Iron Nanoparticles Catalyzing the Asymmetric Transfer Hydrogenation of Ketones

Abstract

Investigation into the mechanism of transfer hydrogenation using trans-[Fe(NCMe)CO(PPh(2)C(6)H(4)CH═NCHR-)(2)][BF(4)](2), where R = H (1) or R = Ph (2) (from R,R-dpen), has led to strong evidence that the active species in catalysis are iron(0) nanoparticles (Fe NPs) functionalized with achiral (with 1) and chiral (with 2) PNNP-type tetradentate ligands. Support for this proposition is given in terms of in operando techniques such as a kinetic investigation of the induction period during catalysis as well as poisoning experiments using substoichiometric amounts of various poisoning agents. Further support for the presence of Fe(0) NPs includes STEM microscopy imaging with EDX analysis, XPS analysis, and SQUID magnetometry analysis of catalytic solutions. Further evidence of Fe NPs acting as the active catalyst is given in terms of a polymer-supported substrate experiment whereby the NPs are too large to permeate the pores of a functionalized polymer. Final support is given in terms of a combined poisoning/STEM/EDX experiment whereby the poisoning agent is shown to be bound to the Fe NPs. This paper provides evidence of a rare example of asymmetric catalysis with nonprecious metal, zerovalent nanoparticles.

The Golden Age of Transfer Hydrogenation

“…284 During mechanistic investigation of the catalytic activity of the i ron − P NNP complex t r a n s -[Fe (NC Me) CO-(PPh 2 C 6 H 4 CHNCHR−) 2 ][BF 4 ] 2 for TH of ketones, Morris found that the real active species formed during catalysis are iron(0) nanoparticles modified with achiral PNNP-type tetradentate ligands (Scheme 23). 285 This proposed mechanism was given strong evidence by various analyses such as in operando techniques, poisoning experiments, scanning transmission electron microscopy (STEM) imaging with energydispersive X-ray spectroscopy (EDX) analysis, X-ray photoelectron spectra (XPS) analysis, superconducting quantum interference device (SQUID) magnetometry analysis, and a polymer supported substrate experiment. 285 Other groups also reported crucial studies on iron−PNNPcatalyzed TH.…”

Determination of the Dominant Catalyst Derived from the Classic [RhCp*Cl2]2 Precatalyst System: Is it Single-Metal Rh1Cp*-Based, Subnanometer Rh4 Cluster-Based, or Rh(0)n Nanoparticle-Based Cyclohexene Hydrogenation Catalysis at Room Temperature and Mild Pressures?

“…This constitutes a relatively rare example in which the poisoned catalyst has been detected as part of studies focused on determining the true active catalyst. 1,12 The poisoning data in Figure 4, when combined with the detection of the [Rh 1 Cp*Cl(1,10-phenanthroline)] + as the poisoned product, add considerable credence to the hypothesis of a Rh 1 Cp*-based species as the dominant catalyst in the 22°C and 2.7 atm initial H 2 pressure cyclohexene hydrogenation system investigated herein.…”

Ru(II) pyridyl-based NNN complex catalysts for (asymmetric) transfer hydrogenation of ketones at room temperature

“…Recently, Baratta and co-authors [11] found that 2-(aminomethyl) pyridine (ampy) accelerated the transfer hydrogenation (TH) of ketones [12][13][14] in Ru(II) and Os(II) complex catalysts. Very recently, Morris group [15,16] reported that chiral NNPP ligands and their iron(II) complexes were effective in the ATH of ketones under mild conditions, representing a rare case of using inexpensive iron as the active metal. Although new ligands and their transition metal complexes have been established in this area [17][18][19][20], the desire for more highly active catalytic systems with better stereoselectivity and broader substrate scopes under mild conditions is still strong.…”