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

The spectroscopic, electrochemical, and photochemical properties of two d4 metal polynuclear complex systems were investigated: the quadruple bond complex, Re2Cl82-, and the M6X142- halide cluster ions of molybdenum(II) and tungsten (II). The vibrationally structured luminescence spectrum of Re2Cl82- at 5 K has confirmed that emission arises from the δδ* singlet state. Both Re2Cl82- and electronically excited Re2Cl82- (Re2Cl82-*) undergo facile one-electron oxidation and reduction reactions. Aromatic amines quench Re2Cl82-* luminescence and a Marcus analysis of the steady state quenching rate constants suggest that Re2Cl83-|D+ is formed in the quenching reaction and that the ion-pair decays rapidly by back electron transfer. The luminescence of Re2Cl82-* 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 Re2Cl92-, thereby circumventing the efficient back reaction. Irradiation (λ > 600 nm) of nonaqueous solutions of Re2Cl82- and chloranil or TCNE in the presence of excess Cl- does, indeed, produce Re2Cl92- 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 Re2Cl82-. Direct two-electron oxidation of Re2Cl82-* by chlorine atom transfer reagents such as PtCl62- to produce Re2Cl9- was also attempted. Irradiation (λ > 590 nm) of dichloromethane solutions containing Re2Cl82- and PtCl62- 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 3A2u ligand field excited state of PtCl62- 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 M6X142- (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 M6X142- cluster ions to undergo reversible single-electron oxidation reactions. For Mo6Cl142- a quasi-reversible one-electron reduction wave was observed in addition to its oxidation wave. The electrochemical and photophysical properties of Mo6Cl142- suggested electrogenerated chemiluminescent behavior of the cluster ion. Emission, characteristic of Mo6Cl142-*, is observed upon electrochemical generation of Mo6Cl14- and Mo6Cl143-.