Cyclopentadienone Iron Complex-Catalyzed Hydrogenation of Ketones: An Operando Spectrometric Study Using Pressurized Sample Infusion-Electrospray Ionization-Mass Spectrometry

Abstract

The sulfonate charge-tagged cyclopentadienone iron complexes [Fe R (MeCN)(CO) 2 -SO 3 ]Na (R = TMS, t Bu) 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 C�O double bond in the ligand is beneficial over the use of silyl groups when conducted in aqueous media.

Zwitterionic Halido Cyclopentadienone Iron Complexes and Their Catalytic Performance in Hydrogenation Reactions

“…Mass spectra recorded under PSI-ESI-MS conditions further corroborated this interpretation. Similar observations were reported previously by our group using a related negatively charge-tagged cyclopentadienone iron complex, showing that TMS hydrolysis ultimately leads to catalytically inactive tricarbonyl species …”

“…Similar observations were reported previously by our group using a related negatively charge-tagged cyclopentadienone iron complex, showing that TMS hydrolysis ultimately leads to catalytically inactive tricarbonyl species. 5 The observation and isolation of FeI(CO) 2 -NMe 3 suggests that, upon in situ activation of the tricarbonyl precatalyst [Fe(CO) 3 -NMe 3 ]I with Me 3 NO as performed by Renaud et al., 6 the iodide counter anion is not innocent, binding reversibly to the Fe center and reducing the steady-state concentration of active catalyst. The same is true for the corresponding chlorido and bromido complexes.…”

“…During our studies to elucidate the reaction mechanism of the hydrogenation reaction using charge-tagged cyclopentadienone iron complexes by mass spectrometry 5 with the permanently positively charged complex reported by Renaud et al, 6 we came across the zwitterionic compound FeI(CO) 2 -NMe 3 . This crystalline and air and moisture stable compound constitutes a close analogue to the compounds reported by Knolker et al With the prospect that halide dissociation in this complex leads to the formation of an active hydrogenation catalyst, we set out to explore the catalytic performance of zwitterionic halido cyclopentadienone iron complexes.…”

“…The setup used for PSI-ESI-MS has been described before. 5 Kinetic experiments were conducted by using the PSI-ESI-MS setup with a shorter piece of tubing coming out of the reaction vessel stoppered by a screw-on stopper. For aliquot collection, the reaction vessel was neither cooled nor vented.…”

“…During our studies to elucidate the reaction mechanism of the hydrogenation reaction using charge-tagged cyclopentadienone iron complexes by mass spectrometry with the permanently positively charged complex reported by Renaud et al ., we came across the zwitterionic compound FeI­(CO) 2 -NMe 3 . This crystalline and air and moisture stable compound constitutes a close analogue to the compounds reported by Knölker et al .…”