EPR Spectra Of Solutions Research Articles

Synthesis, structures, spectral and electrochemical properties of copper(II) complexes of sterically hindered Schiff base ligands

The Schiff base ligands 2-(2,6-diisopropylphenyliminomethyl)phenol H( L1), 5-diethylamino-2-(2,6-diisopropylphenyliminomethyl)phenol H( L2), 2,4-di- tert-butyl-6-(2,6-diisopropylphenyliminomethyl)phenol H( L3), 3-(2,6-diisopropylphenyliminomethyl)naphthalen-2-ol H( L4) and 4-(2,6-diisopropylphenyliminomethyl)-5-hydroxymethyl-2-methylpyridin-3-ol H(L5) have been synthesized by the condensation, respectively, of salicylaldehyde, 4-(diethylamino)salicylaldehyde, 3,5-di- tert-butylsalicylaldehyde, 2-hydroxy-1-napthaldehyde and pyridoxal with 2,6-diisopropylaniline. The copper(II) bis-ligand complexes [Cu(L1) 2] 1, [Cu(L2) 2] 2, [Cu(L3) 2] 3, [Cu(L4) 2] 4 and [Cu(L5) 2] · CH 3OH 5 of these ligands have been isolated and characterized. The X-ray crystal structures of two of the complexes [Cu(L1) 2] 1 and [Cu(L5) 2] · CH 3OH 5 have been successfully determined, and the centrosymmetric complexes possess a CuN 2O 2 chromophore with square planar coordination geometry. The frozen solution EPR spectra of the complexes reveal a square-based CuN 2O 2 chromophore, and the values of g ‖ and g ‖/ A ‖ index reveal enhanced electron delocalization by incorporating the strongly electron-releasing –NEt 2 group ( 2) and fusing a benzene ring on sal-ring ( 4). The Cu(II)/Cu(I) redox potentials of the Cu(II) complexes reveal that the incorporation of electron-releasing –NEt 2 group and fusion of a benzene ring lead to enhanced stabilization of Cu(II) oxidation state supporting the EPR spectral results. The hydrogen bonding interactions between the two molecules present in the unit cell of 5a generate an interesting two-dimensional hydrogen-bonded network topology.

Synthesis, structure and properties of some copper(II) complexes containing an ONO donor Schiff base and substituted imidazole ligands

Five new mixed ligand coper(II) complexes, viz. [Cu(SAA)(H 2O)] ( 1), [Cu(SAA)(MeImH)] ( 2), [Cu(SAA)(EtImH)] ( 3), [Cu(SAA)(BenzImH)] ( 4) and [Cu(SAA)(MebenzImH)] ( 5), where SAA = salicylideneanthranilic acid, MeImH = 2-methylimidazole, EtImH = 2-ethylimidazole, BenzImH = benzimidazole, MebenzImH = 2-methylbenzimidazole, have been synthesized and characterized by means of elemental analysis, FAB mass spectrometry, magnetic susceptibility, X-band EPR, electronic spectroscopy, IR and cyclic voltammetry. The frozen solution EPR spectra of the complexes have shown axial features. Single crystal X-ray analysis of 2 and 3 has revealed the presence of a distorted square planar geometry (N 2O 2) in the complexes. The superoxide dismutase (SOD) activity of the present complexes has also been measured and discussed.

A Proposed EPR Approach to Evaluating Agonist Binding Site of a Peptide Receptor

Angiotensin II (Ang II) and its transmembrane AT1 receptor were selected in order to test an innovative strategy that might allow the assessment of the agonist binding site in the receptor molecule. With the use of the 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC) paramagnetic probe, a biologically active agonist (TOAC1-Ang II), as well as an inactive control (TOAC4-Ang II) analogs were mixed in solution with various synthesized AT1 fragments. Comparative intermolecular interactions, as estimated by analyzing the EPR spectra of solutions, suggested the existence of an agonist binding site containing a sequence composed of portions of the N-terminal (13-17) and the third extracellular loop (266-278) fragments of the AT1 molecule. Therefore, this combined EPR-TOAC approach shows promise as an alternative for use also in other applications related to specific intermolecular association processes.

EPR spectra and structures of dinuclear copper(II) complexes with acyldihydrazones of benzenedicarboxylic acids

Spacer-armed dinuclear copper(II) complexes with condensation products of isophthalic and terephthalic acid dihydrazides with salicylaldehyde and 2-hydroxyacetophenone were synthesized and studied by EPR and X-ray diffraction. The compositions and structures of most of the complexes were determined by elemental analysis, thermogravimetric analysis, and IR spectroscopy. The structure of the copper(II) complex with acyldihydrazone of salicylaldehyde and 1,3-benzenedicarboxylic acid (H4L) with the composition [Cu2L1·2morph·MeOH] (morph is morpholine) was established by X-ray diffraction. The CuII atoms are spaced by 10.29 A and are structurally nonequivalent. One copper cation has a square-planar coordination formed by donor atoms (2 N + O) of the doubly deprotonated acylhydrazine fragment and the N atom of the morpholine molecule. The second copper atom is additionally coordinated by a methanol molecule through the oxygen atom, so that this copper atom is in a tetragonal-pyramidal coordination with the oxygen atom in the axial position. The EPR spectra of liquid solutions of the complexes based on 1,4-benzenedicarboxylic acid acyldihydrazones and 1,3-benzenedicarboxylic acid bis(salicylidene)hydrazone at room temperature show a four-line hyperfine structure with the constant aCu = 54.4–67.0·10−4 cm−1 (g = 2.105–2.147), which is indicative of the independent behavior of the paramagnetic centers. The EPR spectrum of a solution of the complex based on isophthalic acid and 2-hydroxyacetophenone shows the seven-line hyperfine structure corresponding to two equivalent copper nuclei (g = 2.11, aCu = 36.5·10−4 cm−1).

Stable Copper−Nitrosyl Formation by Nitrite Reductase in Either Oxidation State

Nitrite reductase (NiR) is an enzyme that uses type 1 and type 2 copper sites to reduce nitrite to nitric oxide during bacterial denitrification. A copper-nitrosyl intermediate is a proposed, yet poorly characterized feature of the NiR catalytic cycle. This intermediate is formally described as Cu(I)-NO+ and is proposed to be formed at the type 2 copper site after nitrite binding and electron transfer from the type 1 copper site. In this study, copper-nitrosyl complexes were formed by prolonged exposure of exogenous NO to crystals of wild-type and two variant forms of NiR from Alcaligenes faecalis (AfNiR), and the structures were determined to 1.8 A or better resolution. Exposing oxidized wild-type crystals to NO results in the reverse reaction and formation of nitrite that remains bound at the active site. In a type 1 copper site mutant (H145A) that is incapable of electron transfer to the type 2 site, the reverse reaction is not observed. Instead, in both oxidized and reduced H145A crystals, NO is observed bound in a side-on manner to the type 2 copper. In AfNiR, Asp98 forms hydrogen bonds to both substrate and product bound to the type 2 Cu. In the D98N variant, NO is bound side-on but is more disordered when observed for the wild-type enzyme. The solution EPR spectra of the crystallographically characterized NiR-NO complexes indicate the presence of an oxidized type 2 copper site and thus are interpreted as resulting from stable copper-nitrosyls and formally assigned as Cu(II)-NO-. A reaction scheme in which a second NO molecule is oxidized to nitrite can account for the formation of a Cu(II)-NO- species after exposure of the oxidized H145A variant to NO gas.

Electron Exchange Involving a Sulfur-Stabilized Ruthenium Radical Cation

Half-sandwich Ru(II) amine, thiol, and thiolate complexes were prepared and characterized by X-ray crystallography. The thiol and amine complexes react slowly with acetonitrile to give free thiol or amine and the acetonitrile complex. With the thiol complex, the reaction is dissociative. The thiolate complex has been oxidized to its Ru(III) radical cation and the solution EPR spectrum of that radical cation recorded. Cobaltocene reduces the thiol complex to the thiolate complex. The 1H and 31P NMR signals of the thiolate complex in acetonitrile become very broad whenever the thiolate and thiol complexes are present simultaneously. The line broadening is primarily due to electron exchange between the thiolate complex and its radical cation; the latter is generated by an unfavorable redox equilibrium between the thiol and thiolate complexes. Pyramidal inversion of sulfur in the thiol complex is fast at room temperature but slow at lower temperatures; major and minor conformers of the thiol complex were observed by 31P NMR at -98 degrees C in CD2Cl2.

Ruthenium(III) mediated C–H activation of azonaphthol: Synthesis, structural characterization and transfer hydrogenation of ketones

Treatment of [RuCl 3(PPh 3) 3] with 1-(arylazo)naphthol ligands in benzene under reflux afford air-stable new organoruthenium(III) complexes with general composition [Ru(an-R)Cl(PPh 3) 2] (where, R = H, Cl, CH 3, OCH 3, OC 2H 5) in fairly good yield. The 1-(arylazo)naphtholate ligands behave as dianionic tridentate C, N, O donors and coordinates to ruthenium through phenolic oxygen, azo nitrogen and ortho carbon generate two five-membered chelate rings. The composition of the complexes have been established by analytical (elemental analysis and magnetic susceptibility measurement) and spectral (FT-IR, UV–Vis, EPR) methods. The complexes are paramagnetic (low-spin, d 5) in nature and in dichloromethane solution show intense d–d transitions and ligand-to-metal charge transfer (LMCT) transitions in the visible region. The solution EPR spectrum of complex [Ru(an-CH 3)Cl(PPh 3) 2] ( 3) in dichloromethane at 77 K shows rhombic distortion around the ruthenium ion with three different ‘ g’ values ( g x ≠ g y ≠ g z ). The single crystal structure of the complex [Ru(an-OCH 3)Cl(PPh 3) 2] ( 4) has been characterised by X-ray crystallography, indicates the presence of a distorted octahedral geometry in these complexes. All the complexes exhibit one quasi-reversible oxidative response in the range 0.60–0.79 V (Ru IV/Ru III) and two quasi-reversible reductive responses (Ru III/Ru II; Ru II/Ru I) within the range −0.50 to −0.62 V and −0.93 to −0.98 V respectively. The formal potential of all the couples correlate linearly with the Hammett constant of the para substituent in arylazo fragment of the 1-(arylazo)naphtholate ligand. Further, the catalytic efficiency of one of the ruthenium complexes ( 4) was determined for the transfer hydrogenation of ketones with an excellent yield up to 99% in the presence of isopropanol/KOH.

Nitrogen oxide interaction with copper complexes formed by small peptides belonging to the prion protein octa-repeat region

The interaction between NO and copper(II) complexes formed by peptides coming from the N-terminal prion protein octa-repeat region was studied. Aqueous solutions of the Cu-Ac-HGGG-NH(2) and the Cu-Ac-PHGGGWGQ-NH(2) systems around pH 7.5 were tested after the addition of NONOates as a source of NO. UV-Vis, room temperature and frozen solution EPR spectra showed the occurrence of copper(ii) reduction in all these complexes. The reduction of these complexes is probably mediated by the formation of a labile NO adduct, which, after re-oxidation, leads to a relatively stable NO(2)(-) adduct through the apical coordination along the void site of their square pyramidal structure. In fact, the most significant shifts in EPR magnetic parameters (g(||) and A(||) or g(iso) and A(iso)) as well as in the optical parameters (lambda(max) and epsilon(max)) gave a reason for geometrical changes of the copper coordination polyhedron from a distorted square pyramid to a pseudo-octahedron. The presence of oxygen in the aqueous solution hindered the reduction ability of NO towards copper, but it made it easier to return to the original species. In order to elucidate the possible mechanism of this interaction, the reduction of copper complexed by these ligands was followed by means of zinc powder addition. The further addition of nitrite to the solution containing reduced copper led to the conclusion that nitrite could easily form an adduct, which after re-oxidation presented the same spectral features of the species obtained when the NO interaction was followed. The complexity of this interaction could involve both an inner or an outer-sphere electron transfer mechanism.

Binuclear Mixed Valence Oxovanadium(IV/V) Complexes Containing a [OVIV(μ‐Ooxo)(μ‐Ophen)VVO]2+ Core: Synthesis, EPR Spectra, Molecular and Electronic Structure

AbstractBinuclear mixed valence oxovanadium(IV/v) complexes of general formula [V2O3L] containing a [OVIV(μ‐Ooxo)(μ‐Ophen)VVO]2+ core have been synthesised using conformationally labile N4O3‐coordinating heptadentate ligands (H3L). The X‐ray structure of one complex has been examined. Solution EPR spectra revealed that the unpaired electron of the complexes is delocalised between the two vanadium centres. The simulated EPR spectrum of one complex confirms this experimental observation. DFT studies have been performed using crystallographic coordinates in order to obtain further insight into the electronic structure of this type of molecule. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Synthesis and Characterization of Variable-valent Imidorhenium Species. A Family of Imidorhenium(VI) Amide Complexes and their Imidorhenium(V) Imine Precursors Related by Oxygen Atom Transfer

The synthesis of imidorhenium(V) [Re(NAr)Cl3(L)] complexes is reported. Their reaction with dilute HNO3 furnishes [Re(NAr)Cl3(L′)] species incorporating monoionized iminoamide ligands (the ligands are abbreviated as L and L′ respectively). The iminoamide complexes exhibit sextet EPR spectra in CH2Cl2 solution at room temperature. They are electroactive in MeCN solution and to display two quasi reversible responses near 0.2 and 1.6 V which can be attributed to ReV→ReVI and ReVI → ReVII oxidations respectively. One of the [Re(NAr)Cl3(L)] complexes has been structurally characterized. The trans influence of the amide nitrogen considerably lengthens the Re–N bond lying trans to the imido group. The triply bonded ReN–C moiety is linear.

A linear potassium metalated vanadium(IV) polymeric alkoxide: Structural and spectroscopic studies

We have been interested in the preparation of alkoxometallate “bricks” of the M′{M(OR) n } type, with M′ = alkaline metal, M = vanadium(IV) and R = isopropyl or tert-butyl, to be employed as starting materials in salt-elimination reactions with other transition metal complexes. Here, we report the synthesis, spectroscopic characterisation and molecular structure of [{K 2(VO) 2(OPr i ) 6(Pr i OH) 2} ∞] ( 1), prepared by a combination of Lewis acid–base and micro-hydrolysis reactions. The linear polymeric chain contains planar four-membered {(V O) 2(μ-OPr i ) 2} rings with vanadyl groups in anti-coplanar configuration; the rings are connected by “bridging” K(HOPr i ) + units. Powder and solution EPR spectra suggest a spin triplet ground state, with a very weak ferromagnetic interaction between the vanadium(IV) centres at room temperature.

Synthesis and spectroscopic studies on oxovanadium(IV) tetraaza macrocyclic complexes derived from substituted B-diketones and 2,6-diaminopyridine

The in situ reactions of 1-phenyl-3-(4-chlorophenyl/4-nitrophenyl/4-methoxyphenyl)-diketone with 2,6-diaminopyridine in the presence of oxovanadium(iv) sulphate yield macrocyclic complexes of type [VO(mac)]SO 4 . Attempts to synthesize the corresponding metal free macrocyclic ligands did not prove successful. The magnetic moments of the complexes lie in the'range 1.70-1.75 μ B , suitable for complexes with one unpaired electron. The electronic spectra exhibit three bands. The infrared spectra indicate that the ligands coordinate through four azomethine nitrogen atoms. The fluid solution EPR spectra show an eight line pattern typical for a mononuclear VO IV complexes. The spectral studies support square-pyramidal geometry for the vanadium(IV) complexes.

Synthetic, EPR spectroscopic, magnetic and X-ray crystallographic structural studies on copper(II) complexes of the tridentate N 2S donor ligand formed from 6-methyl-2-formylpyridine and S-methyldithiocarbazate (Hmpsme)

New copper(II) complexes of general empirical formula, Cu(mpsme)X · xCH 3COCH 3 (mpsme = anionic form of the 6-methyl-2-formylpyridine Schiff base of S-methyldithiocarbazate; X = Cl, N 3, NCS, NO 3; x = 0, 0.5) have been synthesized and characterized by IR, electronic, EPR and susceptibility measurements. Room temperature μ eff values for the complexes are in the range 1.75–2.1 μ B typical of uncoupled or weakly coupled Cu(II) centres. The EPR spectra of the [Cu(mpsme)X] (X = Cl, N 3, NO 3, NCS) complexes reveal a tetragonally distorted coordination sphere around the mononuclear Cu(II) centre. We have exploited second derivative EPR spectra in conjunction with Fourier filtering (sine bell and Hamming functions) to extract all of the nitrogen hyperfine coupling matrices. While the X-ray crystallography of [Cu(mpsme)NCS] reveals a linear polymer in which the thiocyanate anion bridges the two copper(II) ions, the EPR spectra in solution are typical of a magnetically isolated monomeric Cu(II) centres indicating dissociation of the polymeric chain in solution. The structures of the free ligand, Hmpsme and the {[Cu(mpsme)NO 3] · 0.5CH 3COCH 3} 2 and [Cu(mpsme)NCS] n complexes have been determined by X-ray diffraction. The {[Cu(mpsme)NO 3] 0.5CH 3COCH 3} 2 complex is a centrosymmetric dimer in which each copper atom adopts a five-coordinate distorted square-pyramidal geometry with an N 2OS 2 coordination environment, the Schiff base coordinating as a uninegatively charged tridentate ligand chelating through the pyridine and azomethine nitrogen atoms and the thiolate, an oxygen atom of a unidentate nitrato ligand and a bridging sulfur atom from the second ligand completing the coordination sphere. The [Cu(mpsme)(NCS)] n complex has a novel staircase-like one dimensional polymeric structure in which the NCS − ligands bridge two adjacent copper(II) ions asymmetrically in an end-to-end fashion providing its nitrogen atom to one copper and the sulfur atom to the other.

Synthesis, structure, catalytic transfer hydrogenation and biological activity of cyclometallated ruthenium(III)2-(arylazo)phenolate complexes

A series of mononuclear organoruthenium complexes of the type [RuX(PPh 3) 2(L)] (X = Cl or Br; L = 2-(arylazo)phenolate ligand) have been synthesized from the reaction of five 2-(arylazo)phenol ligands with ruthenium(III) precursors, viz. [RuCl 3(PPh 3) 3] and [RuBr 3(PPh 3) 2(CH 3OH)] in benzene under reflux. In all these reactions, the 2-(arylazo)phenolate ligand replaces one triphenylphosphine molecule, two chlorides or bromides and one methanol from the precursors leading to five-membered cyclometallated species. The 2-(arylazo)phenol ligands behave as dianionic tridentate C, N, O donors and coordinated to ruthenium by dissociation of the phenolic proton and the phenyl proton at the ortho position of the phenyl ring. The compositions of the complexes have been established by elemental analysis, magnetic susceptibility measurement, FT-IR, UV–Vis and EPR spectral data. These complexes are paramagnetic and shows intense d–d and charge transfer transitions in chloroform. The solution EPR spectrum of the complex 7 in dichloromethane at 77 K shows rhombic distortion around the ruthenium ion. The structural conformation of the complex 1 has been carried out by X-ray crystallography. The redox behavior of the complexes has been investigated by cyclic voltammetry and the potentials are observed with respect to the electronic nature of substituents (R) in the 2-(arylazo) phenolate ligands. These complexes catalyze transfer hydrogenation of benzophenone to benzhydrol with up to 99.5% in the presence of i-prOH/KOH. Further, these complexes have shown great promise in inhibiting the growth of both Gram +ve and Gram −ve bacteria, viz. Staphylococcus aureus NCIM 2079 and Escherichia coli NCIM 2065 and fungus Candida albicans NCIM 3102.

Ligand Induced Stereoisomers Revealed in Copper(II) Complex of Nitrolawsone Oxime: EPR and Electronic Spectral Studies

Amphi‐syn stereoisomers with 62:38 compositional ratio existing in 3‐nitrolawsoneoxime [S.Y. Rane et al., 1990] leads to two configurational forms in its copper (II) complex 1 viz. [Cu(NO2‐LwOx)2] · 3H2O. The D4h:D2h geometries of 1 have been detected in 70:30 ratio from solution EPR spectra. Line width alternation effect with superimposition of metal‐ligand hyperfine splits lead to anisotropic magnetic parameters. Best fitted bonding parameters of 1 are compared with O∧O and N∧O donor complexes.

Coordination chemistry of the new ligand 1-(bis-pyridin-2-ylmethyl-amino)-3-chloropropan-2-ol (HPClNOL) with copper(II). X-ray crystal structure, spectroscopic and electrochemical properties of the complex [Cu(HPClNOL)(CH 3CN)](ClO 4) 2

The crystal structure, electrochemical behaviour and spectroscopic properties (FT-IR, UV–vis and EPR) of the mononuclear complex [Cu(HPClNOL)(CH 3CN)](ClO 4) 2 1, are reported. The new tetradentate ligand, HPClNOL, 1-(bis-pyridin-2-ylmethyl-amino)-3-chloropropan-2-ol, reacts with 1 equiv. of [Cu(OH 2) 6](ClO 4) 2, in methanol, to give 1, which crystallizes as blue monoclinic crystals, space group P2 1/ c with a=14.906(3) Å, b=11.371(2) Å, c=14.113(3) Å, β=103.86(3)°, V=2322.4(8) Å 3 and Z=4. In 1, the copper center is pentacoordinated, showing a distorted square pyramidal geometry. The electronic spectrum indicates that, in acetonitrile, the copper atom becomes hexacoordinated ( λ max( ε) 621 nm (87 dm 3 mol −1 cm −1). Frozen solution EPR spectra of 1 in pure acetonitrile ( A ‖=190 G, g ‖=2.24) and of 1 in acetonitrile/tetrabutylammonium perchlorate solution ( A ‖=185 G, g ‖=2.25) indicates a hexacoordinated environment for the copper(II) center. Complex 1 shows a quasi-reversible redox process at 70 mV vs NHE, which is attributed to Cu(II)/Cu(I) redox couple.

Structural variation in the copper(II) complexes with a tetradentate bis-6-methylpyridine-substituted bispidine ligand

Abstract Four single crystal X-ray structures of the complex cation [CuII(L)(X)]n+ (L = 3,7-dimethyl-2,4-di-(6-methyl-2-pyridyl)-9-diol-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylate dimethylester, X = NCCH3, OH2, Cl–, NO3–) are discussed together with their spectroscopic properties. The bispidine ligand L enforces a square pyramidal or cis-octahedral coordination geometry with two cis-disposed tertiary amines (N3 and N7), two trans-disposed pyridines which are co-planar and in-plane with the metal center and one of the two amines (N3), and one or two co-ligands trans to the amine donors (N3 or N7). Three structural types have been found: (i) X = NCCH3, square pyramidal, X trans to N3, in-plane with the pyridine donors, elongation of the Cu–N7 bond; (ii) X = Cl–, OH2, square pyramidal, X trans to N7, perpendicular to the pyridine planes, slight elongation of the Cu–N3 bond; (iii) X = NO3–, octahedral, bidentate NO3–, elongation of the Cu–Npyridine bonds. The three isomeric chromophores (Cu(L)-fragments, the three possible minima of the ‘Mexican hat’ Jahn–Teller potential energy surface) are discussed on the basis of the experimental structural data, the steric contributions to the stabilization/destabilization of the three energetically similar minima are discussed on the basis of empirical force field calculations, and the results of DFT model calculations are used for a qualitative interpretation of electronic effects. Solid state and solution electronic spectra and frozen solution EPR spectra are used for a qualitative analysis of the three electronic ground states and a possible equilibration of the three isomeric forms, and this is supported by a preliminary ligand field analysis, based on AOM model calculations. To cite this article: P. Comba et al., C R Chimie 8 (2005).

The synthesis, structure and properties of copper(ii) complexes of asymmetrically functionalized derivatives of 1,4,7-triazacyclononane

Reaction of 1-propylamino-4-acetato-1,4,7-triazacyclononane (L1), 1-benzyl-4-acetato-1,4,7-triazacyclononane (L2) and 1-benzyl-4-propylamino-1,4,7-triazacyclononane (L3) with a copper(II) salt gave Na2[CuL1](ClO4)3(1a), [CuL2]Cl (2) and [Cu2L32](ClO4)4.5H2O (3), respectively. [CuL4]ClO4 (4) was formed by reacting 1-formyl-4-ethylacetato-1,4,7-triazacyclononane with cupric chloride in aqueous solution. The X-ray crystal structures of the complexes reveal that the ligands generate distorted square pyramidal or square planar coordination environments about the Cu(II) centre, but in three complexes (1b, 3 and 4) weak interactions to an oxygen atom from a perchlorate anion and, in the case of 4, also to an amide nitrogen leading to tetragonally elongated octahedral Cu(II) geometries. In 4, the formyl group is found to reduce the coordinating ability of the macrocyclic nitrogen to which it is attached, as evidenced by the weak CuN interaction. The formation of five-membered chelate rings on coordination of the ligands further contributes to the distortion from the ideal geometries. The crystal lattices contain a number of novel supramolecular features. 1a contains a negatively charged sodium perchlorate chain of composition [Na2(ClO4)3]x(x-), with a complex series of Na-O-Na bridges flanked by [CuL1]+ units, while 3 contains highly complex hydrogen bonded sheets approximately 20 A thick that stack through van der Waals interactions. One-dimensional chains comprised of copper complexes are found in 2 and 4, and are held together by hydrogen bonds in 2 and acetate bridges between the copper cations in 4. The solution EPR spectra indicate that the copper(II) centres exist in isolated distorted square pyramidal (possibly square planar for 4) environments, while in the solid state there is evidence for the existence of weak exchange and dipole-dipole coupling for some complexes.

EPR spectral and redox properties of some 2,9-dimethyl-1,10-phenanthroline copper(II) complexes

The redox behaviour of [Cu(dmp) 2 ](ClO 4 ) 2 (1), [Cu(dmp) 2 ]SO 4 (2), [Cu(dmp) 2 Cl]Cl.H 2 O (3) and [Cu(dmp) 2 Br]Br.H 2 O (4) has been investigated in 50% aqueous-ethanol/0.2 M NaClO 4 and absolute ethanol/TBAP at GCE by means of cyclic voltammetry. All these complexes show a quasi-reversible couple Cu 2 + / + with formal potential, E°' =+430 to +650 mV vs SCE in the potential range +1.0 to -0.2 V vs SCE. Analysis of CV data shows that the electrode process for complex 3 involves ECE mechanism in absolute ethanol/0.1 M TBAP medium. An unusual and complicated electrochemical behaviour has been observed for this complex in 50% aqueous-ethanol/0.2 M NaClO 4 medium due to the presence of more than one complex species in solution. The reduction potentials, E p c 1 (Cu 2 + / + ) for complexes 2 and 4 in 50% aqueous-ethanol medium are almost similar at a given scan rate. However, the ease of reduction decreases (i.e. E p c 1 becomes less positive) in the order : 1 > 4 > 3 in absolute alcohol/0.1 M TBAP medium. Powdered and frozen solution EPR spectra (77 K) of these complexes have also been studied and the results are interpreted.

Single crystal X- and Q-band EPR spectroscopy of a binuclear Mn(2)(III,IV) complex relevant to the oxygen-evolving complex of photosystem II.

The anisotropic g and hyperfine tensors of the Mn di-micro-oxo complex, [Mn(2)(III,IV)O(2)(phen)(4)](PF(6))(3).CH(3)CN, were derived by single-crystal EPR measurements at X- and Q-band frequencies. This is the first simulation of EPR parameters from single-crystal EPR spectra for multinuclear Mn complexes, which are of importance in several metalloenzymes; one of them is the oxygen-evolving complex in photosystem II (PS II). Single-crystal [Mn(2)(III,IV)O(2)(phen)(4)](PF(6))(3).CH(3)CN EPR spectra showed distinct resolved (55)Mn hyperfine lines in all crystal orientations, unlike single-crystal EPR spectra of other Mn(2)(III,IV) di-micro-oxo bridged complexes. We measured the EPR spectra in the crystal ab- and bc-planes, and from these spectra we obtained the EPR spectra of the complex along the unique a-, b-, and c-axes of the crystal. The crystal orientation was determined by X-ray diffraction and single-crystal EXAFS (Extended X-ray Absorption Fine Structure) measurements. In this complex, the three crystallographic axes, a, b, and c, are parallel or nearly parallel to the principal molecular axes of Mn(2)(III,IV)O(2)(phen)(4) as shown in the crystallographic data by Stebler et al. (Inorg. Chem. 1986, 25, 4743). This direct relation together with the resolved hyperfine lines significantly simplified the simulation of single-crystal spectra in the three principal directions due to the reduction of free parameters and, thus, allowed us to define the magnetic g and A tensors of the molecule with a high degree of reliability. These parameters were subsequently used to generate the solution EPR spectra at both X- and Q-bands with excellent agreement. The anisotropic g and hyperfine tensors determined by the simulation of the X- and Q-band single-crystal and solution EPR spectra are as follows: g(x) = 1.9887, g(y) = 1.9957, g(z) = 1.9775, and hyperfine coupling constants are A(III)(x) = |171| G, A(III)(y) = |176| G, A(III)(z) = |129| G, A(IV)(x) = |77| G, A(IV)(y) = |74| G, A(IV)(z) = |80| G.