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Exploring and exploiting Ruthenium-porphyrin complexes : functionalization, atropisomerism, small-molecule recognition, catalysis, and biological implications Rebouças, Júlio Santos

Abstract

This thesis represents a contribution to advances in ruthenium-porphyrin chemistry. The synthesis of Ru(carbonyl) porphyrins via a new metallation strategy using hexakis(N,N-dimethylformamide)ruthenium(III) triflate is examined in detail, covering a broad range of porphyrins. Mechanistic insights, advantages, and limitations of the methodology are also addressed. The method is also shown to be of value in the preparation of other related Ru macrocyclic complexes, such as those of tetradentate Schiff-bases. The direct β-functionalization of Ru(carbonyl) porphyrins is explored, as a convenient alternative for the Ru-metallation of β-functionalized free-bases. Specifically, protocols for β-octachlorination, β-octabromination, and β-mononitration are discussed. Electrochemical studies on Ru(carbonyl) porphyrins are explored as probes for investigating some structural aspects of these complexes. It is proposed that Ru complexation hinders distortion of the porphyrin ring from a planar configuration, and examination of literature crystallographic data is used to rationalize such a "Ru effect". The effect has direct consequences in hampering aryl-ring rotation in meso-tetraarylporphyrins, and this is exploited to yield the first examples of atropisomerism in non-ortho-substituted porphyrins containing non-bulky meta-substituents. Ruthenium porphyrins are used as models for studying several aspects of heme-proteins that contain S-donor ligands proximal to the heme moiety. The coordination capability of the Ru(porphyrin) moiety is explored for preparation of adducts with thiols, organic disulfides, and trisulfides. The reactions between Ru porphyrins and H₂S are also discussed. The steric and electronic effects involved in small-molecule recognition by simple metalloporphyrins using Ru porphyrins and thiols as probes are examined thoroughly. The model proposed to describe thiol coordination is also extended to another coordination system, that with primary amines, and represents a successful, general attempt to depict quantitatively the non-bonding interactions between axial ligands and the porphyrin plane. The implications of the findings within the thiol--Ru-porphyrin systems to the evolution of heme-thiol(ate) proteins are contemplated, and the application of the amine--Ru-porphyrin models to examine the interactions between primary amines and heme-proteins is demonstrated. The first studies on investigation of the O₂-oxidation of thiols catalyzed by Ru porphyrins are presented, together with a tentative mechanism that accommodates generally the kinetic and spectroscopic data. The ability of Ru porphyrins to perform O₂-oxidative N-dealkylations of tertiary amines, which is a classical cytochrome P450-catalyzed reaction, is also demonstrated. Finally, a simple (phosphine-free) RuCl₃-based, H₂-hydrogenation method is shown to be useful for the preparation of mesoporphyrin dimethyl ester.

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