Synthesis and Structural Investigations of [Mn(3)O(4)(phen)(4)(H(2)O)(2)](NO(3))(4).2.5H(2)O: A Water-Bound Complex Obtained by Cerium(IV) Oxidation

The trinuclear manganese complex [Mn(3)O(4)(phen)(4)(H(2)O)(2)](NO(3))(4).2.5H(2)O, 1 (where, phen = 1,10-phenanthroline), has been synthesized by the Ce(IV) oxidation of a concentrated solution of manganese(II) acetate and phen in 1.6 N nitric acid. The complex crystallizes in the triclinic space group P&onemacr; with a = 10.700(2) ?, b = 12.643(3) ?, c = 20.509(4) ?, alpha = 78.37(3) degrees, beta = 83.12(3) degrees, gamma = 82.50(3) degrees, and Z = 2. The structure was solved by direct methods and refined by least-squares techniques to the conventional R (R(w)) factors of 0.055 (0.076) based on 4609 unique reflections with F(o) >/= 6.0sigma(F(o)). The structure of the cation consists of an oxo-bridged Mn(3)O(4)(4+) core, with the geometry of the manganese atoms being octahedral. The coordination polyhedron of one of the manganese atoms (Mn(1)) consists of two &mgr; oxo ligands and two pairs of nitrogen atoms of two phen moieties, whereas that of each of the remaining two manganese atoms consists of three &mgr;-oxo ligands, two nitrogen atoms of a phen moiety, and the oxygen atom of a water molecule. The complex represents the second example for water coordination to manganese(IV) centers in complexes with a Mn(3)O(4)(4+) core. Optical spectra in ligand buffer (pH 4.5) reveal complete conversion of the complex into a Mn(III)Mn(IV) species. The observed room-temperature (298 K) magnetic moment of 3.75 &mgr;(B) indicates the presence of strong antiferromagnetic coupling in the complex.     

Mechanistic studies on the oxidation of ascorbic acid and hydroquinone by a {Mn4O6}4+ core in aqueous media

Described in this work is the kinetics of oxidation of ascorbic acid and hydroquinone by a tetranuclear Mn(IV) oxidant, [Mn(4)(μ-O)(6)(bipy)(6)](4+) (1(4+), bipy =2,2(/)-bipyridine), in aqueous solution over a wide pH range 1.5-6.0. In particular, below pH 3.0, protonation on the oxo-bridge of 1(4+) results in the formation of [Mn(4)(μ-O)(5)(μ-OH)(bipy)(6)](5+) (1H(5+)) as an additional oxidant over 1(4+). Both ascorbic acid and ascorbate whereas only hydroquinone and none of its protolytic species were found to be reactive reducing agents in these reactions. Analysis of the rate data clearly established that the oxo-bridge protonated oxidant 1H(5+) is kinetically far more superior to 1(4+) in oxidizing ascorbic acid and hydroquinone. Rates of these reactions are substantially lowered in D(2)O-enriched media in comparison to that in H(2)O media. An initial one electron one proton transfer electroprotic rate step could be mechanistically conceived. DFT studies established that among the two sets of terminal and central Mn(IV) atoms in the tetranuclear oxidant, one of the two terminal Mn(IV) is reduced to Mn(III) at the rate step that we can intuitively predict considering the probable positive charge distribution on the Mn(IV) atoms.     

Determination of mu-oxo exchange rates in di-mu-oxo dimanganese complexes by electrospray ionization mass spectrometry

A time-resolved mass spectrometric technique has been used for the determination of rates of exchange of mu-O atoms with water for the complexes [(mes-terpy)2Mn2(III/IV)(mu-O)2(H2O)2](NO3)3 (1, mes-terpy = 4'-mesityl-2,2':6',2' '-terpyridine), [(bpy)4Mn2(III/IV)(mu-O)2](ClO4)3 (2, bpy = 2,2'-bipyridine), [(phen)4Mn2(III/IV)(mu-O)2](ClO4)3 (3, phen = 1,10-phenanthroline), [(bpea)2Mn2(III/IV)(mu-O)2(mu-OAc)](ClO4)2 (4, bpea = bis(2-pyridyl)ethylamine), [(bpea)2Mn2(IV/IV)(mu-O)2(mu-OAc)](ClO4)3 (4ox), [(terpy)4Mn4(IV/IV/IV/IV)(mu-O)5(H2O)2](ClO4)6 (5, terpy = 2,2':6',2'-terpyridine), and [(tacn)4Mn4(IV/IV/IV/IV)(mu-O)6]Br(3.5)(OH)0.5.6H2O (6, tacn = 1,4,7-triazacyclononane). The rate of exchange of mu-OAc bridges with free acetate in solution has been measured for complexes 4 and 4ox. These are the first measurements of rates of ligand exchange on biologically relevant high-valent Mn complexes. The data analysis method developed here is of general utility in the quantitation of isotope exchange processes by mass spectrometry. We find that the presence of labile coordination sites on Mn increases mu-O exchange rates, and that all-Mn(IV) states are more inert toward exchange than mixed Mn(III)-Mn(IV) states. The rates of mu-O exchange obtained in this work for a di-mu-oxo Mn2(III/IV) dimer with labile coordination sites are compared with the oxygen isotope incorporation rates from substrate water to evolved dioxygen measured in different S states of the oxygen evolving complex (OEC) of photosystem II (PSII). On the basis of this comparison, we propose that both substrate waters are not bound as mu-O bridges between Mn atoms in the S2 and S3 states of the OEC.     

Hydrocarbon oxidation by Bis-mu-oxo manganese dimers: electron transfer,hydride transfer,and hydrogen atom transfer mechanisms

Described here are oxidations of alkylaromatic compounds by dimanganese mu-oxo and mu-hydroxo dimers [(phen)(2)Mn(IV)(mu-O)(2)Mn(IV)(phen)(2)](4+) ([Mn(2)(O)(2)](4+)), [(phen)(2)Mn(IV)(mu-O)(2)Mn(III)(phen)(2)](3+) ([Mn(2)(O)(2)](3+)), and [(phen)(2)Mn(III)(mu-O)(mu-OH)Mn(III)(phen)(2)](3+) ([Mn(2)(O)(OH)](3+)). Dihydroanthracene, xanthene, and fluorene are oxidized by [Mn(2)(O)(2)](3+) to give anthracene, bixanthenyl, and bifluorenyl, respectively. The manganese product is the bis(hydroxide) dimer, [(phen)(2)Mn(III)(mu-OH)(2)Mn(II)(phen)(2)](3+) ([Mn(2)(OH)(2)](3+)). Global analysis of the UV/vis spectral kinetic data shows a consecutive reaction with buildup and decay of [Mn(2)(O)(OH)](3+) as an intermediate. The kinetics and products indicate a mechanism of hydrogen atom transfers from the substrates to oxo groups of [Mn(2)(O)(2)](3+) and [Mn(2)(O)(OH)](3+). [Mn(2)(O)(2)](4+) is a much stronger oxidant, converting toluene to tolyl-phenylmethanes and naphthalene to binaphthyl. Kinetic and mechanistic data indicate a mechanism of initial preequilibrium electron transfer for p-methoxytoluene and naphthalenes because, for instance, the reactions are inhibited by addition of [Mn(2)(O)(2)](3+). The oxidation of toluene by [Mn(2)(O)(2)](4+), however, is not inhibited by [Mn(2)(O)(2)](3+). Oxidation of a mixture of C(6)H(5)CH(3) and C(6)H(5)CD(3) shows a kinetic isotope effect of 4.3 +/- 0.8, consistent with C-H bond cleavage in the rate-determining step. The data indicate a mechanism of initial hydride transfer from toluene to [Mn(2)(O)(2)](4+). Thus, oxidations by manganese oxo dimers occur by three different mechanisms: hydrogen atom transfer, electron transfer, and hydride transfer. The thermodynamics of e(-), H(*), and H(-) transfers have been determined from redox potential and pK(a) measurements. For a particular oxidant and a particular substrate, the choice of mechanism is influenced both by the thermochemistry and by the intrinsic barriers. Rate constants for hydrogen atom abstraction by [Mn(2)(O)(2)](3+) and [Mn(2)(O)(OH)](3+) are consistent with their 79 and 75 kcal mol(-)(1) affinities for H(*). In the oxidation of p-methoxytoluene by [Mn(2)(O)(2)](4+), hydride transfer is thermochemically 24 kcal mol(-)(1) more facile than electron transfer; yet the latter mechanism is preferred. Thus, electron transfer has a substantially smaller intrinsic barrier than does hydride transfer in this system.     

Water oxidation catalyzed by the tetranuclear Mn complex [Mn(IV)4O5(terpy)4(H2O)2](ClO4)6

The tetranuclear manganese complex [Mn(IV)(4)O(5)(terpy)(4)(H(2)O)(2)](ClO(4))(6) (1; terpy = 2,2':6',2″-terpyridine) gives catalytic water oxidation in aqueous solution, as determined by electrochemistry and GC-MS. Complex 1 also exhibits catalytic water oxidation when adsorbed on kaolin clay, with Ce(IV) as the primary oxidant. The redox intermediates of complex 1 adsorbed on kaolin clay upon addition of Ce(IV) have been characterized by using diffuse reflectance UV/visible and EPR spectroscopy. One of the products in the reaction on kaolin clay is Mn(III), as determined by parallel-mode EPR spectroscopic studies. When 1 is oxidized in aqueous solution with Ce(IV), the reaction intermediates are unstable and decompose to form Mn(II), detected by EPR spectroscopy, and MnO(2). DFT calculations show that the oxygen in the mono-μ-oxo bridge, rather than Mn(IV), is oxidized after an electron is removed from the Mn(IV,IV,IV,IV) tetramer. On the basis of the calculations, the formation of O(2) is proposed to occur by reaction of water with an electrophilic manganese-bound oxyl radical species, (?)O-Mn(2)(IV/IV), produced during the oxidation of the tetramer. This study demonstrates that [Mn(IV)(4)O(5)(terpy)(4)(H(2)O)(2)](ClO(4))(6) may be relevant for understanding the role of the Mn tetramer in photosystem II.     

Distinct mechanisms of bridging-oxo exchange in di-mu-O dimanganese complexes with and without water-binding sites: implications for water binding in the O(2)-evolving complex of photosystem II

Isotopic exchange between oxygens of water and mu-O bridges in the di-mu-O dimanganese complexes, [(mes-terpy)2Mn2(III/IV)(mu-O)2(H2O)2](NO3)3 (1, mes-terpy = 4'-mesityl-2,2':6',2' '-terpyridine) and [(phen)4Mn2III/IV(mu-O)2](ClO4)3 (2, phen = 1,10-phenanthroline), has been investigated by a study of the kinetics of exchange. The data provide evidence for distinct mechanisms of exchange in 1 and 2 and suggest that these differences arise due to the presence and absence of terminal water-binding sites in 1 and 2, respectively. Exchange of oxygen atoms between water and mu-O bridges must involve the elementary steps of bridge protonation, deprotonation, opening, and closing. On the basis of the existing literature on these reactions in oxo-bridged metal complexes and our present data, we propose pathways of exchange in 1 and 2. The mechanism proposed for 1 involves an initial fast protonation of an oxo-bridge by water coordinated to Mn(IV), followed by a slow opening of the protonated bridge as proposed earlier for an analogous complex on the basis of DFT calculations. The mechanism proposed for 2 involves initial dissociation of phen, followed by coordination of water at the vacated sites, as observed for rearrangement of 2 to a trinuclear complex. The subsequent steps are proposed to be analogous to those for 1. Our results are discussed in the context of data on 18O-labeled water isotope exchange in photosystem II and provide support for the existence of fully protonated terminal waters bound to Mn in the O2-evolving complex of photosystem II.     

Structural diversity, optical and magnetic properties of a series of manganese thioarsenates with 1,10-phenanthroline or 2,2'-bipyridine ligands: using monodentate methylamine as an alkalinity regulator

The exploration in two hydro(solvo)thermal reaction systems As/S/Mn(2+)/phen/methylamine aqueous solution and As/S/Mn(2+)/2,2'-bipy/H(2)O affords five new manganese thioarsenates with diverse structures, namely, (CH(3)NH(3)){[Mn(phen)(2)](As(V)S(4))}·phen (1 and 1'), (CH(3)NH(3))(2){[Mn(phen)](2)(As(V)S(4))(2)} (2), {[Mn(phen)(2)](As(III)(2)S(4))}(n) (3), {[Mn(phen)](3)(As(III)S(3))(2)}·H(2)O (4), and {[Mn(2,2'-bipy)(2)](2)(As(V)S(4))}[As(III)S(S(5))] (5). Compound 1 comprises a {[Mn(phen)(2)](As(V)S(4))}(-) complex anion, a monoprotonated methylamine cation and a phen molecule. Compound 2 contains a butterfly like {[Mn(phen)](2)(As(V)S(4))(2)}(2-) anion charge compensated by two monoprotonated methylamine cations. Compound 3 is a neutral chain formed by a helical (1)(∞)(As(III)S(2)(-)) vierer chain covalently bonds to [Mn(II)(phen)](2+) complexes via all its terminal S atoms. Compound 4 features a neutral chain showing the stabilization of noncondensed (As(III)S(3))(3-) anions in the coordination of [Mn(II)(phen)](2+) complex cations. Compound 5 features a mixed-valent As(III)/As(V) character and an interesting chalcogenidometalates structure, where a polycation formed by the connection of two [Mn(2,2'-bipy)(2)](2+) complex cation and a (As(V)S(4))(3-) anion acts as a countercation for a polythioarsenate anion, [As(III)S(S(5))](-). The title compounds exhibit optical gaps in the range 1.58-2.48 eV and blue photoluminescence. Interestingly, compound 1 displays a weak second harmonic generation (SHG) response being about 1/21 times of KTP (KTiOPO(4)). Magnetic measurements show paramagnetic behavior for 1 and dominant antiferromagnetic behavior for 2-5. Of particular interest is 4, which is the first manganese chalcogenide showing spin-canting characteristic.     

mer-[Fe(pcq)(CN)3]-: a novel cyanide-containing building block and its application to assembling cyanide-bridged trinuclear FeIII2MnII complexes [pcq- = 8-(pyridine-2-carboxamido)quinoline anion

A new cyanide-containing building block K[Fe(pcq)(CN)(3)] [1; pcq(-) = 8-(pyridine-2-carboxamido)quinoline anion] containing a low-spin Fe(III) center with three cyanide groups in a meridional arrangement has been successfully designed and synthesized. Three cyanide-bridged trinuclear Fe(III)(2)Mn(II) complexes, [Fe(pcq)(CN)(3)](2)[Mn(CH(3)OH)(2)(H(2)O)(2)].2H(2)O (2), [Fe(pcq)(CN)(3)](2)[Mn(bipy)(2)].CH(3)OH.2H(2)O (3), and [Fe(pcq)(CN)(3)](2)[Mn(phen)(2)].CH(3)OH.2H(2)O (4), have been synthesized and structurally characterized. The magnetic susceptibilities of the three heterometallic complexes have been investigated.     

Fe(Phen)(CN)4]-: a versatile building block for the design of heterometallic systems. Crystal structures and magnetic properties of PPh4[Fe(Phen)(CN)4]*2H2O and [[Fe(Phen)(CN)4]2M(H2O)2]*4H2O [Phen = 1,10-phenanthroline; M = Mn(II) and Zn(II)

The mononuclear PPh4[Fe(phen)(CN)4]*2H2O (1) complex and the cyanide-bridged bimetallic [[Fe(phen)(CN)4]2M(H2O)2]*4H2O compounds [M = Mn(II) (2) and Zn(II) (3); phen = 1,10-phenanthroline; PPh4 = tetraphenylphosphonium cation] have been synthesized and structurally and magnetically characterized. Complex 1 crystallizes in the monoclinic system, space group P2(1)/c, with a = 9.364(4) A, b = 27.472(5) A, c = 14.301(3) A, beta = 97.68(2) degrees, and Z = 4. Complexes 2 and 3 are isostructural and they crystallize in the monoclinic system, space group P2(1)/n, with a = 7.5292(4) A, b = 15.6000(10) A, c = 15.4081(9) A, beta = 93.552(2) degrees, and Z = 2 for 2 and a = 7.440(1) A, b = 15.569(3) A, c = 15.344(6) A, beta = 93.63(2) degrees, and Z = 2 for 3. The structure of complex 1 is made up of mononuclear [Fe(phen)(CN)4]- anions, tetraphenyphosphonium cations, and water molecules of crystallization. The iron(III) is hexacoordinate with two nitrogen atoms of a chelating phen (2.018(6) and 2.021(6) A for Fe-N) and four carbon atoms of four terminal cyanide groups (Fe-C bond lengths varying in the range 1.906(8)-1.95(1) A) building a distorted octahedron around the metal atom. The structure of complexes 2 and 3 consists of neutral double zigzag chains of formula [[Fe(phen)(CN)4]2M(H2O)2] and crystallization water molecules. The [Fe(phen)(CN)4]- entity of 1 is present in 2 and 3 acting as a bridging ligand toward M(H2O)2 units [M = Mn(II) (2) and Zn(II) (3)] through two cyanide groups in cis positions, the other two cyanide remaining terminal. Two water molecules in trans positions and four cyanide-nitrogen atoms from four [Fe(phen)(CN)4]- units build a distorted octahedral surrounding Mn(II) (2) and Zn(II) (3). The M-O bond lengths are 2.185(3) (2) and 2.105(3) A (3), whereas the M-N bond distances vary in the ranges 2.210(3)-2.258(3) A (2) and 2.112(3)-2.186(3) A (3). The structure of the [Fe(phen)(CN)4]- complex ligand in 2 and 3 is as in 1. The shorter intrachain Fe-M distances through bridging cyano are 5.245(5) and 5.208(5) A in 2 and 5.187(1) and 5.132(1) A in 3. The magnetic properties of 1-3 have been investigated in the temperature range 2.0-300 K. Complex 1 is a low-spin iron(III) complex with an appreciable orbital contribution. The magnetic properties of 3 correspond to the sum of two magnetically isolated spin triplets, the magnetic coupling between the low-spin iron(III) centers through the -CN-Zn-NC- bridging skeleton (iron-iron separation larger than 10.2 A) being negligible. More interestingly, 2 exhibits one-dimensional ferrimagnetic behavior due to the noncompensation of the local interacting spins (S(Mn) = 5/2 and S(Fe) = 1/2) which interact antiferromagnetically through bridging cyano groups. A comparison between the magnetic properties of the isostructural compounds 2 and 3 allow us to check the antiferromagnetic coupling in 2.     

A novel carboxylate-free ferromagnetic trinuclear mu(3)-oxo-manganese(III) complex with distorted pentagonal-bipyramidal metal centers

In methanol, the reaction of Mn(ClO(4))(2).6H(2)O and 1,2-bis(biacetylmonoximeimino)ethane (H(2)bamen) in the presence of triethylamine affords a trinuclear complex having the formula [Mn(3)(mu(3)-O)(mu(3)-bamen)(3)]ClO(4).2H(2)O. The structure of this complex shows a symmetric planar central [Mn(III)(3)(mu(3)-O)] unit coordinated to three hexadentate bridging (via oximate groups) ligands. The N(4)O(3) coordination sphere around each metal center is very close to pentagonal-bipyramidal. A cyclic voltammogram of the complex displays two reversible and an irreversible response due to Mn(III)(3) --> Mn(III)(2)Mn(IV), Mn(III)(2)Mn(IV) --> Mn(III)Mn(IV)(2), and Mn(III)Mn(IV)(2) --> Mn(IV)(3) oxidation processes, respectively. Cryomagnetic data reveal that the complex is ferromagnetic.     

A comparative XAS and X-ray diffraction study of new binuclear Mn(III) complexes with catalase activity. indirect effect of the counteranion on magnetic properties

Four new binuclear Mn(III) complexes with carboxylate bridges have been synthesized: [[Mn(nn)(H(2)O)](2)(mu-ClCH(2)COO)(2)(mu-O)](ClO(4))(2) with nn = bpy (1) or phen (2) and [[Mn(bpy)(H(2)O)](2)(mu-RCOO)(2)(mu-O)](NO(3))(2) with RCOO = ClCH(2)COO (3) or CH(3)COO (4). The characterization by X-ray diffraction (1 and 3) and X-ray absorption spectroscopy (XAS) (1-4) displays the relevance of this spectroscopy to the elucidation of the structural environment of the manganese ions in this kind of compound. Magnetic susceptibility data show an antiferromagnetic coupling for all the compounds: J = -2.89 cm(-1) (for 1), -8.16 cm(-1) (for 2), -0.68 cm(-1) (for 3), and -2.34 cm(-1) (for 4). Compounds 1 and 3 have the same cation complex [[Mn(bpy)(H(2)O)](2)(mu-ClCH(2)COO)(2)(mu-O)](2+), but, while 1 shows an antiferromagnetic coupling, for 3 the magnetic interaction between Mn(III) ions is very weak. The four compounds show catalase activity, and when the reaction stopped, Mn(II) compounds with different nuclearity could be obtained: binuclear [[Mn(phen)(2)](mu-ClCH(2)COO)(2)](ClO(4))(2), trinuclear [Mn(3)(bpy)(2)(mu-ClCH(2)COO)(6)], or mononuclear complexes without carboxylate. Two Mn(II) compounds without carboxylate have been characterized by X-ray diffraction: [Mn(NO(3))(2)(bpy)(2)][Mn(NO(3))(bpy)(2)(H(2)O)]NO(3) (5) and [Mn(bpy)(3)](ClO(4))(2).0.5 C(6)H(4)-1,2-(COOEt)(2).0.5H(2)O (8).     

Synthesis and Structure of a New Dimanganese Complex [Mn2(μ-OAc)2(phen)4](ClO4)2

A dinuclear manganese(II) complex, [Mn2(μ-Oac)2(phen)4](ClO4)2 1 (phen = phenanthroline), has been prepared by the reaction of Mn(Oac)2·4H2O, phen, tartaric acid and NaClO4·H2O in MeOH/H2O solution. The crystal structure has been determined by X-ray diffraction. The crystal is of monoclinic, space group P21/c with a = 9.6052(4), b = 14.2353(7), c = 18.8893(8)(A),β= 96.584(1)°, C52H38Cl2Mn2N8O12, Mr = 1147.68, V = 2565.8(2) (A)3, Dc = 1.486 g/cm3, F(000) = 1172, μ = 0.668 cm-1 and Z = 2. The final refinement gave R = 0.0605 and wR = 0.1619 for 3203 reflections with I > 2σ(I). The complex contains a dinuclear [Mn2(μ-Oac)2(phen)4]]2+ cation located at a centre of symmetry with two syn-anti acetate bridges between the Mn atoms. Each Mn atom is coordinated by four N atoms from two chelating phen ligands and two O atoms from a pair of bridging acetate groups to furnish a distorted octahedral geometry. Two ClO4 anions exist around the cation [Mn2(μ-Oac)2(phen)4]2+as counter ions.     

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.     

Mechanistic studies on oxidation of nitrite by a {Mn3O4}4+ core in aqueous acidic media

[MnIV3(micro-O)4(phen)4(H2O)2]4+ (, phen=1,10-phenanthroline) equilibrates with its conjugate base [Mn3(micro-O)4(phen)4(H2O)(OH)]3+ in aqueous solution. Among the several synthetic multinuclear oxo- and/or carboxylato bridged manganese complexes known to date containing metal-bound water, to the best of our knowledge, only deprotonates (right harpoon over left harpoon+H+, pKa=4.00 (+/-0.15) at 25.0 degrees C, I=1.0 M, maintained with NaNO3) at physiological pH. An aqueous solution of quantitatively oxidises NIII (HNO2 and NO2-) to NO3- within pH 2.3-4.1, the end manganese state being MnII. Both and are reactive oxidants in the title redox. In contrast to a common observation that anions react quicker than their conjugate acids in reducing metal centred oxidants, HNO2 reacts faster than NO2- in reducing or . The observed rates of nitrite oxidation do not depend on the variation of 1,10-phenanthroline content of the solution indicating that the MnIV-bound phen ligands do not dissociate in solution under experimental conditions. Also, there was no kinetic evidence for any kind of pre-equilibrium replacement of MnIV-bound water by nitrite prior to electron transfer which indicates the substitution-inert nature of the MnIV-bound waters and the 1,10-phenanthroline ligands. The MnIV3 to MnII transition in the present observation proceeds through the intermediate generation of the spectrally characterised mixed-valent MnIIIMnIV dimer that quickly produces MnII. The reaction rates are substantially lowered when solvent H2O is replaced by D2O and a rate determining 1e, 1H+ electroprotic mechanism is proposed.     

Models for the lower S states of photosystem II: a trinuclear mixed-valent MnII/MnIV/MnII complex

The trinuclear complex Mn(3)(pko)(4)(CH(3)O)(2)(SCN)(2).CH(3)OH, 1, where Hpko is 2,2'-dipyridylketonoxime, is a rare example of a complex simultaneously containing Mn(II) and Mn(IV). X-ray crystallography and XANES spectroscopy clearly distinguish the Mn(II)(2)Mn(IV) valence isomer from the more commonly observed Mn(III)(2)Mn(II) formulation. Fits to variable-temperature magnetic susceptibility data indicate that the Mn(II) and Mn(IV) are ferromagnetically coupled (J = +6.13 cm(-1)) and that 1 has an S = (13)/(2) ground state.     

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).     

Theoretical insights into the ferromagnetic coupling in oxalato-bridged chromium(III)-cobalt(II) and chromium(III)-manganese(II) dinuclear complexes with aromatic diimine ligands

Two novel heterobimetallic complexes of formula [Cr(bpy)(ox)(2)Co(Me(2)phen)(H(2)O)(2)][Cr(bpy)(ox)(2)]·4H(2)O (1) and [Cr(phen)(ox)(2)Mn(phen)(H(2)O)(2)][Cr(phen)(ox)(2)]·H(2)O (2) (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, and Me(2)phen = 2,9-dimethyl-1,10-phenanthroline) have been obtained through the "complex-as-ligand/complex-as-metal" strategy by using Ph(4)P[CrL(ox)(2)]·H(2)O (L = bpy and phen) and [ML'(H(2)O)(4)](NO(3))(2) (M = Co and Mn; L' = phen and Me(2)phen) as precursors. The X-ray crystal structures of 1 and 2 consist of bis(oxalato)chromate(III) mononuclear anions, [Cr(III)L(ox)(2)](-), and oxalato-bridged chromium(III)-cobalt(II) and chromium(III)-manganese(II) dinuclear cations, [Cr(III)L(ox)(μ-ox)M(II)L'(H(2)O)(2)](+)[M = Co, L = bpy, and L' = Me(2)phen (1); M = Mn and L = L' = phen (2)]. These oxalato-bridged Cr(III)M(II) dinuclear cationic entities of 1 and 2 result from the coordination of a [Cr(III)L(ox)(2)](-) unit through one of its two oxalato groups toward a [M(II)L'(H(2)O)(2)](2+) moiety with either a trans- (M = Co) or a cis-diaqua (M = Mn) configuration. The two distinct Cr(III) ions in 1 and 2 adopt a similar trigonally compressed octahedral geometry, while the high-spin M(II) ions exhibit an axially (M = Co) or trigonally compressed (M = Mn) octahedral geometry in 1 and 2, respectively. Variable temperature (2.0-300 K) magnetic susceptibility and variable-field (0-5.0 T) magnetization measurements for 1 and 2 reveal the presence of weak intramolecular ferromagnetic interactions between the Cr(III) (S(Cr) = 3/2) ion and the high-spin Co(II) (S(Co) = 3/2) or Mn(II) (S(Mn) = 5/2) ions across the oxalato bridge within the Cr(III)M(II) dinuclear cationic entities (M = Co and Mn) [J = +2.2 (1) and +1.2 cm(-1) (2); H = -JS(Cr)·S(M)]. Density functional electronic structure calculations for 1 and 2 support the occurrence of S = 3 Cr(III)Co(II) and S = 4 Cr(III)Mn(II) ground spin states, respectively. A simple molecular orbital analysis of the electron exchange mechanism suggests a subtle competition between individual ferro- and antiferromagnetic contributions through the σ- and/or π-type pathways of the oxalato bridge, mainly involving the d(yz)(Cr)/d(xy)(M), d(xz)(Cr)/d(xy)(M), d(x(2)-y(2))(Cr)/d(xy)(M), d(yz)(Cr)/d(xz)(M), and d(xz)(Cr)/d(yz)(M) pairs of orthogonal magnetic orbitals and the d(x(2)-y(2))(Cr)/d(x(2)-y(2))(M), d(xz)(Cr)/d(xz)(M), and d(yz)(Cr)/d(yz)(M) pairs of nonorthogonal magnetic orbitals, which would be ultimately responsible for the relative magnitude of the overall ferromagnetic coupling in 1 and 2.     

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.     

Polynuclear and mixed-ligand mononuclear Cu(I) complexes with N-thiophosphorylated thioureas and 1,10-phenanthroline or PPh3

Deprotonation of the N-thiophosphorylated thioureas RC(S)NHP(S)(OiPr)(2) (R = Me(2)N, HL(I); iPrNH, HL(II); 2,6-Me(2)C(6)H(3)NH, HL(III), 2,4,6-Me(3)C(6)H(2)NH, HL(IV), aza-15-crown-5, HL(V)) and reaction with CuI or Cu(NO(3))(2) in aqueous EtOH leads to the polynuclear complexes [Cu(4)(L(I)-S,S')(4)], [Cu(8)(L(II)-S,S')(8)], and [Cu(3)(L(III-V)-S,S')(3)]. The structures of these compounds were investigated by IR, (1)H, (31)P{(1)H} NMR, UV-vis spectroscopy and elemental analyses. The crystal structures of [Cu(4)L(I)(4)], [Cu(8)L(II)(8)], [Cu(3)L(III,IV)(3)] were determined by single-crystal X-ray diffraction. Reaction of the deprotonated ligands (L(I-V))(-) with a mixture of CuI and 1,10-phenanthroline (phen) or PPh(3) leads to the mixed-ligand mononuclear complexes [Cu(phen)L(I-V)], [Cu(PPh(3))L(I-V)] or [Cu(PPh(3))(2)L(I-V)]. The same mixed-ligand complexes were obtained from the reaction of [Cu(4)L(I)(4)], [Cu(8)L(II)(8)], [Cu(3)L(III-V)(3)] with phen or PPh(3).     

Fe(bipy)(CN)(4)](-) as a versatile building block for the design of heterometallic systems: synthesis, crystal structure, and magnetic properties of PPh(4)[Fe(III)(bipy)(CN)(4)] x H(2)O, [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] x 4H(2)O, and [[Fe(III)(bipy)(CN)(4)](2)Zn(II)] x 2H(2)O [bipy = 2,2'-Bipyridine; M = Mn and Zn

The new cyano complexes of formulas PPh(4)[Fe(III)(bipy)(CN)(4)] x H(2)O (1), [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] x 4H(2)O with M = Mn (2) and Zn (3), and [[Fe(III)(bipy)(CN)(4)](2)Zn(II)] x 2H(2)O (4) [bipy = 2,2'-bipyridine and PPh(4) = tetraphenylphosphonium cation] have been synthesized and structurally characterized. The structure of complex 1 is made up of mononuclear [Fe(bipy)(CN)(4)](-) anions, tetraphenyphosphonium cations, and water molecules of crystallization. The iron(III) is hexacoordinated with two nitrogen atoms of a chelating bipy and four carbon atoms of four terminal cyanide groups, building a distorted octahedron around the metal atom. The structure of complexes 2 and 3 consists of neutral centrosymmetric [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] heterotrinuclear units and crystallization water molecules. The [Fe(bipy)(CN)(4)](-) entity of 1 is present in 2 and 3 acting as a monodentate ligand toward M(H(2)O)(4) units [M = Mn(II) (2) and Zn(II) (3)] through one cyanide group, the other three cyanides remaining terminal. Four water molecules and two cyanide nitrogen atoms from two [Fe(bipy)(CN)(4)](-) units in trans positions build a distorted octahedron surrounding Mn(II) (2) and Zn(II) (3). The structure of the [Fe(phen)(CN)(4)](-) complex ligand in 2 and 3 is close to that of the one in 1. The intramolecular Fe-M distances are 5.126(1) and 5.018(1) A in 2 and 3, respectively. 4 exhibits a neutral one-dimensional polymeric structure containing two types of [Fe(bipy)(CN)(4)](-) units acting as bismonodentate (Fe(1)) and trismonodentate (Fe(2)) ligands versus the divalent zinc cations through two cis-cyanide (Fe(1)) and three fac-cyanide (Fe(2)) groups. The environment of the iron atoms in 4 is distorted octahedral as in 1-3, whereas the zinc atom is pentacoordinated with five cyanide nitrogen atoms, describing a very distorted square pyramid. The iron-zinc separations across the single bridging cyanides are 5.013(1) and 5.142(1) A at Fe(1) and 5.028(1), 5.076(1), and 5.176(1) A at Fe(2). The magnetic properties of 1-3 have been investigated in the temperature range 2.0-300 K. 1 is a low-spin iron(III) complex with an important orbital contribution. The magnetic properties of 3 correspond to the sum of two magnetically isolated spin triplets, the antiferromagnetic coupling between the low-spin iron(III) centers through the -CN-Zn-NC- bridging skeleton (iron-iron separation larger than 10 A) being very weak. More interestingly, 2 exhibits a significant intramolecular antiferromagnetic interaction between the central spin sextet and peripheral spin doublets, leading to a low-lying spin quartet.