Transition Metal Coordination Complexes Research Articles

Mn-L-based multifunctional molecular spintronic device: A first-principles investigation

A transition metal coordination complex Mn-L-based multifunctional molecular spintronic device is proposed and its spin-polarized transport properties in two magnetic configurations (parallel and anti-parallel alignments) are evaluated using the first-principles density functional theory and non-equilibrium Green’s function method. Four high-performance physical effects, including spin-filtering, spin-rectifying, giant magnetoresistance and negative differential resistance, are revealed in the proposed device. The physical mechanisms are analyzed through the spin-resolved bias-dependent transmission spectra, and projected device density of states spectra. These results demonstrate that the proposed system holds great promise in developing the multifunctional molecular spintronic devices.

Ag-based coordination polymer-enhanced photocatalytic degradation of ciprofloxacin and nitrophenol.

Transition-metal coordination complexes have attracted wide attention in molecular chemistry, but their applications still confront a tremendous challenge. Herein, a novel silver coordination polymer with a formula of {[Ag9(TIPA)6](NO3)9·12H2O}n (Ag-TIPA) was prepared by a solvothermal reaction of silver nitrate with triangular tris(4-imidazolylphenyl)amine (TIPA). The crystalline molecular structure was determined by single-crystal X-ray diffraction, which showed that each Ag(I) was coordinated with two nitrogen atoms of TIPA ligands. Such Ag-TIPA was used as a catalyst for the photodegradation of ciprofloxacin and 4-nitrophenol under UV-visible light irradiation. The results exhibited excellent photocatalytic performance and reusability due to high structure stability in an acidic, neutral and alkaline environment. The experimental findings and density functional theory calculations revealed that metal-ligand charge transfer in Ag-TIPA extended the absorption range of light and improved the charge transfer properties of TIPA. To further understand the photodegradation process, the intermediates were predicted and analysed through electrostatic potential, orbital weighted dual descriptor, and liquid chromatography-mass spectrometry techniques. Based on these findings, a possible degradation mechanism was proposed. This study provides new insights into the design and synthesis of Ag-based coordination polymers with novel structures, excellent catalytic activity, and good durability.

Changing Directions: Influence of Ligand Electronics on the Directionality and Kinetics of Photoinduced Charge Transfer in Cu(I)Diimine Complexes.

A key challenge to the effective utilization of solar energy is to promote efficient photoinduced charge transfer, specifically avoiding unproductive, circuitous electron-transfer pathways and optimizing the kinetics of charge separation and recombination. We hypothesize that one way to address this challenge is to develop a fundamental understanding of how to initiate and control directional photoinduced charge transfer, particularly for earth-abundant first-row transition-metal coordination complexes, which typically suffer from relatively short excited-state lifetimes. Here, we report a series of functionalized heteroleptic copper(I)bis(phenanthroline) complexes, which have allowed us to investigate the directionality of intramolecular photoinduced metal-to-ligand charge transfer (MLCT) as a function of the substituent Hammett parameter. Ultrafast transient absorption suggests a complicated interplay of MLCT localization and solvent interaction with the Cu(II) center of the MLCT state. This work provides a set of design principles for directional charge transfer in earth-abundant complexes and can be used to efficiently design pathways for connecting the molecular modules to catalysts or electrodes and integration into systems for light-driven catalysis.

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.

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.

Redox-active ligands - a viable route to reactive main group metal compounds.

Anionic redox-active ligands such as o-amidophenolates, catecholates, dithiolenes, 1,2-benzendithiolates, 2-amidobenzenethiolates, reduced α-diimines, ferrocenyl and porphyrinates are capable of reversible oxidation and thus have the ability to act as sources of electrons for metal centres. These and other non-innocent ligands have been employed in coordination complexes of base transition metals to influence their redox chemistry and afford compounds with useful catalytic, optical, magnetic and conducting properties. Despite the focus in contemporary main group chemistry on designing reactive compounds with potential catalytic activity, comparatively few studies exploring the chemistry of main group metal complexes incorporating redox-active ligands have been reported. This article highlights relevant chemical reactivity and electrochemical studies that probe the oxidation/reduction of main group metal compounds possessing redox-active ligands and comments on the prospects for this relatively untapped avenue of research.

Polypyridine-based architectures for smart electrochromic and energy storage materials

The emerging role of metal–organic coordination compounds in the development of electrochromic materials (ECMs) fueled a growing interest in both the development of the new systems and their effective integration into functional devices. Coordination complexes of late transition metals with polypyridine ligands, like 2′-bipyridine (bpy) and 2,2′:6′,2″-terpyridine (tpy) often feature bright colours as a result of strong metal-to-ligand charge transfer. These colours could be reversibly bleached by oxidation of the metal centre and restored upon the metal centre reduction. The colour and its intensity can be controlled in different ways: by changing the nature of the metal centre(s) and by structural modification of ligand(s); by the construction of various metal–organic motifs and embedding them into devices, as well as by controlled layer-by-layer growth of the coordination assemblies on the surface of interest. The versatility of the available molecular building blocks allows for the systematic programming of the desired properties of the metal–organic units that could be effectively translated into the devices. This makes polypyridine metal complexes advantageous candidates for the design of effective ECMs. Here, we review the evolution of ECMs and devices based on molecularly defined bpy and tpy coordination adducts of late transition metals integrated into functional electrochromic devices over the past decade.

Interfacial and rheological properties of long-lived foams stabilized by rice proteins complexed to transition metal ions in the presence of alkyl polyglycoside

HypothesisTransition-metal coordination complexes are hopeful to make advanced structural materials, since the metal-coordination bonds, unlike typical covalent bonds, can regenerate after rupture, allowing for dynamic, tunable, and reversible mechanical characteristics. Integration of metal-coordinate crosslinking in foam material has rarely been reported. ExperimentsWe developed the hydrolyzed rice proteins (HRP) as the building block for amphiphilic transition-metal coordination complexes that could be used to make long-lived foams with high yield stress. Surface properties of the foaming solution were determined using equilibrium and dynamic tensiometers. Structural information of aggregates in the foaming solution was detected by small-angle X-ray scattering (SAXS) and Cryo-Tem. Visualization of liquid flow in the interfacial liquid film was studied by the reflective optical interference technique. Rheological response of liquid foam was characterized by a rheometer with amplitude-sweep and frequency-sweep modes. FindingsIn the presence of transition metal ions, HRP formed a mechanically strong rigid film. In the absence of transition metal ions or the addition of alkyl polyglycoside (APG), HRP was desorbed to produce a mobile film with a detergent state. The two interfacial states could be actively switched based on facile changes in bulk solution composition (metal ions or alkyl glycoside or chelating agent), and the switching between the two states led to the formation of extremely stable foam with high yield stress or the collapse of foam with a significant decrease in yield limit. The transition-metal coordination complexes adsorbed on the surface of the liquid film could increase the elastic modulus of liquid foam by more than an order of magnitude without increasing the viscosity of the foaming solution. We further revealed the origin of foam stability/instability and used other well-characterized proteins to prepare transition-metal coordination complexes to make long-lived foams. The cases described in this work illustrate the universal nature of the strategy, which in principle can be extended to many types of protein.

Synthesis, Structure, and Bonding of a XeIV Transition-Metal Coordination Complex, F3 XeFb - - -WOF4.

The coordination complex, F3 XeFb - - -WOF4 , was synthesized in CFCl3 solvent by reaction of the weak fluoride-ion donor and strong oxidative fluorinating agent, XeF4 , with the moderate-strength fluoride-ion acceptor, WOF4 . The compound is the only transition-metal coordination complex of XeIV and was characterized at low temperatures by single-crystal X-ray diffraction and Raman spectroscopy. Xenon tetrafluoride and WOF4 coordinate trans to the W=O bond through a W- - -Fb bond. The XeF3 moiety of F3 XeFb - - -WOF4 acquires a degree of [XeF3 ]+ character upon coordination that is reflected by its Xe-F stretching frequencies which are intermediate with respect to those of XeF4 and [XeF3 ]+ . Calculations show W- - -Fb is predominantly an electrostatic, σ-hole bond with a significant orbital contribution that accounts for the bent Xe-Fb - - -W angle. The calculations show F3 XeFb - - -MOF4 (M=Cr, Mo) are less stable than their W analogue, consistent with failed attempts to synthesize F3 XeFb - - -MoOF4 .

Understanding the Role of Inter- and Intramolecular Promoters in Electro- and Photochemical CO2 Reduction Using Mn, Re, and Ru Catalysts.

Recycling of carbon dioxide to fuels and chemicals is a promising strategy for renewable energy storage. Carbon dioxide conversion can be achieved by (i) artificial photosynthesis using photoinduced electrons; (ii) electrolysis using electricity produced by photovoltaics; and (iii) thermal CO2 hydrogenation using renewable H2. The focus of our group's research is on molecular catalysts, in particular coordination complexes of transition metals (e.g., Mn, Re, and Ru), which offer versatile platforms for mechanistic studies of photo- and electrochemical CO2 reduction. The interactions of catalytic intermediates with Lewis or Brønsted acids, hydrogen-bonding moieties, solvents, cations, etc., that function as promoters or cofactors have become increasingly important for efficient catalysis. These interactions may have dramatic effects on selectivity and rates by stabilizing intermediates or lowering transition state barriers, but they are difficult to elucidate and challenging to predict. We have been carrying out experimental and theoretical studies of CO2 reduction using molecular catalysts toward addressing mechanisms of efficient CO2 reduction systems with emphasis on those containing intramolecular (or pendent) and intermolecular (solution phase) additives. This Account describes the identification of reaction intermediates produced during CO2 reduction in the presence of triethanolamine or ionic liquids, the benefits of hydrogen-bonding interactions among intermediates or cofactors, and the complications of pendent phenolic donors/phenoxide bases under electrochemical conditions.Triethanolamine (TEOA) is a common sacrificial electron donor for photosensitizer excited state reductive quenching and has a long history of use in photocatalytic CO2 reduction. It also functions as a Brønsted base in conjunction with more potent sacrificial electron donors, such as 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH). Deprotonation of the BIH•+ cation radical promotes irreversible photoinduced electron transfer by preventing charge recombination. Despite its wide use, most research to date has not considered the broader reactions of TEOA, including its direct interaction with CO2 or its influence on catalytic intermediates. We found that in acetonitrile, TEOA captures CO2 in the form of a zwitterionic adduct without any metal catalyst. In the presence of ruthenium carbonyl catalysts bearing α-diimine ligands, it participates in metal hydride formation, accelerates hydride transfer to CO2 to form the bound formate intermediate, and assists in the dissociation of formate anion from the catalyst ( J. Am. Chem. Soc. 2020, 142, 2413-2428).Hydrogen bonding and acid/base promoters are understood to interact with key catalytic intermediates, such as the metallocarboxylate or metallocarboxylic acid during CO2 reduction. The former is a high energy species, and hydrogen-bonding or Lewis acid-stabilization are beneficial. We have found that imidazolium-based ionic liquid cations can stabilize the doubly reduced form of the [ReCl(bpy)(CO)3] (bpy = 2,2'-bipyridine) electrocatalyst through both hydrogen-bonding and π-π interactions, resulting in CO2 reduction occurring at a more positive potential with a higher catalytic current ( J. Phys. Chem. Lett. 2014, 5, 2033-2038). Hydrogen bonding interactions between Lewis basic methoxy groups in the second coordination sphere of a Mn-based catalyst and the OH group of the Mn-COOH intermediate in the presence of a Brønsted acid were also found to promote C-(OH) bond cleavage, enabling access to a low-energy protonation-first pathway for CO2 reduction ( J. Am. Chem. Soc. 2017, 139, 2604-2618).The kinetics of forming the metallocarboxylic acid can be enhanced by internal acids, and its proton-induced C-OH bond cleavage to the metallocarbonyl and H2O is often the rate-limiting step. Therefore, proton movement organized by pendent hydrogen-bonding networks may also accelerate this step. In contrast, during electrolysis, OH groups in the second coordination sphere are deprotonated to the oxyanions, which deter catalytic CO2 reduction by directly binding CO2 to form the carbonate or by making an M-O bond in competition with CO2 binding ( Inorg. Chem. 2016, 55, 4582-4594). Our results emphasize that detailed mechanistic research is critical in discovering the design principles for improved catalysts.

Bifunctional tetrazole-carboxylate ligand based Zn(ii) complexes: synthesis and their excellent potential anticancer properties.

Transition metal coordination complexes have provided cancer treatment with new insights to overcome the limitations of current chemotherapeutic agents. Utilization of bifunctional tetrazole-carboxylate ligands with Zn(ii) obtained two self-assembled complexes [Zn(HL1)(bipy)3/2(H2O)]·CH3OH·4(H2O) (1) (H3L1 = 1,3,5-tri(2-carboxymethyltetrazol-5-yl) benzene) and [Zn(L2)2(H2O)2]2·2H2O (2) (HL2 = (5-pyridin-3-yl-tetrazol-2-yl)-acetic acid). The X-ray diffraction results showed that the two complexes displayed a two-dimensional (2D) layer structure and a one-dimensional (1D) layer structure. Nanocoprecipitation with DSPE-PEG-2000 resulted in the formation of complex nanoparticles (NPS) with excellent water dispersion. In vitro CCK-8 assay indicated the two NPs exert high cytotoxicity and sensitivity and a low half-maximum inhibitory concentration (IC50) towards HeLa than HepG2 cells. In addition, the cytotoxicity was also confirmed by live/dead co-stained experiments. The presented experimental results showed the 1 and 2 NPs were capable of inhibiting cell proliferation in vitro and may help design coordination complex-based anticancer candidates for cancer cells.

Synthesis, crystallization and Hirshfeld surface analysis of transition metal carboxylate pentapyridines

Single X-ray diffraction studies and Hirshfeld surface analysis of three transition metal coordination complexes of Py5Me2COOH reveals the ligand's diverse coordination utility and the structure directing effect of the carboxylate moiety.

Probing Ligand Effects on the Ultrafast Dynamics of Copper Complexes via Midinfrared Pump-Probe and 2DIR Spectroscopies.

The effects of ligand structural variation on the ultrafast dynamics of a series of copper coordination complexes were investigated using polarization-dependent mid-IR pump-probe spectroscopy and two-dimensional infrared (2DIR) spectroscopy. The series consists of three copper complexes [(R3P3tren)CuIIN3]BAr4F (1PR3, R3P3tren = tris[2-(phosphiniminato)ethyl]amine, BAr4F = tetrakis(pentafluorophenyl)borate) where the number of methyl and phenyl groups in the PR3 ligand are systematically varied across the series (PR3 = PMe3, PMe2Ph, PMePh2). The asymmetric stretching mode of azide in the 1PR3 series is used as a vibrational probe of the small-molecule binding site. The results of the pump-probe measurements indicate that the vibrational energy of azide dissipates through intramolecular pathways and that the bulkier phenyl groups lead to an increase in the spatial restriction of the diffusive reorientation of bound azide. From 2DIR experiments, we characterize the spectral diffusion of the azide group and find that an increase in the number of phenyl groups maps to a broader inhomogeneous frequency distribution (Δ2). This indicates that an increase in the steric bulk of the secondary coordination sphere acts to create more distinct configurations in the local environment that are accessible to the azide group. This work demonstrates how ligand structural variation affects the ultrafast dynamics of a small molecular group bound to the metal center, which could provide insight into the structure-function relationship of the copper coordination complexes and transition-metal coordination complexes in general.

Towards the design, synthesis and preliminary biological evaluation of potential corono and clipcarbenes: Novel bis-imidazolium-bis-heterocycle macrocyclic ligands

The synthesis of novel macrocyclic N-bis-imidazolium-bis-heterocyclic carbenes (NHCs) with rigid and constrained structures to improve the stability of their transition metal coordination complexes in order to obtain efficient photosensitizers is reported. Two pathways have been developed for the preparation of coronocarbenes (L1-L3) containing bipyridine or bithiazole groups. Flexible hemi-macrocycles with potential as clipcarbenes (L1″-L3″) are also described. The crystal structures of ligands L1 and L3 are detailed and the preliminary biological evaluation of L1-L3 is presented.

Transition-metal coordinate bonds for bioinspired macromolecules with tunable mechanical properties

Transition-metal coordination complexes are emerging as a broad class of supramolecular crosslinks that can be used to engineer the mechanical properties of advanced structural materials. Unlike conventional covalent bonds, metal-coordination bonds have the capacity to reform after rupture, thereby enabling dynamic, tunable and reversible (self-healing) mechanical properties. Several biological organisms, such as marine mussels, have been found to take advantage of these unique properties of metal-coordinate complexes in the assembly of load-bearing materials for complex extraorganismal functions. Accordingly, efforts to integrate metal-coordinate crosslinking in bioinspired synthetic protein and polymer hydrogels are an increasingly active area of research. However, a deeper understanding of how metal-coordination bonds affect bulk mechanical properties is still missing, rendering predicting the mechanical properties of metal-coordinated materials challenging. In this Review, we survey recent advances and open questions in our understanding of how chemical properties of metal-coordinate complexes influence multiscale mechanical behaviour, with the aim of presenting metal-coordination bonding as a rich, inorganic crosslinking chemistry tool. We also review applications of metal-coordinate crosslinking in the design of novel materials with tunable mechanical properties, ranging from tough gels to soft robots. These applications highlight the opportunities arising from the integration of this class of load-bearing crosslinks in structural materials design. The reversibility of transition-metal coordination bonds affords broad control over the structural dynamics of materials. This Review surveys the design principles underlying the utilization of this dynamic crosslink chemistry to engineer tunable mechanical properties in biological materials and protein and polymer hydrogels.

Carbazole-based π-conjugated 2,2′-Bipyridines, a new class of organic chromophores: Photophysical, ultrafast nonlinear optical and computational studies

The developmen of donor-acceptor (D-A) architecture based organic chromophores with large two-photon absorption (2 PA) cross-sections are essential for myriad of applications, ranging from nonlinear microscopy to biomedical imaging. Here, we present the results from a comprehensive study of 2 PA cross-sections of a new series of carbazole mono substituted-π-conjugated-2,2′-bipyridine derivatives with D-π-A architecture, possessing carbazole as the donor moiety and bipyridine core as an acceptor. Further, we have extended the π-conjugation by introducing the phenyl and butoxy substituted phenyl linkers. The fluorescence properties of these D-A chromophores are highly sensitive to solvent polarity and alteration of electron donor functionalities. The relevant computation studies support our experimental results, e.g., a shift in emission maxima and band gaps of the molecules. The third-order nonlinear optical (NLO) properties of the title donor-acceptor (D-A) chromophores demonstrated that 2 PA cross-section values are in the range of 6–39 GM, obtained using ~50 fs (fs) laser pulses at a wavelength of 800 nm. Further, the nonlinear refractive indices (n2) of these chromophores were found to be ~10−14 cm2/W, rendering them potential optical switching candidates since the coefficients were obtained with kHz, fs pulses extracting pure electronic nonlinearities. Based on our experimental findings and theoretical calculations presented in this study, we believe that carbazole-based π-conjugated 2,2′-bipyridines, demonstrated in this work, would be potential ligands to obtain a series of transition metal coordination complexes of interesting physical properties (for example, superior NLO behaviour).

Influence of coordination complexes of transition metals on EMI-shielding properties and permeability of polymer blend/carbon nanotube/nickel composites

In this work the influence of transition metal coordination compounds was analyzed on the EMI-shielding characteristics and permeability for a paramagnetic probe of polymer composites based on cross-linked polyurethane or incompatible polymer blend (simultaneous semi-IPN based on cross-linked polyurethane and linear poly(methylmethacrylate) filled in situ with conductive and/or magnetic filler. Cross-linked polyurethane/carbon nanotube composites, polymer blend/carbon nanotube composites and polymer blend/carbon nanotube/nickel composites contained of 1.5 wt% and 3 wt% of fillers were modified with Fe(3+)acetylacetonate or Cr(3+)acetylacetonate coordination compounds of different complexing ability. It was shown that opposite effect of Fe (3+) and Cr(3+) complexes on the phase separation in the polymer blend allows regulate the level of the multiple reflection of the electromagnetic wave at the interfacial boundaries as well as protective characteristics and permeability of the investigated systems. The shielding efficiency of chromium-modified composite in the frequency range of (25.8–37.5) GHz is up to 25 dB in the segregated polymer. It has been shown that the effect of conductive nanofillers on polymer loosening is opposite to that of magnetic fillers and is sensitive to the presence in the system of coordination complexing agent.

Novel Two-Dimensional Porous Materials for Electrochemical Energy Storage: A Minireview.

Two dimensional (2D) porous materials have great potential in electrochemical energy conversion and storage. Over the past five years, our research group has focused on Simple, Mass, Homogeneous and Repeatable Synthesis of various 2D porous materials and their applications for electrochemical energy storage especially for supercapacitors (SCs). During the experimental process, through precisely controlling the experimental parameters, such as reaction species, molar ratio of different ions, concentration, pH value of reaction solution, heating temperature, and reaction time, we have successfully achieved the control of crystal structure, composition, crystallinity, morphology, and size of these 2D porous materials including transition metal oxides (TMOs), transition metal hydroxides (TMHOs), transition metal oxalates (TMOXs), transition metal coordination complexes (TMCCs) and carbon materials, as well as their derivatives and composites. We have also named some of them with CQU-Chen (CQU is the initialism of Chongqing University, Chen is the last name of Lingyun Chen), such as CQU-Chen-Co-O-1, CQU-Chen-Ni-O-H-1, CQU-Chen-Zn-Co-O-1, CQU-Chen-Zn-Co-O-2, CQU-Chen-OA-Co-2-1, CQU-Chen-Co-OA-1, CQU-Chen-Ni-OA-1, CQU-Chen-Gly-Co-3-1, CQU-Chen-Gly-Ni-2-1, CQU-Chen-Gly-Co-Ni-1, etc. The introduction of 2D porous materials as electrode materials for SCs improves the energy storage performances. These materials provide a large number of active sites for ion adsorption, supply plentiful channels for fast ion transport and boost electrical conductivity and facilitate electron transportation and ion penetration. The unique 2D porous structures review is mainly devoted to the introduction of our contribution in the 2D porous nanostructured materials for SC. Finally, the further directions about the preparation of 2D porous materials and electrochemical energy conversion and storage applications are also included.

Direct Conversion of Low-Grade Heat to Electricity Based on Transition Metal Coordination Complexes

Thermo-electrochemical cells (or thermocells), which could convert thermal energy to electrical power, are promising for waste heat utilization. Steady and continuous electricity can be generated through thermocells as long as there is a temperature gradient. The commercialization of thermocells is presently limited by low power output, energy conversion efficiency and high capital investment.Liquid-based thermo-electrochemical cells have great potential for large-scale industrialization. Transition metal (Fe, Ni, Co, Ru, Cr etc.) redox pairs, as well as their coordination complexes, have shown excellent thermo-electrochemical properties. The corresponding electromotive force is linked to the entropy changes of the redox reactions. Further, the thermodynamic and kinetics of the redox reactions are significantly influenced by the type of coordinating ligands, solvents and electrodes materials. Therefore, the design of an efficient thermo-electrochemical cell with outstanding thermal-to-electrical power output requires careful design and consideration of all components within the cell.In this study, transition metal coordination redox couples have been investigated for thermo-electrochemical energy conversion. Iron and cobalt are screened out for their spin states and low cost. By tuning the chelating ligands of transition metals in various solvents, increased Seebeck coefficient and fast ion diffusion were obtained, resulting in significant improvement of the performances in cell voltage and power density. Figure 1

Effect of Molecular Distortion of Ligand Field Splitting in Five-Coordinated Metal Complex

Metal complex indicates the significant physical properties such as magnetism, electrical conductivity and an optical property due to the electronic interaction between the ligand molecule and the central metal ion. We have evaluated the magnetism of the metal complex by controlling the ligand field splitting, which is the energy splitting of the d-orbitals. The magnetic phenomenon of such compounds are realized as a result of the electronic state based on the transition metal coordination complexes, and which can be switched between high spin (HS) and low spin (LS), that is called a spin-crossover (SCO) phenomenon. Our group have been investigated the way how to control the spin state according to the concept of a molecular distortion. The five-coordinated metal complex ML5 has been experimentally confirmed to be a molecular distortion called the Berry pseudorotation (BPR). In the BPR process, both the axial and equatorial ligands can move at the same rate of increasing the angle between the other axial or equatorial ligands in five-coordinated metal complex. The classical BPR mechanism changes its molecular symmetry between two independent trigonal bipyramidals (TBPs) of D3h symmetry via a square pyramidal (SP) of C4v symmetry. The distortion parameters in the five-coordinated metal complexes τ5 are proposed in order to represent the distortion of the BPR process quantitatively. In this work, we predicted the possibility of the SCO phenomenon in intramolecular exchange mechanism through the BPR process in a five-coordinated metal complexes.