Ligand electronic effects in dimetal complexes: Developments toward low ionization energies and correlations with solution properties
AuthorVan Dorn, Laura Olivia
AdvisorLichtenberger, Dennis L.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractThe electronic effects of ligand substitution on paddlewheel-type metal-metal bonded systems with the general formula M₂(L ͡ L )₄ have been studied using ultraviolet photoelectron spectroscopy (UPS) and computational methods. The primary focus was to determine the influence of solvent interaction in the solution phase, and to explore what ligand properties produce low ionization energies when combined with dimetal cores. The first ionization energies (IE₁) of two substituted dimetal systems, determined by UPS in the gas phase, were compared to the E₁/₂ values obtained by electrochemical oxidation potentials in solution phase. The first dimetal system, substituted Mo₂(DPhF)₄ (where DPhF is N, N'-diphenylformamidinate), has a linear correlation of E₁/₂ with IE₁ with a slope of 0.41. This indicates a significant amount of solvent stabilization of the cation, with the effect increasing with electron withdrawing substituents. This is attributed to the removal of electron density from the metal centers, making axial coordination of an electron donating solvent favorable. A linear trend also was found for Rh₂(DPhF)₄ but with a E₁/₂:IE₁ slope of 0.53, indicating less solvent effect than with the molybdenum analogues. A possibility for the difference in slopes of the molybdenum and rhodium may be due to pi-backbonding from the solvent (CH₂Cl₂) into the empty molybdenum π* orbitals. These π* orbitals are filled in the rhodium system, so stabilization of the cation would not be possible by this mechanism. A molecule with IE₁ lower than any element or chemically prepared molecule was found during this research. The factors contributing to low IE₁ of stable, closed-shell molecules were explored by varying metals down a period while retaining the same ligand, and by varying the ligand by geometry constraints and alkyl substitutions while keeping a similar metal core structure. The results indicate two factors contribute to the unusual properties of M₂(hpp)₄ (where M = Cr, Mo or W and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyramido[1,2- a]pyrimidine): (1) strong orbital overlap interactions of a ligand b₂(g) orbital with the metal δ orbital, and (2) the relative inability of the hpp anion to stabilize negative charge.
Degree ProgramGraduate College