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- Sun, L C, et al.
(författare)
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Binuclear ruthenium-manganese complexes as simple artificial models for photosystem II in green plants
- 1997
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 119:30, s. 6996-7004
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Tidskriftsartikel (refereegranskat)abstract
- As part of a project aimed at developing models for photosystem II (PSII) in green plants, we have prepared a series of model compounds (7, 8, and 13). In these compounds, a photosensitizer, ruthenium(II) tris(bipyridyl) complex (to mimic the function of P-680 in PSII), was covalently linked to a manganese(II) ion through different bridging ligands. The structures of the compounds were characterized by electron paramagnetic resonance measurements and electrospray ionization mass spectrometry. The interaction between the ruthenium and manganese moieties within the complex was probed by steady-state and time-resolved emission measurements. When the binuclear complexes are exposed to flash photolysis in the presence of an electron acceptor such as methylviologen (MV2+), it could be shown that after the initial electron transfer from the excited state of Ru(II) in compound 7, forming Ru(III) and MV+., an intramolecular electron transfer from coordinated Mn(II) to the photogenerated Ru(III) occurred with a first-order rate constant of 1.8 x 10(5) s(-1), regenerating Ru(II). This is believed to be the first supramolecular system where a manganese complex has been used as an electron donor to a photo-oxidized photosensitizer, Possible extensions to develop the manganese donor, and thus to approach the function of reaction center in PSII, are indicated.
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| 3. |
- Nivorozhkin, A L, et al.
(författare)
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Structure and electrochemical studies of [(trispicMeen)ClFeIII OFeIII Cl(trispicMeen)](2+). Spectroscopic characterization of the mixed-valence Fe-III OFeII form. Relevance to the active site of dinuclear iron-oxo proteins
- 1997
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 36:5, s. 846-853
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Tidskriftsartikel (refereegranskat)abstract
- The dinuclear species [(trispicMeen)(ClFeOFeCl)-O-III-Cl-III(trispicMeen)]Cl(OH)(H2O)(7) (1) (trispicMeen = N,N,N'-tris-(2-pyridylmethyl)-N'-methylethane- 1,2-diamine) was synthesized. It crystallizes in the monoclinic space group C2/c with a = 33.87(2) Angstrom, b = 17.42(2) Angstrom, c = 23.41(5) Angstrom, beta = 132.88(5)degrees, V = 10 121(25) Angstrom(3), and Z = 8. It contains an almost linear unit (Fe-O-Fe angle = 177.4(7)degrees). The potentially pentadentate Ligand is in fact only tetracoordinated with one pyridine not bound to the metal ion. The octahedral coordination of Fe(Ln) is completed by one chloride ion. The structure of [(bispicMeen)ClFeOFeCl(bispicMeen)]Cl-2 . CH3COCH3 . 2H(2)O (2) (bispicMeen = N,N'-bis(2-pyridylmethyl)-N'-methylethane-1,2-diamine) was also determined. It crystallizes in the monoclinic space group C2/c with a 11.124(4) Angstrom, b = 22.769(9) Angstrom, c = 15.874(6) Angstrom, beta = 97.79(4)degrees, V = 3984(3) Angstrom(3), and Z = 4. The main difference from 1 is that, in 2, the Fe-O-Fe unit is bent with an FeOFe angle 152.3(3)degrees. In cyclic voltammetry, 1 exhibits two reduction peaks at -0.230 and -0.960 V/SCE. They correspond respectively to the reduction to the (FeFeIII)-Fe-II and (FeFeIII)-Fe-II states. Cyclic voltammetry shows that the mixed-valent form [(trispicMeen)(ClFeOFeCl)-O-II-Cl-III(trispicMeen)](+) (E degrees = -0.175 V/SCE) is in equilibrium with another species (E degrees = +0.065 V/SCE) proposed to be [(trispicMeen)(FeOFeCl)-O-II-Cl-III(trispicMeen)](2+) in which a chloride ion has been displaced by the originally unbound pyridine. The equilibrium constant was estimated to be 90 M(-1), and the rate of the recombination of chloride to the [(trispicMeen)(FeOFeCl)-O-III-Cl-III(trispicMeen)](3+) complex was found equal to 3 x 10(5) M(-1) s(-1). Controlled potential electrolysis of an acetonitrile solution of 1 allowed the preparation of the mixed-valent Fe(II)-O-Fe(III) form which gives an almost isotropic EPR signal similar to that already observed with ore-bridged model compounds (Holt, et al. Inorg. Chem. 1993, 32, 5844. Hartman, et al. J. Am. Chem. Sec. 1987, 109, 7387) but different from the rhombic one observed in the mixed valent form of MMO. The mixed-valent forms slowly disproportionate to a mixture of (FeFeIII)-Fe-III and Fe(II) forms. The mixed-valent forms could be generated by radiolysis at 77 K, and an EPR study of the mixed-valent forms obtained by this procedure demonstrated that these species could not be protonated. Radiolysis of 1 at 77 K afforded the EPR spectrum of [(trispicMeen)(ClFeOFeCl)-O-II-Cl-III(trispicMeen)](+); upon annealing at 200 K, the solution gave an EPR spectrum very similar to that observed on the electrochemically reduced solution. This is in agreement with the observation of substitution of a chloride ligand. The mixed-valent form was not detected with the analogous complexes of the bispicen family: [(bispicMeen)ClFeOFeCl(bispicMeen)]Cl-2 . CH3COCH3 . 2H(2)O (2) and [(bispicMe(2)en)ClFeOFeCl(bispicMe(2)en)]Cl-2 (3). Upon reduction, these complexes quickly (at CV time scale) decompose to mononuclear species.
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