Transition Metal Centers Research Articles

Interplay of electron correlations, spin-orbit couplings, and structural effects for Cu centers in the quasi-two-dimensional magnet InCu2/3V1/3O3

Less common ligand coordination of transition-metal centers is often associated with peculiar valence-shell electron configurations and outstanding physical properties. One example is the Fe$^+$ ion with linear coordination, actively investigated in the research area of single-molecule magnetism. Here we address the nature of 3$d^9$ states for Cu$^{2+}$ ions sitting in the center of trigonal bipyramidal ligand cages in the quasi-two-dimensional honeycomb compound InCu$_{2/3}$V$_{1/3}$O$_3$, whose unusual magnetic properties were intensively studied in the recent past. In particular, we discuss the interplay of structural effects, electron correlations, and spin-orbit couplings in this material. A relevant computational finding is a different sequence of the Cu ($xz$, $yz$) and ($xy$, $x^2\!-\!y^2$) levels as compared to existing electronic-structure models, which has implications for the interpretation of various excitation spectra. Spin-orbit interactions, both first- and second-order, turn out to be stronger than previously assumed, suggesting that rather rich single-ion magnetic properties can be in principle achieved also for the 3$d^9$ configuration by properly adjusting the sequence of crystal-field states for such less usual ligand coordination.

Excited-state behavior and photoinduced electron transfer of pH-sensitive Ir(III) complexes with cyclometallation (C/N–) ratios between 0/6 and 3/3

The first coordination sphere of Ir(III) 2,2′-bipyridine / 2-phenylpyridine complexes can be tuned to achieve either C– or N–chelation in ratios that range between 0/6 and 3/3. Of particular interest is the synthesis of Ir(III) complexes bearing a 2,2′-bipyridine ligand coordinated in a N,C3 pattern, leaving an exposed pyridine moiety, accessible for acid-base chemistry or coordination to a second transition metal center. The protonated forms of these “rolled-over” Ir(III) complexes were isolated in a straight-forward procedure using trifluoroacetic acid. The photophysical, photochemical and electrochemical properties of both the protonated and unprotonated Ir(III) complexes were investigated by steady-state and time-resolved spectroscopies, as well as by density functional theory calculations. The nature of the excited states was shown to depend on both the ligand coordination pattern and protonation state of the complex. In addition, the unprotonated and protonated analogues were efficiently quenched by hydroquinone and benzoquinone in acetonitrile with quenching rate constants close to the solvent diffusion limit. The results presented herein have direct implications for proton sensitive photoredox chemistry and the development of photo-acids and photo-bases.

Development of Triggerable, Trackable, and Targetable Carbon Monoxide Releasing Molecules.

Carbon monoxide (CO) is a gaseous signaling molecule produced in humans via the breakdown of heme in an O2-dependent reaction catalyzed by heme oxygenase enzymes. A long-lived species relative to other signaling molecules (e.g., NO, H2S), CO exerts its physiological effects via binding to low-valent transition metal centers in proteins and enzymes. Studies involving the administration of low doses of CO have shown its potential as a therapeutic agent to produce vasodilation, anti-inflammatory, antiapoptotic, and anticancer effects. In pursuit of developing tools to define better the role and therapeutic potential of CO, carbon monoxide releasing molecules (CORMs) were developed. To date, the vast majority of reported CORMs have been metal carbonyl complexes, with the most well-known being Ru2Cl4(CO)6 (CORM-2), Ru(CO)3Cl(glycinate) (CORM-3), and Mn(CO)4(S2CNMe(CH2CO2H)) (CORM-401). These complexes have been used to probe the effects of CO in hundreds of cell- and animal-based experiments. However, through recent investigations, it has become evident that these reagents exhibit complicated reactivity in biological environments. The interpretation of the effects produced by some of these complexes is obscured by protein binding, such that their formulation is not clear, and by CO leakage and potential redox activity. An additional weakness with regard to CORM-2 and CORM-3 is that these compounds cannot be tracked via fluorescence. Therefore, it is unclear where or when CO release occurs, which confounds the interpretation of experiments using these molecules. To address these weaknesses, our research team has pioneered the development of metal-free CORMs based on structurally tunable extended flavonol or quinolone scaffolds. In addition to being highly controlled, with CO release only occurring upon triggering with visible light (photoCORMs), these CO donors are trackable via fluorescence prior to CO release in cellular environments and can be targeted to specific cellular locations.In the Account, we highlight the development and application of a series of structurally related flavonol photoCORMs that (1) sense characteristics of cellular environments prior to CO release; (2) enable evaluation of the influence of cytosolic versus mitochondrial-localized CO release on cellular bioenergetics; (3) probe the cytotoxicity and anti-inflammatory effects of intracellular versus extracellular CO delivery; and (4) demonstrate that albumin delivery of a photoCORM enables potent anticancer and anti-inflammatory effects. A key advantage of using triggered CO release compounds in these investigations is the ability to examine the effects of the molecular delivery vehicle in the absence and presence of localized CO release, thus providing insight into the independent contributions of CO. Overall, flavonol-based CO delivery molecules offer opportunities for triggerable, trackable, and targetable CO delivery that are unprecedented in terms of previously reported CORMs and, thus, offer significant potential for applications in biological systems.

Tunneling matrix element and tunneling pathways of protein electron transfer calculated with a fragment molecular orbital method

Practical ways to calculate the tunneling matrix elements and analyze the tunneling pathways for protein electron-transfer (ET) reactions with a fragment molecular orbital (FMO) method are presented. The straightforward use of minimal basis sets only for the atoms involved in the covalent bond detachment in FMO can properly describe the ETs through the protein main-chains with the cost-effective two-body corrections (FMO2) without losing the quality of double-zeta basis sets. The current FMO codes have been interfaced with density functional theory, polarizable continuum model, and model core potentials, with which the FMO-based protein ET calculations can consider the effects of electron correlation, solvation, and transition-metal redox centers. The reasonable performance of the FMO-based ET calculations is demonstrated for three different sets of protein-ET model molecules: (1) hole transfer between two tryptophans covalently bridged by a polyalanine linker in the ideal α-helix and β-strand conformations, (2) ET between two plastoquinones covalently bridged by a polyalanine linker in the ideal α-helix and β-strand conformations, and (3) hole transfer between ruthenium (Ru) and copper (Cu) complexes covalently bridged by a stretch of a polyglycine linker as a model for Ru-modified derivatives of azurin.

Valence state of transition metal center as an activity descriptor for CO2 reduction on single atom catalysts

Valence state of transition metal center as an activity descriptor for CO2 reduction on single atom catalysts

Synthesis, Crystal Structures, and Thermolysis Studies of Heteronuclear Transition Metal Aluminum Alcoholates

Heteronuclear alcoholate complexes [M{Al(OiPr)4}2(bipy)] (2‐M, M = Fe, Co, Ni, Cu, Zn) and [M{Al(OcHex)4}2(bipy)] (3‐M, M = Fe, Co, Ni, Zn) are formed by adduct formation of [M{Al(OiPr)4}2] (1‐M, M = Fe, Co, Ni, Cu, Zn) with 2,2'‐bipyridine and transesterification reaction with cHexOAc. According to crystal structure analyses, in 2‐M and 3‐M the central transition metal ion M2+ is coordinated by two chelating Al(OR)4– moieties and one bipyridine ligand in an octahedral arrangement. Treating 1‐Cu with 2,2'‐bipyridine leads to a reduction process, whereat the intermediate [Cu{Al(OiPr)4}(bipy)2][Al(OiPr)4] (4) could be structurally characterized. During conversion of the iso‐propanolate ligands in 1‐Cu to cyclohexanolate ligands, Cu2+ is reduced to Cu+ forming [Cu{Al(OcHex)4}(py)2] (5). UV/Vis‐spectra and results of thermolysis studies by TG/DTA‐MS are reported.

MrpH, a new class of metal-binding adhesin, requires zinc to mediate biofilm formation.

Proteus mirabilis, a Gram-negative uropathogen, is a major causative agent in catheter-associated urinary tract infections (CAUTI). Mannose-resistant Proteus-like fimbriae (MR/P) are crucially important for P. mirabilis infectivity and are required for biofilm formation and auto-aggregation, as well as for bladder and kidney colonization. Here, the X-ray crystal structure of the MR/P tip adhesin, MrpH, is reported. The structure has a fold not previously described and contains a transition metal center with Zn2+ coordinated by three conserved histidine residues and a ligand. Using biofilm assays, chelation, metal complementation, and site-directed mutagenesis of the three histidines, we show that an intact metal binding site occupied by zinc is essential for MR/P fimbria-mediated biofilm formation, and furthermore, that P. mirabilis biofilm formation is reversible in a zinc-dependent manner. Zinc is also required for MR/P-dependent agglutination of erythrocytes, and mutation of the metal binding site renders P. mirabilis unfit in a mouse model of UTI. The studies presented here provide important clues as to the mechanism of MR/P-mediated biofilm formation and serve as a starting point for identifying the physiological MR/P fimbrial receptor.

Single transition metal atoms on nitrogen-doped carbon for CO2 electrocatalytic reduction: CO production or further CO reduction?

Electrochemical CO2 reduction reaction (CO2RR) is an effectivestrategy for CO2 conversion and clean fuel generation. Diverse nitrogen-doped carbon-supported transition metal (M-N-C) have been proved as favorable catalysis materials for CO2RR, especially for CO production. Herein, density functional theory is performed to investigate the CO2RR mechanisms over M-Nx-C with twelve types of transition metal centers. Fe-, Co-, Ni-, Cu-, Rh-, Pd-, Ir-, and Pt-Nx embedded on graphene with double vacancies are relatively stable structures contributing to electrocatalysis operation. The adsorption energies of COOH and CO, which are the reaction intermediates during CO2RR, are linearly related and can be used as a descriptor for CO production. Moderate adsorption strengths of CO and COOH are favorablefor CO generation with low limiting potential. Co-N4, Ni-N1, Pd-N1, Pt-N1, and Rh-N4 are the most active sites for CO2RR by thermodynamics analysis. However, the discussion on competition between CO2RR and hydrogen evolution reaction (HER) indicates that these sites show low selectivity. Considering both CO2RR selectivity and activity, Ni-N4 site is proved to be the most effective site for CO production among all the M-Nx sites. It is also noted that Fe-N4 is more favorable to further reduction towards *CO with the products of CH3OH and CH4.

Highly sensitive and selective sensing of acetone and hydrogen sulfide using metal phthalocyanine – carbon nanotube hybrids

Noncovalent functionalization of carbon nanotubes with metal phthalocyanine has great potential for sensor nanodevices, nevertheless, some aspects of their electron transport mechanism behind the sensing properties have not been understood yet. Here, we evaluate the sensing performance of metal phthalocyanine – carbon nanotube hybrids for acetone and hydrogen sulfide (H2S) detection in terms of sensitivity and selectivity, using density functional theory and the non-equilibrium Green’s function method. We systematically investigated the effects of central transition metal atoms (Fe, Co, and Mn) of metal phthalocyanine with different chiralities (quasi-metallic CNT (9,0) zigzag and metallic CNT(5,5) armchair) of single-walled carbon nanotubes and tuning the gate voltages making this nanosensor highly sensitive and selective to acetone and H2S.

Highly surface electron-deficient Co9S8 nanoarrays for enhanced oxygen evolution

Tailoring valence electron delocalization of transition metal center is of importance to achieve highly-active electrocatalysts for oxygen evolution reaction (OER). Herein, we demonstrate a “poor sulfur” route to synthesize surface electron-deficient Co9S8 nanoarrays, where the binding energy (BE) of Co metal center is considerably higher than all reported Co9S8-based electrocatalysts. The resulting Co9S8 electrocatalysts only require the overpotentials (η) of 265 and 326 mV at 10 and 100 mA cm−2 with a low Tafel slope of 56 mV dec−1 and a 60 h-lasting stability in alkaline media. The OER kinetics are greatly expedited with a low reaction activation energy of 27.9 kJ mol−1 as well as abundant OOH∗ key intermediates (24%), thus exhibiting excellent catalytic performances. The surface electron-deficient engineering gives an available strategy to improve the catalytic activity of other advanced non-noble electrocatalysts.

Advanced metal–organic frameworks for aqueous sodium-ion rechargeable batteries

In this comprehensive review, we summarize the recent progress on the metal–organic frameworks (MOFs) as electrode materials for aqueous sodium-ion rechargeable batteries (ASIBs) and discuss future challenges for the MOFs based on current technologies. • Aqueous sodium-ion rechargeable batteries (ASIBs) are cheap, safe, and low toxicity. • Metal-organic frameworks (MOFs) have been studied as electrode materials for ASIBs. • The MOFs were reviewed in terms of type and combination of central transition metals. • The applications of MOFs to aqueous sodium-metal hybrid ion batteries were reviewed. • The comprehensive review gives insight into the future development of MOFs for ASIBs. Inexpensive and abundant sodium resources make energy storage systems using sodium chemistry promising replacements for typical lithium-ion rechargeable batteries (LIBs). Fortuitously, aqueous sodium-ion rechargeable batteries (ASIBs), which operate in aqueous electrolytes, are cheaper, safer, and more ionically conductive than batteries that operate in conventional organic electrolytes; furthermore, they are suitable for grid-scale energy storage applications. As electrode materials for storing Na + ions in ASIBs, a variety of multifunctional metal–organic frameworks (MOFs) have demonstrated great potential in terms of having porous 3D crystal structures, compatibility with aqueous solutions, long cycle lives (≥1000 cycles), and ease of synthesis. The present review describes MOF-derived technologies for the successful application of MOFs to ASIBs and suggests future challenges in this area of research based on the current understanding.

A Search by NBO Analysis for Syndetic Donation in Known X‐ray Structures of Transition Metal Complexes with Close Aromatic Ring C–H···Metal Approaches

Natural bond orbital (NBO) analysis obtained from density functional theory (DFT) calculations shows that where close approaches of aromatic ring C–H bond hydrogens to transition metal centers indicate an agostic interaction might be present, the angle the ring makes with the coordination plane and the metal···Cagostic separation can determine how significant are the agostic, syndetic (π or σ‐electron density in the vicinity of the agostic carbon interacting with a metal center) and back‐donations that are present. The study reveals that in the few X‐ray crystal structures that are known where agostic bonding is present, the interaction is weak, but the presence of syndetic donation is confirmed in the solid state but is small in magnitude. Agostic interactions present in a Co‐aminophosphane, and Ir and Ru 2‐phenylpyridine complexes show syndetic π‐donation components with NBO E(2) values of only 3.2, 2.1 and 2.1 kcal mol–1 respectively whereas a Rh tris‐triphenylphosphine cation shows minimal agostic donation and η2‐π‐donation and a Pd complex with two dicyclohexylbiphenylphosphine ligands has minimal agostic or syndetic components and appears to contain unusual aromatic ring rotations between the anagostic and agostic states. The work reinforces the multicomponent nature of agostic interactions involving aromatic rings.

A nature inspired molecular Ni-catalyst for efficient photocatalytic CO2 reduction to CO under visible light

Development of efficient molecular catalysts for photocatalytic CO2 reduction is desirable but challenging. At present, the majority of reported molecular catalysts consist of the transition metal centers coordinated by N-containing ligands. Inspired by the sulfur-containing active site found in carbon monoxide dehydrogenases, we here report a novel Ni-based molecular catalyst bearing 2,6-bis([(2-pyridinylmethyl)thio]methyl) pyridine) as the pentadentate ligand. The product is assessed for photocatalytic CO2 reduction in acetonitrile/water solution using Ru(bpy)3Cl2 as the photosensitizer and triethanolamine as the electron donor. Under the optimal condition, a total of ca. 25 μmol CO is produced after 7 h visible light irradiation with a large turnover number of 63 and high selectivity of 91%.

Response of Hirshfeld Surface to Structural Modifications in Transition-Metal Coordination Compounds

This work aims at identifying salient parameters of the Hirshfeld surface of transition-metal centers in coordination compounds able to elucidate the nature of metal–ligand interactions. As an alte...

Transition Metal Clusters Constrained by Linear Tetradentate Phosphine Ligands

Rationally designed topological arrangement of multinuclear transition metal centers is of fundamental importance to create diverse functions applicable to catalysts, electronic, photochemical, and...

Subcomponent Self-Assembly of Covalent Metallacycles Templated by Catalytically Active Seven-Coordinate Transition Metal Centers

Coordination geometries of transition metals play vital roles in the self-assembly process of supramolecular coordination complexes. Herein, seven-coordinate 3d metal ions were applied as templates and catalytically active sites for subcomponent self-assembly that resulted in a new category of covalent metallacycles. Single-crystal structures showed that the sizes, configurations, and functionalization of covalent metallacycles could be tuned by the selection of rigid dihydrazide, transition metal ions, and prefunctionalized subcomponents, respectively. Moreover, metallacycles decorated with carboxylic groups could be employed as precursors to prepare aerogels through hierarchical self-assembly, which also exhibited high catalytic activity for cycloaddition of CO2 into cyclic carbonates.

Generation of two-dimensional spectrochemical series in tetrahedral complexes

The physical properties of metal complexes that exist in many places are due to the ligand field splitting and electronic configuration of the d-orbital of the central transition metal, so changing the ligand field splitting of the d-orbital allows to control the physical properties. The arrangement of metals and ligands in order of the ligand field splitting is called a spectrochemical series. There is already a spectrochemical series only in the case of octahedral hexa-coordination complexes, but no equivalent index is shown for any other symmetry such as tetrahedral tetra-coordination complexes. In this study, we have investigated the creation of spectrochemical series in tetrahedral tetra-coordinated metal complexes. As a result, it is possible to accurately grasp the ligand field splitting of a tetrahedral tetra-coordinated metal complex, and to more effectively apply energy for expressing a spin crossover phenomenon in which the magnetism reversibly changes.

Multielectron, Cation and Anion Redox in Lithium-Rich Iron Sulfide Cathodes.

Conventional Li-ion cathodes store charge by reversible intercalation of Li coupled to metal cation redox. There has been increasing interest in new materials capable of accommodating more than one Li per transition-metal center, thereby yielding higher charge storage capacities. We demonstrate here that the lithium-rich layered iron sulfide Li2FeS2 as well as a new structural analogue, LiNaFeS2, reversibly store ≥1.5 electrons per formula unit and support extended cycling. Ex situ and operando structural and spectroscopic data indicate that delithiation results in reversible oxidation of Fe2+ concurrent with an increase in the covalency of the Fe-S interactions, followed by reversible anion redox: 2 S2-/(S2)2-. S K-edge spectroscopy unequivocally proves the contribution of the anions to the redox processes. The structural response to the oxidation processes is found to be different in Li2FeS2 in contrast to that in LiNaFeS2, which we suggest is the cause for capacity fade in the early cycles of LiNaFeS2. The materials presented here have the added benefit of avoiding resource-sensitive transition metals such as Co and Ni. In contrast to Li-rich oxide materials that have been the subject of so much recent study and that suffer capacity fade and electrolyte degradation issues, the materials presented here operate within the stable potential window of the electrolyte, permitting a clearer understanding of the underlying processes.

Computational Screening of Transition Metal–Phthalocyanines for the Electrochemical Reduction of Carbon Dioxide

Molecular complexes containing low-cost transition-metal (TM) centers have been extensively studied for the electrochemical reduction of carbon dioxide. Of all the molecular catalysts reported so f...

Synthesis and Electronic Characterization of Iridium‐Aluminum and Rhodium‐Aluminum Heterobimetallic Complexes Bridged by 3‐Oxypyridine and 4‐Oxypyridine

We demonstrate the synthesis and characterization of regioisomeric late transition metal‐aluminum heterobimetallic complexes containing 3‐ and 4‐oxypyridine ligands which bridge between the aluminum and transition metal center (Rh or Ir). Combined experimental and theoretical data allow for direct comparison of the electronic effect of the two different aluminum‐containing ligands on the late transition metal center. In combination with electronic data regarding previously reported 2‐oxypyridine bridged systems, we report a measurable decrease in ligand donor power as the regiosubstitution of the oxypyridine bridge is varied from 2‐ to 4‐ to 3‐. In all cases, addition of aluminum is found have a minimal effect on the donor ability of the oxypyridine bridge in dichloromethane solution.