Temperature Dependence of the Electron Transfer in a Mn(II)Mn(III)(&mgr;-OH) Mixed-Valence Manganese Complex

The role of carboxylato ligand dissociation in the oxidation of chrysin with H₂O₂ catalysed by [Mn₂(III,IV)(μ-CH₃COO)(μ-O)₂(Me₄dtne)](PF₆)₂

The aqueous and non-aqueous chemistry of the complex [Mn2(III,IV)(μ-CH3COO)(μ-O)2(Me4dtne)](PF6)2 (where Me4dtne = 1,2-bis(4,7-dimethyl-1,4,7-triazacyclonon-1-yl)ethane), which has been demonstrated as an exceptionally active catalyst in the bleaching of raw cotton and especially wood pulp at high pH (>11), is explored by UV/vis absorption, Raman and EPR spectroscopies and cyclic voltammetry. The data indicate that dissociation of the μ-acetato bridge is essential to the catalyst activity and rationalises the effect of sequestrants such as DTPA on its performance.

Mechanism of Cis-Dihydroxylation and Epoxidation of Alkenes by Highly H2O2 Efficient Dinuclear Manganese Catalysts

In the presence of carboxylic acids the complex [Mn(IV)2(micro-O)3(tmtacn)2]2+ (1, where tmtacn = N,N',N''-trimethyl-1,4,7-triazacyclononane) is shown to be highly efficient in catalyzing the oxidation of alkenes to the corresponding cis-diol and epoxide with H2O2 as terminal oxidant. The selectivity of the catalytic system with respect to (w.r.t.) either cis-dihydroxylation or epoxidation of alkenes is shown to be dependent on the carboxylic acid employed. High turnover numbers (t.o.n. > 2000) can be achieved especially w.r.t. cis-dihydroxylation for which the use of 2,6-dichlorobenzoic acid allows for the highest t.o.n. reported thus far for cis-dihydroxylation of alkenes catalyzed by a first-row transition metal and high efficiency w.r.t. the terminal oxidant (H2O2). The high activity and selectivity is due to the in situ formation of bis(micro-carboxylato)-bridged dinuclear manganese(III) complexes. Tuning of the activity of the catalyst by variation in the carboxylate ligands is dependent on both the electron-withdrawing nature of the ligand and on steric effects. By contrast, the cis-diol/epoxide selectivity is dominated by steric factors. The role of solvent, catalyst oxidation state, H2O, and carboxylic acid concentration and the nature of the carboxylic acid employed on both the activity and the selectivity of the catalysis are explored together with speciation analysis and isotope labeling studies. The results confirm that the complexes of the type [Mn2(micro-O)(micro-R-CO2)2(tmtacn)2]2+, which show remarkable redox and solvent-dependent coordination chemistry, are the resting state of the catalytic system and that they retain a dinuclear structure throughout the catalytic cycle. The mechanistic understanding obtained from these studies holds considerable implications for both homogeneous manganese oxidation catalysis and in understanding related biological systems such as dinuclear catalase and arginase enzymes.

Structure and orientation of the Mn4Ca cluster in plant photosystem II membranes studied by polarized range-extended x-ray absorption spectroscopy

X-ray absorption spectroscopy has provided important insights into the structure and function of the Mn(4)Ca cluster in the oxygen-evolving complex of Photosystem II (PS II). The range of manganese extended x-ray absorption fine structure data collected from PS II until now has been, however, limited by the presence of iron in PS II. Using a crystal spectrometer with high energy resolution to detect solely the manganese Kalpha fluorescence, we are able to extend the extended x-ray absorption fine structure range beyond the onset of the iron absorption edge. This results in improvement in resolution of the manganese-backscatterer distances in PS II from 0.14 to 0.09A(.) The high resolution data obtained from oriented spinach PS II membranes in the S(1) state show that there are three di-mu-oxo-bridged manganese-manganese distances of approximately 2.7 and approximately 2.8A in a 2:1 ratio and that these three manganese-manganese vectors are aligned at an average orientation of approximately 60 degrees relative to the membrane normal. Furthermore, we are able to observe the separation of the Fourier peaks corresponding to the approximately 3.2A manganese-manganese and the approximately 3.4A manganese-calcium interactions in oriented PS II samples and determine their orientation relative to the membrane normal. The average of the manganese-calcium vectors at approximately 3.4A is aligned along the membrane normal, while the approximately 3.2A manganese-manganese vector is oriented near the membrane plane. A comparison of this structural information with the proposed Mn(4)Ca cluster models based on spectroscopic and diffraction data provides input for refining and selecting among these models.

High-field EPR investigations of Mn(III)Mn(IV) and Mn(II)Mn(III) states of dimanganese catalase and related model systems

Multi-frequency EPR experiments at 9, 34 and 94 GHz are reported on the antiferromagnetically coupled mixed valence Mn(II)Mn(III) complex of manganese catalase and on several dinuclear manganese model systems. They are compared with similar experiments obtained earlier for the Mn(III)Mn(IV) states. It is demonstrated how accurate information on the G- and 55Mn hyperfine tensors can be derived from this approach. Furthermore, the effect of oxidation state, planarity of the manganese-oxygen core and the type of ligands bridging the manganese ions on the magnetic resonance parameters and the related electronic structure is investigated. 'Broken-symmetry' density functional calculations on two Mn(III)Mn(IV) complexes, including the superoxidized state of the catalase, are presented. The agreement between calculated and experimental EPR parameters and complex geometries is remarkably good. Implications of these results for the structure and function of the dimanganese catalase are discussed.

What we can learn about fast chemical processes from slow diffraction experiments

The potential energy surface is an important determinant of a chemical reaction. Three ways of deducing non-trivial properties of such surfaces from the results of crystal structure analyses are discussed and illustrated with examples. (1) The mapping approach brings together structures of the same molecular fragment from different environments to outline reaction coordinates and vibrations. (2) Correlations between molecular structures and activation energies for a given reaction type reveal general and quantitative relations between seemingly independent entities such as ground state structure, force constants, reaction path length, activation energy and catalysis. (3) The evolution of atomic mean square amplitudes (displacement parameters) with temperature uncovers frequencies and atomic displacement patterns of large-amplitude vibrations in molecular crystals. Examples include the vibrations of molecular zeolite building blocks, the crankshaft motion of stilbenes, the dynamic coupling between pyramidal deformation of the amide NH2 group and hydrogen bonding, the bowl inversion of corannulenes and nucleophilic addition/elimination reactions.

The Mn cluster in the S(0) state of the oxygen-evolving complex of photosystem II studied by EXAFS spectroscopy: are there three Di-mu-oxo-bridged Mn(2) moieties in the tetranuclear Mn complex?

A key component required for an understanding of the mechanism of the evolution of molecular oxygen by the photosynthetic oxygen-evolving complex (OEC) in photosystem II (PS II) is the knowledge of the structures of the Mn cluster in the OEC in each of its intermediate redox states, or S-states. In this paper, we report the first detailed structural characterization using Mn extended X-ray absorption fine structure (EXAFS) spectroscopy of the Mn cluster of the OEC in the S(0) state, which exists immediately after the release of molecular oxygen. On the basis of the EXAFS spectroscopic results, the most likely interpretation is that one of the di-mu-oxo-bridged Mn-Mn moieties in the OEC has increased in distance from 2.7 A in the dark-stable S(1) state to 2.85 A in the S(0) state. Furthermore, curve fitting of the distance heterogeneity present in the EXAFS data from the S(0) state leads to the intriguing possibility that three di-mu-oxo-bridged Mn-Mn moieties may exist in the OEC instead of the two di-mu-oxo-bridged Mn-Mn moieties that are widely used in proposed structural models for the OEC. This possibility is developed using novel structural models for the Mn cluster in the OEC which are consistent with the structural information available from EXAFS and the recent X-ray crystallographic structure of PS II at 3.8 A resolution.

X-ray structure analysis and the intervalent electron transfer in organic mixed-valence crystals with bridged aromatic cation radicals

X-ray crystallography identifies the aromatic donor group D = 2,5-dimethoxy-4-methylphenyl to be a suitable redox center for the construction of organic mixed-valence crystals owing to its large structural change attendant upon 1e oxidation to the cation-radical (D*(+)). The combination of cyclic voltammetry, dynamic ESR line broadening, and electronic (NIR) spectroscopy allows the intervalence electron transfer between the redox centers in the mixed-valence system D-br-D*(+) [where br can be an aliphatic trimethylene or an aromatic (poly)phenylene bridge] to be probed quantitatively. Independent measures of the electronic coupling matrix element (H) for D/D*(+) electron exchange via Mulliken-Hush theory accord with the X-ray crystallographic data-both sufficient to consistently identify the various D-br-D*(+) according to the Robin-Day classification. Thus, the directly coupled biaryl D-D*(+) is a completely delocalized cation in class III with the charge distributed equally over both redox centers. The trimethylene- and biphenylene-bridged cations D(CH(2))(3)D*(+) and D(ph)(2)D*(+) with highly localized charge distributions are prototypical class II systems involving moderately coupled redox centers with H approximately equal to 400 cm(-1). The borderline region between class II/III is occupied by the phenylene-bridged cation D(ph)D*(+); and the X-ray, CV, and NIR analyses yield ambivalent H values (which we believe to be) largely a result of an unusually asymmetric (20/80) charge distribution that is polarized between the D/D*(+) redox centers.

Structural, spectroscopic, and electrochemical studies of binuclear manganese(II) complexes of bis(pentadentate) ligands derived from bis(1,4,7-triazacyclononane) macrocycles

Structural, electrochemical, ESR, and H2O2 reactivity studies are reported for [Mn(dmptacn)Cl]ClO4 (1, dmptacn = 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane) and binuclear complexes of bis(pentadentate) ligands, generated by attaching 2-pyridylmethyl arms to each secondary nitrogen in bis(1,4,7-triazacyclononane) macrocycles and linked by ethyl (tmpdtne, [Mn2(tmpdtne)Cl2](ClO4)2.2DMF, 2), propyl (tmpdtnp, [Mn2(tmpdtnp)Cl2](ClO4)2.3H2O, 3), butyl (tmpdtnb, [Mn2(tmpdtnb)Cl2](ClO4)2.DMF.2H2O, 4), m-xylyl (tmpdtn-m-X, [Mn2(tmpdtn-m-X)-Cl2](ClO4)2, 5) and 2-propanol (tmpdtnp-OH, [Mn2(tmpdtnp-OH)Cl2](ClO4)2, 6) groups. 1 crystallizes in the orthorhombic space group P2(1)2(1)2(1) (No. 19) with a = 7.959(7) A, b = 12.30(1) A, and c = 21.72(2) A; 2, in the monoclinic space group P2(1)/c (No. 14) with a = 11.455(4) A, b = 15.037(6) A, c = 15.887(4) A, and beta = 96.48(2) degrees; 3, in the monoclinic space group P2(1)/c (No. 14) with a = 13.334(2) A, b = 19.926(2) A, c = 18.799(1) A, and beta = 104.328(8) degrees; and [Mn2(tmpdtnb)Cl2](ClO4)2.4DMF.3H2O (4'), in the monoclinic space group P2(1)/n (No. 14) with a = 13.361(3) A, b = 16.807(5) A, c = 14.339(4) A, and beta = 111.14(2) degrees. Significant distortion of the Mn(II) geometry is evident from the angle subtended by the five-membered chelate (ca. 75 degrees) and the angles spanned by trans donor atoms (< 160 degrees). The Mn geometry is intermediate between octahedral and trigonal prismatic, and for complexes 2-4, there is a systematic increase in M...M distance with the length of the alkyl chain. Cyclic and square-wave voltammetric studies indicate that 1 undergoes a 1e- oxidation from Mn(II) to Mn(III) followed by a further oxidation to MnIV at a significantly more positive potential. The binuclear Mn(II) complexes 2-5 are oxidized to the Mn(III) state in two unresolved 1e- processes [MnII2-->MnIIMnIII-->MnIII2] and then to the MnIV state [MnIII2-->MnIIIMnIV-->MnIV2]. For 2, the second oxidation process was partially resolved into two 1e- oxidation processes under the conditions of square-wave voltammetry. In the case of 6, initial oxidation to the MnIII2 state occurs in two overlapping 1e- processes as was found for 2-5, but this complex then undergoes two further clearly separated 1e- oxidation processes to the MnIIIMnIV state at +0.89 V and the MnIV2 state at +1.33 V (vs Fc/Fc+). This behavior is attributed to formation of an alkoxo-bridged complex. Complexes 1-6 were found to catalyze the disproportionation of H2O2. Addition of H2O2 to 2 generated an oxo-bridged mixed-valent MnIIIMnIV intermediate with a characteristic 16-line ESR signal.

Conversion of core oxos to water molecules by 4e-/4H+ reductive dehydration of the Mn4O2(6+) core in the manganese-oxo cubane complex Mn4O4(Ph2PO2)6: a partial model for photosynthetic water binding and activation

Reaction of the Mn4O4(6+) "cubane" core complex, Mn4O4L6 (1) (L = diphenylphosphinate, Ph2PO2-), with a hydrogen atom donor, phenothiazine (pzH), forms the dehydrated cluster Mn4O2L6 (2), which has lost two mu-oxo bridges by reduction to water (H2O). The formation of 2 was established by electrospray mass spectrometry, whereas FTIR spectroscopy confirmed the release of water molecules into solution during the reduction of 1. UV-vis and EPR spectroscopies established the stoichiometry and chemical form of the pzH product by showing the production of 4 equiv of the neutral pz radical. By contrast, the irreversible decomposition of 1 to individual Mn(II) ions occurs if the reduction is performed using electrons provided by various proton-lacking reductants, such as cobaltocene or electrochemical reduction. Thus, cubane 1 undergoes coupled four-electron/four-proton reduction with the release of two water molecules, a reaction formally analogous to the reverse sequence of the steps that occur during photosynthetic water oxidation leading to O2 evolution. 1H NMR of solutions of 2 reveal that all six of the phosphinate ligands exhibit paramagnetic broadening, due to coordination to Mn ions, and are magnetically equivalent. A symmetrical core structure is thus indicated. We hypothesize that this structure is produced by the dynamic averaging of phosphinato ligand coordination or exchange of mu-oxos between vacant mu-oxo sites. The paramagnetic 1H NMR of water molecules in solution shows that they are able to freely exchange with water molecules that are bound to the Mn ion(s) in 2, and this exchange can be inhibited by the addition of coordinating anions, such as chloride. Thus, 2 possesses open or labile coordination sites for water and anions, in contrast to solutions of 1, which reveal no evidence for water coordination. Complex 2 exhibits greater paramagnetism than that of 1, as seen by 1H NMR, and it possesses a broad (440 G wide) EPR absorption, centered at g = 2, that follows a Curie-Weiss temperature dependence (10-40 K) and is visible only at low temperatures, compared to EPR-silent 1. Its comparison to a spin-integration standard reveals that 2 contains 2 equiv of Mn(II), which is in agreement with the formal oxidation state of 2Mn(II)2Mn(III) that was derived from the titration. The EPR and NMR data for 2 are consistent with a loss of two of the intermanganese spin-exchange coupling pathways, versus 1, which results in two "wingtip" Mn(II) S = 5/2 spins that are essentially magnetically uncoupled from the diamagnetic Mn2O2 base. Bond-enthalpy data, which show that O2 evolution via the reaction 1-->2 + O2, is strongly favored thermodynamically but is not observed in the ground state due to an activation barrier, are included. This activation barrier is hypothesized to arise, in part, from the constraining effect of the facially bridging phosphinate ligands.

Structural Variation in Manganase Complexes: Synthesis and Characterization of Manganese Complexes from Carboxylate-containing Chelating Ligands

Three manganese(II) complexes, [Mn(II)(2)L(1)(2)(H(2)O)(4)](ClO(4))(2).H(2)O (1, L(1)H = (bis(2-pyridylmethyl)amino)acetic acid), [Mn(II)(2)L(2)(2)(H(2)O)(2)](BPh(4))(2).2EtOH.2H(2)O (2, L(2)H = 3-(bis(2-pyridylmethyl)amino)propionic acid), {[Mn(II)(2)L(2)(2)(H(2)O)(MeCN)](BPh(4))(2).2MeCN}(infinity) (3), and a manganese(IV) complex [Mn(IV)(2)O(2)L(2)(2)](ClO(4))(2).4H(2)O (4) were synthesized and characterized by X-ray crystallography. The compound 1 was a dinuclear Mn(II)(2) complex which crystallized in the monoclinic crystal system, space group P2(1)/n, with Z = 4, a = 12.19(1) Å, b = 14.623(8) Å, c = 21.72(1) Å, beta = 96.29(6) degrees, V = 3849(4) Å(3). The complex cation had an approximate C(2) symmtery. The two manganeses were both seven-coordinate and doubly bridged by one oxygen atom of carboxylate groups in &mgr;(2),eta(1)-mode. The compound 2 was also a dinuclear Mn(II)(2) complex which crystallized in the monoclinic crystal system, space group P2(1)/n, with Z = 2, a = 16.760(2) Å, b = 9.643(2) Å, c = 23.533(2) Å, beta = 92.984(8) degrees, V = 3798.4(7) Å(3). The complex cation of 2 also had two seven-coordinate manganese ions, but unlike 1 the nonbridging carboxylate oxygens weakly coordinate to the manganese ions. The compound 3 crystallized in the orthorhombic crystal system, space group P2(1)2(1)2(1), with Z = 4, a = 27.888(3) Å, b = 29.054(2) Å, c = 9.428(2) Å, V = 7638(2) Å(3). The cationic portion of 3 consisted of infinite chains of Mn(II) (two Mn(II) ions per an asymmetric unit) bridged by carboxylates in bidentate syn/anti mode. The compound 4 was a dinuclear bis(&mgr;-oxo) Mn(IV)(2) complex which crystallized in the trigonal crystal system, space group R&thremacr;, with Z = 8, a = 23.962(4) Å, c = 17.190(3) Å, V = 8547(3) Å(3). All these structures are made up from a common fragment "L(n)()Mn" assembling in various topologies. Variable-temperature magnetic susceptibility measurements revealed that the Mn(II) ions in 1-3 were weakly antiferromagnetically coupled (J = -0.631(6), -0.655(5), and -0.20(1) cm(-)(1) for 1-3), and that the Mn(IV) ions in 4 were strongly antiferromagnetically coupled (J = -97.5(5) cm(-)(1)). The cyclic voltammogram of 4 showed two reduction waves with E(1/2) values of -0.52 and 0.28 V (vs ferrocene). These E(1/2) values are more negative by 0.1 V than those of the closely related complex [Mn(III)Mn(IV)O(2)L(1)(2)](ClO(4)).