Reactivity and mechanisms in aqueous organometallic chemistry: Carbon-hydrogen bond activation in water catalyzed by molybdocenes.

摘要

The chemistry of three organometallic systems in water was studied with the goal of understanding more fully the reactivity of organometallic complexes in aqueous environments.;Molybdocene complexes were found to catalyze C-H bond activation reactions in water. The catalytically active solutions were prepared from the molybdocene dimer [Cp'2Mo(mu-OH)2MoCp '2](OTs)2 (Cp' = eta 5-C5H4CH3), which was characterized crystallographically. This compound was shown to exist in a monomer-dimer equilibrium at pD 6.9 in D2O. At elevated temperatures, the CpMe hydrogens were exchanged intramolecularly by deuterium with the activation parameters DeltaH≠ = 25.0 kcal mol--1 +/- 0.68 kcal mol--1 and DeltaS≠= --5.3 cal mol--1 K--1 10.47 cal mol--1 K--1. Primary alcohols selectively exchange alpha-hydrogens intermolecularly, and some additional beta-hydrogen exchange is observed in secondary alcohols. Formation of chelate complexes, such as the independently synthesized and crystallographically characterized glycolate complex [Cp'2MoOCH2CH 2OH](OTs), inhibit the H/D exchange in multidentate alcohols. The exchange reaction proceeds via formation of a ketone hydride molybdocene intermediate [Cp'2Mo(OCR1R2)H] +,which can reversibly dissociate the ketone ligand. The molybdocene hydride complex that results from ketone dissociation was identified by independent synthesis and crystallographic characterization of the hydride complex CP '2MoH(OTf). The H/D exchange reaction proceeds stepwise, with the active catalyst being derived from the monomeric complex [Cp '2Mo(OH)(OH2)]+. The activation parameters were determined as DeltaH≠= 19.4 kcal mol --1 +/- 0.2 kcal mol--1 and DeltaS ≠= --22.7 cal mol--1 K--1 +/- 0.7 cal mol--1 K--1 and a primary kinetic isotope effect of kH, pD6.4/kD, pH6.5 = 2.2 was observed.;In addition to C-H bond activation, the molybdocene catalyst also promotes ester and nitrile hydrolysis, aldehyde disproportionation, ether cleavage and reduction of olefins in water under the appropriate conditions.;Photochemical studies of [CpCH2CH2NMe3) 2Mo2]NO3)2 and chemical and electrochemical oxidation studies of (CO)3Fe(TPPTS)2 showed that the 17-electron organometallic radicals generated from these precursors exhibit the same reactivity as their organic-soluble analogs. The iron carbonyl complex was characterized crystallographically.;This dissertation contains previously published, co-authored materials.