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Two new terpyridine dimanganese oxo complexes [Mn2[supIII,IV](7-O)2(terpy)2(CF3CO2)2][sup+] (3) and [Mn2[supIII,III](7-O)(terpy)2(CF3CO2)4] (4) (terpy = 2,2`:6,2`u`-terpyridine) have been synthesized and their X-ray structures determined. In contrast to the corresponding mixed-valent aqua complex [Mn2[supIII,IV](7-O)2(terpy)2(H2O)2][sup3+] (1), the two Mn atoms in 3 are not crystallographically equivalent. The neutral binuclear monooxo manganese(III,III) complex 4 exhibits two crystallographic forms having cis and trans configurations. In the cis complex, the two CF3CO2[sip-] ligands on each manganese adopt a cis geometry to each other; one CF3CO2[sup-] is trans to the oxygen of the oxo bridge while the second is cis. In the trans complex, the two coordinated CF3CO2[sup-] have a trans geometry to each other and are cis to the oxo bridge. The electrochemical behavior of 3 in organic medium (CH3CN) shows that this complex could be oxidized into its corresponding stable manganese(IV,IV) species while its reduced form manganese(III,III) is very unstable and leads by a disproportionation process to Mn(II) and Mn(IV) complexes. Complex 4 is only stable in the solid state, and it disproportionates spontaneously in CH3CN solution into the mixed-valent complex 3 and the mononuclear complex [Mn[supII](terpy)2][sup2+] (2), thereby preventing the observation of its electrochemical behavior.

The enzyme FeFe-hydrogenase catalyzes H 2 evolution and oxidation at an active site that consists of a [4Fe-4S] cluster bridged to a [Fe 2 (CO) 3 (CN) 2 (azadithiolate)] subsite. Previous investigations of its mechanism were mostly conducted on a few "prototypical" FeFe-hydrogenases, such as that from Chlamydomonas reinhardtii (Cr HydA1), but atypical hydrogenases have recently been characterized in an effort to explore the diversity of this class of enzymes. We aim at understanding why prototypical hydrogenases are active in either direction of the reaction in response to a small deviation from equilibrium, whereas the homologous enzyme from Thermoanaerobacter mathranii (Tam HydS) shows activity only under conditions of very high driving force, a behavior that was referred to as "irreversible catalysis". We follow up on previous spectroscopic studies and recent developments in the kinetic modeling of bidirectional reactions to investigate and compare the catalytic cycles of Cr HydA1 and Tam HydS under conditions of direct electron transfer with an electrode. We compare the hypothetical catalytic cycles described in the literature, and we show that the observed changes in catalytic activity as a function of potential, pH, and H 2 concentration can be explained with the assumption that the same catalytic mechanism applies. This helps us identify which variations in properties of the catalytic intermediates give rise to the distinct "reversible" or "irreversible" catalytic behaviors.

The observation that some homologous enzymes have the same active site but very different catalytic properties demonstrates the importance of long-range effects in enzyme catalysis, but these effects are often difficult to rationalize. The NiFe hydrogenases 1 and 2 (Hyd 1 and Hyd 2) from E. coli both consist of a large catalytic subunit that embeds the same dinuclear active site and a small electron-transfer subunit with a chain of three FeS clusters. Hyd 1 is mostly active in H 2 oxidation and resistant to inhibitors, whereas Hyd 2 also catalyzes H 2 production and is strongly inhibited by O 2 and CO. Based on structural and site-directed mutagenesis data, it is currently believed that the catalytic bias and tolerance to O 2 of Hyd 1 are defined by the distal and proximal FeS clusters, respectively. To test these hypotheses, we produced and characterized a hybrid enzyme made of the catalytic subunit of Hyd 1 and the electron transfer subunit of Hyd 2. We conclude that catalytic bias and sensitivity to CO are set by the catalytic subunit rather than by the electron transfer chain. We confirm the importance of the proximal cluster in making the enzyme Hyd 1 resist long-term exposure to O 2 , but we show that other structural determinants, in both subunits, contribute to O 2 tolerance. A similar strategy based on the design of chimeric heterodimers could be used in the future to elucidate various structure-function relationships in hydrogenases and other multimeric metalloenzymes and to engineer useful hydrogenases that combine the desirable properties of distinct, homologous enzymes.

Many enzymes that produce or transform small molecules such as O 2 , H 2 , and CO 2 embed inorganic cofactors based on transition metals. Their active site, where the chemical reaction occurs, is buried in and protected by the protein matrix, and connected to the solvent in several ways: chains of redox cofactors mediate long-range electron transfer; static or dynamic tunnels guide the substrate, product and inhibitors; amino acids and water molecules transfer protons. The catalytic mechanism of these enzymes is therefore delocalized over the protein and involves many different steps, some of which determine the response of the enzyme under conditions of stress (extreme redox conditions, presence of inhibitors, light), the catalytic rates in the two directions of the reaction and their ratio (the "catalytic bias"). Understanding all the steps in the catalytic cycle, including those that occur on sites of the protein that are remote from the active site, requires a combination of biochemical, structural, spectroscopic, theoretical, and kinetic methods. Here we argue that kinetics should be used to the fullest extent, by extracting quantitative information from the comparison of data and kinetic models and by exploring the combination of experimental kinetics and theoretical chemistry. In studies of these catalytic mechanisms, direct electrochemistry, the technique which we use and contribute to develop, has become unescapable. It simply consists in monitoring the changes in activity of an enzyme that is wired to an electrode by recording an electric current. We have described kinetic models that can be used to make sense of these data and to learn about various aspects of the mechanism that are difficult to probe using more conventional methods: long-range electron transfer, diffusion along gas channels, redox-driven (in)activations, active site chemistry and photoreactivity under conditions of turnover. In this Account, we highlight a few results that illustrate our approach. We describe how electrochemistry can be used to monitor substrate and inhibitor diffusion along the gas channels of hydrogenases and we discuss how the kinetics of intramolecular diffusion relates to global properties such as resistance to oxygen and catalytic bias. The kinetics and/or thermodynamics of intramolecular electron transfer may also affect the catalytic bias, the catalytic potentials on either side of the equilibrium potential, and the overpotentials for catalysis (defined as the difference between the catalytic potentials and the open circuit potential). This is understood by modeling the shape of the steady-state catalytic response of the enzyme. Other determinants of the catalytic rate, such as domain motions, have been probed by examining the transient catalytic response recorded at fast scan rates. Last, we show that combining electrochemical investigations and MD, DFT, and TD-DFT calculations is an original way of probing the reactivity of the H-cluster of hydrogenase, in particular its reactions with CO, O 2 , and light. This approach contrasts with the usual strategy which aims at stabilizing species that are presumed to be catalytic intermediates, and determining their structure using spectroscopic or structural methods.

FeFe hydrogenases catalyze H 2 oxidation and formation at an inorganic active site (the "H-cluster"), which consists of a [Fe 2 (CO) 3 (CN) 2 (dithiomethylamine)] subcluster covalently attached to a Fe 4 S 4 subcluster. This active site is photosensitive: visible light has been shown to induce the release of exogenous CO (a reversible inhibitor of the enzyme), shuffle the intrinsic CO ligands, and even destroy the H-cluster. These reactions must be understood because they may negatively impact the use of hydrogenase for the photoproduction of H 2 . Here, we explore in great detail the reactivity of the excited states of the H-cluster under catalytic conditions by examining, both experimentally and using TDDFT calculations, the simplest photochemical reaction: the binding and release of exogenous CO. A simple dyad model can be used to predict which excitations are active. This strategy could be used for probing other aspects of the photoreactivity of the H-cluster.

The mechanism of reaction of FeFe hydrogenases with oxygen has been debated. It is complex, apparently very dependent on the details of the protein structure, and difficult to study using conventional kinetic techniques. Here we build on our recent work on the anaerobic inactivation of the enzyme [Fourmond et al. Nat. Chem. 2014, 4, 336-342] to propose and apply a new method for studying this reaction. Using electrochemical measurements of the turnover rate of hydrogenase, we could resolve the first steps of the inhibition reaction and accurately determine their rates. We show that the two most studied FeFe hydrogenases, from Chlamydomonas reinhardtii and Clostridium acetobutylicum, react with O2 according to the same mechanism, despite the fact that the former is much more O2 sensitive than the latter. Unlike often assumed, both enzymes are reversibly inhibited by a short exposure to O2. This will have to be considered to elucidate the mechanism of inhibition, before any prediction can be made regarding which mutations will improve oxygen resistance. We hope that the approach described herein will prove useful in this respect.

Using direct electrochemistry to learn about the mechanism of electrocatalysts and redox enzymes requires that kinetic models be developed. Here we thoroughly discuss the interpretation of electrochemical signals obtained with adsorbed enzymes and molecular catalysts that can reversibly convert their substrate and product. We derive analytical relations between electrochemical observables (overpotentials for catalysis in each direction, positions, and magnitudes of the features of the catalytic wave) and the characteristics of the catalytic cycle (redox properties of the catalytic intermediates, kinetics of intramolecular and interfacial electron transfer, etc.). We discuss whether or not the position of the wave is determined by the redox potential of a redox relay when intramolecular electron transfer is slow. We demonstrate that there is no simple relation between the reduction potential of the active site and the catalytic bias of the enzyme, defined as the ratio of the oxidative and reductive limiting currents; this explains the recent experimental observation that the catalytic bias of NiFe hydrogenase depends on steps of the catalytic cycle that occur far from the active site [Abou Hamdan et al., J. Am. Chem. Soc. 2012, 134, 8368]. On the experimental side, we examine which models can best describe original data obtained with various NiFe and FeFe hydrogenases, and we illustrate how the presence of an intramolecular electron transfer chain affects the voltammetry by comparing the data obtained with the FeFe hydrogenases from Chlamydomonas reinhardtii and Clostridium acetobutylicum, only one of which has a chain of redox relays. The considerations herein will help the interpretation of electrochemical data previously obtained with various other bidirectional oxidoreductases, and, possibly, synthetic inorganic catalysts.

Direct electron transfer between enzymes and electrodes is now commonly achieved, but obtaining protein films that are very stable may be challenging. This is particularly crucial in the case of hydrogenases, the enzymes that catalyze the biological conversion between dihydrogen and protons, because the instability of the hydrogenase films may prevent the use of these enzymes as electrocatalysts of H(2) oxidation and production in biofuel cells and photoelectrochemical cells. Here we show that two different FeFe hydrogenases (from Chamydomonas reinhardtii and Clostridium acetobutylicum) can be covalently attached to functionalized pyrolytic graphite electrodes using peptidic coupling. In both cases, a surface patch of lysine residues makes it possible to favor an orientation that is efficient for fast, direct electron transfer. High hydrogen-oxidation current densities are maintained for up to one week, the only limitation being the intrinsic stability of the enzyme. We also show that covalent attachment has no effect on the catalytic properties of the enzyme, which means that this strategy can also used be for electrochemical studies of the catalytic mechanism.

Carbon monoxide is often described as a competitive inhibitor of FeFe hydrogenases, and it is used for probing H(2) binding to synthetic or in silico models of the active site H-cluster. Yet it does not always behave as a simple inhibitor. Using an original approach which combines accurate electrochemical measurements and theoretical calculations, we elucidate the mechanism by which, under certain conditions, CO binding can cause permanent damage to the H-cluster. Like in the case of oxygen inhibition, the reaction with CO engages the entire H-cluster, rather than only the Fe(2) subsite.

The trinuclear oxo bridged manganese cluster, [Mn(IV)(3)O(4)(terpy)(terpyO(2))(2)(H(2)O)](S(2)O(8))(2) (5) (terpy = 2,2':2'',6'-terpyridine and terpyO(2) = 2,2':2'',6'-terpyridine 1,1''-dioxide), was isolated in an acidic aqueous medium from the reaction of MnSO(4), terpy, and oxone as chemical oxidant. The terpyO(2) ligands were generated in situ during the synthesis by partial oxidation of terpy. The complex crystallizes in the monoclinic space group P21/n with a = 14.251(5) A, b = 15.245(5) A, c = 24.672(5) A, alpha = 90.000(5) degrees, beta = 92.045(5) degrees, gamma = 90.000(5) degrees, and Z = 4. The triangular {Mn(IV)(3)O(4)}(4+) core observed in this complex is built up of a basal Mn(mu-O)(2)Mn unit where each Mn ion is linked to an apical Mn ion via mono(mu-O) bridges. The facial coordination of the two tridentate terpyO(2) ligands to the Mn(mu-O)(2)Mn unit allows the formation of the triangular core. 5 is also the first structurally characterized Mn complex with polypyridinyl N-oxide ligands. The variable-temperature magnetic susceptibility data for this complex, in the range of 10-300 K, are consistent with an S = 1/2 ground state and were fit using the spin Hamiltonian H(eff) with S(1) = S(2) = S(3) = 3/2, J(a) = -37 (+/-0.5) and J(b) = -53 (+/-1) cm(-1), where J(a) and J(b) are exchange constants through the mono-mu-oxo and the di-mu-oxo bridges, respectively. The doublet ground spin state of 5 is confirmed by EPR spectroscopic measurements. Density functional theory (DFT) calculations based on the broken symmetry approach reproduce the magnetic properties of 5 very well (calculated values: J(a) = -39.4 and J(b) = -55.9 cm(-1)), thus confirming the capability of this quantum chemical method for predicting the magnetic behavior of clusters involving more than two metal ions. The nature of the ground spin state of the magnetic {Mn(IV)(3)O(4)}(4+) core and the role of ancillary ligands on the magnitude of J are also discussed.

Chronoamperometric experiments with adsorbed electrocatalysts are commonly performed either for analytical purposes or for studying the catalytic mechanism of a redox enzyme. In the context of amperometric sensors, the current may be recorded as a function of time while the analyte concentration is being increased to determine a linearity range. In mechanistic studies of redox enzymes, chronoamperometry proved powerful for untangling the effects of electrode potential and time, which are convoluted in cyclic voltammetric measurements, and for studying the energetics and kinetics of inhibition. In all such experiments, the fact that the catalyst's coverage and/or activity decreases over time distorts the data. This may hide meaningful features, introduce systematic errors, and limit the accuracy of the measurements. We propose a general and surprisingly simple method for correcting for electrocatalyst desorption and inactivation, which greatly increases the precision of chronoamperometric experiments. Rather than subtracting a baseline, this consists in dividing the current, either by a synthetic signal that is proportional to the instant electroactive coverage or by the signal recorded in a control experiment. In the latter, the change in current may result from film loss only or from film loss plus catalyst inactivation. We describe the different strategies for obtaining the control signal by analyzing various data recorded with adsorbed redox enzymes: nitrate reductase, NiFe hydrogenase, and FeFe hydrogenase. In each case we discuss the trustfulness and the benefit of the correction. This method also applies to experiments where electron transfer is mediated, rather than direct, providing the current is proportional to the time-dependent concentration of catalyst.

The electrochemical behavior of two mononuclear Mn(II) bis-terpyridine complexes, [Mn(II)(L)(2)](2+) (L = terpy (2,2':6',2''-terpyridine) and (t)Bu(3)-terpy (4,4',4''-tritert-butyl-2,2':6',2''-terpyridine)), has been investigated in dry CH(3)CN. Under these conditions, the cyclic voltammograms of these complexes exhibit not only the well-known Mn(II)/Mn(III) oxidation system but also a second metal-based oxidation one, corresponding to the Mn(III)/Mn(IV) redox couple. These oxidative processes are located at E(1/2) = +0.96 and +1.77 V vs Ag/Ag(+) (+1.26 and +2.07 V vs SCE) for the terpy complex and E(1/2) = +0.85 and +1.56 V vs Ag/Ag(+) (+1.15 and +1.86 V vs SCE) for the (t)Bu(3)-terpy derivative. The one-electron oxidized form of these complexes, [Mn(III)(L)(2)](3+), has been quantitatively generated by exhaustive electrolyses at E = 1.30 V, as previously observed in the case of the oxidation of [Mn(II)(tolyl-terpy)(2)](2+) (tolyl-terpy = 4'-(4-Methylphenyl)-2,2':6',2''-terpyridine) (Romain, S.; Duboc, C.; Neese, F.; Riviere, E.; Hanton, L. R.; Blackman, A. G.; Lepretre, J.-C.; Deronzier, A.; Collomb, M.-N. Chem.Eur. J. 2009, 15, 980-988). Further electrolyses at E = 1.65-1.80 V of [Mn(III)(L)(2)](3+) solutions have shown that the [Mn(IV)(L)(2)](4+) species is only stable for L = (t)Bu(3)-terpy because of the strong electron-donating properties of the tert-butyl substituents. These electrogenerated high-valent complexes are rare examples of mononuclear Mn(III) and Mn(IV) complexes stabilized solely by neutral N ligands. They have been fully characterized in solution by UV-visible and electron paramagnetic resonance (EPR) spectroscopies. A detailed investigation of the EPR spectra of the [Mn(II)((t)Bu(3)-terpy)(2)](2+) and [Mn(IV)((t)Bu(3)-terpy)(2)](4+) has allowed the determination of the spin Hamiltonian parameters for both systems (for Mn(II): |D| = 0.059 cm(-1); |E| = 0.014 cm(-1); E/D = 0.259; g(x) = g(y) = g(z) = 2.00 and for Mn(IV): |D| = 1.33(6) cm(-1); |E| = 0.36(4) cm(-1); E/D = 0.27; g(x) = 1.96(4); g(y) = 1.97(4); g(z) = 1.98(4)).

The synthesis, isolation, and structural characterization of the fully oxidized sulfite-based polyoxotungstate cluster (Pr4N)4{alpha-[W18O54(SO3)2]}.2CH3CN and the one-electron reduced form (Pr4N)5{alpha-[W18O54(SO3)2]}.2CH3CN has been achieved. alpha-[W18O54(SO3)2]5- was obtained as a Pr4N+ salt by reducing the "Trojan Horse" [W18O56(SO3)2(H2O)2]8- cluster via a template orientation transformation. Acetonitrile solutions of pure alpha-[W18O54(SO3)2]5- also were prepared electrochemically by one-electron bulk reductive electrolysis of alpha-[W18O54(SO3)2]4-. Cyclic voltammetry of alpha-[W18O54(SO3)2]4- and alpha-[W18O54(SO3)2]5- in CH3CN (0.1 M Hx4NClO4) produces evidence for an extensive series of reversible one-electron redox processes, that are associated with the tungsten-oxo framework of the polyoxometalate cluster. Hydrodynamic voltammograms in CH3CN exhibit the expected sign and magnitude of the steady-state limiting current values for the alpha-[W18O54(SO3)2]4-/5-/6- series and confirm the existence of a stable one-electron reduced species, alpha-[W18O54(SO3)2]5-. Employment of the Randles-Sevcik (cyclic voltammetry) and Levich (rotating disk electrode) equations at a glassy carbon electrode (d=3 mm) enable diffusion coefficient values of 3.7 and 3.8x10(-6) cm2 s-1 to be obtained for alpha-[W18O54(SO3)2]4- and alpha-[W18O54(SO3)2]5-, respectively. The tungsten polyoxometalates are highly photoactive, since measurable photocurrents and color changes are detected for both species upon irradiation with white light. EPR spectra obtained from both acetonitrile solution and solid samples, down to temperatures as low as 2.3 K, of the chemically and electrochemically prepared one-electron reduced species provided evidence that the unpaired electron in alpha-[W18O54(SO3)2]5- is delocalized over a number of atoms in the polyoxometalate structure, even at very low temperatures.

Cobaloximes are effective electrocatalysts for hydrogen evolution and thus functional models for hydrogenases. Among them, difluoroboryl-bridged complexes appear both to mediate proton electroreduction with low overpotentials and to be quite stable in acidic conditions. We report here a mechanistic study of [Co(dmgBF2)2L] (dmg2- = dimethylglyoximato dianion; L = CH3CN or N,N-dimethylformamide) catalyzed proton electroreduction in organic solvents. Depending on the applied potential and the strength of the acid used, three different pathways for hydrogen production were identified and a unified mechanistic scheme involving cobalt(II) or cobalt(III) hydride species is proposed. As far as working potential and turnover frequency are concerned, [Co(dmgBF2)2(CH3CN)2], in the presence of p-cyanoanilinium cation in acetonitrile, is one of the best synthetic catalysts of the first-row transition-metal series for hydrogen evolution.

To examine the real ability of the binuclear di-mu-oxo complex [Mn2(III,IV)O2(terpy)2(H2O)2]3+ (2) to act as a catalyst for water oxidation, we have investigated in detail its redox properties and that of its mononuclear precursor complex [Mn(II)(terpy)2]2+ (1) in aqueous solution. It appears that electrochemical oxidation of 1 allows the quantitative formation of 2 and, most importantly, that electrochemical oxidation of 2 quantitatively yields the stable tetranuclear Mn(IV) complex, [Mn4(IV)O5(terpy)4(H2O)2]6+ (4), having a linear mono-mu-oxo{Mn2(mu-oxo)2}2 core. Therefore, these results show that the electrochemical oxidation of 2 in aqueous solution is only a one-electron process leading to 4 via the formation of a mono-mu-oxo bridge between two oxidized [Mn2(IV,IV)O2(terpy)2(H2O)2]4+ species. 4 is also quantitatively formed by dissolution of the binuclear complex [Mn2(IV,IV)O2(terpy)2(SO4)2] (3) in aqueous solutions. Evidence of this work is that 4 is stable in aqueous solutions, and even if it is a good synthetic analogue of the "dimers-of-dimers" model compound of the OEC in PSII, this complex is not able to oxidize water. As a consequence, since 4 results from an one-electron oxidation of 2, 2 cannot act as an efficient homogeneous electrocatalyst for water oxidation. This work demonstrates that a simple oxidation of 2 cannot produce molecular oxygen without the help of an oxygen donor.

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.

The dimanganese(II,II) complexes 1a [Mn(2)(L)(OAc)(2)(CH(3)OH)](ClO(4)) and 1b [Mn(2)(L)(OBz)(2)(H(2)O)](ClO(4)), where HL is the unsymmetrical phenol ligand 2-(bis-(2-pyridylmethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminomethyl)-4-methylphenol, react with hydrogen peroxide in acetonitrile solution. The disproportionation reaction was monitored by electrospray ionization mass spectrometry (ESI-MS) and EPR and UV-visible spectroscopies. Extensive EPR studies have shown that a species (2) exhibiting a 16-line spectrum at g approximately 2 persists during catalysis. ESI-MS experiments conducted similarly during catalysis associate 2a with a peak at 729 (791 for 2b) corresponding to the formula [Mn(III)Mn(IV)(L)(O)(2)(OAc)](+) ([Mn(III)Mn(IV)(L)(O)(2)(OBz)](+) for 2b). At the end of the reaction, it is partly replaced by a species (3) possessing a broad unfeatured signal at g approximately 2. ESI-MS associates 3a with a peak at 713 (775 for 3b) corresponding to the formula [Mn(II)Mn(III)(L)(O)(OAc)](+) ([Mn(II)Mn(III)(L)(O)(OBz)](+) for 3b). In the presence of H(2)(18)O, these two peaks move to 733 and to 715 indicating the presence of two and one oxo ligands, respectively. When H(2)(18)O(2) is used, 2a and 3a are labeled showing that the oxo ligands come from H(2)O(2). Interestingly, when an equimolar mixture of H(2)O(2) and H(2)(18)O(2) is used, only unlabeled and doubly labeled 2a/b are formed, showing that its two oxo ligands come from the same H(2)O(2) molecule. All these experiments lead to attribute the formula [Mn(III)Mn(IV)(L)(O)(2)(OAc)](+) to 2a and to 3a the formula [Mn(II)Mn(III)(L)(O)(OAc)](+). Freeze-quench/EPR experiments revealed that 2a appears at 500 ms and that another species with a 6-line spectrum is formed transiently at ca. 100 ms. 2a was prepared by reaction of 1a with tert-butyl hydroperoxide as shown by EPR and UV-visible spectroscopies and ESI-MS experiments. Its structure was studied by X-ray absorption experiments which revealed the presence of two or three O atoms at 1.87 A and three or two N/O atoms at 2.14 A. In addition one N atom was found at a longer distance (2.3 A) and one Mn at 2.63 A. 2a can be one-electron oxidized at E(1/2) = 0.91 V(NHE) (DeltaE(1/2) = 0.08 V) leading to its Mn(IV)Mn(IV) analogue. The formation of 2a from 1a was monitored by UV-visible and X-ray absorption spectroscopies. Both concur to show that an intermediate Mn(II)Mn(III) species, resembling 4a [Mn(2)(L)(OAc)(2)(H(2)O)](ClO(4))(2), the one-electron-oxidized form of 1a, is formed initially and transforms into 2a. The structures of the active intermediates 2 and 3 are discussed in light of their spectroscopic properties, and potential mechanisms are considered and discussed in the context of the biological reaction.

Reactions of the unsymmetrical phenol ligand 2-(bis(2-pyridylmethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminomethyl)-4-methylphenol with Mn(OAc)(2).4H(2)O or Mn(H(2)O)(6)(ClO(4))(2) in the presence of NaOBz affords the dimanganese(II) complexes 1(CH(3)OH), [Mn(2)(L)(OAc)(2)(CH(3)OH)](ClO(4)), and 2(H(2)O), [Mn(2)(L)(OBz)(2)(H(2)O)](ClO(4)), respectively. On the other hand, reaction of the ligand with hydrated manganese(III) acetate furnishes the mixed-valent derivative 3(H(2)O), [Mn(2)(L)(OAc)(2)(H(2)O)](ClO(4))( 2). The three complexes have been characterized by X-ray crystallography. 1(CH(3)OH) crystallizes in the monoclinic system, space group P2(1)/c, with a = 10.9215(6) A, b = 20.2318(12) A, c = 19.1354(12) A, alpha = 90 degrees, beta = 97.5310(10) degrees, gamma = 90 degrees, V = 4191.7 A(3), and Z = 4. 2(H(2)O) crystallizes in the monoclinic system, space group P2(1)/n, with a = 10.9215(6) A, b = 20.2318(12) A, c = 19.1354(12) A, alpha = 90 degrees, beta = 97.5310(10) degrees, gamma = 90 degrees, V = 4191.7 A(3), and Z = 4. 3(H(2)O) crystallizes in the monoclinic system, space group P2(1)/c, with a = 11.144(6) A, b = 18.737(10) A, c = 23.949(13) A, alpha = 90 degrees, beta = 95.910(10) degrees, gamma = 90 degrees, V = 4974(5) A(3), and Z = 4. Magnetic measurements revealed that the three compounds exhibit very similar magnetic exchange interactions -J = 4.3(3) cm(-)(1). They were used to establish tentative magneto-structural correlations which show that for the dimanganese(II) complexes -J decreases when the Mn-O(phenoxo) distance increases as expected from orbital overlap considerations. For the dimanganese(II,III) complexes, crystallographic results show that the Mn(II)-O(phenoxo) and Mn(III)-O(phenoxo) bond lengths are inversely correlated. An interesting magneto-structural correlation is found between -J and the difference between these bond lengths, delta(Mn)(-)(O) = d(Mn)()II(-)(O) - d(Mn)()III(-)(O): the smaller this difference, the larger -J. Electrochemical studies show that the mixed-valence state is favored in 1-3 by ca. 100 mV with respect to analogous complexes of symmetrical ligands, owing to the asymmetry of the electron density as found in the analogous diiron complexes.