Paramagnetic Transition Metal Complexes Research Articles

Paramagnetic transition metal complexes with a redox-active ligand: M(hfac)2(EDO-EDT-TTF-py)n; [M = CuII, n= 1, 2; M = MnII, n= 2

Synthesis, crystal structures, electrochemical and physical properties of the new ligand, EDO-EDT-TTF-py (L1) [4,5-ethylenedioxy-4′,5′-(4-pyridylethylenedithio)tetrathiafulvalene], and the paramagnetic transition metal coordination complexes, M(hfac)2(EDO-EDT-TTF-py)2; M = CuII (2), MnII (3), and CuII(hfac)2(EDO-EDT-TTF-py) (4) are reported where hfac = hexafluoroacetylacetonato. Crystal data: (L1) triclinic system, space group P, a = 8.4060(3), b = 10.5093(3), c = 11.5318(5) A, α = 64.758(2), β = 86.923(2), γ = 72.444(2)°, V = 875.22(6) A3, Z = 2, 2 and 3 are isostructural (data for 3 are given in brackets), triclinic system, space group P, a = 7.1085(4) [7.1013(9)A], b = 11.1210(7) [11.111(2)A], c = 16.3628(11) A [16.346(3) A], α = 91.682(2) [91.662(6)], β = 91.014(3) [91.036(7)], γ = 93.547(3)° [93.651(6)°], V = 1290.3(1) A3 [1286.3(3) A3], Z = 1, (4) triclinic system, space group P, a = 9.2937(1), b = 15.1897(2), c = 24.3174(6) A, α = 92.461(1), β = 93.048(1), γ = 105.491(1)°, V = 3297.6(1) A3, Z = 4. In the compounds 2, 3 and 4, the ligand L1 is coordinated to the transition metals through the nitrogen atom of the pyridine group. In the crystal, the organic and inorganic layers are segregated. The arrangement of the organic layers is similar to the well known β phase in BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene] charge transfer complexes, showing that the crystal packing is mainly governed by π–π stacking of the organic molecules. L1 is a TTF derivative and shows the electron donating ability with oxidation potential E1½ = +0.52 V versus SCE in benzonitrile. The same oxidation potentials were observed for complexes 2 and 3, indicating the negligible interactions between the ligands and the metals in solution.

TiCp2Cl-Catalyzed Living Radical Polymerization of Styrene Initiated by Oxirane Radical Ring Opening

Epoxides and paramagnetic early transition metal complexes are introduced as two new classes of initiators and catalysts, respectively, for living radical polymerizations. Thus, Ti(III)Cp2Cl synthesized in situ from the reduction of TiCp2Cl2 with Zn catalyzes the radical ring opening of oxiranes to initiate the radical polymerization of styrene. A linear dependence of molecular weight on conversion, low polydispersity, and reinitiation of the polymerization in the presence of fresh monomer indicates that the polymerization is living and that it most likely occurs by the reversible endcapping of the macroradical with Ti(III). Moreover, epoxides provide convenient access to alcohol chain ends, suitable for further transformations.

Temperature dependence of the IR reflectance spectra of molecular crystals: κ-(EDDH-TTP) 3[Cr(phen)(NCS) 4]·2CH 2Cl 2 and κ 21-(BDH-TTP) 5[Cr(phen)(NCS) 4] 2·2CH 2Cl 2

Polarised IR reflectance spectra of two organic conductors formed by tetrathiapentalene-derived donors and paramagnetic transition metal complex anions, κ-(EDDH-TTP) 3[Cr(phen)(NCS) 4]·2CH 2Cl 2 and κ 21-(BDH-TTP) 5[Cr(phen)(NCS) 4] 2·2CH 2Cl 2, with κ-type structure of conducting organic layers were measured as a function of temperature. Plasma-like-dispersion in reflectance spectra is analysed and transport parameters are estimated. FT-NIR Raman spectra of powdered compounds in KBr were studied at room temperature. Additionally, IR spectra of neutral EDDH-TTP and BDH-TTP neutral molecules are recorded. Assignment of bands related to the CC stretching vibrations of donor molecules is discussed.

First radical cation salt of paramagnetic transition metal complex containing TTF as ligand, [Cu(II)(hfac)2(TTF-py)2](PF6) x 2CH2Cl2 (hfac = hexafluoroacetylacetonate and TTF-py = 4-(2-tetrathiafulvalenyl-ethenyl)pyridine).

The preparation, X-ray crystal structure, EPR data, and magnetic measurement of [Cu(II)(hfac)(2)(TTF-py)(2)](PF(6)).2CH(2)Cl(2), a novel material where the conducting and the localized spin systems are covalently linked through conjugated bridges, are reported. The partial oxidation of the TTF-type organic donor ligand yielded the first radical cation salt of a paramagnetic transition metal complex. Moreover, this compound shows a mixed valence state at the unimolecular level, and additionally, the arrangement of the molecules in the crystal structure revealed the presence of isolated mixed valence TTF dimers.

Synthesis, X‐ray Studies and Magnetic Properties of Dinuclear NiII and CuII Complexes Bridged by the Azo‐2,2′‐bipyridine Ligand

AbstractTwo new paramagnetic transition metal complexes that contain the bridging ligand azo‐2,2′‐bipyridine (abpy) have been prepared and characterized by X‐ray crystallography and magnetic susceptibility measurements. The compound [Ni2(μ‐abpy)(CH3CN)2(NO3)4]·2CH3CN (1·2CH3CN) crystallizes in the triclinic space group P$\bar 1$ with a = 8.290(5) Å, b = 8.343(5) Å, c = 11.180(5) Å, α = 105.36(3)°, β = 94.39(3)°, γ = 107.56(3)°, V = 700.5(7) Å3 and Z = 1. The related salt [Cu2(μ‐abpy)(CH3CN)8][BF4]4 (2) crystallizes in the triclinic space group P$\bar 1$ with a = 10.240(2) Å, b = 10.597(2) Å, c = 10.712(2) Å, α = 64.23(3)°, β = 78.21(3)°, γ = 78.57(3)°, V = 1026.7(3) Å3 and Z = 1. Solutions of 2 eventually produce the polymeric CuI compound {[Cu(μ‐abpy)][BF4]}∞ (3) which was also crystallographically characterized: monoclinic space group P21/n, with a = 9.295(2) Å, b = 9.541(2) Å, c = 13.486(3) Å, β = 91.58(3)°, V = 1195.5(4) Å3 and Z = 4. The magnetic properties of these dinuclear paramagnetic complexes have been studied in detail, and have provided an opportunity for probing the ability of this ligand to mediate magnetic exchange interactions between paramagnetic metal centers. Compound 1 exhibits antiferromagnetic exchange interactions (J = −7.5 cm−1) whereas the magnetic interactions in compound 2 were found to be negligible. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Synthesis, redox behaviour and X-ray structure of bis-TTF containing a pyridine unit as a potential building block in the construction of conducting magnetic materials

The synthesis of singly and doubly bridged bis-TTF containing a 2,6-bis(methylthio)pyridine group as a linker and potentially usable as a ligand for paramagnetic transition metal complexes is reported. Cyclisation leading to doubly bridged bis-TTF has been achieved without using the high dilution technique generally employed in such cases. The redox behaviour of such precursors has been studied by cyclic voltammetry and square wave techniques. The crystal structure of one of these electron donors has been determined by X-ray diffraction showing a stacking arrangement of the TTF units.

Use of EPR Spectroscopy in Elucidating Electronic Structures of Paramagnetic Transition Metal Complexes

Electronic structure is an important concept in chemistry. Among the techniques that can be used to understand electronic structure, EPR spectroscopy is unique in the sense that it can specifically probe paramagnetic molecules. This paper describes a number of fundamental concepts of EPR spectroscopy and its application to paramagnetic transition metal ions. As examples, EPR spectra of two paramagnetic metalloporphyrin complexes are used to define electronic structure of these molecules. The concepts described can be used in upper-level undergraduate and first-year graduate classes.

Laser-induced ESR shift of a paramagnetic transition metal complex ion [IrBr 6] 2−

Taking a paramagnetic transition metal complex ion [IrBr 6] 2− as an example, the laser-induced ESR shifts in oriented and unoriented samples have been studied and calculated. In oriented and unoriented systems the ESR shifts result from the contribution of the symmetric and antisymmetric polarizabilities. The shifts of the [IrBr 6] 2− complex ion are calculated to be of the order of 1∼10 kHz for a laser beam of 10 W cm −2. The factors affecting the magnitude of the laser-induced ESR shift are discussed. The results indicate that there should be observable ESR shifts in a molecular or radical paramagnetic system on applying linearly or circularly polarized light at a frequency near optical absorption.

Correlations between magnetic resonance parameters of ESR and NMR spectra

Empirically established correlations between magnetic resonance parameters of free radicals (g-factors, isotropic hyperfine coupling constans) and isostructural molecules (chemical shifts, isotropic spin-spin coupling constants) or structurally similar ligands in paramagnetic transition metal complexes (isotropic chemical shifts) are systematized and critically discussed. Quantum-chemical analysis of the suggested spin distribution damping coefficients in model systems and structurally similar chemical compounds is performed. Based on the results obtained, physicochemical interpretation of the observed correlations between the parameters of ESR and NMR spectra is given.

Field-Dependent Proton NMR Relaxation in Aqueous Solutions of Ni(II) Ions. A New Interpretation

A new model is presented for nuclear-spin relaxation in paramagnetic transition metal complexes in solution, allowing the electron-spin relaxation to be outside the Redfield limit. The novel feature is that the transient zero-field splitting (ZFS), modulated by distortions of the solvation shell, is allowed to be of rhombic rather than cylindrical symmetry. The model, which assumes that the static ZFS is absent, is applicable to aqueous solutions of transition metal ions. The magnetic-field dependence of the proton spin–lattice relaxation rate enhancement in aqueous solution has been investigated, and calculations are presented for anS= 1 system such as Ni2+(aq), using different degrees of rhombicity of the ZFS and different motional conditions (Redfield limit and slow-motion regime). The new model is also applied to fit the previously reported data for the field dependence of proton relaxation in aqueous solution of Ni(ClO4)2at low pH [J. Kowalewski, T. Larsson, and P.-O. Westlund,J. Magn. Reson.74, 56 (1987)]. The inclusion of rhombicity, motivated by recent theoretical work, provides a model which performs as well as the earlierad hocmodel.

An electron paramagnetic resonance study of arthritic joints

Much interest has recently been shown in the detection of reactive oxygen species (ROS) in view of the evidence implicating a primary or secondary role for them in the initiation or progression of many human diseases, the majority of which are characterized by an inflammatory reaction [1-3]. Hydrogen peroxide (H202) and superoxide anion ( 0 2 ) , are produced by diverse cell types and induce free radical-mediated cellular and tissue damage. Redox active transition metal complexes can readily promote the generation of the extremely reactive hydroxyl radical (-OH) from hydrogen peroxide and superoxide [4, 5]. Within the inflamed rheumatoid joint, reactive oxygen species oxidatively modify IgG [6], inactivate alpha-l-proteinase inhibitor [7], peroxidize polyunsaturated lipids [6] and depolymerize hyaluronate [8]. Electron paramagnetic resonance (EPR) spectroscopy has been applied in attempts to assess free radical production in many disease states. For example, in a recent EPR study we have employed the spin trap 3,5,dibromo-4-nitrobenzenesulphonate (DBNBS) to demonstrate reperfusion-induced free radical generation in isolated rheumatoid synovium [9]. Also EPR spectroscopy, usually at low temperatures, has been used to study paramagnetic transition metal complexes in a broad range of biological systems, including enzymes, cells and tissues [1016]. In the present study we have employed lowtemperature EPR spectroscopy to investigate the presence of paramagnetic species in isolated synovium and bone from patients suffering from rheumatoid (RA) and osteo-arthritis (OA). The EPR profiles of samples of normal bone and synovium obtained during ~raumatic injury (TI) surgery were employed as controls. Samples of RA synovium (n = 8), OA synovium ( n = 8 ) , RA cancellous bone ( n = 5 ) and OA cancellous bone (n = 10), from arthritic patients were obtained during total kneeand hip-joint replacement surgery at the Royal London Hospital. Synovium samples ( n = 6 ) were obtained from traumatic injury patients undergoing orthopaedic surgery. All samples were frozen in liquid nitrogen ( -196°C) and stored at 7 0 ° C prior to EPR

Paramagnetic Transition Metal Complexes With [sgrave]-Bonded Tetracyanoethylene (TCNE)

Abstract Mono- and dimetallic building blocks are being studied in charge-transfer reactions with organic π-acceptors such as TCNE to form zero-, one-, and two-dimensional hybrid donor/acceptor compounds. In particular, tetracarboxylate-bridged dinuclear and solvated 3d mononuclear complexes with [sgrave]-bonded TCNE ligands are being investigated with regards to their variable temperature magnetic behavior. Results of experiments with dimolybdenum, diruthenium and vanadium complexes are discussed.

Hyperfine-axis orientation in rhombic metal complexes from ENDOR-induced EPR and quadrupolar coupling determination

The ENDOR resonances from ligand nuclei in paramagnetic transition-metal complexes exhibit a shift from the nuclear Larmor frequency because of the dipolar and Fermi contact interactions with the unpaired electron. The dipolar interaction has orientation dependence associated with its magnitude, where the position of the nuclei with respect to the metal atom determines the resonant frequencies and spectral lineshape. In this paper effects of rhombic g and metal hyperfine tensors on the EPR powder pattern and ENDOR spectra are illustrated. The EPR angular selection is modified from that encountered in axial systems and the technique of ENDOR-induced EPR is utilized to elucidate the orientation of magnetic axes with respect to one another. Quadrupole couplings in ENDOR spectra have often been analyzed using an equation from which cross terms have been omitted. The cross terms make important contributions to the spectra in some cases and the effect of these terms has been examined in order to determine when the complete equation for the quadrupole interaction must be utilized to analyze correctly ENDOR spectra.

Ligand-field analysis of transition-metal complexes

Abstract The aim of this review is to give an account of recent theoretical work (‘Ligand-Field Analysis’) that has both led to clarification of the basic structure of ligand-field theory and has facilitated the acquisition of information about chemical bonding in paramagnetic transition-metal complexes that form insulating crystals. The review begins with a justification in modern terms for the classical crystal-field approach based on a well defined dn-configuration for the metal ion; the central idea is that the ground and low-lying electronic states are based on localized electron wavefunctions because the interelectron repulsion energy is so important in these systems. The ligand-field theory is developed in two stages; first, the group product wavefunction method is used to construct a physically important subspace of many-electron wavefunctions for a transition-metal complex. The full n-electron hamiltonian is then studied in this basis using Lowdin partitioning and the chain formalism of Haydock; a full theoretical characterization of the Ligand-Field hamiltonian, which refers explicitly to only the d-electrons, is given. The paper describes the parametrization of the ligand-field based on the Cellular Ligand Field (CLF) model which explicitly introduces the notion of the chemical functional group to ligand-field theory. The {e l k} -parameters associated with the local interactions of the metal ion and its individual ligands (l) are discussed, and the review includes some general remarks about these quantities for the ligand-fields of some typical transition-metal complexes. A short introduction to the modern electronic structure theory of materials is given as a postlude to the main review.

NMR chemical shift method in magnetic susceptibility measurements applied to studies of the reaction of cobalt complexes with cumene hydroperoxide

AbstractThe NMR chemical shift method was applied for the determination of the magnetic susceptibility of paramagnetic transition metal complexes in solution. The method was found to be very useful for the determination of magnetic susceptibility changes during a chemical reaction in which one paramagnetic compound produced another. The concentrations of two paramagnetic cobalt complexes [Co3O(O2CCH3)6(HO2CCH3)3] and Co(O2CCH3)2, were calculated from the chemical shift value observed during their reaction with cumene hydroperoxide. The results are consistent with those obtained from electronic spectra.

On the theory of electron and nuclear spin relaxation in paramagnetic transition metal complexes

Modulation of the crystal field by solvent electric field fluctuations is shown to induce rapid temperature insensitive electron spin relaxation in paramagnetic transition metal complexes in solution. The theory is developed for a d 2 (S = 1) octahedral complex. If the zero field splitting is larger than the applied field strength (D > μB B o) the observed nuclear relaxation times are field insensitive in accord with experiment.

Nuclear spin-lattice relaxation in a multi-spin system. A spin 1/2 particle dipolar coupled to a spin 1 and spin 3/2 particle. 13C spin relaxation in urea

Density matrix theory is used to show that for S = 1/2, 1, 3/2 the decay of the I-magnetization (I = 4) in a two-spin IS dipolar coupled system is governed by the differential equation ��������������� d<Iz>/dt= -k1{J(ωI-ωS)+3J(ωI)+6J(ωI+ωS)}{<Iz>-I0}������ ������������������-k2{6J(ωI+ωS)-J(ωI-ωS)}{<SZ>-S0} where k1 and k2 are positive constants dependent on S and the dipolar interaction constants. It follows that if <S2> = 0, as arises on S- irradiation, or <SZ> = S0, as in paramagnetic transition-metal complexes, the decay of the I-magnetization is exponential.�The theory is used to analyse 14N,2H dipolar-induced 13C spin relaxation in deuterated urea. It is shown that at low temperatures dipolar interactions dominate the relaxation whereas at high temperatures spin rotation effects become important. The relaxation times are very long; at 300 T1 is 114s.

Isostructural triazenido complexes of first row transition metals. Part 3. Low spin cyclopentadienyl complexes of cobalt(II) (?5-C5H5)(PPh3)(RN3R)Co

Reaction of (PPh3)2Cl2CoII with TlC5H5 gave the new (η5-C5H5)(PPh3)ClCoII complex. This complex reacted with Ag(RN3R) or Cu(RN3R) (R=p-MeC6H4,p-ClC6H4 or 3,5-Cl2C6H3) to give the novel compounds (η5-C5H5)(PPh3)(RN3R)CoII, with a spin state of one half, which are representatives of the very small class of paramagnetic monocyclopentadienyl transition metal complexes. The structural conclusions are based on comparison with the isostructural [η5-C5H5)(PPh3)(RN3R)CoIII]PF6 complex, on the i.r. spectra and on the observed redox behaviour. The e.p.r. spectra are reported, but are not well enough resolved for the determination of the hyperfine interactions. The orbital scheme of the new complex is discussed in terms of a distorted octahedral crystal field in relation to the series of isostructural (η5-C5H5)(PPh3)(RN3R)MII (M = Fe, Co or Ni) complexes.

Intramolecular reorientation, an electron spin relaxation process in solutions of discrete paramagnetic transition-metal complexes

Time modulation of the g-tensor by intramolecular reorientation between structurally equivalent molecular arrangements is postulated to dominate electron spin relaxation in solutions of some paramagnetic transition-metal complexes. The process is treated theoretically and it is shown that the resulting electron spin relaxation time depends on the correlation time for intramolecular reorientation. The temperature dependence of the nuclear T1 thus yields information concerning the potential energy profile for intramolecular reorientation. Experimental results on the field dependence of the temperature dependence of T1 of the methyl proton in Ru(acac)3 are in accord with the theory.

Electron and nuclear spin relaxation in paramagnetic transition-metal complexes with S = 1. The effects of a non-zero average zero-field splitting on the spin energy levels

The effects of a non-zero average zero-field splitting on electron spin relaxation in paramagnetic (S = 1) complexes is treated theoretically. The spin-lattice interaction is postulated to be a simple scalar P(t). S process with correlation time Ti. This process is assumed not to modulate the zero-field splitting which, however, is modulated by the molecular tumbling. The frequency dependence of the nuclear relaxation time (Tl) now depends on the magnitude of the zero-field splitting constant (D), and, for large values of D, the value of T1 is independent of the applied field strength.