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Spectroscopic and functional models for Hmd hydrogenase

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Title: Spectroscopic and functional models for Hmd hydrogenase
Author(s): Royer, Aaron M.
Director of Research: Rauchfuss, Thomas B.
Doctoral Committee Chair(s): Rauchfuss, Thomas B.
Doctoral Committee Member(s): Zimmerman, Steven C.; Hartwig, John F.; Girolami, Gregory S.
Department / Program: Chemistry
Discipline: Chemistry
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: Ph.D.
Genre: Dissertation
Subject(s): Hydrogenase (Hmd) transfer hydrogenation, pinacol coupling
Abstract: The hydrogenases are metalloenzymes that catalyze transformations of dihydrogen. Two hydrogenase classes, [FeFe] and [NiFe] catalyze the interconversion of dihydrogen to protons and electrons. In order to facilitate electron transfer, these enzymes contain iron-sulfur clusters. Synthetic models for both of these hydrogenases are thoroughly studied. More recently, a third class of hydrogenase, Hmd or alternatively [Fe]-hydrogenase, was discovered in methanogenic archaea. This enzyme catalyzes the heterolysis of dihydrogen to a proton with hydride transfer to a carbocation and does not require iron-sulfur clusters. Early work on Hmd revealed the presence of iron, carbonyl ligands, and a 2-pyridinol-6-acetic acid derivative. We examined the metal coordination of a similar ligand, 4-methyl-2-hydroxypyridine-6-acetic acid (cmhpH2), on the simplified platforms of Cp*Ir and Cp*Rh. We were able to show the cmhpH ligand chelates to the metal through the carboxylate oxygen and pyridine nitrogen. The hydroxyl group exhibits strong intramolecular hydrogen-bonding and also stabilizes water binding with ionization of Cl- in water. Hydrogen transfer reactions of secondary alcohols mediated by Cp*IrCl(cmhpH) were also examined. Initial work with cmhpH2 complexes of iron resulted in insoluble products. The structure of Hmd was reported in late 2008 and revised in 2009. The revised structure is a ferrous carbonyl fragment with appended thiolate, acyl and 2-pyridone/ 2-hydroxypyridine ligands. The Fe-acyl and pyridinol derivatives are both novel to biological systems. In an effort to confirm the structure reported for Hmd, we synthesized ferrous models containing the unique acyl ligand tethered to a donor ligand. Although Hmd active site incorporates a nitrogen heterocycle, we found phosphine to be a suitable alternative. The addition of thioesters, derived from the reaction of o-diphenylphosphino benzoic acid with a multitude of thiols, to Fe(0) carbonyls resulted in oxidative addition of the thioester to give complexes of the type Fe(SR)(Ph2PC6H4CO)(CO)3 with a chelating phosphine acyl ligand. These complexes readily lost CO to give a dimer of the type Fe2(SR)2(Ph2PC6H4CO)2(CO)3. In two cases, where R=Et or 2,6-dimesitylphenyl, we were able to show phosphine binding, prior to oxidative addition, gave products of the formula Fe[PPh2(C6H4COSR)](CO)4. With the bulky R = 2,6-dimesitylphenyl thioester, the oxidative addition reaction was completely arrested. In the case of R = Ph, we were able to carbonylate the dimer to give the tricarbonyl monomer, which exhibited a similar IR spectrum to the CO inhibited form of Hmd. The substitution reactions of this monomer with CN- and TsCH2NC were stereoselective, similar to the enzyme. The CN- derivative was characterized by EXAFS, XANES, and IR spectroscopy and all demonstrated a remarkable similarity to CN- inhibited Hmd. Protonation of the thiolate in Hmd has been proposed, and we examined protonation of the tricarbonyl monomer. The product was unstable even at -30 ºC and the IR spectrum was found to differ greatly from the Hmd active site. A similar method to the oxidative addition of thioesters was attempted in the addition of o-(diphenylphosphino)benzaldehyde to Fe(0) carbonyls. Although an acyl hydride intermediate was detected, the isolated product is a result of C-C coupling of two aldehyde carbons to give a tetradentate bisphosphine-bisalkoxide ligand bound to a ferrous dicarbonyl fragment. A similar coupling reaction has been reported, but our method is superior in terms of cost and yields. The Fe(P2O2)(CO)2 complex reacts with (ferrocenium)BF4 to give a 50% conversion to a complex with BF3 bound to each alkoxide. The mechanism for this reaction is proposed to involve an iron mediated F- abstraction from BF4-. The binding of Lewis acids to the alkoxides was found to be general. Addition of water to a THF solution of the bis-BF3 complex resulted in loss of the P2O2 ligand as a diol. The novel diphosphine diol was isolated in analytical purity. Hydrogen transfer reactions mediated by 2-pyridone (2hpH) complexes are potentially relevant to the mechanism by which Hmd heterolytically cleaves dihydrogen. The complex Cp*IrCl(2hp) was reported to be an excellent precatalyst for direct dehydrogenation of secondary alcohols. We found this complex reacted with secondary alcohols or dihydrogen to give a transient complex (Cp*IrHCl(2hpH)), followed by formation of a 2hp bridged dimer of the formula [Cp*IrH)2(2hp)]+. This dimer was found to be a resting state of catalysis and not the active catalyst. In the presence of Cl-, the dimer dissociates into monomers that are highly active for dehydrogenation of secondary alcohols.
Issue Date: 2011-01-14
URI: http://hdl.handle.net/2142/18433
Rights Information: Copyright 2010 Aaron M. Royer
Date Available in IDEALS: 2011-01-14
Date Deposited: December 2
 

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