Five-coordinate Mn Research Articles

Is There a Different Mechanism for Water Oxidation in Higher Plants?

The leading mechanism for the formation of O2 in photosystem II (PSII) has, during the past decade, been established as the so-called oxyl-oxo mechanism. In that mechanism, O2 is formed from a binding between an oxygen radical (oxyl) and a bridging oxo group. For the case of higher plants, that mechanism has recently been criticized. Instead, a nucleophilic attack of an oxo group on a five-coordinated Mn(V)═O group forming O2 has been suggested in a so-called water-unbound (WU) mechanism. In the present study, the WU mechanism has been investigated. It is found that the WU mechanism is just a variant of a previously suggested mechanism but with a reactant and a transition state that have much higher energies. The addition of a water molecule on the empty site of the Mn(V)═O center is very exergonic and leads back to the previously suggested oxyl-oxo mechanism.

Theoretical investigation of five-coordinated manganese(III) oxophlorin with different axial ligands at various spin states using different DFT methods

The Mn(III)-oxophlorin complexes with imidazole, pyridine and t-butylcyanide as axial ligands have been studied using B3LYP, Bv86p, and M06-2X methods. All of the possible optimized geometries are specified, while the M06-2X is employed. Results obtained show that the isomers of Mn(III)-oxophlorin with imidazole or pyridine are the most stable at quintet state, compared to singlet and triplet spin states. Besides, there are two and four [Formula: see text]-electrons on manganese in each of these complexes at triplet and quintet states, respectively. Also, Mn(III)-oxophlorin with t-butylcyanide as axial ligand is only stable at singlet state. Non-specific solvent effects show that dispersion and London forces have the basic role in stability of complexes in a solvent. Note that latter interactions can occur in medium with dielectric constant ([Formula: see text]) of [Formula: see text]8, such as [Formula: see text] for position of oxophlorin in heme oxygenase enzyme. NBO analysis show that there is no degeneracy between d orbitals of Mn in the five-coordinated Mn(III)-oxophlorin at singlet and triplet spin states, but two d orbitals of manganese are degenerated in latter complexes at quintet state. Such degeneracy of d orbitals is observed in a complex with square pyramid structure. Then five-coordinated Mn(III)-oxophlorin with imidazole or pyridine is the most stable at quintet spin state, because of its geometry corresponding to square pyramid configuration of atoms. Also, nonbounding interaction between Mn and the ring of oxophlorin or Mn and ligand are more effective in Mn(III)-oxophlorin with imidazole as axial ligand, compared to pyridine and t-butylcyanide.

A redox cascade of NOx− complexes: Structures and nitrogen deoxygenation thermodynamics

The pincer ligand PNNH contains pyridine with its ortho carbons bearing one tBu2PCH2 donor and one pyrazole donor. A DFT study of the energies and structures of (PNNH)Mn(CO)n(NOx) (n = 0–2, x = 0–3) reveals thermodynamic preferences over the suite of compounds. In every NOx case, the cis positioning of two carbonyls gives a more stable isomer than does the trans, which is understood as avoiding the trans conflict of two strong π acid ligands. The calculations also reveal a preference for nitrito (ONO) linkage isomers over the nitro (NO2) alternative. Additionally, the calculations predict loss of one CO when the nitrosyl oxidation level is reached, forming the five-coordinate (PNNH)Mn(CO)(NO). Nitrosyl ligand bending is evaluated with changing manganese coordination number, and diagnostic infrared frequencies are also discussed. We also investigate the thermodynamics of deoxygenation of the original coordinated nitrate. It is shown that reductive deoxygenations among these species are thermodynamically favored using a bis silyl dihydropyridazine reagent (oxygen removed as O(SiMe3)2).

Ring-Shaped Polyoxometalate Built by {Mn 4 PW 9 } and PO 4 Units for Efficient Visible-Light-Driven Hydrogen Evolution

A novel mixed-valence, ring-shaped multinuclear Mn-containing polyoxometalate, [H2N(CH3)2]15NaH8-[MnIII3MnIV(μ3-O)3(OAc)PO4(B-α-PW9O34)]4·36H2O (1) was made and systematically characterized using v...

Spectral, Electrochemical, and ESR Characterization of Manganese Tetraarylporphyrins Containing Four β,β'-Pyrrole Fused Butano and Benzo Groups in Nonaqueous Media.

Two series of β,β'-pyrrole butano- and benzo-substituted mangenese(III) tetraarylporphyrins were synthesized and characterized with regard to their spectral and electrochemical properties. The investigated compounds have the general formula butano(Ar)4PorMnCl and benzo(Ar)4PorMnCl, where Por is the dianion of the porphyrin and Ar is a p-CH3Ph, Ph or p-ClPh group on each of the four meso-positions of the macrocycle. Each manganese(III) butano- or benzoporphyrin was examined in CH2Cl2 and/or pyridine containing 0.1 M tetra- n-butylammonium perchlorate and the data then were compared to that of the parent tetraarylporphyrins having the same meso-substituents. Up to four reductions are observed for each compound, the first being metal-centered to generate a Mn(II) porphyrin, and the second and third being porphyrin ring-centered to give a Mn(II) porphyrin π-anion radical and dianion, respectively. The one-electron reduced manganese porphyrins have an ESR spectrum with signals at g⊥= 5.6-5.8 and g// = 2.0, indicating a mixture of the four- and five-coordinated Mn(II) complexes in a high-spin state (3d5, S = 5/2, I = 5/2). Data from cyclic voltammetry and spectroelectrochemistry both suggest that formation of the porphyrin dianion is followed by a chemical reaction at the electrode surface to give an electroactive phlorin anion. The effects of solvent and porphyrin substituents on ultraviolet-visible light (UV-vis) spectra, redox potentials, and electron transfer mechanisms are discussed.

Tetranuclear Manganese Models of the OEC Displaying Hydrogen Bonding Interactions: Application to Electrocatalytic Water Oxidation to Hydrogen Peroxide.

Toward the development of structural and functional models of the oxygen evolving complex (OEC) of photosystem II, we report the synthesis of site-differentiated tetranuclear manganese complexes featuring three six-coordinate and one five-coordinate Mn centers. To incorporate biologically relevant second coordination sphere interactions, substituents capable of hydrogen bonding are included as pyrazolates with arylamine substituents. Complexes with terminal anionic ligands, OH- or Cl-, bound to the lower coordinate metal center are supported through the hydrogen-bonding network in a fashion reminiscent of the enzymatic active site. The hydroxide complex was found to be a competent electrocatalyst for O-O bond formation, a key transformation pertinent to the OEC. In an acetonitrile-water mixture, at neutral pH, electrochemical water oxidation to hydrogen peroxide was observed, albeit with low (15%) Faradaic yield, likely due to competing reactions with organics. In agreement, 9,10-dihydroanthracene is electrochemically oxidized in the presence of this cluster both via H-atom abstraction and oxygenation with ∼50% combined Faradaic yield.

Room-Temperature Energy-Sampling Kβ X-ray Emission Spectroscopy of the Mn4Ca Complex of Photosynthesis Reveals Three Manganese-Centered Oxidation Steps and Suggests a Coordination Change Prior to O2 Formation.

In oxygenic photosynthesis, water is oxidized and dioxygen is produced at a Mn4Ca complex bound to the proteins of photosystem II (PSII). Valence and coordination changes in its catalytic S-state cycle are of great interest. In room-temperature (in situ) experiments, time-resolved energy-sampling X-ray emission spectroscopy of the Mn Kβ1,3 line after laser-flash excitation of PSII membrane particles was applied to characterize the redox transitions in the S-state cycle. The Kβ1,3 line energies suggest a high-valence configuration of the Mn4Ca complex with Mn(III)3Mn(IV) in S0, Mn(III)2Mn(IV)2 in S1, Mn(III)Mn(IV)3 in S2, and Mn(IV)4 in S3 and, thus, manganese oxidation in each of the three accessible oxidizing transitions of the water-oxidizing complex. There are no indications of formation of a ligand radical, thus rendering partial water oxidation before reaching the S4 state unlikely. The difference spectra of both manganese Kβ1,3 emission and K-edge X-ray absorption display different shapes for Mn(III) oxidation in the S2 → S3 transition when compared to Mn(III) oxidation in the S1 → S2 transition. Comparison to spectra of manganese compounds with known structures and oxidation states and varying metal coordination environments suggests a change in the manganese ligand environment in the S2 → S3 transition, which could be oxidation of five-coordinated Mn(III) to six-coordinated Mn(IV). Conceivable options for the rearrangement of (substrate) water species and metal-ligand bonding patterns at the Mn4Ca complex in the S2 → S3 transition are discussed.

A five-coordinate Mn(iv) intermediate in biological water oxidation: spectroscopic signature and a pivot mechanism for water binding.

Among the four photo-driven transitions of the water-oxidizing tetramanganese-calcium cofactor of biological photosynthesis, the second-last step of the catalytic cycle, that is the S2 to S3 state transition, is the crucial step that poises the catalyst for the final O-O bond formation. This transition, whose intermediates are not yet fully understood, is a multi-step process that involves the redox-active tyrosine residue and includes oxidation and deprotonation of the catalytic cluster, as well as the binding of a water molecule. Spectroscopic data has the potential to shed light on the sequence of events that comprise this catalytic step, which still lacks a structural interpretation. In this work the S2-S3 state transition is studied and a key intermediate species is characterized: it contains a Mn3O4Ca cubane subunit linked to a five-coordinate Mn(iv) ion that adopts an approximately trigonal bipyramidal ligand field. It is shown using high-level density functional and multireference wave function calculations that this species accounts for the near-infrared absorption and electron paramagnetic resonance observations on metastable S2-S3 intermediates. The results confirm that deprotonation and Mn oxidation of the cofactor must precede the coordination of a water molecule, and lead to identification of a novel low-energy water binding mode that has important implications for the identity of the substrates in the mechanism of biological water oxidation.

Effects of porphyrin deformation on the 13C and1H NMR chemical shifts in high-spin five- and six-coordinate manganese(III) porphyrin complexes

As an extension of our study to reveal the effect of porphyrin deformation on the [Formula: see text]C and1H NMR chemical shifts, both five- and six-coordinate high-spin (S [Formula: see text] 2) Mn(III) complexes such as Mn(Por)Cl and [Mn(Por)(CD3OD)2]Cl have been prepared, where Por is a porphyrin dianion such as TPP, OMTPP, and T[Formula: see text]PrP. Molecular structures of five-coordinate Mn(OMTPP)Cl and Mn(TiPrP)Cl have been determined by the X-ray crystallographic analysis. As expected, Mn(OMTPP)Cl and Mn(TiPrP)Cl have exhibited a highly saddled and highly ruffled porphyrin core, respectively. The [Formula: see text]C NMR spectra have revealed that these complexes generally exhibit the [Formula: see text]-pyrrole signals at the downfield positions and [Formula: see text]-pyrrole an. meso signals at the upfield positions. The results suggest that the spin polarization of Mn(III)–NPσ bonds, which occurs in all the high-spin Mn(III) complexes, is the major factor to determine the chemical shifts of the porphyrin carbon signals (Cheng, R.-J.; Chang, S.-H.; Hung, K.-C. Inorg. Chem. 2007; 46: 1948–1950). Although th. meso and [Formula: see text]-pyrrole signals are observed at the upfield and downfield positions, respectively, these signals are widely dispersed depending on the deformation mode of the porphyrin ring. The results have been explained in terms of the strong spin polarization of the Mn–NPbond together with the specific metal-porphyrin orbital interactions such as: (i) the a2u-dz2 interaction in five-coordinate complexes, (ii) the a2u-dxyinteraction in ruffled complexes, and (iii) the a2u-dx2-y2 interaction in saddled complexes.

Manganese as a Substitute for Rhenium in CO2 Reduction Catalysts: The Importance of Acids

Electrocatalytic properties, X-ray crystallographic studies, and infrared spectroelectrochemistry (IR-SEC) of Mn(bpy-tBu)(CO)3Br and [Mn(bpy-tBu)(CO)3(MeCN)](OTf) are reported. Addition of Brönsted acids to CO2-saturated solutions of these Mn complexes and subsequent reduction of the complexes lead to the stable and efficient production of CO from CO2. Unlike the analogous Re catalysts, these Mn catalysts require the addition of Brönsted acids for catalytic turnover. Current densities up to 30 mA/cm(2) were observed during bulk electrolysis using 5 mM Mn(bpy-tBu)(CO)3Br, 1 M 2,2,2-trifluoroethanol, and a glassy carbon working electrode. During bulk electrolysis at -2.2 V vs SCE, a TOF of 340 s(-1) was calculated for Mn(bpy-tBu)(CO)3Br with 1.4 M trifluoroethanol, corresponding to a Faradaic efficiency of 100 ± 15% for the formation of CO from CO2, with no observable production of H2. When compared to the analogous Re catalysts, the Mn catalysts operate at a lower overpotential and exhibit similar catalytic activities. X-ray crystallography of the reduced species, [Mn(bpy-tBu)(CO)3](-), shows a five-coordinate Mn center, similar to its rhenium analogue. Three distinct species were observed in the IR-SEC of Mn(bpy-tBu)(CO)3Br. These were of the parent Mn(bpy-tBu)(CO)3Br complex, the dimer [Mn(bpy-tBu)(CO)3]2, and the [Mn(bpy-tBu)(CO)3](-) anion.

Syntheses, structure, and magnetic properties of hexanuclear MnIII2MIII4 (M = Y, Gd, Tb, Dy) complexes

The synthesis and characterizations of a family of isomorphous [Mn(III)(2)M(III)(4)L(2)(μ(4)-O)(2)(N(3))(2)(CH(3)O)(2)(CH(3)OH)(4)(NO(3))(2)]·2H(2)O (M = Y(1), Gd(2), Tb(3), Dy(4)) are reported, where H(4)L = N,N'-dihydroxyethyl-N,N'-(2-hydroxy-4,5-dimethylbenzyl)ethylenediamine. They were obtained from the reactions of H(4)L with M(NO(3))(3)·6H(2)O, Mn(ClO(4))(2)·6H(2)O, NaN(3) and NEt(3) in a 1:1:1:2:2 molar ratio. The core structure consists of a Mn(2)M(4) unit. The four M(III) ions that are held together by two μ(4)-bridging oxygen atoms form a butterfly M(4) moiety. The M(4) core is further connected to the two five-coordinate trigonal-bipyramidal Mn(III) ions via one μ(4)-O(2-), two alkyloxo and one methoxo triple bridges. Magnetic susceptibility measurements indicate the presence of intramolecular antiferromagnetic interactions in complex 2, and overall intramolecular ferromagnetic interactions in complexes 3 and 4. The alternating current (AC) magnetic susceptibility studies revealed that complexes 3 and 4 showed frequency-dependent out-of-phase signals, which indicates that they exhibit slow relaxation of the magnetization.

Significant effect of spin flip on the oxygen atom transfer reaction from (oxo)manganese(v) corroles to thioanisole: insights from density functional calculations

The electronic and structural features of (oxo)manganese(V) corroles and their catalyzed oxygen atom transfers to thioanisole in different spin states have been investigated by the B3LYP functional calculations. Calculations show that these corrole-based oxidants and their complexes with thioanisole generally have the singlet ground state, and their triplet forms are also accessible in consideration of the spin-orbit coupling interaction. Due to strong d-π conjugation interactions between Mn and the corrole ring arising from the π electron donation of the corrole moiety, the five-coordinated Mn approximately has the stable 18-electron configuration. The predicted free energy barriers for the singlet oxygen atom transfer reactions are generally larger than 22 kcal mol(-1), while the spin flip in reaction may remarkably increase the reactivity. In particular, the bromination on β-pyrrole carbon atoms of the meso-substituted (oxo)manganese(V) corrole strikingly enhances the spin-orbit coupling interaction and results in the dramatic increase of reactivity. The multiple spin changes are predicted to be involved in the low-energy reaction pathway. The present results show good agreement with the experimental observation and provide a detailed picture for the oxygen atom transfer reaction induced by the (oxo)manganese(V) corroles.

Triaquabis{2-methoxy-6-[(phenyliminiumyl)methyl]phenolate-κO1}manganese(II) dinitrate

The crystal structure of the title compound, [Mn(C14H13NO2)2(H2O)3](NO3)2, is comprised of two Schiff base 2-meth­oxy-6-(N-phenyl­carboximido­yl)phenol (HL) ligands and three coordinated water mol­ecules. The MnII ion lies on a twofold axis that bis­ects one water O atom. The coordination sphere of the five-coordinate Mn atom is completed by the two monodentate HL ligands and three coordinated water mol­ecules binding through their O atoms, affording a distorted tetra­gonal–pyramidal geometry. In the phenolate ligands, the hy­droxy H atom transfers to the imine N atom. This H atom is also involved in an intra­molecular N—H⋯O hydrogen bond that imposes a nearly planar conformation on each ligand, with dihedral angles of 2.78 (3) and 2.43 (5)° between the aromatic rings of each ligand. In the crystal, mol­ecules are linked by inter­molecular O—H⋯O hydrogen bonds.

A Novel Five-coordinate Mn Complex with a Redox-active SNNNS Ligand System

Synthesis and characterization of a manganese complex of a new redox-active diaminocyclohexane derivative of a thiourea-based ligand with SNNNS donor atom set is reported. The molecular structure of the complex exhibits a pentadentate SNNNS ligand coordinated in the neutral form to the Mn(II) center. The coordination around the central metal is best described as distorted trigonal bipyramidal with the pyridine nitrogen and the two sulfur atoms being in the equatorial plane and the remaining two nitrogen donors occupying the apical positions of the bipyramid. Cyclic voltammetry in DMSO shows the MnII/MnIII redox couple with the oxidation peak at −0.199 V and the reduction peak at −0.587 V vs. Fc/Fc+ (scan rate 0.5 V s−1). The ligand itself possesses redox activity and exhibits prominent oxidation and reduction peaks at −1.194 and −1.481 V vs. Fc/Fc+, respectively. The results of electrochemical experiments under oxygen atmosphere suggest that upon reaction with electrochemically generated superoxide the redox-active ligand is modified causing decomposition of the complex.

A new aquamanganese(II) oxalate phosphate, Mn(C2O4)Mn3(PO4)2(H2O)2

The title salt, diaquatetra­manganese(II) oxalate bis[ortho­phos­phate(V)], Mn4(C2O4)(PO4)2(H2O)2, was synthesized hydro­thermally and displays a three-dimensional framework structure. The asymmetric unit consists of two different MnII centers, half of an oxalate anion, a phosphate group and a coordinated water mol­ecule. A crystallographic inversion center is located at the mid-point of the oxalate C—C bond. The distorted octa­hedral MnO6 and the tetra­gonal pyramidal MnO5 centers are linked through bridging oxalate and phosphate groups. The water mol­ecule also has a weaker bonding contact to the five-coordinate Mn atom, which consequently exhibits a distorted octa­hedral geometry and also bridges the independent Mn atoms. The water mol­ecule is a donor for intra- and inter­molecular O—H⋯O hydrogen bonds.

Highly Stable Five-Coordinated Mn(II) Polymer [Mn(Hbidc)]n (Hbidc=1H-Benzimidazole-5,6-dicarboxylate): Crystal Structure, Antiferromegnetic Property, and Strong Long-Lived Luminescence

A novel five-coordinated Mn(II) polymer 1H-benzimidazole-5,6-dicarboxylato manganese (II) ([Mn(Hbidc)]n, 1) possessing abundant hydrogen bonds and π−π stacking interactions, synthesized by the hydrothermal reaction between MnSO4 and a multifunctional organic aromatic ligand H3bidc, displays antiferromagnetic character, strong long-lived red luminescence (λmax = 726 nm and τ = 0.3 ms) as well as high thermal stability (up to ca. 500 °C).

Syntheses, structures and magnetic properties of Mn(II) containing 3D polymeric networks

Two novel 3D coordination polymers {[Mn(aip)(DMF)]} n , CPO-9, and {[Mn 3(Hatp) 2(atp) 2](H 2O) 2(DEF) 4} n CPO-10 (aip = 5-aminoisophthalate, atp = 2-aminoterephthalate, DMF = dimethylformamide, DEF = diethylformamide) have been synthesized by solvothermal methods. Their properties have been studied by single-crystal X-ray diffraction, thermogravimetric analysis, high-temperature powder X-ray diffraction and magnetic susceptibility measurements. The crystal structure of CPO-9 is based on infinite chains of carboxylato-bridged five-coordinated Mn(II) ions that are crosslinked via the aip ligands to form a 3D structure. CPO-10 is based on linear trinuclear building units of carboxylato-bridged octahedral Mn(II) ions that are crosslinked by the atp ligands into a 3D structure. Both compounds have 1D channels that contain solvent molecules. The solvent accessible void volume for CPO-10 is 51.9% of the unit cell volume. For both compounds, however, the solvent molecules cannot be removed without the collapse of the structures into amorphous phases at 250 °C. The magnetic susceptibility measurements indicate antiferromagnetic couplings between the Mn(II) ions in both compounds. The magnetic data have been fitted using theoretical approaches.

A Rare μ-Hydroxo-Bridged Species. Synthesis, Structure, and Properties of μ-Hydroxo(tetraphenylporphyrinatomanganese(III))(phthalocyaninato(azido)chromium(III)), [(TPP)Mn−O(H)−CrPc(N3)

A novel ditetrapyrrolic, heteroleptic, and heterometallic (Mn-Cr) mu-hydroxo-bridged complex has been prepared, and its structural and general properties have been studied. The species mu-hydroxo(tetraphenylporphyrinatomanganese(III))(phthalocyaninato(azido)chromium(III)), [(TPP)Mn-O(H)-CrPc(N3)], isolated as a chloronaphthalene (ClNP) solvate, has been structurally characterized by single-crystal X-ray work. The two (TPP)Mn and CrPc(N3) fragments are held together by the bridging mu-hydroxo ion with long Mn-O [1.993(5) A] and Cr-O [1.976(5) A] bond distances and a Mn-O(H)-Cr angle of 163.7(3) degrees . The five-coordinate Mn center in the (TPP)Mn fragment is displaced from the TPP rigorously planar central N4 core by 0.128 A, and the environment is typical of a Mn(III) high-spin site. The six-coordinate Cr(III) in the CrPc(N3) moiety lies practically in the plane of the phthalocyanine macrocycle (displacement toward the azido group: 0.054 A). The average Mn-N(pyr) and Cr-N(pyr) bond distances are 2.011(6) and 1.982(6) A, respectively, and the Mn-Cr bond distance is 3.929(2) A. The porphyrin and phthalocyanine rings are in an almost eclipsed position [5.16(2) degrees ], and the mean planes of the two macrocycles form a dihedral angle of 5.79(4) degrees. Crystal data for [(TPP)Mn-O(H)-CrPc(N3)].2ClNP, C76H45CrMnN15O.2C10H7Cl: a = 16.645(3) A, b = 17.692(4) A, c = 25.828(5) A, alpha = 90 degrees , beta = 98.79(3) degrees , gamma = 90 degrees , space group P2(1)/c (No. 14), V = 7517(3) A(3), Z = 4, R1 = 0.086, and wR2 = 0.267. IR and UV-vis-near-IR spectral and room temperature magnetic susceptibility data of the [Mn-Cr] species are also presented.

C 3N 2H 12] · [MnAl 3P 4O 17] · [H 3O]: A manganese (II)-substituted aluminophosphate with zeotype AFN topology

Using 1,3-propyldiamine (1,3-PDA) as the structure-directing agent, a new manganese (II)-substituted aluminophosphate [C 3N 2H 12] · [MnAl 3P 4O 17] · [H 3O] (denoted MnAPO-14-PDA) containing five-coordinated Mn 2+ ions has been hydrothermally synthesized. The structure has been solved by single-crystal X-ray diffraction analysis and further characterized by X-ray powder diffraction, ICP and TG analyses. Differently from our recently reported MnAPO-14-DABCO built up from MnO 4(H 2O) 2 octohedra, AlO 4 and PO 4 tetrahedra, MnAPO-14-PDA is made of MnO 5 trigonal bipyramid, AlO 6 octohedra, AlO 4 and PO 4 tetrahedra. Its framework is analogous to aluminophosphate zeotype AFN in which 25% of the aluminum sites are replaced by Mn(II) atoms. The diprotonated 1,3-PDA cations and protonated H 2O molecules reside in the 8-membered ring (MR) channels. Computational simulations indicate that the substitution sites and coordination modes of Mn atoms in the framework are determined by the structure-directing agent (SDA) through the host-guest interactions. Crystal data: [C 3N 2H 12] · [MnAl 3P 4O 17] · [H 3O], triclinic P-1 (No.2), a = 9.799(3) Å, b = 9.799(3) Å, c = 11.203(4) Å, α = 72.708(4)°, β = 72.708(4)°, γ = 88.11(0)°, Z = 2, R 1 = 0.0775 ( I > 2 σ( I)) and wR 2 = 0.2079 (all data).

Synthesis, Characterization, and Structures of Mn(DMHP)3·12H2O and Mn(DMHP)2Cl·0.5H2O

This report describes the synthesis, characterization, and X-ray crystal structures of two Mn(III) complexes, Mn(DMHP)3 x 12H2O and Mn(DMHP)2Cl x 0.5H2O (DMHP = 1,2-dimethyl-3-hydroxy-4-pyridinone). Mn(DMHP)2Cl was prepared from the reaction of Mn(II) chloride with 2 equiv of DMHP under reflux in the presence of triethylamine. Mn(DMHP)3 was obtained by reacting Mn(II) acetate with 3 equiv of DMHP in the presence of sodium acetate. Mn(DMHP)3 could also be prepared by reacting Mn(OAc)3 x 2H2O with 3 equiv of DMHP in the presence of triethylamine. Both Mn(III) complexes have been characterized by elemental analysis, infrared spectroscopy, electronic paramagnetic resonance, electrospray ionization spectroscopy, electrochemical method, and X-ray crystallography. The X-ray crystal structure of Mn(DMHP)2Cl x 0.5H2O revealed a rare example of five-coordinated Mn(III) complexes with two bidentate ligands and a square pyramidal coordination geometry. Surprisingly, the average Mn-O (hydroxy) bond distance in Mn(DMHP)2Cl x 0.5H2O is approximately 0.025 A longer than that of the average Mn-O (carbonyl) bond, suggesting an extensive delocalization of electrons in the two pyridinone rings. The structure of Mn(DMHP)3 x 12H2O, a rare example of six-coordinate high-spin Mn(III) complexes without Jahn-Teller distortion, is isostructural to M(DMHP)3 x 12H2O (M = Al, Ga, Fe, and In). The electrochemical data for Mn(DMHP)3 suggests that the Mn(III) oxidation state is highly stabilized by three DMHP ligands. DMHP has the potential as a chelator for the removal of excess intracellular Mn and the treatment of chronic Mn toxicity.