Tetranuclear Manganese Research Articles

A New Quantum Chemical Approach to the Magnetic Properties of Oligonuclear Transition‐Metal Complexes: Application to a Model for the Tetranuclear Manganese Cluster of Photosystem II

The reliable correlation of structural features and magnetic or spectroscopic properties of oligonuclear transition-metal complexes is a critical requirement both for research into innovative magnetic materials and for elucidating the structure and function of many metalloenzymes. We have developed a novel method that for the first time enables the extraction of hyperfine coupling constants (HFCs) from broken-symmetry density functional theory (BS-DFT) calculations on clusters. Using the geometry-optimized tetranuclear manganese complex [Mn(4)O(6)(bpy)(6)](4+/3+) as a model, we first examine in detail the calculation of exchange coupling constants J through the BS-DFT approach. Complications arising from the indeterminacy of experimentally fitted J constants are identified and analyzed. It is found that only the energy levels derived from Hamiltonian diagonalization are a physically meaningful basis for comparing theory and experiment. Subsequently, the proposed theoretical scheme is applied to the calculation of (55)Mn HFCs of the Mn(III,IV,IV,IV) state of the complex, which is similar to the S(2) state of the oxygen-evolving complex (OEC) in photosystem II of oxygenic photosynthesis. The new approach performs reliably and accurately, and yields calculated HFCs that can be directly compared with experimental data. Finally, we carefully examine the dependence of HFC on the J value and draw attention to the sensitivity of the calculated values to the exchange coupling parameters. The proposed strategy extends naturally to hetero-oligonuclear clusters of arbitrary shape and nuclearity, and hence is of general validity and usefulness in the study of magnetic metal clusters. The successful application of the new approach presented here is a first step in the effort to establish correlations between the available spectroscopic information and the structural features of complex metalloenzymes like OEC.

A Tetranuclear Mixed-Valence Manganese Complex of a Diimine Ligand Derived from 1,4-Diformyl-2,3-dihydroxybenzene: Synthesis, Structure, and Magnetic Properties

An acyclic hexadentate diimine ligand, H4L1, was prepared in situ in methanol by the condensation of 1,4-diformyl-2,3-dihydroxybenzene (1) with 2-aminoethanol, and complexed directly with two equivalents of MnII(OAc)2·4H2O or MnIII(OAc)3·2H2O, or with one equivalent of each. All three of these one-pot reactions gave the mixed-valent tetrametallic complex [MnII2MnIII2(L1)2(OAc)2(solvents)4], [2(solvents)4]. An X-ray crystal structure determination on [2(MeOH)4]·2MeOH revealed a centre of inversion in the middle of the complex. The four metal ions are in an almost planar array, sandwiched by two offset ligands which provide all of the equatorial donors, with the axial sites occupied by acetate ions and methanol molecules. The two manganese(II) ions are seven coordinate and centrally located, whereas the two manganese(III) ions are Jahn–Teller elongated octahedra and are located in the outer sites. Magnetic analysis of an air-dried sample, [2(MeOH)2(H2O)2]·3H2O, showed that weak antiferromagnetic interactions between the manganese ions dominate, resulting in a low ground spin state.

Two tetranuclear Mn-complexes as biomimetic models of the oxygen evolving complex in Photosystem II. A synthesis, characterisation and reactivity study

In this work we report the preparation of two metallamacrocyclic tetranuclear manganese(II) complexes, [L1(4)Mn4](ClO4)4 and [L2(4)Mn4](ClO4)4 where L1 and L2 are the anions of the heptadentate ligands 2-((2-(bis(pyridin-2-ylmethyl)amino)ethyl)(methyl)amino)acetic acid and 2-(benzyl(2-(bis(pyridin-2-ylmethyl)amino)ethyl)amino)acetic acid), respectively. The complexes have been fully characterized by ESI-MS, elemental analysis, single-crystal X-ray diffraction, magnetic susceptibility, and EPR spectroscopy. Electrochemical reactions as well as reactions with different chemical redox reagents have been performed and a reversible two electron oxidation per manganese ion has been identified. The reaction of [L1(4)Mn4](ClO4)4 with oxone or cerium(IV) results in the evolution of oxygen which makes this system interesting for future studies in the search for a functional mimic of the oxygen evolving complex in Photosystem II.

Controlling the Structure of Manganese(II) Phosphates by the Choice and Ratio of Organophosphate and Auxiliary Ligands

Tetranuclear manganese(II) phosphates [Mn(dipp)(bpy)](4)4 H(2)O (1) and [Mn(4)(dmpp)(2)(dmppH)(4)(bpy)(4)(H(2)O)(2)]H(2)O (2) have been prepared from Mn(OAc)(2)4 H(2)O and 2,6-diisopropylphenyl phosphate (dippH(2)) or 2,6-dimethylphenyl phosphate (dmppH(2)) in the presence of 2,2'-bipyridine (bpy). In contrast, the reaction between [Mn(bpy)(2)(OAc)(ClO(4))]H(2)O and dippH(2) affords [Mn(bpy)(2)(dippH)](2)2 ClO(4)2 CH(3)OH (3). The reactions of Mn(OAc)(2)4 H(2)O, dippH(2), and pyridine (py) or 3,5-dimethylpyrazole (dmpz) in CH(3)CN under reflux afford hexanuclear complexes [Mn(6)(dipp)(6)(py)(8)]2CH(3)CN (4) and [Mn(6)(dipp)(6)(dmpz)(6)(AcOH)(2)]2 H(2)O (5), respectively. Although compounds 1 and 2 are tetrameric, the former is a closed cubane-like structure resembling the D4R secondary building unit of zeolites, whereas the latter exists in a staircase structure with fused Mn(2)O(4)P(2) rings. The core structure of 3 contains a Mn(2)O(4)P(2) eight-membered ring that resembles the S4R building block of zeolites. Single-crystal X-ray diffraction studies reveal that compounds 4 and 5 have a similar core structure and differ from each other by the neutral ligands coordinated to manganese ions. All six phosphate ligands exist in a doubly deprotonated [(RO)PO(3) (2-)] form and exhibit two types of binding modes [5.222] and [3.111]. An interesting feature of compounds 1-5 is that although they are oligonuclear complexes, there is an absence of oxido bridges. The magnetic properties of compounds 1-5 have been investigated in the temperature range 5-298 K, and it was found that all the compounds obey the Curie law.

A one-dimensional chain structure based on unusual tetranuclear manganese(II) clusters

The title coordination polymer, poly[bis(mu(4)-biphenyl-2,2'-dicarboxylato)(dipyrido[3,2-a:2',3'-c]phenazine)manganese(II)], [Mn(2)(C(14)H(8)O(4))(2)(C(18)H(10)N(4))](n), was obtained through the reaction of MnCl(2).4H(2)O, biphenyl-2,2'-dicarboxylic acid (H(2)dpdc) and dipyrido[3,2-a:2',3'-c]phenazine (L) under hydrothermal conditions. The asymmetric unit contains two crystallographically unique Mn(II) ions, one unique L ligand and two unique dpdc ligands. One Mn ion is six-coordinated by four O atoms from three different dpdc ligands and two N atoms from one L ligand, adopting a distorted octahedral coordination geometry. The distortions from ideal octahedral geometry are largely due to the presence of chelating ligands and the resulting acute N-Mn-N and O-Mn-O angles. The second Mn ion is coordinated in a distorted trigonal bipyramidal fashion by five O atoms from four distinct dpdc ligands. Four Mn(II) ions are bridged by the carboxylate groups of the dpdc ligands to form an unusual tetranuclear Mn(II) cluster. Clusters are further connected by the aromatic backbone of the dicarboxylate ligands, forming a one-dimensional chain structure along the b axis. The title compound is the first example of a chain structure based on a tetranuclear Mn(II) cluster.

Multifrequency Pulsed Electron Paramagnetic Resonance Study of the S2 State of the Photosystem II Manganese Cluster

Multifrequency electron spin-echo envelope modulation (ESEEM) spectroscopy is employed to measure the strength of the hyperfine coupling of magnetic nuclei to the paramagnetic (S = 1/2) S2 form of photosystem II (PSII). Previous X-band-frequency ESEEM studies indicated that one or more histidine nitrogens are electronically coupled to the tetranuclear manganese cluster in the S2 state of PSII. However, the spectral resolution was relatively poor at the approximately 9 GHz excitation frequency, precluding any in-depth analysis of the corresponding bonding interaction between the detected histidine and the manganese cluster. Here we report ESEEM experiments using higher X-, P-, and Ka-band microwave frequencies to target PSII membranes isolated from spinach. The X- to P-band ESEEM spectra suffer from the same poor resolution as that observed in previous experiments, while the Ka-band spectra show remarkably well-resolved features that allow for the direct determination of the nuclear quadrupolar couplings for a single I = 1(14)N nucleus. The Ka-band results demonstrate that at an applied field of 1.1 T we are much closer to the exact cancellation limit (alpha iso = 2nu(14)N) that optimizes ESEEM spectra. These results reveal hyperfine (alpha iso = 7.3 +/- 0.20 MHz and alpha dip = 0.50 +/- 0.10 MHz) and nuclear quadrupolar (e(2)qQ = 1.98 +/- 0.05 MHz and eta = 0.84 +/- 0.06) couplings for a single (14)N nucleus magnetically coupled to the manganese cluster in the S 2 state of PSII. These values are compared to the histidine imidazole nitrogen hyperfine and nuclear quadrupolar couplings found in superoxidized manganese catalase as well as (14)N couplings in relevant manganese model complexes.

Synthesis, crystal structure and magnetic properties of single-molecule magnet: [Mn4(CF3COO)4(hmp)6

The preparation, X-ray structure and magnetic properties are presented for a new mixed-valence tetranuclear manganese complex that functions as a single-molecule magnet (SMM): [Mn4(CF3COO)4(hmp)6], where hmp− is the anion of 2-(hydroxymethyl) pyridine and is a N,O bidentate chelate. The compound crystallizes in a monoclinic system, space group P 21/c (No. 14) with unit cell parameters a = 13.663(3) A, b = 14.705(3) A, c = 14.734(3) A, β = 98.51(3)°, V = 2927.6 A3 and Z = 2. The structure of the complex shows a novel coordination of the trifluoroacetate (TFA) anions, with one anion acting as a monodentate ligand while the second one coordinating through both oxygens to the same Mn center. Direct current magnetic susceptibility measurement in the 2–300 K temperature range supports a high-spin ground state. The presence of a frequency-dependent alternating current susceptibility signal indicates that the individual molecule is acting as magnet.

Novel Manganese(II) Sulfonate−Phosphonates with Dinuclear, Tetranuclear, and Hexanuclear Clusters

Hydrothermal reactions of manganese(II) salts with m-sulfophenylphosphonic acid (m-HO3S-Ph-PO3H2, H3L) and 1,10-phenanthroline (phen) led to six novel manganese(II) sulfonate-phosphonates, namely, [Mn2(HL)2(phen)4][Mn2(HL)2(phen)4(H2O)](2).6H2O (1), [Mn4(L)2(phen)8(H2O)2][ClO4](2).3H2O (2), [Mn(phen)(H2O)4]2[Mn4(L)4(phen)4].10H2O (3), [Mn6(L)4(phen)8(H2O)2].4H2O (4), [Mn6(L)4(phen)8(H2O)2].24H2O (5), and [Mn6(L)4(phen)6(H2O)4].5H2O (6). The structure of 1 contains two types of dinuclear manganese(II) clusters, and 2-3 exhibit two types of tetranuclear manganese(II) cluster units. 4-5 feature two different types of isolated hexanuclear manganese(II) clusters, whereas the hexanuclear manganese(II) clusters in 6 are bridged by sulfonate-phosphonate ligands into a 1D chain. Magnetic property measurements indicate that there exist weak antiferromagnetic interactions between magnetic centers in all six compounds.

Probing the Functional Role of Ca2+ in the Oxygen-Evolving Complex of Photosystem II by Metal Ion Inhibition

Photosynthetic oxygen evolution in photosystem II (PSII) takes place in the oxygen-evolving complex (OEC) that is comprised of a tetranuclear manganese cluster (Mn4), a redox-active tyrosine residue (YZ), and Ca2+ and Cl- cofactors. The OEC is successively oxidized by the absorption of 4 quanta of light that results in the oxidation of water and the release of O2. Ca2+ is an essential cofactor in the water-oxidation reaction, as its depletion causes the loss of the oxygen-evolution activity in PSII. In recent X-ray crystal structures, Ca2+ has been revealed to be associated with the Mn4 cluster of PSII. Although several mechanisms have been proposed for the water-oxidation reaction of PSII, the role of Ca2+ in oxygen evolution remains unclear. In this study, we probe the role of Ca2+ in oxygen evolution by monitoring the S1 to S2 state transition in PSII membranes and PSII core complexes upon inhibition of oxygen evolution by Dy3+, Cu2+, and Cd2+ ions. By using a cation-exchange procedure in which Ca2+ is not removed prior to addition of the studied cations, we achieve a high degree of reversible inhibition of PSII membranes and PSII core complexes by Dy3+, Cu2+, and Cd2+ ions. EPR spectroscopy is used to quantitate the number of bound Dy3+ and Cu2+ ions per PSII center and to determine the proximity of Dy3+ to other paramagnetic centers in PSII. We observe, for the first time, the S2 state multiline electron paramagnetic resonance (EPR) signal in Dy3+- and Cd2+-inhibited PSII and conclude that the Ca2+ cofactor is not specifically required for the S1 to S2 state transition of PSII. This observation provides direct support for the proposal that Ca2+ plays a structural role in the early S-state transitions, which can be fulfilled by other cations of similar ionic radius, and that the functional role of Ca2+ to activate water in the O-O bond-forming reaction that occurs in the final step of the S state cycle can only be fulfilled by Ca2+ and Sr2+, which have similar Lewis acidities.

Synthesis and characterization of a family of tetranuclear manganese(iii) phosphonate complexes

Four tetranuclear manganese(III) complexes containing phosphonate ligands (RPO32−, R = Me, Et and n-Bu), [Mn4O2(MePO3)2(O2CMe)4(bpy)2]·4MeCOOH (1), [Mn4O2(EtPO3)2(O2CMe)4(bpy)2]·7H2O (2), [Mn4O2(n-BuPO3)2(O2CMe)4(bpy)2]·4H2O·(COOH)2 (3) and [Mn4O2(MePO3)2(O2CPh)4(phen)2]·4PhCOOH·2MeCN (4), have been synthesized and characterized by IR spectroscopy, ESI-MS spectrometry, electrochemical analysis, magnetic measurements and TGA. The X-ray analysis shows that the four complexes all have one [Mn4(μ3-O)2]8+ core with four coplanar Mn atoms disposed in an extended “butterfly-like” arrangement and two O atoms triply bridging each “wing”, and the peripheral ligation is provided by four μ2-MeCO2− or four μ2-PhCO2−, two terminal bpy or phen (bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline) groups at both ends of the molecule, and two bicapping μ3-phosphonate ligands, which each binds to three manganese centers above and below the plane. The cyclic voltammetry of 1 and 3 each displays four reduction and one oxidation processes, of which the four reduction processes are possibly attributed to the reduction of four MnIII of the compounds to MnII. The ESI-MS data indicate that the family of complexes are stable in solution. Magnetic susceptibility measurements reveal that compounds 1 and 3 display antiferromagnetic interactions between the adjacent MnIII ions mediated through the μ3-O, O–C–O and O–P–O bridges.

Tetranuclear manganese(III) clusters containing both carboxylate and phosphonate bridging ligands

This paper reports two tetranuclear manganese clusters, namely, [Mn 4O 2(O 2CCH 3) 4(O 3PC 6H 11) 2(phen) 2] ( 1) and [Mn 4O 2(O 2CPh) 4 (O 3PC 6H 11) 2(bpy) 2] ( 2). Both contain a butterfly-like [Mn 4(μ 3-O) 2] 8+ core. The neighboring Mn atoms within the core are bridged by the carboxylate groups, forming approximately a plane. The phosphonate ligands locate above and below the plane, and cap on top of the Mn 3O triangles by using its three phosphonate oxygen atoms. The magnetic measurements of complexes 1 and 2 reveal that dominant antiferromagnetic interactions are propagated between the magnetic centers.

Time-resolved vibrational spectroscopy detects protein-based intermediates in the photosynthetic oxygen-evolving cycle.

Photosynthetic oxygen production by photosystem II (PSII) is responsible for the maintenance of aerobic life on earth. The production of oxygen occurs at the PSII oxygen-evolving complex (OEC), which contains a tetranuclear manganese (Mn) cluster. Photo-induced electron transfer events in the reaction center lead to the accumulation of oxidizing equivalents on the OEC. Four sequential photooxidation reactions are required for oxygen production. The oxidizing complex cycles among five oxidation states, called the S(n) states, where n refers to the number of oxidizing equivalents stored. Oxygen release occurs during the S(3)-to-S(0) transition from an unstable intermediate, known as the S(4) state. In this report, we present data providing evidence for the production of an intermediate during each S state transition. These protein-derived intermediates are produced on the microsecond to millisecond time scale and are detected by time-resolved vibrational spectroscopy on the microsecond time scale. Our results suggest that a protein-derived conformational change or proton transfer reaction precedes Mn redox reactions during the S(2)-to-S(3) and S(3)-to-S(0) transitions.

Syntheses, Structures, and Characterization of Two Manganese(II)‐Aminobenzoic Complexes

AbstractA discrete tetranuclear manganese(II) complex, {(H3O)2Mn4(4‐Haba)2(4‐aba)6(SCN)4(H2O)2]} (1) and a two‐dimensional coordination polymer [Mn(3‐aba)2]n (2) (4‐Haba = 4‐aminobenzoic acid, 3‐Haba = 3‐aminobenzoic acid), have been synthesized and characterized by elemental analyses, IR, X‐ray crystallography, and magnetic susceptibility. In complex 1, 4‐aba in three types of coordination modes connects MnII centers to form a discrete tetranuclear anionic molecule. Hydrogen bonding interaction links the tetranuclear units to generate a one‐dimensional supramolecular chain. In complex 2, 3‐aba in an exo‐tridentate mode bridges six‐coordinate MnII centers forming a two‐dimensional coordination architecture. Studies of the magnetic susceptibilities of 1 and 2 reveal weak antiferromagnetic exchange interactions between adjacent MnII centers. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

A carboxylic residue at the high-affinity, Mn-binding site participates in the binding of iron cations that block the site

The role of carboxylic residues at the high-affinity, Mn-binding site in the ligation of iron cations blocking the site [Biochemistry 41 (2000) 5854] was studied, using a method developed to extract the iron cations blocking the site. We found that specifically bound Fe(III) cations can be extracted with citrate buffer at pH 3.0. Furthermore, citrate can also prevent the photooxidation of Fe(II) cations by Y Z . Participation of a COOH group(s) in the ligation of Fe(III) at the high-affinity site was investigated using 1-ethyl-3-[(3-dimethylamino)propyl] carbodiimide (EDC), a chemical modifier of carboxylic amino acid residues. Modification of the COOH groups inhibits the light-induced oxidation of exogenous Mn(II) cations by Mn-depleted photosystem II (PSII[−Mn]) membranes. The rate of Mn(II) oxidation saturates at ≥10 μM in PSII(−Mn) membranes and ≥500 μM in EDC-treated PSII (−Mn) samples. Intact PSII(−Mn) membranes have only one site for Mn(II) oxidation via Y Z (dissociation constant, K d = 0.64 μM), while EDC-treated PSII(−Mn) samples have two sites ( K d = 1.52 and 22 μM; the latter is the low-affinity site). When PSII(−Mn) membranes were incubated with Fe(II) before modifier treatment (to block the high-affinity site) and the blocking iron cations were extracted with citrate (pH 3.0) after modification, the membranes contained only one site ( K d = 2.3 μM) for exogenous Mn(II) oxidation by Y Z radical. In this case, the rate of electron donation via Y Z saturated at a Mn(II) concentration ≥15 μM. These results indicate that the carboxylic residue participating in Mn(II) coordination and the binding of oxidized manganese cations at the HA Z site is protected from the action of the modifier by the iron cations blocking the HA Z site. We concluded that the carboxylic residue (D1 Asp-170) participating in the coordination of the manganese cation at the HA Z site (Mn4 in the tetranuclear manganese cluster [Science 303 (2004) 1831]) is also involved in the ligation of the Fe cation(s) blocking the high-affinity Mn-binding site.

1,1,1-Tris(hydroxymethyl)propane in manganese carboxylate chemistry: synthesis, structure and magnetic properties of a mixed-valence [MnIII4MnII4] cluster featuring the novel [MnIII4MnII4(μ3-OR)6(μ2-OR)8]6+core

The reaction between MnBr(2).4H(2)O with H(3)tmp (1,1,1-tris(hydroxymethyl)propane) in MeCN in the presence of Na(O(2)CCMe(3)) and NBu(4)Br produces the complex [Mn(8)(O(2)CCMe(3))(2)(tmp)(2)(Htmp)(4)Br(4)(H(2)O)(2)].2MeCN (1.2MeCN) in good yield. The centrosymmetric octanuclear molecule consists of four Mn(III) and four Mn(II) ions assembled together by fourteen alkoxo bridges to give a [Mn(III)(4)Mn(II)(4)(mu(3)-OR)(6)(mu(2)-OR)(8)](6+) rod-like core in which the metal centres are arranged in a planar zigzag fashion. Peripheral ligation is provided by a combination of bridging pivalate ions, terminal bromides and water molecules. Dc magnetic susceptibility measurements reveal the presence of dominant antiferromagnetic interactions leading to a spin ground state of S = 0. A rationalization of this result is attempted by structural comparison with previously reported tetranuclear manganese complexes containing the [Mn(III)(2)Mn(II)(2)(mu(3)-OR)(2)(mu(2)-OR)(4)] core in which the magnetic interactions are ferromagnetic.

Ligand-induced distortion of a tetranuclear manganese butterfly complex

The reaction of the pentadentate Schiff-base ligand 1,3-bis(salicylideneamino)-2-propanol (salproH3) with [Mn3O(O2CR)6(py)3] (R = Me, Et, But) gives the corresponding tetranuclear manganese product [Mn4O2O2CR)5(salpro)] (4Mn(III)). The syntheses, structure and magnetochemical characterization of these complexes are reported. The structure of the [Mn4(mu3-O)2]8+ is butterfly-like much more closed than in previous complexes with this core as a result of the alkoxide oxygen of the salpro ligand bridging the two wingtip Mn atoms. Variable-temperature, solid-state magnetic susceptibility studies reveal that these complexes possess S = 0 ground state spins. Fitting of the magnetic susceptibility data to the theoretical chiMT vs. T expression derived for a C2v symmetry complex, assuming an isotropic Heisenberg spin-Hamiltonian and using the Van Vleck equation, revealed that the various exchange parameters are all antiferromagnetic, and the core thus experiences spin frustration effects.

Tetranuclear Manganese(II) Complexes of Thiacalixarene Macrocycles with Trigonal Prismatic Six-Coordinate Geometries: Synthesis, Structure, and Magnetic Properties

Two tetranuclear manganese(II) complexes [Mn(II)4(thiaS)2] (1) and [Mn(II)4(thiaSO)2] (2) have been synthesized under solvothermal conditions in methanol with p-tert-butylthiacalix[4]arene (thiaS) and p-tert-butylsulfinylthiacalix[4]arene (thiaSO). For both complexes, the structure has been established from single-crystal X-ray diffraction. [Mn4(thiaS)2].H2O (1) crystallizes in the orthorhombic Immm (No. 71) space group with the following parameters: a = 18.213 (5) angstroms, b = 19.037 (5) angstroms, c = 29.159 (5) angstroms, V = 10110 (4) angstroms3, and Z = 4. [Mn4(thiaSO)2].H2O (2) crystallizes in the monoclinic C2/m (No. 12) space group with the following parameters: a = 33.046(1) angstroms, b = 19.5363 (8) angstroms, c = 15.7773 (9) angstroms, beta = 115.176 (2) degrees, V = 9218.3 (8) angstroms3, and Z = 4. The two complexes are neutral and are best described as manganese squares sandwiched between two thiacalixarene macrocycles. In both complexes, each manganese center is six-coordinated in a trigonal prismatic geometry with four phenoxo oxygen atoms plus two sulfur atoms for 1 or two oxygen atoms from SO groups for 2. The two tetranuclear complexes exhibit identical magnetic behaviors resulting from antiferromagnetic interactions between the four manganese centers. The simulation of the magnetic susceptibility was done considering a single exchange-coupling constant between the manganese(II) ions, J (H = -J(S1S2 + S2S3 + S3S4 + S1S4)). The best fits give the same result for the two complexes: g = 1.94 and J = -5.57 cm(-1).

Electronic and spin structures of manganese clusters in the photosynthesis II system

Electronic and spin structures of manganese clusters, Mn 4O 4 ( 1), CaMn 3O 4 ( 2), Mn 3O 4 ( 3), MnX ( 4), and CaMn 4O 4 ( 5), in the photosynthesis II system are investigated using the classical and quantum Heisenberg models. The molecular orbital calculations by the use of general spin orbitals (GSO) are performed for cubane-type calcium manganese cluster 2 with noncollinear spin alignment, which has also been concluded in our previous studies of 1 and 3. The calculated results, together with available experiments, enable us to propose possible electronic states (from S 0 to S 4) of tetranuclear manganese cluster 5, which is the active site of oxygen evolution center (OEC). The low-spin (LS) ground states of 2– 5 are consistent with the ESR and other magnetic observations. A new reaction scheme for oxygen evolution from water is resulted from both theoretical and experimental results for manganese clusters 1– 5 in OEC on the basis of the newly determined X-ray structure by Ferreira et al. [Science 303 (2004) 1831]. In this mechanism, both calcium and manganese ions play important roles for formation of peroxide anion bridge, which is the precursor of molecular oxygen. Implications of computational results are discussed in relation to a key role of the high-valent manganese oxo (Mn(V) O) species in OEC for oxygen evolution from water. Our previous and present computations conclude that Mn(V) O plays a crucial role in both native and artificial OEC systems.

N,N,N‘,N‘-Tetrakis(2-quinolylmethyl)-2-hydroxy-1,3-propanediamine (Htqhpn) as a Supporting Ligand for a Low-Valent (μ-O)2 Tetranuclear Manganese Core

A new heptadentate N6-O1 ligand, N,N,N',N'-tetrakis(2-quinolylmethyl)-2-hydroxy-1,3-propanediamine (Htqhpn), was synthesized and used to generate compounds with linearly ordered MnIIMnIIIMnIIIMnII tetranuclear cores. This is the lowest valent tetranuclear manganese complex that exhibits a (mu2-O)2Mn2 core in the molecule. The electron paramagnetic resonance and magnetic measurements of these tetranuclear complexes suggest moderately strong antiferromagnetic coupling for the central MnIII2 core, with weak coupling between the MnII and MnIII centers.

Syntheses, structures and single-molecule magnetic behaviors of two dicubane Mn4 complexes

Abstract The reaction of MnX2 · 4H2O (X = Cl or Br) with 2,6-bis(hydroxymethyl)-4-methylphenol (H3L) and NaOH in methanol solution yielded two tetranuclear manganese complexes, [Mn4(HL)4(MeOH)4Cl2] (1) and [Mn4(HL)4(MeOH)4Br2] (2). Both compounds crystallize in the monoclinic space group C2/c with cell parameters: a = 26.0945(19) A, b = 11.4999(8) A, c = 21.2188(16) A, β = 121.050(1)° and z = 4 for 1 · 2Et2O; a = 25.8145(3) A, b = 11.6734(2) A, c = 21.3956(3) A, β = 120.1277(6)° and z = 4 for 2 · 2Et2O. Both complexes consist of a mixed-valence dicubane structure, which comprises two MnII and two MnIII ions. Magnetic susceptibilities and magnetization of complexes 1 and 2 in the solid state indicate that two clusters have an S = 9 ground state. Frequency-dependent out-of-phase signals of alternating current magnetic susceptibilities were observed in the low temperature range (