Dimer-of-dimers model for the oxygen-evolving complex of photosystem II. Synthesis and properties of [MnIV4O5(terpy)4(H2O)2](ClO4)6

A dimer-of-dimers model compound for the oxygen-evolving complex of photosystem II, [[(H(2)O)(terpy)Mn(IV)(micro-O)(2)Mn(IV)(terpy)](2)(micro-O)](ClO(4))(6) (terpy = 2,2':6',2' '-terpyridine), has been prepared and characterized by X-ray crystallography and ESI-MS. Low pH was found to promote the disproportionation of [Mn(III/IV)(2)O(2)(terpy)(2)(OH(2))(2)](3+) to Mn(2+) and a Mn(IV/IV)(2)O(2)(terpy)(2) species; the latter complex slowly dimerizes to form the title complex. Protonation of a micro-oxo bridge is proposed to initiate the disproportionation, based on analogy with the [Mn(III/)(IV)(2)O(2)(bpy)(4)](3+) system.     

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.     

Photoinduced magnetization in copper octacyanomolybdate

This article describes the studies of a photomagnetic cyanide-bridged Cu-Mo bimetallic assembly, Cu(II)(2)[Mo(IV)(CN)(8)].8H(2)O (Cu(II), S = (1)/(2); Mo(IV), S = 0) (1), which has an intervalence transfer (IT) band from Mo(IV)-CN-Cu(II) to Mo(V)-CN-Cu(I) around 480 nm. Wide-angle X-ray scattering and X-ray spectroscopic studies provide precise information about the 3D connectivity and the local environment of the transition metal ions. Irradiating with blue light causes solid 1 to exhibit a spontaneous magnetization (Curie temperature = 25 K). The thermal reversibility is carefully studied and shows the long-time stability of the photoinduced state up to 100 K. Photoreversibility is also observed; i.e., the magnetization is induced by irradiation with light below 520 nm, while the magnetization is reduced by irradiation with light above 520 nm. The UV-vis absorption spectrum after irradiation shows a decrease of the IT band and the appearance of the reverse-IT band in the region of 600-900 nm (lambda(max) = 710 nm). This UV-vis absorption spectrum is recovered to the original spectrum by irradiation with 658-, 785-, and 840-nm light. In this photomagnetic effect, the excitation of the IT band causes an electron transfer from Mo(IV) to Cu(II), producing a ferromagnetic mixed-valence isomer of Cu(I)Cu(II)[Mo(V)(CN)(8)].8H(2)O (Cu(I), S = 0; Cu(II), S = (1)/(2); Mo(V), S = (1)/(2)) (1'). 1' returns to 1 by irradiation of the reverse-IT band, which obeys the scheme for the potential energy surface in mixed-valence class II compounds.     

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.     

Oxidation state dependence of the geometry, electronic structure, and magnetic coupling in mixed oxo- and carboxylato-bridged manganese dimers

Approximate density functional theory has been used to investigate changes in the geometry and electronic structure of the mixed oxo- and carboxylato-bridged dimers [Mn(2)(mu-O)(2)(O(2)CH)(NH(3))(6)](n+)and [Mn(2)(mu-O)(O(2)CH)(2)(NH(3))(6)](n+)in the Mn(IV)Mn(IV), Mn(III)Mn(IV), and Mn(III)Mn(III) oxidation states. The magnetic coupling in the dimer is profoundly affected by changes in both the bridging ligands and Mn oxidation state. In particular, change in the bridging structure has a dramatic effect on the nature of the Jahn-Teller distortion observed for the Mn(III) centers in the III/III and III/IV dimers. The principal magnetic interactions in [Mn(2)(mu-O)(2)(O(2)CH)(NH(3))(6)](n+)() involve the J(xz/xz)and J(yz/yz) pathways but due to the tilt of the Mn(2)O(2) core, they are less efficient than in the planar di-mu-oxo structure and, consequently, the calculated exchange coupling constants are generally smaller. In both the III/III and III/IV dimers, the Mn(III) centers are high-spin, and the Jahn-Teller effect gives rise to axially elongated Mn(III) geometries with the distortion axis along the Mn-O(c) bonds. In the III/IV dimer, the tilt of the Mn(2)O(2) core enhances the crossed exchange J(x)()()2(-)(y)()()2(/)(z)()()2 pathway relative to the planar di-mu-oxo counterpart, leading to significant delocalization of the odd electron. Since this delocalization pathway partially converts the Mn(IV) ion into low-spin Mn(III), the magnetic exchange in the ground state can be considered to arise from two interacting spin ladders, one is the result of coupling between Mn(IV) (S = 3/2) and high-spin Mn(III) (S = 2), the other is the result of coupling between Mn(IV) (S = 3/2) and low-spin Mn(III) (S = 1). In [Mn(2)(mu-O)(O(2)CH)(2)(NH(3))(6)](n+)(), both the III/III dimer and the lowest energy structure for the III/IV dimer involve high-spin Mn(III), but the Jahn-Teller axis is now orientated along the Mn-oxo bond, giving rise to axially compressed Mn(III) geometries with long Mn-O(c) equatorial bonds. In the IV/IV dimer, the ferromagnetic crossed exchange J(yz)()(/)(z)()()2 pathway partially cancels J(yz/yz) and, as a consequence, the antiferromagnetic J(xz/xz) pathway dominates the magnetic coupling. In the III/III dimer, the J(yz/yz) pathway is minimized due to the smaller Mn-O-Mn angle, and since the ferromagnetic J(yz)()(/)(z)()()2 pathway largely negates J(xz/xz), relatively weak overall antiferromagnetic coupling results. In the III/IV dimer, the structures involving high-spin and low-spin Mn(III) are almost degenerate. In the high-spin case, the odd electron is localized on the Mn(III) center, and the resulting antiferromagnetic coupling is similar to that found for the IV/IV dimer. In the alternative low-spin structure, the odd electron is significantly delocalized due to the crossed J(yz)()(/)(z)()()2 pathway, and cancellation between ferromagnetic and antiferromagnetic pathways leads to overall weak magnetic coupling. The delocalization partially converts the Mn(IV) ion into high-spin Mn(III), and consequently, the spin ladders arising from coupling of Mn(IV) (S = 3/2) with high-spin (S = 2) and low-spin (S = 1) Mn(III) are configurationally mixed. Thus, in principle, the ground-state magnetic coupling in the mixed-valence dimer will involve contributions from three spin-ladders, two associated with the delocalized low-spin structure and the third arising from the localized high-spin structure.     

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

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

The formation of a luminescent Mn(III,IV) intermediate of bis(2-pyridylmethyl)amine and acetone assistant its intramolecular C-H oxidation

The dinuclear Mn(II) complexes of bis(2-pyridylmethyl)amine (dpa) reacted with H(2)O(2) producing a fluorescent dioxodimanganese(III,IV) intermediate [(dpa)Mn(2)Cl(2)(μ-O(2))(OHdpa)](3+), which was characterized by IR, UV, ESR, ES-MS and fluorescence spectra. ES-MS data show that this intermediate could bind an acetone molecule forming dioxodimanganese(III,IV)-acetone adduct [(dpa)Mn(2)Cl(2)(μ-O)(CH(3)COCH(3))(OHdpa)](3+). The emission of dioxodimanganese(III,IV)-acetone at 378 nm was stronger than that of dioxodimanganese(III,IV) complex. Excess acetone molecules promoted the intramolecular C-H oxidation and the formation of one dimensional chain Mn(II) complex [(2-picolinic-acid)Mn(H(2)O)(2)Cl(O)](n) through possible intramolecular oxygen transfer reaction.     

Photodynamic effect of iron excess on photosystem II function in pea plants

An earlier mechanistic phase of iron toxicity in photosynthetic cells was interpreted in terms of enhanced photodynamic action by the cytochrome b6/f complex (Cyt b6/f) via singlet oxygen (1O2) on the photosystem II complex (PS II). Iron excess was induced in hydroponically cultured pea (Pisum sativum L.) plants, and its effect on the function of PS II in vivo as well as in vitro was studied under high-irradiance conditions. Iron excess in plants gave rise to a significant increase in Cyt b6/f content of thylakoids. It appeared that the larger the content of Cyt b6/f, the more susceptible PS II was to photoinhibition, and the higher the rate of 1O2 photoproduction in thylakoids was. The action spectrum for degradation of the D1 protein in thylakoids revealed that photosensitization by nonporphyrin chromophore(s) was apparently associated with near UV to blue light-induced deterioration of PS II. The results are pertinent to the concept that photooxidative damage to PS 11, exacerbated by iron accumulation in thylakoid membranes in the form of Cyt b6/f, is involved in the mechanism of iron toxicity in leaf cells.     

Manganese(II/II/II) and Manganese(III/II/III) Trinuclear Compounds. Structure and Solid and Solution Behavior

Two mixed-valence Mn(III)Mn(II) complexes and a homo-valence Mn(II) trinuclear manganese complex of stoichiometry Mn(III)Mn(II)Mn(III)(5-Cl-Hsaladhp)(2)(AcO)(4)(MeOH)(2).4CH(3)OH (1a), Mn(III)Mn(II)Mn(III) (Hsaladhp)(2)(AcO)(2)(5-Cl-Sal)(2)(thf)(2) (3a) and Mn(II)Mn(II)Mn(II) (AcO)(6)(pybim)(2) (1b) where H(3)saladhp is a tridentate Schiff base ligand and pybim a neutral bidentate donor ligand, have been structurally characterized by using X-ray crystallography. The structurally characterized mixed-valence complexes have strictly 180 degrees Mn(III)-Mn(II)-Mn(III) angles as required by crystallographic inversion symmetry. The complexes are valence trapped with two terminal Mn(III) ions showing Jahn-Teller distortion along the acetate or salicylate-Mn(III)-X axis. The Mn.Mn separation is 3.511 ? and 3.507 ? respectively. The mixed-valence complexes have S = (3)/(2) ground state and the homovalence complex S = (5)/(2), with small antiferromagnetic exchange J couplings, -5.6 and -1.8 cm(-1), respectively, while the powder ESR spectra at 4 K show a broad low field signal with g approximately 4.3 for Mn(III)Mn(II)Mn(III) and a broad temperature-dependent signal at g = 2 for Mn(II)Mn(II)Mn(II). Crystal data for 1a: [C(36)H(60)O(20)N(2)Cl(2)Mn(3)], triclinic, space group P&onemacr;, a = 9.272(7) ?, b = 11.046(8) ?, c = 12.635(9) ?, alpha = 76.78(2) degrees, beta = 81.84(2) degrees, gamma = 85.90(2) degrees, Z = 1. Crystal data for 3a: [C(48)H(56)O(18)N(2)Cl(2)Mn(3)], monoclinic, space group P2(1)/n, a = 8.776(3) ?, b = 22.182(7) ?, c = 13.575(4) ?, beta = 94.44(1) degrees, Z = 2. Crystal data for 1b: [C(36)H(36)O(12)N(6)Mn(3)], triclinic, space group P&onemacr;, a = 13.345(6) ?, b = 8.514(4) ?, c = 9.494(4) ?, alpha = 75.48(1) degrees, beta = 75.83(1) degrees, gamma = 76.42(1) degrees, Z = 1.     

Slow magnetic relaxation in a mixed-valence Mn(II/III) Complex: [MnII2(bispicen)2(mu 3-Cl)2Mn(III)(Cl4Cat)2Mn(III)(Cl4Cat)2(H2O)2] infinity

A layered mixed-valence manganese complex, [Mn(II)(2)(bispicen)(2)(mu(3)-Cl)(2)Mn(III)(Cl(4)Cat)(2)Mn(III)(Cl(4)Cat)(2)(H(2)O)(2)](infinity), is synthesized and characterized structurally. It displays a slow magnetic relaxation and hysteresis effect.     

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.     

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.     

FTIR spectra and normal-mode analysis of a tetranuclear manganese adamantane-like complex in two electrochemically prepared oxidation states: relevance to the oxygen-evolving complex of photosystem II

The IR spectra and normal-mode analysis of the adamantane-like compound [Mn(4)O(6)(bpea)(4)](n+) (bpea = N,N-bis(2-pyridylmethyl)ethylamine) in two oxidation states, Mn(IV)(4) and Mn(III)Mn(IV)(3), that are relevant to the oxygen-evolving complex of photosystem II are presented. Mn-O vibrational modes are identified with isotopic exchange, (16)O-->(18)O, of the mono-micro-oxo bridging atoms in the complex. IR spectra of the Mn(III)Mn(IV)(3) species are obtained by electrochemical reduction of the Mn(IV)(4) species using a spectroelectrochemical cell, based on attenuated total reflection [Visser, H.; et al. Anal. Chem. 2001, 73, 4374-4378]. A novel method of subtraction is used to reduce background contributions from solvent and ligand modes, and the difference and double-difference spectra are used in identifying Mn-O bridging modes that are sensitive to oxidation state change. Two strong IR bands are observed for the Mn(IV)(4) species at 745 and 707 cm(-1), and a weaker band is observed at 510 cm(-1). Upon reduction, the Mn(III)Mn(IV)(3) species exhibits two strong IR bands at 745 and 680 cm(-1), and several weaker bands are observed in the 510-425 cm(-1) range. A normal-mode analysis is performed to assign all the relevant bridging modes in the oxidized Mn(IV)(4) and reduced Mn(III)Mn(IV)(3) species. The calculated force constants for the Mn(IV)(4) species are f(r)(IV)= 3.15 mdyn/A, f(rOr) = 0.55 mdyn/A, and f(rMnr) = 0.20 mdyn/A. The force constants for the Mn(III)Mn(IV)(3) species are f(r)(IV)= 3.10 mdyn/A, f(r)(III)= 2.45 mdyn/A, f(rOr) = 0.40 mdyn/A, and f(rMnr) = 0.15 mdyn/A. This study provides insights for the identification of Mn-O modes in the IR spectra of the photosynthetic oxygen-evolving complex during its catalytic cycle.     

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.     

Two new terpyridine dimanganese complexes: a manganese(III,III) complex with a single unsupported oxo bridge and a manganese(III,IV) complex with a dioxo bridge. Synthesis, structure, and redox properties

Two new terpyridine dimanganese oxo complexes [Mn(2)(III,IV)(mu-O)(2)(terpy)(2)(CF(3)CO(2))(2)](+) (3) and [Mn(2)(III,III)(mu-O)(terpy)(2)(CF(3)CO(2))(4)] (4) (terpy = 2,2':6,2' '-terpyridine) have been synthesized and their X-ray structures determined. In contrast to the corresponding mixed-valent aqua complex [Mn(2)(III,IV)(mu-O)(2)(terpy)(2)(H(2)O)(2)](3+) (1), the two Mn atoms in 3 are not crystallographically equivalent. The neutral binuclear monooxo manganese(III,III) complex 4 exhibits two crystallographic forms having cis and trans configurations. In the cis complex, the two CF(3)CO(2)(-) ligands on each manganese adopt a cis geometry to each other; one CF(3)CO(2)(-) is trans to the oxygen of the oxo bridge while the second is cis. In the trans complex, the two coordinated CF(3)CO(2)(-) have a trans geometry to each other and are cis to the oxo bridge. The electrochemical behavior of 3 in organic medium (CH(3)CN) shows that this complex could be oxidized into its corresponding stable manganese(IV,IV) species while its reduced form manganese(III,III) is very unstable and leads by a disproportionation process to Mn(II) and Mn(IV) complexes. Complex 4 is only stable in the solid state, and it disproportionates spontaneously in CH(3)CN solution into the mixed-valent complex 3 and the mononuclear complex [Mn(II)(terpy)(2)](2+) (2), thereby preventing the observation of its electrochemical behavior.     

Insights into the mechanism of O₂ formation and release from the Mn₄O₄L₆ "cubane" cluster

To probe photoinduced water oxidation catalyzed by the Mn?O?L? cubane clusters, we have computationally studied the mechanism and controlling factors of the O? formation from the [Mn?O?L?] catalyst, 6. It was demonstrated that dissociation of an L = H?PO?? ligand from 6 facilitates the direct O-O bond formation that proceeds with a 28.3 (33.4) kcal/mol rate-determining energy barrier at the transition state TS1. This step (the O-O single bond formation) of the reaction is a two-electron oxidation/reduction process, during which two oxo ligands are transformed into to μ2:η2-O?2? unit, and two ("distal") Mn centers are reduced from the 4+ to the 3+ oxidation state. Next two-electron oxidation/reduction occurs by "dancing" of the resulted O?2? fragment between the Mn1 and Mn2/Mn(2')-centers, keeping its strong coordination to the Mn(1')-center. As a result of this four-electron oxidation/reduction process Mn centers of the Mn?-core of I transform from {Mn1(III)-Mn(1')(III)-Mn2(IV)-Mn(2')(IV)} to {Mn1(II)-Mn(1')(II)-Mn2(III)-Mn(2')(III)} in IV. In other words, upon O? formation in cationic complex [Mn?O?L?](+), I, all four Mn-centers are reduced by one electron each. The overall reaction I → TS1 → II → III → TS2 → IV → TS3 → V → VI + O? is found to be exothermic by 15.4 (10.5) kcal/mol. We analyze the lowest spin states and geometries of all reactants, intermediates, transition states, and products of the targeted reaction.     

Rare tetranuclear mixed-valent [Mn(II)2Mn(IV)2] clusters as building blocks for extended networks

We report the synthesis of a series of mixed valence Mn(II/IV) tetranuclear clusters [Mn(II)(2)Mn(IV)(2)O(2)(heed)(2)(EtOH)(6)Br(2)]Br(2) (), [Mn(II)(2)Mn(IV)(2)O(2)(heed)(2)(H(2)O)(2)Cl(4)].2EtOH.H(2)O (.2EtOH.H(2)O), [Mn(II)(2)Mn(IV)(2)O(2)(heed)(2)(heedH(2))(2)](ClO(4))(4) (), [Mn(II)(2)Mn(IV)(2)O(2)(heed)(2)(MeCN)(2)(H(2)O)(2)(bpy)(2)](ClO(4))(4) () and [Mn(II)(2)Mn(IV)(2)O(2)(heed)(2)(bpy)(2)Br(4)].2MeOH (.2MeOH). Clusters are constructed from the tripodal ligand N,N-bis(2-hydroxyethyl)ethylene diamine (heedH(2)) and represent rare examples of tetranuclear Mn clusters possessing the linear trans zig-zag topology, being the first Mn(II/IV) mixed-valent clusters of this type. The molecular clusters can then be used as building blocks in tandem with the (linear) linker dicyanamide ([N(CN)(2)](-), dca(-)) for the formation of a novel extended network {[Mn(II)(2)Mn(IV)(2)O(2)(heed)(2)(H(2)O)(2)(MeOH)(2)(dca)(2)]Br(2)}(n) (), which exhibits a rare form of the 2D herring bone topology.     

General synthesis of di-mu-oxo dimanganese complexes as functional models for the oxygen evolving complex of photosystem II

A series of complexes with the formula [Mn(III/IV)2(mu-O)2(L)2(X)2]3+ have been prepared in situ from Mn(II)LCl2 precursors by a general preparative method (L = terpy, Cl-terpy, Br-terpy, Ph-terpy, tolyl-terpy, mesityl-terpy, t Bu3-terpy, EtO-terpy, py-phen, dpya, Me2N-terpy, or HO-terpy, and X = a labile ligand such as water, chloride, or sulfate). The parent complex, where L = terpy and X = water, is a functional model for the oxygen-evolving complex of photosystem II (Limburg, et al. J. Am. Chem. Soc. 2001, 123, 423-430). Crystals of Mn(II)(dpya)Cl2, Mn(II)(Ph-terpy)Cl2, Mn(II)(mesityl-terpy)Cl2, and an organic-soluble di-mu-oxo di-aqua dimanganese complex, [Mn(III/)(IV)2(mu-O)2(mesityl-terpy)2(OH2)2](NO3)3, were obtained and characterized by X-ray crystallography. Solutions of the in situ-formed di-mu-oxo dimanganese complexes were characterized by electrospray mass spectrometry, EPR spectroscopy, and UV-visible spectroscopy, and the rates of catalytic oxygen-evolving activity were assayed. The use of Mn(II)LCl2 precursors leads to higher product purity of the Mn dimers while achieving the 1:1 ligand to Mn stoichiometry appropriate for catalytic activity assay. These methods can be used to screen the catalytic activity of other di-mu-oxo dimanganese complexes generated by using a ligand library.     

Manganese-depleted/reconstituted photosystem II core complexes in solution and liposomes

Chlorophyll fluorescence transients measurements were employed to study the functioning of spinach photosystem II (PS II) core complexes in solution or reconstituted into liposomes. Lipid vesicles were prepared from soybean phospholipids (asolectine) or a mixture of spinach thylakoid lipids. In comparison with intact PS II core complexes comprising two distinct fluorescence phases, designated as O-J and J-P, complete suppression of the latter phase in Mn-depleted samples was observed. An increase of magnitude of the J-P phase in the presence of exogenous MnCl(2) (4 Mn/RC) indicate in favor of partial restoring of oxygen-evolution activity of PS II. The J-P phase observed in PS II in solution was characterized by a lifetime of ~320 ms, while in liposome-reconstituted samples this phase was accelerated up to ~20 ms in case of asolectine and up to ~9 ms in case of a mixture of thylakoid lipids. These data clearly suggest that lipid environment stimulates the steady-state rate of oxygen evolution. The effect of lipids is likely based on keeping the embedded proteins in optimal structure for efficient functioning.