Five-coordinate Mn Research Articles

The manganese complex of oxygenic photosynthesis: conversion of five-coordinated Mn(III) to six-coordinated Mn(IV) in the S2-S3 transition is implied by XANES simulations

Photosynthetic water oxidation proceeds at a tetra-manganese complex bound to a cofactor-protein complex denoted as Photosystem II (PSII). To understand this process of fundamental importance for life on Earth, the crucial changes of the active-site occurring upon the individual transitions in the catalytic S-state cycle have been investigated by x-ray absorption spectroscopy at the Mn K-edge. For the S2-S3 transition, the changes in position and shape of the x-ray edge have been interpreted to be either associated with oxidation of Mn(III) to Mn(IV) or to formation of a ligand radical without manganese oxidation. Here we simulate XANES spectra of simple Mn-complexes using modern multiple-scattering code. Features of the XANES spectra are assigned to molecular orbitals by means of comparative semi-empirical calculations. It is concluded that the changes in position and shape of the edge spectra observed for the S2-S3 transition are explainable by the transformation of five-coordinated Mn(III) in a square-pyramidal geometry to six-coordinated Mn(IV).

Spectroscopic and computational studies of the azide-adduct of manganese superoxide dismutase: definitive assignment of the ligand responsible for the low-temperature thermochromism.

A variety of spectroscopic and computational techniques have been used to examine the thermochromic transition previously reported for the oxidized state of Mn-dependent superoxide dismutase from E. coli in the presence of substrate analog azide (N(3)-Mn(3+)SOD).[Whittaker, M. M.; Whittaker, J. W. Biochemistry 1996, 35, 6762-6770.] Although previous spectroscopic studies had shown that this thermochromic event corresponds to a change in coordination number of the active-site Mn(3+) ion from 6 to 5 as temperature is increased, the ligand that dissociates in this conversion had yet to be identified. Through the use of electronic absorption, circular dichroism (CD), and magnetic CD (MCD) spectroscopies, both d-->d and ligand-to-metal charge-transfer (LMCT) transition energies have been determined for native Mn(3+)SOD (possessing a five-coordinate Mn(3+) center) and Y34F N(3)-Mn(3+)SOD (forming a six-coordinate N(3)-Mn(3+) adduct at all temperatures). These two systems provide well-defined reference points from which to analyze the absorption and CD data obtained for N(3)-Mn(3+)SOD at room temperature (RT). Comparison of excited-state spectroscopic data reveals that Mn(3+)SOD and RT N(3)-Mn(3+)SOD exhibit virtually identical d-->d transition energies, suggesting that these two species possess similar geometric and electronic structures and, thus, that azide does not actually coordinate to the active-site Mn(3+) ion at RT. However, resonance Raman spectra of both N(3)-Mn(3+)SOD and Y34F N(3)-Mn(3+)SOD at 0 degrees C exhibit azide-related vibrations, indicating that azide does interact with the active site of the native enzyme at this temperature. To gain further insight into the nature of the azide/Mn(3+) interaction in RT N(3)-Mn(3+)SOD, several viable active-site models designed to promote either dissociation of coordinated solvent, Asp167, or azide were generated using DFT computations. By utilizing the time-dependent DFT method to predict absorption spectra for these models of RT N(3)-Mn(3+)SOD, we demonstrate that only azide dissociation is consistent with experimental data. Collectively, our spectroscopic and computational data provide evidence that the active site of N(3)-Mn(3+)SOD at RT exists in a dynamic equilibrium, with the azide molecule either hydrogen-bonded to the second-sphere Tyr34 residue or coordinated to the Mn(3+) ion. These results further highlight the role that second-sphere residues, especially Tyr34, play in tuning substrate (analog)/metal ion interactions.

Structural characterization and electronic properties determination by high-field and high-frequency EPR of a series of five-coordinated Mn(II) complexes.

The isolation, structural characterization, and electronic properties of a series of high-spin mononuclear five-coordinated Mn(II) complexes, [Mn(terpy)(X)(2)] (terpy = 2, 2':6',2' '-terpyridine; X = I(-) (1), Br(-) (2), Cl(-) (3), or SCN(-) (4)), are reported. The X-ray structures of the complexes reveal that the manganese ion lies in the center of a distorted trigonal bipyramid for complexes 1, 2, and 4, while complex 3 is better described as a distorted square pyramid. The electronic properties of 1-4 were investigated by high-field and high-frequency EPR spectroscopy (HF-EPR) performed between 5 and 30 K. The powder HF-EPR spectra have been recorded in high-field-limit conditions (95-285 GHz) (D << gbetaB). The spectra are thus simplified, allowing an easy interpretation of the experimental data and an accurate determination of the spin Hamiltonian parameters. The magnitude of D varies between 0.26 and 1.00 cm(-)(1) with the nature of the anionic ligand. Thanks to low-temperature EPR experiments, the sign of D was unambiguously determined. D is positive for the iodo and bromo complexes and negative for the chloro and thiocyano ones. A structural correlation is proposed. Each complex is characterized by a significant rhombicity with E/D values between 0.17 and 0.29, reflecting the distorted geometry observed around the manganese. Finally, we compared the spin Hamiltonian parameters of our five-coordinated complexes and those previously reported for other analogous series of dihalo four- and six-coordinated complexes. The effect of the coordination number and of the geometry of the Mn(II) complexes on the spin Hamiltonian parameters is discussed.

A Mn12 single-molecule magnet [Mn12O12(OAc)12(dpp)4] (dppH = diphenyl phosphate) with no coordinating water molecules.

The preparation and physical characterization are reported for a novel single-molecule magnet [Mn(12)O(12)(OAc)(12)(dpp)(4)] (dppH = diphenyl phosphate) with no coordinating water molecules. The crystal structure analysis reveals that there are four five-coordinate Mn(III) ions with Mn.H approaches. Addition of water in CD(2)Cl(2) solution was monitored by (1)H NMR, which showed that H(2)O could coordinate to a vacant site of a five-coordinate Mn(III) ion in solution. The measurements and analyses of magnetization hysteresis and ac magnetic susceptibility indicate that the title complex is a single-molecule magnet with a quantum tunneling behavior, whose ground state was tentatively assigned to S = 10 with g = 1.78 and D = -0.60 K.

Synthesis, structures, and magnetic properties of novel mononuclear, tetranuclear, and 1D chain Mn(III) complexes involving three related asymmetrical trianionic ligands.

The manganese(III) complexes studied in this report derive from asymmetrical trianionic ligands abbreviated H(3)L(i) (i = 4-6). These ligands are obtained through reaction of salicylaldehyde with "half-units", the latter resulting from monocondensation of different diamines with phenylsalicylate,. Upon deprotonation, L(i) (i = 4-6) possess an inner N(2)O(2) coordination site with one amido, one imine, and two phenoxo functions, and an outer amido oxygen donor. The trianionic character of such ligands yields original neutral complexes with the L/Mn stoichiometry. The crystal and molecular structures of three complexes have been determined at 190 K (1) or 180 K (2 and 3). Complex 1 crystallizes in the triclinic space group P (No. 2): a = 7.8582(14) A, b = 10.9225(16) A, c = 12.4882(18) A, alpha = 67.231(14) degrees, beta = 72.134(14) degrees, gamma = 82.589(13) degrees, V = 940.6(3) A(3), Z = 2. Complex 2 crystallizes in the orthorhombic space group Pbcn (Nuomicron. 60): a = 23.8283(15) A, b = 11.1605(7) A, c = 26.152(2) A, V = 6954.8(8) A(3), Z = 8, while complex 3 crystallizes in the monoclinic space group P2(1)/c (No. 14) with a = 11.7443(14) A, b = 7.5996(10) A, c = 18.029(2) A, beta = 100.604(10) degrees, V = 1581.6(3) A(3), Z = 4. Owing to hydrogen bonds and pi-pi stackings, the mononuclear neutral molecules of 1 are arranged in a 2D network while complexes 2 and 3 are tetranuclear and polymeric (1D chain) species, respectively, owing to the bridging ability of the oxygen atom of the amido function. The experimental magnetic susceptibilities of complexes 2 and 3 indicate the occurrence of similarly weak Mn(III)-Mn(III) antiferromagnetic interactions (J = -1.1 cm(-1)). Single ion zero-field splitting of manganese(III) must be taken into account for satisfactorily fitting the data by exact calculation of the energy levels associated to the spin Hamiltonian through diagonalization of the full matrix for axial symmetry in 2 (J = - 1.1 cm(-1), D(1) = 2.2 cm(-1), D(2) = -2.8 cm(-1)), D(1) and D(2) being associated to the six- and five-coordinate Mn ions, respectively. A weaker antiferromagnetic interaction (J = - 0.2 cm(-1)) operates through pi-pi stacking in complex 1. Complex 3 is a weak ferromagnet (ordering temperature approximately 7 K) as a result of the spin canting originating from the crystal packing.

Synthesis, crystal structure and magnetic properties of novel Mn12 single-molecule magnets with thiophenecarboxylate, [Mn12O12(O2CC4H3S)16(H2O)4], and its tetraphenylphosphonium salt

The preparation and physical characterization are reported for novel Mn12 single-molecule magnets having thiophenecarboxylate bridges, [Mn12O12(O2CC4H3S)16(H2O)4] (1), and its PPh4 salt (2). The reaction of the excess amount of 2-thiophenecarboxylic acid (tpcH) and [Mn12O12(OAc)16(H2O)4] in CH2Cl2 gave black crystals of 1, which could be reduced by PPh4I to give 2. From the crystal structure analysis of 2, it is revealed that there are two five-coordinated Mn ions, one of which is assigned to a MnII ion being in the vicinity of the tetraphenylphosphonium cation. Both complexes exhibit out-of-phase a.c. magnetic susceptibility (χ″M) signals in the 5.0–6.5 K range at 997 Hz a.c. frequency, which indicate that they are single-molecule magnets. From Arrhenius plots of the frequency dependence of the temperature of the χ″M peaks, the effective energy barriers Ueff were estimated to be 69 and 57 K for high-temperature phases of 1 and 2, respectively, and 40 K for the low-temperature phase of 1. The reduced magnetization measurement and its analysis indicate that 2 has S=19/2 ground state with g=1.99 and D=−0.61 K.

Aggregate manganese Schiff base moieties by terephthalate or acetate: dinuclear manganese and trinuclear mixed metal Mn2/Na complexes.

A reaction system consisting of terephthalic acid, NaOH, inorganic Mn(II) or Mn(III) salt, and salicylidene alkylimine resulted in dinuclear manganese complexes (salpn)(2)Mn(2)(mu-phth)(CH(3)OH)(2) (1, salpn = N,N'-1,3-propylene-bis(salicylideneiminato); phth = terephthalate dianion), (salen)(2)Mn(2)(mu-phth)(CH(3)OH)(2) (2, salen = N,N'-ethylene-bis(salicylideneiminato)), (salen)(2)Mn(2)(mu-phth)(CH(3)OH)(H(2)O) (3), and (salen)(2)Mn(2)(mu-phth) (4), while the absence of NaOH in the reaction led to a mononuclear Mn complex (salph)Mn(CH(3)OH)(NO(3)) (5, salph = N,N'-1,2-phenylene-bis(salicylideneiminato)). In addition, a trinuclear mixed metal complex H[Mn(2)Na(salpn)(2)(mu-OAc)(2)(H(2)O)(2)](OAc)(2) (6) was obtained from the reaction system by using maleic acid instead of terephthalic acid. Five-coordinate Mn ions were found in 4 giving rise to an intermolecular interaction and constructing a one-dimensional linear structure. Antiferromagnetic exchange interactions were observed for 1-3, and a total ferromagnetic exchange of 4 was considered to stem from intermolecular magnetic coupling. (1)H NMR signals of phenolate ring and alkylene (or phenylene) backbone of the diamine are similar to those reported in the literature, and the phth protons are at -2.3 to -10.1 ppm. Studies on structure, bond valence sum analysis, and magnetic properties indicate the oxidation states of the Mn ions in 6 to be +3, which are also indicated by ESR spectra in dual mode. Ferromagnetic exchange interaction between the Mn(III) sites was observed with J = 1.74 cm(-1). A quasireversible redox pair at -0.29V/-0.12V has been assigned to the redox of Mn(2)(III)/Mn(III)Mn(II), implying the intactness of the complex backbone in solution.

Trends in metal-biradical exchange interaction for first-row M(II)(nitronyl nitroxide-semiquinone) complexes.

We report molecular structures and temperature-dependent magnetic susceptibility data for several new metal complexes of heterospin triplet ground-state biradical ligands. The ligands are comprised of both nitronyl-nitroxide (NN) and semiquinone (SQ) spin carriers. Five compounds are five-coordinate M(II) complexes (M = Mn, Co, Ni, Cu, and Zn), and one is a six-coordinate Ni(II) complex. Five compounds were structurally characterized. During copper complex formation a reaction with methanol occurs to form a unique methoxy-substituted SQ ring. Variable-temperature magnetic susceptibility studies are consistent with strong intraligand (NN-SQ and NN-PhSQ) ferromagnetic exchange coupling. For the five-coordinate Mn, Co, and Ni complexes, the S = 1 ligand is antiferromagnetically coupled to the metal. For both the five-coordinate Cu complex and the six-coordinate Ni complex, the ligand is ferromagnetically coupled to the metal spins in accordance with orbital symmetry arguments. Despite the low molecular symmetries, the predicted trend in metal-ligand exchange interactions is supported by spin dimer analysis based on extended Hückel calculations. For (NN-SQ)NiTp(Cum,Me)() (Tp(Cum,Me)() = hydro-tris(3-cumenyl-5-methylpyrazolyl)borate), an antisymmetric exchange term was required for the best fit of the magnetic susceptibility data. Antisymmetric exchange was less important for the other complexes due to inherently smaller Deltag. Finally, it is shown that intraligand exchange coupling is of paramount importance in stabilizing high-spin states of mixed metal-biradical complexes.

Crystal structure of a genomically encoded fosfomycin resistance protein (FosA) at 1.19 A resolution by MAD phasing off the L-III edge of Tl(+).

The fosfomycin resistance protein (FosA) catalyzes the Mn(II)- and K+-dependent addition of glutathione to the oxirane of the antibiotic fosfomycin. The crystal structure of FosA from Pseudomonas aeruginosa was solved at a resolution of 1.19 A by multiwavelength anomalous diffraction at the L-III edge of a Tl+ derivative. The structure solution took advantage of the ability of Tl+ to substitute for K+. The existence of multiple Tl sites in the asymmetric unit suggests that this may be a generally useful technique for phasing protein crystal structures. A 1.35 A resolution structure with phosphate bound in the active site shows that the Mn(II) center has a rare four-coordinate geometry. The structure of the fosfomycin complex at 1.19 A resolution indicates that the Mn(II) center is close to five-coordinate with trigonal bipyramidal geometry and a ligand set consisting of two histidines (H7 and H64) and one phosphonate oxygen occupying the equatorial sites and the carboxylate of E110 at one of the apical sites. The oxirane oxygen of the substrate is located at the other apical site but is 0.2 A beyond the average Mn-O distance for five-coordinate Mn(II). The Mn(II) center is proposed to stabilize the alkoxide in the transition state, while the nearby hydroxyl group of T9 acts as a proton donor in the reaction. The K+ ion located 6.5 A from the Mn(II) appears to help orient the substrate for nucleophilic attack.

Preparation, crystal structure and chelate substitution reactions of [Mn 4O 2(O 2CPh) 6(dpm) 2] (dpm=the anion of dipivaloylmethane)

The reaction of [N n Bu 4][Mn 4O 2(O 2CPh) 9(H 2O)] with 2 equiv. of Na(dpm) (dpm − is the anion of dipivaloylmethane) gives [N n Bu 4][Mn 4O 2(O 2CPh) 7(dpm) 2] ( 1), which undergoes chelate substitution with picolinic acid (picH) or dibenzoylmethane (dbmH) to give the corresponding pic − and dbm − complexes, 2 and 3, respectively. Reaction of 1 with 1 equiv. of Me 3SiCl converts it to [Mn 4O 2(O 2CPh) 6(dpm) 2] ( 4), whose crystal structure shows a butterfly-like [Mn 4O 2] 8+ core but with two five-coordinate Mn III ions. Compound 4 also undergoes chelate substitution with picH, dbmH, 2-(hydroxymethyl)pyridine and hexafluoroacetylacetone to give the corresponding complexes. With trans-1,2- bis(4-pyridyl)ethylene (bpe), a bridged compound [Mn 4O 2(O 2CPh) 6(dpm) 2(bpe)] 2 is obtained, analogous to the dbm − compound previously characterized crystallographically. Magnetic susceptibility studies of 4 in the 5.00–300 K range give J bb=−27.5 cm −1, J wb=−0.4 cm −1 ( H ̂ =−2J S ̂ i S ̂ j convention), and g=1.87, indicating 4 to have a quintuply degenerate ( n, 0, n) ground state ( n=4, 3, 2, 1, 0) as a result of the large J bb/ J wb ratio.

Characterization of a dinuclear Mn(II) tri(μ-carboxylato) complex with the hindered hydrotris(3,5-diisopropyl-1-pyrazolyl)borate (=Tp iPr 2) ligand: intramolecular hydrogen bonding interaction between the protonated Tp iPr 2 and Mn-coordinating carboxylate ligands

A dinuclear Mn(II) di(μ-hydroxo) complex having hydrotris(3,5-diisopropyl-1-pyrazolyl)borate (=Tp iPr 2 ) reacted with benzoic acid to yield a dinuclear Mn(II) tri(μ-carboxylato) complex, Tp iPr 2 Mn-(μ-OBz) 3-Mn(Tp iPr 2 H). X-ray crystallography reveals the unsymmetrical coordination environments for the manganese centers. One of the two Tp iPr 2 ligands, which bound to the five-coordinated Mn center, is protonated by the action of the third carboxylic acid and the resulting non-Mn-binding N–H moiety forms an intramolecular hydrogen bond with the oxygen donor of a carboxylate ligand. Steric congestion in the bimetallic core results in the large separation of the manganese centers bridged by the syn- anti carboxylate ligand.

Molecular structure and spectral properties of bis(2,6-dimethoxybenzoato)(2,2′:6′,2″-terpyridine)manganese(II): a five-coordinate Mn(II) complex

Download options Please wait... Supplementary files Crystal structure data TXT (24K) Article information DOI https://doi.org/10.1039/B002119I Article type Paper Submitted 14 Mar 2000 Accepted 03 Jul 2000 First published 14 Aug 2000 Download Citation New J. Chem., 2000,24, 725-728 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Molecular structure and spectral properties of bis(2,6-dimethoxybenzoato)(2,2′: 6′,2″-terpyridine)manganese(II): a five-coordinate Mn(II) complex L. S. Erre, G. Micera, E. Garribba and A. Cs. Bényei, New J. Chem., 2000, 24, 725 DOI: 10.1039/B002119I To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Liliana Strinna Erre Giovanni Micera Eugenio Garribba Attila Cs. Bényei

Structural and Magnetochemical Properties of Mono-, Di-, and Trinuclear Manganese(III) Dithiolate Complexes.

Aerial oxidation of Mn(II)/ptt(3)(-) (ptt(3)(-) = propane-1,2,3-trithiolate) mixtures gives [Mn(2)(pttd)(2)](2)(-), where pttd(4)(-) is the mono(disulfide) of ptt(3)(-). (NEt(3)Bz)(2)[Mn(2)(pttd)(2)] (2) crystallizes in space group P2(1)/c with (at -158 degrees C) a = 11.540(2) Å, b = 12.115(2) Å, c = 17.478(4) Å, beta = 101.78(1) degrees, and Z = 2. The anion contains a doubly-bridged [Mn(2)S(8)] core (Mn.Mn = 3.598(2) Å) with five-coordinate Mn(III) ions, very similar to previously reported [Mn(2)(edt)(4)](2)(-) (anion of 1; edt(2)(-) = ethane-1,2-dithiolate). Aerial oxidation of Mn(II)/pdt(2)(-) (pdt(2)(-) = propane-1,3-dithiolate) mixtures gives [Mn(3)(pdt)(5)](2)(-), which is mixed valent (Mn(II), 2Mn(III)). (PPh(4))(2)[Mn(3)(pdt)(5)] (3) crystallizes in space group P&onemacr; with (at -161 degrees C) a = 14.385(6) Å, b = 23.734(11) Å, and Z = 2. The anion contains a near-linear Mn(III)Mn(II)Mn(III) unit with five-coordinate Mn(III), six-coordinate Mn(II), and three thiolate bridges between each Mn(2) pair; Mn.Mn separations are 3.123(3) and 3.101(3) Å. Aerial oxidation of Mn(II)/edt(2)(-)/ImH (ImH = imidazole) mixtures gives [Mn(edt)(2)(ImH)](-). (NEt(4))[Mn(edt)(2)(ImH)] (4) crystallizes in space group P2(1)/n with (at -72 degrees C) a = 13.974(5) Å, b = 14.317(5) Å, c = 10.564(3) Å, beta = 90.13(2) degrees, and Z = 4. The anion is five-coordinate and square-pyramidal. Aerial oxidation of Mn(II)/edt(2)(-)/Im(-) mixtures gave [Mn(2)(Im)(edt)(4)](3)(-), which contains two Mn(III) ions. (NMe(4))(3)[Mn(2)(Im)(edt)(4)] (5) crystallizes in space group Pna2(1) with (at -160 degrees C) a = 17.965(5) Å, b = 16.094(4) Å, c = 14.789(3) Å, and Z = 4. The five-coordinate Mn(III) ions are bridged by the Im(-) group across a Mn.Mn separation of 6.487(2) Å. The anion of 4 contains high-spin Mn(III) (S = 2) and exhibits inter-anion antiferromagnetic exchange interactions (J = -0.15 cm(-)(1), g = 1.91) propagated by interanion NH.S hydrogen bonds. Complexes 1-3 and 5 all possess intraanion antiferromagnetic exchange interactions; the fitting parameters are as follows: 1, J = -19.0 cm(-)(1), g = 1.96, D = -0.22 cm(-)(1); 2, J = -16.4 cm(-)(1), g = 1.96, D = -0.22 cm(-)(1); 3, J = -18.8 cm(-)(1), g = 2.00; 5, J = -1.75 cm(-)(1), g = 1.84, D = -0.028 cm(-)(1) (H = -2JS(i)()S(j)() convention). Complexes 1, 2, and 5 have S = 0 ground states, while that of 3 is S = (3)/(2).

Monomeric and Trimeric Manganese(III) Complexes of 2-Hydroxy-5,10,15,20-tetraphenylporphyrin. Synthesis and Characterization.

The hydrolysis of the monomeric five-coordinate (2-BzO-TPP)Mn(III)Cl complex has been investigated.(1) Evidence for the formation of the cyclic trimeric complex [(2-O-TPP)Mn(III)](3) is presented. The (1)H NMR spectroscopic evidence indicates that the trimeric manganese(III) complex has a head-to-tail cyclic trimeric structure with the pyrrolic alkoxide groups forming bridges from one macrocycle to the manganese(III) ion in the adjacent macrocycle PMn-O-PMn-O-PMn-O. The three manganese(III) porphyrin subunits are not equivalent. The characteristic upfield shift of the 3-H pyrrole resonance (-111.5 ppm at 291 K) was determined and considered as the diagnostic feature for the high-spin d(4) manganese(III)-pyrrole alkoxide coordination. The strong upfield shift of the 3-H resonance has been accounted for by the donation of the electron density from the filled orbital of the 2-O atom on the half-occupied d(z)()()2 orbital of the external manganese(III) ion. The other pyrrole resonances produce the complex multiplet at the typical -5 to -40 ppm region. The (1)H NMR spectra of the series of monomeric 2-substituted manganese(III) 5,10,15,20-tetraphenylporphyrin complexes (2-X-TPP)Mn(III)Cl have been obtained and analyzed. The pattern of the assigned seven pyrrole resonances reflects the asymmetry imposed by 2-substitution and has been used as a (1)H NMR spectroscopic probe to map the spin density distribution. The electronic effect is strongly localized at the beta-substituted pyrrole. The upfield shift of the 3-H resonance increases in the order (2-NO(2)-TPP)Mn(III)Cl < (2-BzO-TPP)Mn(III)Cl < (2-OCH(3)-TPP)Mn(III)Cl < (2-OH-TPP)Mn(III)Cl < (2-NH(2)-TPP)Mn(III)Cl < [(2-O-TPP)Mn(III)(OH)](-) following the increasing electron-donating properties of the beta-substituent.

Air Oxidation of a Five-Coordinate Mn(III) Dimer to a High-Valent Oxomanganese(V) Complex

Air Oxidation of a Five-Coordinate Mn(III) Dimer to a High-Valent Oxomanganese(V) Complex

Methanol bonding to and electron-transfer reactivity of chloro(tetraphenylporphinato)manganese(III)

The ability of axial ligation to modulate redox potentials and electrode half-reactions of metalloporphyrins is illustrated by coordination of methanol to Mn(TPP)Cl (1) and its reduced and oxidized products in 1,2-dichloroethane. Visible spectrophotometry, spectroelectrochemistry, and cyclic and microelectrode voltammetry are used to characterize reactions whereby six-coordinate Mn(III) (Mn(TPP)(CH{sub 3}OH)Cl (3) and Mn(TPP)(CH{sub 3}OH){sub 2}{sup +} (4)) and five-coordinate Mn(II) (Mn(TPP)(CH{sub 3}OH)(6)) species are formed from 1. The axial ligand preferences of these two oxidation states lead to a biphasic behavior in the potential of Mn{sup III/II} reduction. At low CH{sub 3}OH concentrations, five-coordinate 1 is reduced to five-coordinate 6, causing E{sub 1/2} to shift to positive values with increasing (CH{sub 3}OH); at larger methanol concentrations, reduction of 4 to 6 predominates and the shift is in the opposite direction. In addition, the ring- and metal-centered oxidations of 1 at 1.22 and 1.56 V vs Ag/AgCl, respectively, coalesce into a single two-electron process at 1.31 V upon CH{sub 3}OH coordination. The negative shift in the metal-centered oxidation potential is an anticipated consequence of the greater donor strength of two CH{sub 3}OH ligands versus one Cl{sup {minus}} ligand. The positive shift in the ring-centered potential is believed to result from reduced deformationmore » of the porphyrin core and consequent stabilization of the porphyrin HOMO as axial coordination by CH{sub 3}OH returns the Mn atoms to an in-plate position from its significant out-of-plane displacement (0.27 {angstrom}) in 1.« less

Structure determination of CaMnO 3 and CaMnO 2.5 by X-ray and neutron methods

Low-temperature synthesis of the oxygen-deficient compound CaMnO 2.5 from polycrystalline CaMnO 3 preserves the existing structural framework of the oxidized precursor. The crystal structure of CaMnO 2.5 was determined using neutron powder diffraction data analyzed by the Reitveld profile refinement method. The structure of the reduced phase can be described by the orthorhombic distortion (a 0 + b 0 2 ) 2 = a c 2 1 2 , where a c is the simple cubic distance (∼3.7 Å) characteristic of the Mn 4+O 2−Mn 4+ framework in CaMnO 3. The unique features of the structure are five-coordinate Mn 3+ cations with nearly square pyramidal (∼ C 4 v ) coordination and ordered oxygen vacancies. The preparation, structure refinement, and noncubic distortions of single crystals of CaMnO 3 are also described. Attempts to transform single crystals of CaMnO 3 into CaMnO 2.5 by well-ordered topotactic changes have not been successful.