Synthesis and Hydrogenation Activity of Iron Dialkyl Complexes with Chiral Bidentate Phosphines

Abstract

The activity of bis(phosphine) iron dialkyl complexes for the asymmetric hydrogenation of alkenes has been evaluated. High-throughput experimentation was used to identify suitable iron−phosphine combinations using the displacement of pyridine from py 2 Fe(CH 2 SiMe 3 ) 2 for precatalyst formation. Preparative-scale synthesis of a family of bis(phosphine) iron dialkyl complexes was also achieved using both ligand substitution and salt metathesis methods. Each of the isolated organometallic iron complexes was established as a tetrahedral and hence high-spin ferrous compound, as determined by Mossbauer spectroscopy, magnetic measurements, and, in many cases, X-ray diffraction. One example containing a Josiphos-type ligand, (SL-J212-1)Fe(CH 2 SiMe 3 ) 2 , proved more active than other isolated iron dialkyl precatalysts. Filtration experiments and the lack of observed enantioselectivity support dissociation of the phosphine ligand upon activation with dihydrogen and formation of catalytically active heterogeneous iron. The larger six-membered chelate is believed to reduce the coordination affinity of the phosphine for the iron center, enabling metal particle formation.

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles

“…There is some experimental proof on the in situ transformation of organometallic compounds into metal nanoparticles, which can work as the active sites for hydrogenation reactions. 294 In a recent work, Gieshoff et al have isolated series of subnanometric Fe clusters (Fe 4 , Fe 6 , and Fe 7 ) from the reaction mixture when using mononuclear Fe compound as the precatalyst in the presence of ligands for hydrogenation of alkenes. 295 Kinetic studies and structural analysis showed that those subnanometric Fe clusters were more efficient than Fe nanoparticles and were able to perform hydrogenation of alkenes under very mild conditions (1–4 bar of H 2 at 20 °C), being promising substitutes for noble metal catalysts.…”

Chiral (Cyclopentadienone)iron Complexes for the Catalytic Asymmetric Hydrogenation of Ketones

“…In particular, Fe appears to be the most appealing base metal due to its abundance (5 % of Earth's crust), low cost and scarce toxicity 3. In spite of these attractive features, the application of Fe in AH is still very limited:4 a few chiral catalysts have been developed for the hydrogenation of ketones5 and ketoimines,5b,6 while no examples were reported with olefins 7…”

Cobalt-catalyzed asymmetric hydrogenation of enamides enabled by single-electron reduction

“…Therefore, understanding and controlling ligand coordination and dissociation equilibria are key to enabling catalyst stability and communicating stereochemical information. Unlike other tactics for improving catalyst performance, rational control of catalyst activation and deactivation are challenging and often circumvented by increased catalyst loadings (9)(10)(11). With classic transition metal catalysts such as Wilkinson's (Ph 3 P) 3 RhCl (12) and (Ph 3 P) 3 RuCl 2 (13) (Ph, phenyl), catalyst activation and, ultimately, performance is limited by phosphine dissociation equilibria and halide coordination ( Fig.…”