Iridium(III) Bis-Pyridine-2-Sulfonamide Complexes as Efficient and Durable Catalysts for Homogeneous Water Oxidation

Efficiency Conceptualization Model: A Theoretical Method for Predicting the Turnover of Catalysts

In recent times, the theoretical prediction of catalytic efficiency is of utmost urgency. With the advent of density functional theory (DFT), reliable computations can delineate a quantitative aspect of the study. To this state-of-the-art approach, valuable incorporation would be a tool that can acknowledge the efficiency of a catalyst. In the current work, we developed the efficiency conceptualization model (ECM) that utilizes the quantum mechanical tool to achieve efficiency in terms of turnover frequency (TOF). Twenty-six experimentally designed transition metal (TM) water oxidation catalysts were chosen under similar experimental conditions of temperature, pressure, and pH to execute the same. The computations conclude that the Fe-based [Fe(OTf)2(Me2Pytacn)] (MWOC-17) is a highly active catalyst and, therefore, can endure for more time in the catalytic cycle. Our results conclude that the Ir-based catalysts [Cp*Ir(κ2-N,O)X] with MWOC-23: X=Cl; and MWOC-24: X=NO3 report the highest computed turnover numbers (TONs),τ c o m p u t e d T O N 0 ${\tau _{computed\;TON}^0 }$ of 406 and 490 against the highest experimental TONs,τ e x p e r i m e n t a l T O N ${\tau _{experimental\;TON} }$ of 1200 and 2000 respectively, whereas the Co-based [Co(12-TMC)]2+ (MWOC-19) has the lowest TONs (τ c o m p u t e d T O N 0 ${\tau _{computed\;TON}^0 }$ =19, τexperimental TON=16) among the chosen catalysts and thereby successful in corroborating the previous experimental results.

Toward Rational Design of Mononuclear Nickel Complexes as Water Oxidation Catalysts Exploring the Ligand Effects on the Rate-Determining Step

The major impediment in realizing a carbon-neutral hydrogen fuel economy is the cost and inadequacy of contemporary electrochemical water splitting approaches towards the energy intensive oxygen evolution reaction (OER). The O-O bond formation in the water oxidation half-cell reaction is both kinetically and thermodynamically challenging and amplifies the overpotential requirement in most of the active water oxidation catalysts. Herein, density functional theory is employed to interrogate 20 Ni(II) complexes, out of which 17 are in silico designed molecular water oxidation catalysts, coordinated to electron-rich tetra-anionic redox non-innocent phenylenebis(oxamidate) and dibenzo-1,4,7,10-tetraazacyclododecane-2,3,8,9-tetraone parent ligands and their structural analogues, and identify the role of substituent changes or ligand effects in the order of their reactivity. Importantly, our computational mechanistic analyses predict that the activation free energy of the rate-determining O-O bond formation step obeys an inverse scaling relationship with the global electrophilicity index of the intermediate generated on two-electron oxidation of the starting complex. Additionally, the driving force is directly correlated with this OER descriptor which enables two-dimensional volcano representation and thereby extrapolation towards the ideal substitution with the chosen ligand. Our study, therefore, establish fundamental insights to overcome the imperative overpotential issue with simple and precise computational rationalization preceding experimental validation.

Evolution in the Design of Water Oxidation Catalysts with Transition-Metals: A Perspective on Biological, Molecular, Supramolecular, and Hybrid Approaches

Increased demand for a carbon-neutral sustainable energy scheme augmented by climatic threats motivates the design and exploration of novel approaches that reserve intermittent solar energy in the form of chemical bonds in molecules and materials. In this context, inspired by biological processes, artificial photosynthesis has garnered significant attention as a promising solution to convert solar power into chemical fuels from abundantly found H2O. Among the two redox half-reactions in artificial photosynthesis, the four-electron oxidation of water according to 2H2O → O2 + 4H+ + 4e- comprises the major bottleneck and is a severe impediment toward sustainable energy production. As such, devising new catalytic platforms, with traditional concepts of molecular, materials and biological catalysis and capable of integrating the functional architectures of the natural oxygen-evolving complex in photosystem II would certainly be a value-addition toward this objective. In this review, we discuss the progress in construction of ideal water oxidation catalysts (WOCs), starting with the ingenuity of the biological design with earth-abundant transition metal ions, which then diverges into molecular, supramolecular and hybrid approaches, blurring any existing chemical or conceptual boundaries. We focus on the geometric, electronic, and mechanistic understanding of state-of-the-art homogeneous transition-metal containing molecular WOCs and summarize the limiting factors such as choice of ligands and predominance of environmentally unrewarding and expensive noble-metals, necessity of high-valency on metal, thermodynamic instability of intermediates, and reversibility of reactions that create challenges in construction of robust and efficient water oxidation catalyst. We highlight how judicious heterogenization of atom-efficient molecular WOCs in supramolecular and hybrid approaches put forth promising avenues to alleviate the existing problems in molecular catalysis, albeit retaining their fascinating intrinsic reactivities. Taken together, our overview is expected to provide guiding principles on opportunities, challenges, and crucial factors for designing novel water oxidation catalysts based on a synergy between conventional and contemporary methodologies that will incite the expansion of the domain of artificial photosynthesis.

Cationic N,S-chelate half-sandwich iridium complexes: synthesis, characterization, anticancer and antiplasmodial activity

A series of pyrazole-based ligands and their corresponding cationic N,S-chelate half-sandwich iridium complexes were successfully synthesized. All iridium complexes exhibited good anticancer activity against the MCF-7 and MDA-MB-231 human breast cancer cells. The cytotoxic activity of unsubstituted iridium complex 1 is greater than that of cisplatin against MCF-7 cells. In addition, the cationic half-sandwich iridium complexes are also efficient in antiplasmodial study and complex 1 displayed the best activity as its IC50 was observed to be approximately 0.11 μM against the CQS-NF54 strain. These iridium complexes generally exhibited enhanced activity against the CQS-NF54 strain in comparison with that against the CQR-K1 strain. An "IC50 speed assay" investigation against the CQS-NF54 strain indicated complexes 1-3 to be fast-acting complexes that reach their lowest IC50 values within 16 hours. All complexes were fully characterized by IR spectroscopy, NMR spectroscopy, and elemental analysis, and the structure of the iridium complex was confirmed by single-crystal X-ray diffraction.

Metal Chelates of Sulfafurazole Azo Dye Derivative: Synthesis, Structure Affirmation, Antimicrobial, Antitumor, DNA Binding, and Molecular Docking Simulation

A series of divalent and one trivalent metal chelates of the azo ligand resulting from coupling of sulfafurazole diazonium chloride with resorcinol have been designed and synthesized. Structure investigation of the isolated chelates have been achieved by applying spectroscopic and analytical tools which collaborated to assure the formation of the metal chelates in the molar ratios of 1L: 1M for Ni(II), Co(II), and Fe(III) chelates, where Cu(II) and Zn(II) complexes formed in the ratio 2L : 1M. The geometrical arrangement around the metal canters was concluded from UV-Vis spectra to be octahedral for all metal chelates. The attachment of the ligand to the metal ions took place through the azo group nitrogen and o-hydroxyl oxygen through proton displacement leading to the ligand being in monobasic bidentate binding mode. Antimicrobial and antitumor activities of the interested compounds have been evaluated against alternative microorganisms and cancer cells, respectively, in a trial to investigate their extent of activity in addition to docking studies. The mode of interaction of the compounds with SS-DNA has been examined by UV-Vis spectra and viscosity studies.

Bio-Inspired Molecular Catalysts for Water Oxidation

The catalytic tetranuclear manganese-calcium-oxo cluster in the photosynthetic reaction center, photosystem II, provides an excellent blueprint for light-driven water oxidation in nature. The water oxidation reaction has attracted intense interest due to its potential as a renewable, clean, and environmentally benign source of energy production. Inspired by the oxygen-evolving complex of photosystem II, a large of number of highly innovative synthetic bio-inspired molecular catalysts are being developed that incorporate relatively cheap and abundant metals such as Mn, Fe, Co, Ni, and Cu, as well as Ru and Ir, in their design. In this review, we briefly discuss the historic milestones that have been achieved in the development of transition metal catalysts and focus on a detailed description of recent progress in the field.

Synthesis of triarylpyridines with sulfonate and sulfonamide moieties via a cooperative vinylogous anomeric-based oxidation

Herein, novel magnetic nanoparticles with pyridinium bridges namely Fe3O4@SiO2@PCLH-TFA through a multi-step pathway were designed and synthesized. The desired catalyst and its corresponding precursors were characterized with different techniques such as Fourier transform infrared (FT-IR) spectroscopy, 1H NMR, 13C NMR, Mass spectroscopy, energy dispersive X-ray (EDX) analysis, thermogravimetric/derivative thermogravimetry (TG/DTG) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM). In addition, the catalytic application of the prepared catalyst in the synthesis of new series of triarylpyridines bearing sulfonate and sulfonamide moieties via a cooperative vinylogous anomeric-based oxidation was highlighted. The current trend revealed that the mentioned catalyst shows high recoverability in the reported synthesis.

The Influence of the Ligand in the Iridium Mediated Electrocatalyic Water Oxidation

Electrochemical water oxidation is the bottleneck of electrolyzers as even the best catalysts, iridium and ruthenium oxides, have to operate at significant overpotentials. Previously, the position of a hydroxyl on a series of hydroxylpicolinate ligands was found to significantly influence the activity of molecular iridium catalysts in sacrificial oxidant driven water oxidation. In this study, these catalysts were tested under electrochemical conditions and benchmarked to several other known molecular iridium catalysts under the exact same conditions. This allowed us to compare these catalysts directly and observe whether structure–activity relationships would prevail under electrochemical conditions. Using both electrochemical quartz crystal microbalance experiments and X-ray photoelectron spectroscopy, we found that all studied iridium complexes form an iridium deposit on the electrode with binding energies ranging from 62.4 to 62.7 eV for the major Ir 4f_7/2 species. These do not match the binding energies found for the parent complexes, which have a broader binding energy range from 61.7 to 62.7 eV and show a clear relationship to the electronegativity induced by the ligands. Moreover, all catalysts performed the electrochemical water oxidation in the same order of magnitude as the maximum currents ranged from 0.2 to 0.6 mA cm^–2 once more without clear structure–activity relationships. In addition, by employing ¹H NMR spectroscopy we found evidence for Cp* breakdown products such as acetate. Electrodeposited iridium oxide from ligand free [Ir(OH)₆]^2– or a colloidal iridium oxide nanoparticles solution produces currents almost 2 orders of magnitude higher with a maximum current of 11 mA cm^–2. Also, this deposited material contains, apart from an Ir 4f_7/2 species at 62.4 eV, an Ir species at 63.6 eV, which is not observed for any deposit formed by the molecular complexes. Thus, the electrodeposited material of the complexes cannot be directly linked to bulk iridium oxide. Small IrO_x clusters containing few Ir atoms with partially incorporated ligand residues are the most likely option for the catalytically active electrodeposit. Our results emphasize that structure–activity relationships obtained with sacrificial oxidants do not necessarily translate to electrochemical conditions. Furthermore, other factors, such as electrodeposition and catalyst degradation, play a major role in the electrochemically driven water oxidation and should thus be considered when optimizing molecular catalysts.

Tuning PtII -Based Donor-Acceptor Systems through Ligand Design: Effects on Frontier Orbitals, Redox Potentials, UV/Vis/NIR Absorptions, Electrochromism, and Photocatalysis

Asymmetric platinum donor-acceptor complexes [(pimp)Pt(Q2- )] are presented in this work, in which pimp=[(2,4,6-trimethylphenylimino)methyl]pyridine and Q2- =catecholate-type donor ligands. The properties of the complexes are evaluated as a function of the donor ligands, and correlations are drawn among electrochemical, optical, and theoretical data. Special focus has been put on the spectroelectrochemical investigation of the complexes featuring sulfonyl-substituted phenylendiamide ligands, which show redox-induced linkage isomerism upon oxidation. Time-dependent density functional theory (TD-DFT) as well as electron flux density analysis have been employed to rationalize the optical spectra of the complexes and their reactivity. Compound 1 ([(pimp)Pt(Q2- )] with Q2- =3,5-di-tert-butylcatecholate) was shown to be an efficient photosensitizer for molecular oxygen and was subsequently employed in photochemical cross-dehydrogenative coupling (CDC) reactions. The results thus display new avenues for donor-acceptor systems, including their role as photocatalysts for organic transformations, and the possibility to introduce redox-induced linkage isomerism in these compounds through the use of sulfonamide substituents on the donor ligands.

An iridium-based nanocomposite prepared from an iridium complex with a hydrocarbon-based ligand

For the first time, a chlorobis(cyclooctene)iridium(i) dimer with only a simple hydrocarbon-based ligand is investigated as a heterogeneous catalyst for the oxygen-evolution reaction in the presence of cerium(iv) ammonium nitrate.

Post Synthetic Defect Engineering of UiO-66 Metal–Organic Framework with An Iridium(III)-HEDTA Complex and Application in Water Oxidation Catalysis

Clean production of renewable fuels is a great challenge of our scientific community. Iridium complexes have demonstrated a superior catalytic activity in the water oxidation (WO) reaction, which is a crucial step in water splitting process. Herein, we have used a defective zirconium metal–organic framework (MOF) with UiO-66 structure as support of a highly active Ir complex based on EDTA with the formula [Ir(HEDTA)Cl]Na. The defects are induced by the partial substitution of terephthalic acid with smaller formate groups. Anchoring of the complex occurs through a post-synthetic exchange of formate anions, coordinated at the zirconium clusters of the MOF, with the free carboxylate group of the [Ir(HEDTA)Cl]− complex. The modified material was tested as a heterogeneous catalyst for the WO reaction by using cerium ammonium nitrate (CAN) as the sacrificial agent. Although turnover frequency (TOF) and turnover number (TON) values are comparable to those of other iridium heterogenized catalysts, the MOF exhibits iridium leaching not limited at the first catalytic run, as usually observed, suggesting a lack of stability of the hybrid system under strong oxidative conditions.

Molecular water oxidation catalysis by zwitterionic carboxylate bridge-functionalized bis-NHC iridium complexes

Carboxylate functionalized bis-NHC ligands allow for the stabilization of high-valent iridium intermediate species involved in homogeneous water oxidation catalysis.

Crystal structure of 2-(4-fluorophenyl)-N-phenyl-2-(phenylamino)ethanesulfonamide – toluene (1/0.5), C23.5H23FN2O2S

C23.5H23FN2O2S, Mr = 416.50, monoclinic, C2/c (no. 15), a = 27.2594(19) Å, b = 5.7351(4) Å, c = 26.1139(18) Å, β = 102.009(2)°, V = 3993.2(5) Å3, Z = 8, Rgt(F) = 0.0358, wRref(F2) = 0.0958 T = 100(2) K.

Iridium-Coordinated Histidyl Bolaamphiphile Self-Assemblies as Heterogeneous Catalysts for Water Oxidation

Catalysts that can promote oxygen evolution from water are necessary for green energy production. In this study, colloidal heterogeneous catalysts for oxygen evolution were prepared by coordination of Ir species to self-assemblies of histidyl bolaamphiphiles. When dissolved in water, the histidyl bolaamphiphiles self-assembled to form particulate structures with the exposure of densely packed histidine imidazoles on their surface. Subsequent coordination of the Ir species to the bolaamphiphile assembly gave rise to catalytic activity toward the oxygen evolution reaction. The oxygen evolution was examined by using the catalytic assemblies in the presence of a sacrificial oxidant, cerium ammonium nitrate. The Ir-coordinated assemblies showed a turnover frequency of 13 min^-1 , which was comparable to those previously reported for molecular water oxidation catalysts. The catalytic activity increased with increasing histidine imidazole/Ir molar ratio, which suggested that multiple coordination of Ir to imidazoles facilitated the formation of active Ir intermediates. This study demonstrates the feasibility of constructing catalytically active interfaces from colloidal bolaamphiphile assemblies with biochemical ligands.

Electrocatalytic oxygen evolution with a cobalt complex

The development of an earth-abundant, first-row water oxidation catalyst that operates at a high TOF and a low overpotential remains a fundamental chemical challenge. Cobalt complexes are important members of water oxidation catalysts. Herein, we report a cobalt-based robust homogeneous water oxidation catalyst, which can electrocatalyze water oxidation at a high pH and a low overpotential (η = 520 mV) in phosphate buffer. This homogeneous system exhibits a high turnover frequency (about 5 s^-1) of catalyzing water oxidation to produce oxygen at η = 720 mV. We speculate the mechanism of the reaction that O-O bond formation prefers a HO-OH coupling in catalytic water oxidation.

Synthesis and structural characterisation of unprecedented primary N-nitrosamines coordinated to iridium(iv)

The one electron oxidation of the N-nitrosamine complexes [Ir^IIICl₅(RN(H)NO)]^2– (R = benzyl or n-butyl) was studied in detail.

Benchmarking Water Oxidation Catalysts Based on Iridium Complexes: Clues and Doubts on the Nature of Active Species

Water oxidation (WO) is a central reaction in the photo- and electro-synthesis of fuels. Iridium complexes have been successfully exploited as water oxidation catalysts (WOCs) with remarkable performances. Herein, we report a systematic study aimed at benchmarking well-known Ir WOCs, when NaIO₄ is used to drive the reaction. In particular, the following complexes were studied: cis-[Ir(ppy)₂ (H₂ O)₂ ]OTf (ppy=2-phenylpyridine) (1), [Cp*Ir(H₂ O)₃ ]NO₃ (Cp*=1,2,3,4,5-pentamethyl-cyclopentadienyl anion) (2), [Cp*Ir(bzpy)Cl] (bzpy=2-benzoylpyridine) (3), [Cp*IrCl₂ (Me₂ -NHC)] (NHC=N-heterocyclic carbene) (4), [Cp*Ir(pyalk)Cl] (pyalk=2-pyridine-isopropanoate) (5), [Cp*Ir(pic)NO₃ ] (pic=2-pyridine-carboxylate) (6), [Cp*Ir{(P(O)(OH)₂ }₃ ]Na (7), and mer-[IrCl₃ (pic)(HOMe)]K (8). Their reactivity was compared with that of IrCl₃ ⋅n H₂ O (9) and [Ir(OH)₆ ]^2- (10). Most measurements were performed in phosphate buffer (0.2 m), in which 2, 4, 5, 6, 7, and 10 showed very high activity (yield close to 100 %, turnover frequency up to 554 min^-1 with 10, the highest ever observed for a WO-driven by NaIO₄ ). The found order of activity is: 10>2≈4>6>5>7>1>9>3>8. Clues concerning the molecular nature of the active species were obtained, whereas its exact nature remains doubtfully.

A Robust Molecular Catalyst Generated In Situ for Photo- and Electrochemical Water Oxidation

Water splitting is the key step towards artificial photosystems for solar energy conversion and storage in the form of chemical bonding. The oxidation of water is the bottle-neck of this process that hampers its practical utility; hence, efficient, robust, and easy to make catalytic systems based on cheap and earth-abundant materials are of exceptional importance. Herein, an in situ generated cobalt catalyst, [Co^II (TCA)₂ (H₂ O)₂ ] (TCA=1-mesityl-1,2,3-1H-triazole-4-carboxylate), that efficiently conducts photochemical water oxidation under near-neutral conditions is presented. The catalyst showed high stability under photolytic conditions for more than 3 h of photoirradiation. During electrochemical water oxidation, the catalytic system assembled a catalyst film, which proved not to be cobalt oxide/hydroxide as normally expected, but instead, and for the first time, generated a molecular cobalt complex that incorporated the organic ligand bound to cobalt ions. The catalyst film exhibited a low overpotential for electrocatalytic water oxidation (360 mV) and high oxygen evolution peak current densities of 9 and 2.7 mA cm^-2 on glassy carbon and indium-doped tin oxide electrodes, respectively, at only 1.49 and 1.39 V (versus a normal hydrogen electrode), respectively, under neutral conditions. This finding, exemplified on the in situ generated cobalt complex, might be applicable to other molecular systems and suggests that the formation of a catalytic film in electrochemical water oxidation experiments is not always an indication of catalyst decomposition and the formation of nanoparticles.

Enhanced durability of an iridium-bipyridine complex embedded into organosilica nanotubes for water oxidation

Robust bipyridine-incorporated organosilica nanotubes were used to fabricate heterogeneous iridium-bipyridine catalysts, which showed enhanced durability in the water oxidation reaction, benefiting from the stable nanotube structures as well as the isolated active sites.

Chemical and photochemical water oxidation catalyzed by novel ruthenium complexes comprising a negatively charged NCNHCO ligand

Pincer-type ruthenium complexes incorporating an asymmetric negatively charged imidazolylidene were prepared and capable to oxidize water chemically and photo-chemically with high stability.

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.