Synthesis, Characterization and Investigation of Metal-Metal Bonded Dirhodium Complexes
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This dissertation focuses on the design and tailoring of dirhodium complexes and the investigation of their structural, spectroscopic and theoretical properties. The variation of the ligands occupying the equatorial (eq) positions results in appreciable changes in the structure and the electronic properties, which renders them suitable for different applications. The coordination of methyl isocyanide ligands to dirhodium centers bridged by electron-rich formamidinate or orthometalated phosphine ligands led to a family of unprecedented dirhodium complexes of general formula cis-[Rh_(2)(μ-L)_(2)(CNCH_(3))_(6)]^(2+) (L = bridging ligand). These complexes exhibit unusually long Rh-Rh bond distances and short Rh-C (CH_(3)NC) contacts and exhibit the unexpected bonding situation in which the Rh_(2)(π*) levels lie lower in energy than the Rh_(2)(σ) orbitals. Reactions between cis-[Rh_(2)(Form)_(2)(CH_(3)CN)_(6)]^(2+) (Form = formamidinate) and the electron accepting chelating diimine ligands dpq = dipyrido[3,2-f:2',3'-h]-quinoxaline, dppz = dipyrido[3,2-a:2’,3’-c]phenazine and dppn = benzo[i]dipyrido[3,2-a:2’,3’-h]quinoxaline produce complexes of the general type cis-[Rh_(2)(Form)_(2)(N-N)_(2)]^(2+) with intriguing photophysical properties. The lowest energy absorption in their electronic absorption spectra is a Ligand-to-Ligand Charger-Transfer (LL’CT) transition which leads to interesting excited state reactivity patterns, most notably water reduction to produce hydrogen. Three 6-R-hydroxyl-pyridine ligand with different substituents (R= -CH_(3), -F, -Cl) was selected as a bridging ligand for the series of partial paddlewheel complexes cis-[Rh_(2)(μ-L)_(2)(CH_(3)CN)_(6)]^(2+). The goal was to probe the effect of the bridging ligand substituents on the lability of the eq CH_(3)CN ligands. It was shown the substitution rate of the eq CH_(3)CN ligands is intimately correlated to the electronic characteristics of the bridging ligands with decreased lability occurring in the order of –F < –Cl < –CH_(3), in accord with the stronger trans effect exerted by the more electron donating ligands. Additional efforts to tune the electronic properties of dirhodium complexes with chelating diimine ligands include the search for new bridging ligands, e.g. [Ph_(2)P(C_(6)H_(4))]^(-). Density Functional Theory (DFT) and Time Dependent (TD) DFT calculations on the synthesized cis-[Rh_(2)[Ph_(2)P(C_(6)H_(4))]_(2)(N-N)_(2)]^(2+) (N-N = dpq, dppz, and dppn) complexes indicate that they exhibit very different electronic structures as compared to the dirhodium complexes with formamidinate bridging ligands. Finally, additional studies were directed at the synthesis of ligands with hydrophilic functional groups to improve the water solubility of the complexes in order to enhance their activity as photodynamic anticancer therapy agents and water reduction photocatalysts.
Li, Zhanyong (2014). Synthesis, Characterization and Investigation of Metal-Metal Bonded Dirhodium Complexes. Doctoral dissertation, Texas A & M University. Available electronically from