State Transition Metal Research Articles

Stabilization of oxidation states in transition metals

Stabilization of oxidation states (OSs) for transition elements is considered. Distinctions between methods for stabilizing OSs in compounds in solution and in a solid state are discussed. The factors influencing stabilization are elucidated. Generation of high OSs of metals in alkali solutions is noticed. The specifics of stabilization of high OSs in solid compounds, including those in which mixed-valence effects are observed, and in solid matrix systems, in particular, in perovskite-related ones, are discussed. New data are cited concerning the stabilization of metal OSs in trans-dioxo complexes [MO2L4] 2 , where M = Mo, Tc, Ru, W, Re, or Os; and in the paramagnetic clusters that were discovered in neutral tungstates and hydroxo complexes of Rh, Ir, Pt, Pu, or Fe, and other elements in unusual high OSs.

Macrocyclic coordination chemistry

Highlights for 2005 include the characterisation of a number of unusual oxidation state transition metal compounds, such as non-heme Fe(V) and a stable Co(I) species. These compounds can be of particular relevance to oxidation states encountered in biochemical pathways. Another highlight is the exciting study involving protein crystallography that was carried out to determine the first structural example of a cyclam type complex interacting with the protein lysozyme. There have also been many advances in the areas of functional MRI agents and luminescent compounds for cellular sensing.

Stabilization of Highest‐Oxidation States of Transition Metals and the Induced Electronic Phenomena in Oxidic Perovskite Structures

AbstractIn oxides the stabilization of the highest oxidation states n+ of transition metals leads to the stabilization of strong covalent Mn+‐O bonds. Such very covalent bonds can induce electronic phenomena as spin transitions through the increase of the local crystal‐field energy (Dq) or in the specific case of an anisotropic eg1 electronic configuration some strong interactions with the perovskite structure (3D character, structural distortion…︁) can lead to a insulator→metal transition or a disproportionation phenomenon. [2Mn+⇔(M(n—ϵ)+)+(M(n+ϵ)+)] In order to illustrate such a topic, three unusual oxidation states have been selected: Co3+(d6), Fe4+(d4) and Ni3+(d7) in perovskite‐type structures.

Suppression of Angular Forces in Collisions of Non-S-State Transition Metal Atoms

Angular momentum transfer is expected to occur rapidly in collisions of atoms in states of nonzero angular momenta due to the large torque of angular forces. We show that despite the presence of internal angular momenta transition metal atoms interact in collisions with helium effectively as spherical atoms and angular momentum transfer is slow. Thus, magnetic trapping and sympathetic cooling of transition metal atoms to ultracold temperatures should be readily achievable. Our results open up new avenues of research with a broad class of ultracold atoms.

4p States and X-Ray Spectroscopy

The 4p states in transition metals and their compounds usually play minor roles on their physical quantities. Recent development of resonant x-ray scattering (RXS) at the K-edge of transition metals, however, casts light on the 4p states, because the signals on orbital and magnetic superlattice spots are brought about by the modulation in the 4p states. The 4p states are extending in solids and thereby sensitive to electronic states at neighboring sites. This characteristic determines the mechanism of RXS that the intensity on orbital superlattice spots are mainly generated by the lattice distortion and those on magnetic superlattice spots by the coupling of the 4p states with the orbital polarization in the 3d states at neighboring sites. Taking up typical examples for orbital and magnetic RXS, we demonstrate these mechanisms on the basis of the band structure calculation. Finally, we study the magnetic circular dichroism spectra at the K-edge, demonstrating that the same mechanism as the magnetic RXS is working.

Ozonolysis for the preparation of high-oxidation state transition metal complexes, and the X-ray crystal structure of Co(py) 2(OAc) 2(H 2O) 2

The use of an ozone/oxygen mixture for the preparation of pyridinecarboxylate complexes of silver(II), [AgL 2], (L = 2-, 3- and 4-pyridinecarboxylate; 2,3-, 2,4-, 2,5- and 3,5-pyridinedicarboxylate), Mn III(picolinate) 3, and the heteropolyanion [Co IIIW 12O 40] 5− are reported. It is shown that previous literature work on oxidation of alcohols with O 3/O 2 using Co(OAc) 2 are erroneous in that the latter has no catalytic effect. In the course of this work the title complex was made and its X-ray crystal structure determined.

Influence of cointercalated HF on the electrochemical behavior of highly fluorinated graphite

Highly fluorinated graphite was prepared at room temperature using high oxidation state transition metal complex fluoride (K2PdF6, K2MnF6, K2NiF6 or KAgF4) and elemental fluorine under pressure ((5.9–11.8) ×105 Pa) in anhydrous liquid HF (aHF). The composition of the fluorinated graphite samples ranged from C1.1F to C1.9F containing small amounts of HF. IR absorption spectra revealed that stage 1 phase of CxF contained several different phases with planar (sp2) and puckered (sp3) graphene layers. Electrochemical discharge of the fluorinated graphite showed that profile of discharge potential and discharge capacity varied depending on the amount of cointercalated HF. The CxF samples with less amounts of HF and HF2δ− had relatively flat discharge potentials and large discharge capacities. The discharge capacity reached 500–600 mAh/g in 1 mol/dm3 LiClO4–propylene carbonate solution at 25 °C. Chemical diffusion coefficients of Li+ ion in the intermediate discharge products were (4.4–13) ×10−12 cm2/s from impedance measurement.

Analysis of low oxidation state transition metal clusters by laser desorption/ionization time-of-flight mass spectrometry.

A variety of homonuclear and heteronuclear transition metal carbonyl clusters have been analyzed by ultraviolet laser desorption/ionization time-of-flight mass spectrometry. The spectra were recorded in negative and positive ion modes, using both linear and reflective techniques. A range of different clusters based on different nuclearities, geometries, and ligand types, which include hydrides, phosphines, nitriles, and cyclopentadienyl ligands and naked main group atoms, were studied. These experiments have allowed us to construct a detailed picture of the technique for the analysis of transition metal carbonyl clusters and their derivatives. In general, extensive reactions are observed, cluster aggregation reactions in particular, and from a comparison of the spectra obtained, some mechanistic inferences concerning the aggregation processes have been drawn.

Evidence for Penning Ionization in the Generation of Electronically Excited States of Transition Metal Cations by Laser Vaporization

We report the first evidence for the involvement of Penning ionization in the production of electronically excited state transition metal ions by laser vaporization in the presence of argon carrier gas. The use of argon as a carrier gas has been usually considered to be a more efficient cooling and quenching gas of the excited-state species. The current results show that this view is no longer valid and under certain conditions, where metastable argon atoms can be generated through the laser plasma vaporization, efficient production of electronic excited states of transition metal ions can result. The effect of Penning ionization is demonstrated for the production of excited states of Ni+, Nb+, and Pt+ ions through the coupling of the laser vaporization/ionization of metals and the mass-selected ion mobility techniques. The current finding has significant implications not only for understanding the mechanisms of ion production by laser-surface interactions, but also for developing new approaches to study state selected ion−molecule reactions.

On the catalytic activity of cluster anions in styrene hydrogenation: considerable enhancements in ionic liquids compared to molecular solvents

Many low oxidation state transition metal (carbonyl) clusters are salts and are highly soluble in ionic liquids. Since many are also effective catalysts/precatalysts for hydrogenation reactions, we have conducted a catalytic study of these compounds in ionic liquids and found that the activity of certain clusters to hydrogenate alkene substrates, when immobilised in ionic liquids, is up to 3.6-fold faster than that observed in organic solvents. Using high-pressure NMR it has been possible to trace the improvements in activity to the increased stability of the cluster species in the ionic liquid. The ionic liquid also gives rise to higher regioselectivity in the hydrogenation of cyclic dienes to monoenes compared to that observed in organic solvents.

Calculation of exchange coupling constants in solid state transition metal compounds using localized atomic orbital basis sets

The application of theoretical methods based on density functional theory using hybrid functionals and localized, atomic orbital type basis sets is shown to provide good estimates for exchange coupling constants in non-metallic, solid state transition metal compounds with relatively complex crystal structures. The accuracy of the calculated exchange coupling constants is similar to that previously obtained for dinuclear and polynuclear molecular compounds. As an application of this procedure, the magnetic properties of the high-temperature phase of CuGeO 3, the recently synthesized silver copper oxide Ag 2Cu 2O 3, and the family of M[N(CN) 2] 2 ( M=Cr(II), Mn(II), Fe(II), Co(II), Ni(II) and Cu(II)) compounds are analyzed via the computation of their most relevant exchange coupling constants.

Hyperfine Interactions and Magnetism of 3d Transition-Metal-Impurities in II–VI and III–V Compound-Based Diluted Magnetic Semiconductors

The electronic structure of II–VI and III–V compound-based diluted magnetic semiconductors is calculated based on the local density approximation (LDA) using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation. The magnetism of 3d transition-metal-atom-doped ZnO, ZnS, ZnSe, ZnTe, GaN, GaAs is investigated from first-principles. It is suggested that the double exchange mechanism stabilizes the ferromagnetism in these DMSs. In order to obtain microscopic information on the electronic structure of transition-metal-impurities in semiconductors, the hyperfine field of respective impurities in each host material is calculated. It is found that the agreement with the experimental values is not good, probably because the LDA is not sufficient to describe the core states of transition metals. However, it is suggested that the hyperfine fields clearly reflect the local magnetic moments for 3d impurities.

The Interactions of Low Oxidation State Transition Metal Clusters with DNA: Potential Applications in Cancer Therapy

A series of ruthenium and platinum clusters have been examined for DNA binding activity in methanol-water and water solutions. The clusters [H4Ru4(C6H6)4]2+and Ru3(CO)9(PTA)3(PTA=1,3,5-triaza-7-phosphatricyclo[3.3.1.1] decane) proved to be most effective; the former is believed to cross link DNA and the latter possibly intercalates. These clusters are highly water soluble and combined with their DNA damaging activity and size represent potential anticancer drugs.

Ab initio model potential embedded cluster calculation of the absorption spectrum of Cs2GeF6:Mn4+. Large discrepancies between theory and experiment

Ab initio model potential (AIMP) embedded cluster calculations of the absorption spectrum of Cs2GeF6:Mn4+ show unusually large dicrepancies with very accurate experimental data in electronic transition energies that involve an effective single excitation within the octahedral Mn(3d3) manifold: \documentclass{article}\pagestyle{empty}\begin{document}$t_{2g}^{3}\to t_{2g}^{2}e_{g}$\end{document}; good results are obtained only for lower, intraconfigurational excitations that can be approximately described as metal \documentclass{article}\pagestyle{empty}\begin{document}$t_{2g}^{3}\to t_{2g}^{3}$\end{document} transitions. The large discrepancies are in clear contrast with previous (good) results obtained for similar materials with the same methods; their origins have been investigated here. Peculiar to the Cs2GeF6:Mn4+ material, as compared to V2+ and Cr3+ isoelectronic impurity ions in similar fluoride hosts, are a shorter metal–ligand distance, larger crystal field, and higher oxidation state. Consequently, it has been considered that the usual, truncated configuration interaction treatments might be failing in describing the proper role of ligand-to-metal charge transfer and non-negligible F–F bonding interactions in the tight MnF62− cluster. Complete and restricted active space self-consistent-field (CASSCF, RASSCF), and averaged coupled pair aproximation (ACPF) calculations have been done in the embedded MnF62− cluster in an attempt to investigate these factors. The effects of the dn and charge transfer (CT) configuration interactions on the wave functions and energies of the ground and excited states are found to be very state dependent. In particular, the inclusion of different CT configurations in the multiconfigurational space leads to very large variations on the calculated transition energies. These large differential nondynamic interactions appear to be peculiar to this intermediate oxidation state transition metal system and suggests that dynamic correlation treatments based on larger multireferences than those that are nowadays computationally feasible are needed. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 623–635, 2000

Catalytic hydrogenation of carbon oxides – a 10-year perspective

There are major developments in the science and technology of catalyzed hydrogenation of carbon oxides. Experiments reveal the structural sensitivity of H 2 and CO bonding and the potential energy surface leading to dissociation. The theoretical descriptions of H 2, CO, and CO 2 bond activation become more complete and show good correlation with experiments. CO 2 hydrogenation to methanol dominates over Cu/ZnO catalysts, while methanol is produced primarily by the CO hydrogenation route over palladium-based systems. The oxide–metal interface sites are often better catalysts for C–O bond activation than the transition metal itself because of the Lewis acidity of certain high oxidation state transition metal oxides that resist reduction. Secondary reactions leading to oligomerization and polymerization of CH x fragments often occur. When higher molecular weight hydrocarbon production is desired, usually promoted binary alloy Co–Re and Co–Ru catalyst systems are utilized. Developments obtaining heat balance during steam reforming and in catalytic reactor technology have greatly improved the economics of producing liquid fuels from natural gas.

Low temperature thermoelectric power of amorphous Al–Y–TM alloys

The low temperature variation of thermoelectric power of amorphous alloys, Al85Y10TM5 (TM=Cu, Ni, Co, Fe), was measured. The thermoelectric power was measured by a differential method in a range 20–300 K. The characteristic `knee' of the thermopower is related to the electron–phonon mass enhancement. The Debye temperature and electron–phonon mass enhancement parameter determined from experimental data increase with increasing number of empty states in transition metals when passing from Cu to Fe. This unusual dependence is caused by a transfer of electrons from 3p of Al to 3d state of transition metals (Cu, Ni, Co, Fe) and 4d Y states. This attractive bonding force may be responsible for the density of nucleation centres during the crystallization process.

2H NMR Theory of Transition Metal Dihydrides: Coherent and Incoherent Quantum Dynamics

In this paper a simple phenomenological description of the effects of coherent quantum and incoherent mutual exchange of two deuteron nuclei in solid state transition metal complexes on their 2H NMR spectra is given. This description is based on the quantum-mechanical density matrix formalism developed by Alexander and Binsch. Only the nuclear spin system is treated quantum mechanically. The quantum exchange interaction in NMR is included in the nuclear spin Hamiltonian, and the interaction with the surrounding bath and incoherent exchange processes are treated as phenomenological rate processes described by rate constants. The incoherent exchange corresponds formally to 180° rotations or jumps of the D−D vector around an axis perpendicular to this vector and averages the different quadrupole splitting of the two deuterons. In principle the dideuteron pair will exist in several rovibrational states. However, if the interconversion among these states is fast, the dideuteron exchange can be described by an average exchange coupling or tunnel frequency X12 and a single average rate constant k12 of the incoherent exchange. It is shown that the incoherent exchange gives rise to a relaxation of rate −2k12 between coherences created between states of different symmetry. The 2H NMR line shape of a dideuteron pair in the solid state as a function of tunnel and incoherent exchange rate is studied numerically. For single crystals, the effects of coherent and incoherent exchange are strongly different, in particular if the rate constants are on the order of the quadrupole splitting. The spectra of nonoriented powder samples are more similar to each other. Nevertheless, our calculations show that there are still pronounced differences, which should allow the distinction between coherent and incoherent exchange even in nonoriented samples.

Changes in the oxidation state of transition metal containing MCM-41 during deNOx reactions

The oxidation state of Co, Pt and Rh containing MCM-41 after co-adsorption of NO, CH4, and O2 in the presence of water vapor was studied by in situ XANES experiments. All catalysts were oxidized after the adsorption of NO and reduced after the co-adsorption of NO and CH4. The presence of water vapor did not influence the oxidation/reduction of Rh/MCM-41 and Pt/MCM-41, while Co/MCM-41 was oxidized by the presence of H2O at reaction temperature.

Sinusoidal and Helical Spin Density Wave States in Transition Metals

The difference between sinusoidal and helical spin density wave states is discussed in reference to these states of chromium and fcc iron. The relative stability between these states against the magnitude of local magnetic moments is investigated. It is shown that for chromium the stable spin density wave state changes from a sinusoidal one to a helical one as the magnitude of local magnetic moments becomes large while for fcc iron the stable spin density wave state is always a helical one. The effect of charge modulation associated with sinusoidal spin density wave on the stability is further investigated. It is shown that the charge modulation stabilizes the sinusoidal spin density wave state. The difference in electronic structure between sinusoidal and helical spin density wave states is also elucidated.

Stabilization of high oxidation states in transition metals. An electrochemical and computational study of structurally comparable molybdenum and tungsten complexes

Molybdenum and tungsten chlorides, chloride oxides and chloride nitrides as well as thiazene and phosphazene metallacycles were studied by electrochemical methods in CH2Cl2. The different ligand systems have a profound influence on the redox potentials. The stabilization of high valency increases from the hexachloro to pentachlorooxo to chloronitrido complexes and the metallacycles. There is a systematic difference in the electrode potentials between molybdenum and tungsten redox couples. The geometries of the d1 and d0 species, optimized using density functional methods, are in good agreement with the available experimental data and trends in redox data are reproduced by the computed energy difference between the appropriate d1/d0 pair. The relative stability of the higher oxidation state is directly linked to the donor properties of the ligands present, and increases in the order: Cl6 < F6 < Cl5O < Cl4(N3S2) < Cl3(N3P2) < Cl4N.