Mn Distance Research Articles

The diversity of structural features in binuclear cyclobutadiene manganese carbonyls: Relationship to homoleptic manganese carbonyls and cyclopentadienyl chromium carbonyls

The structures and energetics of the cyclobutadiene manganese carbonyls (C4H4)2Mn2(CO)n (n=6, 5, 4, 3) have been investigated using density functional theory. In this connection the lowest energy (C4H4)2Mn2(CO)6 structure consists of two C4H4Mn(CO)3 units coupled through an Mn–Mn bond of length ∼3.0Å. This rather fragile dimer is predicted to dissociate readily into C4H4Mn(CO)3 radicals. A higher energy (C4H4)2Mn2(CO)6 structure has an agostic hydrogen atom and an Mn⋯Mn distance of ∼4.3Å, too long for a direct bond. The unsaturated pentacarbonyl (C4H4)2Mn2(CO)5 system resembles its (C5H5)2Cr2(CO)5 counterpart by having low energy triplet spin state structures and being disfavored relative to (C4H4)2Mn2(CO)6+(C4H4)2Mn2(CO)4. A singlet tetracarbonyl (C4H4)2Mn2(CO)4 structure is found with a short MnMn distance of ∼2.2Å suggesting a formal triple bond analogous to the known (C5H5)2Cr2(CO)4 structure. However, the lowest energy (C4H4)2Mn2(CO)4 state is a novel triplet spin state octahedral Mn2C4 cluster with Mn(CO)3 and (η4-C4H4)Mn(CO) vertices. The lowest energy (C4H4)2Mn2(CO)3 structure by ∼13kcal/mol is an unsymmetrical triplet spin state structure with a C4H4 ligand bridging a dative formal MnMn triple bond connecting an Mn(CO)3 group to a (η4-C4H4)Mn group.

Magnetic properties of the layered oxypnictides (LnO)MnAs (Ln = La, Ce, Pr, Nd)

We have investigated the rare earth elements dependence on the magnetism to understand the contribution to physical properties of the 4f electrons of (LnO)MnAs (Pn = La, Ce, Pr, Nd). (CeO)MnAs, (PrO)MnAs and (NdO)MnAs shows the antiferromagnetic behaviors at low temperature. (CeO)MnAs and (NdO)MnAs have the magnetic anomalies around 34 K and 24 K, respectively. So, it is speculated that the anomalies depend on the Mn -Mn distance directly

A novel dinuclear manganese(II) compound incorporating two bis(imidazolyl) based ligands with single μ2-η1:η1-carboxylato bridges. Molecular and crystal structure and magnetic properties of [Mn2Cl(BIP)(HBIMAM)(H2O)5]Cl3·4H2O (BIP = 3,3-di(2-1H-2-imidazolyl)propanoate; BIMAM = N-methyldi(1H-2-imidazolyl)methanamine)

This paper reports the synthesis, X-ray structure and magnetic characterization of [Mn2Cl(BIP) (HBIMAM)(H2O)5]Cl3·4H2O a novel dinuclear manganese(II) incorporating two related bis(imidazolyl)-based ligands, BIP (3,3-di(2-1H-2-imidazolyl)propanoate) and BIMAM (N-methyldi(1H-2-imidazolyl)methanamine). The metal atoms in the dinuclear entity are bridged by a μ2-η1:η1 carboxylate group (from the propionate arm of the BIP ligand) acting in an anti–anti coordination bridging mode, with an intramolecular Mn⋯Mn distance of 6.47Å. The coordination around the Mn(II) ions shows a distorted octahedral geometry for both MnClN2O3 and MnN2O4 chromophores. Between the imidazole rings of neighbouring dinuclear units there significant aromatic–aromatic interactions. The propagation of these interactions generates chains of π-stacked dinuclear units, roughly running along the [100] direction. Magnetic susceptibility measurements in the range 2–300K show weak antiferromagnetic exchange interactions which were analysed by means of the isotropic S1=S2=5/2 spin-coupled dimer model Hˆ=-2J(Sˆ1·Sˆ2). Finally, the observed magnetic behavior (2J=−0.38(1)cm−1) is compared with that observed in related manganese(II) compounds with single [Mn(μ2-η1:η1-carboxylato)Mn] core.

The Variation in the Depth of Overburden at Different Ves Points within Samaru Using D.C. Resistivity Technique

The interpretation of 32 Schlumberger vertical electrical sounding (VES) data was carried out at Samaru College of Agriculture, A.B.U, Zaria, Sabongari Local government area of Kaduna State, Nigeria. This is an attempt to investigate the groundwater potential and the geologic characteristics of the overburden of the area. Terrameter signal averaging system (SAS) model 300 was the instrument used. No booster was used as the expected depth is within the range of penetration of the instrument. In this instrument consecutive readings are taken automatically and the results averaged continuously and displayed. The schlumberger electrode configuration was used in the data acquisition. The field procedure consists of expanding AB (distance between current electrodes) while MN (distance between potential electrodes) is fixed. This process yields a rapidly decreasing potential difference across MN, which eventually exceeds the measuring capacity of the instrument; therefore a larger value for MN was taken to continue with the survey. The VES curves were interpreted using IPI2Win resistivity computer software. The survey area is dominated by mainly four layers, namely: Overburden, Weathered basement, fractured basement and Fresh basement. The overburden consists of laterites, clay and fadama loam. The results of the interpreted VES data showed that The Overburden thickness varies from 1.3 to 5.2m, with an average of 3.1m. The lowest overburden depth is at VES20 where the depth to basement is as low as 6m. A map was produced by contouring all the overburden depths at each VES point at an interval of 0.5m. The map shows the variation of the topsoil depth from one place to another within the survey Area which is an indication of the inhomogenuity of the subsurface structures. The thickness of the aquifer varied from 1-35m with an average of 18m. DOI: 10.5901/ajis.2013.v2n12p95

Reprint of “Theory of chemical bonds in metalloenzymes XVIII. Importance of mixed-valence configurations for Mn5O5, CaMn4O5 and Ca2Mn3O5 clusters revealed by UB3LYP computations. A bio-inspired strategy for artificial photosynthesis”

Full geometry optimizations of mixed-valence (MV) Mn(II)RMn(II)aMn(IV)b Mn(IV)cMn(III)d (1), Ca(II)RMn(III)aMn(III)bMn(IV)cMn(III)d (2) and Ca(II)RCa(II)a Mn(IV)bMn(IV)cMn(III)d (3) clusters by UB3LYP have been performed to elucidate possible roles of substitutions of Mn(II) with Ca(II) in parent manganese oxides. The optimized Mn–Mn and Mn–Ca distances for 1, 2 and 3 have been compared with the EXAFS and XRD experimental structures of the oxygen evolving complex (OEC) of photosystem II (PSII) to elucidate variations of geometrical structures and valence fluctuations by the substitutions. The optimized Mn–O distances of 1, 2 and 3 have been examined to elucidate Jahn–Teller distortions induced by the Mn(III) ions. The computational results have illuminated possible origins of the elongated Mn–Mn distances and Mn–O distances in the high-resolution XRD structure by Umena et al. Implications of the computational results have been discussed in relation to chemical modifications of multi-nuclear manganese complexes with substitutions of Mn(II) with Ca(II) for rational design of artificial catalysts for water oxidation. A new bio-inspired strategy for artificial photosynthesis is also proposed based on a guiding principle, namely use of hole- and electron-doped strongly correlated electron systems (SCES) for oxidation and reduction reactions instead of conventional semiconductor materials.

First MnIII complexes with tetradentate (N2O2) Schiff bases and tricyanomethanide: synthesis, crystal structure, and magnetic properties

The first MnIII complexes with Schiff bases and tricyanomethanide-anion were synthesized: [Mn(salen)C(CN)3(H2O)] (1), [Mn(5-Brsalen)C(CN)3(H2O)] (2), [Mn(salpn)C(CN)3(H2O)] (3), [Mn(3-MeOsalen)C(CN)3(H2O)] (4), [Mn(5-Brsalen)(MeOH)(H2O)][C(CN)3] (5), and [Mn(3-MeOsalpn)(H2O)2][C(CN)3] (6), where SalenH2 is N,N′-bis(salicylidene)ethylenediamine, 5-BrsalenH2 is N,N′-bis(5-bromosalicylidene)ethylenediamine, SalpnH2 is N,N′-bis-(salicylidene)-1,3-diaminopropane, 3-MeOsalenH2 is N,N′-bis(3-methoxysalicylidene)-ethylenediamine, 3-MeOsalpnH2 — N,N′-bis(3-methoxysalicylidene)-1,3-diaminopropane. The tricyanomethanide anion in complexes 1–4 acts as a the terminal ligand, whereas in complexes 5 and 6 tricyanomethanide is not coordinated by MnIII and acts as an out-of-sphere counterion. The structures of complexes 1–4 are characterized by the formation of dimers due to hydrogen bonds between the water molecules and oxygen atoms of the Schiff bases. The Mn...Mn distances inside the dimers are 4.69–5.41 A. Complex 6 has a zigzag chain structure consisting of the [Mn(3-MeOsalpn)(H2O)2]+ cations bound by double bridging aqua ligands. The study of the magnetic properties of complexes 1, 3, 4, and 6 showed the existence of antiferromagnetic interactions between the MnIII ions through the system of hydrogen bonds.

Study of local disorder in LiMn(Cr,Ni)O2 compounds by extended X-ray absorption fine structure measurements

We have studied local structure of LiMnO2, LiMn0.65Cr0.35O2 and LiMn0.5Ni0.5O2 compounds by Mn K-edge extended X-ray absorption fine structure measurements. The local structure of LiMnO2 is found to be consistent with Jahn–Teller distorted MnO6 octahedra characterized by two different Mn–O bond distances. The Jahn–Teller distortions are suppressed in the Cr and Ni substituted compounds, resulting a single Mn–O distance. However, the Cr atoms tend to occupy a site at a longer distance from Mn in the host lattice (Mn–Cr distance is longer than Mn–Mn distance), unlike the Ni atoms which prefer a site closer to the Mn atoms (Mn–Ni distance is shorter than Mn–Mn distance). Incidentally, Mn–O and Mn–Mn bonds are substantially stiffer in the Cr and Ni substituted compounds. In addition, the static atomic disorder is confined around Cr atoms in the LiMn0.65Cr0.35O2, that is different from the case of LiMn0.5Ni0.5O2 in which larger static disorder appears in the proximity of the Mn atoms. The results suggest that the differences in the local structure of different compounds should be the likely reason for their differing battery characteristics.

The ternary germanides UMnGe and U2Mn3Ge

The title compounds were prepared by induction levitation melting of the elemental components and subsequent annealing. UMnGe (Pnma, a = 686.12(9), b = 425.49(6) and c = 736.5(1) pm) adopts the orthorhombic structure of TiNiSi and U2Mn3Ge (P63/mmc, a = 524.3(2) and c = 799.2(3) pm) possesses the hexagonal Mg2Cu3Si-type structure (ordered variant of the hexagonal Laves phase MgZn2). Both structures were refined from X-ray powder data to residuals of RI = 0.021 and 0.014 for UMnGe and U2Mn3Ge, respectively. The manganese and germanium atoms in UMnGe build up a three-dimensional [MnGe] network of ordered Mn3Ge3 hexagons with Mn–Ge distances ranging from 248 to 259 pm. The uranium atoms are coordinated by two tilted Mn3Ge3 hexagons. The manganese atoms in U2Mn3Ge build up Kagomé networks with 252 and 272 pm Mn–Mn distances which are connected via the germanium atoms (254 pm Mn–Ge) to a three-dimensional network. A remarkable feature of the U2Mn3Ge structure is a short U–U distance of 278 pm between adjacent cavities of the [Mn3Ge] network. From DFT based electronic structure calculations both germanides are found more cohesive than the Laves phase UMn2, thus underpinning the substantial role of Mn–Ge bonding. Calculations for both germanides show ferrimagnetic ground states with antiparallel spin alignments between U and Mn. The valence bands show bonding characteristics for interactions of atoms of different chemical natures and significant Mn–Mn bonding in U2Mn3Ge. Preliminary investigation of UMnGe by magnetization measurements confirms an antiferromagnetic arrangement below TN = 240 K.

Theory of chemical bonds in metalloenzymes XVIII. Importance of mixed-valence configurations for Mn5O5, CaMn4O5 and Ca2Mn3O5 clusters revealed by UB3LYP computations. A bio-inspired strategy for artificial photosynthesis

Full geometry optimizations of mixed-valence (MV) Mn(II)RMn(II)aMn(IV)b Mn(IV)cMn(III)d (1), Ca(II)RMn(III)aMn(III)bMn(IV)cMn(III)d (2) and Ca(II)RCa(II)a Mn(IV)bMn(IV)cMn(III)d (3) clusters by UB3LYP have been performed to elucidate possible roles of substitutions of Mn(II) with Ca(II) in parent manganese oxides. The optimized Mn–Mn and Mn–Ca distances for 1, 2 and 3 have been compared with the EXAFS and XRD experimental structures of the oxygen evolving complex (OEC) of photosystem II (PSII) to elucidate variations of geometrical structures and valence fluctuations by the substitutions. The optimized Mn–O distances of 1, 2 and 3 have been examined to elucidate Jahn–Teller distortions induced by the Mn(III) ions. The computational results have illuminated possible origins of the elongated Mn–Mn distances and Mn–O distances in the high-resolution XRD structure by Umena et al. Implications of the computational results have been discussed in relation to chemical modifications of multi-nuclear manganese complexes with substitutions of Mn(II) with Ca(II) for rational design of artificial catalysts for water oxidation. A new bio-inspired strategy for artificial photosynthesis is also proposed based on a guiding principle, namely use of hole- and electron-doped strongly correlated electron systems (SCES) for oxidation and reduction reactions instead of conventional semiconductor materials.

Structure and electronic properties of a Mn nanowire encapsulated in carbon nanotubes

Abstract First-principles calculations using density functional theory are performed studying the stability and the electronic structure of an one-dimensional chain of Mn atoms as a nanowire encapsulated in a single-walled (5,5) armchair carbon nanotube. The results show a ferromagnetic ground state for a dimerized nanowire where the interatomic Mn–Mn distance is 2.65 A and the interdimer separation is 3.40 A. The average Mn moment is 4.35 μ B . A charge transfer from the nanotube to the nanowire is observed. A polarized spin channel is identified for conduction electrons in the filled nanotube.

Major Differences between the Binuclear Manganese Boronyl Carbonyl Mn2(BO)2(CO)9 and Its Isoelectronic Chromium Carbonyl Analogue Cr2(CO)11

The lowest energy structures of the manganese boronyl carbonyl Mn2(BO)2(CO)9 by more than 8 kcal/mol are found to have a single end-to-end bridging BO group bonding to one manganese atom through its boron atom and to the other manganese atom through its oxygen atom. The long Mn···Mn distances in these structures indicate the lack of direct manganese-manganese bonding as confirmed by essentially zero Wiberg bond indices. These Mn2(BO)2(CO)9 structures are favored thermochemically by more than 25 kcal/mol over dissociation into mononuclear fragments and thus appear to be viable synthetic objectives. This contrasts with the isoelectronic Cr2(CO)11 system, which is predicted to be disfavored relative to the mononuclear fragments Cr(CO)6 + Cr(CO)5. Analogous Mn2(BO)2(CO)9 structures with an end-to-end bridging CO group lie ∼17 kcal/mol in energy above the corresponding structures with end-to-end bridging BO groups. The lowest energy Mn2(BO)2(CO)9 structures without an end-to-end bridging BO group provide unprecedented examples of the coupling of two terminal BO groups to form a terminal dioxodiborene (B2O2) ligand with a B-B distance of ∼1.9 Å. Still higher energy Mn2(BO)2(CO)9 structures include singly bridged and doubly semibridged structures analogous to the previously optimized lowest energy Cr2(CO)11 structures.

Crystal structure of chloro-(5,10,15,20-tetraphenylporphyrinato)-manganese, C44H28ClMnN4

Abstract C44H28ClMnN4, monoclinic, P21/c (no. 14), a = 12.3044(6) Å, b = 21.688(1) Å, c = 17.0016(8) Å, β = 123.536(3)°, V = 3781.9 Å3, Z = 4, Rgt(F) = 0.0645, wRref(F2) = 0.0939, T = 293 K

Investigation of the crystal and molecular structure of a novel coordination polymer formed by manganese(II) nitrate and 4,4,10,10-tetramethyl-1,3,7,9-tetraazospiro[5.5]undecan-2,8-dione

A coordination polymer {[Mn(H2O)2(C11H20N4O2)2]2+·2(NO3−)}n is synthesized and its structure is determined. The crystals are monoclinic: space group P21/c, a = 12.3771(3) A, b = 14.8775(3) A, c = 18.1388(4) A, β = 106.611(2)°, V = 3200.70(12) A3, dx = 1.44 g/cm3, Z = 4. Manganese ions are coordinated with four oxygen atoms of four organic ligands (two unique) and two water molecules. The coordination polyhedron is a distorted octahedron; the O-Mn-O angles between the adjacent oxygen atoms fall within 83.96(5)–98.11(5)°. The nitrate anions are in the outer coordination sphere. The Mn...Mn distances in the polymer are 8.56 A. In the crystal, polymeric coordination chains are joined in layers perpendicular to the b axis by hydrogen bonds involving the organic ligands, water molecules, and nitrate anions.

Ti8(TixMn1−x)6Mn39 (‘TiMn∼4’): a metallic spin fluctuation system

The crystal structure of Ti8(TixMn1−x)6Mn39, x = 0.187, was obtained from x-ray single-crystal diffraction data, confirming it to have rhombohedral symmetry (space group ; ahex = 1.100 70(2) nm, chex = 1.944 11(4) nm; RF = 0.0293) and isotypism with the prototype Mo0.38Cr0.16Co0.46 (the so-called R-phase). On the basis of electron probe micro-analyser results and structure determination, the homogeneity region of the phase TiMn∼4 was determined for temperatures in the range 800 °C < T < 1200 °C and is in between 16.0 at.% Ti and 20 at.% Ti. Various physical properties, determined in the temperature range from ∼2 K to room temperature, characterize the compound with composition TiMn4.26 as a metallic spin fluctuation system, evidenced from a T3lnT dependence of the heat capacity in combination with large values of the electronic Sommerfeld constant of the order of 140 mJ mol−1 K−2. The occurrence of a small anomaly in the heat capacity and magnetization data around 10 K is attributed to a scenario involving spin freezing phenomena, since a fraction of the order of 10% of all Mn–Mn distances within the unit cell are above a critical distance, where Mn atoms carry a spontaneous magnetic moment.

Short-range and long-range order of phyllomanganate nanoparticles determined using high-energy X-ray scattering

High-energy X-ray scattering (HEXS) is used to explore the pH-dependent structure of randomly stacked manganese oxide nanosheets of nominal formula δ-MnO2. Data are simulated in real space by pair distribution function (PDF) analysis and in reciprocal space by both the Bragg-rod method and the Debye equation in order to maximize the information gained from the total scattering measurements. The essential new features of this triple-analysis approach are (1) the use of a two-dimensional supercell in PDF modeling to describe local distortions around Mn layer vacancies, (2) the implementation in Bragg-rod calculations of a lognormal crystal size distribution in the layer plane and an empirical function for the effect of strain, and (3) the incorporation into the model used with the Debye equation of an explicit elastic deformation of the two-dimensional nanocrystals. The PDF analysis reveals steady migration at acidic pH of the Mn atoms from layer to interlayer sites, either above or below the Mn layer vacancies, and important displacement of the remaining in-layer Mn atoms toward vacancies. The increased density of the vacancy–interlayer Mn pairs at low pH causes their mutual repulsion and results in short-range ordering. The layer microstructure, responsible for the long-range lateral disorder, is modeled with spherically and cylindrically bent crystallites having volume-averaged radii of 20–40 Å. Thebunit-cell parameter from the hexagonal layer has different values in PDF, Bragg-rod and Debye equation modeling, because of the use of different weighting contributions from long-range and short-range distances in each method. The PDFbparameter is in effect a measure of the average inlayer Mn...Mn distance and consistently deviates from the average structure value determined by the Bragg-rod method by 0.02 Å at low pH, as a result of the local relaxation induced by vacancies. The layer curvature increases the Bragg-rod value by 0.01–0.02 Å with the cylindrical model and as much as 0.04–0.05 Å with the spherical model. Therefore, in principle, the diffraction alone can unambiguously determine with good accuracy only a volume-averaged apparent layer dimension of the manganese oxide nanosheets. Thebparameter is model dependent and has no single straightforward interpretation, so comparison ofbbetween different samples only makes sense if done in the context of a single specified model.

Structural and magnetic characterizations of the first manganese(iii) Schiff base complexes involving hexathiocyanidoplatinate(iv) bridges

The reactions of [MnIII(L4a)Cl] or [MnIII(L4b)Cl] with [Pt(SCN)6]2− resulted in the formation of a 1D polymeric complex [{Mn(L4a)}2{μ-Pt(SCN)6}]n (1) and a trinuclear compound [{Mn(L4b)(H2O)}2{μ-Pt(SCN)6}] (2), where L4a and L4b are tetradentate dianionic Schiff base ligands, H2L4a = N,N′-ethylenebis(salicylimine) and H2L4b = N,N′-3-methylbenzenebis(3-ethoxysalicylimine). In compound 1, the adjacent [Mn(L4a)]+ subunits are bound together through the phenolic oxygen atoms, thus forming [{Mn(L4a)}2]2+ dimers with the shortest Mn⋯Mn distance of 3.2101(5) Å, while in compound 2, the intermolecular bifurcated O–H⋯O hydrogen bonds between the neighbouring molecules form [{Mn(L4b)(H2O)}2]2+ dimers, resulting in a supramolecular 1D chain structure. The Mn⋯Mn distance within the supramolecular dimer is 4.7007(9) Å, while the shortest Mn⋯Mn distance between the dimers is 9.2347(10) Å. The magnetic analyses comprising the zero-field splitting resulted in a ferromagnetic exchange interaction within the phenolato-bridged dimer in 1 (J = +1.73 cm−1, DMn = −2.65 cm−1) and the antiferromagnetic exchange interaction within the supramolecular dimer in 2 (J = −0.88 cm−1, DMn = −3.06 cm−1). The experimental findings regarding the dominant as well as negligible exchange pathways are in good agreement with the results from DFT calculations at the B3LYP/TZVP level of theory.

Structures and magnetic properties of copper(ii) and manganese(ii) polymers derived from pseudohalides and a flexible zwitterionic dicarboxylate ligand

The flexible zwitterionic dicarboxylate ligand 1,4-bis(4-carboxylato-1-pyridinium)butane (bcpb) assumes different conformations to collaborate with pseudohalides in various coordination modes to produce coordination polymers in which distinct anionic motifs with mixed carboxylate and pseudohalide bridges are interlinked by the cationic butylenebis(pyridinium) tethers. The Cu(II) compound, [Cu2(bcpb)(N3)4]n·nH2O (1), is a 1D coordination polymer based on the defective dicubane-like [Cu4(μ3-1,1,1-N3)2(μ-1,1-N3)2(μ-1,1-OCO)2] cluster. With Mn(II), four distinct 3D coordination polymers, [Mn4(bcpb)4(N3)(H2O)4]n(ClO4)7n·nCH3OH·3nH2O (2), [Mn2.5(bcpb)(N3)5(H2O)2]n (3), [Mn2(bcpb)(N3)4]n·nH2O (4), and [Mn2(bcpb)(NCO)4]n·nH2O (5), were characterized. 2 is the first Mn(II) compound with the rare μ4-1,1,3,3 azide bridge and exhibits an unusual 3D framework based on the [Mn4(μ4-1,1,3,3-N3)(μ-1,3-OCO)6] cluster. In 3, the unique undulated honeycomb-like [Mn2(μ-1,3-N3)3]n layers are interlinked into a 3D framework by disordered [Mn(μ-1,1-N3)4(μ-1,3-OCO)2] and [(O(aqua)-H)2···OCO]2 moieties, and the bcpb ligands serve as additional interlayer linkers to lead to the rare self-catenated 6(6) net. 4 and 5 show 3-fold interpenetrated 3D frameworks based on the chains with (μ-1,1-N3)2(μ-1,3-OCO) and (μ-N,N-NCO)2(μ-1,3-OCO) bridges, respectively. Magnetic studies indicated that 1 shows competing ferromagnetic and antiferromagnetic interactions. Compounds 2-5 all show antiferromagnetic coupling between Mn(ii) ions, while 3 shows 3D ordering. Analyses of magneto-structural data suggest a general trend that the antiferromagnetic interaction through (μ-1,1-N3)2(μ-1,3-OCO) or (μ-N,N-NCO)2(μ-1,3-OCO) increases with a decrease of the Mn···Mn distance.

Theoretical insight in to hydrogen-bonding networks and proton wire for the CaMn4O5 cluster of photosystem II. Elongation of Mn–Mn distances with hydrogen bonds

Quantum mechanical (QM) and QM/molecular mechanics (MM) calculations of three different cluster models have been performed to shed light on hydrogen bonding networks and proton wires for proton release pathways (PRP) of water oxidation reactions in the oxygen evolving complex (OEC) of photosystem II (PSII). Positions of all the hydrogen atoms in an extended QM Model III including the second coordination sphere for the active-site CaMn4O5 complex of OEC of PSII have been optimized assuming the geometry of heavy atoms determined by the recent high-resolution X-ray diffraction (XRD) experiment of PSII refined to 1.9 A resolution. Full geometry optimizations of the first coordination sphere model (QM Model I) embedded in the Model III and QM (QM Model I plus seven water molecules, namely QM Model II)/MM models, together with full QM Model III, have also been conducted to elucidate confinement effects for geometrical parameters of the CaMn4O5 cluster by proteins. Computational results by these methods have elucidated the O⋯O(N), O⋯H distances and O(N)–H⋯O angles for hydrogen bonds in proton release paths (PRP) I and II that construct a proton wire from Asp61 toward His190. The hydrogen-bonding structures revealed have also been examined in relation to the possibilities of protonation of bridge oxygen dianions within the CaMn4O5 cluster. The optimized inter-atomic distances by QM Models I and III, together with QM(Model II)/MM, have elucidated the elongation of the Mn–Mn distances with hydrogen bonds and variations of the Mnd–O(5) length with confinement effects by protein. Implications of the computational results are discussed in relation to the available EXAFS experiments, and internal, semi-internal and external reductions of Mn ions and long Mn–Mn distances of the high-resolution (SP8) XRD, and rational design of artificial catalysts for water oxidation that are current topics in the field of OEC of PSII.

Coulombic Aggregations of MnIII salen-Type Complexes and Keggin-Type Polyoxometalates: Isolation of Mn2 Single-Molecule Magnets

The reaction of Mn(III) salen-type complexes with di- and tetraanionic α-Keggin-type polyoxometalates (POMs) was performed, and three types of Coulombic aggregations containing Mn(III) out-of-plane dimeric units (abbreviated as [Mn(2)](2+)) that are potentially single-molecule magnets (SMMs) with an S(T) = 4 ground state were synthesized: [Mn(2)(5-MeOsaltmen)(2)(acetone)(2)][SW(12)O(40)] (1), [Mn(2)(salen)(2)(H(2)O)(2)](2)[SiW(12)O(40)] (2), and [Mn(5-Brsaltmen)(H(2)O)(acetone)](2)[{Mn(2)(5-Brsaltmen)(2)}(SiW(12)O(40))] (3), where 5-Rsaltmen(2-) = N,N'-(1,1,2,2-tetramethylethylene)bis(5-R-salicylideneiminate) with R = MeO (methoxy), Br (bromo) and salen(2-) = N,N'-ethylenebis(salicylideneiminate). Compound 1 with a dianionic POM, [SW(12)O(40)](2-), is composed of a 1:1 aggregating set of [Mn(2)](2+)/POM, and 2, with a tetraanionic POM, [SiW(12)O(40)](4-), is a 2:1 set. Compound 3 with [SiW(12)O(40)](4-) forms a unique 1D coordinating chain with a [-{Mn(2)}-POM-](2-) repeating unit, for which a hydrogen-bonded dimeric unit ([Mn(5-Brsaltmen)(H(2)O)(acetone)](2)(2+)) is present as a countercation. Independent of the formula ratio of [Mn(2)](2+)/POM, Mn(III) dimers and POM units in 1-3 form respective segregated columns along a direction of the unit cell, which make an alternate packing to separate evenly identical species in a crystal. The nearest intermolecular Mn···Mn distance is found in the order 2 < 3 < 1. The segregation of the [Mn(2)](2+) dimer resulted in interdimer distances long enough to effectively reduce the intermolecular magnetic interaction, in particular in 1 and 3. Consequently, an intrinsic property, SMM behavior, of Mn(III) dimers has been characterized in this system, even though the interdimer interactions are still crucial in the case of 2, where a long-range magnetic order competitively affects slow relaxation of the magnetization at low ac frequencies.

Substituent Effects on Core Structures and Heterogeneous Catalytic Activities of MnIII(μ-O)2MnIV Dimers with 2,2′:6′,2″-Terpyridine Derivative Ligands for Water Oxidation

[(OH(2))(R-terpy)Mn(μ-O)(2)Mn(R-terpy)(OH(2)) ](3+) (R-terpy = 4'-substituted 2,2':6',2″-terpyridine, R = butoxy (BuO), propoxy (PrO), ethoxy (EtO), methoxy (MeO), methyl (Me), methylthio (MeS), chloro (Cl)) have been synthesized as a functional oxygen-evolving complex (OEC) model and characterized by UV-vis and IR spectroscopic, X-ray crystallographic, magnetometric, and electrochemical techniques. The UV-vis spectra of derivatives in water were hardly influenced by the 4'-substituent variation. X-ray crystallographic data showed that Mn centers in the Mn(III)(μ-O)(2)Mn(IV) cores for derivatives with R = H, MeS, Me, EtO, and BuO are crystallographically indistinguishable, whereas the derivatives with R = MeO and PrO gave the significantly distinguishable Mn centers in the cores. The indistinguishable Mn centers could be caused by rapid electron exchange between the Mn centers to result in the delocalized Mn(μ-O)(2)Mn core. The exchange integral values (J = -196 to -178 cm(-1)) for delocalized cores were lower than that (J = -163 to -161 cm(-1)) for localized cores, though the Mn···Mn distances are nearly the same (2.707-2.750 Å). The half wave potential (E(1/2)) of a Mn(III)-Mn(IV)/Mn(IV)-Mn(IV) pair of the derivatives decreased with an increase of the electron-donating ability of the substituted groups for the delocalized core, but it deviated from the correlation for the localized cores. The catalytic activities of the derivatives on mica for heterogeneous water oxidation were remarkably changed by the substituted groups. The second order rate constant (k(2)/mol(-1) s(-1)) for O(2) evolution was indicated to be correlated to E(1/2) of a Mn(III)-Mn(IV)/Mn(IV)-Mn(IV) pair; k(2) increased by a factor of 29 as E(1/2) increased by 28 mV.