One-electron contributions to the g-tensor for second-order Douglas-Kroll-Hess theory

The electric g-tensor is a central quantity for the interpretation of electron paramagnetic resonance spectra. In this paper, a detailed derivation of the 1-electron contributions to the g-tensor is presented in the framework of linear response theory and the second-order Douglas-Kroll-Hess (DKH) transformation. Importantly, the DKH transformation in the presence of a magnetic field is not unique. Whether or not the magnetic field is included in the required Foldy-Wouthuysen transformation, different transformation matrices and, consequently, Hamiltonians result. In this paper, a detailed comparison of both approaches is presented, paying particular attention to the mathematical properties of the resulting Hamiltonians. In contrast to previous studies that address the g-tensor in the framework of DKH theory, the resulting terms are compared to those of the conventional Pauli theory and are given a physical interpretation. Based on these mathematical and physical arguments, we establish that the proper DKH transformation for systems with constant magnetic fields is based on a gauge-invariant Foldy-Wouthuysen transformation, i.e., a Foldy-Wouthuysen transformation including the magnetic field. Calculations using density functional theory (DFT) are carried out on a set of heavy, diatomic molecules, and a set of transition-metal complexes. Based on these calculations, the performance of the relativistic calculation with and without inclusion of picture-change effects is compared. Additionally, the g-tensor is calculated for the Lanthanide dihydrides. Together with the results from the other two molecular test sets, these calculations serve to quantify the magnitude of picture-change effects and elucidate trends across the periodic table.     

Efficient and accurate approximations to the molecular spin-orbit coupling operator and their use in molecular g-tensor calculations

Approximations to the Breit-Pauli form of the spin-orbit coupling (SOC) operator are examined. The focus is on approximations that lead to an effective quasi-one-electron operator which leads to efficient property evaluations. In particular, the accurate spin-orbit mean-field (SOMF) method developed by Hess, Marian, Wahlgren, and Gropen is examined in detail. It is compared in detail with the "effective potential" spin-orbit operator commonly used in density functional theory (DFT) and which has been criticized for not including the spin-other orbit (SOO) contribution. Both operators contain identical one-electron and Coulomb terms since the SOO contribution to the Coulomb term vanishes exactly in the SOMF treatment. Since the DFT correlation functional only contributes negligibly to the SOC the only difference between the two operators is in the exchange part. In the SOMF approximation, the SOO part is equal to two times the spin-same orbit contribution. The DFT exchange contribution is of the wrong sign and numerically shown to be in error by a factor of 2-2.5 in magnitude. The simplest possible improvement in the DFT-SOC treatment [Veff(-2X)-SOC] is to multiply the exchange contribution to the Veff operator by -2. This is verified numerically in calculations of molecular g-tensors and one-electron SOC constants of atoms and ions. Four different ways of handling the computationally critical Coulomb part of the SOMF and Veff operators are discussed and implemented. The resolution of the identity approximation is virtually exact for the SOC with standard auxiliary basis sets which need to be slightly augmented by steep s functions for heavier elements. An almost as efficient seminumerical approximation is equally accurate. The effective nuclear charge model gives results within approximately 10% (on average) of the SOMF treatment. The one-center approximation to the Coulomb and one-electron SOC terms leads to errors on the order of approximately 5%. Small absolute errors are obtained for the one-center approximation to the exchange term which is consequently the method of choice [SOMF(1X)] for large molecules.     

Polyoxometalate tri-supported transition metal complexes containing mixed-valent transition metal ions

Three compounds, [AsMo₈V₆O₄₂][Cu(2,2?-bpy)₂]₂[Cu(2,2?-bpy)]·4H₂O (1), [PMo₈V₆O₄₂][Cu(2,2?-bpy)₂]₂[Cu(2,2?-bpy)]·3H₂O (2) and [PMo₈V₆O₄₂][Cu(2,2?-bpy)₂]₂[Cu(2,2?-bpy)]·3.5H₂O (3), have been synthesized under hydrothermal conditions and characterized by IR, UV–vis, XRD, TG, elemental analysis, and X-ray diffraction analysis. Single-crystal X-ray structure analysis reveals that 1 and 2 are isostructural and isomorphous, whereas 2 and 3 are polymorphs. Polymorphs of 1 have not been synthesized yet. The mixed-valent transition metal ion in 1–3 has been further confirmed by TG analyses. Catalytic properties of 1 and 2 have also been studied.     

Electrostatic and covalent contributions in the coordination bonds of transition metal complexes

To develop a molecular mechanics force field for modeling complexes of transition metals and organic ligands, the electrostatic and covalent contributions in the coordination bonds were investigated using quantum mechanical density functional theory and model complexes of glyoxal diimine and the 2+ cations of the first row transition metals. The VDD and Hirshfeld charges are found to be closely correlated with the extent of the electron transfer between the ligands and the cations. Assuming the electrostatic contribution can be represented by the atomic partial charges, the covalent contributions in the coordination bonds are estimated to be in a range of 54-92% for the systems calculated. A simple force field was parametrized to validate the partial charge representation.     

Polyoxometalates containing late transition and noble metal atoms

This review describes the state of the art in the field of polyoxometalates containing noble metal atoms (ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold). The structures of the various species are listed together with their applications (mainly in catalysis).     

Aqueous solutions of transition metal containing micelles

Incorporation of d- or f-block metals into ligand systems that renders a metal complex surface-active or drives its partitioning into surfactant phases enables the localisation of chemical functionality at interfaces. This article discusses a number of fundamental aspects of these interesting materials and examines potential applications.     

Calculation of electronic g-tensors for transition metal complexes using hybrid density functionals and atomic meanfield spin-orbit operators

We report the first implementation of the calculation of electronic g-tensors by density functional methods with hybrid functionals. Spin-orbit coupling is treated by the atomic meanfield approximation. g-Tensors for a set of small main group radicals and for a series of ten 3d and two 4d transition metal complexes have been compared using the local density approximation (VWN functional), the generalized gradient approximation (BP86 functional), as well as B3-type (B3PW91) and BH-type (BHPW91) hybrid functionals. For main group radicals, the effect of exact-exchange mixing is small. In contrast, significant differences between the various functionals arise for transition metal complexes. As has been shown previously, local and in particular gradient-corrected functionals tend to underestimate the "paramagnetic" contributions to the g-tensors in these cases and thereby recover only about 40-50% of the range of experimental g-tensor components. This is improved to ca. 60% by the B3PW91 functional, which also gives slightly reduced standard deviations. The range increases to almost 100% using the half-and-half functional BHPW91. However, the quality of the correlation with experimental data worsens due to a significant overestimate of some intermediate g-tensor values. The worse performance of the BHPW91 functional in these cases is accompanied by spin contamination. Although none of the functionals tested thus appears to be ideal for the treatment of electronic g-tensors in transition metal complexes, the B3PW91 hybrid functional exhibited the overall most satisfactory performance. Apart from the validation of hybrid functionals, some aspects in the treatment of spin-orbit contributions to the g-tensor are discussed.     

Performance of density functionals for first row transition metal systems

This article investigates the performance of five commonly used density functionals, B3LYP, BP86, PBE0, PBE, and BLYP, for studying diatomic molecules consisting of a first row transition metal bonded to H, F, Cl, Br, N, C, O, or S. Results have been compared with experiment wherever possible. Open-shell configurations are found more often in the order PBE0>B3LYP>PBE approximately BP86>BLYP. However, on average, 58 of 63 spins are correctly predicted by any functional, with only small differences. BP86 and PBE are slightly better for obtaining geometries, with errors of only 0.020 A. Hybrid functionals tend to overestimate bond lengths by a few picometers and underestimate bond strengths by favoring open shells. Nonhybrid functionals usually overestimate bond energies. All functionals exhibit similar errors in bond energies, between 42 and 53 kJmol. Late transition metals are found to be better modeled by hybrid functionals, whereas nonhybrid functionals tend to have less of a preference. There are systematic errors in predicting certain properties that could be remedied. BLYP performs the best for ionization potentials studied here, PBE0 the worst. In other cases, errors are similar. Finally, there is a clear tendency for hybrid functionals to give larger dipole moments than nonhybrid functionals. These observations may be helpful in choosing and improving existing functionals for tasks involving transition metals, and for designing new, improved functionals.     

Applications of transition metal complexes containing aminophosphine ligand to transfer hydrogenation of ketones

Hydrogen transfer reduction processes are attracting increasing interest from synthetic chemists in view of their operational simplicity. Reaction of [Ph₂PNHCH₂‐C₄H₃S] with [Ru(η⁶‐benzene)(µ‐Cl)Cl]₂, [Rh(µ‐Cl)(cod)]₂ and [Ir(η⁵‐C₅Me₅)(µ‐Cl)Cl]₂ gave a range of new monodendate complexes [Ru(Ph₂PNHCH₂‐C₄H₃S)(η⁶‐benzene)Cl₂], 1, [Rh(Ph₂PNHCH₂‐C₄H₃S)(cod)Cl], 2, and [Ir(Ph₂PNHCH₂‐C₄H₃S)(η⁵‐C₅Me₅)Cl₂], 3, respectively. All new complexes were fully characterized by analytical and spectroscopic methods. ¹H? ³¹P NMR, ¹H? ¹³C HETCOR or ¹H? ¹H COSY correlation experiments were used to confirm the spectral assignments. 1–3 are suitable catalyst precursors for the transfer hydrogenation of acetophenone derivatives. Notably [Ru(Ph₂PNHCH₂‐C₄H₃S)(η⁶‐benzene)Cl₂], 1, acts as an excellent catalyst, giving the corresponding alcohols in 98–99% yields in 30 min at 82 °C (TOF ≤200 h^?1) for the transfer hydrogenation reaction in comparison to analogous rhodium or iridium complexes. This transfer hydrogenation is characterized by low reversibility under these conditions. Copyright © 2011 John Wiley & Sons, Ltd.     

The role of additional solvents in transition metal complex catalyzed asymmetric reductions in ionic liquid containing systems

When ionic liquids (ILs) are employed as solvents for transition metal complex (TMC) catalyzed reductions, a second solvent can be added to increase the efficiency of the catalytic cycle and the solubility of the reactant in the IL phase. Two industrially relevant asymmetric hydrogenations, the enantioselective reductions of methyl 2-acetamidoacrylate with Rh-EtDuPHOS and methyl acetoacetate with Ru-BINAP, were performed in different catalytic systems including 1-butyl-3-methylimidazolium hexafluorophosphate/ tetrafluoroborate as ILs. Product separation and TMC recycling was performed by extracting the product from the reaction mixture. This can be accomplished by cooling the system, by adding an excess of the second solvent or by adding a third solvent. A high solubility of the second solvent in the IL catalytic phase favors the reaction activity, but can induce leaching of the IL and TMC.     

Preparation and properties of anionic transition metal complexes containing triheterocarbenium ions

Summary Thermally stable anionic tetracarbonylcobalt complexes containing triheterocarbenium ions, [Co(CO)₄]^–[cation]⁺, have been synthesized by the ion exchange reaction of [Co(CO)₄]^–PPN⁺ with the corresponding carbenium ions. Similar molybdenum complexes containing cyclopentadienyl and carbonyl ligands were also prepared. The complexes were characterized by elemental analyses and by i.r. and n.m.r. spectroscopies. The ionic structures of the complexes are confirmed on the basis of their large electric conductivities.     

三取代过渡金属钨镓杂多配合物的磁性及导电性能研究

合成了过渡金属三取代的钨镓杂多配合物:α-Na~nH~m[GaW~9O~3~7M~3(H~2O)~3]·xH~2O[M=Fe(Ⅲ), Cu(Ⅱ), Co(Ⅱ)]。通过元素分析、红外、紫外、^1^8^3W NMR以及热分析等手段进行了表征, 变温磁化率数据显示了配合物具有反铁磁特征。电导率测定结果表明所合成的配合物是迄今未见文献报道的过渡金属三取代杂多配合物高质子导体, 室温下GaW~9Fe~3的σ值可达10^-^3S·cm^-^1, 其特点是热稳定范围宽, 不易失水, 可望成为能实用化的杂多酸型固体电解质。     

Synthesis and reactivity of transition metal complexes containing halogenated boryl ligands

The synthesis and characterization of iron and manganese complexes containing the tetrachlorocatecholboryl (BO₂C₆Cl₄) ligand are reported. Crystallographic study of the methylcyclopentadienyl derivative (η⁵-C₅H₄Me)Fe(CO)₂BO₂C₆Cl₄ allows comparison of structure and bonding with related complexes of the type (η⁵-C₅R₅)Fe(CO)₂B(OR)₂ and reveals that the relative orientation of (η⁵-C₅H₄Me)Fe(CO)₂ and BO₂C₆Cl₄ moieties is influenced by intramolecular CH?O hydrogen bonding. Additionally, an alternative route to catecholboryl complexes from dilithiocatechol is reported.     

Electronic phase separation in transition metal oxide systems

Electronic phase separation is increasingly getting recognized as a phenomenon of importance in understanding the magnetic and electron transport properties of transition metal oxides. The phenomenon dominates the rare-earth manganates of the formula Ln(1-x)A(x)MnO(3)(Ln = rare earth and A = alkaline earth) which exhibit ferromagnetism and metallicity as well as charge-ordering, depending on the composition, size of A-site cations and external factors such as magnetic and electric fields. We discuss typical phase separation scenarios in the manganates, with particular reference to Pr(1-x)Ca(x)MnO(3)(x= 0.3-0.4), (La(1-x)Ln(x))(0.7)Ca(0.3)MnO(3)(Ln = Pr, Nd, Gd and Y) and Nd(0.5)Sr(0.5)MnO(3). Besides discussing the magnetic and electron transport properties, we discuss electric field effects. Rare-earth cobaltates of the type Pr(0.7)Ca(0.3)CoO(3) and Gd(0.5)Ba(0.5)CoO(3) also exhibit interesting magnetic and electron transport properties which can be understood in terms of phase separation.     

Peculiarities of cyanide-faujasite systems incorporating transition metal ions

Fragments of the corresponding hexacyanoferrates are formed when transition metal ions replace Na⁺ in faujasite containing hexacyanoferrate(II) encapsulated in the large pores.Translated from Teoreticheskaya i Èksperimental'naya Khimiya, Vol. 30, No. 6, pp. 332–336, November–December, 1994.     

Effect of gamma-radiation on Methanosarcina hydrogenase containing transition metal ions

Tracer studies using⁶⁵Zn and⁵⁸Co showed that of the four forms ofMethanosarcina hydrogenases, the A form has about 15% of acid labile zinc, while the hydrogenase D has about 50% of cobalt of the total bound activity in the cell and the other two forms B and C have neither zinc nor cobalt. However, all hydrogenases are known to contain iron, sulfur and probably nickel in trace amounts. All air-oxidized forms of hydrogenases catalyze the reduction of methyl viologen after a finite incubation period. The reduction is revealed by an increase in the absorption peak at 602 nm. On -irradiation, all the four hydrogenases changed to more stable oxidized forms, as indicated by an increase in the optical absorption in the visible region at 405 nm. The irradiated samples showed a greater time lag before they could reduce methyl viologen, the time lag increasing with the -dose. The irradiated enzymes could be reactivated by flushing with H₂. The zinc-bearing hydrogenase A alone appeared to be immune to -radiation in its ability to reduce methyl viologen. This may be due to the zinc having no unpaired electrons to interact with -radiation or the primary radiolytic products.     

Determination of spin-orbit coupling contributions in the framework of density functional theory

We present a noniterative method to calculate spin-orbit coupling by means of a theoretical approach that provides the use of the full Breit-Pauli operator. This method was applied to compute one and two-electron spin-orbit coupling contributions between singlet and triplet, and doublet and doublet states, respectively. These states have been represented by monodeterminantal wave functions and optimized using the PW91 gradient-corrected exchange-correlation functional and the hybrid B3LYP one. They have been supplied by the conventional density functional theory packages, and thus coupled by our spin-orbit coupling code. Different size basis sets have been employed and the obtained results have been compared with the corresponding ones provided by some of the already existing methods and with the experimental data. They have been found to be in good quantitative agreement.