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

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.