Photoinduced hole transfer from tris(bipyridine)ruthenium dye to a high-valent iron-based water oxidation catalyst

Factors that Impact Photochemical Cage Escape Yields

The utilization of visible light to mediate chemical reactions in fluid solutions has applications that range from solar fuel production to medicine and organic synthesis. These reactions are typically initiated by electron transfer between a photoexcited dye molecule (a photosensitizer) and a redox-active quencher to yield radical pairs that are intimately associated within a solvent cage. Many of these radicals undergo rapid thermodynamically favored "geminate" recombination and do not diffuse out of the solvent cage that surrounds them. Those that do escape the cage are useful reagents that may undergo subsequent reactions important to the above-mentioned applications. The cage escape process and the factors that determine the yields remain poorly understood despite decades of research motivated by their practical and fundamental importance. Herein, state-of-the-art research on light-induced electron transfer and cage escape that has appeared since the seminal 1972 review by J. P. Lorand entitled "The Cage Effect" is reviewed. This review also provides some background for those new to the field and discusses the cage escape process of both homolytic bond photodissociation and bimolecular light induced electron transfer reactions. The review concludes with some key goals and directions for future research that promise to elevate this very vibrant field to even greater heights.

Crystal structure of a water oxidation catalyst solvate with composition (NH4)2[FeIV(L-6H)]·3CH3COOH (L = clathrochelate ligand)

The synthetic availability of mol-ecular water oxidation catalysts containing high-valent ions of 3d metals in the active site is a prerequisite to enabling photo- and electrochemical water splitting on a large scale. Herein, the synthesis and crystal structure of di-ammonium {μ-1,3,4,7,8,10,12,13,16,17,19,22-dodeca-aza-tetra-cyclo-[8.8.4.13,17.18,12]tetra-cosane-5,6,14,15,20,21-hexa-onato}ferrate(IV) acetic acid tris-olvate, (NH4)2[FeIV(C12H12N12O6)]·3CH3COOH or (NH4)2[FeIV(L-6H)]·3CH3COOH is reported. The FeIV ion is encapsulated by the macropolycyclic ligand, which can be described as a dodeca-aza-quadricyclic cage with two capping tri-aza-cyclo-hexane fragments making three five- and six six-membered alternating chelate rings with the central FeIV ion. The local coord-ination environment of FeIV is formed by six deprotonated hydrazide nitro-gen atoms, which stabilize the unusual oxidation state. The FeIV ion lies on a twofold rotation axis (multiplicity 4, Wyckoff letter e) of the space group C2/c. Its coordination geometry is inter-mediate between a trigonal prism (distortion angle φ = 0°) and an anti-prism (φ = 60°) with φ = 31.1°. The Fe-N bond lengths lie in the range 1.9376 (13)-1.9617 (13) Å, as expected for tetra-valent iron. Structure analysis revealed that three acetic acid mol-ecules additionally co-crystallize per one iron(IV) complex, and one of them is positionally disordered over four positions. In the crystal structure, the ammonium cations, complex dianions and acetic acid mol-ecules are inter-connected by an intricate system of hydrogen bonds, mainly via the oxamide oxygen atoms acting as acceptors.

Crystal structure and Hirshfeld surface analysis of poly[[tetra-aqua-(μ-1,3,4,7,8,10,12,13,16,17,19,22-dodeca-aza-tetra-cyclo-[8.8.4.13,17.18,12]tetra-cosane-5,6,14,15,20,21-hexaonato)iron(IV)dilithium] tetra-hydrate]

The title compound, [FeLi2(C12H12N12O6)(H2O)4]·4H2O, consists of iron complex anions, lithium cations and water mol-ecules. The complex anion shows a clathrochelate topology. The coordination geometry of the FeIV centre is inter-mediate between a trigonal prism and a trigonal anti-prism. In the crystal, the complex anions are connected through two Li cations into dimers, which are connected by Li-O bonds, forming infinite chains along the b-axis direction.

Synthesis, Structure, and Magnetic Properties of Ditopic Ferrocenylboron-Capped Tris-Pyridineoximate Iron, Cobalt, and Nickel(II) Pseudoclathrochelates

Abstract

Tris-pyridineoximate iron, cobalt, and nickel(II) pseudoclathrochelates with apical ferrocenyl substituent were obtained in the reasonable yields (50–70%) in a boiling ethanol by the template condensation of 2-acetylpyridineoxime with ferrocenylboronic acid on the corresponding M2+ ion as a matrix. The composition and structure of new ditopic compounds, isolated in the forms of their ionic associates with perchlorate anion, were determined using elemental analysis, UV-vis spectroscopy, MALDI-TOF mass spectrometry, and NMR spectroscopy. According to the magnetometry data, the iron(II) pseudoclathrochelate is a diamagnetic compound, while the temperature dependences of magnetic susceptibility of the nickel and cobalt(II) complexes are characteristic of the high-spin systems with S = 1 and 3/2, respectively. As follows from the X-ray diffraction data for the iron and nickel(II) pseudoclathrochelates, the Ni–N distances (2.15–2.17 Å) are characteristic of the high-spin Ni2+ complexes, while they in its iron(II)-containing analog, slightly exceed of 2 Å, thus suggesting the low-spin state of this ion.

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.

Hybrid iron(II) phthalocyaninatoclathrochelates with a terminal reactive vinyl group and their organo-inorganic polymeric derivatives: synthetic approaches, X-ray structures and copolymerization with styrene

Hybrid metallo(IV)phthalocyaninate-capped tris-dioximate iron(II) complexes (termed as "phthalocyaninatoclathrochelates") with non-equivalent apical fragments and functionalized with one terminal reactive vinyl group were prepared for the first time using three different synthetic approaches: (i) transmetallation (capping group exchange) of the appropriate labile boron,antimony-capped cage precursors, (ii) capping of the initially isolated reactive semiclathrochelate intermediate, and (iii) direct one-pot template condensation of their ligand synthons on the iron(II) ion as a matrix. The obtained polytopic cage complexes were characterized using elemental analysis, ¹H NMR, MALDI-TOF MS and UV-vis spectra, and the single-crystal X-ray diffraction experiments. One of the obtained vinyl-terminated iron(II) phthalocyaninatoclathrochelates and its semiclathrochelate precursor were tested as monomers in a copolymerization reaction with styrene as the main component. These vinyl-terminated (semi)clathrochelate iron(II) complexes were found to be successfully copolymerized with this industrially important monomer, affording the intensely colored copolymer products. Because of a low solubility of the tested zirconium(IV) phthalocyaninate-capped tris-nioximate monomer in styrene as a solvent, a molar ratio of 1 : 500 was used. The obtained copolymer products and the kinetics of their formation were studied using GPC, FTIR, UV-vis, TGA and DSC methods. Even at such a low concentration of the Fe,Zr-binuclear metallocomplex component, an increase in the rate of the UV-light degradation of the organo-inorganic products, as well as in their thermal stability, was observed.

Expanding manganese(IV) aqueous chemistry: unusually stable water-soluble hexahydrazide clathrochelate complexes

Mn cage complexes are rare, and the ones successfully isolated in the solid state are not stable in water and organic solvents. Herein, we present the first report of mononuclear Mn clathrochelates, in which the encapsulated metal exists in the oxidation state +4. The complexes are extremely stable in the crystalline state and in solutions and show rich redox chemistry.

Nanosecond heme-to-heme electron transfer rates in a multiheme cytochrome nanowire reported by a spectrally unique His/Met-ligated heme

Multiheme cytochromes have been identified as essential proteins for electron exchange between bacterial enzymes and redox substrates outside of the cell. In microbiology, these proteins contribute to efficient energy storage and conversion. For biotechnology, multiheme cytochromes contribute to the production of green fuels and electricity. Furthermore, these proteins inspire the design of molecular-scale electronic devices. Here, we report exceptionally high rates of heme-to-heme electron transfer in a multiheme cytochrome. We expect similarly high rates, among the highest reported for ground-state electron transfer in biology, in other multiheme cytochromes as the close-packed hemes adopt similar configurations despite very different amino acid sequences and protein folds.

Proteins achieve efficient energy storage and conversion through electron transfer along a series of redox cofactors. Multiheme cytochromes are notable examples. These proteins transfer electrons over distance scales of several nanometers to >10 μm and in so doing they couple cellular metabolism with extracellular redox partners including electrodes. Here, we report pump-probe spectroscopy that provides a direct measure of the intrinsic rates of heme–heme electron transfer in this fascinating class of proteins. Our study took advantage of a spectrally unique His/Met-ligated heme introduced at a defined site within the decaheme extracellular MtrC protein of Shewanella oneidensis. We observed rates of heme-to-heme electron transfer on the order of 10⁹ s⁻¹ (3.7 to 4.3 Å edge-to-edge distance), in good agreement with predictions based on density functional and molecular dynamics calculations. These rates are among the highest reported for ground-state electron transfer in biology. Yet, some fall 2 to 3 orders of magnitude below the Moser–Dutton ruler because electron transfer at these short distances is through space and therefore associated with a higher tunneling barrier than the through-protein tunneling scenario that is usual at longer distances. Moreover, we show that the His/Met-ligated heme creates an electron sink that stabilizes the charge separated state on the 100-μs time scale. This feature could be exploited in future designs of multiheme cytochromes as components of versatile photosynthetic biohybrid assemblies.

Crystal structure and Hirshfeld surface analysis of poly[[bis-[μ4-N,N'-(1,3,5-oxadiazinane-3,5-di-yl)bis(carbamoyl-methano-ato)]nickel(II)tetra-potassium] 4.8-hydrate]

The title compound, {[K4Ni2(C7H6N4O7)2]·4.8H2O} n , was obtained as a result of a template reaction between oxalohydrazide-hydroxamic acid, formaldehyde and nickel(II) nitrate followed by partial hydrolysis of the formed inter-mediate. The two independent [Ni(C7H6N4O7)]2- complex anions exhibit pseudo-C S symmetry and consist of an almost planar metal-containing fragment and a 1,3,5-oxadiazinane ring with a chair conformation disposed nearly perpendicularly with respect to the former. The central NiII atom has a square-planar N2O2 coordination arrangement formed by two amide N and two carboxyl-ate O atoms. In the crystal, the nickel(II) complex anions form layers parallel to the ab plane. Neighboring complex anion layers are connected by layers of potassium cations for which two of the four independent cations are disordered over two sites [ratios of 0.54 (3):0.46 (3) and 0.9643 (15):0.0357 (15)]. The framework is stabilized by an extensive system of hydrogen bonds where the water mol-ecules act as donors and the carb-oxy-lic O atoms, the amide O atoms and the oxadiazinane N atoms act as acceptors.

Hybrid compound based on diethylenetriaminecopper(ii) cations and scarce V-monosubstituted β-octamolybdate as water oxidation catalyst

Herein, we report on a new hybrid compound (NH₄){[Cu(dien)(H₂O)₂]₂[β-VMo₇O₂₆]}·1.5H₂O (1), where dien = diethylenetriamine, containing an extremely rare mixed-metal pseudo-octamolybdate cluster. An ex situ EPR spectroscopy provided insights into the formation of paramagnetic species in reaction mixture and in solution of 1. The magneto-structural correlations revealed weak antiferromagnetic exchange interactions between the [Cu(dien)]²⁺ cations transmitted by intermolecular pathways. The cyclic voltammetry showed the one-electron process associated with the Cu³⁺/Cu²⁺ oxidation followed by the multi-electron catalytic wave due to water oxidation with a faradaic yield of 86%. The title compound was thus employed in homogeneous water oxidation catalysis using tris(bipyridine)ruthenium photosensitizer. At pH 8.0, efficiency of the catalytic system attained 0.19 turnovers per second supported by the relatively mild water oxidation overpotential of 0.54 V.

A new hybrid compound (NH₄){[Cu(dien)(H₂O)₂]₂[β-VMo₇O₂₆]}·1.5H₂O was employed in homogeneous water oxidation catalysis. At pH 8.0, its efficiency attains 0.19 turnovers per second, supported by the relatively mild water oxidation overpotential of 0.54 V.

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.

Interacting Ru(bpy) 3 2 + Dye Molecules and TiO2 Semiconductor in Dye-Sensitized Solar Cells

Solar energy is an alternative source of energy that can be used to replace fossil fuels. Various types of solar cells have been developed to harvest this seemingly endless supply of energy, leading to the construction of solar cell devices, such as dye-sensitized solar cells. An important factor that affects energy conversion efficiency of dye-sensitized solar cells is the distribution of dye molecules within the porous semiconductor (TiO 2 ). In this paper, we formulate a continuum model for the interaction between the dye molecule Tris(2,2 ′ -bipyridyl)ruthenium(II) (Ru(bpy) 3 2 + ) and titanium dioxide (TiO 2 ) semiconductor. We obtain the equilibrium position at the minimum energy position between the dye molecules and between the dye and TiO 2 nanoporous structure. Our main outcome is an analytical expression for the energy of the two molecules as a function of their sizes. We also show that the interaction energy obtained using the continuum model is in close agreement with molecular dynamics simulations.