A new chapter in the organometallic porphyrin story
Abstract
Metalloporphyrins comprise the prosthetic group of numerous metalloproteins, such as cytochromes, hemoglobin, and certain photosynthetic proteins. This biological relevance has resulted in a long and colorful history to metalloporphyrin chemistry. We have selected organometallic Ru porphyrin complexes as model compounds for the Fe-carbon and N-carbon bonded species formed in reactions of cytochrome P-450. The complexes of Ru(OEP)R$\sb{\rm n}$ (n = 1, 2 and R = methyl, p-X-C$\sb6$H$\sb4$, and C$\sb6$F$\sb5$) have been studied using electrochemical, chemical, and spectroscopic methods. These studies reveal the oxidation state-dependent stability for the various $\sigma$-bonded R groups. Cyclic voltammetry of Ru(OEP) (Ar)$\sb2$ in THF reveals two reversible reductions and one irreversible oxidation. The first reduction is chemically reversible, while the second reduction is coupled to loss of aryl$\sp-$ on the cyclic voltammetric time scale. The oxidation is irreversible due to a rapid Ru-to-N aryl migration. (Ru(OEP-N-C$\sb6$H$\sb5$) (C$\sb6$H$\sb5$)) (BF$\sb4$) has been crystallographically characterized. Voltammetry at 10 kV/s captures the Ru(OEP(Ar)$\sb2\sp+$ species and provides a measure of its lifetime on the order of $\mu$s. The voltammetry of Ru(OEP)(Ar) complexes reveal a reversible oxidation and a reversible reduction. These electrochemical observations of the Ru(OEP)(Ar)$\sb{\rm n}$ complexes were verified using sodium naphthalenide as a chemical reductant and AgBF$\sb4$ as a chemical oxidant. The $\sp1$H NMR spectra of mixtures of neutral and ionic species reveal rapid electron self-exchange. These physical measurements have been extended to the analogous Ru(OEP)(CH$\sb3$)$\sb{\rm n}$ complexes. Ru(OEP)(CH$\sb3$)$\sb2$ exhibits irreversible oxidations and reductions; here the first reduction results in loss of $\cdot$CH$\sb3$. The initial oxidation product, Ru(OEP)(CH$\sb3$)$\sb2\sp+$, rapidly N-methylates and further reacts to a bridging methylene product, (Ru(OEP-N-$\mu$-CH$\sb2$) (CH$\sb3$)) (BF$\sb4$), confirmed by an X-ray crystal structure. The mono-CH$\sb3$ complex shows the same redox chemistry as observed with the mono-aryls. A combination, at least conceptually, of these two classes of Ru(OEP)R$\sb{\rm n}$ complexes lies in the Ru(OEP)(C$\sb6$H$\sb5$)(CH$\sb3$) complex, synthesized from Ru(OEP)(C$\sb6$H$\sb5$)$\sp-$ and CH$\sb3$I. This mixed aryl-alkyl complex is the first example of an aryl-alkyl porphyrin complex. The Ru(OEP) (C$\sb6$H$\sb5$)(CH$\sb3$) complex thermally decomposes via Ru-CH$\sb3$ bond homolysis, thus providing the opportunity to measure the Ru-C bond energy, 26.5 kcal/mol, a quantity of considerable interest within the field.
Degree
Ph.D.
Advisors
Leidner, Purdue University.
Subject Area
Chemistry
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