Single crystal EPR study of Mn(II)-doped magnesium potassium Tutton's salt

Single crystal EPR studies of Mn(II)-doped magnesium potassium Tutton's salt, MgK₂(SO₄)₂.6H₂O, was studied at room temperature. The spin-Hamiltonian parameters obtained are: g=2.0036(3), A = −96(3), D = 350(5), a = 14(2) and F = −5(1) (A, D, a and F are in units of 10⁻⁴ cm⁻¹). The tetragonal distortion axis corresponds to one of the MgO bond directions. The zero-field splitting parameter (D) shows a linear dependence in the temperature range 77–370 K. The percentage of covalency of the MnO bond has been estimated to be 8 per cent.     

Two-dimensional catena–polymer [[[bis (µ-aqua)(bis (µ-cpiap-K2O1:O2)sodium(I))] (µ-chlorido) bis(µ-cpiap-K5N1:O1:O2:O3,O3′)dicopper(II)]bis(µ-aqua)(aqua)bis(cpiap-K2O1,O2)copper(II)] hexa hydrate

The new title two-dimensional hetero-tetra nuclear Cu₃–Na coordination polymer {[NaCu₃Cl(cpiap)₂(H₂O)₃]_n·6nH₂O} (1) consists of crystallographically two-independent copper(II) centers, each bridged by a sodium cation through carboxylate-oxygen of the deprotonated H₃cpiap ligand (H₃cpiap = 2-(carboxyphenyl)iminoaceticpropanoic acid) to Cu^II (2) and Cu^II (2⁻) cations, and through water molecules to Cu^II (1) cation. Cu^II (2) and Cu^II (1) cations are bridged by carboxylate-oxygen atoms of the ligand in a syn-anti mode which, alternate regularly within the chain being bridged by a tetra coordinated sodium cation. Each Cu^II (2) and Cu^II (2⁻) cation in (1) is in an octahedral environment formed by four carboxylate-oxygens from two cpiap³⁻ ligands, one nitrogen atom and a bridging chloride atom. Cu^II (1) cation is in a square pyramidal environment formed by three water molecules and two carboxylate-oxygens from two cpiap³⁻ ligands. The ligand acts simultaneously as monodentate and tridentate toward Cu^II (1) and Cu^II (2) cations respectively. The lattice water molecules involved in OH···O hydrogen bonding are situated in the void spaces between layers. The zigzag chains, which run along the b-axes further construct three-dimensional metal-organic framework via hydrogen bonding and weak face-to-face π-π interactions. Weak CH···O interactions are also present.     

Jahn-Teller activity in eight-coordinate Oh fx systems

The angular overlap model is used for the calculation of the Jahn-Teller coupling constants for O_h, MX₈, f^x systems in the (LSJM_J) basis. For the 5f¹ anion, [UF₈]^3?, the Γ₈ ground state is predicted to show substantial Jahn-Teller activity for coupling to the τ_2g bending mode, with E_JT comparable to ω, a result consistent with the available magnetic susceptibility data.     

Spectral and voltammetric studies on titanium substituted Keggin-type heteropolyanions

Bonding and redox properties of mono- and dititanium substituted 12-heteropolytungsto-phosphates and their complexes with bivalent Mn, Co, Ni and Cu are studied by UV-Vis, ESR and cyclicvoltammetric techniques. The heteropoly blue analogue [Ti^IIITi^IVW₁₀PO₄₀]⁸⁻ is unstable and its presence in solution is proved by optical and ESR spectroscopy. ESR results reveal that the odd electron is trapped on the titanium atom at 77 K. The optical electronegativity of Ti(III) in this heteropoly blue is estimated to be 2.4 from the intervalence charge transfer (IVCT) bands. X-ray powder diffraction results show that the complexes K₅M^II[Ti₂W₁₀PO₄₀].xH₂O(M^II = Cu or Mn) are isomorphous with K₇[Ti₂W₁₀PO₄₀].6H₂O. Electronic spectroscopy indicates very high symmetry around the bivalent metal ion M in the K₅M^II[Ti₂W₁₀PO₄₀].xH₂O complexes. This high symmetry is confirmed in K₅Cu[Ti₂W₁₀PO₄₀] by the observation of dynamic Jahn-Teller (JT) distortion at 300 K and static JT distortion at 77 K.     

Exact relation between the spectrum and the eigenfunctions of a generalized Jahn-Teller hamiltonian for different parameter values

A generalized E ? ? Jahn-Teller hamiltonian with a parameter δ (δ = ?1/4 in the JT case) is studied. As in the Jahn-Teller case, the angular momentum is a constant of the motion with the eigenvaluesj + 1/2 where j is a non-negative integer. For j fixed, consider two values δ₁ = ?j/2 ?1/2 and δ₂ = ?gd₁ ?1/2. The spectrum and the eigenfunctions for δ₂ are completely determined by the spectrum and the eigenfunctions for δ₁.     

Coordination-Induced N−H Bond Splitting of Ammonia and Primary Amine of CuI−MOFs

We report a porous three-dimensional anionic tetrazolium based Cu^I−MOF 1 , which is capable of cleaving the N−H bond of ammonia and primary amine, as well as the O−H bond of H₂O along with spontaneous H₂ evolution. In the gas-solid phase reaction of 1 with ammonia and water vapor, Cu^I−MOF 1 was gradually oxidized to NH₂−Cu^II−MOF and OH−Cu^II−MOF, through single-crystal-to-single-crystal (SCSC) structural transformations, which was confirmed by XPS, PXRD and X-ray single-crystal diffraction. Density functional theory (DFT) demonstrated that Cu^I−MOF could lower N−H bond dissociation free energy of ammonia through coordination-induced bond weakening and promote H₂ evolution by the reduction potential of 1 . To our knowledge, this is the first example of MOFs that activate ammonia and amine in gas-solid manner.     

Coordinating behaviour of a new pyridylhydrazone; tris-complexes of manganese(II), cobalt(II), nickel(II), copper(II) and zinc(II) with 2-pyridylcarbaldehyde-N,N-dimethylhydrazone

Summary The preparation and characterization oftris-complexes of Mn^II, Co^II, Ni^II, Cu^II and Zn^II with a new pyridylhydrazone, 2-pyridylcarbaldehyde-N,N-dimethylhydrazone (pch), are described. In all the complexes pch behaves as a bidentate ligand binding through the pyridine and azomethyne nitrogen atoms. The complexes appear to be monomeric, high spin six-coordinate, and a distorted octahedral stereochemistry around the metal is suggested. The e.p.r. results for both Cu^II compounds indicate a mainly d_x ²–y² ground state with a static Jahn-Teller distortion, whilst for the Mn^II complex the e.p.r. data indicates a very low symmetry for the MnN₆ polyhedron.     

Titanates de cuivre substitués à structure bixbyite: Les composés Cu1−xTi1−xFe2xO3 (0.15 ≤ x ≤ 0.33)

A new bixbyite family, Cu_1?xTi_1?xFe_2xO₃ (0.15 ≤ x ≤ 0.33) has been synthesized and characterized. The unit cell is cubic: a ～ 9.40Å. The X-ray powder diffraction study shows up an isotypism with the (Fe, Mn)₂O₃ compounds. There is a disordered distribution of Cu^II, Ti^IV, and Fe^III over the two cyrstallographic sites: P_I and P_II. P_II is highly distorted (two long MO distances) by the Jahn-Teller effect of Cu^II. The bixbyite structure is described in terms of polyhedra arrangement, as a particular case of the CM₂O₃ family. The cation packing is discussed in relation with the existence of the bixbyite structure for the Cu_1?xTi_1?xFe_2xO₃ compounds. The electrical properties (σ ～ 10^?5(Ω cm)^?1 for x = 0.286 at room temperature) show an electron conduction with probably a hopping mechanism.     

Cyclovoltammetric and spectroscopic characterization of optically active cobalt(II) and copper(II) complexes with the Schiff base derived from (N,N′)-(1R,2R)-(−)-cyclohexylenediamine and 2-hydroxyacetophenone

The Schiff base prepared by reacting (–)-(1R,2R)-1,2-cyclohexanediamine with 2-hydroxyacetophenone was used as a ligand for Co^II and Cu^II. The coordination compounds were studied by u.v.–vis. absorption and by circular dichroism (c.d.) spectroscopy in solution. The complexes are four-coordinated in a slightly distorted square planar symmetry. The distortion from planarity is a main factor influencing the chiral surroundings of the metal ion. The d–d and c.t. transitions are consistent with the observed distortion, which arises from intramolecular interactions between the methyl groups attached to the Schiff base imine carbon and hydrogen atoms of the cyclohexane ring. The electrochemical properties of the Co^II and Cu^II complexes were observed in MeCN but investigations revealed weaker oxygen activation than of Co^II analogue with salicylaldehyde. The Cu^II complex is reduced in H₂O to Cu^I which disproportionates to Cu^II and Cu⁰.     

Investigation of EPR parameters and the local structure of the rhombic Cu2+ center in zinc fluoride

Based on a cluster approach, the local structure of the rhombic Cu²⁺ center in a ZnF₂ crystal is investigated by using the high-order perturbation formulas of electron paramagnetic resonance (EPR) parameters g factors g _i (i = x, y, z) and hyperfine structure constants A _i in rhombically compressed octahedral symmetry. According to the studies, the local axial distortion angle Δα′(≈ 2.9°) in the impurity center is found to be by ～2.5° larger than the host value, which is characterized as a contraction and stretching of the parallel and perpendicular bond lengths by about 0.116 Å and 0.058 Å respectively. This results in a more compressed ligand octahedron because of the Jahn-Teller effect via the interaction of the vibrations of impurity-ligand bonds in the Cu²⁺ center. The reasonableness of the results is also discussed.     

Jahn–Teller coupling and the influence of strain in Tg and Eg ground and excited states – A ligand field and DFT study on halide MIIIX6 model complexes [M = TiIII–CuIII; X = F−, Cl−]

In contrast to well established experimental results of vibronic coupling effects in octahedral dⁿ complexes with E_g ground states (Cu²⁺, Ag²⁺; Cr²⁺, Mn³⁺ etc.), not much useful material is available for the Jahn–Teller (JT) effect in orbital triplet ground states. The present study is concerned with this deficiency, providing data for octahedral halide model complexes with 3dⁿ cations – in particular for Ti^III, V^III and high-spin Co^III, Ni^III with T_2g and T_1g ground states, which involve, to first-order, solely splitting of the π-antibonding t_2g MOs. Besides experimental results – structural and spectroscopic, mainly from d–d spectra – data from computations are needed for a quantitative treatment of the T_g ? (?_g + τ_2g) vibronic interaction as well as in the E_g ? ?_g coupling case (Mn^III, low-spin Ni^III); DFT was the method of choice, if only critically selected outcomes are utilised. The theoretical bases of the treatment are the dⁿ ligand field matrices in O_h, extended by the inclusion of lower-symmetry distortion parameters, and the conventional theory of vibronic coupling. Caution is needed when classifying the effects of interelectronic repulsion; DFT does not reproduce the magnitudes of the Racah parameters B, C, as deduced from the d–d spectra, properly – the presumed reasons are analysed. DFT even allows one to deduce reliable vibronic coupling constants via the analysis of orbitally degenerate excited states (Cr^III, ⁴A_2g ground state). The group-theoretical analysis of the interaction with the JT-active ?_g and τ_2g modes yields D_4h, D_3d and D_2h as the possible distortion symmetries in the case of a T_g ground state. The DFT-calculations give clear evidence, that the D_4h stationary points represent the absolute minima in the T_g ? (?_g + τ_2g) potential surface – in agreement with experiment, where available. For the first time, vibronic coupling constants, characterising JT splitting of ground and excited T_g states, can be presented for trivalent 3dⁿ cations in octahedral halide ligand fields. They turn out to be smaller by a factor of almost 3 in comparison to those, which determine the coupling in σ-antibonding e_g MOs.The tetragonal splitting of T_g states is typically only small, around 0.1 eV, and suggests that strain influences from a specific ligand arrangement and/or the presence of different ligands may modify the potential surface considerably. We have studied such effects via compounds A^IM^IIIF₄, where an elastic strain induced by the host structure, and a binding strain, due to the simultaneous existence of (largely) terminal and of bridging ligands, are active. A novel strain model, in its interplay with JT coupling, is proposed and applied – using energies from the d–d spectra, structural results and data from DFT.Chloride complexes are only known for Ti^III to Fe^III; the rather small electronegativity already of Co^III suggests a reducing ligand-to-metal (3dⁿ) electron transfer for n ≥ 6. Similarly, the low-lying ligand-to-metal charge transfer bands in the d–d spectra of the Cu^IIIF₆^3? complex and the reduced T_g ? ?_g coupling strength suggest a pronounced covalency of the Cu^III–F, and, even more distinctly, of the Cu^III–O bond, which is of interest for superconductivity. The Ni^IIIF₆^3? polyhedron possesses a low-spin configuration in the elpasolite structure. The spectroscopic evidence and the DFT data indicate, that the minimum positions of the alternative _a²A_1g(_a²E_g) and _a⁴A_2g (_a⁴T_1g) potential curves are only ≤0.02 eV apart, giving rise to interesting high-spin/low-spin phenomena. It is the strong E_g ? ?_g as compared to the T_1g ? ?_g coupling, which finally stabilises a spin-doublet ground state in D_4h.We think, that the selected class of solids is unique particularly for the study of Jahn–Teller coupling in T ground states, with model character for other systems. In our overview a procedure is sketched, which uses reliable computational results (here from DFT) for supplementing incomplete experimental data, and presents – on a semiquantitative scale – convincing statements, consistent with chemical intuition. It is also a pleading for ligand field theory, which rationalises d-d spectra in terms of chemical bonding; though the latter spectra provide frequently only rather coarse information, their assistance in the energy analysis is crucial.     

Struktur und Bindung in Übergangsmetall-Fluoriden MIIMeIVF6: Neutronenbeugungs-Strukturuntersuchungen an CaSnF6, FeZrF6, und CrZrF6

Compounds M^IIMe^IVF₆ requently undergo phase transitions from the cubic ordered ReO₃ to the trigonal LiSbF₆ structure when lowering the temperature. In case of a strongly Jahn-Teller unstable cation in the M^II position additional phases may occur. Results of powder neutron-diffraction studies on CaSnF₆, FeZrF₆, and CrZrF₆ at different temperatures are reported. The high-temperature phases have the space group Fm3m; the F^? ligands are either statistically displaced from the M^IIMe^IV directions or undergo a strong thermal motion perpendicular to these directions ($\text{?M}{}^{\mathrm{<mtext>II</mtext>}}\text{}\text{FMe}{}^{\mathrm{IV}}$: 165–180°). The thermal ellipsoids of the CrF bonds are strongly indicative of a dynamical Jahn-Teller effect in addition. In the low-temperature phases of CaSnF₆ and FeZrF₆ (space group $\text{R}\text{3}$) the $\text{?M}{}^{\mathrm{<mtext>II</mtext>}}\text{}\text{FMe}{}^{\mathrm{IV}}$ is more distinctly bent (?155–160°). CrZrF₆ undergoes two reversible phase transitions, which are determined to occur at 415 ± 5 K (cubic → tetragonal, dynamic to static Jahn-Teller distortion of CrF₆ octahedra and 150 ± 10 K (tetragonal → (pseudo)monoclinic).     

The Jahn-Teller distortion and cation ordering in the perovskite Sr2MnSbO6

The perovskite Sr₂MnSbO₆ has been synthesized using conventional ceramic techniques and structurally characterized using high-resolution powder X-ray and neutron diffraction. The structure is tetragonal in space group I4/m. The octahedra were found to feature Jahn-Teller (JT) distortion due to the presence of Mn³⁺, and this is identified as strongly contributing to the octahedral tilting. Evidence for B-site cation ordering is presented however there is extensive anti-site disorder. The disordering of the Mn³⁺ and Sb⁵⁺ cations is believed to be a result of the similar size of these two cations and the polarizability of the Sb⁵⁺ cation. The structure was found to undergo a transition to cubic symmetry at 521 °C with removal of the octahedral tilting leading to the quenching of the JT distortion. This phase transition was found to be continuous and tricritical in nature.     

The Jahn-Teller effect in solid state chemistry of transition metal compounds

The general features of the local and cooperative Jahn-Teller effect of dⁿ cations with E_g ground states in octahedral coordination and simple model structures are discussed. Examples of different cooperative Jahn-Teller ordering patterns and of phase transitions from static to partially dynamic and finally fully dynamic Jahn-Teller determined structures are given. While in general a tetragonally elongated coordination of the Jahn-Teller ions is favored, the compressed configuration may be stabilized under certain conditions also. It is demonstrated by some examples that the extent of the Jahn-Teller distortion depends on the symmetry and connection pattern of the polyhedra in the host lattice structure. Finally it is shown that the crossover between high- and low-spin configurations of d⁷ cations is strongly influenced by the additional Jahn-Teller stabilization of the low-spin ²E_g state.     

Synthesis and reactivity of metal-containing monomers

Acid and neutral Co^II, Cu^II, Ni^II, Zn^II, Fe^II, and Fe^III maleates, fumarates, and itaconates were obtained and characterized. The methods for their synthesis were optimized, and the valence state and coordination of metals were studied. Co^II and Fe^II hydrogen maleates, Co^II maleate, and Co^II fumarate were examined by X-ray diffraction analysis. The ligands based on unsaturated dicarboxylic acids can be mono-, bi-, and tetradentate, which results in the formation of acid salts, chain and three-dimensional coordination polymers, whose double bond is not involved in the coordination. The strong antiferromagnetic exchange (μ_elf=1.41 and 0.34 μ_B at 290 and 80 K, respectively) was detected in Cu^II itaconate. Based on the data of Mössbauer spectroscopy, the partial reduction of Fe^III to Fe^II during the synthesis of Fe^III maleate was shown to occur: δFe=0.43 and 1.27 mm s^?1, ΔE _Q=0.57 and 3.13 mm s^?1 and Γ=0.37 and 0.28 mm s^?1 atT=298 K for Fe^III and Fe^II, respectively.     

A new family of magnetic 2D coordination polymers based on [MV(CN)8]3− (M=Mo,W) and pre-programmed Cu2+ centres

The self-assembly of [M(CN)₈]³⁻ (M=Mo, W) anion and polyamine complexes of Cu^II[Cu(tetren)]²⁺ and [Cu(dien)(H₂O)₂]²⁺ (tetren=tetraethylenepentamine, dien=diethylenetriamine) in acidic aqueous solution gives (tetrenH₅)_0.8{Cu^II ₄[W^V(CN)₈]₄}·7.2H₂O 1, (tetrenH₅)_0.8{Cu^II ₄[Mo^V(CN)₈]₄}·7.2H₂O 2, (dienH₃){Cu^II ₃[W^V(CN)₈]₃}·4H₂O 3 and (dienH₃){Cu^II ₃[Mo^V(CN)₈]₃}·4H₂O 4 2D coordination polymers. All compounds are structure-related: the crystal structures of isomorphous 1–2 and 3–4, respectively, consist of double-layered cyano-bridged {Cu^II[W^V(CN)₈]⁻}_n square grid backbones and non-coordinated fully protonated polyamine countercations as well as H₂O molecules located between the sheets. The magnetic measurements reveal long range ferromagnetic ordering with sharp phase transitions at T_C in range 28–37 K and coercivity in range 30–225 Oe at liquid helium temperature, T=4.3 K.     

Jahn-Teller-Verzerrungen von Übergangsmetallionen in tetraedrischer Koordination – die Strukturen von Cat[MII(NCS)4] (MII: Co,Ni, Cu) und von Spinellmischkristallen MIICr2O4(MII: Zn?Ni,Zn?Cu,Cu?Ni)

Jahn-Teller Distortions of Transition Metal Ions in Tetrahedral Coordination — The Structures of Cat[M^II(NCS)₄]^II (M^II: Co, Ni, Cu) and of Mixed Crystals M^IICr₂O₄(M^II: Zn? Ni, Zn? Cu, Cu? Ni) of the Spinel Type The structure determination of compounds Cat[M^II(NCS)₄] with Cat = p-xylylenebis(triphenylphosphonium)²⁺ and M^II = Co, Ni, Cu [space group P2₁/n, Z = 4] yielded pseudotetrahedral M^IIN₄-polyhedra, which are distorted by packing forces and vibronic coupling effects of the Jahn-Teller type. Spinel mixed crystals with M^II = Zn? Ni, Zn? Cu, Ni? Cu in the tetrahedral sites exhibit phase transition to tetragonal and o-rhombic structures, induced by cooperative Jahn-Teller interactions. The distortion symmetries of the M^IIN₄ and M^IIO₄ tetrahedra are analysed on the basis of the respective electronic groundstate and the possible Jahn-Teller active vibrational modes.     

The CuII−CuI−Cu0(Hg) system in the presence of benzotriazole: Part II. A chronocoulometric investigation

Both the oxidation of Cu⁰ at dropping amalgam electrodes immersed in solutions of benzotriazole (BTA) and the reduction of Cu^II at a dropping mercury electrode from BTA solutions have been investigated by the single potential-step chronocoulometric technique. The dependence of the charge Q(t) flowing as a consequence of a given potential jump E_i→E_f upon the initial and final potentials E_i and E_f, as well as upon the time t elapsed from the instant of the potential jump provides direct evidence for the presence of a single adsorbed monolayer of a Cu^I compound on a mercury electrode immersed in a Cu^II solution containing BTA, at applied potentials positive to ≈?0.4 V/SCE. Analogous measurements carried out at dropping amalgam electrodes reveal the presence of a single adsorbed monolayer of a Cu^I compound, or else of an adsorbed multilayer, depending on the potential range investigated. The results of the chronocoulometric measurements are in agreement with those of the polarographic measurements of Part I.     

Electrical,magnetic, and EPR studies of the quaternary chalcogenides Cu2AIIBIVX4 prepared by iodine transport

Electrical, magnetic, and electron paramagnetic resonance (EPR) measurements have been made on crystals and powders of several quaternary chalcogenides of the type Cu₂A^IIB^IVX₄, where A^II = Zn, Mn, Fe, or Co, B^IV = Si, Ge, or Sn, and X = S or Se. The electrical properties of these compounds are extrinsic, but their magnetic properties do not appear to be affected by impurities. The magnetic moments of the Cu₂MnBX₄ compounds decrease with increasing covalency of the MnX bond, and those of Cu₂FeGeS₄ and Cu₂CoGeS₄ reflect an orbital contribution to the moment. Both the Weiss constants and magnetic ordering temperatures in these compounds show an evolution from antiferromagnetism to ferromagnetism with increasing separation between the moments. Magnetic measurements on single crystals of Cu₂MnGeS₄, Cu₂CoGeS₄, and Cu₂FeGeS₄ indicate that only the latter is anisotropic. EPR measurements on crystals and powders of Cu₂ZnGeS₄ doped nominally with 0.1% Mn reveal that Mn²⁺ experiences an axial distortion and that the bond ionicity is the same as in ZnS.     

The Nasicon-like copper(II) zirconium phosphate Cu0.5Zr2(PO4)3 and related compounds

A new family of Nasicon-like zirconium phosphates of formula M_0.5Zr₂(PO₄)₃ with M = Ca, Cu, Sr, Cd or Pb has been described. The physical properties of the copper compound have been investigated in detail in which Cu²⁺ induces a Jahn-Teller distortion below 520°C. Crystal field, electron paramagnetic resonance, and magnetic data are consistent with an elongated octahedral surrounding of the Cu²⁺ ions.