Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 109, Issue 7, Pages 2246-2250Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1118934109
Keywords
organometallic catalysis; reaction intermediates; ruthenium; kinetics
Categories
Funding
- Center for Molecular Analysis and Design at Stanford University [1123893-1-AABGE, 1134697-1-EANUU]
- Swiss National Science Foundation [PBEZP2-133126]
- Department of Defense
- Air Force Office of Scientific Research [FA 9550-10-1-0235]
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Desorption electrospray ionization (DESI) coupled to high-resolution Orbitrap mass spectrometry (MS) was used to study the reactivity of a (beta-amino alcohol)(arene) RuCl transfer hydrogenation catalytic precursor in methanol (CH3OH). By placing [(p-cymene) RuCl2](2) on a surface and spraying a solution of beta-amino alcohol in methanol, two unique transient intermediates having lifetimes in the submillisecond to millisecond range were detected. These intermediates were identified as Ru (II) and Ru (IV) complexes incorporating methyl formate (HCOOCH3). The Ru (IV) intermediate is not observed when the DESI spray solution is sparged with Ar gas, indicating that O-2 dissolved in the solvent is necessary for oxidizing Ru (II) to Ru (IV). These proposed intermediates are supported by high-resolution and high mass accuracy measurements and by comparing experimental to calculated isotope profiles. Additionally, analyzing the bulk reaction mixture using gas chromatography-MS and nuclear magnetic resonance spectroscopy confirms the formation of HCOOCH3. These results represent an example that species generated from the (beta-amino alcohol)(arene) RuCl (II) catalytic precursor can selectively oxidize CH3OH to HCOOCH3. This observation leads us to propose a pathway that can compete with the hydrogen transfer catalytic cycle. Although bifunctional hydrogen transfer with Ru catalysts has been well-studied, the ability of DESI to intercept intermediates formed in the first few milliseconds of a chemical reaction allowed identification of previously unrecognized intermediates and reaction pathways in this catalytic system.
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