Distinguishing homogeneous from nanoparticle asymmetric iron catalysis

Abstract

This perspective will examine the use of a wide range of techniques for differentiating homogeneous from nanoparticle asymmetric catalysis as it pertains to two highly active systems developed within our group. The 6,5,6 and 5,5,5-precatalysts, trans-[Fe(NCMe)CO(PPh₂C₆H₄CHNCHPh–)₂][BF₄]₂ and trans-[Fe(CO)Br(PR₂CH₂CHNCHPh–)₂][BF₄], respectively, are highly active and selective asymmetric transfer hydrogenation (ATH) catalysts. Here, we will review the series of tests that were undertaken to support the idea that the 6,5,6-precatalyst forms iron nanoparticles (Fe NPs) during catalysis, whereas the 5,5,5-system remains homogeneous. Techniques include the use of NMR and DFT to investigate intermediates and activation steps, reaction profile and induction period analysis, substoichiometric poisoning, electron microscopy imaging, dynamic light scattering (DLS), X-ray photoelectron microscopy (XPS), magnetometry, and multiphasic analysis. We also elaborate on the wider applicability of these and other tests to probe the true nature of an active catalyst, with emphasis on the importance of using a wide range of techniques for insightful mechanistic evaluations.