Light-induced multistep oxidation of dinuclear manganese complexes for artificial photosynthesis

Solution Equilibria Formation of Manganese(II) Complexes with Ethylenediamine, 1,3-Propanediamine and 1,4-ButanediaMine in Methanol

Manganese is an abundant element that plays critical roles and is at the reaction center of several enzymes. In order to promote an understanding of the behavior of manganese(II) ion with several aliphatic ligands, in this work, the stability and spectral behavior of the complexes with manganese(II) and ethylenediamine, 1,3-propanediamine or 1,4-butanediamine were explored. A spectrophotometric study of its speciation in methanol was performed at 293 K. The formation constants obtained for these systems were: manganese(II)-ethylenediamine log β110 = 3.98 and log β120 = 7.51; for the manganese(II)-1,3-propanediamine log β110 = 5.08 and log β120 = 8.66; and for manganese(II)-1,4-butanediamine log β110 = 4.36 and log β120 = 8.46. These results were obtained by fitting the experimental spectrophotometric data using the HypSpec software. The complexes reported in this study show a spectral pattern that could be related to a chelate effect in which the molar absorbance is not directly related to the increase in the carbon chain of the ligands.

Non-heme manganese catalysts for on-demand production of chlorine dioxide in water and under mild conditions

Two non-heme manganese complexes are used in the catalytic formation of chlorine dioxide from chlorite under ambient temperature at pH 5.00. The catalysts afford up to 1000 turnovers per hour and remain highly active in subsequent additions of chlorite. Kinetic and spectroscopic studies revealed a Mn(III)(OH) species as the dominant form under catalytic conditions. A Mn(III)(μ-O)Mn(IV) dinuclear species was observed by EPR spectroscopy, supporting the involvement of a putative Mn(IV)(O) species. First-order kinetic dependence on the manganese catalyst precludes the dinuclear species as the active form of the catalyst. Quantitative kinetic modeling enabled the deduction of a mechanism that accounts for all experimental observations. The chlorine dioxide producing cycle involves formation of a putative Mn(IV)(O), which undergoes PCET (proton coupled electron-transfer) reaction with chlorite to afford chlorine dioxide. The ClO2 product can be efficiently removed from the aqueous reaction mixture via purging with an inert gas, allowing for the preparation of pure chlorine dioxide for on-site use and further production of chlorine dioxide.

High-valent [MnFe] and [FeFe] cofactors in ribonucleotide reductases

Ribonucleotide reductases (RNRs) are essential for DNA synthesis in most organisms. In class-Ic RNR from Chlamydia trachomatis (Ct), a MnFe cofactor in subunit R2 forms the site required for enzyme activity, instead of an FeFe cofactor plus a redox-active tyrosine in class-Ia RNRs, for example in mouse (Mus musculus, Mm). For R2 proteins from Ct and Mm, either grown in the presence of, or reconstituted with Mn and Fe ions, structural and electronic properties of higher valence MnFe and FeFe sites were determined by X-ray absorption spectroscopy and complementary techniques, in combination with bond-valence-sum and density functional theory calculations. At least ten different cofactor species could be tentatively distinguished. In Ct R2, two different Mn(IV)Fe(III) site configurations were assigned either L(4)Mn(IV)(μO)(2)Fe(III)L(4) (metal-metal distance of ~2.75Å, L = ligand) prevailing in metal-grown R2, or L(4)Mn(IV)(μO)(μOH)Fe(III)L(4) (~2.90Å) dominating in metal-reconstituted R2. Specific spectroscopic features were attributed to an Fe(IV)Fe(III) site (~2.55Å) with a L(4)Fe(IV)(μO)(2)Fe(III)L(3) core structure. Several Mn,Fe(III)Fe(III) (~2.9-3.1Å) and Mn,Fe(III)Fe(II) species (~3.3-3.4Å) likely showed 5-coordinated Mn(III) or Fe(III). Rapid X-ray photoreduction of iron and shorter metal-metal distances in the high-valent states suggested radiation-induced modifications in most crystal structures of R2. The actual configuration of the MnFe and FeFe cofactors seems to depend on assembly sequences, bound metal type, valence state, and previous catalytic activity involving subunit R1. In Ct R2, the protonation of a bridging oxide in the Mn(IV)(μO)(μOH)Fe(III) core may be important for preventing premature site reduction and initiation of the radical chemistry in R1.

Dinuclear manganese centers in the manganese-lead-tellurate glasses

FTIR, UV-VIS and EPR spectra of manganese doped lead-tellurate glasses with composition xMnO·(100-x)[4TeO2·PbO2] where x=0, 1, 5, 10, 20, 30, 40mol% have been studied. The FTIR spectra show the formation of the Mn-O-Pb and Mn-O-Te bridging bonds by increasing of MnO concentration. The UV-VIS spectra show the Mn(+3) species exhibit pronounced absorption, which masks the Mn(+2) spin-forbidden absorption bands when Mn(+2) ions are in high concentrations in these glasses. The EPR spectra exhibit resonance signals characteristic of Mn(+2) ions. The resonance signal located at g≈2 is due to Mn(+2) ions in an environment close to octahedral symmetry, whereas the resonance at g≈4.3 and 3.3 are attributed to the rhombic surroundings of the Mn(+2) ions. The increase in the MnO content gives rise to absorption at g≈2.4 and the paramagnetic ions are involved in dinuclear manganese centers.

Coupled electron transfers in artificial photosynthesis

Light-induced charge separation in molecular assemblies has been widely investigated in the context of artificial photosynthesis. Important progress has been made in the fundamental understanding of electron and energy transfer and in stabilizing charge separation by multi-step electron transfer. In the Swedish Consortium for Artificial Photosynthesis, we build on principles from the natural enzyme photosystem II and Fe-hydrogenases. An important theme in this biomimetic effort is that of coupled electron-transfer reactions, which have so far received only little attention. (i) Each absorbed photon leads to charge separation on a single-electron level only, while catalytic water splitting and hydrogen production are multi-electron processes; thus there is the need for controlling accumulative electron transfer on molecular components. (ii) Water splitting and proton reduction at the potential catalysts necessarily require the management of proton release and/or uptake. Far from being just a stoichiometric requirement, this controls the electron transfer processes by proton-coupled electron transfer (PCET). (iii) Redox-active links between the photosensitizers and the catalysts are required to rectify the accumulative electron-transfer reactions, and will often be the starting points of PCET.

Oxygen evolving reactions catalysed by synthetic manganese complexes: A systematic screening

A set of six multinuclear manganese complexes was screened for the ability to catalyse reactions yielding O(2) under coherent experimental conditions; we identify a much larger number of manganese compounds than previously known that catalyse oxygen formation.

Consistent simulation of X- and Q-band EPR spectra of an unsymmetric dinuclear Mn2II,III complex

Simulation of X- and Q-band electron paramagnetic resonance (EPR) spectra of an unsymmetric dinuclear [Mn(2)(II,III)L(mu-OAc)(2)]ClO(4) complex (1), (L is the dianion of 2-{[N,N-bis(2-pyridylmethyl)amino]methyl}-6-{[N-(3,5-di-tert-butyl-2-hydroxybenzyl)-N-(2-pyridylmethyl)amino]methyl}-4-methylphenol) was performed using one consistent set of simulation parameters. Rhombic g-tensors and hyperfine tensors were necessary to obtain satisfactory simulation of the EPR spectra. The anisotropy of the effective hyperfine tensors of each individual (55)Mn ion was further analyzed in terms of intrinsic hyperfine tensors. Detailed analysis shows that the hyperfine anisotropy of the Mn(III) ion is a result of the Jahn-Teller effect and thus an inherent character. In contrast, the anomalous hyperfine anisotropy of the Mn(II) ion is attributed as being transferred from the Mn(III) ion through the spin exchange interaction. The anisotropy parameter for the Mn(II) is deduced as D(II)=-1.26+/-0.2cm(-1). This is the first reported D(II) value for a Mn(II) ion in a weakly exchange coupled mixed-valence Mn(2)(II,III) complex with a bis-mu-acetato-bridge. The [see text] electronic configuration of the Mn(III) ion in 1 is revealed by the negative sign of its intrinsic hyperfine tensor anisotropy, Deltaa(III)=a(z)-a(x,y)=-46cm(-1). Lower spectral resolution of the Q-band EPR spectrum as compared to the X-band EPR spectrum is associated to large line width broadening of the x- and y-components in contrast to the z-component. The origins of the unequal distribution of line width between the z- and x-, y-components are discussed.

Mimicking the electron donor side of Photosystem II in artificial photosynthesis

This review focuses on our recent efforts in synthetic ruthenium-tyrosine-manganese chemistry mimicking the donor side reactions of Photosystem II. Tyrosine and tryptophan residues were linked to ruthenium photosensitizers, which resulted in model complexes for proton-coupled electron transfer from amino acids. A new mechanistic model was proposed and used to design complexes in which the mechanism could be switched between concerted and step-wise proton-coupled electron transfer. Moreover, a manganese dimer linked to a ruthenium complex could be oxidized in three successive steps, from Mn (2) (II,II) to Mn (2) (III,IV) by the photo-oxidized ruthenium sensitizer. This was possible thanks to a charge compensating ligand exchange in the manganese complex. Detailed studies of the ligand exchange suggested that at high water concentrations, each oxidation step is coupled to a proton-release of water-derived ligands, analogous to the oxidation steps of the manganese cluster of Photosystem II.

Ligand exchange upon oxidation of a dinuclear Mn complex–detection of structural changes by FT-IR spectroscopy and ESI-MS

The structural rearrangements triggered by oxidation of the dinuclear Mn complex [Mn(2)(bpmp)(mu-OAc)2]+(bpmp = 2,6-bis[bis(2-pyridylmethyl)amino]methyl-4-methylphenol anion) in the presence of water have been studied by combinations of electrochemistry with IR spectroscopy and with electrospray ionization mass spectrometry (ESI-MS). The exchange of acetate bridges for water (D2O) derived ligands in different oxidation states could be monitored by mid-IR spectroscopy in CD(3)CN-D(2)O mixtures following the v(as(C-O)) bands of bound acetate at 1594.4 cm(-1)(II,II), 1592.0 cm(-1)(II,III) and 1586.5 cm(-1)(III,III). Substantial loss of bound acetate occurs at much lower water content (< 0.5% v/v) in the III,III state than in the II,II and II,III states (> or = 10%). The ligand-exchange reactions do not initially reduce the overall charge of the complex but facilitate further oxidation by proton-coupled electron transfer as the water-derived ligands are increasingly deprotonated in higher oxidation states. In the IR spectra deprotonation could be followed by the formation of acetic acid (DOAc, approximately 1725 cm(-1), v(C-O)) from the released acetate (1573.6 cm(-1), v(as(C-O))). By the on-line combination of an electrochemical flow cell with ESI-MS several product complexes could be identified. A di-mu-oxo bridged III,IV dimer [Mn(2)(bpmp)(mu-O)(2)](2+)(m/z 335.8) can be generated at potentials below the III,III/II,III couple of the di-mu-acetato complex (0.61 V vs. ferrocene). The ligand-exchange reactions allow for three metal-centered oxidation steps to occur from II,II to III,IV in a potential range of only 0.5 V, explaining the formation of a spin-coupled III,IV dimer by photo-oxidation with [Ru[bpy)(3)](3+) in previous EPR studies.

Elucidating excited state electronic structure and intercomponent interactions in multicomponent and supramolecular systems

Rational design of supramolecular systems for application in photonic devices requires a clear understanding of both the mechanism of energy and electron transfer processes and how these processes can be manipulated. Central to achieving these goals is a detailed picture of their electronic structure and of the interaction between the constituent components. We review several approaches that have been taken towards gaining such understanding, with particular focus on the physical techniques employed. In the discussion, case studies are introduced to illustrate the key issues under consideration.