Biopyrrin Pigments as Redox-Active Ligands in Complexes of Noble Metals

Abstract

Biopyrrins are naturally occurring oligopyrroles formed from the oxidative metabolism ofheme and linear tetrapyrrolic bile pigments. These tripyrrolic and dipyrrolic fragments maintain the effective metal chelation properties of their parent tetrapyrroles and offer compact platforms for robust ligand-centered redox chemistry. As such, metal complexes containing the tripyrrin-1,14-dione and dipyrrin-1,9-dione frameworks have found applications as electron reservoirs, in reversible ð-ð assembly, and as redox-switchable fluorophores. This dissertation focuses on the coordination chemistry of the tripyrrin-1,14- dione and dipyrrin-1,9-dione ligands to several noble metals: palladium, platinum, and gold. Chapter 1 provides a background on the biological origin of tetrapyrrolic bile pigments and their low-order biopyrrin metabolites and offers an overview of the coordination chemistry and redox activity of metal complexes of the tripyrrindione and dipyrrindione pigments. Chapter 2 describes the synthesis of three heteroleptic palladium(II) complexes of dipyrrindione containing supporting primary amine ligands. Characterization of these compounds by X-ray diffractometry revealed the presence of intramolecular hydrogen bonding interactions between the amine ligands and the dipyrrindione scaffold. Electrochemical and spectroelectrochemical studies demonstrated that these cationic complexes undergo ligand-centered reduction at more modest potentials than previously examined complexes. In Chapter 3, the dimerization of a bis(aqua) PdII complex of dipyrrin-1,9-dione to form a binuclear ì-hydroxo dimer is investigated. The two-electron reduction product of the dimer was isolated and characterized as ligand-based triplet diradical by X-ray diffraction, EPR spectroscopic, and DFT computational 21 techniques. Further examination on the colorimetric water sensing abilities of the bis(aqua) complex in organic solvents is also discussed. In Chapter 4, the synthesis two platinum(II) tripyrrindione complexes, containing either aqua or tert-butyl isocyanide ligands within the primary coordination sphere, is described. The influence of these two supporting ligands on the redox profiles and ð- dimerization is explored, wherein incorporation of the isocyanide group modifies the thermodynamics of dimer formation. Additionally, the presence of multicenter bonding interactions between dimers of oxidized cationic PtII tripyrrindione complexes is shown through DFT computations and crystallographic analysis. Chapter 5 investigates the coordination of tripyrrindione to trivalent gold(III), wherein electrochemical analysis reveals the combination of both ligand- and metal-centered redox events. The chemical reduction of the gold center, to form a proposed Au(II) intermediate, results in activation of C-Cl bonds in dichloromethane aided by the tripyrrindione framework. The work described in this dissertation was performed with financial support from NSF grant 1454047.