Efficient Hydrogenation of Ketones and Aldehydes Catalyzed by Well-Defined Iron(II) PNP Pincer Complexes: Evidence for an Insertion Mechanism

Abstract

We have prepared and structurally characterized a new class of Fe(II) PNP pincer hydride complexes [Fe(PNP-iPr)(H)(CO)(L)]n (L = Br–, CH3CN, pyridine, PMe3, SCN–, CO, BH4–; n = 0, +1) based on the 2,6-diaminopyridine scaffold where the PiPr2 moieties of the PNP ligand are connected to the pyridine ring via NH and/or NMe spacers. Complexes [Fe(PNP-iPr)(H)(CO)(L)]n with labile ligands (L = Br–, CH3CN, BH4–) and NH spacers are efficient catalysts for the hydrogenation of both ketones and aldehydes to alcohols under mild conditions, while those containing inert ligands (L = pyridine, PMe3, SCN–, CO) are catalytically inactive. Interestingly, complex [Fe(PNPMe-iPr)(H)(CO)(Br)], featuring NMe spacers, is an efficient catalyst for the chemoselective hydrogenation of aldehydes. The first type of complexes involves deprotonation of the PNP ligand as well as heterolytic dihydrogen cleavage via metal-alkoxide cooperation, but no reversible aromatization/deprotonation of the PNP ligand. In the case of the N-methylated complex the mechanism remains unclear, but obviously does not allow bifunctional activation of dihydrogen. The experimental results complemented by DFT calculations strongly support an insertion of the C=O bond of the carbonyl compound into the Fe–H bond.

Divergent Coupling of Alcohols and Amines Catalyzed by Isoelectronic Hydride MnI and FeII PNP Pincer Complexes

“…Simplified catalytic cycles with 1 and 3 as catalyst and pre‐catalyst, respectively, are shown in Schemes and . Key species are A Mn and A Fe with the latter being formed initially from 3 in the presence of base . Both compounds feature a deprotonated PNP ligand and are coordinatively unsaturated with the important difference that the first contains two, the latter only one inert CO co‐ligand, but additionally a hydride ligand, which can participate in the catalytic reaction.…”

Highly Active and Selective Manganese C=O Bond Hydrogenation Catalysts: The Importance of the Multidentate Ligand, the Ancillary Ligands, and the Oxidation State

“…A very good functional group tolerance including the quantitative and selective hydrogenation of a ketone in the presence of a non-shielded olefin is observed. Since PN 5 Pligands and the related PN 3 P-ligands, [8] introduced by Haupt and co-workers [9] and intensively used by the Kirchner group in recent years, [10] are simple to vary, ligand or catalyst libraries can be used to identify catalytically active species. This observation emphasizes an advantage and the importance of homogeneous catalysts in 3d-metal catalysis.…”

“…[1] The key motivation for this shift of interest results from the need of the conservation of our elemental resources as a central issue of a more sustainable future. Since PN 5 Pligands and the related PN 3 P-ligands, [8] introduced by Haupt and co-workers [9] and intensively used by the Kirchner group in recent years, [10] are simple to vary, ligand or catalyst libraries can be used to identify catalytically active species. [2] The application of homogeneous hydrogenation catalysts is especially promising for the reduction of C = O bonds since a bifunctional mechanism involving the ligand can operate for efficient H 2 activation.…”

Chemoselective transfer hydrogenation of aldehydes in aqueous media catalyzed by a well-defined iron(II) hydride complex

“…For example, the hydrido carbonyl complex 1 was found to efficiently promote the chemoselective hydrogenation of aldehydes exhibiting unprecedented activity and productivity under mild conditions (Scheme 1) [13, 14]. The same catalyst could successfully be employed in the production of dihydrogen by catalytic decomposition of formic acid [15].…”