Synthesis, characterization, and reactivities of manganese(V)-oxo porphyrin complexes

The reactions of manganese(III) porphyrin complexes with terminal oxidants, such as m-chloroperbenzoic acid, iodosylarenes, and H(2)O(2), produced high-valent manganese(V)-oxo porphyrins in the presence of base in organic solvents at room temperature. The manganese(V)-oxo porphyrins have been characterized with various spectroscopic techniques, including UV-vis, EPR, 1H and 19F NMR, resonance Raman, and X-ray absorption spectroscopy. The combined spectroscopic results indicate that the manganese(V)-oxo porphyrins are diamagnetic low-spin (S = 0) species with a longer, weaker Mn-O bond than in previously reported Mn(V)-oxo complexes of non-porphyrin ligands. This is indicative of double-bond character between the manganese(V) ion and the oxygen atom and may be attributed to the presence of a trans axial ligand. The [(Porp)Mn(V)=O](+) species are stable in the presence of base at room temperature. The stability of the intermediates is dependent on base concentration. In the absence of base, (Porp)Mn(IV)=O is generated instead of the [(Porp)Mn(V)=O](+) species. The stability of the [(Porp)Mn(V)=O](+) species also depends on the electronic nature of the porphyrin ligands: [(Porp)Mn(V)=O](+) complexes bearing electron-deficient porphyrin ligands are more stable than those bearing electron-rich porphyrins. Reactivity studies of manganese(V)-oxo porphyrins revealed that the intermediates are capable of oxygenating PPh(3) and thioanisoles, but not olefins and alkanes at room temperature. These results indicate that the oxidizing power of [(Porp)Mn(V)=O](+) is low in the presence of base. However, when the [(Porp)Mn(V)=O](+) complexes were associated with iodosylbenzene in the presence of olefins and alkanes, high yields of oxygenated products were obtained in the catalytic olefin epoxidation and alkane hydroxylation reactions. Mechanistic aspects, such as oxygen exchange between [(Porp)Mn(V)=16O](+) and H(2)(18)O, are also discussed.     

Electrochemically driven generation of manganese(IV,V)-oxo multiporphyrin arrays and their redox properties with manganese(III) species in Langmuir-Blodgett films

High-valency manganese (IV,V)-oxo porphyrins have been electrochemically generated and in situ spectrally characterized in multiporphyrin arrays, which were formed by an interfacial coordination reaction of Na2PdCl4 with manganese (III) tetrapyridylporphyrin (MnTPyP). Multilayers of the Pd-MnTPyP multiporphyrin arrays were obtained by the Langmuir-Blodgett (LB) method. The redox behaviors of manganese in the multiporphyrin arrays were pH-dependent. Spectroelectrochemical experiments revealed a reversible redox process between Pd-Mn(III)TPyP and its Mn(IV)-oxo species, but an irreversible process between Pd-Mn(III)TPyP and its Mn(V)-oxo species. The Pd-Mn(IV)TPyP multiporphyrin arrays could be spontaneously reduced to their Mn(III) complex, while the Pd-Mn(V)TPyP arrays were rather stable in basic solutions (pH > 10.5). However, when the Pd-Mn(V)TPyP multiporphyrin arrays were washed by or immersed in water, they were immediately reduced to their Mn(III) complex. Because these well-organized multiporphyrin arrays are of high thermal and chemical stability, they are potential molecular materials in the studies of natural and artificial catalytic processes as well as redox-based molecular switches.     

Low-energy states of manganese-oxo corrole and corrolazine: multiconfiguration reference ab initio calculations

Manganese(V)-oxo corrole and corrolazine have been studied with ab initio multiconfiguration reference methods (CASPT2 and RASPT2) and large atomic natural orbital (ANO) basis sets. The calculations confirm the expected singlet d(δ)(2) ground states for both complexes and rule out excited states within 0.5 eV of the ground states. The lowest excited states are a pair of Mn(V) triplet states with d(δ)(1)d(π)(1) configurations 0.5-0.75 eV above the ground state. Manganese(IV)-oxo macrocycle radical states are much higher in energy, ≥1.0 eV relative to the ground state. The macrocyclic ligands in the ground states of the complexes are thus unambiguously 'innocent'. The approximate similarity of the spin state energetics of the corrole and corrolazine complexes suggests that the latter macrocycle on its own does not afford any special stabilization for the Mn(V)O center. The remarkable stability of an Mn(V)O octaarylcorrolazine thus appears to be ascribable to the steric protection afforded by the β-aryl groups.     

A comparative study of high-spin manganese and iron complexes

Different metal complexes of the general form M(OH) _n (H₂O)_{6– n} have been studied for manganese and iron. Oxidation states considered for manganese are Mn(III), Mn(IV) and Mn(V) and for iron Fe(II), Fe(III) and Fe(IV). Oxygen containing ligands are used throughout with varying numbers of hydroxyl and water ligands. Some metal-oxo and some charged complexes were also studied. Large Jahn-Teller distortions were found for the Mn(III) and Fe(IV) complexes. Consequences of these distortions are that water ligands have to be placed along the weak JT-axis and that five-coordination by a loss of one of these water ligands is quite competitive with six-coordination in particular for manganese. For Fe(II) and Fe(III) lower coordinations than six are preferred due to the presence of two repulsive e _g electrons. For the metal-oxo complexes five-coordination is also preferred due to the strong trans effect from the oxo ligand. All complexes studied have high-spin ground states. An interesting effect is that the spin is much more delocalized on the ligands for the iron complexes than for the manganese complexes. This effect, which is chemically important for certain iron enzymes, is rationalized by the large number of 3d electrons on iron. For manganese with only five 3d electrons no spin delocalization is needed to obtain the proper high-spin states. Received: 4 February 1997 / Accepted: 24 February 1997     

Structure,stability, and electronic and NMR properties of various oxo- and nitrido-derivatives of [L(Salen)Mn(III)]+, where L = none and imidazole. A density functional study

Structure, stability, and electronic and NMR properties of [(Salen)Mn(III)](+)-derived intermediates/reactants in the epoxidation/amination of unfunctionalized olefins, namely [(Salen)Mn(V)O](+) (1-oxo), [(Salen)Mn(IV)O] (2-oxo), and [(Salen)Mn(V)N] (3), have been studied with the B3LYP density functional method. It has been shown that the (1)A, (3)A, and (5)A states of cationic 1-oxo species are virtually degenerate, while for the neutral 2-oxo species the ground (4)A state lies 6.4 kcal/mol lower than (2)A. In the nitrido species 3, the (1)A state has been shown to be the ground state in agreement with experiment. We have investigated isomerization of 1-oxo and 2-oxo species into unusual [(OSalen)Mn(III)](+) (1-N-oxo and 1-peroxo) and [(OSalen)Mn(II)] (2-N-oxo and 2-peroxo) species, respectively. For cationic species 1, the 1-N-oxo isomers are more stable (by 10-12 kcal/mol) than the 1-oxo isomer and are separated from the latter by 21-22 kcal/mol barriers. On the other hand, 1-peroxo isomers are calculated to be 14-16 kcal/mol higher than the 1-oxo isomer. For neutral species 2, however, both 2-N-oxo and 2-peroxo isomers lie significantly higher in energy than the 2-oxo isomer. It has been shown that coordination of axial imidazole ligand alters relative energies of spin states for 1- and 2-oxo species, destabilizing low-spin states. For singlet states of H(2)Salen, 1-oxo, and 3, we have calculated (1)H, (13)C, (15)N, and (17)O NMR chemical shifts using the gauge-independent-atomic orbital (GIAO) approach.     

cis-Stilbene and (1 alpha,2 beta,3 alpha)-(2-ethenyl-3-methoxycyclopropyl)benzene as mechanistic probes in the Mn(III)(salen)-catalyzed epoxidation: influence of the oxygen source and the counterion on the diastereoselectivity of the competitive concerted and radical-type oxygen transfer

cis-Stilbene (1) has been epoxidized by a set of diverse oxygen donors [OxD], catalyzed by the Mn(III)(salen)X complexes 3 (X = Cl, PF(6)), to afford a mixture of cis- and trans-epoxides 2. The cis/trans ratios range from 29:71 (extensive isomerization) to 92:8, which depends both on the oxygen source [OxD] and on the counterion X of the catalyst. When (1 alpha,2 beta,3 alpha)-(2-ethenyl-3-methoxycyclopropyl)-benzene (4) is used as substrate, a mechanistic probe which differentiates between radical and cationic intermediates, no cationic ring-opening products are found in this epoxidation reaction; thus, isomerized epoxide product arises from intermediary radicals. The dependence of the diastereoselectivity on the oxygen source is rationalized in terms of a bifurcation step in the catalytic cycle, in which concerted Lewis-acid-activated oxygen transfer competes with stepwise epoxidation by the established Mn(V)(oxo) species. The experimental counterion effect is attributed to the computationally assessed ligand-dependent reaction profiles and stereoselectivities of the singlet, triplet, and quintet spin states available to the manganese species.     

Insights into the structure and reactivity of acylperoxo complexes in the Kochi-Jacobsen-Katsuki catalytic system. A density functional study

Structural properties of the acylperoxo complexes [(Salen)Mn(III)RCO(3)] (2) and [(Salen)Mn(IV)RCO(3)] (3), the critical intermediates in the Kochi-Jacobsen-Katsuki reaction utilizing organic peracids or O(2)/aldehydes as oxygen source, have been studied with the density functional theory. Four distinct isomers, cis(O,N), cis(N,O), cis(N,N), and trans, of these complexes have been located. The isomer 2-cis(O,N) in its quintet ground state, and nearly degenerate isomers 3-cis(O,N) and 3-cis(N,O) in their quartet ground states are found to be the lowest in energy among the other isomers. The O-O bond cleavage in the cis(O,N), cis(N,O), and trans isomers of 2 and 3 has been elucidated. In complex 3, the O-O bond is inert. On the contrary, in complex 2, the O-O bond cleaves via two distinct pathways. The first pathway occurs exclusively on the quintet potential energy surface (PES) and corresponds to heterolytic O-O bond scission coupled with insertion of an oxygen atom into an Mn-N(Salen) bond to form 2-N-oxo species; this pathway has the lowest barrier of 14.9 kcal/mol and is 15.6 kcal/mol exothermic. The second pathway is tentatively a spin crossover pathway. In particular, for 2-cis(O,N) and 2-cis(N,O) the second pathway proceeds through a crucial minimum on the seam of crossing (MSX) between the quintet and triplet PESs followed by heterolytic O-O cleavage on the triplet PES, and produces unusual triplet 2-cis(O,N)- and 2-cis(N,O)-oxo ([(Salen)Mn(V)(O)RCO(2)]) species; this pathway requires 12.8 kcal/mol and is 1.4 kcal/mol endothermic. In contrast, for the 2-trans isomer, spin crossing is less crucial and the O-O cleavage proceeds homolytically to generate 2-trans-oxo [(Salen)Mn(IV)(O)] species with RCO(2) radical; this pathway, however, cannot compete with that in 2-cis because it needs 21.9 kcal/mol for activation and is 15.3 kcal/mol endothermic. In summary, the O-O cleavage occurs predominantly in the 2-cis complexes, and may proceed either through pure high spin or spin crossover heterolytic pathway to produce 2-cis-oxo and 2-N-oxo species.     

Cooperative pull and push effects on the O-O bond cleavage in acylperoxo complexes of [(Salen)MnIIIL]: ensuring formation of manganese(V) oxo species

The acidity (pull) and the axial ligand (push) effects on the O-O bond cleavage in the [(Salen)Mn(III)(RCO(3))L] acylperoxo complexes, with model L = none, NH(3), and HCO(2)(-) (1), have been studied with B3LYP density functional calculations. The acidic conditions have been mimicked by explicit protonation of 1 to afford a variety of [(Salen)Mn(III)(RCO(3)H)L] (2) and [(SalenH)Mn(III)(RCO(3))L] (3) complexes in ground quintet states. The protonation assists the O-O bond heterolysis, thus primarily forming highly reactive Mn(V)(O) species, and consequently suppresses formation of the less reactive Mn(IV)(O) species through homolytic channel described earlier in 1 [Khavrutskii, I. V.; Rahim, R. R.; Musaev, D. G.; Morokuma, K. J. Phys. Chem. B 2004, 108, 3845-3854]. In addition to the qualitative change of the O-O bond cleavage mode, the protonation affects the rate of the O-O bond cleavage. Therefore, varying the acidity of the reaction media helps control the O-O bond cleavage mode and rate.     

咔咯锰(Ⅲ)与锰(V)-氧配合物与含氮配体的轴向配位性质

采用密度泛函理论(DFT)的BP86方法对含氮配体咪唑、甲基咪唑、异丙基咪唑和吡啶与5,10,15-三(五氟苯基)咔咯锰[(TPFC)Mn^Ⅲ]和5,10,15-三(五氟苯基)咔咯锰氧[(TPFC)Mn^VO]的轴向配位性质进行理论研究.计算结果表明配体能与五重态下的(TPFC)Mn^Ⅲ形成有效的轴向配位作用,结合能绝对值次序为：咪唑>4-甲基咪唑>吡啶,与实验结果一致. 另外,结合能和轴向配位键长数据显示,这些配体不能与基态(单重态)或三重态(TPFC)Mn^VO中的Mn^V原子形成有效的轴向配位作用,自然键轨道(NBO)分析表明其Mn^V没有空的3d 轨道来接受配体的孤对电子,但配体可与三重态下的(TPFC)Mn^VO形成弱的配位作用.     

Catalytic sulfoxidation and epoxidation with a Mn(III) triazacorrole: evidence for a "third oxidant" in high-valent porphyrinoid oxidations

The reaction between (TBP)8(Cz)Mn(III) (1) and the oxygen atom donors iodosylbenzene (PhIO) or p-cyanodimethylaniline-N-oxide (CDMANO) leads to the manganese(V)-oxo complex (TBP)8(Cz)Mn(V)O (2), which has been isolated and characterized previously. Under catalytic conditions with 1 as added catalyst and PhIO as oxidant, both sulfoxidation of PhSMe and epoxidation of cis-stilbene is observed, whereas with CDMANO no sulfoxidation takes place, suggesting that 2 is not the active oxidant. Examination of the independent reactivity of isolated 2 toward PhSMe and cis-stilbene supports this conclusion. A mechanism which relies on a novel type of oxidant involving Lewis acid activation of PhIO by the Mn(V)-oxo complex 2 accounts for these observations and is confirmed by 18O-labeling experiments.     

Laser flash photolysis generation and kinetic studies of corrole-manganese(V)-oxo intermediates

Corrole-manganese(V)-oxo intermediates were produced by laser flash photolysis of the corresponding corrole-manganese(IV) chlorate complexes, and the kinetics of their decay reactions in CH2Cl2 and their reactions with organic reductants were studied. The corrole ligands studied were 5,10,15-tris(pentafluorophenyl)corrole (H3TPFC), 5,10,15-triphenylcorrole (H3TPC), and 5,15-bis(pentafluorophenyl)-10-(p-methoxyphenyl)corrole (H3BPFMC). In self-decay reactions and in reactions with substrates, the order of reactivity of (Cor)Mn(V)(O) was TPC > BPFMC > TPFC, which is inverted from that expected based on the electron-demand of the ligands. The rates of reactions of (Cor)Mn(V)(O) were dependent on the concentration of the oxidant and other manganese species, with increasing concentrations of various manganese species resulting in decreasing rates of reactions, and the apparent rate constant for reaction of (TPFC)Mn(V)(O) with triphenylamine was found to display fractional order with respect to the manganese-oxo species. The kinetic results are consistent in part with a reaction model involving disproportionation of (Cor)Mn(V)(O) to give (Cor)Mn(IV) and (Cor)Mn(VI)(O) species, the latter of which is the active oxidant. Alternatively, the results are consistent with oxidation by (Cor)Mn(V)(O) which is reversibly sequestered in non-reactive complexes by various manganese species.     

High-valent iron and manganese complexes of corrole and porphyrin in atom transfer and dioxygen evolving catalysis

Manganese(V) imido complexes of 5,10,15-tris(pentafluorophenyl)corrole (H(3)tpfc) can be prepared by the reaction of Mn(III)(tpfc) and organic nitrene generated from either photolytic or thermal activation of organic azides. The terminal imido complexes of manganese(V) were among the first structurally characterized examples of Mn(V) terminal imido complexes in the literature. They feature a short Mn≡N triple bond and a nearly linear M[triple bond, length as m-dash]N-C angle. The ground state of (tpfc)Mn(V)(NAr) is singlet. Contrary to expectations, arylimido complexes of manganese(V) were stable to moisture and did not undergo [NR] group transfer to olefins. Manganese(V) imido corrole with an activated tosyl imido ligand was prepared from iodoimine (ArINTs) and manganese(III) corrole. The resulting complex (tpfc)Mn(NTs) is paramagnetic (S = 1), hydrolyzes to (tpfc)Mn(O) in the presence of water, abstracts hydrogen atoms from benzylic C-H bonds, and catalyzes aziridination of alkenes. Mechanistic studies on the aziridination and hydrogen atom transfer reactions are reviewed. This perspective also describes the reaction chemistry of the heme enzyme chlorite dismutase, the mechanism by which dioxygen is formed on a single-metal site, and recent advances in functional modelling of this enzyme. We also compare the reactivity of water-soluble iron versus manganese porphyrins towards the chlorite anion.     

Activation of nitrous oxide and selective epoxidation of alkenes catalyzed by the manganese-substituted polyoxometalate, [Mn(III)2ZnW(Zn2W9O34)2]10-

A manganese(III)-substituted polyoxometalate of the "sandwich" structure, [MnIII2ZnW(ZnW9O34)2]10-, catalyzed the highly selective (>99.9%) epoxidation of alkenes, such as 1-octene, 2-octene, and cyclohexene with nitrous oxide. Reactions occurred in homogeneous media at 150 degrees C under 1 atm N2O. The epoxidation had a linear reaction profile; turnover frequencies of 0.5-1.4 h-1 were measured. The reactions were also stereoselective; for example, cis-stilbene gave cis-stilbene oxide. From ESR spectroscopy, it was shown that a Mn(II) octahedral species is reversibly formed by reaction between the original Mn(III) polyoxometalate and N2O. Therefore, it would appear that a Mn(V)-oxo active species is not formed; it is possible that the activation of nitrous oxide was by its oxidation by the Mn(III) polyoxometalate.     

Nitrogen atom transfer between manganese complexes of salen, porphyrin, and corrole and characterization of a (nitrido)manganese(VI) corrole

The investigations of complete nitrogen atom transfer reactions from (nitrido)manganese(V) salen to manganese(III) complexes of porphyrins and corroles revealed that stabilization of the [Mn(N)]2+ moiety is in the order of corrole > porphyrin > salen. The first kinetic examination of this quite fundamental reaction exposed a large solvent effect on both the enthalpy and entropy activation energies. Oxidation of the (nitrido)manganese(V) corroles leads to the first (nitrido)manganese(VI) complexes that are coordinated by tetrapyrrolic ligands.     

Electron spin multiplicities of transition-metal aromatic radicals and ions: M[C6(CH3)6] and M(+)[(C6(CH3)6] (M = Ti, V, and Co)

Determination of electron spin multiplicities of transition-metal radicals and ions challenges both experimentalists and theoreticians. In this work, we report preferred electron spin states of M[C(6)(CH(3))(6)] and M(+)(C(6)(CH(3))(6)], where M = Ti, V, and Co. The neutral radicals were formed in a supersonic metal cluster beam source, and their masses were measured with time-of-flight mass spectrometry. Precise ionization energies of the radicals and metal-ligand stretching frequencies of the ions were measured by pulsed field ionization zero electron kinetic energy spectroscopy. C-H stretching frequencies of the methyl group in the radicals were obtained by infrared-ultraviolet two-photon ionization. Electron spin multiplicities of the radicals and ions were investigated by combining the spectroscopic measurements, density functional theory, and Franck-Condon factor calculations. The preferred spin states are quintet, sextet, and quartet for the neutral Ti, V, and Co radicals, respectively; for the corresponding singly charged cations, they are quartet, quintet, and triplet. In these high-spin states, the aromatic ring remains nearly planar. This finding contrasts to the previous study of Sc(hmbz), for which low-spin states are favored, and the aromatic ring is severely bent.     

Does B3LYP correctly describe magnetism of manganese complexes with various oxidation numbers and various structural motifs?

We assessed the applicability and basis set dependency of the B3LYP functional to investigate magnetic interactions of Mn complexes. For the purpose, we constructed a test set consisting of 16 Mn complexes with various oxidation states and structural motifs.The B3LYP results correctly reproduced magnetism and magneto–redox correlation of the standard μ-oxo motifs with superexchange paths, while it does not work for weak magnetic complexes. We also showed that a modest basis set yields results similar to those of triple-zeta plus diffuse-and-polarization functions. This basis set is expected to be a standard basis set for investigating magnetism of manganese complexes.     

Preparation and properties of a monomeric high-spin Mn(V)-oxo complex

Oxomanganese(V) species have been implicated in a variety of biological and synthetic processes, including their role as a key reactive center within the oxygen-evolving complex in photosynthesis. Nearly all mononuclear Mn(V)-oxo complexes have tetragonal symmetry, producing low-spin species. A new high-spin Mn(V)-oxo complex that was prepared from a well-characterized oxomanganese(III) complex having trigonal symmetry is now reported. Oxidation experiments with [FeCp(2)](+) were monitored with optical and electron paramagnetic resonance (EPR) spectroscopies and support a high-spin oxomanganese(V) complex formulation. The parallel-mode EPR spectrum has a distinctive S = 1 signal at g = 4.01 with a six-line hyperfine pattern having A(z) = 113 MHz. The presence of an oxo ligand was supported by resonance Raman spectroscopy, which revealed O-isotope-sensitive peaks at 737 and 754 cm(-1) assigned as a Fermi doublet centered at 746 cm(-1)(Δ(18)O = 31 cm(-1)). Mn Kβ X-ray emission spectra showed Kβ' and Kβ(1,3) bands at 6475.92 and 6490.50 eV, respectively, which are characteristic of a high-spin Mn(V) center.     

Dioxo-bridged dinuclear manganese(III) and -(IV) complexes of pyridyl donor tripod ligands: combined effects of steric substitution and chelate ring size variations on structural,spectroscopic, and electrochemical properties

The syntheses and structural, spectral, and electrochemical characterization of the dioxo-bridged dinuclear Mn(III) complexes [LMn(mo-O)(2)MnL](ClO(4))(2), of the tripodal ligands tris(6-methyl-2-pyridylmethyl)amine (L(1)) and bis(6-methyl-2-pyridylmethyl)(2-(2-pyridyl)ethyl)amine (L(2)), and the Mn(II) complex of bis(2-(2-pyridyl)ethyl)(6-methyl-2-pyridylmethyl)amine (L(3)) are described. Addition of aqueous H(2)O(2) to methanol solutions of the Mn(II) complexes of L(1) and L(2) produced green solutions in a fast reaction from which subsequently precipitated brown solids of the dioxo-bridged dinuclear complexes 1 and 2, respectively, which have the general formula [LMn(III)(mu-O)(2)Mn(III)L](ClO(4))(2). Addition of 30% aqueous H(2)O(2) to the methanol solution of the Mn(II) complex of L(3) ([Mn(II)L(3)(CH(3)CN)(H(2)O)](ClO(4))(2) (3)) showed a very sluggish change gradually precipitating an insoluble black gummy solid, but no dioxo-bridged manganese complex is produced. By contrast, the Mn(II) complex of the ligand bis(2-(2-pyridyl)ethyl)(2-pyridylmethyl)amine (L(3a)) has been reported to react with aqueous H(2)O(2) to form the dioxo-bridged Mn(III)Mn(IV) complex. In cyclic voltammetric experiments in acetonitrile solution, complex 1 shows two reversible peaks at E(1/2) = 0.87 and 1.70 V (vs Ag/AgCl) assigned to the Mn(III)(2) <--> Mn(III)Mn(IV) and the Mn(III)Mn(IV) <--> Mn(IV)(2) processes, respectively. Complex 2 also shows two reversible peaks, one at E(1/2) = 0.78 V and a second peak at E(1/2) = 1.58 V (vs Ag/AgCl) assigned to the Mn(III)(2) <--> Mn(III)Mn(IV) and Mn(III)Mn(IV) <--> Mn(IV)(2) redox processes, respectively. These potentials are the highest so far observed for the dioxo-bridged dinuclear manganese complexes of the type of tripodal ligands used here. The bulk electrolytic oxidation of complexes 1 and 2, at a controlled anodic potential of 1.98 V (vs Ag/AgCl), produced the green Mn(IV)(2) complexes that have been spectrally characterized. The Mn(II) complex of L(3) shows a quasi reversible peak at an anodic potential of E(p,a) of 1.96 V (vs Ag/AgCl) assigned to the oxidation Mn(II) to Mn(III) complex. It is about 0.17 V higher than the E(p,a) of the Mn(II) complex of L(3a). The higher oxidation potential is attributable to the steric effect of the methyl substituent at the 6-position of the pyridyl donor of L(3).     

Electronic structure of the Mn4OxCa cluster in the S0 and S2 states of the oxygen-evolving complex of photosystem II based on pulse 55Mn-ENDOR and EPR spectroscopy

The heart of the oxygen-evolving complex (OEC) of photosystem II is a Mn4OxCa cluster that cycles through five different oxidation states (S0 to S4) during the light-driven water-splitting reaction cycle. In this study we interpret the recently obtained 55Mn hyperfine coupling constants of the S0 and S2 states of the OEC [Kulik et al. J. Am. Chem. Soc. 2005, 127, 2392-2393] on the basis of Y-shaped spin-coupling schemes with up to four nonzero exchange coupling constants, J. This analysis rules out the presence of one or more Mn(II) ions in S0 in methanol (3%) containing samples and thereby establishes that the oxidation states of the manganese ions in S0 and S2 are, at 4 K, Mn4(III, III, III, IV) and Mn4(III, IV, IV, IV), respectively. By applying a "structure filter" that is based on the recently reported single-crystal EXAFS data on the Mn4OxCa cluster [Yano et al. Science 2006, 314, 821-825] we (i) show that this new structural model is fully consistent with EPR and 55Mn-ENDOR data, (ii) assign the Mn oxidation states to the individual Mn ions, and (iii) propose that the known shortening of one 2.85 A Mn-Mn distance in S0 to 2.75 A in S1 [Robblee et al. J. Am. Chem. Soc. 2002, 124, 7459-7471] corresponds to a deprotonation of a mu-hydroxo bridge between MnA and MnB, i.e., between the outer Mn and its neighboring Mn of the mu3-oxo bridged moiety of the cluster. We summarize our results in a molecular model for the S0 --> S1 and S1 --> S2 transitions.     

Isolation of the first ferromagnetically coupled Mn(III/IV) complex

Binuclear manganese complexes Mn2(III/IV)(dtsalpn)2DCBI, 1, Mn2(III/III)(dtsalpn)2HDCBI, 2, containing the ligand dicarboxyimidazole (DCBI) have been prepared in order to address the issue of imidazole bridged and ferromagnetically coupled Mn sites in high oxidation states of the OEC in Photosystem II (PS II). Temperature dependent magnetic susceptibility studies of 1 indicates that the interaction between the two Mn(III)/Mn(IV) ions is ferromagnetic (J = +1.4 cm(-1)). Variable temperature EPR spectra of 1 shows that a g = 2 multiline is as an excited state signal corresponding to S = 1/2.