Transition Metal Centers Research Articles

Phendione-Transition-Metal Complexes with Bipolar Redox Activity for Lithium Batteries.

1,10-Phenanthroline-5,6-dione (phendione)-based transition-metal complexes are known for their use in pharmacological and catalysis applications. However, their application in electrochemical energy storage has not been investigated thus far. Herein, the feasibility of employing phendione-transition-metal complexes was investigated for electrochemical charge storage by taking advantage of the reversible redox activity of both carbonyl groups and transition metal center, contributing to augmented charge storage. Interestingly, the chemistry of the counter ion in the studied complexes effectively tuned the solubility and improved the cycling stability. Although further studies are required to limit the solubility and active-species shuttle, this study explores the bottlenecks of phendione-transition-metal complexes as electrode materials for solid-electrode-format batteries.

Determining the Orientation and Vibronic Couplings between Electronic and Vibrational Coordinates with Polarization-Selective Two-Dimensional Vibrational-Electronic Spectroscopy.

We experimentally demonstrate polarization-selective two-dimensional (2D) vibrational-electronic (VE) spectroscopy on a transition-metal mixed-valence complex where the cyanide stretching vibrations are coupled to the metal-to-metal charge-transfer transition. A simultaneous fitting of the parallel and crossed polarized 2D VE spectra quantifies the relative vibronic coupling strengths and angles between the charge-transfer transition and three coupled cyanide stretching vibrations in a mode-specific manner. In particular, we find that the bridging vibration, which modulates the distance between the transition-metal centers, is oriented nearly parallel to the charge-transfer axis and is 9 times more strongly coupled to the electronic transition than the radial vibration, which is oriented almost perpendicular to the charge-transfer axis. The results from this experiment allow us to map the spectroscopically observed vibronic coordinates onto the molecular frame providing a general method to spatially resolve vibronic energy transfer on a femtosecond time scale.

Penta-coordinated transition metal macrocycles as electrocatalysts for the oxygen reduction reaction

The oxygen reduction reaction (ORR) is a highly important reaction in electrochemistry. The following short review details recent advances in novel non-precious metal catalysts containing transition metal macrocycles for use in the ORR. Unbound, many of these electrodes were found to generate high levels of side products such as O2− and H2O2 via 2-electron processes, and for this reason, it is aimed to create systems which would favor a 4-electron process which would completely convert oxygen to H2O. The 4-electron reduction of O2 releases the most energy in a fuel cell. Novel catalytic materials containing metal macrocycles were created mimicking the structure of enzyme metal centers, the metal in the macrocycle bound to a fifth axial ligand. These structures were observed to exhibit improved catalytic activity, and in the case of cobalt, phthalocyanine systems were observed to move away from the inefficient 2-electron process towards the more complete 4-electron process in alkaline media. Both experimental results (XPS, EPR, cyclic voltammetry and polarization curves) and theoretical models were gathered for various pentacoordinate systems and various electronic effects of the axial ligand on metal center were proposed. Penta-coordinate macrocycles are an important tool for further manipulating and tuning the electronic behavior of transition metal centers for catalysis of the ORR.

Conformation controllable inelastic charge transport and shot noise behavior in metal-string single molecular devices

It is often intriguing experimentally to take stock of how conformational changes in the device configuration may impact the overall charge transport behavior of single-molecule junctions. Based on the allied approach of density functional theory and non-equilibrium Green's function formalism, we explore here the effect of junction heterogeneity on inelastic charge transport in various metal-string based single-molecule devices. The constituent active elements being sensitive to the resonant levels, transition metal centers are found to influence stretching, bending, and torsional excitation modes, while rocking and scissoring modes are controlled largely by the axial ligands. For certain molecular conformations and electrode orientations, phonon-assisted quantum interference effect may crop up, leading to the suppression of higher wavenumber vibrational modes. The resulting inelastic spectra are likely to take the shape of dominant Fano resonance or anti-resonance, depending on whether phonons are emitted or absorbed. Such nanoscale quantum interference effect is manifested especially in those metal-string molecular junctions for which the energy gap (between localized and delocalized virtual states) lies well within the optical phonon energies (ΔE|HOMO-LUMO| < 40 meV). It also turns out that single molecular shot noise can exhibit nearly Poissonian behavior if the inter-channel tunneling through frontier orbitals is accompanied by phonon absorption or emission following a slow relaxation process. Charge transport properties across metal-string complexes can thus potentially be tuned by selective architecture of the metal centers and also, by preferred orientation of nanoscale electrodes in a bid to build up molecular devices with desirable controllability.

(De)lithiation of spinel ferrites Fe3O4, MgFe2O4, and ZnFe2O4: a combined spectroscopic, diffraction and theory study.

Iron based materials hold promise as next generation battery electrode materials for Li ion batteries due to their earth abundance, low cost, and low environmental impact. The iron oxide, magnetite Fe3O4, adopts the spinel (AB2O4) structure. Other 2+ cation transition metal centers can also occupy both tetrahedral and/or octahedral sites in the spinel structure including MgFe2O4, a partially inverse spinel, and ZnFe2O4, a normal spinel. Though structurally similar to Fe3O4 in the pristine state, previous studies suggest significant differences in structural evolution depending on the 2+ cation in the structure. This investigation involves X-ray absorption spectroscopy and X-ray diffraction affirmed by density functional theory (DFT) to elucidate the role of the 2+ cation on the structural evolution and phase transformations during (de)lithiation of the spinel ferrites Fe3O4, MgFe2O4, and ZnFe2O4. The cation in the inverse, normal and partially inverse spinel structures located in the tetrahedral (8a) site migrates to the previously unoccupied octahedral 16c site by 2 electron equivalents of lithiation, resulting in a disordered [A]16c[B2]16dO4 structure. DFT calculations support the experimental results, predicting full displacement of the 8a cation to the 16c site at 2 electron equivalents. Substitution of the 2+ cation results in segregation of oxidized phases in the charged state. This report provides significant structural insight into the (de)lithiation mechanisms for an intriguing class of iron oxide materials.

Photoactive metal complexes that bind DNA and other biomolecules as cell probes, therapeutics, and theranostics.

This review discusses the advantages of using luminescent d6-transition-metal complexes as cell probes for optical microscopy. In particular it focusses on the Thomas group's use of specific complexes as "building blocks" toward the construction of biomolecular binding substrates, with DNA being a particular target. Using this approach, a range of new imaging probes for conventional optical microscopy, nanoscopy and transmission electron microscopy have been identified. Through selection of specific metal centres and by substitution of coordinated ligands we illustrate how new chemotherapeutics, photo-therapeutics, and theranostics have been identified and developed from the original architectures.

Modular bonding picture for aromatic borometallic molecular wheels

Borometallic molecular wheel is a special class of clusters with a planar ring of boron atoms surrounding a transition metal center, and its distinctive geometry and atypical electronic structure make this class of clusters an interesting target for theoretical analysis. Previous adaptive natural density partitioning analyses have provided a bonding picture for them. In this work, we take a slightly different perspective on their bonding, by considering such clusters as coordination complexes. We find that these clusters can be understood through a simple modular bonding picture, which could be applied to a number of related complexes/clusters.

Oxidative Formation and Redox Properties of 9‐Methylisocorroles

Cationic nickel(II), palladium(II), and copper(II) isocorrole complexes were obtained from linear tetrapyrrolic 2,2′‐bidipyrrin precursors by metal‐templated oxidative macrocyclization reactions using silver(I) tetrafluoroborate as the oxidant in 40–69 % yield. The new compounds were characterized by optical and resonance methods. Two products from overoxidation and attack of nucleophiles were identified by means of single‐crystal X‐ray diffraction, elucidating possible H atom transfer processes during and after the macrocyclization step. The redox behavior was studied by UV/Vis‐spectroelectrochemistry, and the complexes were found to show different stabilities of the electrogenerated radicals, depending on the nature of the central transition metal ion. A one‐electron reduction step at low potential is common for all species, but reversible only in the case of nickel, and an irreversible reaction of a copper derivative was found to produce copper corrole.

Quantum Chemical Modeling of Pressure-Induced Spin Crossover in Octahedral Metal-Ligand Complexes.

Spin state switching on external stimuli is a phenomenon with wide applicability, ranging from molecular electronics to gas activation in nanoporous frameworks. Here, we model the spin crossover as a function of the hydrostatic pressure in octahedrally coordinated transition metal centers by applying a field of effective nuclear forces that compress the molecule towards its centroid. For spin crossover in first‐row transition metals coordinated by hydrogen, nitrogen, and carbon monoxide, we find the pressure required for spin transition to be a function of the ligand position in the spectrochemical sequence. While pressures on the order of 1 GPa are required to flip spins in homogeneously ligated octahedral sites, we demonstrate a fivefold decrease in spin transition pressure for the archetypal strong field ligand carbon monoxide in octahedrally coordinated Fe2+ in [Fe(II)(NH3)5CO]2+.

A hexagonal planar transition-metal complex.

Transition-metal complexes are widely used in the physical and biological sciences. They have essential roles in catalysis, synthesis, materials science, photophysics and bioinorganic chemistry. Our understanding of transition-metal complexes originates from Alfred Werner's realization that their three-dimensional shape influences their properties and reactivity1, and the intrinsic link between shape and electronic structure is now firmly underpinned by molecular-orbital theory2-5. Despite more than a century of advances in this field, the geometries of transition-metal complexes remain limited to a few well-understood examples. The archetypal geometries of six-coordinate transition metals are octahedral and trigonal prismatic, and although deviations from ideal bond angles and bond lengths are frequent6, alternative parent geometries are extremely rare7. The hexagonal planar coordination environment is known, but it is restricted to condensed metallic phases8, the hexagonal pores of coordination polymers9, or clusters that contain more than one transition metal in close proximity10,11. Such a geometry had beenconsidered12,13 for [Ni(PtBu)6]; however,an analysis of the molecular orbitals suggested that this complex isbest described as a 16-electron specieswith a trigonal planar geometry14. Here we report the isolation and structural characterization of a simple coordination complex in which six ligands form bonds with a central transition metal in a hexagonal planar arrangement. The structure contains a central palladium atom surrounded by three hydride and three magnesium-based ligands. This finding has the potential to introduce additional design principles for transition-metal complexes, with implications for several scientific fields.

Central (S) to Central (M=Ir, Rh) to Planar (Metallocene, M=Fe, Ru) Chirality Transfer Using Sulfoxide‐Substituted Mesoionic Carbene Ligands: Synthesis of Bimetallic Planar Chiral Metallocenes

Enantiopure bimetallic systems containing three different elements of chirality, namely a main-group-based chiral center (sulfur), a transition-metal chiral center (rhodium or iridium), and a planar chiral element (ferrocene or ruthenocene), have been prepared by a sequence of diastereoselective reactions. The chirality of the chiral sulfur center attached to C-5 of a 1,2,3-triazolylidene mesoionic carbene (MIC) ligand coordinated to a metal (Ir, Rh) was transferred through the formation of bimetallic complexes having a chiral-at-metal center and a planar chiral metallocene by C-H activation of the sandwich moiety (M=Fe, Ru). The sense of the planar chirality formed in this sequence of reactions depended on the nature of the ligands at the metal center of the starting complex. The configurations of these species were assigned on the basis of a combination of X-ray diffraction and CD measurements. An electrochemical study of these bimetallic complexes in coordinating solvents showed an equilibrium between the cationic complexes and the neutral species. The effect of the half-sandwich moiety on the oxidation potentials of the system is remarkable, producing notable cathodic displacements. DFT calculations support these findings.

Syntheses and magnetic properties of three transition metal complexes based on 4′-p-tolyl-2,2′:6′,2″-terpyridine and SCN−

Three transition metal complexes based on 4′-p-tolyl-2,2′:6′,2″-terpyridine (ttpy) and SCN−, [Mn2(ttpy)2(SCN)3Cl] (1), [Co2(ttpy)2(SCN)4]·H2O (2) and [Ni(ttpy)(SCN)2]2 (3), have been hydrothermally synthesized. The single-crystal X-ray diffraction analyses reveal that the three complexes display discrete molecules with three distinct structures caused by the different coordination geometry of the transition metal centers. Complex 1 contains two neutral independent components of mononuclear [Mn(ttpy)(SCN)2] and binuclear [Mn(ttpy)(SCN)Cl]2. Complex 2 shows an ion-pair bicomponent structure of [Co(ttpy)2]2+ and [Co(SCN)4]2−. Complex 3 shows a binuclear structure bridged by two end-to-end bridging SCN− anions. Magnetic properties of the three complexes have been investigated.

E-Selective Synthesis and Coordination Chemistry of Pyridine-Phosphaalkenes: Five Ligands Produce Four Distinct Types of Ru(II) Complexes.

Pyridine-phosphaalkene (PN) ligands 2a-e were prepared in an E-selective fashion using phospha-Wittig methodology. Treatment of these five ligands, varying only in their 6-substituent with RuCl2(PPh3)3, produced four distinct types of coordination complexes: pyridine-phosphaalkene-derived 3b,d, cyclized 4e, and six-coordinate 5a and 6c. Prolonged heating of 3b,d in THF resulted in C-H activation of the Mes* group and cyclization to give 4b,d featuring a bidentate pyridine-phospholane ligand bound to the metal center. Complex 5a, also possessing a newly formed phospholane ring, contained a different spatial arrangement of donors to Ru(II) with an agostic Ru-H-C interaction serving as the sixth donor to the transition metal center. Ligands 2b,d,e and Ru(II) complexes 3b, 4b,e and 5a were all characterized by X-ray crystallography. Six-coordinate 6c featured a structure similar to 4b,d,e, but with the CF3 substituent acting as a weakly bound sixth ligand to the Ru(II) center, as observed by 31P{1H} and19F NMR spectroscopy. The calculated structure of 6c established that the closest Ru- - -F contact was at 2.978 Å.

Tunable giant magnetoresistance in a single-molecule junction

Controlling electronic transport through a single-molecule junction is crucial for molecular electronics or spintronics. In magnetic molecular devices, the spin degree-of-freedom can be used to this end since the magnetic properties of the magnetic ion centers fundamentally impact the transport through the molecules. Here we demonstrate that the electron pathway in a single-molecule device can be selected between two molecular orbitals by varying a magnetic field, giving rise to a tunable anisotropic magnetoresistance up to 93%. The unique tunability of the electron pathways is due to the magnetic reorientation of the transition metal center, resulting in a re-hybridization of molecular orbitals. We obtain the tunneling electron pathways by Kondo effect, which manifests either as a peak or a dip line shape. The energy changes of these spin-reorientations are remarkably low and less than one millielectronvolt. The large tunable anisotropic magnetoresistance could be used to control electronic transport in molecular spintronics.

Building Up a Picture of the Electrocatalytic Nitrogen Reduction Activity of Transition Metal Single-Atom Catalysts.

The lack of chemical understanding and efficient catalysts impedes the development of electrocatalytic nitrogen reduction reaction (eNRR) for ammonia production. In this work, we employed density functional theory calculations to build up a picture (activity trends, electronic origins, and design strategies) of single-atom catalysts (SACs) supported on nitrogen-doped carbons as eNRR electrocatalysts. To construct such a picture, this work presents systematic studies of the eNRR activity of SACs covering 20 different transition metal (TM) centers coordinated by nitrogen atoms contained in three types of nitrogen-doped carbon substrates, which gives 60 SACs. Our study shows that the intrinsic activity trends could be established on the basis of the nitrogen adatom adsorption energy (Δ EN*). Furthermore, the influence of metal and support (ligands) on Δ EN* proved to be related to the bonding/antibonding orbital population and regulating the scaling relations for adsorption of intermediates, respectively. Accordingly, a two-step strategy is proposed for improving the eNNR activity of TM-SACs, which involves the following: (i) selection of the most promising family of SACs (g-C3N4 supported SACs as predicted in this work) and (ii) further improvement of the activity of the best candidate in the aforementioned family via tuning the adsorption strength of the key intermediates. Also, the stability of N-doped carbon supports and their selectivity in comparison to the competing hydrogen evolution need to be taken into consideration for screening the durable and efficient candidates. Finally, an effective strategy for designing active, stable, and selective SACs based on the mechanistic insights is elaborated to guide future eNRR studies.

Jahn-Teller Distorted Effects To Promote Nitrogen Reduction over Keggin-Type Phosphotungstic Acid Catalysts: Insight from Density Functional Theory Calculations.

Molecular geometry, electronic structure, and possible reaction mechanism of a series of mono-transition-metal-substituted Keggin-type polyoxometalate (POM)-dinitrogen complexes [PW11O39M(N2)] n- (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, Au, and Hg) have been investigated by using density functional theory (DFT) calculations with M06L functional. The calculated adsorption energy of N2 molecule, N-N bond length, N-N stretching frequency, and the NBO charge on the coordinated N2 moiety indicate that MoII-, TcII-, WII-, ReII-, and OsII-POM complexes are significant for binding and activation of the inert N2 molecule. The degree of the N2 activation can be classified into the "moderately activated" category according to Tuczek's sense [ J. Comput. Chem. 2006 , 27 , 1278 ]. Electronic structure and NBO analysis indicate that the terminal N atom of the coordinated N2 molecule in these POM-dinitrogen complexes possesses more negative charge relative to the bridge N atom because Jahn-Teller distorted effects lead to an effective orbital mixture between σ2s* orbital of N2 and d z2 orbital of transition metal center. And the mono-lacunary Keggin-type POM ligand with five oxygen donor atoms serves as a strong electron donor to the bivalent metal center. Meanwhile, a catalytic cycle for direct conversion of N2 into NH3 has been systematically investigated based on a Re-POM complex along distal, alternating, and enzymatic pathways. The calculated free energy profile of the three catalytic cycles indicates that the distal mechanism is the favorable pathway in the presence of proton and electron donors.

Investigation of ORR Performances on Graphene/Phthalocyanine Nanocomposite in Neutral Medium.

The drive to replace scarce and expensive Pt-based electrocatalysts for oxygen reduction reaction (ORR) has led to the development of a group of electrocatalysts composed of transition-metal ion centers coordinated with four nitrogen groups (M-N4). Among these, metal phthalocyanines (MPcs), due to low cost of preparation, highly conjugated structure as well as high thermal and chemical stability, have received a great interest. The catalytic activity of MPcs can be improved by employing conducting supports. Here, in this report, we have solvothermally synthesized graphene-supported zinc phthalocyanine nanostructures, and their ORR kinetics and mechanism have been investigated in neutral solution (pH = 7) by using the rotating disk electrode technique. The as-synthesized nanocomposite followed a 4e- reduction pathway. The onset potential (-0.04 V versus Ag/AgCl) found in this work can be comparable with other state-of-the-art material, demonstrating good performance in neutral solution. The fascinating performance leads the nanocomposite material toward future energy applications.

Metal transition complexes of tridentate Schiff base ligands derived from 2-hydrazinopyridine: synthesis, spectroscopic characterization and X-ray structures

Mononuclear complexes of 1-(pyridin-2-ylmethylidene)-2-(pyridin-2-yl)hydrazine (HL1) and 1-(pyridin-2-yl)-2-(1-(pyridin-2-yl)ethylidene)hydrazine (HL2), [Mn(HL1)(Cl2)(H2O)] (1), [Zn(HL1)(CH3COO)2]·(H2O)3 (2), [Mn(HL2)2]·(ClO4)2 (3) and [Cu(HL2)(NO3)2(H2O)] (4) were synthesized and characterized by physicochemical and spectroscopic methods and X-ray structure determination. The mononuclear compounds 1, 2 and 4 contain one ligand molecule per metal atom while the manganese (II) atom in compound 3 is coordinated to two ligand molecules. Both ligands coordinated to the transition metal center in a tridentate fashion through two Npyridyl atoms and one Nimino atom. The chloride and acetate anions coordinate in monodentate manner, respectively, in complex 1 and in complex 2. In complex 3, the perchlorate groups remain uncoordinated. In complex 4, the nitrate anions act in unidentate fashion. The molar conductance value indicates that the complexes obtained from HL1 are non-electrolytes while those obtained from HL2 are 2:1 electrolytes in DMF solutions. The X-ray structures reveal octahedral geometry for complexes 1, 3, 4 and trigonal bipyramidal environment for 2.

Bridging the Homogeneous-Heterogeneous Divide: Modeling Spin for Reactivity in Single Atom Catalysis.

Single atom catalysts (SACs) are emergent catalytic materials that have the promise of merging the scalability of heterogeneous catalysts with the high activity and atom economy of homogeneous catalysts. Computational, first-principles modeling can provide essential insight into SAC mechanism and active site configuration, where the sub-nm-scale environment can challenge even the highest-resolution experimental spectroscopic techniques. Nevertheless, the very properties that make SACs attractive in catalysis, such as localized d electrons of the isolated transition metal center, make them challenging to study with conventional computational modeling using density functional theory (DFT). For example, Fe/N-doped graphitic SACs have exhibited spin-state dependent reactivity that remains poorly understood. However, spin-state ordering in DFT is very sensitive to the nature of the functional approximation chosen. In this work, we develop accurate benchmarks from correlated wavefunction theory (WFT) for relevant octahedral complexes. We use those benchmarks to evaluate optimal DFT functional choice for predicting spin state ordering in small octahedral complexes as well as models of pyridinic and pyrrolic nitrogen environments expected in larger SACs. Using these guidelines, we determine Fe/N-doped graphene SAC model properties and reactivity as well as their sensitivities to DFT functional choice. Finally, we conclude with broad recommendations for computational modeling of open-shell transition metal single-atom catalysts.

Large dual spin-rectifying and high-efficiency dual spin-filtering in cyclooligomeric Mn-phthalocyanine dimer molecular junction

Abstract Based on density functional theory combined with nonequilibrium Green’s function technique, we investigate the spin-polarized transport properties of the cyclooligomeric Mn-phthalocyanine dimer and Fe-phthalocyanine dimer with carbon nanotube electrodes. The results show that the spin-polarized transport properties can be effectively tuned by changing the central transition metal atoms, and the Mn-phthalocyanine dimer system can exhibit large dual spin-rectifying and high-efficiency dual spin-filtering effects simultaneously under the [1, −1] magnetic configuration. The underlying mechanism is analyzed by spin-resolved transmission spectra, molecular projected self-consistent Hamiltonian orbitals and their spatial distribution.