Transition Metal Coordination Complexes Research Articles

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.

Antimicrobial activity of selective transition metal co-ordination complexes of myristic acid

A series of co-ordination complexes of Mn(II), Cu(II) and Zn(II) with myristic acid have been synthesized by simple precipitation method. The shift in the absorption peaks of these selective transition metal complexes were well characterized by UV–Visible spectroscopy. Elemental and EDX analysis of these complexes were consistent with the proposed formula. FT-IR spectrum confirmed the transition metal linkage with myristic acid. Thermogravimetric studies of these co-ordination complexes depicted its thermal stability and molecular structure arrangement. Further, these transition metal complexes and its ligand have been screened for their antibacterial and antifungal activity. The cell viability of the solid complexes was also studied from MTT assay.

Automated oxidation-state assignment for metal sites in coordination complexes in the Cambridge Structural Database.

The Cambridge Structural Database (CSD) currently contains over 400 000 transition-metal-containing entries, however many entries still lack curated oxidation-state assignments. Surveying and editing the remaining entries would be far too resource- and time-intensive to be carried out manually. Here, a highly reliable automated workflow for oxidation-state assignment in transition-metal coordination complexes via CSD Python API (application programming interface) scripts is presented. The strengths and limitations of the bond-valence sum (BVS) method are discussed and the use of complementary methods for improved assignment confidence is explored. In total, four complementary techniques have been implemented in this study. The resulting workflow overcomes the limitations of the BVS approach, widening the applicability of an automated procedure to more CSD entries. Assignments are successful for 99% of the cases where a high consensus between different methodologies is observed. Out of a total number of 54 999 unique metal atoms in a test dataset, the procedure yielded the correct oxidation state in 47 072 (86%) of cases.

Structural and Spectroscopic Properties of Metal Complexes with Ruhemann’s Purple Compounds Calculated Using Density Functional Theory

Structural and spectroscopic properties of Ruhemann’s purple (RP) and its transition metal coordination complexes were calculated using theoretical chemistry techniques. The obtained information described RP and its coordination complexes with the transition metal ions [Cr(II), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II)]. The procedures involved calculations of what are called density functional theory (DFT) and time-dependent DFT (TD-DFT). These methods optimized what is called, in the codes of theoretical chemistry, the hybrid density B3LYP function employing the 6‐311++G(d,p) and LANL2DZ basis sets. The RP geometries, bond lengths, angles, quantum chemical parameters, and excitation spectra indicate that the RP is well able to coordinate with a transition element ion. Then the correlation of these theoretical results with experimental observations provides a detailed description of the structural and spectroscopic properties of RP compounds. The inclusion of solvent effects causes a blue shift in all theoretical excitation spectra. In summary, this work leads to an understanding of the characteristics of transition metal complexes with Ruhemann’s purple. These materials can be applied in forensic chemistry as reagents in developing latent fingerprints.

(Invited) What Did the Metals Know, and When Did They Know It? Femtosecond M-Edge XANES Reveals Short-Lived States in Transition Metal Catalysts

X-ray absorption near edge spectroscopy (XANES or NEXAFS) is a powerful technique for electronic structure determination. However, widespread use of femtosecond XANES is limited by the need for scare free-electron laser beamtime. We have developed M2,3-edge XANES, corresponding to 3p→3d transitions, as a reliable technique for measuring the electronic structure of first-row transition metal coordination complexes. The tabletop high-harmonic source provides femtosecond time resolution and 24/7 acessiblility. This talk present the photophysics of three transition metal complexes relevant to photocatalysis: Iron(III) tetraphenyl porphyrin chloride, which relaxes through a ligand-to-metal charge transfer state; Nickel (II) octaethylporphyrin, which relaxes through a 3(d,d) state; and a Co4O4 cubane that catalyzes water oxidation through one of two proposed pathways. We demonstrate M-edge XANES as a powerful new tool for identification of catalytic intermediates.

Structural characterization and emission properties of mixed-ligand transition metal coordination complexes with dicarboxylic acids and pyrazinecarboxamide

Five new transition metal coordination compounds based on pyrazinecarboxamide and dicarboxylic acids have been prepared, and their structures have been characterized by IR spectroscopy and single crystal X-ray diffraction. The crystalline solids include one dinuclear compound [Cu2(mal)2(pca)2(H2O)2]·H2O (1), three one-dimensional coordination polymers [M(mal)(pca)(H2O)]n M = Co(II), Zn(II) (2, 3), [Cu2(CH3COO)4(pca)]n (4) and one two-dimensional coordination polymer [Cd(bdc)(pca)(H2O)]n (5), where pca = pyrazinecarboxamide, H2mal = malonic acid and H2bdc = 1,4-benzenedicarboxylic acid. The pca coordinates to the metal centers as a neutral ligand in chelating (1, 2, 3), bidentate bridging (4) and monodentate modes (5), while the bidentate acetate, malonate and benzenedicarboxylato anionic ligands provide extensions of structures. The new compounds 1, 3 and 5 reveal violet and ultraviolet luminescence upon excitation with ultraviolet light.

A Fast Transient Absorption Study of Co(AcAc)3

The study of transition metal coordination complexes has played a key role in establishing quantum chemistry concepts such as that of ligand field theory. Furthermore, the study of the dynamics of their excited states is of primary importance in determining the de-excitation path of electrons to tailor the electronic properties required for important technological applications. This work focuses on femtosecond transient absorption spectroscopy of Cobalt tris(acetylacetonate) (Co(AcAc)3) in solution. The fast transient absorption spectroscopy has been employed to study the excited state dynamics after optical excitation. Density functional theory coupled with the polarizable continuum model has been used to characterize the geometries and the electronic states of the solvated ion. The excited states have been calculated using the time dependent density functional theory formalism. The time resolved dynamics of the ligand to metal charge transfer excitation revealed a biphasic behavior with an ultrafast rise time of 0.07 ± 0.04 ps and a decay time of 1.5 ± 0.3 ps, while the ligand field excitations dynamics is characterized by a rise time of 0.07 ± 0.04 ps and a decay time of 1.8 ± 0.3 ps. Time dependent density functional theory calculations of the spin-orbit coupling suggest that the ultrafast rise time can be related to the intersystem crossing from the originally photoexcited state. The picosecond decay is faster than that of similar cobalt coordination complexes and is mainly assigned to internal conversion within the triplet state manifold. The lack of detectable long living states (>5 ps) suggests that non-radiative decay plays an important role in the dynamics of these molecules.

Hard x-ray spectroscopy of the itinerant magnets RFe4Sb12 (R=Na, K, Ca, Sr, Ba)

Ordered states in itinerant magnets may be related to magnetic moments displaying some weak local moment characteristics, as in intermetallic compounds hosting transition metal coordination complexes. In this paper, we report on the Fe $K$-edge X-ray absorption spectroscopy (XAS) of the itinerant magnets $R$Fe$_{4}$Sb$_{12}$ ($R=$ Na, K, Ca, Sr, Ba), aiming at exploring the electronic and structural properties of the octahedral building block formed by Fe and the Sb ligands. We find evidence for strong hybridization between the Fe $3d$ and Sb $5p$ states at the Fermi level, giving experimental support to previous electronic structure calculations of the $R$Fe$_{4}$Sb$_{12}$ skutterudites. The electronic states derived from Fe 3$d$ Sb $5p$ mixing are shown to be either more occupied and/or less localized in the cases of the magnetically ordered systems, for which $R=$ Na or K, connecting the local Fe electronic structure to the itinerant magnetic properties. Moreover, the analysis of the extended region of the XAS spectra (EXAFS) suggests that bond disorder may be a more relevant parameter to explain the suppression of the ferromagnetic ordered state in CaFe$_{4}$Sb$_{12}$ than the decrease of the density of states.

Scrutinizing metal-ligand covalency and redox non-innocence via nitrogen K-edge X-ray absorption spectroscopy.

Nitrogen K-edge X-ray absorption spectra (XAS) were obtained for 19 transition metal complexes bearing bipyridine, ethylenediamine, ammine, and nitride ligands. Time-dependent density functional theory (TDDFT) and DFT/restricted open configuration interaction singles (DFT/ROCIS) calculations were found to predict relative N K-edge XAS peak energies with good fidelity to experiment. The average difference (|ΔE|) between experimental and linear corrected calculated energies were found to be 0.55 ± 0.05 eV and 0.46 ± 0.04 eV, respectively, using the B3LYP hybrid density functional and scalar relativistically recontracted ZORA-def2-TZVP(-f) basis set. Deconvolution of these global correlations into individual N-donor ligand classes gave improved agreement between experiment and theory with |ΔE| less than 0.4 eV for all ligand classes in the case of DFT/ROCIS. In addition, calibration method-dependent values for the N 1s → 2p radial dipole integral of 25.4 ± 1.7 and 26.8 ± 1.9 are obtained, affording means to estimate the nitrogen 2p character in unfilled frontier molecular orbitals. For the complexes studied, nitrogen covalency values correlate well to those calculated by hybrid DFT with an R 2 = 0.92 ± 0.01. Additionally, as a test case, a well-characterized PNP ligand framework (PNP = N[2-P(CHMe2)2-4-methylphenyl]2 1-) coordinated to NiII is investigated for its ability to act as a redox non-innocent ligand. Upon oxidation of (PNP)NiCl with [FeCp2](OTf) to its radical cation, [(PNP)NiCl](OTf) (OTf = triflate), a new low-energy feature emerges in the N K-edge XAS spectra. This feature is assigned as N 1s to a PNP-localized acceptor orbital exhibiting 27 ± 2% N 2p aminyl radical character, obtained using the aforementioned nitrogen covalency calibration. Combined, these data showcase a direct spectroscopic means of identifying redox-active N-donor ligands and also estimating nitrogen 2p covalency of frontier molecular orbitals in transition metal complexes.

Experimental and Theoretical Comparison of Transition-Metal and Actinide Tetravalent Schiff Base Coordination Complexes.

A series of homoleptic tetravalent transition-metal and actinide Schiff-base coordination complexes, ML2 {M = Zr, Hf, Th, U; L = N, N'-bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediamine}, have been synthesized that feature a rigid phenyl backbone. These complexes create the opportunity for comparing a series of complexes containing metal cations in the formal IV+ oxidation state by structural, spectroscopic, and theoretical analysis that also incorporate the previously reported Ce(IV) and Pu(IV) analogues. X-ray crystallographic analysis reveals that all complexes are isomorphous and feature a co-facial ligand geometry. TD-DFT and other quantum mechanical methods were used to explore bonding differences across between the complexes, and resulting calculated absorbance spectra for ML2 are in good agreement with the experimental data. The computational results also suggest that U(IV) and Pu(IV) analogs have more covalent character in their bonding than found with the other metal cations reported here.

Computer-aided design of short-lived phosphorescent Ru(II) polarity probes

Fluorescent polarity probes are usually based on intramolecular charge transfer excited states of selected dyes, the behavior of which in different solvents is traditionally rationalized by the well-known Lippert-Mataga treatment of the “general solvents effect”. Less often transition metal coordination complexes are used as luminescent probes, even though the spectroscopic properties of these dyes are usually dependent on the environment. This is the case of Ru(II) polypyridyls, which are good candidates to develop robust sensitive polarity probes because of their lowest-lying metal-to-ligand charge transfer triplet emissive state, provided their chelating ligands structure is judiciously tuned. The aim of this work has been to design a computational strategy to forecast the behavior of polarity-sensitive Ru(II) complexes without the need to prepare a large set of candidates. In particular, we have analysed a number of complexes derived from [Ru(bpy)3]2+ by introducing different pairs of substituents in the 4,4′ positions of one of the three equivalent 2,2′-bipyridine (bpy) moieties. In this way, we have investigated if a direct relationship may be established between the electronic features of the substituent and the Stokes shift sensitivity to the solvent polarity. Our computational data satisfactorily agree with our experimental results, but they demonstrate that only by explicitly performing the calculation of the Stokes shift in different media for each candidate, it is possible to select the best Ru(II) dyes to be used as polarity probes.

Crystal structure and Hirshfeld surface analysis of bis (2-(2-(1H-benzo[d]imidazol-2-yl)hydrazono)propan-1-ol) nickel(II) chloride

Abstract The present work describes the crystal structure and Hirshfeld surface analysis of a transition metal coordination complex, bis (2-(2-(1H-benzo[d]imidazol-2-yl)hydrazono)propan-1-ol) nickel(II) chloride.Recrystallization of the title complex from aqueous methanol has crystallized the complex in monoclinic crystal system with P21/c space group. The asymmetric unit of the complex has two monomeric distorted octahedral coordination entities, along with four chloride anions and seven water molecules as crystallization solvent. The ring puckering analysis, intramolecular hydrogen and halogen bonding interactions and the C H⋯π and π–π interactions of the complex are explored. Further, the Hirshfeld surface analysisis carried out to reveal the nature and contributions of individual intermolecular interactions and the importance of the C H⋯π and π–π interactions in the crystal packing.

Dimers, monomers and pentacoordination in a series of earth-abundant transition metal dibromido complexes supported by a neutral SNS ligand framework

Pincer-type architectures represent an important class of ligand frameworks for the design and synthesis of transition metal coordination complexes and catalysts. The first systematic application of bis(thioether)pyridine “SNS” ligands in coordination chemistry of first-row transition-metal bromides and the first structurally characterized “SNS” complexes of Mn and Fe are reported. The complexes were characterized by single crystal X-ray analysis, UV–Vis and FTIR spectroscopy, mass-spectrometry, elemental analysis, and where appropriate by Evans method. Analysis of the resulting complexes reveals a set of monomeric and dinuclear species, whose coordination mode is associated with a delicate balance of structural features arising from the metal’s d-orbital occupancy and the steric load of the ligand. Dimerization of the solid state structure appears to be primarily favored through lower steric load from the ligand and lower metal d-electron count.

Role of hydrogen-bonds in the crystal packing of three novel strandberg-type polyoxoanion compounds

Three new supramolecular compounds [Cu0.5(H3O)2](H2prz)1.5[Mo5P2O23]·H2O (1); [4,4′-bpy]2[4,4′-H2bpy][H4Mo5P2O23]·4H2O (2); [4,4′-Hbpy][4,4′-bpy][H3Mo5P2O23]·2H3O (3); (pyz = pyrazine, bpy = 4,4′-bpyridine) have been hydrothermally synthesized and characterized by IR, UV-Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), powder XRD analyses, TG analyses, cyclic voltammogram analyses, elemental analyses and single crystal X-ray diffraction analyses. Single crystal X-ray diffraction analysis reveals that there are multipoint hydrogen-bonding interactions in compounds 1–3. The structural features of the three compounds are that distinct one-dimensional (1D) chains are firstly formed, then the chains extend to form novel two-dimensional (2D) networks, finally unprecedented three-dimensional (3D) supramolecular host-guest structures were generated by Strandberg-type polyoxometaltes (POMs) and transition metal coordination complexes (TMCs) or organic ligands, and small organic guest molecules are exactly entrapped in void spaces via H-bonding interactions. The formations of the three distinct 3D supramolecular hybrid materials demonstrate the important role of H-bonds (non-covalent bond) in the construction of high dimensional architectures. Compounds 2–3 also show outstanding photocatalysis properties.

One new manganese polymer with a bidentate pyrazole-based ligand: crystal structure, fluorescence and Hirshfeld surface analysis

One new transition metal coordination complex {[MnL1(H2O)3]·2H2O}n (1) (where H2L1 is 1-(carboxymethyl)-1H-pyrazole-3-carboxylic acid) were synthesized by one-pot in situ hydrolysis. The complex is a 1D polymer and assembled into a 3D structure by hydrogen bonding interactions and intermolecular contacts. The structure has been established by single-crystal X-ray diffraction, and characterized by FT-IR and solid state fluorescent spectroscopy. The maximum emission of 1 is at 362 nm (λex = 298 nm), which is red-shift to the free ligand λmax = 327 nm (λex = 298 nm). Hirshfeld surface analysis was also carried on complex 1. According to the 3D Hirshfeld surface and 2D fingerprint plots, the main interactions in the complex are O···H/H···O contacts.

Series of Single-Ion and 1D Chain Complexes Based on Quinolinic Derivative: Synthesis, Crystal Structures, HF-EPR, and Magnetic Properties.

By utilizing the quinolinic derivative, 8-carboxymethoxy-2-carboxylicquinoline (L), five transition metal coordination complexes, [M(L)(H2O)3]·H2O] (M = Mn (1), Co (2)), [Ni(L)(H2O)2] (3), and {[M(L)](H2O)} n (M = Ni (4), Cu (5)), were synthesized by hydrothermal methods employing similar synthetic strategies. The crystal structures, magnetism and high-field EPR were characterized for the obtained compounds. 1-3 are mononuclear compounds. 1 and 2 have pentagonal bipyramidal geometry, while 4 and 5 exhibit one-dimensional zig-zag chain. Direct current magnetic and EPR studies demonstrate that compound 2 has large and positive D value (∼70.4 cm-1), indicating the easy plane magnetic anisotropies of 2. This D value is the largest one in the reported Co(II) complexes with pentagonal bipyramidal geometry. Field-induced slow magnetic relaxation behavior was observed for 2 by the dynamic ac magnetic susceptibility measurements. The dc magnetic susceptibility studies of 4 and 5 give similar weak MII-MII antiferromagnetic interactions ( J = -1.50 and -3.55 K for 4 and 5, respectively). High-field EPR results show that 4 can be considered as a quantum antiferromagnet.

Transition metal coordination complexes based on V-shaped bis-triazole ligand: syntheses, structures, and properties

Hydrothermal reactions of 1,3-bis(1,2,4-triazol-1-yl)benzene (btb) and M(NO3)2 (M = Co2+ (1), Cu2+ (2)) afforded two new coordination polymers, [Co(btb)2(NO3)(H2O)]n·NO3·H2O (1) and [Cu(btb)2(NO3)2]n (2), respectively. Single-crystal X-ray diffraction reveals that 1 crystallizes in the space group P21/m and 2 crystallizes in the space group Pī, both showing a double-stranded chain structure. The 1-D chains are interconnected via π⋯π interactions to lead to 2-D ladder-like supramolecular architectures. In addition, magnetic behavior and thermal stability of 1 and 2 have been investigated. For 1, weak antiferromagnetic interactions are observed at low temperature, and the data obey the Curie–Weiss law χM = C/(T−θ), with C = 3.22 cm3·mol−1·K and θ = −10.39 K. For 2, the decrease of the χT vs. T curve at low temperature is the result of intermolecular antiferromagnetic magnetic interactions.

Two new compounds of polyoxoanions, transition metal complexes and organic amines

Abstract Two compounds [Cu(biim)]2(TEA)2[HPW12O40]·H2O (1); [Cu(im)2](im)[H2PMo12O40] (2), (biim = 2,2′-biimidazole, im = imidazole, TEA = triethylamine) have been hydrothermally synthesized and characterized by IR, XPS, XRD, UV-Vis, fluorescent spectra analysis, elemental analysis, X-ray diffraction analysis, cyclic voltammetric measurement and single crystal X-ray diffraction analysis. Single crystal X-ray diffraction analysis reveals that compounds 1–2 represent new examples formed by polyoxoanions, transition metal coordination complexes (TMCs) and organic amines. Compound 1 exhibits a 3-D supermolecular structure of which layers A and C are stacked alternatively, while compound 2 first shows a 1-D chain structure with an –A↑–A↓–A↑–A↓ linking mode, then along a, b and c axis, respectively exhibits different beautiful 2-D supramolecular networks via hydrogen bond interactions.The photodegradation properties of compounds 1–2 have been investigated.