After their treatment with LiAlH4 and then alcohol, new iron dicarbonyl complexes mer-trans-[Fe(Br)(CO)2(P-CH═N-P')][BF4] (where P-CH═N-P' = R2PCH2CH═NCH2CH2PPh2 and R = Cy or iPr or P-CH═N-P' = (S,S)- Cy2PCH2CH═NCH(Me)CH(Ph)PPh2) are catalysts for the hydrogenation of ketones in THF solvent with added KOtBu at 50 °C and 5 atm H2. Complexes with R = Ph are not active. With the enantiopure complex, alcohols are produced with an enantiomeric excess of up to 85% (S) at TOF up to 2000 h(-1), TON of up to 5000, for a range of ketones. An activated imine is hydrogenated to the amine in 90% ee at a TOF 20 h(-1)and TON 99. This is a significant advance in asymmetric pressure hydrogenation using iron. The complexes are prepared in two steps: (1) a one-pot reaction of phosphonium dimers ([cyclo-(PR2CH2CH(OH)(-))2][Br]2), KOtBu, FeBr2, and Ph2PCH2CH2NH2 (or (S,S)-Ph2PCH(Ph)CH(Me)NH2 for the enantiopure complex) in THF under a CO atmosphere to produce the complexes cis- and trans-[Fe(Br)2(CO)(P-CH═N-P')]; (2) the reaction of these with AgBF4 under CO(g) to afford the dicarbonyl complexes in high yield (50-90%). NMR and DFT studies of the process of precatalyst activation show that the dicarbonyl complexes are converted first to hydride-aluminum hydride complexes where the imine of the P-CH═N-P' ligand is reduced to an amide [P-CH2N-P'](-) with aluminum hydrides still bound to the nitrogen. These hydride species react with alcohol to give monohydride amine iron compounds FeH(OR')(CO)(P-CH2NH-P'), R' = Me, CMe2Et as well as the iron(0) complex Fe(CO)2(P-CH2NH-P') under certain conditions.

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.

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(PPh2C6H4CH=NCHPh-)2][BF4]2 and trans-[Fe(CO)Br(PR2CH2CH=NCHPh-)2][BPh4], respectively, are highly active and selective asymmetric transfer hydrogenation (ATH) catalysts. Here, we will review the series of tests that were undertaken to support 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. Table of Contents Overview Statement: We highlight the use of multiple techniques to differentiate homogeneous from nanoparticle catalysis as it pertains to systems developed in our group.

A series of precatalysts of the general formula [Fe(NCMe)(L)(PPh2C6H4CH=NCHR-)2][BF4]2 (where L = CO or NCMe, and R = Ph or H) were tested for the dehydrogenation of amine-boranes. They have already been used in our lab for the transfer hydrogenation or direct hydrogenation of ketones and the oxidative kinetic resolution of alcohols. We compared a series of sterically-(R = H or Ph) and electronically-(L = NCMe or CO) varied precatalysts in both protic and aprotic solvents for the release of hydrogen from ammonia-borane (AB) and studied the products by NMR. At room temperature in THF we optimized our systems, and achieved maximum turnover frequencies (TOF) of up to 3.66 H2/sec and 1.8 total H2 equivalents, and in isopropanol we were able to release a maximum of 2.9 equivalents H2 and reuse some of our catalytic systems. In previous mechanistic studies we provided strong evidence that the active species during TH and oxidation catalysis are zero-valent iron nanoparticles formed by the reduction of the Fe-PNNP precatalysts with base. To probe the dehydrogenation active species we successfully show comparable activity between preformed catalysts, and those generated in situ using commercially available Fe 2+ sources and sub-stoichiometric amounts of PNNP ligand. This result, when paired with transmission electron microscope images of ~4 nm iron nanoparticles of reaction solutions provide evidence that the highly active systems studied are heterogeneous in nature. This would be the first report of iron nanoparticles catalysing H2 evolution from AB in non-protic solvents. We also report the evolution of hydrogen from dimethylamine-borane and the resultant product mixtures using the same catalyst series.

Always cite the published version, so the author(s) will receive recognition through services that track citation counts, e.g. Scopus. If you need to cite the page number of the author manuscript from TSpace because you cannot access the published version, then cite the TSpace version in addition to the published version using the permanent URI (handle) found on the record page.ABSTRACT: Our group previously reported the development of iron carbonyl catalysts bearing chiral tridentate P-N-P' ligands for the asymmetric hydrogenation of prochiral ketones in THF. An NMR study into the activation process identified the amine hydride alkoxide complexes Fe(P-NH-P')(CO)(H)(OR 1 ) with R 1 = Me, tBu or tAmyl and P-NH-P' = PPh2CH2CH2NHCH2CH2PiPr2 or (S,S)-PPh2CHPhCHMeNHCH2CH2PCy2. These still required treatment with excess KOtBu and H2(g) to be catalytically active in THF. Both experimental methods and Density Functional Theory (DFT) calculations were used to show that this treatment leads to the formation of a hydride amide complex Fe(P-N-P')(CO)(H) which reacts with dihydrogen to form cis and trans dihydride complexes Fe(P-NH-P')(CO)(H)2, identified by NMR spectroscopy. In the presence of KOtBu, NaOtBu or KOtBu/2,2,2-cryptand and H2(g), these species are active for the catalytic hydrogenation of acetophenone, while in the absence of H2(g), inactive Fe(0) complexes are formed. Ketone hydrogenation is proposed to occur in an outer sphere stepwise process and this enantio-determining step has been modeled by DFT. The calculations suggest that the energy barriers for either hydride attack on the ketone, or dihydrogen splitting either to the nitrogen of the amide complex in the inner coordination sphere or to the oxygen of an alkoxide group in the outer sphere are similar and that either hydride transfer or dihydrogen splitting could determine the turn-over frequency depending of the nature of the ketone.