Geometric isomers of dichloridoiron(III) complexes of CTMC (5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradecane)

Di­chlorido­iron(III) com­plexes of two stereoisomers of a tetra­aza­macrocycle are examined. The stereoisomerism of the macrocycle is shown to determine the geometric isomerism of the resulting metal com­plex, and these results are com­pared to relevant previous reports.

Both trans and cis iron–CTMC com­plexes, namely, trans-di­chlorido­[(5SR,7RS,12RS,14SR)-5,7,12,14-tetra­methyl-1,4,8,11-tetra­aza­cyclo­tetra­deca­ne]iron(III) tetra­chlorido­ferrate, [Fe(C₁₄H₃₂N₄)Cl₂][FeCl₄] (1a), the analogous chloride methanol monosolvate, [Fe(C₁₄H₃₂N₄)Cl₂]Cl·CH₃OH (1b), and cis-di­chlo­rido­[(5SR,7RS,12SR,14RS)-5,7,12,14-tetra­methyl-1,4,8,11-tetra­aza­cyclo­tetra­dec­ane]iron(III) chloride, [Fe(C₁₄H₃₂N₄)Cl₂]Cl (2), were successfully synthesized and structurally characterized using X-ray diffraction. The coordination geometry of the macrocycle is dependent on the stereoisomerism of CTMC. The packing of these com­plexes appears to be strongly influenced by extensive hydro­gen-bonding inter­actions, which are in turn determined by the nature of the counter-anions (1aversus1b) and/or the coordination geometry of the macrocycle (1a/1bversus2). These observations are extended to related ferric cis- and trans-di­chloro macrocyclic com­plexes.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Electrocatalytic Water Oxidation with α-[Fe(mcp)(OTf)2] and Analogues

The complex α-[Fe(mcp)(OTf)₂] (mcp = N,N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)-cyclohexane-1,2-diamine and OTf = trifluoromethanesulfonate anion) was reported in 2011 by some of us as an active water oxidation (WO) catalyst in the presence of sacrificial oxidants. However, because chemical oxidants are likely to take part in the reaction mechanism, mechanistic electrochemical studies are critical in establishing to what extent previous studies with sacrificial reagents have actually been meaningful. In this study, the complex α-[Fe(mcp)(OTf)₂] and its analogues were investigated electrochemically under both acidic and neutral conditions. All the systems under investigation proved to be electrochemically active toward the WO reaction, with no major differences in activity despite the structural changes. Our findings show that WO-catalyzed by mcp–iron complexes proceeds via homogeneous species, whereas the analogous manganese complex forms a heterogeneous deposit on the electrode surface. Mechanistic studies show that the reaction proceeds with a different rate-determining step (rds) than what was previously proposed in the presence of chemical oxidants. Moreover, the different kinetic isotope effect (KIE) values obtained electrochemically at pH 7 (KIE ∼ 10) and at pH 1 (KIE = 1) show that the reaction conditions have a remarkable effect on the rds and on the mechanism. We suggest a proton-coupled electron transfer (PCET) as the rds under neutral conditions, whereas at pH 1 the rds is most likely an electron transfer (ET).

Homogeneous Catalysts Based on First-Row Transition-Metals for Electrochemical Water Oxidation

Strategies that enable the renewable production of storable fuels (i. e. hydrogen or hydrocarbons) through electrocatalysis continue to generate interest in the scientific community. Of central importance to this pursuit is obtaining the requisite chemical (H+ ) and electronic (e- ) inputs for fuel-forming reduction reactions, which can be met sustainably by water oxidation catalysis. Further possibility exists to couple these redox transformations to renewable energy sources (i. e. solar), thus creating a carbon neutral solution for long-term energy storage. Nature uses a Mn-Ca cluster for water oxidation catalysis via multiple proton-coupled electron-transfers (PCETs) with a photogenerated bias to perform this process with TOF 100∼300 s-1 . Synthetic molecular catalysts that efficiently perform this conversion commonly utilize rare metals (e. g., Ru, Ir), whose low abundance are associated to higher costs and scalability limitations. Inspired by nature's use of 1st row transition metal (TM) complexes for water oxidation catalysts (WOCs), attempts to use these abundant metals have been intensively explored but met with limited success. The smaller atomic size of 1st row TM ions lowers its ability to accommodate the oxidative equivalents required in the 4e- /4H+ water oxidation catalysis process, unlike noble metal catalysts that perform single-site electrocatalysis at lower overpotentials (η). Overcoming the limitations of 1st row TMs requires developing molecular catalysts that exploit biomimetic phenomena - multiple-metal redox-cooperativity, PCET and second-sphere interactions - to lower the overpotential, preorganize substrates and maintain stability. Thus, the ultimate goal of developing efficient, robust and scalable WOCs remains a challenge. This Review provides a summary of previous research works highlighting 1st row TM-based homogeneous WOCs, catalytic mechanisms, followed by strategies for catalytic activity improvements, before closing with a future outlook for this field.

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.

A dinuclear iron complex as an efficient electrocatalyst for homogeneous water oxidation reaction

A novel dinuclear iron complex of a Schiff base ligand has been exploited as a homogeneous water splitting electrocatalyst having possible real life application in renewable energy.

Water oxidation by Ferritin: A semi-natural electrode

Ferritin is a protein (ca. 12 nm) with a central pocket of 6 nm diameter, and hydrated iron oxide stored in this central cavity of this protein. The protein shell has a complicated structure with 24 subunits. Transmission electron microscopy images of ferritin showed nanosized iron oxides (ca. 4-6 nm) in the protein structure. In high-resolution transmission electron microscopy images of the iron core, d-spacings of 2.5-2.6 Å were observed, which is corresponded to d-spacings of ferrihydrite crystal structure. Our experiments showed that at pH 11, the modified electrode by this biomolecule is active for water oxidation (turnover frequency: 0.001 s^-1 at 1.7 V). Using affected by bacteria, we showed that Fe ions in the structure of ferritin are critical for water oxidation.

Design of Iron Coordination Complexes as Highly Active Homogenous Water Oxidation Catalysts by Deuteration of Oxidation-Sensitive Sites

The nature of the oxidizing species in water oxidation reactions with chemical oxidants catalyzed by α-[Fe(OTf)₂(mcp)] (1α; mcp = N, N'-dimethyl- N, N'-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine, OTf = trifluoromethanesulfonate anion) and β-[Fe(OTf)₂(mcp)] (1β) has been investigated. Mössbauer spectroscopy provides definitive evidence that 1α and 1β generate oxoiron(IV) species as the resting state. Decomposition paths of the catalysts have been investigated by identifying and quantifying ligand fragments that form upon degradation. This analysis correlates the water oxidation activity of 1α and 1β with stability against oxidative damage of the ligand via aliphatic C-H oxidation. The site of degradation and the relative stability against oxidative degradation are shown to be dependent on the topology of the catalyst. Furthermore, the mechanisms of catalyst degradation have been rationalized by computational analyses, which also explain why the topology of the catalyst enforces different oxidation-sensitive sites. This information has served in creating catalysts where sensitive C-H bonds have been replaced by C-D bonds. The deuterated analogues D₄-α-[Fe(OTf)₂(mcp)] (D₄-1α), D₄-β-[Fe(OTf)₂(mcp)] (D₄-1β), and D₆-β-[Fe(OTf)₂(mcp)] (D₆-1β) were prepared, and their catalytic activity has been studied. D₄-1α proves to be an extraordinarily active and efficient catalyst (up to 91% of O₂ yield); it exhibits initial reaction rates identical with those of its protio analogue, but it is substantially more robust toward oxidative degradation and yields more than 3400 TON ( n(O₂)/ n(Fe)). Altogether this evidences that the water oxidation catalytic activity is performed by a well-defined coordination complex and not by iron oxides formed after oxidative degradation of the ligands.

Catalytic Activity of an Iron-Based Water Oxidation Catalyst: Substrate Effects of Graphitic Electrodes

The synthesis, characterization, and electrochemical studies of the dinuclear complex [(MeOH)Fe(Hbbpya)-μ-O-(Hbbpya)Fe(MeOH)](OTf)₄ (1) (with Hbbpya = N,N-bis(2,2′-bipyrid-6-yl)amine) are described. With the help of online electrochemical mass spectrometry, the complex is demonstrated to be active as a water oxidation catalyst. Comparing the results obtained for different electrode materials shows a clear substrate influence of the electrode, as the complex shows a significantly lower catalytic overpotential on graphitic working electrodes in comparison to other electrode materials. Cyclic voltammetry experiments provide evidence that the structure of complex 1 undergoes reversible changes under high-potential conditions, regenerating the original structure of complex 1 upon returning to lower potentials. Results from electrochemical quartz crystal microbalance experiments rule out that catalysis proceeds via deposition of catalytically active material on the electrode surface.

Armed by Asp? C-terminal carboxylate in a Dap-branched peptide and consequences in the binding of CuII and electrocatalytic water oxidation

C-Terminal carboxylate in branched peptide allows insight into water oxidation electrocatalysis by Cu-complexes, revealing differences to homologues with varied modules.

In operando studies on the electrochemical oxidation of water mediated by molecular catalysts

This feature article describes on-line studies regarding the water oxidation reaction mediated by molecular catalysts.

Water oxidation using earth-abundant transition metal catalysts: opportunities and challenges

Catalysts for the oxidation of water are a vital component of solar energy to fuel conversion technologies. This Perspective summarizes recent advances in the field of designing homogeneous water oxidation catalysts (WOCs) based on Mn, Fe, Co and Cu.

An efficient and inexpensive water-oxidizing manganese-based oxide electrode

A cheap and very simple method to synthesize an efficient water-oxidizing manganese-based oxide coated fluorine doped tin oxide electrode was reported.