Photophysics and Electrochemistry of d⁴ Metal Clusters

Citation

Zietlow, Thomas Christopher (1985) Photophysics and Electrochemistry of d⁴ Metal Clusters. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/x6gp-2861. https://resolver.caltech.edu/CaltechTHESIS:01042019-095546695

Abstract

The metal-ligand and metal-metal interactions in d⁴ metal cluster systems have been examined using emission spectroscopy and electrochemical techniques. A detailed picture of the electronic structure of the hexanuclear molybdenum and tungsten clusters has not yet been achieved, but significant insight has been obtained through studying the effects of ligand, temperature, and molecular structure on the properties of the emissive excited state. Through these studies, and through analogy to the (d⁴)₂ quadruply bonded molybdenum and tungsten dimers, the dynamics of the excited state, and its relevance to the chemistry out of that state, can be better understood.

The work done on the excited state properties of the Mo₆X^2-₁₄ (X = Cl, Br, I) clusters is outlined in Chapter 2. The energy of the emissive excited state is independent of halide ligand and is best described as a metallocalized state with very little ligand character. Energy transfer quenching data have determined that the emissive excited states are spin triplets.

The unusual emissive behavior of the tungsten series W₆X₈Y^2-₆ (X, Y = Cl, Br, I) is discussed in Chapters 3 and 4. The energy of the emissive states order: W₆Cl^2-₁₄ (1.83 eV) < W₆Br^2-₁₄(1.85 eV) < W₆I^2-₁₄(2.05 eV), the opposite trend from that expected by including charge transfer character into the transition. Variation of axial and facial halides leads to the conclusion that the µ³-bridging halides dominate the electronic structure of the cluster, but that the axial halides have a strong influence on the non-radiative rate. The structural studies show that the W-W bonds are longer in the W₆I^2-₁₄ cluster than in the W₆Cl^2-₁₄ structure, indicating that the metal-metal bond overlap is not the sole determining factor in the energy of the emitting excited states.

Chapters 5, 6, and 7 describe the chemistry and properties of derivatives of these hexanuclear clusters. The W₆Br^-₁₄ anion has been synthesized and its crystal structure determined. The effect of alkylphosphine ligands on the electrochemistry and photophysics of the Mo₆Cl₁₂ cluster has been found to be very small. Although the Mo₅X^n-₁₃ (n = 1, 2, 3) clusters do not appear to have any interesting photochemical properties, the observation of the splitting between spin-orbit states in a high nuclearity cluster has implications in the photochemistry of the hexanuclear clusters.

Finally, the last two chapters describe work done on the electrochemistry of Mo₂X₄ (PR₃) ₄ dimers. An unexpected ordering of the redox potentials is ascribed to π-backbonding from the metal center to the halide ligand. Efficient electrogenerated chemiluininescence has been observed for several of these dimers.

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