title: 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 PiPr2moieties of the PNP ligand are connected to the pyridine ring via NH and/or NMe spacers. Complexes [Fe(PNP-iPr)(H)(CO)(L)]nwith 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.
authors.affiliation: Institute of Applied Synthetic Chemistry and Institute of Chemical Technologies and Analytics, Vienna University of Technology , Getreidemarkt 9, A-1060 Vienna, Austria.
authors.forename: Nikolaus
authors.initials: N
authors.lastname: Gorgas
authors.affiliation: Institute of Applied Synthetic Chemistry and Institute of Chemical Technologies and Analytics, Vienna University of Technology , Getreidemarkt 9, A-1060 Vienna, Austria.
authors.forename: Berthold
authors.initials: B
authors.lastname: Stöger
authors.affiliation: Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa , Avenida Rovisco Pais No. 1, 1049-001 Lisboa, Portugal.
authors.forename: Luis F
authors.initials: LF
authors.lastname: Veiros
authors.affiliation: Institute of Applied Synthetic Chemistry and Institute of Chemical Technologies and Analytics, Vienna University of Technology , Getreidemarkt 9, A-1060 Vienna, Austria.
authors.forename: Ernst
authors.initials: E
authors.lastname: Pittenauer
authors.affiliation: Institute of Applied Synthetic Chemistry and Institute of Chemical Technologies and Analytics, Vienna University of Technology , Getreidemarkt 9, A-1060 Vienna, Austria.
authors.forename: Günter
authors.initials: G
authors.lastname: Allmaier
authors.affiliation: Institute of Applied Synthetic Chemistry and Institute of Chemical Technologies and Analytics, Vienna University of Technology , Getreidemarkt 9, A-1060 Vienna, Austria.
authors.forename: Karl
authors.initials: K
authors.lastname: Kirchner