Tuning the redox properties of manganese(II) and its implications to the electrochemistry of manganese and iron superoxide dismutases

Superoxide dismutases (SODs) catalyze the disproportionation of superoxide to dioxygen and hydrogen peroxide. The active metal sites of iron and manganese superoxide dismutases are structurally indistinguishable from each other. Despite the structural homology, these enzymes exhibit a high degree of metal selective activity suggesting subtle redox tuning of the active site. The redox tuning model, however, up to now has been challenged by the existence of so-called cambialistic SODs that function with either metal ion. We have prepared and investigated two sets of manganese complexes in which groups of varying electron-withdrawing character, as measured by their Hammett constants sigma Para, have been introduced into the ligands. We observed that the Mn(III)/Mn(II) reduction potential for the series based on 4'-X-terpyridine ligands together with the corresponding values for the iron-substituted 4'-X-terpyridine complexes changed linearly with sigma Para. The redox potential of the iron and manganese complexes could be varied by as much as 600 mV by the 4'-substitution with the manganese complexes being slightly more sensitive to the substitution than iron. The difference was such that in the case where the 4'-substituent was a pyrrolidine group both the manganese and the iron complex were thermodynamically competent to catalytically disproportionate superoxide, making this particular ligand "cambialistic". Taking our data and those available from the literature together, it was found that in addition to the electron-withdrawing capacity of the 4'-substituents the overall charge of the Mn(II) complexes plays a major role in tuning the redox potential, about 600 mV per charge unit. The ion selectivity in Mn and FeSODs and the occurrence of cambialistic SODs are discussed in view of these results. We conclude that the more distant electrostatic contributions may be the source of metal specific enzymatic activity.     

EPR and ligand field studies of iron superoxide dismutases and iron-substituted manganese superoxide dismutases: relationships between electronic structure of the active site and activity

The problem of metal selectivity of iron/manganese superoxide dismutases (SODs) is addressed through the electronic structures of active sites using electron paramagnetic resonance and ligand field calculations. Studies of wild-type iron(III) SOD (FeSOD) from Escherichia coli and from Methanobacterium thermoautotrophicum and iron-substituted manganese(III) SOD (Fe(sub)MnSOD) from E. coli and from Serratia marcescens are reported. EPR spectroscopy of wild-type enzymes shows transitions within all three Kramers doublets identified by their g values. From the temperature dependence of the observed transitions, the zero-field splitting is found to be negative, D = -2 +/- 0.2 cm-1. The electronic structure is typical of a distorted trigonal bipyramid, all the EPR features being reproduced by ligand field analysis. This unique and necessary electronic structure characterizes wild-type enzymes whatever their classification from the amino acid sequence into iron or manganese types, as E. coli FeSOD or M. thermoautotrophicum FeSOD. In iron-substituted manganese SODs, reduced catalytic activity is found. We describe how inhomogeneity of all reported substituted MnSODs might explain the activity decrease. EPR spectra of substituted enzymes show several overlapping components. From simulation of these spectra, one component is identified which shares the same electronic structure of the wild-type FeSODs, with the proportion depending on pH. Ligand field calculations were performed to investigate distortions of the active site geometry which induce variation of the excitation energy of the lowest quartet state. The corresponding coupling between the ground state and the excited state is found to be maximum in the geometry of the native SODs. We conjecture that such coupling should be considered in the electron-transfer process and in the contribution of the typical electronic structure of FeSOD to the activity.     

Temperature-dependent coordination in E. coli manganese superoxide dismutase

Two different temperature dependences of the manganese(II) high-field electron paramagnetic resonance spectrum of manganese superoxide dismutase from E. coli were observed. In the 25-200 K range, the zero-field interaction steadily decreased with increasing temperature. This was likely due to the thermal expansion of the protein. From these results, it was possible to deduce an approximately r(-)(2.5) dependence of Mn(II) zero-field interaction on ligand-metal distance. At temperatures above 240 K, a distinct six-line component was detected, the amplitude of which decreased with increasing temperature. On the basis of similarities to the six-line spectrum observed for the azide-complexed E. coli manganese superoxide dismutase, the newly detected six-line spectrum was assigned to a hexacoordinate Mn(II) center resulting from the coordination of a nearby water molecule to the normally five-coordinate center. The changes in enthalpy and entropy characterizing the hexacoordinate-pentacoordinate equilibrium in the 240-268 K range were -5 kcal/mol and -24 cal/mol.K, respectively. The structural implications of the zero-field parameters of the newly found hexacoordinate form in comparison to those of the Mn(II) centers in concanavalin-A and manganese-containing R. spheroides photosynthetic reaction centers and the values predicted by the superposition model are discussed.     

Comparative Analysis and Modeling of Superoxide Dismutases (SODs) in Brachypodium distachyon L.

Superoxide dismutase (SOD, EC 1.15.1.1) is an enzyme catalyzing the dismutation of superoxide radical to hydrogen peroxide and dioxygen. To date, four types of SODs — Cu/ZnSOD, MnSOD, FeSOD, and NiSOD — have been identified. In this study, SOD proteins of Brachypodium distachyon (L.) Beauv. were screened by utilization of bioinformatics approaches. According to our results, Mn/FeSODs and Cu/ZnSODs of B. distachyon were found to be in basic and acidic character, respectively. Domain analyzes of SOD proteins revealed that iron/manganese SOD and copper/zinc SOD were within studied SOD proteins. Based on the seconder structure analyzes, Mn/FeSODs and Cu/ZnSODs of B. distachyon were found as having similar sheets, turns and coils. Although helical structures were noticed in the types of Mn/FeSODs, no the type of Cu/ZnSODs were identified having helical structures. The predicted binding sites of Fe/MnSODs and Cu/ZnSODs were confirmed for having His-His-Asp-His and His-His-His-Asp-Ser residues with different positions, respectively. The 3D structure analyzes of SODs revealed that some structural divergences were observed in patterns of SODs domains. Based on phylogenetic analysis, Mn/FeSODs were found to have similarities whereas Cu/ZnSODs were clustered independently in phylogenetic tree.     

Superoxide dismutase activity of iron(II)TPEN complex and its derivatives

Superoxide is involved in the pathogenesis of various diseases, such as inflammation, ischemia-reperfusion injury and carcinogenesis. Superoxide dismutases (SODs) catalyze the disproportionation reaction of superoxide to produce oxygen and hydrogen peroxide, and can protect living cells against the toxicity of free radicals derived from oxygen. Thus, SODs and their functional mimics have potential value as pharmaceuticals. We have previously reported that Fe(II)tetrakis-N,N,N',N'-(2-pyridylmethyl)ethylenediamine (Fe(II)TPEN) has an excellent SOD activity (IC50 = 0.5 microM) among many iron complexes examined (J. Biol. Chem., 264, 9243-9249 (1989)). Fe(II)TPEN can act like native SOD in living cells, and protect Escherichia coli cells from free radical toxicity caused by paraquat. In order to develop more effective SOD functional mimics, we synthesized Fe(II)TPEN derivatives with electron-donating or electron-withdrawing groups at the 4-position of all pyridines of TPEN, and measured the SOD activities and the redox potentials of these complexes. Fe(II) tetrakis-N,N,N',N'-(4-methoxy-2-pyridylmethyl)ethylenediamine (Fe(II)(4MeO)4TPEN) had the highest SOD activity (IC50 = 0.1 microM) among these iron-based SOD mimics. In addition, a good correlation was found between the redox potential and the SOD activity of 15 Fe(II) complexes, including iron-based SOD mimics reported in the previous paper (J. Organometal. Chem., in press). Iron-based SOD mimics may be clinically applicable, because these complexes are generally tissue-permeable and show low toxicity. Therefore our findings should be significant for the development of clinically useful SOD mimics.     

EPR and optical absorption spectral studies on sphalerite mineral

The mineral sphalerite (Zn,Fe)S has been characterized by a combination of X-ray diffraction, EPR and NIR spectroscopy. The optical absorption spectrum of mineral sphalerite is due to an iron impurity only, which is in a distorted octahedral environment. The g=2.2 is attributed to iron and g and A value observed in the spectrum 1.999 and 6.0 mT are assigned to Mn(II) impurity in the mineral. These results indicate that iron and Mn(II) impurity have entered the lattice by substitution. The EPR results confirm the presence of manganese in a distorted octahedral environment. It is evident from the chemical analysis that iron is present in higher concentrations. NIR results are due to the presence of water and sulphide fundamentals which also support the formula of the mineral. No sulphate in the sphalerite mineral was observed.     

Versatility of 2,6-diacetylpyridine (dap) hydrazones in stabilizing uncommon coordination geometries of Mn(II): synthesis, spectroscopic, magnetic and structural characterization

Five seven- or eight-coordinate manganese complexes of hydrazone ligands have been prepared. Three seven-coordinate neutral Mn(II) complexes: [Mn(dapA2)]n (1), [Mn(dapB2)(H2O)2] (2), [Mn(dapS2)(H2O)2] (3) have been synthesized from the bis-Schiff bases of 2,6-diacetylpyridine: dap(AH)2, dap(BH)2 and dap(SH)2 (AH = anthraniloyl hydrazide, BH = benzoyl hydrazide, SH = salicyloyl hydrazide), respectively. Two eight-coordinate Mn(II) complexes: [Mn(dapS)2] (4) and [Mn(dapB)2].3H2O (5) have been synthesized from the mono-Schiff bases dapBH and dapSH, respectively. The complexes have been characterized by elemental analyses and by IR, UV-Vis., FAB mass, EI mass and EPR spectroscopy. The molecular structures of 1, 3.DMF and 4.DMF have been determined by single-crystal X-ray diffraction. The mono-Schiff bases are monoanionic and the bis-Schiff bases are dianionic. The octa-coordinated mono-Schiff base complex 4 adopts a dodecahedral geometry, while the hepta-coordinated bis-Schiff base complex 1 forms a one-dimensional linear polymeric chain. A weak antiferromagnetic exchange interaction (J=-0.15 cm(-1)) between the Mn(II) ions in is attributed to weak Mn...Mn interaction through the PhNH(2) moiety of the ligand, as indicated by extended-Hückel molecular orbital calculations. A good simulation of the EPR spectrum of a frozen solution (DMSO at 4 K) of compound 1 was obtained with g=2.0, D=0.1 cm(-1), E=0.01 cm(-1). The EPR spectrum of a powdered sample of compound 1 shows a large broadening of the signal, due in part, to the important zero-field splitting of the hepta-coordinated Mn(II) ion.     

A geometrically constraining bis(benzimidazole) ligand and its nearly tetrahedral complexes with Fe(II) and Mn(II)

2,2'-Bis[2-(1-propylbenzimidazol-2-yl)]biphenyl), 4, and its bis complexes with Fe(II) and Mn(II) have been prepared and characterized structurally and spectroscopically. Ligand 4 adopts an open, "trans" conformation in the solid state with the benzimidazole (BzIm) groups on opposite sides of the biphenyl unit. In its complexes with metal ions, a "cis" conformation is observed, and 4 behaves as a geometrically constraining bidentate ligand with four planar groups connected by three "hinges". Reaction of 4 with Fe(II) or Mn(II) yielded isomorphous crystals (space group Pnn2) of Fe(II)(4)2.(ClO4)2 and Mn(II)(4)2.(ClO4)2, in which the M(II)(4)2 cations exhibit distorted-tetrahedral coordination geometries (N-M-N angles, 109 +/- 11 degrees ) enforced by rigid, chiral nine-membered M(4) rings in the twist-boat-boat conformation. Individually, the cations show R,R or S,S stereochemistry, and the crystals are racemates. Mn(II)(4)2.(ClO4)2 exhibits a quasi-reversible Mn(II) --> Mn(III) oxidation at E(1/2) = 0.64 V; the corresponding Fe(II) --> Fe(III) oxidation occurs at E(1/2) = 1.76 V. The electrochemical stability of the Fe(III) oxidation state in this system suggests the possibility of isolating an unusual pseudotetrahedral Fe(III)N(BzIm)(4) species. Ultraviolet spectra of the iron and manganese complexes are dominated by absorptions of the ligand 4 blue-shifted by approximately 2000-3000 cm(-1). Ligand-field absorptions were observed for the Fe(II) complex; those for the Mn(II) complex were obscured by tailing ultraviolet absorptions. Electron paramagnetic resonance and magnetic susceptibility measurements are consistent with a high-spin Mn(II) complex, while for the Fe(II) complex, the falloff of the magnetic moment with decreasing temperature is indicative of zero-field splitting with D approximately 4 cm(-1).     

Synthesis, Structure, and Paramagnetism of Manganese(II) Iminophosphate Complexes

The coordination chemistry of the bidentate bis(imino)bis(amino)phosphate ligands [Me(3)SiN═P{NR}{N(H)R}(2)](-), where R = n-propyl is [L(1)H(2)](-), R = cyclohexyl is [L(2)H(2)](-), and R = tert-butyl is [L(3)H(2)](-), with manganese(II), is described. The bis(imino)bis(amino)phosphate-manganese(II) complexes [(η(5)-Cp)Mn(μ-L(1)H(2))](2) (1), [Mn(L(2)H(2))(2)]·THF (2·THF), and [(η(5)-Cp)Mn(L(3)H(2))] (3) were synthesized by monodeprotonation of the respective pro-ligands by manganocene, Cp(2)Mn. The molecular structures of 1-3 reveal that the steric demands of the ligand N-substituents play a dominant role in determining the aggregation state and overall composition of the manganese(II) complexes. The coordination geometries of the Mn(II) centers are six-coordinate pseudotetrahedral in 1, four-coordinate distorted tetrahedral in 2, and five-coordinate in 3, resulting in formal valence electron counts of 17, 13, and 15, respectively. EPR studies of 1-3 at Q-band reveal high-spin manganese(II) (S = (5)/(2)) in each case. In the EPR spectrum of 1, no evidence of intramolecular magnetic exchange was found. The relative magnitudes of the axial zero-field splitting parameter, D, in 2 and 3 are consistent with the symmetry of the manganese environment, which are D(2d) in 2 and C(2v) in 3.     

EPR study of substrate binding to the Mn(II) active site of the bacterial antibiotic resistance enzyme FosA: a better way to examine Mn(II)

FosA is a manganese metalloglutathione transferase that confers resistance to the broad-spectrum antibiotic fosfomycin, (1R,2S)-epoxypropylphosphonic acid. The reaction catalyzed by FosA involves the attack by glutathione on fosfomycin to yield the product 1-(S-glutathionyl)-2-hydroxypropylphosphonic acid. The enzyme is a dimer of 16 kDa subunits, each of which harbors one mononuclear Mn(II) site. The coordination environment of the Mn(II) in the FosA x Mn(2+) complex is composed of a glutamate and two histidine ligands and three water molecules. Here we report EPR spectroscopic studies on FosA, in which EPR spectra were obtained at 35 GHz and 2 K using dispersion-detection rapid-passage techniques. This approach provides an absorption envelope line shape, in contrast to the conventional (slow-passage) derivative line shape, and is a more reliable way to collect spectra from Mn(II) centers with large zero-field splitting. We obtain excellent spectra of FosA bound with substrate, substrate analogue phosphate ion, and product, whereas these states cannot be studied by X-band, slow-passage methods. Simulation of the EPR spectra shows that binding of substrate or analogue causes a profound change in the electronic parameters of the Mn(II) ion. The axial zero-field splitting changes from [D] = 0.06 cm(-1) for substrate-free enzyme to 0.23 cm(-1) for fosfomycin-bound enzyme, 0.28 (1) cm(-1) for FosA with phosphate, and 0.27 (1) cm(-1) with product. Such a large zero-field splitting is uncommon for Mn(II). A simple ligand field analysis of this change indicates that binding of the phosphonate/phosphate group of substrate or analogue changes the electronic energy levels of the Mn(II) 3d orbitals by several thousand cm(-1), indicative of a significant change in the Mn(II) coordination sphere. Comparison with related enzymes (glyoxalase I and MnSOD) suggests that the change in the coordination environment on substrate binding may correspond to loss of the glutamate ligand.     

Spectroscopic and computational investigation of second-sphere contributions to redox tuning in Escherichia coli iron superoxide dismutase

In Fe- and Mn-dependent superoxide dismutases (SODs), second-sphere residues have been implicated in precisely tuning the metal ion reduction potential to maximize catalytic activity (Vance, C. K.; Miller, A.-F. J. Am. Chem. Soc. 1998, 120, 461-467). In the present study, spectroscopic and computational methods were used to characterize three distinct Fe-bound SOD species that possess different second-coordination spheres and, consequently, Fe(3+/2+)reduction potentials that vary by approximately 1 V, namely, FeSOD, Fe-substituted MnSOD (Fe(Mn)SOD), and the Q69E FeSOD mutant. Despite having markedly different metal ion reduction potentials, FeSOD, Fe(Mn)SOD, and Q69E FeSOD exhibit virtually identical electronic absorption, circular dichroism, and magnetic circular dichroism (MCD) spectra in both their oxidized and reduced states. Likewise, variable-temperature, variable-field MCD data obtained for the oxidized and reduced species do not reveal any significant electronic, and thus geometric, variations within the Fe ligand environment. To gain insight into the mechanism of metal ion redox tuning, complete enzyme models for the oxidized and reduced states of all three Fe-bound SOD species were generated using combined quantum mechanics/molecular mechanics (QM/MM) geometry optimizations. Consistent with our spectroscopic data, density functional theory computations performed on the corresponding active-site models predict that the three SOD species share similar active-site electronic structures in both their oxidized and reduced states. By using the QM/MM-optimized active-site models in conjunction with the conductor-like screening model to calculate the proton-coupled Fe(3+/2+) reduction potentials, we found that different hydrogen-bonding interactions with the conserved second-sphere Gln (changed to Glu in Q69E FeSOD) greatly perturb the p K of the Fe-bound solvent ligand and, thus, drastically affect the proton-coupled metal ion reduction potential.     

Speciation of iron and manganese in dam water particles using electron spectroscopy for chemical analysis (ESCA)

The speciation of iron and manganese compounds retained by membrane filtration of dam water samples was studied by use of electron spectroscopy for chemical analysis (ESCA). Samples were taken at various depths and times of year from North Pine Dam near Brisbane, Australia. Both surface and bulk properties of samples representative of the water column profile were investigated. ESCA results showed that iron(III) compounds were found to predominate in the whole water column in any season of the year while the significance of iron(II) species varied in the hypolimnion (the bottom layer). In summer, although various ratios of manganese(II), manganese(III) and manganese(IV) compounds were found to occur down the water column, manganese(IV) compounds were predominant in the epilimnion (the top layer), while both manganese(II) and (IV) compounds predominated in the metalimnion (the middle layer) and the hypolimnion. The majority of Mn(IV) compounds were found throughout the water column after heavy rain and winter season. The ratios of atomic concentrations of iron and manganese as determined by atomic absorption spectrophotometry and ESCA are also discussed.     

Rapid automated in-situ monitoring of total dissolved iron and total dissolved manganese in underground water by reverse-flow injection with chemiluminescence detection during the process of water treatment

Measurement of iron and manganese is very important in evaluating the quality of natural waters. We have constructed an automated Fe(II), total dissolved iron(TDI), Mn(II), and total dissolved manganese(TDM) analysis system for the quality control of underground drinking water by reverse flow injection analysis and chemiluminescence detection(rFIA-CL). The method is based on the measurement of the metal-catalyzed light emission from luminol oxidation by potassium periodate. The typical signal is a narrow peak, in which the height is proportional to light emitted and hence to the concentration of metal ions. The detection limits were 3 x 10(-6)mug ml(-1) for Fe(II) and the linear range extents up to 1.0 x 10(-4) and 5 x 10(-6)mug ml(-1) for Mn(II) cover a linear range to 1.0 x 10(-4)mug ml(-1). This method was used for automated in-situ monitoring of total dissolved iron and total dissolved manganese in underground water during water treatment.     

Temperature dependence of X- and Q-band EPR spectra of the dinuclear manganese(II) complex [(NO2Bpmp)Mn2(mu-OAc)2]+: determination of the exchange constant and of the spin parameters for the S=1, 2, and 3 spin states

A new dinuclear manganese(II) complex was synthesised with the biscompartimental ligand 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-nitrophenol (NO(2)BpmpH) and characterised by X-ray crystallography. Magnetic susceptibility measurements revealed that the two high-spin Mn(II) ions are antiferromagnetically coupled with a singlet-to-triplet separation of 7.2 cm(-1). The powder EPR spectra were recorded for both X- and Q-bands between 1.8 K and 35 K. A detailed analysis of these spectra led to the determination of three out of five individual spin-state zero-field splitting parameters. From the proposed simulations, the exchange coupling constant J and the intermetallic distance have been computed.     

Water exchange on seven-coordinate Mn(II) complexes with macrocyclic pentadentate ligands: insight in the mechanism of Mn(II) SOD mimetics

Seven-coordinate manganese(II) complexes [Mn(L)(H2O)2]2+, where L represents an equatorial pentadentate macrocyclic ligand with five nitrogen donor atoms, were studied with regard to their acid-base properties, water-exchange rate constants, and corresponding activation parameters (DeltaH, DeltaS, and DeltaV). Three of the studied complexes without imine bonds in the macrocyclic ligand are proven superoxide dismutase (SOD) mimetics. Their water-exchange parameters were compared with those of the imino groups containing complex [Mn(L1)(Cl)2] (dichloro-2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]-octadeca-1(18),2,12,14,16-pentaenemanganese(II)), which does not show SOD activity. In addition the X-ray crystal structure of a new complex, dichloro-2,6-bis[1-(2-(N-methylamino)ethylimino)ethyl]pyridine-manganese(II) [Mn(L2)(Cl)2], which is the acyclic analog of [Mn(L1)(Cl)2], is reported. Stability constants of the complexes and the pKa values of the ligands were measured by potentiometric titration. The titrations of [Mn(L1)(H2O)2]2+ and [Mn(L2)(H2O)2]2+ led to complicated species distribution curves because of their ligands containing imine bonds. Water exchange was measured by temperature- and pressure-dependent 17O NMR techniques. In addition to the measurements on [Mn(EDTA)(H2O)]2- and its derivatives, this is the only study of water exchange on seven-coordinate manganese complexes. The water exchange rate constants vary between 1.6 x 107 s-1 and 5.8 x 107 s-1 at 25 degrees C and are mainly controlled by the pi-acceptor abilities of the ligands. The exchange rate constant of the diaqua-1,4,7,10,13-pentaazacyclopentadecanemanganese(II) [Mn([15]aneN5)(H2O)2]2+ complex seems to be even higher but could not be exactly determined. On the basis of the obtained activation parameters, the exchange mechanism of the studied seven-coordinate manganese(II) complexes follows a dissociative pathway (Id mechanism). DFT calculations (UB3LYP/LANL2DZp) were performed to obtain the energy required for the dissociation of the coordinated water molecule, that is, the energy difference between the starting seven-coordinate complex and a six-coordinate intermediate. The results have been discussed in terms of the catalytic mechanism of the proven SOD mimetics.     

Dimetallic complexes of a structurally versatile pyridazine-containing Schiff-base macrocyclic ligand with pendant pyridine arms

The new [2 + 2] Schiff-base macrocyclic ligand L2, containing pyridazine head units and pyridine pendant arms, was synthesised as [Ba(II)2L2(ClO4)4(OH2)] 1 from the barium(II) ion templated condensation reaction of 3,6-diformylpyridazine and N1-(2-aminoethyl)-N1-(methylene-2-pyridyl)-ethane-1,2-diamine. Subsequent transmetallation reactions of 1 with copper(II), iron(II) and manganese(II) perchlorates led to the formation of [Cu(II)2L2](ClO4)4.2MeCN 2, [Fe(II)2L2(MeCN)2](ClO4)4 3 and two manganese complexes, 4 and 5, with the same formula, [Mn(II)2L2(MeCN)(OH2)](ClO4)4, but slightly different crystal structures, respectively. Single-crystal X-ray structural analyses reveal the variety of structures which can be supported by L2 in order to meet the coordination environment preferences of the incorporated metal ions. The barium(II) ions in 1 have an irregular ten-coordinate geometry whereas the copper(II) ions in 2 have a square pyramidal geometry and the iron(II) ions in 3 have an octahedral geometry, while in 4 and 5 every manganese(II) ion is seven-coordinate and the environment can be best described as distorted pentagonal bipyramidal. In 1, 4 and 5 the pyridazine moieties bridge the metal centres [Ba(1)...Ba(2) 4.9557(3)A 1; Mn(1)...Mn(2) 4.520(1)A 4; Mn(1)[dot dot dot]Mn(2) 4.3707(8)A 5] but this is not observed in the copper(II) and iron(II) complexes, 2 and 3, in which the metal ions are well separated [Cu(1)...Cu(2) 5.9378(6)A 2; Fe(1)...Fe(2) 5.7407(12)A 3]. In the cyclic voltammogram of [Cu2(II)L2](ClO4)4.2MeCN 2 in MeCN vs. Ag/AgCl two separate reversible one-electron transfer steps are observed [E(1/2)=0.04 V, DeltaE= 0.12 V and E(1/2)= 0.20 V, DeltaE=0.12 V; K(c)=510; in this system E(1/2)(Fc+/Fc)=0.42 V and DeltaE(Fc+/Fc)=0.08 V]. The other complexes cannot be reversibly reduced/oxidised.     

Mononuclear and binuclear manganese(II) complexes with tridentate bis-benzimidazolyl ligands

Summary Manganese(II) complexes of bis(2-benzimidazolylmethyl) ether (DGB), bis(2-benzimidazolylmethyl) sulphide (TGB) and the n-butyl derivative of DGB (BDGB) were prepared and characterised. The solution e.p.r. spectrum of [Mn(TGB)Cl₂] in DMF at 143 K is commensurate with an axially distorted monomeric manganese(II) complex, room temperature magnetic moment (6.04 B.M.) per manganese(II) atom being in the range found for other d⁵ monomeric manganese(II) complexes. The solution e.p.r. spectrum of [Mn(BDGB)Cl₂]-2H₂O in DMF at 143 K indicates the presence of two equivalent manganese(II) ions coupled by an exchange interaction, fostered by bridging chlorides. Evidence for this is provided by a nearly isotropic 11 line hyperfine structure of ⁵⁵Mn, with a coupling constant 45 ± 5G. Contact-shifted ¹H n.m.r. data also supports an exchange coupled dimeric manganese complex. The room temperature magnetic moment, 5.64 B.M., per manganese(II) indicates quenching of the magnetic moment below that of monomeric manganese(II) ion. The [Mn(DGB)Cl₂]·H₂O complex exhibits a magnetic moment of 6.02 B.M. per manganese, indicating a monomeric manganese complex. E.p.r. data of the complex diluted in an analogous Zn-DGB complex (1∶20) correlates well for D = 0.22cm⁻¹ and λ ∼- 0.267. The [Mn(DGB)-(C1O₄)₂] and [Mn(BDGB)(ClO₄)₂] complexes, diluted in analogous Zn-DGB and Zn-BDGB complexes (1∶20), show a strong single e.p.r. line at g _eff ∼- 2. The complexes have low magnetic moments; 4.44 B.M./Mn and 4.39 B.M./Mn, at room temperature.     

Single crystal EPR studies on Mn(II)-doped sarcosine cadmium chloride and sarcosine cadmium bromide: study of zero-field splitting tensor in iso-structural complexes

EPR spectra of single crystals of Mn(II)-doped sarcosine cadmium chloride and sarcosine cadmium bromide are studied in Q-band and in X-band at room temperature. Two magnetically inequivalent sites are observed in both the lattices in a distorted octahedral environment. The spin-Hamiltonian parameters are extracted and are found to have a rhombic symmetry. The angular variation of the zero-field transitions is simulated for one of the sites with an asymmetric zero-field tensor D = 480 x 10(-4) cm(-1), E = -115 x 10(-4) cm(-1) and a = 10 x 10(-4) cm(-1) for Mn(II) in sarcosine cadmium chloride and with D = 460 x 10(-4) cm(-1) E = -98 x 10(-4) cm(-1) and a = 10 x 10(-4) cm(-1) for Mn(II) in sarcosine cadmium bromide. The observed large value of zero-field tensor is due to the steric effects of the crystal packing caused by the ligands. Matumura's plot predicts an average covalency of 8.8 and 7.7% for the manganese-ligand bond in SCC and SCB lattices respectively.