Highly Modular Piano-Stool N‑Heterocyclic Carbene Iron Complexes: Impact of Ligand Variation on Hydrosilylation Activity

The piano-stool configuration combined with N-heterocyclic carbene (NHC) ligation constitutes an attractive scaffold for employing iron in catalysis. Here, we have expanded this scaffold by installing a pentamethyl cyclopentadienyl (Cp*) ligand as a strong electron donor compared to the traditionally used unsubstituted cyclopentadiene (Cp). Moreover, decarboxylation is introduced as a method to prepare these iron­(II) NHC complexes, which avoids the isolation of air-sensitive free carbenes. In addition to the Cp/Cp* variation, the complexes have been systematically modulated at the NHC scaffold, the NHC wingtip groups, and the ancillary ligands in order to identify critical factors that govern the catalytic activity of the iron center in the hydrosilylation of aldehydes. These modulations reveal the importance of steric tailoring and optimization of electron density for high catalytic performance. The data demonstrate a critical role of the NHC scaffold with triazolylidenes imparting consistently higher activity than imidazolylidenes and a correlation between catalytic activity and steric rather than electronic factors. Moreover, the implementation of steric bulk is strongly dependent on the nature of the NHC and severely limited by the Cp* iron precursor. The best performing catalytic systems reach turnover frequencies, TOFmax’s, of up to 360 h–1 at 60 °C. Mechanistic investigations by 1H NMR and in situ IR spectroscopies indicate a catalyst activation that involves CO release and aldehyde coordination to the [Fe­(Cp)­(NHC)­I] fragment.