Spectroscopy, Electrochemistry, and Photochemistry of Polynuclear Metal-Metal Bonded Complexes

Citation

Nocera, Daniel George (1984) Spectroscopy, Electrochemistry, and Photochemistry of Polynuclear Metal-Metal Bonded Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/T14G-4N32. https://resolver.caltech.edu/CaltechETD:etd-02012005-160716

Abstract

The spectroscopic, electrochemical, and photochemical properties of two d⁴ metal polynuclear complex systems were investigated: the quadruple bond complex, Re₂Cl₈^2-, and the M₆X₁₄^2- halide cluster ions of molybdenum(II) and tungsten (II).

The vibrationally structured luminescence spectrum of Re₂Cl₈^2- at 5 K has confirmed that emission arises from the δδ^* singlet state. Both Re₂Cl₈^2- and electronically excited Re₂Cl₈^2- (Re₂Cl₈^2-*) undergo facile one-electron oxidation and reduction reactions. Aromatic amines quench Re₂Cl₈^2-* luminescence and a Marcus analysis of the steady state quenching rate constants suggest that Re₂Cl₈^3-|D⁺ is formed in the quenching reaction and that the ion-pair decays rapidly by back electron transfer. The luminescence of Re₂Cl₈^2-* is also quenched by electron acceptors (TCNE and chloranil) in nonaqueous solutions, forming Re₂Cl₈^- and reduced acceptor. The back electron transfer reactions are near the diffusion controlled limit. Electrochemical measurements suggested that photogenerated Re₂Cl₈^- anion could be trapped by Cl^- to produce Re₂Cl₉^2-, thereby circumventing the efficient back reaction. Irradiation (λ > 600 nm) of nonaqueous solutions of Re₂Cl₈^2- and chloranil or TCNE in the presence of excess Cl^- does, indeed, produce Re₂Cl₉^2- 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 Re₂Cl₉^-/2- (0.53 vs. SCE) yielded Re₂Cl₉^- as a final product. Thus, a single low energy photon facilitates a two-electron oxidation of Re₂Cl₈^2-. Direct two-electron oxidation of Re₂Cl₈^2-* by chlorine atom transfer reagents such as PtCl₆^2- to produce Re₂Cl₉^- was also attempted. Irradiation (λ > 590 nm) of dichloromethane solutions containing Re₂Cl₈^2- and PtCl₆^2- quantitatively yield Re₂Cl₉^-. Qualitative kinetic experiments measuring the wavelength dependence of the reaction rate suggest that the photochemical reaction proceeds by a free radical pathway involving the ³A_2u ligand field excited state of PtCl₆^2- 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 NBu₄Re₂Br₉ is also presented.

The M₆X₁₄^2- (M=Mo,W; X=Cl,Br,I) 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 M₆X₁₄^- 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 M₆X₁₄^2- cluster ions to undergo reversible single-electron oxidation reactions. For Mo₆Cl₁₄^2- a quasi-reversible one-electron reduction wave was observed in addition to its oxidation wave. The electrochemical and photophysical properties of Mo₆Cl₁₄^2- suggested electrogenerated chemiluminescent behavior of the cluster ion. Emission, characteristic of Mo₆Cl₁₄^2-*, is observed upon electrochemical generation of Mo₆Cl₁₄^- and Mo₆Cl₁₄^3-.

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