Mixed-valence Metal Complexes Research Articles

First Principles Investigation of Stable LiMn2O4 Surfaces for Lithium-Ion Batteries

The spinel LiMn2O4 (LMO) has been widely studied as a lithium-ion battery cathode to replace the currently-used LiCoO2 materials due to its low cost and fast Li+ diffusivity.[1] However, a severe capacity fading during LMO cycling occurs because of the Mn dissolution, possibly initiating from the cathode surface. Thus, it is essential to completely understand the stable LMO surface structure to overcome current limitations and further to suppress the Mn dissolution in LMO. In this work, we investigate the effects of surface termination, slab reconstruction, relaxation schemes, magnetic ordering, and different U value for Mn to determine the relative stabilities of the LMO surfaces. We find that it is necessary to relax all atomic positions with symmetry-broken DFT calculations to fully capture the local variation of Mn-O and Li-O polyhedral clusters to obtain the most consistent surface energies. We finally comment on the possible implication of our work on the future surface studies of the mixed-valence transition metal complex oxides such as the LMO system. Acknowledgement: This work has been supported by Northwestern-Argonne Institute of Science and Engineering (NAISE) and by The Dow Chemical Company. Reference: [1] R. J. Gummow, A. Kock, M.M. Thackeray, Solid State Ionics 1994, 69, 59. Figure 1

Mixed-valent, heteroleptic homometallic diketonates as templates for the design of volatile heterometallic precursors.

A novel series of mixed-valent, heteroleptic transition metal diketonates that can be utilized as prospective single-source precursors for the low-temperature preparation of oxide materials are reported. The first mixed-valent iron β-diketonates with different FeIII/FeII ratios have been synthesized by applying the mixed-ligand approach. Based on nearly quantitative reaction yields and analysis of iron-oxygen bonds, these compounds were formulated as [FeIII(acac)3][FeII(hfac)2] (1) and [FeII(hfac)2][FeIII(acac)3][FeII(hfac)2] (2). In the above heteroleptic complexes, the Lewis acidic, coordinatively unsaturated FeII centers chelated by two hfac (hexafluoroacetylacetonate) ligands with electron-withdrawing substituents maintain bridging interactions with oxygen atoms of electron-donating acac (acetylacetonate) groups that chelate the neighboring FeIII atoms. Switching the ligands on FeIII and FeII atoms in starting reagents resulted in the instant ligand exchange between iron centers and in yet another polynuclear homometallic diketonate [FeII(hfac)2][FeIII(acac)2(hfac)][FeII(hfac)2] (3) that adheres to the same bonding pattern as in complexes 1 and 2. The proposed synthetic methodology has been extended to design heterometallic diketonates with different M : M' ratios. Homometallic parent molecules have been used as templates to obtain heterometallic mixed-valent [FeIII(acac)3][MnII(hfac)2] (4) and [NiII(hfac)2][FeIII(acac)3][NiII(hfac)2] (5) complexes. The combination of two different diketonate ligands with electron-donating and electron-withdrawing substituents was found to be crucial for maintaining the above mixed-valent heterometallic assemblies. Theoretical investigation of two possible "isomers", [FeIII(acac)3][MnII(hfac)2] (4) and [MnIII(acac)3][FeII(hfac)2] (4') provided an additional support for the metal site assignment giving a preference of 9.78 kcal mol-1 for the molecule 4. Heterometallic complexes obtained in the course of this study have been found to act as effective single-source precursors for the synthesis of mixed-transition metal oxide materials M x M'2-xO3 and M x M'1-xO. The title highly volatile precursors can be used for the low-temperature preparation of both amorphous and crystalline heterometallic oxides in the form of thin films or nanosized particles that are known to operate as efficient catalysts in oxygen evolution reaction.

Chemisorption and Electrochemical Activity of Thiophenols at Well-Defined Pd(111) Surfaces: Studies by LEED, AES, HREELS, and Electrochemistry

The chemisorption and electrochemical activity of 2 ,5 -dihydroxythiophenol (DHT) and 2-(8mercaptooctyl)-1,4-benzenediol (DHOT) on well-defined Pd(111) surfaces were studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), high resolution electron energyloss spectroscopy (HREELS), and electrochemistry (EC). Results confirm that DHT is chemisorbed in two discrete orientations such that at low concentrations, DHT is oxidatively bound to the surface through the diphenol and mercapto groups as quinonoid and S moieties, respectively, whereas at high concentrations, the molecule is coordinated oxidatively through the –SH group in a vertical S- η 1

外部刺激に応答する金属錯体の自己集合とそのダイナミックな構造制御

外部刺激に応答する金属錯体の自己集合とそのダイナミックな構造制御

Self‐Assembly of Tubular Microstructures from Mixed‐Valence Metal Complexes and Their Reversible Transformation by External Stimuli

Self‐Assembly of Tubular Microstructures from Mixed‐Valence Metal Complexes and Their Reversible Transformation by External Stimuli

Soliton-induced optical absorption of halogen-bridged mixed-valence binuclear metal complexes

Employing the one-dimensional single-band extended Peierls-Hubbard model, we investigate optical conductivity for solitonic excitations in halogen-bridged binuclear metal (MMX) complexes. Photoinduced soliton absorption spectra for MMX chains possibly split into two bands, forming a striking contrast to those for conventional mononuclear metal (MX) analogs, due to the broken electron-hole symmetry combined with relevant Coulomb and/or electron-phonon interactions.

Electron-Spin Dynamics and Structures in Halogen-Bridged 1-D Metal Complexes Studied by Solid NMR

Abstract Magnetic properties characteristic in 1-D systems were investigated in halogen-bridged mixed-valence and averaged-valence metal complexes by applying magnetic resonance techniques. Neutral-solitons and polarons formed from impurity amounts of paramagnetic Pt3+ and Pd3+ sites in 1-D chains were sensitively detected by the NMR relaxation measurement. Their rapid 1-D diffusion comparable to that in trans-polyacetylene was revealed in [MII(en)2][MVIBr2(en)2](ClO4)4 (M: Pt, Pd). From 13C NMR spectra and 1H NMR relaxation studies, [NiIIIX(chxn)2]X2 (X: Cl, Br) was shown to be in an averaged-valence state forming S = 1/2 1-D Heisenberg antiferromagnetic chains. [NiBr(chxn)2]Br2 revealed to have the largest exchange interaction so far reported, was expected to transform into a new state at low-temperatures below 100 K. From studies on thermal motions and phase transitions associated with the 1-D chain-structure in [Pt(en)2][PtX2(en)2](ClO4)4 (X: Cl, Br), the presence of the dynamic disorder of the Pt(en)2 chelate-ring conformation in a highly asymmetric double-well potential undetected by the X-ray diffraction was detected by the NMR relaxation measurement.

Ultrafast dynamics of lattice relaxed states in quasi-one-dimensional MX-chain compounds

Ultrafast relaxation processes of photoexcited states in a quasi-one-dimensional halogen-bridged mixed-valence metal complex [Pt(en) 2][Pt(en) 2I 2](SO 4) 2.6H 2O have been investigated by femtosecond transient absorption spectra measurements. A photoinduced absorption band caused by non-thermalized self-trapped excitons has been found. Oscillation of the absorption band caused by the symmetric Pt–I stretching mode of the self-trapped exciton with a frequency of 103 cm −1 has been observed. The rise time of a polaron absorption band has been measured to be 1.2 ps. This corresponds to the decay time, 1.1 ps, of a photoinduced absorption band which oscillates with a frequency of 74 cm −1.

Near-infrared absorbing organic materials

Abstract Organic solids and polymers that absorb in the near-infrared (NIR) region (1000–2000 nm) represent a class of emerging materials and show a great potential for use in photonics and telecommunications. The radical anions of stacked aromatic imides, fused phorphyrin arrays, polythiophenes, sandwich-type lanthanide bisphthalocyanines, semiquinones, and mixed-valence dinuclear metal complexes are a few known examples of NIR-absorbing organic materials. Most of these NIR-absorbing materials are also electro- chemically active or electrochromic (EC). This brief review covers several types of NIR-absorbing organic materials and discusses their potential for applications in EC variable optical attenuators (VOAs).

Calorimetric Study of a Halogen-Bridged MMX Chain Complex Having Alkyl Chains, Pt2(n-PrCS2)4I (n-Pr = Propyl Group)

The heat capacity of a halogen-bridged quasi-one-dimensional (Q1D) mixed-valence binuclear metal complex (the so-called MMX chain), Pt2(n-PrCS2)4I, was measured by adiabatic calorimetry. A higher order phase transition due to structural disorder was observed at 209 K. Another first-order phase transition having a large tail on the low-temperature side was detected at 358.8 K. Their enthalpy and entropy of transition were determined and analyzed. As evidenced by the magnitude of the entropy of transition, similar structural disorder occurs at the corresponding phase transition in Pt2(n-PrCS2)4I and Pt2(n-BuCS2)4I, but the shape of thermal anomaly is largely different between the two. The magnitude of the entropy of transition shows that the change of both structural disorder and electronic states occurs simultaneously in a gradual manner on the low-temperature tail of the thermal anomaly at 358.8 K. The change in entropy assignable to the Q1D electron system was estimated as about 2.3 J K-1 mol-1.

Vibrational Mode Coupling to Ultrafast Electron Transfer in [(CN)5OsCNRu(NH3)5]-Studied by Femtosecond Infrared Spectroscopy

We report studies of the coupling of vibrational excitations to photoinduced electron transfer in a mixed-valence transition metal complex [(CN)5OsCNRu(NH3)5]-. Static metal−metal charge transfer (MMCT) spectra, infrared (IR) absorption spectra at different temperatures, resonance Raman spectra, and polarized light transient spectra were taken to reveal the solvent and vibrational coupling to reverse electron transfer. The measured reverse electron transfer rates are greater than 3 × 1012 s-1. We observe excitations in the non-totally symmetric vibrational mode accompanying the reverse electron transfer process in both formamide (FA) and deuterium oxide (D2O) solutions. A simulation of the spectral dynamics in FA and D2O solutions is presented using a kinetic model for vibrational excitation and relaxation. The simulation includes the effect of solvent heating through coupling a low-frequency mode to a medium-frequency oscillator. The vibrational relaxation times of trans CN obtained from the spectral fits show faster vibration relaxation in D2O than FA, reflecting an overlap of the IR absorption spectra of the solvent and the CN vibrations.

Intrinsic inhomogeneity due to nonlinearity

Presence of strong nonlinearity in a material, either explicit, or effective (resulting from the interaction of coupled degrees of freedom), can lead to intrinsic inhomogeneity even in a periodic extended system. We briefly discuss two examples where such a breaking of the translational symmetry due to nonlinearity (i) determines the optical features (in particular the resonance Raman scattering spectra) of a quasi-one-dimensional halogen-bridged mixed-valence transition metal complex, and (ii) enhances the trapping of a charge in a model of DNA, which is likely to affect the long-range transport properties in this biomolecule.

Interfacial electron transfer in FeII(CN)6 4- -sensitized TiO2 nanoparticles: a study of direct charge injection by electroabsorption spectroscopy.

Electroabsorption (Stark) spectroscopy has been used to study the dye sensitized interfacial electron transfer in an Fe(II)(CN)(6)(4)(-) donor complex bound to a TiO(2) nanoparticle. The average charge-transfer distance determined from the Stark spectra is 5.3 A. This value is similar to the estimated distance between the Fe(II) center of the complex and the Ti(IV) surface site coordinated to the nitrogen end of a bridging CN ligand in (CN)(5)Fe(II)-CN-Ti(IV)(particle). This finding suggests that the electron injection is to either an individual titanium surface site or a small number of Ti centers localized around the point of ferrocyanide coordination to the particle and not into a conduction band orbital delocalized over the nanoparticle. The polarizability change, Tr(Deltaalpha), between the ground and the excited states of the Fe(II)(CN)(6)(4)(-)-TiO(2)(particle) system is approximately 3 time larger than normally observed in mixed-valence dinuclear metal complexes. It is proposed that the large polarizability of the excited state increases the dipole-moment changes measured by Stark spectroscopy.

Calorimetric study of the halogen-bridged mixed-valence binuclear metal chain complexPt2(n−BuCS2)4I(Bu=butyl chain)

Heat capacity of a halogen-bridged quasi-one-dimensional mixed-valence binuclear metal complex (the so-called $\mathrm{MMX}$ chain), ${\mathrm{Pt}}_{2}(n\ensuremath{-}\mathrm{Bu}{\mathrm{CS}}_{2}{)}_{4}\mathrm{I},$ was measured by adiabatic calorimetry. First-order phase transitions were observed at 213.5 K and 323.5 K. For the former, the enthalpy and entropy of transition were $4.29{\mathrm{kJ}\mathrm{}\mathrm{mol}}^{\ensuremath{-}1}$ and $20.09{\mathrm{J}\mathrm{}\mathrm{K}}^{\ensuremath{-}1}{\mathrm{mol}}^{\ensuremath{-}1},$ respectively. Those of the latter were $2.41{\mathrm{kJ}\mathrm{}\mathrm{mol}}^{\ensuremath{-}1}$ and $7.46{\mathrm{J}\mathrm{}\mathrm{K}}^{\ensuremath{-}1}{\mathrm{mol}}^{\ensuremath{-}1},$ respectively. Another thermal anomaly probably due to a higher-order phase transition was detected at 114 K. The magnitude of the entropy of transition shows that, upon heating, the butyl chains in one-third complexes in crystalline ${\mathrm{Pt}}_{2}(n\ensuremath{-}\mathrm{Bu}{\mathrm{CS}}_{2}{)}_{4}\mathrm{I}$ are changed from an ordered state to a disordered state through the phase transition at 213.5 K, and resume the ordered state from this disordered state at 323.5 K. The transition at 213.5 K involves a ``spin-Peierls'' contribution beyond the structural one.

ChemInform Abstract: Long‐Distance Intervalence Electron Transfer

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Visible to Near-Infrared Ultrafast Spectroscopy of a Quasi-One-Dimensional Halogen-Bridged Mixed-Valence Metal Complex [Pt(en)2][Pt(en)2Cl2](ClO4)4

Ultrafast relaxation dynamics is studied in a quasi-one-dimensional halogen-bridged mixed-valence metal complex [Pt(en)2][Pt(en)2Cl2](ClO4)4 by pump−probe spectroscopy. A photoinduced absorption spectrum is measured in a broadband energy region of 0.6−2.6 eV, applying optical parametric amplification. The measurement is performed at both 293 and 4 K. The photoinduced absorption spectrum consists of the three contributions with different time and temperature dependence. One is the absorption band ascribed to the self-trapped exciton (STE), and the second is that due to the spatially uncorrelated neutral-soliton pair. Just after the photoexcitation, the STE is in a nonequilibrium state, and the time required for the thermalization is evaluated to be about 1 ps. The formation time of the third one is coincident with the thermalization time of the STE, and it is attributed to the spatially confined neutral-soliton pair.

UlTRAFAST LATTICE RELAXATION DYNAMICS OF EXCITON IN A QUAISI-1-D METAL-HALOGEN COMPLEX

A quasi-one-dimensional halogen-bridged mixed-valence metal complex is studied by time-resolved pump and probe spectroscopy with sub-5 fs time resolution. Two kinds of oscillatory signals are observed, which are attributed to the wave packet motions both in an electronic ground state and in a self-trapped exciton (STE) state. The onset of the wave packet motion is found to be delayed by about 50 fs, comparing with the ideal wave packet in the electronic excited state. The delay reflects the thermalization process in a free exciton (FE) state and a lattice relaxation process from FE to STE states.

Long-distance intervalence electron transfer

Long-distance intervalence electron transfer can be monitored by the quantitative study of intervalence transitions in series of binuclear mixed-valence metal complexes, with metal–metal distances reaching 25 A. This allows the determination of the electronic coupling Vab, and its rate of decay with distance. Comparisons are made between several series of compounds, and also with organic mixed-valence systems. The decay is exponential with a coefficient varying in a relatively narrow range (0.08 to 0.12 A−1). Applications for molecular switching are discussed.

Ultrafast relaxation processes of solitons, polarons, and self-trapped excitons in MX-chain

Ultrafast relaxation processes of photoexcited states of a quasi-one-dimensional halogen-bridged mixed-valence metal complex [Pt(en) 2][Pt(en) 2Cl 2](PF 6) 4 have been investigated by femtosecond transient absorption spectra measurements. It has been found that a considerable part of the neutral soliton pairs is generated directly from photogenerated charge transfer excitons. Charged soliton pairs have been found to be generated from free electron–hole pairs. The rise time and the decay time of the charged soliton absorption band are 2.2 and 6.5 ps, respectively. The polaron pairs are generated from free electron–hole pairs within time resolution.

Wave packet dynamics in a quasi-one-dimensional metal-halogen complex studied by ultrafast time-resolved spectroscopy.

An ultrafast optical response is studied in a quasi-one-dimensional halogen-bridged mixed-valence metal complex [Pt(en)(2)] [Pt(en)2I2] (ClO4)(4) with ultrafast time resolution. Wave packet motions both in the ground and self-trapped exciton (STE) states are observed as oscillatory modulations in the time-resolved reflectivity. The wave packet motion on the STE potential surface begins after about 50 fs with respect to the photoexcitation. This delay is attributed to the lattice relaxation from the free exciton state to the STE state.