Manganese and Iron Coordination Complexes for Biological Imaging and Oxidative Catalysis
Type of Degreedissertation
Chemistry and Biochemistry
MetadataShow full item record
The ligand N,N’-bis(2-pyridylmethyl)-bis(ethylacetate)-1,2-ethanediamine (debpn) coordinates divalent transition metal ions in either a pentadentate or hexadentate fashion. The coordination number correlates with the ionic radius of the metal ion, with larger cations being heptacoordinate as assessed by solid-state analysis. With Mn(II), the debpn ligand is hexadentate and remains bound to the oxophilic metal ion, even when dissolved in water. The ligand’s incomplete coordination of the manganous ion allows water molecules to coordinate to the metal center. These two properties, coupled with the high paramagnetism associated with the S = 5/2 metal center, enable [Mn(debpn)(H2O)](ClO4)2 to serve as a stable and effective magnetic resonance imaging contrast agent despite the ligand’s lack of both a macrocyclic component and an anionic charge. The Mn(II) and Fe(II) complexes of debpn are capable of catalyzing alkene epoxidation and aliphatic C-H activation reactions, although these activities are inferior to those of related complexes with less coordinating ligands. The hydrocarbon oxidation catalyzed by iron is more severely disrupted. Cyclic voltammetry indicates that the +2 oxidation states for both debpn complexes’ metal ions are stabilized by the two additional chelate arms. Product analysis of the C–H activation and olefin epoxidation chemistries suggest that ligand-substrate steric interactions may exert additional inhibitory effects on the reactivity for the manganese catalysts. The compound N,N’-bis(2-pyridylmethyl)-N,N’-bis(neopentyl)-1,2-ethanediamine(dnbpn) and its ferrous complex [Fe(dnbpn)(OTf)2] were synthesized. The Fe(II) complex was used to catalyze the oxidation of hydrocarbons by both H2O2 and O2. Although the catalyzed alkane oxidation by H2O2 displays a higher preference for secondary over tertiary carbons than those associated with previously reported non-heme iron catalysts, the catalytic activity is markedly inferior. In addition to directing the catalyzed oxidation towards the less sterically congested C-H bonds of the substrates, the neopentyl groups destabilize the metal-based oxidants generated from H2O2 and the Fe(II) complex. The oxidant generated from O2 reacts with allylic and benzylic C-H bonds in the absence of a sacrificial reductant; less dehydrogenation activity is observed than with related previously described systems that use O2 as a terminal oxidant. The formation of a ferric hydroperoxide species from [Fe(bbpc)(MeCN)2]2+ (bbpc = N,N’-di(phenylmethyl)-N,N’-bis(2-pyridinylmethyl)-1,2-cyclohexanediamine) and its subsequent decomposition were analyzed with stopped-flow kinetics. The rate of decay does not scale linearly with the concentrations of either water or substrate, suggesting that the ferric hydroperoxide degrades through homolysis of the O-O bond and is not the relevant metal-based oxidant in aliphatic C-H activation. The rate law corresponding to the complex’s formation from O2 is consistent with the intermediacy of a mononuclear ferric superoxo species.