X-band Electron Paramagnetic Resonance Spectra Research Articles

A spin-frustrated trinuclear copper complex based on triaminoguanidine with an energetically well-separated degenerate ground state.

We present the synthesis and crystal structure of the trinuclear copper complex [Cu3(saltag)(bpy)3]ClO4·3DMF [H5saltag = tris(2-hydroxybenzylidene)triaminoguanidine; bpy = 2,2'-bipyridine]. The complex crystallizes in the trigonal space group R3̅, with all copper ions being crystallographically equivalent. Analysis of the temperature dependence of the magnetic susceptibility shows that the triaminoguanidine ligand mediates very strong antiferromagnetic interactions (JCuCu = -324 cm(-1)). Detailed analysis of the magnetic susceptibility and magnetization data as well as X-band electron spin resonance spectra, all recorded on both powdered samples and single crystals, show indications of neither antisymmetric exchange nor symmetry lowering, thus indicating only a very small splitting of the degenerate S = (1)/2 ground state. These findings are corroborated by density functional theory calculations, which explain both the strong isotropic and negligible antisymmetric exchange interactions.

Electron paramagnetic resonance (EPR) study of free radicals in irradiated zinc poly methacrylate

Abstract We report the analysis of the X-band EPR spectrum of the free radical species formed in Zn(II) polymethacrylate (Zn(II) PMA), after degradation was induced in this polymer by 60 Co γ- radiation at room temperature. We begin by describing the synthesis and characterization of our samples and its polymerization before applying them several doses to induce degradation. Doses from 10 to 100 kGy in steps of 10 kGy were applied to the samples. At 10 kGy dose, the EPR spectrum shows the superimposed spectra of two species, namely: a typical pattern of an axially symmetrical singlet and a broad symmetric single line. The former can be attributed to peroxide radicals. After applying further doses, the EPR spectra of the samples display a series of superimposed lines on top of the mentioned axial singlet and the broad line. In these spectra, we recognize three sets of lines: a weak doublet, a quartet and quintet in addition to the already detected axially symmetrical line and the broad singlet. These can be assigned to scission radicals trapped in the polymeric matrix.

Ni(II), Cu(II), AND Zn(II) COMPLEXES DERIVED FROM A NEW SCHIFF BASE 2-((Z)-(3-METHYLPYRIDIN-2-YLEIMINO)METHYL)PHENOL AND SYNTHESIS OF NANO SIZED METAL OXIDE PARTICLES FROM THESE COMPOUNDS

Synthesis, spectral identification, and magnetic properties of three complexes of Ni(II), Cu(II), and Zn(II) are described. All three compounds have the general formula [M(L)2(H2O)2], where L = deprotonated phenol in the Schiff base 2-((z)-(3-methylpyridin-2-yleimino)methyl)phenol. The three complexes were synthesized in a one-step synthesis and characterized by elemental analysis, Fourier transform infrared spectroscopy, electronic spectra, X-ray diffraction (XRD), and room temperature magnetic moments. The Cu(II) and Ni(II) complexes exhibited room temperature magnetic moments of 1.85 B.M. per copper atom and 2.96 B.M. per nickel atom. The X-band electron spin resonance spectra of a Cu(II) sample in dimethylformamide frozen at 77 K (liquid nitrogen temperature) showed a typical ΔMS = ± 1 transition. The complexes ([M(L)2(H2O)2]) were investigated by the cyclic voltammetry technique, which provided information regarding the electrochemical mechanism of redox behavior of the compounds. Thermal decomposition of the complexes at 750 oC resulted in the formation of metal oxide nanoparticles. XRD analyses indicated that the nanoparticles had a high degree of crystallinity. The average sizes of the nanoparticles were found to be approximately 54.3, 30.1, and 44.4 nm for NiO, CuO, and ZnO, respectively.

The neutral complex [CrIII4(μ3-O)2(μ2-CH3CO2)7(tbpy0)(tbpy•)]0 — a tetranuclear Cr(III) species containing a neutral (bpy0) and a π-radical anion (bpy•)1–

The tetranuclear neutral species [CrIII4(μ3-O)2(μ2-CH3CO2)7(tbpy0)(tbpy•)]0·3THF has been prepared from a mixture of [CrII2(μ2-CH3CO2)4]·2H2O and 4,4′-di-tert-butyl-2,2′-bipyridine (1:2) in tetrahydrofuran (THF) under anaerobic conditions. Black crystals of [CrIII4(μ-O)2(μ-CH3CO2)7(tbpy)2]0 were grown and investigated by X-ray crystallography: a [Cr4O2]8+ butterfly core structure has been identified. In addition, the presence of seven μ2-acetate bridges, a neutral (tbpy0) ligand, and a single π-radical anion (tbpy•)1– have been clearly identified. The magnetic properties have been investigated by solid-state SQUID (superconducting quantum interference device) measurements (2–300 K) and a coupling model is presented. Its X-band electron paramagnetic resonance spectrum has been recorded at 10 and 300 K in toluene solution and successfully simulated. The spin of the (tbpy•)1– radical couples strongly antiferromagnetically to one Cr(III) center (J4 = –129 cm−1). These properties are compared with those reported previously for the monocationic species [CrIII4(μ3-O)2(μ2-CH3CO2)7(bpy0)2](PF6) in Bino et al. (Inorg. Chem. 1991, 30, 856).

Copper(II)-mediated transformation of a tridentate non-innocent ligand into a tetradentate salen-type innocent ligand.

A non-innocent ligand, H4L(CH2NH2), was synthesized by introducing a -CH2NH2 group at the ortho carbon atom to the aniline moiety of 2-anilino-4,6-di-tert-butylphenol. The new ligand was characterized by IR and NMR spectroscopy and mass spectrometry techniques. Upon treatment with CuCl2⋅2H2O, this non-innocent ligand provided a mononuclear four-coordinate salen-type Cu(II) complex by complete modification of the ligand backbone. The complex was characterized by IR spectroscopy, mass spectrometry, X-ray single-crystal diffraction, electron paramagnetic resonance (EPR) spectroscopy, and UV/Vis/near-IR spectroscopy techniques. X-ray crystallographic analysis showed an asymmetric environment around the Cu(II) center with a small (≈12°) twist between the two biting planes. Analysis of the X-band EPR spectrum also supported the asymmetric environment and also indicated the presence of an unpaired electron on the dx2-y2 orbital. The UV/Vis/near-IR spectrum showed strong absorption bands for metal-to-ligand charge transfer and ligand-to-metal charge transfer along with a Cu(II) -centered d-d transition. Mechanistic investigation of the formation of complex 1 indicated that modification of the ligand backbone proceeded through ligand-centered amine to imine oxidation as well as through C-N bond-breaking processes. During these processes, 3,5-di-tert-butyl-1,2-benzoquinone and 2-aminobenzylidene were produced. Ammonia, generated in situ through hydrolysis of the imine to the aldehyde, reacted with 3,5-di-tert-butyl-1,2-benzoquinone to form the corresponding 3,5-di-tert-butyl-1,2-iminobenzoquinone moiety, which upon two-electron reduction in the reaction medium formed 3,5-di-tert-butyl-1,2-aminophenol. This aminophenol underwent condensation with the H2L5 ligand that was formed by self-condensation of two molecules of 2-aminobenzaldehyde and provided the modified ligand backbone.

Rapid-scan EPR of immobilized nitroxides

X-band electron paramagnetic resonance spectra of immobilized nitroxides were obtained by rapid scan at 293K. Scan widths were 155G with 13.4kHz scan frequency for 14N-perdeuterated tempone and for T4 lysozyme doubly spin labeled with an iodoacetamide spirocyclohexyl nitroxide and 100G with 20.9kHz scan frequency for 15N-perdeuterated tempone. These wide scans were made possible by modifications to our rapid-scan driver, scan coils made of Litz wire, and the placement of highly conducting aluminum plates on the poles of a Bruker 10″ magnet to reduce resistive losses in the magnet pole faces. For the same data acquisition time, the signal-to-noise for the rapid-scan absorption spectra was about an order of magnitude higher than for continuous wave first-derivative spectra recorded with modulation amplitudes that do not broaden the lineshapes.

ESR spectroscopy of FeIII ions in sodium silicate glasses

The state of FeIII ions doped in sodium silicate glasses of the composition (100 − x)Na2O-xSiO2−yFe2O3 (x = 75, 81 mol.%; y = 0.05−13 wt.%) was studied by electron spin resonance (ESR). X-band ESR spectra exhibited three resonance signals at g ≈ 2.0, 4.3, and 6. The computer simulation of ESR spectra was performed on the basis of spin-Hamiltonian of rhombic symmetry. The nature of observed signals was interpreted as a combination of five types of FeIII complexes. Three of them were clustered iron ions and two other were isolated paramagnetic ones. The ratio of observed forms was found for different glass compositions and various total amounts of FeIII ions in a matrix containing 0.55, 2.5, 4, 7, and 13 wt.% of Fe2O3.

Conduction electron spin resonance in AlB2

This work reports on electron spin resonance experiments in oriented single crystals of the hexagonal AlB2 diboride compound (P6/mmm, D16h structure) which display conduction electron spin resonance. The X-band electron spin resonance spectra showed a metallic Dysonian resonance with g-value and intensity independent of temperature. The thermal broadening of the anisotropic electron spin resonance linewidth ΔH tracks the T-dependence of the electrical resistivity below T ≃ 100 K. These results confirm the observation of a conduction electron spin resonance in AlB2 and are discussed in comparison with other boride compounds. Based on our main findings for AlB2 and the calculated electronic structure of similar layered honeycomb-like structures, we conclude that any array of covalent B–B layers potentially results in a conduction electron spin resonance signal. This observation may shed new light on the nature of the non-trivial conduction electron spin resonance-like signals of complex f-electron systems such as β-YbAlB4.

Electron Paramagnetic Resonance Spectroscopic Study of Copper Hopping in Doped Bis(l-histidinato)cadmium Dihydrate

Electron paramagnetic resonance (EPR) spectroscopy was used to study Cu(II) dynamic behavior in a doped biological model crystal, bis(L-histidinato)cadmium dihydrate, in order to gain better insight into copper site stability in metalloproteins. Temperature-dependent changes in the low temperature X-band EPR spectra became visible around 100 K and continued up to room temperature. The measured 298 K g-tensor (principal values: 2.17, 2.16, 2.07) and copper hyperfine coupling tensor (principal values: -260, -190, -37 MHz) were similar to the average of the 77 K tensor values pertaining to two neighboring histidine binding sites. The observed temperature dependence was interpreted using Anderson's theory of motional narrowing, where the magnetic parameters for the different states are averaged as the copper rapidly hops between sites. The EPR pattern was also found to undergo a sharp sigmoidal-shaped, temperature-dependent conversion between two species with a critical temperature T(c) ≈ 160 K. The species below T(c) hops between the two low temperature site patterns, and the one above T(c) represents an average of the molecular spin Hamiltonian coupling tensors of the two 77 K sites. In addition, the low and high temperature species hop between one another, contributing to the dynamic averaging. Spectral simulations using this 4-state model determined a hop rate between the two low temperature sites ν(h4) = 4.5 × 10(8) s(-1) and between the low and high temperature states ν(h2) = 1.7 × 10(8) s(-1) at 160 K. An Arrhenius relationship of hop rate and temperature gave energy barriers of ΔE4 = 389 cm(-1) and ΔE2 = 656 cm(-1) between the two low temperature sites and between the low and high temperature states, respectively.

Vanadocene de Novo: Spectroscopic and Computational Analysis of Bis(η5-cyclopentadienyl)vanadium(II)

The magnetic and electronic properties of the long-known organometallic complex vanadocene (VCp2), which has an S = 3/2 ground state, were investigated using conventional (X-band) electron paramagnetic resonance (EPR) and high-frequency and -field EPR (HFEPR), electronic absorption, and variable-temperature magnetic circular dichroism (VT-MCD) spectroscopies. Frozen toluene solution X-band EPR spectra were well resolved, yielding the 51V hyperfine coupling constants, while HFEPR were also of outstanding quality and allowed ready determination of the rigorously axial zero-field splitting of the spin quartet ground state of VCp2: D = +2.836(2) cm–1, g⊥ = 1.991(2), g∥ = 2.001(2). Electronic absorption and VT-MCD studies on VCp2 support earlier assignments that the absorption signals at 17 000, 19 860, and 24 580 cm–1 are due to ligand-field transitions from the 4A2g ground state to the 4E1g, 4E2g, and 4E1g excited states, using symmetry labels from the D5d point group (i.e., staggered VCp2). Contributions to...

Conformationally Distinct Five-Coordinate Heme–NO Complexes of Soluble Guanylate Cyclase Elucidated by Multifrequency Electron Paramagnetic Resonance (EPR)

Soluble guanylate cyclase (sGC) is a heme-containing enzyme that senses nitric oxide (NO). Formation of a heme Fe-NO complex is essential to sGC activation, and several spectroscopic techniques, including electron paramagnetic resonance (EPR) spectroscopy, have been aimed at elucidating the active enzyme conformation. Of these, only EPR spectra (X-band ~9.6 GHz) have shown differences between low- and high-activity Fe-NO states, and these states are modeled in two different heme domain truncations of sGC, β1(1-194) and β2(1-217), respectively (Derbyshire et al., Biochemistry 2008, 47, 3892-3899). The EPR signal of the low-activity sGC Fe-NO complex exhibits a broad lineshape that has been interpreted as resulting from site-to-site inhomogeneity, and simulated using g strain, a continuous distribution about the principal values of a given g tensor. This approach, however, fails to account for visible features in the X-band EPR spectra as well as the g anisotropy observed at higher microwave frequencies. Herein we analyze X-, Q-, and D-band EPR spectra and show that both the broad lineshape and the spectral structure of the sGC EPR signal at multiple microwave frequencies can be simulated successfully with a superposition of only two distinct g tensors. These tensors represent different populations that likely differ in Fe-NO bond angle, hydrogen bonding, or the geometry of the amino acid residues. One of these conformations can be linked to a form of the enzyme with higher activity.

Coordination and RedOx ratio of iron in sodium‐silicate glasses

Mössbauer and Electron paramagnetic resonance (EPR) spectroscopy studies of iron status in doped sodium-silicate glasses (100−x, mol%)Na2O(x, mol%)SiO2(y, wt.%) Fe2O3 (x=75; 81; y=0.05–13) were made as a function of glass matrix and total iron content.55Total iron content is expressed as Fe2O3 in the paper. Mössbauer spectroscopy identified three signals: Fe3+ doublet, Fe2+ doublet and Fe3+ broad hyperfine splitting (HFS) at room temperature dealing with slow relaxation effect of ferric ions. X-band EPR spectra exhibits resonance signals at g≈2.0, g≈4.3 and a shoulder in the region of g≈6.0. EPR spectra were simulated on the basis of rhombic spin-Hamiltonian with Zeeman and fine-structure terms. The nature of EPR absorption signals was interpreted as a combination of five types of Fe3+ complexes. Three of them were clustered ferric ions and two others were isolated paramagnetic ferric ions in distorted octahedral sites. The iron RedOx ratio (Fe2+/Fe3+) has been estimated from Mössbauer spectroscopy, while proportion of Fe3+ ions in adjacent sites (Fe3+isolated)/Fe3+clustered) has been obtained from Mössbauer spectra and simulated EPR spectra.

Characteristics of the Mn2+ EPR spectra in the oxyfluoride glass ceramics containing SrF2 nanocrystals

Characteristics of the electron paramagnetic resonance (EPR) spectra of the Mn2+ ion in the oxyfluoride glass ceramics containing SrF2 crystalline counterpart has been investigated. Explicit superhyperfine structure (SHF) has been observed in the X-Band EPR spectra taken at 77 K after thermal treatment of the glass precursor. Such SHF structure is characteristic to Mn2+ ion in the fluorine coordinated nearest environment which is characteristic to cubic structure of the alkaline earth fluorides. Manganese replaces strontium in the SrF2 lattice where it has pronounced SHF interaction with surrounding fluorine nuclei. Thus Mn2+ ion provides information about crystalline structure of the particular oxyfluoride glass ceramics.

EPR spectra of the Mn2+ ion in the oxyfluoride glass ceramics containing BaF2 nanocrystalline phase

Electron paramagnetic resonance (EPR) spectroscopy has been used to investigate structure of the oxyfluoride glass ceramics containing BaF2 fluoride component. Mn2+ ion has been used as a paramagnetic probe. After successful material synthesis and thermal treatment at temperatures above first crystallization temperature transparent glass ceramics samples were obtained. For these samples superhyperfine (SHF) structure in the X-band EPR spectra taken at 77 K has been observed. Such SHF structure is characteristic to fluorine hyperfine interaction observed in the fluoride crystals. Obtained data allow analyzing the structure of the BaF2 crystallites and possible location of the Mn2+ ion in the BaF2 crystalline lattice.

Spectroscopic Studies on Threonine Doped Polyaniline Composites

ABSTRACT The structural and magnetic properties of polyaniline (PANI)- and threonine (T)-doped polyaniline composites were characterized by using FTIR, UV-Vis, and electron paramagnetic resonance (EPR) spectroscopic techniques. The FTIR, UV-Vis, and X-band EPR spectra were recorded for PANI- and threonine-doped polyaniline (PANI-T) composites. The increased intensity of the IR bands in PANI-T composites is due to the increasing dopant concentration. The observed red shift corresponding to the N-H ··· N mode indicates the hydrogen bond formation in between the PANI and threonine molecule. The observed decrease in spectral intensity of the UV-Vis spectrum of the PANI-T composite confirms the occurrence of the protonation at the imine nitrogen sites. The EPR parameters such as line width, g factor, line asymmetry parameter, signal intensity, and spin concentration values were estimated. The EPR spectrum of PANI clearly indicates the presence of unpaired polaron. The EPR signal intensity and spin concentration values decrease with increasing dopant concentration, which reveals the polaron to bipolaron formation in PANI-T composites. EPR lineshape analysis showed that the absorption profile was Gaussian shape. The Gaussian-shaped line is generally due to the dipole–dipole interactions in the PANI and PANI-T composites. The line asymmetry parameter and g values indicated the isotropic nature of PANI and PANI-T composites.

Nitroxide sensing of a DNA microenvironment: mechanistic insights from EPR spectroscopy and molecular dynamics simulations.

The behavior of the nitroxide spin labels 1-oxyl-4-bromo-2,2,5,5-tetramethylpyrroline (R5a) and 1-oxyl-2,2,5,5-tetramethylpyrroline (R5) attached at a phosphorothioate-substituted site in a DNA duplex is modulated by the DNA in a site- and stereospecific manner. A better understanding of the mechanisms of R5a/R5 sensing of the DNA microenvironment will enhance our capability to relate information from nitroxide spectra to sequence-dependent properties of DNA. Toward this goal, electron paramagnetic resonance (EPR) spectroscopy and molecular dynamics (MD) simulations were used to investigate R5 and R5a attached as R(p) and S(p) diastereomers at phosphorothioate (pS)C(7) of d(CTACTG(pS)C(7)Y(8)TTAG). d(CTAAAGCAGTAG) (Y = T or U). X-band continuous-wave EPR spectra revealed that the dT(8) to dU(8) change alters nanosecond rotational motions of R(p)-R5a but produces no detectable differences for S(p)-R5a, R(p)-R5, and S(p)-R5. MD simulations were able to qualitatively account for these spectral variations and provide a plausible physical basis for the R5/R5a behavior. The simulations also revealed a correlation between DNA backbone B(I)/B(II) conformations and R5/R5a rotational diffusion, thus suggesting a direct connection between DNA local backbone dynamics and EPR-detectable R5/R5a motion. These results advance our understanding of how a DNA microenvironment influences nitroxide motion and the observed EPR spectra. This may enable use of R5/R5a for a quantitative description of the sequence-dependent properties of large biologically relevant DNA molecules.

Spin S = 1 centers: a universal type of paramagnetic defects in nanodiamonds of dynamic synthesis

Intrinsic paramagnetic defects in ∼5 nm sized nanodiamonds, produced by various dynamic synthesis (DySND) techniques (detonation, shock-wave, pulsed laser ablation of solid carbon containing targets), have been studied by multi-frequency electron paramagnetic resonance (EPR). X-band (9–10 GHz) EPR spectra of DySND, in addition to the main intensive singlet Lorentzian-like EPR signal, reveal a low intensity doublet pattern within the half-field (HF) region (g ∼ 4). On transferring spectra to the Q-band (34 GHz) the shape of the HF pattern changes and splitting between doublet components is reduced from 10.4 to 2.6 mT. The HF patterns observed are attributed to the ‘forbidden’ ΔMS = 2 transitions between the Zeeman levels of some spin-triplet (S = 1) centers. The model of two triplet centers with g ∼ 2.003 and zero-field splitting parameters D1 = 0.095 cm−1 (TR1) and D2 = 0.030 cm−1 (TR2) satisfactorily describes experimental results at both microwave frequencies. The spin-triplet-type defects are observed in a wide variety of DySND samples irrespective of industrial supplier, cooling and carbon soot refinement methods, initial purity, disintegration, or subsequent targeted chemical modification. This indicates that the intrinsic defects with S = 1 in DySND systems are of universal origin.

Evaluation of the binding of oxovanadium(iv) to human serum albumin

The understanding of the biotransformations of insulin mimetic vanadium complexes in human blood and its transport to target cells is an essential issue in the development of more effective drugs. We present the study of the interaction of oxovanadium(iv) with human serum albumin (HSA) by electron paramagnetic resonance (EPR), circular dichroism (CD) and visible absorption spectroscopy. Metal competition studies were done using Cu(II) and Zn(II) as metal probes. The results show that V(IV)O occupies two types of binding sites in albumin, which compete not only with each other, but also with hydrolysis of the metal ion. In one of the sites the resulting V(IV)O-HSA complex has a weak visible CD signal and its X-band EPR spectrum may be easily measured. This was assigned to amino acid side chains of the ATCUN site. The other binding site shows stronger signals in the CD in the visible range, but has a hardly measurable EPR signal; it is assigned to the multi metal binding site (MBS) of HSA. Studies with fatted and defatted albumin show the complexity of the system since conformational changes, induced by the binding of fatty acids, decrease the ability of V(IV)O to bind albumin. The possibility and importance of ternary complex formation between V(IV)O, HSA and several drug candidates - maltol (mal), picolinic acid (pic), 2-hydroxypyridine-N-oxide (hpno) and 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone (dhp) was also evaluated. In the presence of maltol the CD and EPR spectra significantly change, indicating the formation of ternary VO-HSA-maltol complexes. Modeling studies with amino acids and peptides were used to propose binding modes. Based on quantitative RT EPR measurements and CD data, it was concluded that in the systems with mal, pic, hpno, and dhp (V(IV)OL(2))(n)(HSA) species form, where the maximum value for n is at least 6 (mal, pic). The degree of formation of the ternary species, corresponding to the reaction V(IV)OL(2) + HSA -->/<-- V(IV)OL(2)(HSA) is hpno > pic ≥ mal > dhp. (V(IV)OL)(n)(HSA) type complexes are detected exclusively with pic. Based on the spectroscopic studies we propose that in the (V(IV)OL(2))(n)(HSA) species the protein bounds to vanadium through the histidine side chains.

Paramagnetic Oxotungsten(V) Complexes Containing the Hydrotris(3,5-dimethylpyrazol-1-yl)borate Ligand

Sky-blue Tp*WOCl(2) has been synthesized from the high-yielding reaction of Tp*WO(2)Cl with boron trichloride in refluxing toluene. Dark-red Tp*WOI(2) was prepared via thermal decarbonylation followed by aerial oxidation of Tp*WI(CO)(3) in acetonitrile. From these precursors, an extensive series of mononuclear tungstenyl complexes, Tp*WOXY [X = Cl(-), Y = OPh(-), SPh(-); X = Y = OPh(-), 2-(n-propyl)phenolate (PP(-)), SPh(-), SePh(-); XY = toluene-3,4-dithiolate (tdt(2-)), quinoxaline-2,3-dithiolate (qdt(2-)), benzene-1,2-diselenolate (bds(2-)); Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate], was prepared by metathesis with the respective alkali-metal salt of X(-)/XY(2-) or (NHEt(3))(2)(qdt). The complexes were characterized by microanalysis, mass spectrometry, electrochemistry, IR, electron paramagnetic resonance (EPR), and electronic absorption spectroscopies, and X-ray crystallography (for X = Y = OPh(-), PP(-), SPh(-); XY = bds(2-)). The six-coordinate, distorted-octahedral tungsten centers are coordinated by terminal oxo [W≡O = 1.689(6)-1.704(3) Å], tridentate Tp*, and monodentate or bidentate O/S/Se-donor ligands. Spin Hamiltonian parameters derived from the simulation of fluid-solution X-band EPR spectra revealed that the soft-donor S/Se ligand complexes had larger g values and smaller (183)W hyperfine coupling constants than the less covalent hard-donor O/Cl species. The former showed low-energy ligand-to-metal charge-transfer bands in the near-IR region of their electronic absorption spectra. These oxotungsten(V) complexes display lower reduction potentials than their molybdenum counterparts, underscoring the preference of tungsten for higher oxidation states. Furthermore, the protonation of the pyrazine nitrogen atoms of the qdt(2-) ligand has been examined by spectroelectrochemistry; the product of the one-electron reduction of [Tp*WO(qdtH)](+) revealed usually intense low-energy bands.

Long-Range Spin Coupling: A Tetraphosphine-Bridged Palladium Dimer

The dipalladium compound [{(adt)Pd}(2)(μ-tpbz)] (1) (adt = bis(p-anisyl)-1,2-ethylenedithiolate, tpbz = 1,2,4,5-tetrakis(diphenylphosphino)benzene) has been synthesized from [{Cl(2)Pd}(2)(μ-tpbz)] by transmetalation employing (adt)SnMe(2). The cyclic voltammogram (CV) of 1 reveals reversible oxidation waves at 0.00 V and +0.50 V (vs [Fc](+)/Fc) with current amplitude twice that for identical processes in the monopalladium compound [(adt)Pd(dppb)] (2) (dppb = 1,2-bis(diphenylphosphino)benzene), an observation indicating each wave involves simultaneous one-electron oxidations at each metallodithiolene fragment. This assignment is affirmed by density functional theory (DFT) calculations that show the redox-active molecular orbital (MO) is principally composed of the dithiolene S(2)C(2) π-system, and by spectroelectrochemical UV-vis of [1](2+), which displays hallmark low energy charge transfer (CT) bands. Dication [1](2+) is a diradical with a near degenerate singlet-triplet ground state; fluid solution electron paramagnetic resonance (EPR) spectra validate the DFT-derived isotropic exchange coupling, J' = -6.3 cm(-1). The frozen solution X-band EPR spectrum of [1](2+) is consistent with a spin-triplet bearing a very faint half-field ("ΔM(S) = 2") signal. It is successfully simulated with an amazingly small zero field splitting, D = -15 × 10(-4) cm(-1) and negligible rhombicity (E/D = 0.008). These zero-field splitting parameters, which stem from the long-range dipolar spin coupling, are very accurately reproduced using a multipoint dipole model with an optimized interspin distance of 12.434 Å. With the framework reported herein for understanding how the weak interaction of two spins is mediated by tpbz, this bridging ligand can now be incorporated into extended systems with tailored chemical and physical properties for use in a variety of molecular-based electronic and magnetic devices.