Ruthenium clusters of the type [Ru3(μ3-O)(OAc)6(CO)(L)(nic)], where L = 4-dimethylaminopyridine (dmap) and nic = isonicotinic acid, form hydrogen-bonded mixed-valence dimers upon a single electron reduction. Electrochemical responses show two overlapping reduction waves, indicating the presence of a thermodynamically stable mixed-valence dimer with considerable electronic coupling across the hydrogen bond. Electronic spectra of the singly reduced hydrogen-bonded mixed-valence dimer reveal two intervalence charge transfer bands in the near-infrared region consistent with a Robin-Day class II system. These bands are assigned as metal-to-metal and metal-to-bridge charge transfer, and their behavior is best described by a semiclassical three state model. Infrared spectroscopy suggests localized behavior indicating electron transfer between the two clusters is slower than 10(10) s(-1).
The preparation, electrochemistry, and spectroscopic characterization of three new species, (ZnTPPpy)Ru3O(OAc)6(CO)-pz-Ru3O(OAc)6(CO)L, where ZnTPPpy = zinc(II) 5-(4-pyridyl)-10,15,20-triphenylporphyin, L = pyridyl ligand, and pz = pyrazine, are reported. These porphyrin-coordinated Ru3O–BL–Ru3O (BL = bridging ligand) dyads are capable of undergoing intramolecular electron transfer from the photoexcited Zn porphyrin to Ru3O donor–bridge–acceptor dimer systems. Seven reversible redox processes are observed in the cyclic voltammograms of the newly synthesized dyads, showing no significant electrochemical interaction between the redox active porphyrin and the pyrazine-bridged ruthenium dimer of Ru3O trimers. From the electrochemical behavior of the dyads, large comproportionation constants (Kc = 6.0 × 10(7) for L = dmap) were calculated from the reduction potentials of the Ru(III)Ru(III)Ru(II) clusters, indicating a stable mixed-valence state. Electronic absorption spectra of the singly reduced mixed-valence species show two intervalence charge transfer (IVCT) bands assigned within the Brunschwig–Creutz–Sutin semiclassical three-state model as metal-to-bridge and metal-to-metal in character. The progression from most to least delocalized mixed-valence dimer ions, as determined by the divergence of the IVCT bands and in agreement with electrochemical data, follows the order of L = 4-dimethylaminopyridine (dmap) > pyridine (py) > 4-cyanopyridine (cpy). These systems show dynamic coalescence of the infrared spectra in the ν(CO) region of the singly reduced state. This sets the time scale of electron exchange at <10 ps. The electron transfer from the S1 excited state of the coordinated porphyrin to the dimer is predicted to be thermodynamically favorable, with ΔGFET(0) ranging from −0.54 eV for L = dmap to −0.62 eV for L = cpy. Observation of IVCT band growth under continual photolysis (λexc = 568 nm) confirms a phototriggered intramolecular electron transfer process resulting in a strongly coupled singly reduced mixed-valence species.
Presented here is the first effort to study the formation and dynamics of the triruthenium cluster (Ru 3 O) pyrazine-bridged dimer mixed-valence state. Femtosecond transient absorption spectroscopy was implemented to follow photoinduced electrontransfer reactions in a series of asymmetric porphyrin-coordinated dyads, which form strongly coupled mixed-valence species upon single reduction. Excitation of the porphyrin subunit resulted in electron transfer to the Ru 3 O dimer with a time constant τ ≈ 0.6 ps. The intramolecular electron transfer was confirmed by excitation of the Ru 3 O MLCT, which resulted in the formation of a vibrationally unrelaxed porphyrin ground state. Under both excitation experiments, the back electron transfer was extremely fast (τ CR < 0.1 ps), preventing complete time-resolved exploration of the mixed-valence state. These complexes enabled the observation of excited product states following electron-transfer processes, resulting from porphyrin S 1 and S 2 excitation. Although the charge recombination itself could not be observed, the yield of unrelaxed ground states supports the conclusion that delocalization takes place at least partially on a sub-100 fs time scale.
Transient absorption decay rate constants (k obs) for reactions of electronically excited zinc tetraphenylporphyrin (3ZnTPP*) with triruthenium oxo-centered acetate-bridged clusters [Ru3(μ3-O)(μ-CH3CO2)6(CO)(L)]2(μ-pz), where pz = pyrazine and L = 4-cyanopyridine (cpy) (1), pyridine (py) (2), or 4-dimethylaminopyridine (dmap) (3), were obtained from nanosecond flash-quench spectroscopic data (quenching constants, k q, for 3ZnTPP*/1–3 are 3.0 × 109, 1.5 × 10 9, and 1.1 × 109 M–1 s–1, respectively). Values of k q for reactions of 3ZnTPP* with 1–3 and Ru3(μ3-O)(μ-CH3CO2)6(CO)(L)2 [L = cpy (4), py (5), dmap (6)] monomeric analogues suggest that photoinduced electron transfer is the main pathway of excited-state decay; this mechanistic proposal is consistent with results from a photolysis control experiment, where growth of characteristic near-IR absorption bands attributable to reduced (mixed-valence) Ru3O-cluster products were observed.
Summary Members of the actinomycete genus Streptomyces are non-motile, filamentous bacteria that are well known for the production of biomedically relevant secondary metabolites. While considered obligate aerobes, little is known about how these bacteria respond to periods of reduced oxygen availability in their natural habitats, which include soils and ocean sediments. Here we provide evidence that the marine streptomycete strain CNQ-525 can reduce MnO2 via a diffusible mechanism. We investigated the effects of hypoxia on secondary metabolite production and observed a shift away from the antibiotic napyradiomycin towards 8-amino-flaviolin, an intermediate in the napyradiomycin biosynthetic pathway. We purified 8-amino-flaviolin and demonstrated that it is reversibly redox-active (midpoint potential –474.5 mV), indicating that it has the potential to function as an endogenous extracellular electron shuttle. This study provides evidence that environmentally triggered changes in secondary metabolite production may provide clues to the ecological functions of specific compounds, and that Gram-positive bacteria considered to be obligate aerobes may play previously unrecognized roles in biogeochemical cycling through mechanisms that include extracellular electron shuttling.
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