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Spectroscopy, electrochemistry, and photochemistry of polynuclear metal-metal bonded complexes

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  • Chemistry
  • Physics


NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The spectroscopic, electrochemical, and photochemical properties of two [...] metal polynuclear complex systems were investigated: the quadruple bond complex, [...], and the [...] halide cluster ions of molybdenum(II) and tungsten (II). The vibrationally structured luminescence spectrum of [...] at 5 K has confirmed that emission arises from the [...] singlet state. Both [...] and electronically excited [...] undergo facile one-electron oxidation and reduction reactions. Aromatic amines quench [...] luminescence and a Marcus analysis of the steady state quenching rate constants suggest that [...] is formed in the quenching reaction and that the ion-pair decays rapidly by back electron transfer. The luminescence of [...] is also quenched by electron acceptors (TCNE and chloranil) in nonaqueous solutions, forming [...] and reduced acceptor. The back electron transfer reactions are near the diffusion controlled limit. Electrochemical measurements suggested that photogenerated [...] anion could be trapped by [...] to produce [...], thereby circumventing the efficient back reaction. Irradiation [...] of nonaqueous solutions of [...] and chloranil or TCNE in the presence of excess [...] does, indeed, produce [...] in quantitative yield. Photolysis reactions employing a quencher (e.g. 2,3-dichloro-5,6-dicyano-benzoquinone) possessing a redox couple with a potential greater than that of [...] yielded [...] as a final product. Thus, a single low energy photon facilitates a two-electron oxidation of [...]. Direct two-electron oxidation of [...] by chlorine atom transfer reagents such as [...] to produce [...] was also attempted. Irradiation [...] of dichloromethane solutions containing [...] and [...] quantitatively yield [...]. Qualitative kinetic experiments measuring the wavelength dependence of the reaction rate suggest that the photochemical reaction proceeds by a free radical pathway involving the [...] ligand field excited state of [...] and does not involve an atom transfer reaction mechanism. These experiments are not without their ambiguities, however, and further areas of research are discussed. The crystal structure analysis of [...] is also presented. The [...] ions are intensely luminescent in the solid state and solution and the photophysical properties of the six cluster ions are documented. These results in conjunction with those of the EPR spectra of the electrochemically generated [...] anions are discussed in terms of recent theoretical models for the electronic structure of the cluster ions. Electrochemical experiments were also conducted and showed the [...] cluster ions to undergo reversible single-electron oxidation reactions. For [...] a quasi-reversible one-electron reduction wave was observed in addition to its oxidation wave. The electrochemical and photophysical properties of [...] suggested electrogenerated chemiluminescent behavior of the cluster ion. Emission, characteristic of [...], is observed upon electrochemical generation of [...] and [...].

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