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.

The zwitterionic halido cyclopentadienone iron complexes FeX(CO) 2 -NMe 3 (X = Cl, Br, I) were prepared and characterized by NMR, XRD, MS, IR, and elemental analysis. Their catalytic performance in hydrogenation and transfer hydrogenation was assessed. Transfer hydrogenation in boiling i PrOH with acetophenone as the test substrate showed no conversion with FeI(CO) 2 -NMe 3 . Hydrogenation reactions under H 2 pressure (7.5 bar) in water as solvent showed up to 93% conversion with FeI(CO) 2 -NMe 3 (2.5 mol %) using acetophenone as the test substrate. The overall relative reactivity order was established to be Cl < Br < I, reflecting the relative bond strengths of the Fe−X bonds. Although the compounds presented in this study can be used as precatalysts for hydrogenation reactions in water, the need to employ high temperatures, leading to more catalyst decomposition, as evidenced by pressurized sample infusion-electrospray ionization−mass spectrometry (PSI-ESI-MS), and high catalyst loading limits their usefulness as catalysts. The limit can be circumvented in part by salt effects analogous to those in classical solvolysis chemistry. ■ EXPERIMENTAL SECTIONAll chemicals were obtained from common chemical suppliers. Acetophenone (Fluka) was used as received without further purification, drying, or degassing. NMR spectra were recorded on

Irradiation of a toluene solution containing cyclopentadienone tricarbonyl iron complexes and isocyanides with blue LEDs afforded the formation and isolation of 12 triisocyanide complexes, two of which, namely tris(2,6-dimethylphenyl isocyanide)(η4-tetraphenylcyclopenatedienone)iron, [Fe(C9H9N)3(C29H20O)], and tris(naphthalen-2-yl isocyanide)(η4-tetraphenylcyclopenatedienone)iron acetone hemisolvate, [Fe(C11H7N)3(C29H20O)]2·C3H6O, could be characterized crystallographically. The air-stable compounds were purified by column chromatography and were characterized by 1H NMR, 13C NMR, elemental analysis and HRMS. NMR and XRD data indicate generally more electron-rich Fe0 centers compared to the corresponding tricarbonyl compounds.