Metal Coordination Complexes Research Articles

Nitrogen, sulfur coordinated metal complex and reduced graphene oxide hybrid materials as counter electrodes for application in dye-sensitized solar cells

Graphene, as one of the carbon-based materials, is considered to be a potential replacement for platinum and has been widely used as a counter electrode in dye-sensitized solar cells to achieve low-cost and high electrocatalytic materials. In this work, we synthesized a nitrogen-sulfur-coordinated iron complex as a source of heteroatoms, and prepared iron-nitrogen-sulfur co-doped reduced graphene oxide (FeNS-rGO) composite material by mixing with different mass ratios of graphene oxide (GO) and thermal annealing under nitrogen at 400 °C. The sintered GO was transformed into rGO, which increased the conductivity of materials, while the doping of nitrogen and sulfur atoms enhanced the carrier mobility and conductivity of the material. The distribution of Fe on graphene also improved the catalytic performance of material. FeNS-rGO (5:1), used as the CE in DSSC, showed superior reduction catalytic activity for I−/I3− compared to platinum and demonstrated an excellent power conversion efficiency of 8.17 %, which outperformed platinum. This proves that the graphene composite material doped with iron, nitrogen, and sulfur has excellent performance and can be used as a substitute for platinum.

Predictive analytics of conductance and HOMO-LUMO gaps with topological descriptors of porphyrin nanosheets

Porphyrins are planar tetrapyrolic aromatic molecules that serve as a host for the formation of metal coordination complexes, which enable additional capabilities. The 2D porphyrin derivative sheets attracted interest due to their versatility and capacity to interact with other chemicals due to the existence of a core metal ion. Topological descriptors are employed as a predictive technique to determine the physical, chemical, and structural characteristics of molecules by considering the molecular structure of compounds as molecular graphs. This paper investigates the degree and degree sum based descriptors of some potential porphyrin derivative nanosheets, using the edge partition method. We also demonstrate a predictive model for analyzing the electrical conductance of porphyrin derivative nanosheets using degree and degree sum based topological descriptors. Furthermore, the Shannon’s information entropies of these porphyrin derivatives are investigated, and the HOMO-LUMO gap of these nanostructures is predicted using these entropy.

A Voltammetric Investigation of the Electrocatalytic Cycle Mechanism of Fe(I) and Mn(I) Complexes

Earth-abundant, first row, low coordinate transition metal complexes are promising candidates for the electrocatalysis of a variety of organic synthesis reactions. However, transition metal complexes in the M(I) state have the propensity to undergo disproportionation, hindering their ability to carry out the electrocatalytic cycle. In this study, we seek to identify and understand the mechanism by which the M(I) complexes may act as an electrocatalyst for a simple organic coupling reaction, beginning with the radicalization of benzyl bromide as a representative starting material. Initial characterization of the complex and its components by cyclic voltammetry demonstrate general stability of the catalyst, while equivalent additions of benzyl bromide exhibit the execution of the electron transfer and radical generation. Cyclic voltammetry simulations provide supplementary guidance into the relationship between benzyl bromide and the catalyst. Additional experiments conducted using analytical techniques such as mass spectrometry, x-ray spectroscopy, and nuclear magnetic resonance help to further characterize portions of the proposed mechanism and supporting details. Analysis to date suggests that the model electrocatalytic cycle is interrupted to an extent, with contributions from both disproportionation and substituted coordination at the transition metal center. Experimental and simulated voltammograms are qualitatively consistent with this proposed catalytic cycle interruption.

Characterization of the Ligand Field in Pseudo‐Octahedral Ni(II) Complexes of Pincer‐Type Amido Ligands: Magnetism, Redox Behavior, Electronic Absorption and High‐Frequency and ‐Field EPR Spectroscopy

AbstractThe description of π‐donor amido moieties as ‘weak‐field’ ligands can belie the influence of metal‐ligand covalency on the overall ligand field of coordination complexes, which can in turn influence properties including the magnetic ground state and those of their excited states. In this contribution, the ligand fields of pseudo‐octahedral Ni(II) complexes supported by diarylamido pincer‐type amido ligands – three previously reported examples supported by asymmetric (2‐R‐phenanthridin‐4‐yl)(8‐quinolinyl)amido ligands (R = Cl, CF3,tBu;RL1) along with a new congener bearing a symmetricbis(8‐quinolinyl)amido ligand (BQA;L2) – were investigated in two ways. First, high‐frequency and ‐field electron paramagnetic resonance spectroscopy (HFEPR), SQUID magnetometry, and electronic absorption spectroscopy were used to determine the ligand field parameters. Second, the ability to electrochemically address ligand‐based oxidations despite metal‐centered SOMOs in the parentS=1 paramagnets was investigated, supported by time‐dependent density functional theory (TDDFT) identification of strong intervalence charge‐transfer (IVCT) transitions attributed to electronic communication between two Namidomoieties mediated by a Ni(II) bridge. These findings are discussed in the broader context of 3d transition metal coordination complexes of weak‐field π‐donor ligands.

Photoredox-active Cr(0) luminophores featuring photophysical properties competitive with Ru(II) and Os(II) complexes

Coordination complexes of precious metals with the d6 valence electron configuration such as Ru(II), Os(II) and Ir(III) are used for lighting applications, solar energy conversion and photocatalysis. Until now, d6 complexes made from abundant first-row transition metals with competitive photophysical and photochemical properties have been elusive. While previous research efforts focused mostly on Fe(II), we disclose that isoelectronic Cr(0) gives access to higher photoluminescence quantum yields and excited-state lifetimes when compared with any other first-row d6 metal complex reported so far. The luminescence behaviour of the metal-to-ligand charge transfer excited states of these Cr(0) complexes is competitive with Os(II) polypyridines. With these Cr(0) complexes, the metal-to-ligand charge transfer states of first-row d6 metal complexes become exploitable in photoredox catalysis, and benchmark chemical reductions proceed efficiently under low-energy red illumination. Here we demonstrate that appropriate molecular design strategies open up new perspectives for photophysics and photochemistry with abundant first-row d6 metals.

Bioactive Thiosemicarbazone Coordination Metal Complexes: Synthesis, Characterization, Theoretical analysis, Biological Activity, Molecular Docking and ADME analysis.

Mn(II) and Cu(II) complexes having the formula [M(L)2 ]X2 of ligand, i. e., 2-acetyl-5-methylfuranthiosemicarbazone were synthesized. Various analytical and spectroscopic techniques described the structure of synthesized complexes. Molar conductance confirmed the electrolytic nature of the complexes. The theoretical study of the complexes explained the structural property and reactivity. The chemical reactivity, interaction and stability of the ligand and metal complexes were studied with the help of global reactivity descriptors. MEP analysis was used to investigate the charge transfer in the ligand. The biological potency was evaluated against two bacteria and two fungi. Complexes demonstrated superior inhibitory action to ligand. The inhibitory effect was also checked at the atomic scale using molecular docking, which confirmed the experimental results. Cu(II) complex was shown to have the most inhibitory effect in experimental and theoretical studies. To check the bioavailability and drug-likeness, ADME analysis was also done.

Synthesis, characterization, spectroscopic study and biological evaluation of Zn(II), Ni(II), and Cu(II)complexes derived from(1-methyl-1H-imidazol-2-yl)methanol

This study focuses on the synthesis, the structural studies of four new divalent metal coordination complexes of (1-methyl-1H-imidazol-2-yl)methanol (Hmim), and the assessment of their antimicrobial activity against five pathogenic microorganisms as well as their antioxidant potential. M(II) complexes were prepared by condensation of Hmim with Zn(OAc)2, Cu(OAc)2, NiCl2·6H2O, or CuCl2 in dry 1,2-dimethoxyethane at rt.. The ligand and its complexes have been characterized systematically by elemental analysis, FT-IR and UV–vis spectroscopy, and single-crystal X-ray diffraction. In [Zn(Hmim)2(OAc)2] (1)four donor atoms surround the metal ion displaying tetra-coordinated compound in distorted tetrahedral geometry, while in [Cu(Hmim)3OAc].AcOH (2), the Cu(II) ion is coordinated by three Hmim ligands and one acetate ligand displaying a tetragonally distorted octahedral environment. In [Ni(Hmim)2(H2O)2]Cl2(3), and [Cu(Hmim)3]Cl2 (4), the ligand is coordinated to M(II) ions in bidentate fashion exhibiting a distorted octahedral environment, the chloride anions act only as counter ions. The free ligand and its complexes 1–4 were tested in vitro for their antimicrobial activity against the growth of five strains including Gram (+) and Gram (-) bacteria, a Gram (+) phytopathogenic bacteria, a yeast and a fungus using the agar diffusion method. The SAR study of the antibacterial activity revealed significant differences in their inhibition activity profile, where 1 gave the best results displaying significant inhibition effects with a broad spectrum. The free-radical scavenging activity of synthesized compounds was investigated through their DPPH radical scavenging capacity, and has shown moderate to interesting antioxidant potential.

Investigation of some new main group metal complexes of hydrazine and 2‐mercaptopyridine‐3‐carboxylic acid mixed–ligands

AbstractSynthesis of some new coordination complexes of main group metals (Ba, Ca, Mg, Sr, and Pb) with 2‐mercaptopyridine‐3‐carboxylic acid (H2Mpc) and hydrazine mixed ligands and their characterization through various chemical and spectral analyses are described in this article. With the help of analytical data obtained from mass, elemental (CHNS), hydrazine content, and metal content analysis, the molecular formulae of the complexes have been proposed as [Ba(N2H4)2(HMpc)H2O], [Ca(N2H4)2(HMpc)(H2O)2], [Sr(N2H4)(HMpc)(H2O)3], [Mg(N2H4)4(HMpc)].4H2O and [Pb(N2H4)5(HMpc)2]. The vibrational (FT‐IR and Raman) spectral data exposes that the 2‐mercaptopyridine‐3‐carboxylic acid coordinates with metal ion via both carboxylate‐O and thiolate‐S in the mode of bi‐dentate dianion while the hydrazine acts as bridging bi‐dentate. On thermal decomposition, these complexes dehydrate to give metal 2‐mercaptopyridine‐3‐carboxylate intermediates, which on further decomposition (heating upto 750°C) give the metal carbonates in the cases of Ba, Ca and Sr, and metal oxides for Mg and Pb. The in vitro antibacterial and antifungal activity of the ligand and its complexes are studied against five bacteria (Bacillus cereus, Staphylococcus aureus, Proteus vulgaris, Pseudomonas aeruginosa and Escherichia coli) and four fungi (Candida albicans, Aspergillus niger, Aspergillus fumigatus, and Penicilium variance).

Metal-centric organic compounds: boon to third-order nonlinear optical applications

Abstract In the recent past the molecular engineering of coordination metal complexes has attracted new interest in the field of nonlinear optics (NLO), which find their applications in optoelectronics and optical data storage technology it is the transition metal along with the organic moieties that induce the control over the optical nonlinearity these properties of the materials not only enhance the intensity but also have a drastic effect on the polarization of incident laser light. This is an important criterion for all-optical switching applications. Coordination metal complexes are a very good target to aim at because of their robustness, physical and chemical stability, and other variable degrees that lead to an increment in NLO responses, most importantly all these properties can be either manipulated or tailored or tunable according to the requirement. Apart from the metal center, these molecules acting as legend must be chromophoric with donor-acceptor nature. In these molecules, the NLO response is intrinsically based on the ‘push-pull’ mechanism of the electrons. Obviously to these molecules, when a metal is in contact, the electronic push-pull mechanism alters rendering the molecule non-symmetric. This review article mainly concentrates on small mononuclear metal complexes for NLO application.

Structure-Activity Relationships of Metal-Based Inhibitors of the Mitochondrial Calcium Uniporter.

The mitochondrial calcium uniporter (MCU) is a transmembrane protein that is responsible for mediating mitochondrial calcium (mCa2+ ) uptake. Given this critical function, the MCU has been implicated as an important target for addressing various human diseases. As such, there has a been growing interest in developing small molecules that can inhibit this protein. To date, metal coordination complexes, particularly multinuclear ruthenium complexes, are the most widely investigated MCU inhibitors due to both their potent inhibitory activities as well as their longstanding use for this application. Recent efforts have expanded the metal-based toolkit for MCU inhibition. This concept paper summarizes the development of new metal-based inhibitors of the MCU and their structure-activity relationships in the context of improving their potential for therapeutic use in managing human diseases related to mCa2+ dysregulation.

Metal Coordination Complexes as Therapeutic Agents for Ischemia-Reperfusion Injury.

Ischemia-reperfusion injury (IRI), which describes the cell damage and death that occurs after blood and oxygen are restored to ischemic or hypoxic tissue, is a significant factor within the mortality rates of heart disease and stroke patients. At the cellular level, the return of oxygen triggers an increase in reactive oxygen species (ROS) and mitochondrial calcium (mCa2+) overload, which both contribute to cell death. Despite the widespread occurrence of IRI in different pathological conditions, there are currently no clinically approved therapeutic agents for its management. In this Perspective, we will briefly discuss the current therapeutic options for IRI and then describe in great detail the potential role and arising applications of metal-containing coordination and organometallic complexes for treating this condition. This Perspective categorizes these metal compounds based on their mechanisms of action, which include their use as delivery agents for gasotransmitters, inhibitors of mCa2+ uptake, and catalysts for the decomposition of ROS. Lastly, the challenges and opportunities for inorganic chemistry approaches to manage IRI are discussed.

Synthesis, spectral characterization, computational studies, antifungal, DNA interaction, antioxidant and fluorescence property of novel Schiff base ligand and its metal chelates

Cobalt, Copper, Nickel and Zinc coordinated metal complexes were synthesized by novel thiazole Schiff base ligand 2-ethoxy-4-((5-methylthiazol-2-ylimino)methyl) phenol derived from 3-ethoxy-4-hydroxybenzaldehyde and 2-amino- 5-methylthiazol. The synthesized compounds were spectrochemically characterized by elemental analysis, molar conductance, FT-IR, UV–Vis, Mass spectral analysis, Powdered-XRD and cyclic voltammetry. The thermal stability of synthesized complexes were investigated by using thermogravimetric analysis (TGA). Theoretical computational study were performed for all the synthesized compounds utilizing the DFT/B3LYP method at the 6-31G basic set for Schiff base ligand whereas LANL2DZ basis set for metal complexes. Molecular Electrostatic Potential (MEP), HOMO-LUMO, Mulliken charges and global reactivity descriptors, including chemical potential (μ), global softness (S), chemical hardness (η), and electrophilicity index (ω) were measured and correlated with antimicrobial activity. The synthesized thiazole Schiff base ligand and its coordinated metal complexes shows good antifungal agreement against Fusarium Oxysporum and Aspergillus Niger species. These compounds also exhibit DNA binding, DNA cleaving and antioxidant activity. All the synthesized molecules indicate potential fluorescence property.

Novel Metal Coordination Complexes Based on 4-Aminophenol: Spectroscopic Analysis and Antibacterial Test

The aims of this research are to synthesize and determine the formula, characteristics, and complex structure of Cu(II) and Co(II) with 4-aminophenol and to investigate their antibacterial activity. The complexes were synthesized by refluxing a solution of CuSO4·5H2O and CoSO4·7H2O, respectively, with 4-aminophenol in methanol for 1 h. The products were characterized using UV-Vis spectroscopy, atomic absorption spectroscopy, thermal analysis, conductivity, FTIR, and magnetic moment. The formation of the complex was indicated by shifting of maximum wavelength of the metal solution toward shorter, i.e., 817 to 421 nm for Cu(II) and 566 to 450 nm for Co(II). From the characterization, the proposed formulas of the complexes are [Cu(4-aminophenol)4]SO4 and [Co(4-aminophenol)4(H2O)2]SO4·5H2O forming square planar and octahedral geometry, respectively. Both complexes are paramagnetic with negligible antibacterial activity against Staphylococcus aureus, Staphylococcus epidermis, Escherichia coli, and Pseudomonas aeruginosa.

Cover Feature: Coupling Dielectric Functionality with Magnetic Properties in Coordination Metal Complexes (ChemPlusChem 6/2023)

The cover feature image illustrates the conceptual design of synergetic or coexistence of magnetic and dielectric functionalities in the coordination metal complexes. The coordination metal complexes can exhibit flexibility in their electronic/magnetic state based on the combination of metal ions and the design of organic coordination ligands. This cover image shows that coordination metal complexes have tunable structures and designs, with coexistence or synergy between the ferroelectric behavior and magnetic properties. More information can be found in the Review by Y. Sekine, S. Hayami, and co-workers.

Coupling Dielectric Functionality with Magnetic Properties in Coordination Metal Complexes.

Significant research has been conducted on molecular ferroelectric materials, including pure organic and inorganic compounds; however, studies on ferroelectric materials based on coordination metal complexes are scarce. Ferroelectric materials based on coordination metal complexes have tunable structures and designs, with coexistence or synergy between the ferroelectric behavior and magnetic properties. Compared to inorganic compounds, few coordination metal complexes exhibit coupling between the magnetic and dielectric properties. Coordination metal complexes with strong coupling between the magnetic and dielectric properties exhibit dielectric permittivity variations under external magnetic fields. Therefore, they have attracted substantial interest for their potential use in magnetoelectric devices. In this review, we discuss recent advances in coordination metal complexes, that exhibit coupled magnetic functionalities and ferroelectricity or dielectric properties, including single-molecule magnets, electron delocalization systems, and external stimuli responsive compounds.

Crystal Design for Tuning the Mechanical Flexibilities of M(salophen) Complexes

Elastic crystals are a novel class of materials for use in optoelectronics and solar cells. Although controlling mechanical flexibility is critical when developing functional soft crystals, the number of reported metal-coordination-complex-based elastic crystals is limited. When developing such crystals, the flexibility of the crystals may be tuned via ligand substitution of the central metals or by exchange of the central metal ions; however, this has not been demonstrated experimentally to date. Thus, we herein report the preparation of mechanically flexible compounds using a synthetic procedure based on metal coordination complexes. It was found that the packing structure was considerably affected by the crystallization solvent, and control over the packing process was critical in obtaining complex molecules with designed flexibilities and structures. Furthermore, we demonstrate new Pd and Pt complexes with the salophen ligand, which exhibit the second and third highest elastic strains (ε values) reported to date, respectively, and the highest and second highest ε values (ε = 11.95% for Pd; 10.65% for Pt), respectively, among metal coordination complexes. These effects can be attributed to the microscopic changes in bond distance between the metals and ligands. Our findings are expected to inspire the tuning and development of metal-complex-based flexible crystals.

Conductivity in Thin Films of Transition Metal Coordination Complexes.

Two coordination complexes have been made by combining the dithiolene complexes [M(mnt)2]2- (mnt = maleonitriledithiolate; M = Ni2+ or Cu2+) as anion, with the copper(II) coordination complex [Cu(Stetra)] (Stetra = 6,6'-bis(4,5-dihydrothiazol-2-yl)-2,2'-bipyri-dine) as cation. The variation of the metal centers leads to a dramatic change in the conductivity of the materials, with the M = Cu2+ variant (Cu-Cu) displaying semiconductor behavior with a conductivity of approximately 2.5 × 10-8 S cm-1, while the M = Ni2+ variant (Ni-Cu) displayed no observable conductivity. Computational studies found Cu-Cu enables a minimization of reorganization energy losses and, as a result, a lower barrier to the charge transfer process, resulting in the reported higher conductivity.

Electrochemiluminescence Amplification in Bead-Based Assays Induced by a Freely Diffusing Iridium(III) Complex.

Heterogeneous electrochemiluminescence (ECL) assays employing tri-n-propylamine as a co-reactant and a tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy)3]2+) derivative as an emissive label are integral to the majority of academic and commercial applications of ECL sensing. This model system is an active research area and constitutes the basis of successfully commercialized bead-based ECL immunoassays. Herein, we propose a novel approach to the enhancement of such conventional ECL assays via the incorporation of a second metal coordination complex, [Ir(sppy)3]3- (where sppy = 5'-sulfo-2-phenylpyridinato-C2,N), to the experimental system. By employing ECL microscopy, we are able to map the spatial distribution of ECL emission at the surface of the bead, from [Ru(bpy)3]2+ labels, and solution-phase emission, from [Ir(sppy)3]3-. The developed [Ir(sppy)3]3--mediated enhancement approach elicited a significant improvement (70.9-fold at 0.9 V and 2.9-fold at 1.2 V vs Ag/AgCl) of the ECL signal from [Ru(bpy)3]2+ labels immobilized on the surface of a polystyrene bead. This dramatic enhancement in ECL signal, particularly at low oxidation potentials, has important implications for the improvement of existing heterogeneous ECL assays and ECL-based microscopy, by amplifying the signal, opening new bioanalytical detection schemes, and reducing both electrode surface passivation and deleterious side reactions.

One-Step Soaking Strategy toward Anti-Swelling Hydrogels with a Stiff "Armor".

Double-network (DN) hydrogels consisting of noncovalent interacting networks are highly desired due to their well-controlled compositions and environmental friendliness, but the low water resistance always impairs their mechanical strength. Here, an anti-swelling hydrogel possessing the core/shell architecture through rational regulation of multiple weak noncovalent interactions is prepared. A composite hydrogel consists of chitosan (CS) and poly(N-acryloyl 2-glycine) (PACG), readily forming the shell-structured DN hydrogel after soaking in a FeCl3 solution because of in situ formation of chain entanglements, hydrogen bonds, and ionic coordination. The produced DN hydrogels exhibit excellent anti-swelling behaviors and mechanical durability for over half a year, even in some strict situations. Taking the merits of noncovalent bonds in adjustability and reversibility, the swelling property of these hydrogels can be easily customized through control of the ion species and concentrations. A dynamically reversible transition from super-swelling to anti-swelling is realized by breaking up and rebuilding the metal-coordination complexes. This facile but efficient strategy of turning the noncovalent interactions and consequently the mechanics and anti-swelling properties is imperative to achieve the rational design of high-performance hydrogels with specific usage requirements and expand their applicability to a higher stage.

Front Cover: Metal Doping and Ligand Engineering as Tools for Tailoring the Electronic Structure of Coordination Polymers and their Oxygen Evolution Electrocatalytic Activity (Eur. J. Inorg. Chem. 5/2023)

The Front Cover shows the process of water oxidation to molecular oxygen, which is fundamental for providing electrons and protons for the generation of renewable fuels, electrocatalyzed by coordination polymers (CPs), under basic conditions. The two key components of CPs, namely the metal coordination complexes (orange octahedrons) and the organic linkers, whose suitable selection and engineering are crucial for rationally optimizing their catalytic performance, are evidenced. A thorough understanding of the structure-activity relationships and reaction mechanism underpins the development of electrocatalysts with ever-improving performance for possible large-scale applications. We would like to thank Leonardo Tensi for designing and creating the cover graphic that fully captures the essence of the Review. More information can be found in the Review by G. Menendez Rodriguez and A. Macchioni.