Iron Coordination Complexes Research Articles

Coordination complexes of iron(II) phthalocyanine with cationic, anionic and neutral ligands

A series of coordination complexes of iron(II) phthalocyanine (Fe[Formula: see text]Pc) with anionic, cationic and neutral ligands has been obtained in crystalline form. The salt of Fe[Formula: see text]Pc with MDABCO[Formula: see text] cations (MDABCO[Formula: see text][Fe[Formula: see text](Pc[Formula: see text]][Formula: see text]⋅(I[Formula: see text]⋅2TPC⋅2C6H4Cl2 (1) contains Fe[Formula: see text]Pc reduced to [Fe[Formula: see text](Pc[Formula: see text]][Formula: see text], and the Fe[Formula: see text] ion coordinates two cationic MDABCO[Formula: see text] ligands with the Fe-N(MDABCO[Formula: see text] bond length of 2.49–2.50 Å. The I[Formula: see text] anions are introduced into the phthalocyanine layers to compensate positive charge of the second MDABCO[Formula: see text] cation and form short hydrogen bonds with Pc. Preparation of dianionic salt of Fe[Formula: see text]Pc with the Bu3MeP[Formula: see text] cations by their reduction with sodium fluorenone ketyl yields a green solution characteristic of neutral Fe[Formula: see text]Pc. The solved structure shows deprotonation of the cations and the formation of Bu3PCH2 ylide which coordinates to Fe[Formula: see text]Pc in (Bu3PCH[Formula: see text][Fe[Formula: see text](Pc[Formula: see text]] (2). Two anionic acetylide (C[Formula: see text]CH[Formula: see text] or imidazole (Im[Formula: see text] ligands coordinate to Fe[Formula: see text]Pc by dissolving with cryptand and yielding {Cryptand(Na[Formula: see text]}2{(CCH[Formula: see text]Fe[Formula: see text](Pc[Formula: see text]}⋅2C6H4Cl2 (3) and {Cryptand(Na[Formula: see text]}2{(Im[Formula: see text]Fe[Formula: see text](Pc[Formula: see text]}⋅2C6H4Cl2 (4) complexes. In contrast to 1, complexes 2–4 are mainly neutral with weak charge transfer from ligands to Fe[Formula: see text], which is accompanied by the formation of Fe[Formula: see text] in 2 and 3.

Design and Processing as Ultrathin Films of a Sublimable Iron(II) Spin Crossover Material Exhibiting Efficient and Fast Light-Induced Spin Transition.

Materials based on spin crossover (SCO) molecules have centered the attention in molecular magnetism for more than 40 years as they provide unique examples of multifunctional and stimuli-responsive materials, which can be then integrated into electronic devices to exploit their molecular bistability. This process often requires the preparation of thermally stable SCO molecules that can sublime and remain intact in contact with surfaces. However, the number of robust sublimable SCO molecules is still very scarce. Here, we report a novel example of this kind. It is based on a neutral iron(II) coordination complex formulated as [Fe(neoim)2], where neoimH is the ionogenic ligand 2-(1H-imidazol-2-yl)-9-methyl-1,10-phenanthroline. In the first part, a comprehensive study, which covers the synthesis and magnetostructural characterization of the [Fe(neoim)2] complex as a bulk microcrystalline material, is reported. Then, in the second part, we investigate the suitability of this material to form thin films through high-vacuum sublimation. Finally, the retainment of all present SCO capabilities in the bulk when the material is processed is thoroughly studied by means of X-ray absorption spectroscopy. In particular, a very efficient and fast light-induced spin transition (LIESST effect) has been observed, even for ultrathin films of 15 nm.

Ferric Pyrophosphate Forms Soluble Iron Coordination Complexes with Zinc Compounds and Solubilizing Agents in Extruded Rice and Predicts Increased Iron Solubility and Bioavailability in Young Women

BackgroundCo-extrusion of ferric pyrophosphate (FePP) with solubilizers, citric acid/trisodium citrate (CA/TSC), or ethylenediaminetetraacetic acid (EDTA) sharply increases iron absorption. Whether this can protect against the inhibition of iron absorption by phytic acid (PA) is unclear. Sodium pyrophosphate (NaPP) may be a new enhancer of iron absorption from FePP. ObjectivesOur objectives were to 1) investigate the ligand coordination of iron, zinc, and solubilizers in extruded rice and test associations with iron solubility and absorption, 2) assess whether co-extrusion of FePP + CA/TSC rice can protect against inhibition of iron absorption by PA; 3) determine the effect of zinc oxide (ZnO) compared with zinc sulfate (ZnSO4), and 4) quantify iron absorption from FePP + NaPP rice. MethodsWe produced labeled 57FePP rice cofortified with ZnSO4 and EDTA, CA/TSC or NaPP, and FePP + EDTA rice with ZnO. We used electron paramagnetic resonance (EPR) to characterize iron-ligand complexes. We measured in vitro iron solubility and fractional iron absorption (FIA) in young women (n = 21, age: 22 ± 2 y, BMI: 21.3 ± 1.5 kg/m2 geometric mean plasma ferritin, 28.5 μg/L) compared with ferrous sulfate (58FeSO4). FIA was compared by linear mixed-effect model analysis. ResultsThe addition of zinc and solubilizers created new iron coordination complexes of Fe(III) species with a weak ligand field at a high-spin state that correlated with solubility (r2 = 0.50, P = 0.02) and absorption (r2 = 0.72, P = 0.02). Phytic acid reduced FIA from FePP + CA/TSC rice by 50% (P < 0.001), to the same extent as FeSO4. FIA from FePP + EDTA + ZnO and FePP + EDTA + ZnSO4 rice did not significantly differ. Mean FIAs from FePP + EDTA + ZnSO4, FePP + CA/TSC + ZnSO4, and FePP + NaPP + ZnSO4 rice were 9% to 11% and did not significantly differ from each other or from FeSO4. ConclusionRice extrusion of FePP with solubilizers resulted in bioavailable iron coordination complexes. In the case of FePP + CA/TSC, PA exerted similar inhibition of FIA as with FeSO4. FePP + NaPP could be a further viable solubilizing agent for rice fortification.This study was registered at clinicaltrials.gov as NCT03703739.

Theoretical, Semiempirical, and Experimental Solvatochromic Comparison Methods for the Construction of the α1 Scale of Hydrogen-Bond Donation of Solvents.

Today, the hydrogen bonding donation (HBD) ability parameter of new solvents, α, is generally determined either by the Kamlet-Taft solvatochromic comparison of two probes, Reichardt betaine dye B(30) and 4-nitroanisole, or by the measurement of a single probe (e.g., solvatochromism of an iron coordination complex). This work highlights the shortcomings of these probes and recommends three replacement methods: (a) the theoretical comparison of the experimental and PCM-TD-DFT calculated transition energies ET(30) of B(30), (b) the semiempirical comparison of the experimental and McRae calculated ET(30), and, (c) for ionic liquids, the experimental comparison of ET(30) and ET(33) lying on the lower basicity of the betaine dye B(33) compared to B(30). These methods yield a new HBD parameter, α1, for 101 molecular solvents and 30 ionic liquids. The novelty is emblematic for water, with α1 = 1.54 instead of α (Kamlet-Taft) = 1.17. The solvent parameter α1 is not equivalent to the solute hydrogen-bond acidity parameter α2H, partly because of the self-association of HBD solvents.

(±)-Catechin-A Mass-Spectrometry-Based Exploration Coordination Complex Formation with FeII and FeIII.

Catechin is an extensively investigated plant flavan-3-ol with a beneficial impact on human health that is often associated with antioxidant activities and iron coordination complex formation. The aim of this study was to explore these properties with FeII and FeIII using a combination of nanoelectrospray-mass spectrometry, differential pulse voltammetry, site-specific deoxyribose degradation assay, FeII autoxidation assay, and brine shrimp mortality assay. Catechin primarily favored coordination complex formation with Fe ions of the stoichiometry catechin:Fe in the ratio of 1:1 or 2:1. In the detected Fe–catechin coordination complexes, FeII prevailed. Differential pulse voltammetry, the site-specific deoxyribose degradation, and FeII autoxidation assays proved that coordination complex formation affected catechin’s antioxidant effects. In situ formed Fe–catechin coordination complexes showed no toxic activities in the brine shrimp mortality assay. In summary, catechin has properties for the possible treatment of pathological processes associated with ageing and degeneration, such as Alzheimer’s and Parkinson’s diseases.

Elucidating Instabilities Contributing to Capacity Fade in Bipyridine‐Based Materials for Non‐aqueous Flow Batteries

AbstractMetal‐based non‐aqueous redox flow batteries have the potential for long‐term energy storage if stability requirements can be achieved. One such stability issue in metal coordination complexes arises from ligand shedding in the anolyte upon reduction. Recognizing that the free ligands are relatively more stable than the metal coordination complex under highly reducing conditions, we evaluated a family of metal‐free bipyridinium materials as flow battery anolytes with the corresponding iron coordination complex as the catholyte. Bipyridinium compounds were functionalized for increased electrochemical stability, cycled in flow cells to understand efficiencies, and analyzed for degradation products. Methylation of the bipyridine nitrogens increased electrochemical stability yet left the reduced molecule susceptible to radical‐induced bond cleavage. Subsequent functionalization of bipyridine with carbomethoxy groups resulted in good battery performance with 96 % Coulombic and 90 % voltage efficiency, and improved cycling stability over a methoxy‐substituted anolyte with 14 % vs 36 % capacity fade in the first charge‐discharge cycle.

Anion-assisted Fe(III)-coordination supramolecular systems based on 2,6-diacetylpyridine dihydrazone

Six new iron coordination complexes were obtained as a result of the direct interaction of the Fe3+ ion and Schiff base ligands in the presence of anions with weak, moderate and high coordinating ability in order to study their role in the construction of diverse supramolecular assemblies. All the crystalline solids present mononuclear complexes: [Fe(H2L1)(H2O)1.5(MeOH)0.5](ClO4)3·1.75H2O (1), [Fe(H2L2)(H2O)2](ClO4)3·H2O (2), [Fe(HL1)(NCS)2]·0.25H2O (3), [Fe(HL2)(NCS)2]·2.33H2O (4), [Fe(L1)(N3)(H2O)]·H2O (5) and [Fe(L2)(H2O)2]OH·4H2O (6), where H2L1 = 2,6-diacetylpyridine bis(isonicotinoylhydrazone) and H2L2 = 2,6-diacetylpyridine bis(nicotinoylhydrazone), which were characterized by IR spectroscopy and single crystal X-ray diffraction. The Schiff base ligands in the perchlorate-assisted compounds adopt a double zwitterion tautomeric form and these coordinate as neutral ligands. In the isothiocyanate complexes, the Schiff base coordinates as a single zwitterionic monoprotonated ligand, while the presence of the N3– anion in the reaction leads to the bisdeprotonation of both 2,6-diacetylpyridine dihydrazones. The crystal structures of all the coordination compounds show different types of weak intermolecular interactions, which were confirmed by the analysis of their Hirshfeld surfaces and the corresponding fingerprint diagrams.

Two state "ON-OFF" NLO switch based on coordination complexes of iron and cobalt containing isomeric ligand: a DFT study.

Coordination complexes are interesting materials for nonlinear optical (NLO) applications due to their large hyperpolarizability values. Moreover, switchable NLO response is also important in coordination complexes. Herein, we report two state ON–OFF switchable NLO contrast of coordination complexes of Fe and Co containing isomeric ligands. The optical, UV-visible, and electronic properties besides the “ON–OFF” switching effect are calculated using the CAM-B3LYP/6-31+G (d) method. The NLO responses of ligand–metal isomers are qualitatively evaluated through variation in charge transference (CT) style through TD-DFT. The higher βo in each isomeric pair is strongly dependent on the HOMO–LUMO gap. The isomer 4b with lowest HOMO–LUMO gap shows the highest NLO response. The charge transfer pattern in these complexes results in variation of their βo values. The notable βo contrast of 21.15 in isomeric pairs 3a and 3b makes these complexes a favorable material for genuine NLO switches. Hence, the outcome of the current investigation reveals that these ligand–metal isomeric complexes exhibit a two-state switch “ON–OFF” effect.

Intramolecular Arene Bonds of Complexes Featuring Terphenyl Bis(carbene) Macrocyclic Ligands

A series of macrocyclic bis(carbene) ligands were synthesized by linking 2,2″-bis(imidazole)terphenyl with a methylene, ethylene, propylene, or borate unit. Coordination complexes of iron were synt...

Synthesis and Characterization of Bis(phosphine)carbodicarbene Complexes of Iron, Cobalt, and Nickel

The synthesis and characterization of nickel, cobalt, and iron complexes of the bis(phosphine)carbodicarbene (CDC) ligand 1 is described. The square-planar complexes can be synthesized without the isolation of the deprotonated carbodicarbene ligand, and three different synthetic methods were used to make these first-row transition-metal compounds. The reaction between ligand 1-BPh4 and Ni(COD)2 afforded the compound [(CDC)Ni(η1-C8H13)][BPh4] (2-BPh4) via the insertion of COD into the Ni–H bond produced by cyclometalation. The complexes [(CDC)MCl]+ for nickel and cobalt were synthesized from the reaction of carbodicarbene ligand 1 with NiCl2 or CoCl2 in the presence of triethylamine. For the synthesis of the iron coordination complex of CDC ligand 1, Fe2Mes4 was used to produce [(CDC)Fe-Mes][BPh4] (5), an intermediate-spin Fe(II) compound. Complexes 3–7 were characterized by spectroscopic methods and X-ray single-crystal diffraction analysis.

Iron(III), cobalt(II) and zinc(II) coordination compounds with a carboximidamide ligand: Synthesis, structures and properties

Coordination complexes of iron(III), cobalt(II) and zinc(II) with the ligand N′-{[1-(pyridin-2-yl)ethylidene]amino}pyridine-2-carboximidamide (pmhpa) have been synthesised and were structurally characterized by elemental analysis, mass spectrometry, X-ray diffraction and infrared spectroscopy. The mononuclear iron(III) complex, [Fe(pmhap)2][FeCl4], (1), adopts the orthorhombic non-centric space group P212121. The iron is present in an octahedral environment in the ion [Fe(pmhap)2]+. The crystallographically-determined formula of the cobalt(II) complex (2) is [Co2(pmhap)2Cl3]2[CoCl4].4MeOH. The compound crystallizes in the non-centric space group P4̄21c with a single cobalt ion in the asymmetric unit. The complex is dinuclear, with two cobalt ions bridged by chloride and the two nitrogen atoms of N=N of the pmhap ligand. The zinc(II) complex; [Zn2(pmhpa)3](NO3)4.2.75MeOH (3) crystallizes in the cubic space group Pa3̄ with disordered methanol molecules present. This is dinuclear with the two zinc atoms being bridged by three N=N links from the ligand to form a threefold symmetric rotor. Infrared spectroscopy confirms that coordination occurs through the azide bridge. Measurements of magnetic susceptibility for the iron(III) and cobalt(II) complexes show that they are paramagnetic between 5 and 300 K. Mössbauer spectroscopic data supports the structure of the iron(III) complex being composed of an octahedral and a tetrahedral iron atom.

Front Cover: Binding and Release of FeIII Complexes from Glucan Particles for the Delivery of T1 MRI Contrast Agents (ChemMedChem 12/2020)

The Front Cover shows yeast-derived β-glucan particles (GPs) loaded with a paramagnetic, high spin iron(III) coordination complex. The iron(III) complex is under development as a magnetic resonance imaging (MRI) contrast agent. The complex remains encapsulated in GPs under physiological conditions, and its water proton relaxivity is masked. Addition of the maltol chelator or mild acid releases the iron(III) complex from the GPs to restore the proton relaxivity of the complex as an MRI contrast agent. The cover art was inspired by Vincent van Gogh′s Starry Night. More information can be found in the Full Paper by Janet R. Morrow et al.

Electrolyte Lifetime in Aqueous Organic Redox Flow Batteries: A Critical Review.

Aqueous organic redox flow batteries (RFBs) could enable widespread integration of renewable energy, but only if costs are sufficiently low. Because the levelized cost of storage for an RFB is a function of electrolyte lifetime, understanding and improving the chemical stability of active reactants in RFBs is a critical research challenge. We review known or hypothesized molecular decomposition mechanisms for all five classes of aqueous redox-active organics and organometallics for which cycling lifetime results have been reported: quinones, viologens, aza-aromatics, iron coordination complexes, and nitroxide radicals. We collect, analyze, and compare capacity fade rates from all aqueous organic electrolytes that have been utilized in the capacity-limiting side of flow or hybrid flow/nonflow cells, noting also their redox potentials and demonstrated concentrations of transferrable electrons. We categorize capacity fade rates as being "high" (>1%/day), "moderate" (0.1-1%/day), "low" (0.02-0.1%/day), and "extremely low" (≤0.02%/day) and discuss the degree to which the fade rates have been linked to decomposition mechanisms. Capacity fade is observed to be time-denominated rather than cycle-denominated, with a temporal rate that can depend on molecular concentrations and electrolyte state of charge through, e.g., bimolecular decomposition mechanisms. We then review measurement methods for capacity fade rate and find that simple galvanostatic charge-discharge cycling is inadequate for assessing capacity fade when fade rates are low or extremely low and recommend refining methods to include potential holds for accurately assessing molecular lifetimes under such circumstances. We consider separately symmetric cell cycling results, the interpretation of which is simplified by the absence of a different counter-electrolyte. We point out the chemistries with low or extremely low established fade rates that also exhibit open circuit potentials of 1.0 V or higher and transferrable electron concentrations of 1.0 M or higher, which are promising performance characteristics for RFB commercialization. We point out important directions for future research.

Miquel Costas.

"The secret of being a successful scientist is, I guess, a combination of intelligence, hard work, perseverance, and serendipity. I admire intelligent originality …" Find out more about Miquel Costas in his Author Profile.

Assessment of cytotoxic and genotoxic potentials of a mononuclear Fe(II) Schiff base complex with photocatalytic activity in Trigonella

BackgroundIn recent times, coordination complexes of iron in various oxidation states along with variety of ligand systems have been designed and developed for effective treatment of cancer cells without adversely affecting the normal cell and tissues of various organs. MethodsIn this study, we have evaluated the mechanism of action of a Fe(II) Schiff base complex in the crop plant Trigonella foenum-graecum L. (Fenugreek) as the screening system by using morphological, cytological, biochemical and molecular approaches. Further functional characterization was performed using MCF-7 cell line and solid tumour model for the assessment of anti-tumour activity of the complex. ResultsOur results indicate efficiency of the Fe(II) Schiff base complex in the induction of double strand breaks in DNA. Complex treatment clearly induced cytotoxic and genotoxic damage in Trigonella seedlings. The Fe-complex treatment caused cell cycle arrest via the activation of ATM-ATR kinase mediated DNA damage response pathway with the compromised expression of CDK1, CDK2 and CyclinB1 protein in Trigonella seedlings. In cultured MCF-7 cells, the complex induces cytotoxicity and DNA fragmentation through intracellular ROS generation. Fe-complex treatment inhibited tumour growth in solid tumour model with no additional side effects. ConclusionThe growth inhibitory and cytotoxic effects of the complex result from activation of DNA damage response along with oxidative stress and cell cycle arrest. General significanceOverall, our results have provided comprehensive information on the mechanism of action and efficacy of a Fe(II) Schiff base complex in higher eukaryotic genomes and indicated its future implications as potential therapeutic agent.

Iron(II) coordination complexes with panchromatic absorption and nanosecond charge-transfer excited state lifetimes.

Replacing current benchmark rare-element photosensitizers with ones based on abundant and low-cost metals such as iron would help facilitate the large-scale implementation of solar energy conversion. To do so, the ability to extend the lifetimes of photogenerated excited states of iron complexes is critical. Here, we present a sensitizer design in which iron(II) centres are supported by frameworks containing benzannulated phenanthridine and quinoline heterocycles paired with amido donors. These complexes exhibit panchromatic absorption and nanosecond charge-transfer excited state lifetimes, enabled by the combination of vacant, energetically accessible heterocycle-based acceptor orbitals and occupied molecular orbitals destabilized by strong mixing between amido nitrogen atoms and iron. This finding shows how ligand design can extend metal-to-ligand charge-transfer-type excited state lifetimes of iron(II) complexes into the nanosecond regime and expand the range of potential applications for iron-based photosensitizers.

Supramolecular Iron Complex Formed Between Nitrogen Riched Phenanthroline Derivative and Iron With Improved Oxygen Reduction Activity in Alkaline Electrolyte.

In this work, the synthesis and evaluation of a new type non-noble metal oxygen reduction reaction (ORR) catalyst is reported. The catalyst is a complex containing iron ions and multiple N active sites, which displayed excellent oxygen reduction activity in alkaline medium. 2-(2-(4-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)pyridin-2-yl)pyridin-4-yl)-1H-imidazo[4,5-f][1,10]phenanthroline (PIPhen) was synthesized and used as a ligand to form a rich nitrogen iron coordination complex (Fe-PIPhen), and the complex was then loaded onto the carbon powder to form the target catalyst of Fe-PIPhen/C. The physical characterization of the catalyst was conducted by using Scanning Electron Microscopy (SEM), nitrogen adsorption-desorption and X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller analysis etc. Electrochemical characterizations were realized by taking cyclic voltammetry (CV), linear sweep voltammetry (LSV) and rotating ring disk electrode (RRDE). The results show that Fe-PIPhen/C possesses the good performance; it exhibits a high electrocatalytic activity, which is mainly via a four electron ORR pathway, with a low hydrogen peroxide yield of 2.58%. And, the average electron transfer number of 3.93 was obtained in alkaline electrolyte. In summary, Fe-PIPhen/C will likely become a promising alternative to Pt catalyst in fuel cell.

Benchmark of Density Functionals for the Calculation of the Redox Potential of Fe3+/Fe2+ Within Protein Coordination Shells.

Iron is a very important transition metal often found in proteins. In enzymes specifically, it is often found at the core of reaction mechanisms, participating in the reaction cycle, more often than not in oxidation/reduction reactions, where it cycles between its most common Fe(III)/Fe(II) oxidation states. QM and QM/MM computational methods that study these catalytic reaction mechanisms mostly use density functional theory (DFT) to describe the chemical transformations. Unfortunately, density functional is known to be plagued by system-specific and property-specific inaccuracies that cast a shadow of uncertainty over the results. Here we have modeled 12 iron coordination complexes, using ligands that represent amino acid sidechains, and calculated the accuracy with which the most common density functionals reproduce the redox properties of the iron complexes (specifically the electronic component of the redox potential at 0 K, ), using the same property calculated with CCSD(T)/CBS as reference for the evaluation. A number of hybrid and hybrid-meta density functionals, generally with a large % of HF exchange (such as BB1K, mPWB1K, and mPW1B95) provided systematically accurate values for , with MUEs of ~2 kcal/mol. The very popular B3LYP density functional was found to be quite precise as well, with a MUE of 2.51 kcal/mol. Overall, the study provides guidelines to estimate the inaccuracies coming from the density functionals in the study of enzyme reaction mechanisms that involve an iron cofactor, and to choose appropriate density functionals for the study of the same reactions.

Antioxidant Properties and the Formation of Iron Coordination Complexes of 8-Hydroxyquinoline.

Background: The alkaloid 8-hydroxyquinoline (8HQ) is well-known for various biological activities, including antioxidant effects and especially for the formation of coordination complexes with various transition metals, such as iron, amongst others. Therefore, 8HQ was extensively explored as a promising antineurodegenerative agent. However, other authors noted pro-oxidant effects of 8HQ. Here, we explore the pro- and antioxidant properties of 8HQ, especially in context of coordination complexes with iron (II) and iron (III). Methods: Nano-electrospray−mass spectrometry, differential pulse voltammetry, deoxyribose degradation, iron (II) autoxidation, and brine shrimp mortality assays were used. Results: 8HQ formed a complex mixture of coordination complexes with iron (II) and iron (III). Furthermore, 8HQ showed antioxidant effects but no pro-oxidant ones. In the brine shrimp mortality assay, 8HQ demonstrated toxicity that decreased in the presence of iron (III). Conclusions: 8HQ is a potent antioxidant whose effects depend not only on the formation of the coordination complexes with iron ions, but surely on the scavenging activities due to the redox properties of the 8-hydroxyl group. No pro-oxidant effects were observed in the set of the used assays.

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.