Field Electron Paramagnetic Resonance Research Articles

Low-Spin Hexacoordinate Mn(III): Synthesis and Spectroscopic Investigation of Homoleptic Tris(pyrazolyl)borate and Tris(carbene)borate Complexes

Three complexes of Mn(III) with "scorpionate" type ligands have been investigated by a variety of physical techniques. The complexes are [Tp(2)Mn]SbF(6) (1), [Tp(2)*Mn]SbF(6) (2), and [{PhB(MeIm)(3)}(2)Mn](CF(3)SO(3)) (3a), where Tp(-) = hydrotris(pyrazolyl)borate anion, Tp*(-) = hydrotris(3,5-dimethylpyrazolyl)borate anion, and PhB(MeIm)(3)(-) = phenyltris(3-methylimidazol-2-yl)borate anion. The crystal structure of 3a is reported; the structures of 1 and 2 have been previously reported, but were reconfirmed in this work. The synthesis and characterization of [{PhB(MeIm)(3)}(2)Mn]Cl (3b) are also described. These complexes are of interest in that, in contrast to many hexacoordinate (pseudo-octahedral) complexes of Mn(III), they exhibit a low-spin (triplet) ground state, rather than the high-spin (quintet) ground state. Solid-state electronic absorption spectroscopy, SQUID magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy were applied. HFEPR, in particular, was useful in characterizing the S = 1 spin Hamiltonian parameters for complex 1, D = +19.97(1), E = 0.42(2) cm(-1), and for 2, D = +15.89(2), E = 0.04(1) cm(-1). In addition, frequency domain Fourier-transform THz-EPR spectroscopy, using coherent synchrotron radiation, was applied to 1 only and gave results in good agreement with HFEPR. Variable-temperature dc magnetic susceptibility measurements of 1 and 2 were also in good agreement with the HFEPR results. This magnitude of zero-field splitting (zfs) is over 4 times larger than that in comparable hexacoordinate Mn(III) systems with S = 2 ground states. Complexes 3a and 3b (i.e., regardless of counteranion) have a yet much larger magnitude zfs, which may be the result of unquenched orbital angular momentum so that the spin Hamiltonian model is not appropriate. The triplet ground state is rationalized in each complex by ligand-field theory (LFT) and by quantum chemistry theory, both density functional theory and unrestricted Hartree-Fock methods. This analysis also shows that spin-crossover behavior is not thermally accessible for these complexes as solids. The donor properties of the three different scorpionate ligands were further characterized using the LFT model that suggests that the tris(carbene)borate is a strong σ-donor with little or no π-bonding.

Facile hydrothermal synthesis and photocatalytic activity of rod-like nanosized silver tungstate

Rod-like nanosized silver tungstate (Ag2WO4) was synthesised via a facile hydrothermal method and characterised by field emission scanning electron microscopy, X-ray diffraction, Raman spectroscopy and electron spin resonance spectroscopy. Under ultraviolet (UV) light, nearly complete degradation of rhodamine B (RhB) was observed in Ag2WO4 suspension after irradiation for 60 min, around 15% RhB was absorbed in the dark. However, more than 60% of methyl blue was absorbed in the dark and the rest was observed to be completely degraded upon UV light irradiation for 10 min. The formed superoxide (O2−.) and hydroxyl (·OH) radicals, as well as photoinduced holes were considered to be the dominant active species in the photocatalytic process.

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...

Developments of Multi-extreme High Field ESR in Kobe

Recent developments of “multi-extreme” high magnetic field electron spin resonance (ESR) in Kobe will be reviewed. Our high magnetic field ESR covers the frequency region between 0.03 and 7 THz and the temperature region between 1.8 and 300 K. With this high magnetic field ESR system we can apply the magnetic field up to 55 T using a Cu-Ag pulsed magnet and a 300 kJ (10 kV) capacitor bank. Under this high magnetic field we can also apply the high pressure up to 1.4 GPa. As we can make the measurement under low temperature, high magnetic field and high pressure simultaneously, we name it as “multi-extreme” ESR. Moreover, in order to gain the sensitivity of our high magnetic field ESR, we have developed a micro-cantilever ESR system using a torque method, which enables the ESR measurement of micrometer size single crystal at low temperature. At the moment we are in the process of extending the magnetic field region of micro-cantilever ESR. Recently we have succeeded in making the measurement up to 369 GHz and the achieved sensitivity is about 1010 spins/G, which is much higher than that using the conventional transmission method. Finally our development of magnetization detected ESR using SQUID magnetometer (SQUID ESR) will be also presented.

Investigating Magnetostructural Correlations in the Pseudooctahedraltrans-[NiII{(OPPh2)(EPPh2)N}2(sol)2] Complexes (E = S, Se; sol = DMF, THF) by Magnetometry, HFEPR, and ab Initio Quantum Chemistry

In this work, magnetometry and high-frequency and -field electron paramagnetic resonance spectroscopy (HFEPR) have been employed in order to determine the spin Hamiltonian (SH) parameters of the non-Kramers, S = 1, pseudooctahedral trans-[Ni(II){(OPPh(2))(EPPh(2))N}(2)(sol)(2)] (E = S, Se; sol = DMF, THF) complexes. X-ray crystallographic studies on these compounds revealed a highly anisotropic NiO(4)E(2) coordination environment, as well as subtle structural differences, owing to the nature of the Ni(II)-coordinated solvent molecule or ligand E atoms. The effects of these structural characteristics on the magnetic properties of the complexes were investigated. The accurately HFEPR-determined SH zero-field-splitting (zfs) D and E parameters, along with the structural data, provided the basis for a systematic density functional theory (DFT) and multiconfigurational ab initio computational analysis, aimed at further elucidating the electronic structure of the complexes. DFT methods yielded only qualitatively useful data. However, already entry level ab initio methods yielded good results for the investigated magnetic properties, provided that the property calculations are taken beyond a second-order treatment of the spin-orbit coupling (SOC) interaction. This was achieved by quasi-degenerate perturbation theory, in conjunction with state-averaged complete active space self-consistent-field calculations. The accuracy in the calculated D parameters improves upon recovering dynamic correlation with multiconfigurational ab initio methods, such as the second-order N-electron valence perturbation theory NEVPT2, the difference dedicated configuration interaction, and the spectroscopy-oriented configuration interaction. The calculations showed that the magnitude of D (∼3-7 cm(-1)) in these complexes is mainly dominated by multiple SOC contributions, the origin of which was analyzed in detail. In addition, the observed largely rhombic regime (E/D = 0.16-0.33) is attributed to the highly distorted metal coordination sphere. Of special importance is the insight by this work on the zfs effects of Se coordination to Ni(II). Overall, a combined experimental and theoretical methodology is provided, as a means to probe the electronic structure of octahedral Ni(II) complexes.

How Do Heavier Halide Ligands Affect the Signs and Magnitudes of the Zero-Field Splittings in Halogenonickel(II) Scorpionate Complexes? A Theoretical Investigation Coupled to Ligand-Field Analysis.

This work presents a detailed analysis of the physical origin of the zero-field splittings (ZFSs) in a series of high-spin (S = 1) nickel(II) scorpionate complexes Tp*NiX (Tp* = hydrotris(3,5-dimethylpyrazole)borate, X = Cl, Br, I) using quantum chemical approaches. High-frequency and -field electron paramagnetic resonance studies have shown that the complexes with heavier halide ligands (Br, I) have greater magnitudes but opposite signs of the ZFSs compared with the chloro congener (Desrochers, P. J.; Telser, J.; Zvyagin, S. A.; Ozarowski, A.; Krzystek, J.; Vicic, D. A. Inorg. Chem.2006, 45, 8930-8941). To rationalize the experimental findings, quantum chemical calculations of the ZFSs in this Ni(II) halide series have been conducted. The computed ZFS using wave-function-based ab initio methods (state-averaged CASSCF, NEVPT2, and SORCI) are in good agreement with the experiment. For comparison, density functional theory was only marginally successful. The ligand-field analysis demonstrates that the signs and magnitudes of the ZFSs are subtly determined by the trade-off between the negative contributions from the (1,3)A1(1e→2e) transitions relative to the positive contributions from the remaining d-d excited states. The term from (1,3)A1(1e→2e) stems from the structural feature that the metal center displaces out of the equatorial plane, and gains the importance when heavier halide ligand is involved.

High Spin Co(I): High-Frequency and -Field EPR Spectroscopy of CoX(PPh3)3 (X = Cl, Br)

The previously reported pseudotetrahedral Co(I) complexes, CoX(PR(3))(3), where R = Me, Ph, and chelating analogues, and X = Cl, Br, I exhibit a spin triplet ground state, which is uncommon for Co(I), although expected for this geometry. Described here are studies using electronic absorption and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy on two members of this class of complexes: CoX(PR(3))(3), where R = Ph and X = Cl and Br. In both cases, well-defined spectra corresponding to axial spin triplets were observed, with signals assignable to three distinct triplet species, and with perfectly axial zero-field splitting (zfs) given by the parameter D = +4.46, +5.52, +8.04 cm(-1), respectively, for CoCl(PPh(3))(3). The crystal structure reported for CoCl(PPh(3))(3) shows crystallographic 3-fold symmetry, but with three structurally distinct molecules per unit cell. Both of these facts thus correlate with the HFEPR data. The investigated complexes, along with a number of structurally characterized Co(I) trisphosphine analogues, were analyzed by quantum chemistry calculations (both density functional theory (DFT) and unrestricted Hartree-Fock (UHF) methods). These methods, along with ligand-field theory (LFT) analysis of CoCl(PPh(3))(3), give reasonable agreement with the salient features of the electronic structure of these complexes. A spin triplet ground state is strongly favored over a singlet state and a positive, axial D value is predicted, in agreement with experiment. Quantitative agreement between theory and experiment is less than ideal with LFT overestimating the zfs, while DFT underestimates these effects. Despite these shortcomings, this study demonstrates the ability of advanced paramagnetic resonance techniques, in combination with other experimental techniques, and with theory, to shed light on the electronic structure of an unusual transition metal ion, paramagnetic Co(I).

Electronic Structure of Nickel(II) and Zinc(II) Borohydrides from Spectroscopic Measurements and Computational Modeling

The previously reported Ni(II) complex, Tp*Ni(κ(3)-BH(4)) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate anion), which has an S = 1 spin ground state, was studied by high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy as a solid powder at low temperature, by UV-vis-NIR spectroscopy in the solid state and in solution at room temperature, and by paramagnetic (11)B NMR. HFEPR provided its spin Hamiltonian parameters: D = 1.91(1) cm(-1), E = 0.285(8) cm(-1), g = [2.170(4), 2.161(3), 2.133(3)]. Similar, but not identical parameters were obtained for its borodeuteride analogue. The previously unreported complex, Tp*Zn(κ(2)-BH(4)), was prepared, and IR and NMR spectroscopy allowed its comparison with analogous closed shell borohydride complexes. Ligand-field theory was used to model the electronic transitions in the Ni(II) complex successfully, although it was less successful at reproducing the zero-field splitting (zfs) parameters. Advanced computational methods, both density functional theory (DFT) and ab initio wave function based approaches, were applied to these Tp*MBH(4) complexes to better understand the interaction between these metals and borohydride ion. DFT successfully reproduced bonding geometries and vibrational behavior of the complexes, although it was less successful for the spin Hamiltonian parameters of the open shell Ni(II) complex. These were instead best described using ab initio methods. The origin of the zfs in Tp*Ni(κ(3)-BH(4)) is described and shows that the relatively small magnitude of D results from several spin-orbit coupling (SOC) interactions of large magnitude, but with opposite sign. Spin-spin coupling (SSC) is also shown to be significant, a point that is not always appreciated in transition metal complexes. Overall, a picture of bonding and electronic structure in open and closed shell late transition metal borohydrides is provided, which has implications for the use of these complexes in catalysis and hydrogen storage.

High-frequency and -field EPR and FDMRS study of the [Fe(H 2O) 6] 2+ ion in ferrous fluorosilicate

The complex [Fe(H 2O) 6]SiF 6 is one of the most stable and best characterized high-spin Fe(II) salts and as such, is a paradigm for the study of this important transition metal ion. We describe high-frequency and -field electron paramagnetic resonance studies of both pure [Fe(H 2O) 6]SiF 6 and [Zn(H 2O) 6]SiF 6 doped with 8% of Fe(II). In addition, frequency domain magnetic resonance spectroscopy was applied to these samples. High signal-to-noise, high resolution spectra were recorded which allowed an accurate determination of spin Hamiltonian parameters for Fe(II) in each of these two, related, environments. For pure [Fe(H 2O) 6]SiF 6, the following parameters were obtained: D = +11.95(1) cm −1, E = 0.658(4) cm −1, g = [2.099(4), 2.151(5), 1.997(3)], along with fourth-order zero-field splitting parameters: B 4 0 = 17 ( 1 ) × 10 - 4 cm - 1 and B 4 4 = 18 ( 4 ) × 10 - 4 cm - 1 , which are rarely obtainable by any technique. For the doped complex, D = +13.42(1) cm −1, E = 0.05(1) cm −1, g = [2.25(1), 2.22(1), 2.23(1)]. These parameters are in good agreement with those obtained using other techniques. Ligand-field theory was used to analyze the electronic absorption data for [Fe(H 2O) 6]SiF 6 and suggests that the ground state is 5A 1, which allows successful use of a spin Hamiltonian model. Density functional theory and unrestricted Hartree–Fock calculations were performed which, in the case of latter, reproduced the spin Hamiltonian parameters very well for the doped complex.

A high field and frequency electron paramagnetic resonance study of the S = 2 chromium(II) complex trans-[Cr(NCCH 3) 4(FBF 3) 2] and investigation of its reaction with dioxygen: Crystal structure of [(CH 3CN) 5CrOCr(NCCH 3) 5](BF 4) 4·2CH 3CN

From high field and frequency electron paramagnetic resonance (EPR) powder spectra of the high spin chromium(II) complex trans-[Cr(NCCH 3) 4(FBF 3) 2] ( 1) recorded at T = 5–40 K the EPR parameters were found to be D = −2.19 cm −1, E ≈ 0 cm −1, and g ‖ = g ⊥ = 1.9833. The reaction between a MeCN solution of 1 and dioxygen gave dark green crystals of [(CH 3CN) 5CrOCr(NCCH 3) 5](BF 4) 4·2CH 3CN ( 2). From the X-ray crystal structure of 2 at T = 122 K a Cr–O–Cr angle of 180° and a Cr–O distance of 1.7556(5) Å was found. This is the shortest among the known Cr–O distances in dinuclear complexes containing a CrOCr 4+ core, and this can partly explain the strong antiferromagnetic coupling between the two chromium(III) centers in 2.

Effect of Metal Ions on the Indigenous Radicals of Humic Acids: High Field Electron Paramagnetic Resonance Study

The interaction of indigenous radicals of humic acid (HA) with metal cations has been studied using high magnetic field (10.5T-285 GHz) electron paramagnetic resonance (HFEPR) spectroscopy. Strong [HA]-[metal] interaction was observed in the case of heavy metals, Cd(2+), Pb(2+), and Sr(2+), leading to formation of covalent bonds with the radicals of HA. On the contrary, alkaline earth metal ions, such as Mg(2+), generate only electrostatic interaction. The two types of indigenous radicals that exist in all HAs are influenced by the metal cations in a unified manner. This provides evidence that the two types of indigenous radicals in HAs originate from a unique, phenolic, moiety in HA. Mg(2+) ions dramatically changed the pH profile of the two radical types of HA, downshifting their interconversion pK(a) by ca. 3 pH units. This is the first experimental observation of the effect of metals on the H-dissociation of the radical centers in HAs.

Theoretical investigations of the local distortion and electron paramagnetic resonance parameter for CdCl2:V2+ and CsMgX3:V2+ (X = Cl, Br) systems

This paper systematically investigates the local distortion and electron paramagnetic resonance (EPR) parameter for CdCl2:V2+ and CsMgX3:V2+ (X = Cl, Br) systems on the basis of the complete energy matrix, in which not only the contributions due to the spin–orbit coupling of the central ions but also that of the ligands are considered. To describe the difference of overlapping between d-orbits and p orbit, two spin–orbit coupling coefficients are introduced. By simulating the crystal field parameter and EPR parameter, the local distortion parameters are studied and the relationships between the EPR parameter and the spin–orbit coupling coefficients as well as divergent parameter are discussed. These results show that the local structures exhibit compression distortion for CdCl2:V2+ and elongation distortions for CsMgX3:V2+ (X = Cl, Br), respectively. It notes that the empirical formula R ≈ RH + (ri – rb)/2 is not suitable for CdCl2:V2+ and CsMgX3:V2+ (X = Cl, Br) systems. The contributions of ligand to spin–orbit coupling interaction cannot be neglected for strong covalent systems, especially for V2+ doped in CsMgBr3:V2+.

Spin relaxation in a ferromagnetically coupled triangular Cu 3 complex

The trinuclear complex [Bu 4N 2[Cu 3(μ 3-Cl) 2(μ-pz) 3Cl 3] (pz = pyrazolato anion) ( 1) exhibits ferromagnetic interactions resulting in an S = 3/2 ground state which is characterized by a small zero field splitting tensor [17]. Herein, the S = 3/2 state is further characterized using High-Frequency and -Field Electron Paramagnetic Resonance spectroscopy. The spin relaxation properties of 1 are studied with alternating current susceptometry in the presence of external magnetic fields. In the 4.5–8.0 K temperature range, the system relaxes via an Orbach mechanism involving transitions between the S = 3/2 and the excited S = 1/2 spin manifolds.

Cobalt(II) “Scorpionate” Complexes as Models for Cobalt-Substituted Zinc Enzymes: Electronic Structure Investigation by High-Frequency and -Field Electron Paramagnetic Resonance Spectroscopy

A series of complexes of formula Tp(R,R')CoL, where Tp(R,R'-) = hydrotris(3-R,5-R'-pyrazol-1-yl)borate ("scorpionate") anion (R = tert-butyl, R' = H, Me, 2'-thienyl (Tn), L = Cl(-), NCS(-), NCO(-), N(3)(-)), has been characterized by electronic absorption spectroscopy in the visible and near-infrared (near-IR) region and by high-frequency and -field electron paramagnetic resonance (HFEPR). Reported here are also crystal structures of seven members of the series that have not been reported previously: R' = H, L = NCO(-), N(3)(-); R' = Me, L = Cl(-), NCS(-), NCO(-), N(3)(-); R' = Tn, L = Cl(-), NCS(-). These include a structure for Tp(t-Bu,Me)CoCl different from that previously reported. All of the investigated complexes contain a four-coordinate cobalt(II) ion (3d(7)) with approximate C(3v) point group symmetry about the metal ion and exhibit an S = (3)/(2) high-spin ground state. The use of HFEPR allows extraction of the full set of intrinsic S = (3)/(2) spin Hamiltonian parameters (D, E, and g values). The axial zero-field splitting parameter, D, for all investigated Tp(R,R')CoL complexes is always positive, a fact not easily determined by other methods. However, the magnitude of this parameter varies widely: 2.4 cm(-1) <or= D <or= 12.7 cm(-1), indicating the extreme sensitivity of this parameter to environment. The spin Hamiltonian parameters are combined with estimates of 3d energy levels based on the visible-near-IR spectra to yield ligand-field parameters for these complexes following the angular overlap model (AOM). This description of electronic structure and bonding in pseudotetrahedral cobalt(II) complexes can enhance the understanding of similar sites in metalloproteins, specifically cobalt-substituted zinc enzymes.

High Field ESR Spectroscopy on GdO1−x F x FeAs

In the present work we have studied polycrystalline samples of the GdO1−x F x FeAs superconductor by means of high field electron spin resonance (HF-ESR) spectroscopy. A set of the samples with different levels of fluorine doping was measured in the frequency range from 10 GHz to 400 GHz in magnetic fields up to 15 T. Surprising results have been obtained on the GdO0.85F0.15FeAs sample with the superconducting transition temperature T c =20 K. Gd ESR gives clear indications of the enhancement of (quasi)-static magnetic correlations which set in below ∼80 K and continue to develop even in the superconducting state. This suggests an occurrence of an intimate interplay between magnetism and superconductivity in the FeAs planes which evolves in GdO1−x F x FeAs compound upon the fluorine doping.

Multifrequency EPR studies of manganese catalases provide a complete description of proteinaceous nitrogen coordination.

Pulse electron paramagnetic resonance (EPR) spectroscopy is employed at two very different excitation frequencies, 9.77 and 30.67 GHz, in the study of the nitrogen coordination environment of the Mn(III)Mn(IV) state of the dimanganese-containing catalases from Lactobacillus plantarum and Thermus thermophilus. Consistent with previous studies, the lower-frequency results reveal one unique histidine nitrogen-Mn cluster interaction. For the first time, a second, more strongly hyperfine-coupled (14)N atom is unambiguously observed through the use of higher frequency/higher field EPR spectroscopy. The low excitation frequency spectral features are rationalized as arising from the interaction of a histidine nitrogen that is bound to the Mn(IV) ion, and the higher excitation frequency features are attributed to the histidine nitrogen bound to the Mn(III) ion. These results allow for the computation of intrinsic hyperfine coupling constants, which range from 2.2 to 2.9 MHz, for sp(2)-hybridized nitrogens coordinating equatorially to high-valence Mn ions. The relevance of these findings is discussed in the context of recent results from analogous higher frequency EPR studies of the Mn cluster in photosystem II and other exchange-coupled, transition metal-containing systems.

Formation of magnetic polarons in lightly Ca doped LaCoO3

We performed high field electron spin resonance, nuclear magnetic resonance and static magnetization measurements on a powder sample of lightly hole-doped La1−xCaxCoO3, x ~ 0.002 in order to study the influence of the size of the substitution ion on the formation of the hole induced spin-state polaron. Previous works [1, 2] showed that doping of LaCoO3 with Sr in very small concentrations (x ~ 0.002) yields the formation of a magnetic polaron with a big spin value and large spin orbital coupling. The Ca2+ ion, in contrast to the Sr2+ion, has almost the same ionic radius as the La3+ ion. Therefore, the substitution of Ca for La provides mainly a hole to the system without creation of a sizeable crystal field distortion around the substituted Ca ion. The data obtained on La0.998Ca0.002CoO3 provide experimental evidence that the introduced hole indeed plays the main role in the formation of the spin-state polaron. Accompanying crystal field distortions seem to play a minor role, e.g. influencing the fine splitting of the spin polaron energy levels and the contribution of the spin orbital coupling.

Family of V(III)-Tristhiolato Complexes Relevant to Functional Models of Vanadium Nitrogenase: Synthesis and Electronic Structure Investigations by Means of High-Frequency and -Field Electron Paramagnetic Resonance Coupled to Quantum Chemical Computations.

A series of V(III) complexes of varying coordination number (5, 6, and 7) all containing the PS3 ligand (PS3 = trianion of tris(2-thiophenyl)phosphine and its derivatives with other phenyl substituents) has been prepared and structurally characterized. The complexes have general formula [V(PS3)L(n)](0,-), where n = 1 (from L = Cl(-), 1-Me-Im, N(3)(-)), 2 (from L = 2,2'-bpy; counting each N of the bidentate ligand), and 3 (from L = 1-Me-Im, N(2)H(4)). The complexes have also been investigated by direct current (DC) magnetic susceptibility and high-frequency and -field electron paramagnetic resonance (HFEPR). HFEPR, supported by magnetometry, has provided accurate spin Hamiltonian parameters that describe the S = 1 spin ground state of the complexes. Of particular interest are the zero-field splitting (zfs) parameters which, together with structural data, are the empirical starting point for detailed computational studies. The computational methods included density functional theory (DFT), which was only marginally successful, and more advanced ab initio methods (CASSCF and SORCI). The zfs in these complexes is relatively small in magnitude (|D| approximately 1 cm(-1)) and is the result of multiple, often counteracting, spin-orbit coupling (SOC) and spin-spin coupling (SSC) contributions. The specific origin of each of these contributions is described in detail. The results indicate the level of electronic structure calculation possible for transition metal complexes even with multiple unpaired electrons and highly covalent, heavier atom donor ligands.

Influence of Pb(II) Ions on the EPR Properties of the Semiquinone Radicals of Humic Acids and Model Compounds: High Field EPR and Relativistic DFT Studies

X-band (9.76 GHz) and high field (416.00 GHz) electron paramagnetic resonance spectroscopy (EPR) was used to study the interactions between Pb(II) ions and semiquinone radicals of natural humic acids and their simple models. The EPR experiments were performed on powder samples. The formation of Pb(II) complexes with the radicals was accompanied by a significant decrease of g parameters as compared to those observed for parent radicals. Two types of complexes were identified depending on the initial concentration of Pb(II) ions. For one of them the anisotropic hyperfine coupling with the (207)Pb nucleus was observed. Systematic DFT calculations were carried out for complexes with different forms of radical ligands (L(2)(-*), HL(-*), and H(2)L*) derived from 3,4-dihydroxybenzoic acid representing different ligation schemes. The g parameters calculated for the structure characterized by a significant accumulation of spin density on the Pb atom are strongly deviated from the values observed experimentally. Moreover, a decrease of the spin population on all oxygen atoms as a result of complexation of Pb(II) via carboxyl oxygens and protonation of hydroxyl oxygens is required to reproduce the experimental g parameters.

Synthesis and spectroscopic investigations of four-coordinate nickel complexes supported by a strongly donating scorpionate ligand

Two four-coordinate nickel complexes, HB( t BuIm) 3NiBr and HB( t BuIm) 3NiNO, were prepared by reaction of a bulky tris(carbene)borate ligand with NiBr 2(PPh 3) 2 and NiBr(NO)(PPh 3) 2, respectively, and structurally and spectroscopically characterized. In addition to standard techniques, high-frequency and -field electron paramagnetic resonance (HFEPR) was employed to understand the spin triplet ( S = 1) ground state of the bromo complex. HFEPR, combined with electronic absorption spectroscopy allows comparison of this novel complex with other paramagnetic four-coordinate Ni(II) species. The tris(carbene)borate ligand is a stronger σ-donor than corresponding tris(pyrazolyl)borates (traditional “scorpionate” ligands). The tris(carbene)borate ligand may also act as a π-acceptor, in contrast to tris(pyrazolyl)borates, which show relatively little π-bonding interactions. The influence of tris(carbene)borate substituents on the donor strength of the ligand have been elucidated from IR spectroscopic investigations of {NiNO} 10 derivatives. HFEPR spectra of HB( t BuIm) 3NiBr exhibit hyperfine coupling from Br, which indicates the strong electronic interaction between Ni(II) and this halide ligand, consistent with studies on tris(pyrazolyl)borate Ni(II) complexes.