FTIR study of manganese dimers with carboxylate donors as model complexes for the water oxidation complex in Photosystem II

The carboxylate stretching frequencies of two high-valent, di-μ-oxido bridged, manganese dimers has been studied with IR spectroscopy in three different oxidation states. Both complexes contain one monodentate carboxylate donor to each Mn ion, in one complex, the carboxylate is coordinated perpendicular to the Mn-(μ-O)(2)-Mn plane, and in the other complex, the carboxylate is coordinated in the Mn-(μ-O)(2)-Mn plane. For both complexes, the difference between the asymmetric and the symmetric carboxylate stretching frequencies decrease for both the Mn(2)(IV,IV) to Mn(2)(III,IV) transition and the Mn(2)(III,IV) to Mn(2)(III,III) transition, with only minor differences observed between the two arrangements of the carboxylate ligand versus the Mn-(μ-O)(2)-Mn plane. The IR spectra also show that both carboxylate ligands are affected for each one electron reduction, i.e., the stretching frequency of the carboxylate coordinated to the Mn ion that is not reduced also shifts. These results are discussed in relation to FTIR studies of changes in carboxylate stretching frequencies in a one electron oxidation step of the water oxidation complex in Photosystem II.     

High-frequency and -field EPR investigation of a manganese(III) N-confused porphyrin complex, [Mn(NCTPP)(py)2

We report the first high-frequency and -field electron paramagnetic resonance (HFEPR) study of a Mn(III) N-confused porphyrin (NCP) complex (NCP is also known as inverted porphyrin or 2-aza-21-carbaporphyrin). We have found a striking variation in the electronic properties of the S = 2 Mn(III) ion coordinated by NCP compared to other Mn(III) porphyrinoid complexes. Thus, inversion of a single pyrrole ring greatly changes the equatorial ligand field exerted and leads to large magnitudes of both the axial and rhombic zero-field splitting [respectively, D = -3.084(3) cm(-1), E = -0.608(3) cm(-1)], which are unprecedented in other Mn(III) porphyrinoids.     

Synthesis and structural characterization of manganese(III,IV) and ruthenium(III) complexes derived from 2-hydroxy-1-naphthaldehydebenzoylhydrazone

Reaction of 2-hydroxy-1-naphthaldehydebenzoylhydrazone(napbhH₂) with manganese(II) acetate tetrahydrate and manganese(III) acetate dihydrate in methanol followed by addition of methanolic KOH in molar ratio (2 : 1 : 10) results in [Mn(IV)(napbh)₂] and [Mn(III)(napbh)(OH)(H₂O)], respectively. Activated ruthenium(III) chloride reacts with napbhH₂ in methanolic medium yielding [Ru(III)(napbhH)Cl(H₂O)]Cl. Replacement of aquo ligand by heterocyclic nitrogen donor in this complex has been observed when the reaction is carried out in presence of pyridine(py), 3-picoline(3-pic) or 4-picoline(4-pic). The molar conductance values in DMF (N,N-dimethyl formamide) of these complexes suggest non-electrolytic and 1 : 1 electrolytic nature for manganese and ruthenium complexes, respectively. Magnetic moment values of manganese complexes suggest Mn(III) and Mn(IV), however, ruthenium complexes are paramagnetic with one unpaired electron suggesting Ru(III). Electronic spectral studies suggest six coordinate metal ions in these complexes. IR spectra reveal that napbhH₂ coordinates in enol-form and keto-form to manganese and ruthenium metal ions in its complexes, respectively. ESR studies of the complexes are also reported.     

Structural investigation of high-valent manganese-salen complexes by UV/Vis,Raman, XANES,and EXAFS spectroscopy

XANES and EXAFS spectroscopic studies at the Mn-K- and Br-K-edge of reaction products of (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride ([(salen)Mn(III)Cl], 1) and (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) bromide ([(salen)Mn(III)Br], 2) with 4-phenylpyridine N-oxide (4-PPNO) and 3-chloroperoxybenzoic acid (MCPBA) are reported. The reaction of the Mn(III) complexes with two equivalents of 4-PPNO leads to a hexacoordinated compound, in which the manganese atom is octahedrally coordinated by four oxygen/nitrogen atoms of the salen ligand at an average distance of approximately 1.90 A and two additional, axially bonded oxygen atoms of the 4-PPNO at 2.25 A. The oxidation state of this complex was determined as approximately +IV by a comparative study of Mn(III) and Mn(V) reference compounds. The green intermediate obtained in reactions of MCPBA and solutions of 1 or 2 in acetonitrile was investigated with XANES, EXAFS, UV/Vis, and Raman spectroscopy, and an increase of the coordination number of the manganese atoms from 4 to 5 and the complete abstraction of the halide was observed. A formal oxidation state of IV was deduced from the relative position of the pre-edge 1s-->3d feature of the X-ray absorption spectrum of the complex. The broad UV/Vis band of this complex in acetonitrile with lambda(max)=648 nm was consistent with a radical cation structure, in which a MCPBA molecule was bound to the Mn(IV) central atom. An oxomanganese(V) or a dimeric manganese(IV) species was not detected.     

Anion-controlled assembly of four manganese ions: structural, magnetic, and electrochemical properties of tetramanganese complexes stabilized by xanthene-bridged Schiff base ligands

The reaction of manganese(II) acetate with a xanthene-bridged bis[3-(salicylideneamino)-1-propanol] ligand, H(4)L, afforded the tetramanganese(II,II,III,III) complex [Mn(4)(L)(2)(μ-OAc)(2)], which has an incomplete double-cubane structure. The corresponding reaction using manganese(II) chloride in the presence of a base gave the tetramanganese(III,III,III,III) complex [Mn(4)(L)(2)Cl(3)(μ(4)-Cl)(OH(2))], in which four Mn ions are bridged by a Cl(-) ion. A pair of L ligands has a propensity to incorporate four Mn ions, the arrangement and oxidation states of which are dependent on the coexistent anions.     

High-frequency and field EPR investigation of (8,12-diethyl-2,3,7,13,17,18-hexamethylcorrolato)manganese(III)

High-field and frequency electron paramagnetic resonance (HFEPR) of solid (8,12-diethyl-2,3,7,13,17,18-hexamethylcorrolato)manganese(III), 1, shows that in the solid state it is well described as an S = 2 (high-spin) Mn(III) complex of a trianionic ligand, [Mn(III)C(3)(-)], just as Mn(III) porphyrins are described as [Mn(III)P(2)(-)](+). Comparison among the structural data and spin Hamiltonian parameters reported for 1, Mn(III) porphyrins, and a different Mn(III) corrole, [(tpfc)Mn(OPPh(3))], previously studied by HFEPR (Bendix, J.; Gray, H. B.; Golubkov, G.; Gross, Z. J. Chem. Soc., Chem. Commun. 2000, 1957-1958), shows that despite the molecular asymmetry of the corrole macrocycle, the electronic structure of the Mn(III) ion is roughly axial. However, in corroles, the S = 1 (intermediate-spin) state is much lower in energy than in porphyrins, regardless of axial ligand. HFEPR of 1 measured at 4.2 K in pyridine solution shows that the S = 2 [Mn(III)C(3)(-)] system is maintained, with slight changes in electronic parameters that are likely the consequence of axial pyridine ligand coordination. The present result is the first example of the detection by HFEPR of a Mn(III) complex in solution. Over a period of hours in pyridine solution at ambient temperature, however, the S = 2 Mn(III) spectrum gradually disappears leaving a signal with g = 2 and (55)Mn hyperfine splitting. Analysis of this signal, also observable by conventional EPR, leads to its assignment to a manganese species that could arise from decomposition of the original complex. The low-temperature S = 2 [Mn(III)C(3)(-)] state is in contrast to that at room temperature, which is described as a S = 1 system deriving from antiferromagnetic coupling between an S = (3/2) Mn(II) ion and a corrole-centered radical cation: [Mn(II)C(*)(2-)] (Licoccia, S.; Morgante, E.; Paolesse, R.; Polizio, F.; Senge, M. O.; Tondello, E.; Boschi, T. Inorg. Chem. 1997, 36, 1564-1570). This temperature-dependent valence state isomerization has been observed for other metallotetrapyrroles.     

Assembly of azido- or cyano-bridged binuclear complexes containing the bulky [Mn(phen)2]2+ building block: syntheses, crystal structures, and magnetic properties

Two new cyano-bridged heterobinuclear complexes, [Mn(II)(phen)2Cl][Fe(III)(bpb)(CN)2] x 0.5CH3CH2OH x 1.5H2O (1) and [Mn(II)(phen)2Cl][Cr(III)(bpb)(CN)2] x 2H2O (2) [phen = 1,10-phenanthroline; bpb(2-) = 1,2-bis(pyridine-2-carboxamido)benzenate], and four novel azido-bridged Mn(II) dimeric complexes, [Mn2(phen)4(mu(1,1)-N3)2][M(III)(bpb)(CN)2]2 x H2O [M = Fe (3), Cr (4), Co (5)] and [Mn2(phen)4(mu(1,3)-N3)(N3)2]BPh4 x 0.5H2O (6), have been synthesized and characterized by single-crystal X-ray diffraction analysis and magnetic studies. Complexes 1 and 2 comprise [Mn(phen)2Cl]+ and [M(bpb)(CN)2]- units connected by one cyano ligand of [M(bpb)(CN)2]-. Complexes 3-5 are doubly end-on (EO) azido-bridged Mn(II) binuclear complexes with two [M(bpb)(CN)2]- molecules acting as charge-compensating anions. However, the Mn(II) ions in complex 6 are linked by a single end-to-end (EE) azido bridging ligand with one large free BPh4(-) group as the charge-balancing anion. The magnetic coupling between Mn(II) and Fe(III) or Cr(III) in complexes 1 and 2 was found to be antiferromagnetic with J(MnFe) = -2.68(3) cm(-1) and J(MnCr) = -4.55(1) cm(-1) on the basis of the Hamiltonian H = -JS(Mn)S(M) (M = Fe or Cr). The magnetic interactions between two Mn(II) ions in 3-5 are ferromagnetic in nature with the magnetic coupling constants of 1.15(3), 1.05(2), and 1.27(2) cm(-1) (H = -JS(Mn1)S(Mn2)), respectively. The single EE azido-bridged dimeric complex 6 manifests antiferromagnetic interaction with J = -2.29(4) cm(-1) (H = -JS(Mn1)S(Mn2)). Magneto-structural correlationship on the EO azido-bridged Mn(II) dimers has been investigated.     

Orbital Configuration of the Valence Electrons, Ligand Field Symmetry, and Manganese Oxidation States of the Photosynthetic Water Oxidizing Complex: Analysis of the S(2) State Multiline EPR Signals

A theoretical framework is presented for analysis of all three "multiline" EPR spectra (MLS) arising from the tetramanganese (Mn(4)) cluster in the S(2) oxidation state of the photosynthetic water oxidizing complex (WOC). Accurate simulations are presented which include anisotropy of the g and (four) (55)Mn hyperfine tensors, chosen according to a database of (55)Mn(III) and (55)Mn(IV) hyperfine tensors obtained previously using unbiased least-squares spectral fitting routines. In view of the large (30%) anisotropy common to Mn(III) hyperfine tensors in all complexes, previous MLS simulations which have assumed isotropic hyperfine constants have required physically unrealistic parameters. A simple model is found which offers good simulations of both the native "19-21-line" MLS and the "26-line" NH(3)-bound form of the MLS. Both a dimer-of-dimers and distorted-trigonal magnetic models are examined to describe the symmetry of the Heisenberg exchange interactions within the Mn(4) cluster and thus define the initial electronic basis states of the cluster. The effect of rhombic symmetry distortions is explicitly considered. Both magnetic models correspond to one of several possible structural models for the Mn(4) cluster proposed independently from Mn EXAFS studies. Simulated MLS were constructed for each of the eight (or seven) doublet states of the Mn(4) cluster in the WOC for the two viable oxidation models (3Mn(III)-1Mn(IV) or 3Mn(IV)-1Mn(III)), and using a wide range of axial Mn hyperfine tensors, with either coaxial or orthogonal tensor alignments. We find accurate simulations using the 3Mn(III)-1Mn(IV) oxidation model. In the dimer-of-dimers coupling model, the spin state conversion between two doublet states |S(12),S(34),S(T)|(7)/(2),4,(1)/(2)> and |(7)/(2),3,(1)/(2)> is found to explain the large (25%) contraction in the hyperfine splitting observed upon conversion from the native MLS to the NH(3)-bound MLS. Stabilization of this excited state as the new ground state is caused by change in the intermanganese exchange coupling, without appreciable change in the intrinsic hyperfine tensors. The lack of good simulations of the Ca(2+)-depleted MLS suggests that Ca(2+)-depletion changes both Mn ligation and intermanganese exchange coupling. The 3Mn(IV)-1Mn(III) oxidation model is disfavored because only approximate simulations could be found for the native MLS and no agreement with the NH(3)-bound MLS was obtained. The scalar part of the hyperfine tensors for both Mn(III) and Mn(IV) ions were found to approximate (+/-5%) the values for the dimanganese(III,IV) catalase enzyme, suggesting similar overall ligand types. However, the large (30%) anisotropic part of the Mn(III) hyperfine interaction is opposite in sign to that found in all tetragonally extended six-coordinate Mn(III) ions (i.e., the usual Jahn-Teller splitting). The distribution of spin density from the high-spin d(4) electron configuration of each Mn(III) ion corresponds to a flattened (oblate) ellipsoid. This electronic distribution is favored in five-coordinate ligand fields having trigonally compressed bipyramidal geometry, but it could also arise, in principle, in strained six-coordinate ligand fields having tetragonally compressed geometry, i.e. [Mn(2)(&mgr;-O)](4+) (reverse Jahn-Teller distortion). The resulting valence electronic configurations are described as e'(2)e"(2) and (d(pi))(3)(d(x)()()2(-)(y)()()2)(1), respectively, in contrast to the (d(pi))(3)(d(z)()()2)(1) configuration common to unstrained six-coordinate tetragonally-extended Mn(III) ions, such as found in the [Mn(2)(&mgr;-O)(2)](3+) core in several synthetic dimers and catalase. Both of the former geometries predict strongly oxidizing Mn(III) ions, thereby suggesting a structural basis for the oxidative reactivity of the Mn(4) cluster in the WOC. The magnetic model needed to explain the MLS is not readily reconciled with the simplest structural and electronic models deduced from EXAFS studies of the WOC.     

Synthesis, characterization, and physicochemical properties of manganese(III) and manganese(V)-oxo corrolazines

The structural and physicochemical properties of the manganese-corrolazine (Cz) complexes (TBP8Cz)Mn(V)O (1) and (TBP8Cz)Mn(III) (2) (TBP = p-tert-butylphenyl) have been determined. Recrystallization of 2 from toluene/MeOH resulted in the crystal structure of (TBP8Cz)Mn(III)(CH3OH) (2 x MeOH). The packing diagram of 2 x MeOH reveals hydrogen bonds between MeOH axial ligands and meso N atoms of adjacent molecules. Solution binding studies of 2 with different axial ligands (Cl-, Et3PO, and Ph3PO) reveal strong binding, corroborating the preference of the Mn(III) ion for a five-coordinate environment. High-frequency and field electron paramagnetic resonance (HFEPR) spectroscopy of solid 2 x MeOH shows that 2 x MeOH is best described as a high-spin (S = 2) Mn(III) complex with zero-field splitting parameters typical of corroles. Structural information on 1 was obtained through an X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure (EXAFS) study and compared to XANES/EXAFS data for 2 x MeOH. The XANES data for 1 shows an intense pre-edge transition characteristic of a high-valent metal-oxo species, and a best fit of the EXAFS data gives a short Mn-O bond distance of 1.56 A, confirming the structure of the metal-oxo unit in 1. Detailed spectroelectrochemical studies of 1 and 2 were performed revealing multiple reversible redox processes for both complexes, including a relatively low potential for the Mn(V) --> Mn(IV) process in 1 (near 0.0 V vs saturated calomel reference electrode). Chemical reduction of 1 results in the formation of a Mn(III)Mn(IV)(mu-O) dimer as characterized by electron paramagnetic resonance spectroscopy.     

Dioxo-bridged dinuclear manganese(III) and -(IV) complexes of pyridyl donor tripod ligands: combined effects of steric substitution and chelate ring size variations on structural,spectroscopic, and electrochemical properties

The syntheses and structural, spectral, and electrochemical characterization of the dioxo-bridged dinuclear Mn(III) complexes [LMn(mo-O)(2)MnL](ClO(4))(2), of the tripodal ligands tris(6-methyl-2-pyridylmethyl)amine (L(1)) and bis(6-methyl-2-pyridylmethyl)(2-(2-pyridyl)ethyl)amine (L(2)), and the Mn(II) complex of bis(2-(2-pyridyl)ethyl)(6-methyl-2-pyridylmethyl)amine (L(3)) are described. Addition of aqueous H(2)O(2) to methanol solutions of the Mn(II) complexes of L(1) and L(2) produced green solutions in a fast reaction from which subsequently precipitated brown solids of the dioxo-bridged dinuclear complexes 1 and 2, respectively, which have the general formula [LMn(III)(mu-O)(2)Mn(III)L](ClO(4))(2). Addition of 30% aqueous H(2)O(2) to the methanol solution of the Mn(II) complex of L(3) ([Mn(II)L(3)(CH(3)CN)(H(2)O)](ClO(4))(2) (3)) showed a very sluggish change gradually precipitating an insoluble black gummy solid, but no dioxo-bridged manganese complex is produced. By contrast, the Mn(II) complex of the ligand bis(2-(2-pyridyl)ethyl)(2-pyridylmethyl)amine (L(3a)) has been reported to react with aqueous H(2)O(2) to form the dioxo-bridged Mn(III)Mn(IV) complex. In cyclic voltammetric experiments in acetonitrile solution, complex 1 shows two reversible peaks at E(1/2) = 0.87 and 1.70 V (vs Ag/AgCl) assigned to the Mn(III)(2) <--> Mn(III)Mn(IV) and the Mn(III)Mn(IV) <--> Mn(IV)(2) processes, respectively. Complex 2 also shows two reversible peaks, one at E(1/2) = 0.78 V and a second peak at E(1/2) = 1.58 V (vs Ag/AgCl) assigned to the Mn(III)(2) <--> Mn(III)Mn(IV) and Mn(III)Mn(IV) <--> Mn(IV)(2) redox processes, respectively. These potentials are the highest so far observed for the dioxo-bridged dinuclear manganese complexes of the type of tripodal ligands used here. The bulk electrolytic oxidation of complexes 1 and 2, at a controlled anodic potential of 1.98 V (vs Ag/AgCl), produced the green Mn(IV)(2) complexes that have been spectrally characterized. The Mn(II) complex of L(3) shows a quasi reversible peak at an anodic potential of E(p,a) of 1.96 V (vs Ag/AgCl) assigned to the oxidation Mn(II) to Mn(III) complex. It is about 0.17 V higher than the E(p,a) of the Mn(II) complex of L(3a). The higher oxidation potential is attributable to the steric effect of the methyl substituent at the 6-position of the pyridyl donor of L(3).     

Spectroscopic approach in characterization of chromium(III), manganese(II), iron(III) and copper(II) complexes with a nitrogen donor tetradentate, 14-membered azamacrocyclic ligand

The complexes of Cr(III), Mn(II), Fe(III) and Cu(II) were synthesized with the macrocyclic ligand i.e. 2,3,9,10-tetraketo-1,4,8,11-tetraazacyclotetradecane. The ligand was prepared by the [2 + 2] condensation reaction of diethyloxalate and 1,3-diamino propane. These complexes were found to have the general composition M(L)X3 and M'(L)X2 [where M = Mn(II) and Cu(II), M' = Cr(III) and Fe(III), L = ligand (N4) and X = Cl-, NO3-, 1/2SO4(2-) and [CH3COO-]. The ligand and its transition metal complexes were characterized by the elemental analyses, molar conductance, magnetic susceptibility, mass, IR, electronic, and EPR spectral studies. On the basis of IR, electronic and EPR spectral studies an octahedral geometry has been assigned for Cr(III), Mn(II) and Fe(III) and a tetragonal geometry for Cu(II) complexes.     

Oxygen evolution catalysis by a dimanganese complex and its relation to photosynthetic water oxidation

[Mn2(III/IV)(mu-O) 2(terpy)2(OH 2)2](NO3)3 (1, where terpy = 2,2':6'2'-terpyridine) acts as a water-oxidation catalyst with HSO5(-) as the primary oxidant in aqueous solution and, thus, provides a model system for the oxygen-evolving complex of photosystem II (Limburg, J.; et al. J. Am. Chem. Soc. 2001, 123, 423-430). The majority of the starting [Mn2(III/IV)(mu-O)2](3+) complex is converted to the[Mn2(IV/IV)(mu-O)2](4+) form (2) during this reaction (Chen, H.; et al. Inorg. Chem. 2007, 46, 34-43). Here, we have used stopped-flow UV-visible spectroscopy to monitor UV-visible absorbance changes accompanying the conversion of 1 to 2 by HSO5(-). With excess HSO5(-), the rate of absorbance change was found to be first-order in [1] and nearly zero-order in [HSO5(-)]. At relatively low [HSO5(-)], the change of absorbance with time is distinctly biphasic. The observed concentration dependences are interpreted in terms of a model involving the two-electron oxidation of 1 by HSO5(-), followed by the rapid reaction of the two-electron-oxidized intermediate with another molecule of 1 to give two molecules of 2. In order to rationalize biphasic behavior at low [HSO5(-)], we propose a difference in reactivity of the [Mn2(III/)(IV)(mu-O)2](3+) complex upon binding of HSO5(-) to the Mn(III) site as compared to the reactivity upon binding HSO5(-) to the Mn(IV) site. The kinetic distinctness of the Mn(III) and Mn(IV) sites allows us to estimate upper limits for the rates of intramolecular electron transfer and terminal ligand exchange between these sites. The proposed mechanism leads to insights on the optimization of 1 as a water-oxidation catalyst. The rates of terminal ligand exchange and electron transfer between oxo-bridged Mn atoms in the oxygen-evolving complex of photosystem II are discussed in light of these results.     

Catalytic epoxidation of cyclohexene by covalently linked manganese porphyrin–viologen complex

When a viologen-linked Mn(III)porphyrin complex with a short methylene-chain, in which a viologen is covalently linked by the methylene-chain into one phenyl group of 5,10,15,20-tetraphenylporphyrinatomanganese(III)chloride (Mn(III)(tpp)Cl), was used as a catalyst for a monooxygenation of cyclohexene in an air-equilibrated acetonitrile solution containing insoluble zinc powder as a reductant, more cyclohexene oxide was obtained as a single product than when Mn(tpp)Cl was used as a catalyst. Benzoic acid as a cleaving reagent of the dioxygen double-bond and 1-methylimidazole as a ligand to Mn porphyrin were further contained in the reaction mixture. This result implies that the viologen moiety in the viologen-linked Mn(III)porphyrin acted effectively as a mediator for electron transfer from zinc powder to the Mn(III)porphyrin moiety in the epoxidation cycle activating molecular dioxygen reductively. Though Mn(tpp)Cl was remarkably demetallated by H⁺ ion from benzoic acid during the epoxidation reaction in the mixed system of Mn(III)(tpp)Cl and viologen, the demetallation of the viologen-linked Mn porphyrin with the short methylene-chain was partly prevented because the reduction of a Mn(II)porphyrin-dioxygen adduct was easily caused by fast intramolecular electron-transfer between the two moieties of the viologen and the Mn porphyrin, proceeding the epoxidation cycle smoothly.     

Reconstruction of the water-oxidizing complex in manganese-depleted Photosystem II using synthetic manganese complexes

The efficiency of a trinuclear and two binuclear manganese complexes in reconstituting electron transport and O(2) evolution activity in Mn-depleted Photosystem II preparations is analyzed. The trinuclear Mn-complex is more efficient than two binuclear Mn-complexes in restoring oxygen evolution, but it is less effective as an electron donor than binuclear Mn-complexes. It is inferred from our results that recovery of electron transport and O(2) evolution with polynuclear Mn-complexes is affected with different factors. Moreover, the trinuclear Mn-complex is extremely sensitive to the addition of CaCl(2). It is suggested that there is an interaction between Ca(2+) and carboxyl within the trinuclear Mn-complex during photoactivation and this interaction benefits the ligation of Mn atom to the apo-WOC and form an active WOC. Binuclear Mn(III)Mn(III) complex shows slightly higher efficiency than binuclear Mn(III)Mn(IV) complex in restoration of O(2) evolution activity. The efficiency of three Mn-complexes in the reconstitution of WOC is in an order: trinuclear Mn(3)(III)>binuclear Mn(III)Mn(III)>binuclear Mn(III)Mn(IV).     

Synthesis, structure, and characterisation of a new phenolato-bridged manganese complex [Mn2(mL)2]2+: chemical and electrochemical access to a new mono-mu-oxo dimanganese core unit

The dinuclear phenolato-bridged complex [(mL)Mn(II)Mn(II)(mL)](ClO(4))(2) (1(ClO(4))(2)) has been obtained with the new [N(4)O] pentadentate ligand mL(-) (mLH=N,N'-bis-(2-pyridylmethyl)-N-(2-hydroxybenzyl)-N'-methyl-ethane-1,2-diamine) and has been characterised by X-ray crystallography. X- and Q-band EPR spectra were recorded and their variation with temperature was examined. All spectra exhibit features extending over 0-800 mT at the X band and over 100-1450 mT at the Q band, features that are usually observed for dinuclear Mn(II) complexes. Cyclic voltammetry of 1 exhibits two irreversible oxidation waves at E(1)(p)=0.89 V and E(2)(p)=1.02 V, accompanied on the reverse scan by an ill-defined cathodic wave at E(1')(p)=0.56 V (all measured versus the saturated calomel electrode (SCE)). Upon chemical oxidation with tBuOOH (10 equiv) at 20 degrees C, 1 is transformed into the mono-mu-oxo species [(mL)Mn(III)-(mu-O)-Mn(III)(mL)](2+) (2), which eventually partially evolves into the di-mu-oxo species [(mL)Mn(III)-(mu-O)(2)-Mn(IV)(mL)](n+) (3) in which one of the aromatic rings of the ligand is decoordinated. The UV/Vis spectrum of 2 displays a large absorption band at 507 nm, which is attributed to a phenolate-->Mn(III) charge-transfer transition. The cyclovoltammogram of 2 exhibits two reversible oxidation waves, at 0.65 and 1.16 V versus the SCE, corresponding to the Mn(III)Mn(III)/Mn(III)Mn(IV) and Mn(III)Mn(IV)/Mn(IV)Mn(IV) oxidation processes, respectively. The one-electron electrochemical oxidation of 2 leads to the mono-mu-oxo mixed-valent species [(mL)Mn(III)-(mu-O)-Mn(IV)(mL)](3+) (2 ox). The UV/Vis spectrum of 2 ox exhibits one large band at 643 nm, which is attributed to the phenolate-->Mn(IV) charge-transfer transition. 2 ox can also be obtained by the direct electrochemical oxidation of 1 in the presence of an external base. The 2 ox and 3 species exhibit a 16-line EPR signal with first peak to last trough widths of 125 and 111 mT, respectively. Both spectra have been simulated by using colinear rhombic Mn-hyperfine tensors. Mechanisms for the chemical formation of 2 and the electrochemical oxidation of 1 into 2 ox are proposed.     

Deliberate synthesis for magnetostructural study of linear tetranuclear complexes BIIIMnIIMnIIBIII, MnIIIMnIIMnIIMnIII, MnIVMnIIMnIIMnIV, FeIIIMnIIMnIIFeIII, and CrIIIMnIIMnIICrIII. Influence of terminal ions on the exchange coupling

As part of an ongoing effort to deliberate synthesis of polynuclear heterometal complexes, we are exploring synthetic routes to high-nuclearity complexes using "metal oximates" as building blocks. Series of tetranuclear linear complex ions of the general types M(A)M(B)M(B)M(A), where M(A) is a trivalent or tetravalent metal ion and M(B) is a divalent metal ion, e.g., Mn(II), have been synthesized by using the dimetal(II) anionic cores, [(M(II)(B))(2)(dfmp)(3)](5)(-) as a bridging ligand for the terminal LM(A) fragments where H(3)dfmp is a dinucleating phenol-oxime ligand, 2,6-diformyl-4-methylphenol oxime, and L denotes a facially coordinating cyclic tridentate amine, 1,4,7-trimethyl-1,4,7-triazacyclononane. The following combinations are reported here, B(III)Mn(II)Mn(II)B(III) (1), Mn(III)Mn(II)Mn(II)Mn(III) (2), Mn(IV)Mn(II)Mn(II)Mn(IV) (3), Fe(III)Mn(II)Mn(II)Fe(III) (4), and Cr(III)Mn(II)Mn(II)Cr(III) (5). The compounds have been characterized spectroscopically and by magnetic susceptibility measurements in the temperature range 2.0-290 K at different field strengths. Complexes 1-4 have also been structurally characterized by single-crystal X-ray diffraction techniques at 100 K. The magnetic behaviors of the compounds indicate weak antiferromagnetic coupling between the manganese(II) centers in the central trisphenoxo-bridged dimanganese(II) core, whereas the coupling between the terminal M(A) and its neighboring Mn(II) center varies and is weak ferromagnetic or antiferromagnetic. The relative interaction intensity in such a series of complexes is discussed. Finally, a profound influence of the charge on the terminal metal ions on the strength of the exchange coupling in the central dimanganese(II) core has been observed and discussed in relation to the covalency of the metal-ligand bonding.     

Novel Schiff base ligand and its metal complexes with some transition elements. Synthesis,spectroscopic, thermal analysis,antimicrobial and in vitro anticancer activity

A novel Schiff base ligand (H₂L) was prepared through condensation of 2,6‐diaminopyridine and o‐benzoylbenzoic acid in a 1:2 ratio. This Schiff base ligand was characterized using elemental and spectroscopic analyses. A new series of Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) metal complexes of H₂L were prepared and characterized using elemental analysis, spectroscopy (¹H NMR, mass, UV–visible, Fourier transform infrared, electron spin resonance), magnetic susceptibility, molar conductivity, X‐ray powder diffraction and thermal analysis. The complexes are found to have trigonal bipyramidal geometry except Cr(III), Mn(II) and Fe(III) complexes which have octahedral geometry based on magnetic moment and solid reflectance measurements. The infrared spectral studies reveal that H₂L behaves as a neutral bidentate ligand and coordinates to the metal ions via the two azomethine nitrogens. ¹H NMR spectra confirm the non‐involvement of the carboxylic COOH proton in complex formation. The presence of water molecules in all reported complexes is supported by thermogravimetric studies. Kinetic and thermodynamic parameters were determined using Coats–Redfern and Horowitz–Metzger equations. The synthesized ligand and its complexes were screened for antimicrobial activities against two Gram‐positive bacteria (Bacillus subtilis and Staphylococcus aureus), two Gram‐negative bacteria (Escherichia coli and Neisseria gonorrhoeae) and one fungus (Candida albicans). Anticancer activities of the ligand and its metal complexes against human breast cancer cell line (MCF7) were investigated. Copyright © 2016 John Wiley & Sons, Ltd.     

Synthesis,characterization, spectroscopic and theoretical studies of transition metal complexes of new nano Schiff base derived from l‐histidine and 2‐acetylferrocene and evaluation of biological and anticancer activities

A new Schiff base derived from the condensation of 2‐acetylferrocene with l ‐histidine was prepared and characterized using elemental analyses and spectroscopic methods. Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) complexes of the Schiff base were prepared and characterized using various physicochemical methods such as elemental analysis, Fourier transform infrared and UV–visible spectroscopies, molar conductance, thermal analysis and scanning electron microscopy (SEM). Both ligand and complexes were investigated for their biological and anticancer activities. The elemental analyses showed that complexes were formed in a metal‐to‐ligand ratio of 1:1 stoichiometry. The spectral analyses proved that the ligand was tridentate and all complexes had an octahedral geometry, except the Zn(II) complex that was tetrahedral. SEM showed that the ligand and its Cd(II) complex were of nanometric structure. The molecular and electronic structure of the free ligand was optimized theoretically and the quantum chemical parameters were calculated. The molecular structure can be used to investigate the coordination sites and the total charge density around each atom. According to anticancer studies, Cd(II) complex was recommended to be used as anti‐breast cancer drug as it had very low IC₅₀ (3.5 μg ml^?1). Molecular docking was used to predict the binding between the free ligand and its Cd(II) complex and crystal structure of Staphylococcus aureus (PDB ID: 3Q8u), receptors of breast cancer mutant oxidoreductase (PDB ID: 3Hb5) and crystal structure of Escherichia coli (PDB ID: 3 T88) and to identify the binding mode and the crucial functional groups interacting with the three proteins.     

New cyanide-bridged Mn(III)-M(III) heterometallic dinuclear complexes constructed from [M(III)(AA)(CN)4]- building blocks (M = Cr and Fe): synthesis, crystal structures and magnetic properties

Three Mn(III)-M(III) (M = Cr and Fe) dinuclear complexes have been obtained by assembling [Mn(III)(SB)(H(2)O)](+) and [M(III)(AA)(CN)(4)](-) ions, where SB is the dianion of the Schiff-base resulting from the condensation of 3-methoxysalicylaldehyde with ethylenediamine (3-MeOsalen(2-)) or 1,2-cyclohexanediamine (3-MeOsalcyen(2-)): [Mn(3-MeOsalen)(H(2)O)(μ-NC)Cr(bipy)(CN)(3)]·2H(2)O (1), [Mn(3-MeOsalen)(H(2)O)(μ-NC)Cr(ampy)(CN)(3)][Mn(3-MeOsalen)(H(2)O)(2)]ClO(4)·2H(2)O (2) and [Mn(3-MeOsalcyen)(H(2)O)(μ-NC)Fe(bpym)(CN)(3)]·3H(2)O (3) (bipy = 2,2'-bipyridine, ampy = 2-aminomethylpyridine and bpym = 2,2'-bipyrimidine). The [M(AA)(CN)(4)](-) unit in 1-3 acts as a monodentate ligand towards the manganese(III) ion through one of its four cyanide groups. The manganese(III) ion in 1-3 exhibits an elongated octahedral stereochemistry with the tetradentate SB building the equatorial plane and a water molecule and a cyanide-nitrogen atom filling the axial positions. Remarkably, the neutral mononuclear complex [Mn(3-MeOsalen)(H(2)O)(2)]ClO(4) co-crystallizes with the heterobimetallic unit in 2. The values of the Mn(III)-M(III) distance across the bridging cyanide are 5.228 (1), 5.505 (2) and 5.265 ? (3). The packing of the neutral heterobimetallic units in the crystal is governed by the self-complementarity of the [Mn(SB)(H(2)O)](+) moieties, which interact each other through hydrogen bonds established between the aqua ligand from one fragment with the set of phenolate- and methoxy-oxygens from the adjacent one. The magnetic properties of the three complexes have been investigated in the temperature range 1.9-300 K. Weak antiferromagnetic interactions between the Mn(III) and M(III) ions across the cyanido bridge were found: J(MnM) = -5.6 (1), -0.63 (2) and -2.4 cm(-1) (3) the Hamiltonian being defined as H = -JS(Mn)·S(M). Theoretical calculations based on density functional theory (DFT) have been used to substantiate both the nature and magnitude of the exchange interactions observed and also to analyze the dependence of the magnetic coupling on the structural parameters within the Mn(III)-N-C-M(III) motif in 1-3.