X-band EPR Spectra Research Articles

Synthesis, Structural Characterization and Biological Studies of Schiff Base Complexes Bearing Indole Moiety

Schiff base complexes of N 2 O 2 type derived from indole-3-carboxaldehyde(ind) and amino acids such as glycine(gly), L-valine(val), and L-tryptophan(try) with Cu(II), Ni(II), Zn(II), Co(II) and Mn(II) were synthesized and characterized by physical and analytical techniques. The complexes are of non-electrolytic nature having 1:2 (metal:ligand) ratio. The geometry of the complexes was predicted with the combined data obtained from electronic spectral and magnetic susceptibility measurements. The mode of coordination of the Schiff base ligands with the metal ions is through the carboxylate oxygen and the azomethine nitrogen as confirmed by FT-IR spectra. The cyclic voltammogram of the Cu(II)complexes exhibit Cu(II)®Cu(I) peak in the cathodic side and the corresponding quasi-reversible type peak is obtained in the anodic side. The thermal behavior (TG/DTA) of the synthesized complexes shows that the complexes loose water molecules in the first step followed by decomposition of the ligand, which follows first order kinetics. The SEM analysis reveals the particle size and the morphology of the complexes. The X-band EPR spectrum of Cu-indgly complex in DMSO solution at 300 and 77 K was recorded. The calculated g tensor values agree with the geometry proposed by the electronic absorption spectral measurements. The M-indval complexes were studied for their cleaving ability with calf thymus DNA by gel electrophoresis method. The antibacterial activity of the Schiff base metal complexes was tested in vitro against some common bacteria using ampicillin as control. Antifungal activities were determined using nystatin as control. The results are compared with control; most of the metal chelates exhibited higher biological activities.

A variable-temperature Q- and X-band EPR study of spin-crossover iron(III) Schiff base complex

The mononuclear iron(III) Schiff base complex was analysed by EPR spectroscopy. The first derivative Q- and X-band EPR spectra of spin-crossover iron(III) complex exhibit the presence of the high spin (S = 5/2) and low spin (S = 1/2) iron(III) state. The variable-temperature EPR measurement confirmed a gradual spin-crossover transition (S = 5/2 ↔ S = 1/2). The possible hysteresis in the thermally induced spin-crossover effect was not detected. The EPR data are in agreement with observed trends in the magnetic measurements reported previously. Note that the EPR studies of this complex have not been performed before. It is obvious that EPR spectroscopy is a powerful method for investigation of the spin-crossover transition and their temperature evolution in the iron(III) complexes.

Efficient hydrolytic cleavage of DNA and antiproliferative effect on human cancer cells by two dinuclear Cu(II) complexes containing a carbohydrazone ligand and 1,10-phenanthroline as a coligand.

We report the synthesis, crystal structures and biological activities of two dinuclear Cu(II) complexes [Cu(o-phen)LCu(OAc)] (1) and [Cu(o-phen)LCu(o-phen)](OAc) (2), where o-phen = 1,10-phenanthroline, H3L = o-HOC6H4C(H)=N-NH-C(OH)=N-N=C(H)-C6H4OH-o, and OAc=CH3COO-. Both compounds display strong and broad X-band EPR spectra at RT in their powder state confirming that these are paramagnetic. The intercalative DNA binding of the compounds as revealed from spectrophotometric studies was found to be consistent with the results of fluorescence spectroscopic studies for ethidium bromide displacement assay as well as enhanced viscosity of DNA in the presence of these compounds. The compounds effectively catalyze hydrolytic cleavage of supercoiled pUC19 DNA and show remarkable cytotoxicity toward human lung cancer A549 cell line (IC50 values are 4.34 and 8.46µM for 1 and 2, respectively) and breast cancer MCF7 cell line (IC50 values are 6.50 and 8.68µM for 1 and 2, respectively) and are found to be relatively less toxic toward keratinocyte HaCaT normal cell line (IC50 values are 11.19 and 16.01µM for 1 and 2, respectively). Annexin-V/PI dual staining results analyzed by flow cytometry strongly suggest the induction of apoptotic pathway for the anticancer activity of these complexes. Flow cytometry experiment for cell cycle analysis showed considerable increase in the G2/M phase in both A549 and MCF7 cell lines by these two compounds. On the other hand, compounds 1 and 2 activate reactive oxygen species (ROS) level in A549 cells, but act as scavengers or inhibitors of ROS in MCF7 cell line as analyzed by DCFDA staining using flow cytometry. Two dinuclear Cu(II) complexes exhibit efficient hydrolytic cleavage of DNA and display remarkable cytotoxicity against human lung cancer A549 and breast cancer MCF7cells. The ROS level in A549 cells is activated, but the ROS level in MCF7 cells is decreased in the presence of these complexes. Cell cycle analysis by flow cytometry shows G2/M phase arrest in both these cell lines.

Evaluation of EPR parameters for compressed and elongated local structures of VO2+ and Cu2+ spin probes in BaO-TeO2-B2O3 glasses

EPR parameters and the local configurations of the transition metal (TM) ions VO2+ and Cu2+ in BaO-TeO2-B2O3 glasses were experimentally evaluated and also theoretically verified. From the recorded X-band EPR spectra the spin-Hamiltonian parameters (SHP) were determined. The optical spectra of vanadium doped glass samples revealed a single absorption band at 596 nm which was assigned to 2B2g → 2B1g transition, whereas copper doped glasses revealed two absorption bands around 818 and 602 nm which were assigned to 2B1g → 2B2g and 2B1g → 2E2g transitions respectively. Theoretical studies were accomplished on the EPR and optical factors from perturbation theory. It was observed from both the experimental and theoretical SHP values of vanadium containing glasses that the VO2+ ions are in tetragonally compressed octahedral sites with dxy (2B2g) ground state, while the copper containing glasses found to be in tetragonally elongated octahedral sites with dx2−y2 (2B1g) as the ground state. It is found that the magnitude of the relative tetragonal compression ratio (φ) of vanadium ion is less than the relative tetragonal elongation ratio (τ) of copper ion. The nature of the bonding between TM ions (i.e. vanadium and copper) and their ligands was assessed. Theoretical and experimental EPR parameters are in great concurrence with each other.

N-alkylamino analogs of Vitamin K3: Electrochemical, DFT and anticancer activity of their oxidized and one electron reduced form

Synthesis, characterization and anticancer activity of reduced form of n-alkylamino analogs of Vitamin K3 (1Na to 8Na) are undertaken. Molecular structure and molecular association of oxidized form of 2-ethylamino-3-methyl-1,4-naphthoquinone (2) and 2-hexylamino-3-methyl-1,4-naphthoquinone (6) were studied by single crystal X-ray analysis. 2 crystallize in monoclinic C2/c and 6 in P21 space group. The reduced form of eight homologated analogs of 2-(n-alkylamino)-3-methyl-1,4-naphthoquinone (1Na to 8Na) were synthesized using sodium metal as reductant at 0 °C in methanol. The formations of naphthosemiquinone radical in 1 to 6 have been confirmed from their EPR spectra. Polycrystalline powder X-band EPR spectra of 1 to 6 shows signals ∼2.0020 ± 0.0026 at 133 K. Anticancer activity of 2-(n-alkylamino)-3-methyl-1,4-naphthoquinone (1Na to 8Na) and one electron reduced forms have been evaluated against breast cancer (HeLa) cell line, 1 and 1Na showed promising anticancer activity.

EPR and Magnetization Studies of Polymer-Derived Fe-Doped SiCN Nanoceramics Annealed at Various Temperatures: Blocking Temperature, Superparamagnetism and Size Distributions

X-band EPR spectra on SiCN ceramics, doped with Fe(III) ions, annealed at 800 °C, 1000 °C, 1100 °C, 1285 °C, and 1400 °C have been simulated to understand better their magnetic properties, accompanied by new magnetization measurements in the temperature range of 5–400 K for zero-field cooling (ZFC) and field cooling (FC) at 100C. The EPR spectra reveal the presence of several kinds of Fe-containing nanoparticles with different magnetic properties. The maxima of the temperature variation of ZFC magnetization were exploited to estimate (i) the blocking temperature, which decreased with annealing temperature of the samples and (ii) the distribution of the size of Fe-containing nanoparticles in the various samples, which was found to become more uniform with increasing annealing temperature, implying that more homogenous magnetic SiCN/Fe composites can be fabricated by annealing at even higher temperatures than 1400 °C to be used as sensors. The hysteresis curves showed different behaviors above (superparamagnetic), below (ferromagnetic), and about (butterfly shape) the respective average blocking temperatures, 〈TB〉. An analysis of the coercive field dependence upon temperature reveals that it follows Stoner–Wohlfarth model for the SiCN/Fe samples annealed above 1100 °C, from which the blocking temperatures was also deduced.

Supramolecular assembled networks in crystal structure built up of copper(II) dipicolinates with pyrazine- and pyridinecarboxamides connected through hydrogen bonds

New copper(II) dipicolinate complexes containing pyrazine- or pyridinecarboxamides, namely {[Cu(pca)(dipic)(H2O)]·H2O} (1), [Cu(nia)(dipic)(H2O)] (2), {[Cu(mnia)(dipic)(H2O)]·2H2O} (3), {[Cu(inia)(dipic)]n·2nCH3OH} (4) {[Cu(hmnia) (dipic)(H2O)]2·2[Cu(hmnia)(dipic)(H2O)2]·4H2O} (5) and [Cu(denia)(dipic)(H2O)] (6) (dipic = dipicolinate, anion derived from pyridine-2,6-dicarboxylic acid, pca = pyrazinecarboxamide, nia = nicotinamide, mnia = N-methylnicotinamide, inia = isonicotinamide, hmnia = N-(hydroxymethyl)nicotinamide and denia = N,N-diethylnicotinamide) were synthesized, structurally characterised by single-crystal X-ray diffraction analysis and their properties were studied using IR, UV–Vis and EPR spectroscopic techniques. The Cu(II) compounds are of square-pyramidal (1–4 and 6) or square-bipyramidal geometry (5), in which copper(II) centres are coordinated by N donor atom of a nicotinamide or its derivatives, one N donor atom and two O donor atoms of dipicolinate anions and the coordination sphere around the Cu(II) atoms is completed by terminal aqua ligands (besides 4). The complex 4 is a polymer, in which are dipicolinate ligands bridging via O atoms of their carboxylate groups neighbouring Cu(II) atoms. The compound 5 is made up of two different types of complex molecules, dinuclear [Cu(hmnia)(dipic)(H2O)]2 and mononuclear [Cu(hmnia)(dipic)(H2O)2], and lattice solvent molecules in the ratio of 1:2:4. Copper(II) atoms in the dinuclear complex molecules of 5 [Cu(hmnia)(dipic)(H2O)]2 are interconnected through carboxamide O donor atoms of bridging N-(hydroxymethyl)nicotinamide ligands. In most cases are complex molecules to each other and solvent molecules linked together via NH⋯O and OH⋯O hydrogen bonds and form 2D or 3D supramolecular networks. The X-band EPR spectra of six copper(II) complexes were recorded at room and low temperature (98 K). The g-factor relation g// > g⊥ > 2.0023 is consistent with the dx2-y2 ground electronic state.

Lanthanide coordination compounds with benzimidazole-based ligands. luminescence and EPR

LnIII coordination compounds were synthesized with two benzimidazolic ligands, 2 pb = 2-(2-pyridyl)benzimidazole and 4tb = 2-(4-thiazolyl)benzimidazole, with Eu, Gd, Tb and Dy, namely: [Ln (NO3)3 (Ln)2]⋅S, where Ln = 2 pb, 4tb and S = solvent. They were characterized by IR, 1D and 2D 1H and 13C NMR, solid state UV–Vis–NIR absorption and emission and EPR spectroscopies, X-ray powder diffraction, elemental analyses, electrical conductivity and magnetic susceptibility. The compounds of EuIII with 2 pb and 4tb, TbIII and DyIII with 4tb showed luminescence at RT in the solid state; their emission life time ranged from 0.026 to 1.82 ms. 2 pb and 4tb sensitized lanthanides luminescence. Results of DFT theoretical calculations for the ligand (H2L) are used to assign the ligand singlet and triplet excited state energy levels. Additionally, a full analysis of the X-band EPR spectra in the solid state gave the following results, at 297 K for [Gd(NO3)3 (2 pb)2]·CH3COCH3, g = 1.990, |D| = 0.0687 cm−1, E = 0.0165 cm−1 and λ = |E/D| = 0.240; for [Gd(NO3)3 (4tb)2]⋅CH3COCH3, g = 2.000; |D| = 0.0670 cm−1, E = 0.000 cm−1 and λ = |E/D| = 0.000. At lower temperature, 128 K, compounds showed an increase of the rhombicity as the temperature decreased.

Walking the seven lines: binuclear copper A in cytochrome c oxidase and nitrous oxide reductase.

The enzymes nitrous oxide reductase (N2OR) and cytochrome c oxidase (COX) are constituents of important biological processes. N2OR is the terminal reductase in a respiratory chain converting N2O to N2 in denitrifying bacteria; COX is the terminal oxidase of the aerobic respiratory chain of certain bacteria and eukaryotic organisms transforming O2 to H2O accompanied by proton pumping. Different spectroscopies including magnetic resonance techniques, were applied to show that N2OR has a mixed-valent Cys-bridged [Cu1.5+(CyS)2Cu1.5+] copper site, and that such a binuclear center, called CuA, does also exist in COX. A sequence motif shared between the CuA center of N2OR and the subunit II of COX raises the issue of a putative evolutionary relationship of the two enzymes. The suggestion of a binuclear CuA in COX, with one unpaired electron delocalized between two equivalent Cu nuclei, was difficult to accept originally, even though regarded as a clever solution to many experimental observations. This minireview in honor of Helmut Sigel traces several of the critical steps forward in understanding the nature of CuA in N2OR and COX, and discusses its unique electronic features to some extent including the contributions made by the development of methodology and the discovery of a novel multi-copper enzyme. Left: X-band (9.130 GHz) and C-band (4.530 GHz, 1st harmonic display of experimental spectrum) EPR spectra of bovine heart cytochrome c oxidase, recorded at 20K. Right: Ribbon presentation of the CuA domain in cytochrome c oxidase and nitrous oxide reductase.

Syntheses, characterization, superoxide dismutase, antimicrobial, crystal structure and molecular studies of copper (II) and nickel (II) complexes with 2-((E)-(2, 4-dibromophenylimino) methyl)-4-bromophenol as Schiff base ligand

Three new copper (II) and nickel (II) complexes viz. [Cu(L)2](1a), [Cu(L)2](1b) and [Ni(L)2].DMF(2), where HL = 2-((E)-(2, 4-dibromophenylimino) methyl)-4-bromophenol, have been synthesized and characterized by using various physico-chemical and spectroscopic techniques. The crystal structures of Schiff base (HL) and their metal complexes (1a), (1b) and (2) were determined by single crystal X-ray diffraction. IR and UV–Vis spectra and magnetic susceptibility measurements agree with the observed crystal structures. The crystallographic and spectroscopic studies confirmed four coordinate environments around the metal (II) ions. The synthesized Schiff base ligand (HL) crystallizes in the orthorhombic system of the space group Pbca. Complex (1a) of HL was crystallized in the monoclinic system of the space group P21/c, a = 10.1712(9) Å, b = 10.9299(10) Å,c = 12.7684(11) Å,α = 90̊,β = 104.649(2)̊, γ = 90̊ and Z = 2 whereas complex (1b) and (2) crystallized in the triclinic system of the space group P-1, a = 11.499(5)Å, b = 11.598(5)Å, c = 12.211(5)Å, α = 98.860(5), β = 115.653(5),γ = 100.906(5) and Z = 2 for (1b), a = 9.080(6) Å, b = 9.545(8)Å, c = 9.545(8)Å, α = 101.43(4)º,β = 99.63(3)̊, γ = 117.71(2)º and Z = 1 for (2). The synthesized ligand (HL) was behaved as monobasic bidentate Schiff base ligand having N and O donor sites. The electron paramagnetic resonance spectra indicate a dx2–y2ground state (g|| > g⊥> 2.0023) for (1a) and (1b). Copper (II) complexes display X-band EPR spectra in 100% DMSO and 77 K, giving indicating dx2–y2ground state. Superoxide dismutase-like activities of HL and its complexes were investigated by nitrobluetetrazolium chloride-DMSO assay and IC50 values were evaluated. These complexes were also tested for their in vitro antimicrobial activities against two bacteria (E. coli and Salmonella typhi) and two fungi (Pencillium, Aspergillus sp.) comparing with the Schiff base. The antimicrobial results showed that the complexes were more biologically active compounds to the Schiff base (HL).

The EPR Spectrum of Triplet Mesitylphosphinidene: Reassignment and New Assignment

Low-temperature UV-photolysis of mesitylphosphiranes under highly anaerobic conditions leads to the formation of the triplet mesitylphosphinidene (MesP). The recorded X-band EPR spectrum of triplet MesP and the derived zero-field splitting parameter D=4.116 cm-1 differ significantly from those reported previously for this intermediate. New magnetic parameters of mesitylphosphinidene are discussed along with the results of DFT calculations.

Hydrogen Evolution Catalysis by a Sparsely Substituted Cobalt Chlorin

A sparsely substituted chlorin macrocycle containing a Co(II) center (1-Co) has been synthesized and structurally characterized. The Co(II) atom resides in a square planar coordination environment and induces significant out-of-plane distortion of the chlorin macrocycle. The paramagnetic Co(II) center resides in the macrocycle in a S = 1/2 spin state, which displays an axial doublet signal (g⊥ ≈ 2.3 and g∥ ≈ 2.03) in the X-band EPR spectrum. The open-shell d-orbital configuration is manifest in the transient absorption spectrum, which reveals an excited-state lifetime of 8.6 ± 0.2 ps for 1-Co. The Co(II) chlorin exhibits a rich oxidation–reduction chemistry with five reversible one-electron waves (three oxidative processes and two reductive processes) observed in the cyclic voltammogram. The reduction processes of 1-Co drive hydrogen evolution catalysis. Electrochemical kinetics analysis of HER by 1-Co in trifluoroacetic acid reveals a hydrogen evolution mechanism that proceeds by an ECEC mechanism. Bench...

Syntheses, single crystal structures, DFT and antioxidant superoxide dismutase studies of some new mono-/binuclear copper(II) complexes

A series of mono- and binuclear copper(II) complexes have been synthesized using HL1 and other mono- and bidentate ligands (L2) [HL1=N′-[(E)-phenyl(pyridin-2-yl)methylidene]thiophene-2-carbohydrazide, L2=2-benzoyl pyridine (BP), neocuprin, imidazole (ImH) and pyrazine] by a one pot synthesis. These complexes were characterized by routine physico-chemical techniques. The single crystal X-ray structures of 1, 2, 3 and 5 were determined. X-ray single crystal analyses reveal that complex 1 is six coordinated while complex 2 is a penta coordinated complex. Complex 3 is pseudo binuclear while 4 and 5 are binuclear complexes. Intermolecular π…π stacking interactions could be detected in the solid medium and classical H-bonding interactions lead to infinite 1D chains and to an infinite 2D network in 2. Variable temperature magnetic susceptibility data display antiferromagnetic couplings in 3–5. The X-band epr spectra for polycrystalline samples of 3–5 exhibit copper(II) hyperfine structure as well as zero-field splitting which are appropriate for the triplet state of such a dimer. Epr data also indicate antiferromagnetic behavior for binuclear complexes 3–5. Binuclear complexes were also studied using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The conproportion constant (Kcon) of complexes 3–5 have been evaluated and discussed. Modeling by DFT and TD-DFT is done to elucidate the nature and location of the excited state in superoxide dismutase (SOD) mimics. In addition, antioxidant SOD activity showed that HL1 and its copper(II) complexes possess significant scavenging effect against O2− free radicals.

Determination of ZFS parameters from the EPR spectra of mono-, di- and trinuclear MnII complexes: impact of magnetic coupling.

A family of new MnII compounds, consisting of seven dinuclear, three mononuclear, and four trinuclear ones, were synthesised using benzoic acid derivatives n-RC6H4COOH, where n-R = 2-MeO, 3-MeO, 4-MeO, or 4-tBu, and 2,2'-bipyridine (bpy) or 1,10-phenantroline (phen) as blocking ligands. The crystal structures of nine of these compounds and the magnetic studies of all of them are reported here. Each type of compound was formed depending on the presence or absence of ClO4- ions, the solvent used, and/or the presence of a small amount of water in the reaction medium. The use of the tert-buthylbenzoate ligand gave unexpected results, very likely due to the steric hindrance caused by the voluminous tBu groups. The EPR spectra of each type of compound give some peculiar features that allow its identification. Attempts to fit these spectra have been made in order to determine the ZFS parameters, D and E, of the MnII ion (for mononuclear and dinuclear systems) or of the ground state (for trinuclear systems). For trinuclear systems, the single-ion ZFS parameters estimated from those of the ground state provided a good simulation of the EPR spectra of these compounds. The EPR signals observed in each case have been rationalised according to the energy level distribution and the plausible population in the excited states. In some particular situations, the sign of DMn could be determined from the fit of the EPR spectra of the antiferromagnetic dinuclear compounds, the source of the difference between the spectra lying in the second excited state.

Low temperature EPR investigation of Co2+ ion doped into rutile TiO2 single crystal: Experiments and simulations

In this paper, we present the results of X-band EPR spectra of Co2+ ion doped rutile (TiO2) which is one of the most promising memristor material. We obtained the angular variation of spectra in three mutually perpendicular planes at liquid helium (7–13K) temperatures. Since the impurity ions have ½ effective spin and 7/2 nuclear spin, a relatively simple spin Hamiltonian containing only electronic Zeeman and hyperfine terms was utilized. Two different methods were used in theoretical analysis. Firstly, a linear regression analysis of spectra based on perturbation theory was studied. However, this approach is not sufficient for analyzing Co+2 spectra and leads to complex eigenvectors for G and A tensors due to large anisotropy of eigenvalues. Therefore, all spectra were analyzed again with exact diagonalization of spin Hamiltonian and the high accuracy eigenvalues and eigenvectors of G and A tensors were obtained by taking into account the effect of small sample misalignment from the exact crystallographic planes due to experimental conditions. Our results show that eigen-axes of g and A tensors are parallel to crystallographic directions. Hence, our EPR experiments proves that Co2+ ions substitute for Ti4+ ions in lattice. The obtained principal values of g tensor are gx=2.110(6), gy=5.890(2), gz=3.725(7) and principal values of hyperfine tensor are Ax=42.4, Ay=152.7, Az=26 (in 10−4/cm).

EPR and optical properties of Eu2+ and Mn2+ co-doped MgSrAl10O17 blue–green light emitting powder phosphors

Strong blue–green light emitting MgSrAl10O17:Eu2+,Mn2+ phosphor was synthesized by a low-temperature initiated, self-propagating and gas producing combustion process in a very short time (<5min). Structural characterization of the luminescent material was studied with X-ray diffraction analysis and energy-dispersive X-ray analysis. The absorption spectrum exhibits bands due to Eu2+ and Mn3+ ions. The excitation spectrum shows a peak at 337nm. Upon excitation at 337nm, the emission spectrum exhibits an intense band centered at 462nm due to transitions from the 4f65d1 to the 4f7 configuration of the Eu2+ ions, whereas sharp peak at 513nm attributed to 4T1→6A1 transition of Mn2+ ions. The X-band EPR spectra of MgSrAl10O17:Eu2+,Mn2+ showed the presence of Eu2+ and Mn2+ ions.

New bis-o-iminosemiquinonate aluminium(III) complexes

The first aluminium(III) complexes based on sterically hindered 4,6-di-tert-butyl-N-(2,6-di-iso-propyl-phenyl)-o-iminobenzoquinone (imQ) of general formula imSQ2AlR (where imSQ — is a radical anion of imQ, R=I (1), NCS (2) and N3 (3)) were synthesized. Compounds 1–3 demonstrate biradical X-band EPR spectra in frozen toluene matrix. The crystalline samples of 1–3 were characterized using magnetic susceptibility measurements. The antiferromagnetic coupling between spins of o-iminosemiquinonate radicals in 1–3 is predominant and, as the result, the singlet ground state for pentacoordinated biradical compounds is observed. The molecular structures of all the complexes obtained have been established by the single crystal X-ray analysis.

Divalent manganese, cobalt and nickel chloride complexes with neutral N2- and N3-based ligands derived from 2-pyridinecarbaldehyde: Synthesis, structural features and spectroscopic studies

A series of dihalide M(II) mononuclear complexes in a cis configuration, Mn(HL1)2Cl2·CH3OH·H2O (1), Co(HL1)2Cl2·CH3OH·H2O (2), Ni(HL1)2Cl2·2H2O (3) and CoL2Cl2·0.5dmf (4) (HL1 – Schiff base ligand 4-N-(2′-pyridylimine)benzoic acid; L2 – pyridin-2-yl(3-(pyridin-2-yl)imidazo[1,5-a]pyridin-1-yl) methanone, both formed in situ from 2-pyridinecarbaldehyde), have been obtained as single crystals and characterized by elemental analysis, IR and EPR spectroscopy, and single-crystal X-ray diffraction techniques. The neutral M(L1)2Cl2 and CoL2Cl2 molecules in 1–3 and 4, respectively, show no crystallographically imposed symmetry with the metal atoms in a MN4Cl2 distorted octahedral (1–3) and CoN3Cl2 (4) elongated trigonal–bipyramidal geometries. In the crystal lattice molecules of 1–3 are arranged in columns propagating along the c axis; the structure of 4 can be described as layers of CoL2Cl2 and dmf molecules alternating along the a axis. The minimum M⋯M separations in a column (1–3) and inside a layer (4) are above 8Å. The X-band solid-state EPR spectra of 1 at room temperature and 77K exhibit broad fine structure signals at 1600 and 3000G (geff∼4.14 and 2.22, respectively) resulting from ZFS of the spin states for the high-spin d5 metal ion. The solid-state EPR spectra of 2 are temperature dependent showing a wide unresolved line at geff∼2.09 at room temperature and a very broad signal (ΔHpp=2500 G) at geff∼4.21 at 77K. The Ni analogue 3 is EPR-silent. The EPR spectra of 4 at both temperatures show a broad asymmetrical signal centred at g∼2 suggesting the low-spin configuration; the line width is nearly independent of temperature.

Rapid-scan coherence signals in X-band EPR spectra of semiquinones with small hyperfine splittings

Rapid-scan EPR signals for semiquinones with very-small well-resolved hyperfine splittings exhibit coherence signals at a time after passing through the EPR line that is proportional to the reciprocal of the hyperfine splitting. Such coherences are a general phenomenon due to constructive interference of the responses to transient excitation of spins by rapid scan of the magnetic field across equally spaced spin packets. Examples are shown for 2,3,5,6-tetramethoxy-1,4-benzosemiquinone with aH=46mG for 12 protons and for 2,5-di-t-butyl-1,4-benzosemiquinone with aH=59mG for 18 protons.

Indirect Magnetic Exchange between o-Iminosemiquinonate Ligands Controlled by Apical Substituent in Pentacoordinated Gallium(III) Complexes.

A number of pentacoordinated gallium complexes iSQ2GaR (1-7) (R = Et (1), Me (2), N3 (3), Cl (4), Br (5), I (6), NCS (7)) where iSQ is a radical anion of 4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-iminobenzoquinone were synthesized, and crystalline samples of 1-7 were characterized using magnetic susceptibility measurements. The character of magnetic exchange interaction between spins of o-iminosemiquinonate radicals was found to be strongly influenced by the nature of the apical substituent. The antiferromagnetic coupling is predominant when the apical position is occupied by halogens or other tested inorganic anions, and the value of exchange interaction parameter varies from -99 to -176 K for R = I and NCS, respectively. In the case of alkyl groups the ferromagnetic exchange prevails and, as the result, the triplet ground state for pentacoordianted biradical compounds was observed. Compounds 1-7 demonstrate a biradical X-band EPR spectrum in frozen toluene matrix. The molecular structures of 4, 6, and 7 have been established by single-crystal X-ray analysis. A computational DFT UB3LYP/6-31G(d,p) study was performed on complexes 1-7 in order to understand the reason for changes in the magnetic behavior of the related diradical gallium compounds. The calculations showed that the magnetic behavior of the complexes with inorganic anions is conditioned by the presence of antiferromagnetic exchange channel formed as a consequence of overlapping between donor atomic orbitals of iminoquinone with π-orbitals of halogen atoms (4-6) or nitrogen atom (3, 7).