂O₈^2– system at pH 8.0, but that activity is demonstrated to arise from the rapid degradation in the buffered solution leading to the formation of catalytically active amorphous iron oxide/hydroxide (FeOOH), which subsequently lost the catalytic activity by forming more extensive and structured FeOOH species. The detailed analysis of the redox and water-binding properties employing electrochemistry, X-ray absorption spectroscopy (XAS), UV–vis spectroscopy, and density-functional theory (DFT) showed that all complexes were able to undergo the M^III/M^II oxidation, but none was able to yield a detectable amount of a M^IV state in our potential window (up to +2 V vs SHE). This inability was traced to (i) the preference for binding Cl^– or acetonitrile instead of water-derived species in the apical position, which excludes redox leveling via proton coupled electron transfer, and (ii) the lack of sigma donor ligands that would stabilize oxidation states beyond M^III. On that basis, design features for next-generation molecular WOCs are suggested.

We scrutinize the water oxidation activity for pentapyridyl metal complexes [M^II(Py5R)Cl]⁺ (M = Mn, Fe, Co, Ni; R = OH, OMe). Analysis of their stability, redox, and water-binding properties shows that the complexes are not able to reach high-valent intermediate states and do not catalyze water oxidation in their molecular form.

Molecular Photocatalytic Water Splitting by Mimicking Photosystems I and II

In nature, water is oxidized by plastoquinone to evolve O₂ and form plastoquinol in Photosystem II (PSII), whereas NADP⁺ is reduced by plastoquinol to produce NADPH and regenerate plastoquinone in Photosystem I (PSI), using homogeneous molecular photocatalysts. However, water splitting to evolve H₂ and O₂ in a 2:1 stoichiometric ratio has yet to be achieved using homogeneous molecular photocatalysts, remaining as one of the biggest challenges in science. Herein, we demonstrate overall water splitting to evolve H₂ and O₂ in a 2:1 ratio using a two liquid membranes system composed of two toluene phases, which are separated by a solvent mixture of water and trifluoroethanol (H₂O/TFE, 3:1 v/v), with a glass membrane to combine PSI and PSII molecular models. A PSII model contains plastoquinone analogs [p-benzoquinone derivatives (X-Q)] in toluene and an iron(II) complex as a molecular oxidation catalyst in H₂O/TFE (3:1 v/v), which evolves a stoichiometric amount of O₂ and forms plastoquinol analogs (X-QH₂) under photoirradiation. On the other hand, a PSI model contains nothing in toluene but contains X-QH₂, 9-mesityl-10-methylacridinium ion (Acr⁺-Mes) as a photocatalyst, and a cobalt(III) complex as an H₂ evolution catalyst in H₂O/TFE (3:1 v/v), which evolves a stoichiometric amount of H₂ and forms X-Q under photoirradiation. When a PSII model system is combined with a PSI model system with two glass membranes and two liquid membranes, photocatalytic water splitting with homogeneous molecular photocatalysts is achieved to evolve hydrogen and oxygen with the turnover number (TON) of >100.

The most reactive iron and manganese complexes with N-pentadentate ligands for dioxygen activation—synthesis, characteristics, applications

The iron and manganese complexes that activate oxygen atom play multiple role in technologically relevant reactions as well as in biological transformations, in which exist in different redox states. Among them, high-valent oxo intermediate seems to be the most important one. Iron, and/or manganese-based processes have found application in many areas, starting from catalysis and sustainable technologies, through DNA oxidative cleavage, to new substances useful in chemotherapeutic drugs. This review is not only the latest detailed list of uses of homogeneous N-pentadentate iron and manganese catalysts for syntheses of valuable molecules with huge applications in green technologies, but also a kind of "a cookbook", collecting "recipes" for the discussed complexes, in which the sources necessary to obtain a full characterization of the compounds are presented. Following the catalytic activity of metalloenzymes, and taking into account the ubiquity of iron and manganese salts, which in combination with properly designed ligands may show similarity to natural systems, the discussed complexes can find application as new anti-cancer drugs. Also, owing to ability of oxygen atom to exchange in reaction with H₂O, they can be successfully applied in photodriven reactions of water oxidation, as well as in chemically regenerated fuel cells as a redox catalyst.

Iron-Based Catalytically Active Complexes in Preparation of Functional Materials

Iron complexes are particularly interesting as catalyst systems over the other transition metals (including noble metals) due to iron’s high natural abundance and mediation in important biological processes, therefore making them non-toxic, cost-effective, and biocompatible. Both homogeneous and heterogeneous catalysis mediated by iron as a transition metal have found applications in many industries, including oxidation, C-C bond formation, hydrocarboxylation and dehydration, hydrogenation and reduction reactions of low molecular weight molecules. These processes provided substrates for industrial-scale use, e.g., switchable materials, sustainable and scalable energy storage technologies, drugs for the treatment of cancer, and high molecular weight polymer materials with a predetermined structure through controlled radical polymerization techniques. This review provides a detailed statement of the utilization of homogeneous and heterogeneous iron-based catalysts for the synthesis of both low and high molecular weight molecules with versatile use, focusing on receiving functional materials with high potential for industrial application.

Photocatalytic Hydrogen Evolution from Plastoquinol Analogues as a Potential Functional Model of Photosystem I

The recent development of a functional model of photosystem II (PSII) has paved a new way to connect the PSII model with a functional model of photosystem I (PSI). However, PSI functional models have yet to be reported. We report herein the first potential functional model of PSI, in which plastoquinol (PQH₂) analogues were oxidized to plastoquinone (PQ) analogues, accompanied by hydrogen (H₂) evolution. Photoirradiation of a deaerated acetonitrile (MeCN) solution containing hydroquinone derivatives (X-QH₂) as a hydrogen source, 9-mesityl-10-methylacridinium ion (Acr⁺-Mes) as a photoredox catalyst, and a cobalt(III) complex, Co^III(dmgH)₂pyCl (dmgH = dimethylglyoximate monoanion; py = pyridine) as a redox catalyst resulted in the evolution of H₂ and formation of the corresponding p-benzoquinone derivatives (X-Q) quantitatively. The maximum quantum yield for photocatalytic H₂ evolution from tetrachlorohydroquinone (Cl₄QH₂) with Acr⁺-Mes and Co^III(dmgH)₂pyCl and H₂O in deaerated MeCN was determined to be 10%. Photocatalytic H₂ evolution is started by electron transfer (ET) from Cl₄QH₂ to the triplet ET state of Acr⁺-Mes to produce Cl₄QH₂^•+ and Acr^•-Mes with a rate constant of 7.2 × 10⁷ M^-1 s^-1, followed by ET from Acr^•-Mes to Co^III(dmgH)₂pyCl to produce [Co^II(dmgH)₂pyCl]^-, accompanied by the regeneration of Acr⁺-Mes. On the other hand, Cl₄QH₂^•+ is deprotonated to produce Cl₄QH^•, which transfers either a hydrogen-atom transfer or a proton-coupled electron transfer to [Co^II(dmgH)₂pyCl]^- to produce a cobalt(III) hydride complex, [Co^III(H)(dmgH)₂pyCl]^-, which reacts with H⁺ to evolve H₂, accompanied by the regeneration of Co^III(dmgH)₂pyCl. The formation of [Co^II(dmgH)₂pyCl]^- was detected by electron paramagnetic resonance measurements.

An Iron(III) Complex with Pincer Ligand—Catalytic Water Oxidation through Controllable Ligand Exchange

Pincer ligands occupy three coplanar sites at metal centers and often support both stability and reactivity. The five-coordinate [FeIIICl2(tia-BAI)] complex (tia-BAI− = 1,3-bis(2’-thiazolylimino)isoindolinate(−)) was considered as a potential pre-catalyst for water oxidation providing the active form via the exchange of chloride ligands to water molecules. The tia-BAI− pincer ligand renders water-insolubility to the Fe–(tia-BAI) assembly, but it tolerates the presence of water in acetone and produces electrocatalytic current in cyclic voltammetry associated with molecular water oxidation catalysis. Upon addition of water to [FeIIICl2(tia-BAI)] in acetone the changes in the Fe3+/2+ redox transition and the UV-visible spectra could be associated with solvent-dependent equilibria between the aqua and chloride complex forms. Immobilization of the complex from methanol on indium-tin-oxide (ITO) electrode by means of drop-casting resulted in water oxidation catalysis in borate buffer. The O2 detected by gas chromatography upon electrolysis at pH 8.3 indicates >80% Faraday efficiency by a TON > 193. The investigation of the complex/ITO assembly by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) before and after electrolysis, and re-dissolution tests suggest that an immobilized molecular catalyst is responsible for catalysis and de-activation occurs by depletion of the metal.

Iron and manganese oxo complexes, oxo wall and beyond

High-valent metal-oxo species with multiply-bonded M-O groups have been proposed as key intermediates in many biological and abiological catalytic oxidation reactions. These intermediates are implicated as active oxidants in alkane hydroxylation, olefin epoxidation and other oxidation reactions. For example, [Fe^ivO(porphyrinato^•-)]⁺ cofactors bearing π-radical porphyrinato^•- ligands oxidize organic substrates in cytochrome P450 enzymes, which are common to many life forms. Likewise, high-valent Mn-oxo species are active for H₂O oxidation in photosystem II. The chemistry of these native reactive species has inspired chemists to prepare highly oxidized transition-metal complexes as functional mimics. Although many synthetic Fe-O and Mn-O complexes now exist, the analogous oxo complexes of the late transition metals (groups 9-11) are rare. Indeed, late-transition-metal-oxo complexes of tetragonal (fourfold) symmetry should be electronically unstable, a rule commonly referred to as the 'oxo wall'. A few late metal-oxos have been prepared by targeting other symmetries or unusual spin states. These complexes have been studied using spectroscopic and theoretical methods. This Review describes mononuclear non-haem Fe-O and Mn-O species, the nature of the oxo wall and recent advances in the preparation of oxo complexes of Co, Ni and Cu beyond the oxo wall.