CpNi(diselenolene)] neutral radical complexes: electron paramagnetic resonance and density functional theory investigations

77Se-enriched CpNi(bds) (bds = 1,2-benzenediselenolate), has been synthesized and its g tensor and 77Se hyperfine tensors have been obtained from its frozen solution electron paramagnetic resonance (EPR) spectrum. These parameters are consistent with those calculated by density functional theory (DFT); it is shown that 10% of the spin is localized on each selenium and that the direction associated to the maximum 77Se couplings is aligned along the gmin direction, perpendicular to the Ni(bds) plane. EPR measurements and DFT calculations are also carried out on the 77Se enriched complex CpNi(dsit) as well on the two dithiolene analogues CpNi(bdt) and CpNi(dmit). The optimized structures of the isolated CpNi(bds) and CpNi(bdt) complexes have been used to generate the idealized dimers (bds)NiCp...CpNi(bds) and (bdt)NiCp...CpNi(bdt) characterized by Cp...Cp overlap. The exchange parameters J calculated at the DFT level for these systems are in reasonable accord with the experimental values. The influence of the geometry of the dimer on its magnetic properties is assessed by calculating the variation of J as a function of the relative orientation of the two Ni(diselenolene) or Ni(dithiolene) planes.     

Density functional restricted-unrestricted approach for nonlinear properties: application to electron paramagnetic resonance parameters of square planar copper complexes

The density functional restricted-unrestricted approach for treatments of spin polarization effects in molecular properties using spin restricted Kohn-Sham theory has been extended from linear to nonlinear properties. It is shown that the spin polarization contribution to a nonlinear property has the form of a quadratic response function that includes the zero-order Kohn-Sham operator, in analogy to the lower order case where the spin polarization correction to an expectation value has the form of a linear response function. The developed approach is used to formulate new schemes for computation of electronic g-tensors and hyperfine coupling constants, which include spin polarization effects within the framework of spin restricted Kohn-Sham theory. The proposed computational schemes are in the present work employed to study the spin polarization effects on electron paramagnetic resonance spin Hamiltonian parameters of square planar copper complexes. The obtained results indicate that spin polarization gives rise to sizable contributions to the hyperfine coupling tensor of copper in all investigated complexes, while the electronic g-tensors of these complexes are only marginally affected by spin polarization and other factors, such as choice of exchange-correlation functional or molecular structures, will have more pronounced impact on the accuracy of the results.     

1H NMR, electron paramagnetic resonance, and density functional theory study of dinuclear pentaammineruthenium dicyanamidobenzene complexes

Paramagnetic (1)H NMR and electron paramagnetic resonance (EPR) spectroscopies and density functional theory (DFT) spin density calculations were selectively performed on the [{(NH(3))(5)Ru}(2)(μ-L)](3+,?4+,?5+) complexes, where L is 2,3,5,6-tetrachloro-, 2,5-dichloro-, 2,5-dimethyl-, and unsubstituted 1,4-dicyanamidobenzene dianion, to characterize the electronic structure of these complexes. EPR spectra of the [{(NH(3))(5)Ru}(2)(μ-L)](3+) complexes in N,N'-dimethylformamide at 4 K showed a ruthenium axial signal, and thus the complexes are [Ru(II),L(2-), Ru(III)] mixed-valence systems. DFT spin density calculations of [{(NH(3))(5)Ru}(2)(μ-L)](3+) where L = 1,4-dicyanamidobenzene dianion gave mostly bridging-ligand centered spin distribution for both vacuum and implicit solvent calculations, in poor agreement with EPR, but more realistic results were obtained when explicit electrostatic interactions between solute and solvent were included in modeling. For the [{(NH(3))(5)Ru}(2)(μ-L)](4+) complexes, EPR spectroscopy showed no signal down to 4 K. Nevertheless, solvent-dependent (1)H NMR data and analysis support a [Ru(III),L(2-), Ru(III)] state. Hyperfine coupling constants (A(c)/h) of trans- and cis-ammine and phenyl hydrogens were determined to be 17.2, 3.8, and -1.5 MHz respectively. EPR studies of the [{(NH(3))(5)Ru}(2)(μ-L)](5+) complexes showed a metal-radical axial signal and based on previously published (1)H NMR data, a [Ru(IV),L(2-), Ru(III)] state is favored over a [Ru(III),L(-), Ru(III)] state.     

Hydrogen bond geometries from electron paramagnetic resonance and electron-nuclear double resonance parameters: density functional study of quinone radical anion-solvent interactions

Density functional theory was used to study the impact of hydrogen bonding on the p-benzosemiquinone radical anion BQ(*-) in coordination with water or alcohol molecules. After complete geometry optimizations, (1)H, (13)C, and (17)O hyperfine as well as (2)H nuclear quadrupole coupling constants and the g-tensor were computed. The suitability of different model systems with one, two, four, and 20 water molecules was tested; best agreement between theory and experiment could be obtained for the largest model system. Q-band pulse (2)H electron-nuclear double resonance (ENDOR) experiments were performed on BQ(*-) in D(2)O. They compare very well with the spectra simulated by use of the theoretical values from density functional theory. For BQ(*-) in coordination with four water or alcohol molecules, rather similar hydrogen-bond lengths between 1.75 and 1.78 A were calculated. Thus, the computed electron paramagnetic resonance (EPR) parameters are hardly distinguishable for the different solvents, in agreement with experimental findings. Furthermore, the distance dependence of the EPR parameters on the hydrogen-bond length was studied. The nuclear quadrupole and the dipolar hyperfine coupling constants of the bridging hydrogens show the expected dependencies on the H-bond length R(O.H). A correlation was obtained for the g-tensor. It is shown that the point-dipole model is suitable for the estimation of hydrogen-bond lengths from anisotropic hyperfine coupling constants of the bridging (1)H nuclei for H-bond lengths larger than approximately 1.7 A. Furthermore, the estimation of H-bond lengths from (2)H nuclear quadrupole coupling constants of bridging deuterium nuclei by empirical relations is discussed.     

Dinuclear copper(II) complexes with ethylenediamine derivative and bridging oxalato ligands: solvatochromism and density functional theory studies

Two oxalato-bridged copper(II) complexes of formula, [Cu₂(L¹)₂(µ-ox)](NO₃)₂·H₂O, 1 and [Cu₂(L²)₂(µ-ox)](NO₃)₂·H₂O, 2 (ox = oxalato dianion, L¹ = N,N-dimethyl,N′-benzylethane-1,2-diamine, L² = N,N-diethyl,N′-benzylethane-1,2-diamine), have been synthesized and characterized by elemental analyses, spectroscopic (IR, UV–Vis) data and molar conductance measurements. The crystal structure of complex 1 was determined by X-ray diffraction analysis, revealing two centrosymmetric dinuclear units. The first consists of a [Cu₂(L¹)₂(µ-ox)(NO₃)₂] molecule, in which each Cu(II) center is in a square-pyramidal environment, providing two nitrogen atoms from the diamine-chelating ligands plus two oxygen atoms from the oxalate in the basal plane and an oxygen of the nitrate group in the axial position. The second unit [Cu₂(L¹)₂(µ-ox)(H₂O)₂](NO₃)₂ has a similar structure, but the apical sites are occupied by water ligands and the nitrate anions are free from coordination. Both complexes are solvatochromic. Their solvatochromism was investigated with different solvent parameter models using SPSS/PC and DFT methodology. The solvatochromic behaviors of the complexes were also explored by TD-DFT in ethanol and acetonitrile solvents. The calculated visible absorption spectra were in accord with the experimental results.     

High frequency electron paramagnetic resonance (HFEPR) study of a high spin Co(II) complex

Variable high-frequency electron paramagnetic resonance data were collected for a single crystal of [Zn(hmp)(dmb)Cl]₄ (1) doped with a small quantity of high spin Co(II), where dmb is 3,3-dimethyl-1-butanol and hmp- is the monoanion of 2-hydroxy-methylpyridine. The lack of solvent in the lattice of complex 1 results in very little disorder. Consequently, the EPR spectra are extremely sharp, enabling precise comparisons with theoretical simulations. We find the ground state of the Co(II) ions to be an effective spin S′ = 1/2 Kramers’ doublet with a highly anisotropic g-tensor. The anisotropy is found to be of the easy-axis type, with the single-ion easy axis directions tilted away from the crystallographic c direction by 58°.     

Structure and electron paramagnetic resonance parameters of the manganese site of concanavalin A studied by density functional methods

The EPR parameters of the manganese site in the saccharide-binding protein concanavalin A have been studied by density functional methods, with an emphasis on metal (⁵⁵Mn) and ligand (¹H and ¹⁷O) hyperfine couplings, in comparison with high-field EPR and ENDOR data. Results for gradient-corrected and hybrid functionals with different exact-exchange admixture have been compared with experiment for the ⁵⁵Mn and the ¹H ligand hyperfine coupling and have been predicted for ¹⁷O hyperfine coupling based on comparison with experiment for the related [Mn(H₂O)₆]²⁺. Appreciable exact-exchange admixture in the hybrid functional is needed to obtain an adequate spin-density distribution and thus near-quantitative agreement with experimental EPR parameters. The common use of experimental proton hyperfine coupling tensors together with the point-dipole approximation for determination of bond lengths is evaluated by explicit calculations.     

Some new rhodium(I)-iridium(III) complexes with bridging hydride and chloride ligands

[IrH₅(PEt₃)₂] reacts with [(PR₃)₂Rh(μ₂-Cl)₂(PR₃)₂] (PR₃ = PEt₃ or 2 PR₃ = Ph₂PCH₂CH₂PPh₂) to give the hydrido-bridged binuclear species [(PR₃)₂Rh(μ₂-H)(μ₂-Cl)IrH₂(PR₃)₂] which show catalytic activity in alkene hydrogenation.     

Spin hamiltonian parameters for Cu(II)-prion peptide complexes from L-band electron paramagnetic resonance spectroscopy

Cu(II) is an essential element for life but is also associated with numerous and serious medical conditions, particularly neurodegeneration. Structural modeling of crystallization-resistant biological Cu(II) species relies on detailed spectroscopic analysis. Electron paramagnetic resonance (EPR) can, in principle, provide spin hamiltonian parameters that contain information on the geometry and ligand atom complement of Cu(II). Unfortunately, EPR spectra of Cu(II) recorded at the traditional X-band frequency are complicated by (i) strains in the region of the spectrum corresponding to the g(∥) orientation and (ii) potentially very many overlapping transitions in the g(⊥) region. The rapid progress of density functional theory computation as a means to correlate EPR and structure, and the increasing need to study Cu(II) associated with biomolecules in more biologically and biomedically relevant environments such as cells and tissue, have spurred the development of a technique for the extraction of a more complete set of spin hamiltonian parameters that is relatively straightforward and widely applicable. EPR at L-band (1-2 GHz) provides much enhanced spectral resolution and straightforward analysis via computer simulation methods. Herein, the anisotropic spin hamiltonian parameters and the nitrogen coordination numbers for two hitherto incompletely characterized Cu(II)-bound species of a prion peptide complex are determined by analysis of their L-band EPR spectra.     

Cu(II) complexes in bacterial growth medium: electron spin resonance study

In this study we report a spectroscopic investigation on the structure and stability of Cu(II)-complexes that are formed in a minimum growth medium (MM), normally used for Bacillus subtilis cultures. As other transition metals, Cu(II) compounds are toxic to this bacterium and the toxicity depends on the Cu(II) concentration. MM contained NH4+ ions and asparagine (asn) as the source of inorganic and organic nitrogen. Both ESR and electronic spectra demonstrated the very important role played by the amino acid asparagine in the coordinative behaviour of Cu(II). In particular, three different complexes were evidenced: Cu(H2O)6(2+); Cu(asn)+ and Cu(asn)2. The relative amount of these three species strongly depended on pH, on Cu:asn ratio and on the presence of the phosphate ions. They were identified and evaluated quantitatively by extensive simulation of the electron spin resonance (ESR) spectra recorded in different experimental conditions. The bis-complex was found to be more stable in MM than in an asparagine-containing water solution with the same Cu:asn ratio. A comparison of the spectroscopic results with microbiological investigations is also made.     

Dinuclear palladium(ii) complexes with bridging amidate ligands

New homonuclear dimeric Pd(ii) complexes have been synthesized by the reaction of Pd(en)(2+) or Pd(bipy)(2+) (where en = ethylenediamine and bipy = 2,2'-bipyridine) units with acetamide or by the Pd(ii) mediated hydrolysis of CH(3)CN. In these dimers the two metal centers are bridged by either two amidates or by the combination of one hydroxo group and one amidate ligand. The crystal structures of complexes {[Pd(bipy)](2)(micro-1,3-CH(3)CONH)(2)}(NO(3))(2).H(2)O.1/2(CH(3))(2)CO.1/2CH(3)CN () and {[Pd(bipy)](2)(micro-1,3-CH(3)CONH)(2)}(OTf)(2) () showed intrametallic Pd-Pd distances of 2.8480(8) A () and 2.8384(7) A (), respectively, in accordance with the accepted values for a strong Pd-Pd interaction. The presence of pi[dot dot dot]pi interactions between the bipyridine ligands on the di-micro-amidate complexes of Pd(bipy)(2+) shortens the distance between the two Pd centers and allows the formation of the metal-metal interaction. By contrast, the crystal structure of complex {[Pd(en)](2)(micro-1,3-CH(3)CONH)(2)}(OTf)(2).H(2)O (), (where OTf = triflate) where there is no pi[dot dot dot]pi interaction between the ligands on the metal centers, is also reported, and no Pd-Pd interaction is observed. Additionally, one of the complexes, {[Pd(en)](2)(micro-OH)(micro-CH(3)CONH)}(NO(3))(2) (), presents an interesting hydrogen bonded 3-D network formed by nitrate ions and water molecules. All complexes have been characterized by infrared and (1)H NMR spectroscopy.     

Comparative density-functional study of the electron paramagnetic resonance parameters of amavadin

Electronic g tensors and hyperfine coupling tensors have been calculated for amavadin, an unusual eight-coordinate vanadium(IV) complex isolated from Amanita muscaria mushrooms. Different density-functional methods have been compared, ranging from local via gradient-corrected to hybrid functionals with a variable Hartree-Fock exchange admixture. For both electron paramagnetic resonance (EPR) properties, hybrid functionals with an appreciable exact-exchange admixture provide the closest agreement with experimental data. Second-order spin-orbit corrections provide non-negligible contributions to the 51V hyperfine tensor. The orientation of g and A tensors relative to each other also depends on spin-orbit corrections to the A tensor. A rationalization for the close resemblance of the EPR parameters of amavadin to those of the structurally rather different vanadyl complexes is provided, based on the nature of the relevant frontier orbitals.     

A density functional study of the electronic structure and spin Hamiltonian parameters of mononuclear thiomolybdenyl complexes

The electron paramagnetic resonance spin Hamiltonian parameters of mononuclear thiomolybdenyl complexes based upon the tris(pyrazolyl)borate ligand, together with their molybdenyl analogues, are calculated using density functional theory. The electronic g matrix and 95Mo hyperfine matrix are calculated as second-order response properties from the coupled-perturbed Kohn-Sham equations. The scalar relativistic zero-order regular approximation (ZORA) is used with an all-electron basis and an accurate mean-field spin-orbit operator which includes all one- and two-electron terms. The principal values and relative orientations of the g and A interaction matrices obtained from the experimental spectra in a previous EPR study are compared with those obtained from unrestricted Kohn-Sham calculations at the BP86 and B3LYP level, and the latter are found to be in good quantitative agreement. A quasi-restricted approach is used to analyze the influence of the various molecular orbitals on g and A. In all complexes the ground state magnetic orbital is dX2-Y2-based and the orientation of the A matrix is directly related to the orientation of this orbital. The largest single contribution to the orientation of the g matrix arises from the spin-orbit coupling of the dYZ-based lowest-unoccupied molecular orbital into the ground state. A number of smaller, cumulative charge-transfer contributions augment the d-d contributions. A comparison of the theoretical EPR parameters obtained using both crystallographic and gas-phase geometry-optimized structures of Tp*MoO(bdt) (Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate, bdt = 1,2-benzenedithiolate) suggests a correspondence between the metal-dithiolate fold angle and the angle of noncoincidence between g and A.     

Electron spin resonance spectroscopy of copper(II) complexes with schiff bases as ligands

The ESR spectra of a series of Cu^II Schiff base complexes have been investigated in liquid and frozen solutions. The structures of these complexes are estimated on the basis of superhyperfine interactions and molecular orbital coefficients α² and β²₁ calculated from copper hyperfine parameters.     

Isotropic paramagnetic shifts of the phosphorus and proton magnetic resonance and the spin density distribution in paramagnetic complexes of triphenylphosphine with nickel acetylacetonate

The Isotropic paramagnetic shifts in the H¹ and P³¹ NMR spectra of triphenylphosphine, forming labile complexes in chloroform with the acetylacetonato compound of nickel(II), have been measured. The concentration dependence of the shifts has been used to calculate the hyperfine interaction constants and the spin densities in an individual complex, on the assumption that the complex has the composition [Ni(AA)₂] (PPh₃)₂. It has been found that the spin density in the sp³ hybrid orbital of the phosphorus atom is positive and amounts to not less than 10%, indicating a -mechanism of transfer of spin density from the nickel to the phosphorus. The spin densities on the carbon atoms of the phenyl rings are smaller fay about two orders of magnitude than those on the phosphorus atom, and this is attributed to the orientation of these rings, which is unsuitable for conjugation with the unshared pair. The contributions of the different mechanisms to the delocalization of the spin density from the phosphorus atom to the -system of the rings have been estimated.The authors wish to express their gratitude to the late Academician V. V. Voevodskii, whose initiative and constant support made this joint work possible. The authors thank A. Nesmei, for assistance in the construction of the phosphorus resonance measurement apparatus, and G. M. Zhidomirov, for discussion of the results.     

Electron paramagnetic resonance of pyridine and thicyanate nitrosyl complexes of Cr(I)

Russian Chemical Bulletin -     

High-frequency and -field electron paramagnetic resonance of high-spin manganese(III) in tetrapyrrole complexes

High-field and -frequency electron paramagnetic resonance (HFEPR) spectroscopy has been used to study three complexes of high spin Manganese(III), 3d4, S = 2. The complexes studied were tetraphenylporphyrinatomanganese(III) chloride (MnTPPCI), phthalocyanatomanganese(III) chloride (MnPcCl), and (8,12-diethyl-2,3,7,13,17,18-hexamethylcorrolato)manganese(III) (MnCor). We demonstrate the ability to obtain both field-oriented (single-crystal like) spectra and true powder pattern HFEPR spectra of solid samples. The latter are obtained by immobilizing the powder, either in an n-eicosane mull or KBr pellet. We can also obtain frozen solution HFEPR spectra with good signal-to-noise, and yielding the expected true powder pattern. Frozen solution spectra are described for MnTPPCl in 2:3 (v/v) toluene/CH2Cl2 solution and for MnCor in neat pyridine (py) solution. All of the HFEPR spectra have been fully analyzed using spectral simulation software and a complete set of spin Hamiltonian parameters has been determined for each complex in each medium. Both porphyrinic complexes (MnTPPCl and MnPcCl) are rigorously axial systems, with similar axial zero-field splitting (zfs): D approximately -2.3 cm(-1), and g values quite close to 2.00. In contrast, the corrole complex, MnCor, exhibits slightly larger magnitude, rhombic zfs: D approximtely -2.6 cm(-1), absolute value(E) approximately 0.015 cm(-1), also with g values quite close to 2.00. These results are discussed in terms of the molecular structures of these complexes and their electronic structure. We propose that there is a significant mixing of the triplet (S = 1) excited state with the quintet (S= 2) ground state in Mn(III) complexes with porphyrinic ligands, which is even more pronounced for corroles.