Journal
ORGANOMETALLICS
Volume -, Issue -, Pages -Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.organomet.2c00341
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Funding
- ETH Zurich
- NCCR Catalysis [180544]
- National Centre of Competence in Research - Swiss National Science Foundation
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This study investigated the hydrogenation of acetophenone using sulfonate charge-tagged cyclopentadienone iron complexes, revealing that replacement of the TMS groups in the catalyst can enhance the activity and longevity.
The sulfonate charge-tagged cyclopentadienone iron complexes [Fe-R(MeCN)(CO)(2)-SO3]Na (R = TMS, Bu-t) were prepared and used for mechanistic investigations using pressurized sample infusion-electrospray ionization-mass spectrometry in the hydrogenation of acetophenone. Reactions were conducted in a mixed aqueous/alcoholic solvent. Based on kinetic and mass spectrometric experiments, information about the operating reaction mechanism was obtained. Furthermore, analysis of the kinetic profiles and mass spectra indicated catalyst decomposition. By analysis of the mass spectrometric results, the decomposition cascade was found to start by solvolysis of the trimethylsilyl (TMS) groups flanking the carbonyl group in the cyclopentadienone ligand of the catalyst. Subsequent dimerization, comproportionation to form Fe(I) radical species, and formation of catalytically inactive iron tricarbonyl species were observed, limiting the catalyst lifetime. Replacement of the TMS groups by non-hydrolyzable tert-butyl groups leads to a significant increase in the observed turnover frequency and catalyst longevity. The turnover number, determined to be approximately 65 under standardized reaction conditions, could be increased to >1000 by the mechanism-guided structural change in the catalyst. No compounds corresponding to Fe(I) species or dimerization products could be identified in this case. The present study suggests that for hydrogenations with cyclopentadienone iron complexes, the use of alkyl groups flanking the CO double bond in the ligand is beneficial over the use of silyl groups when conducted in aqueous media.
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