Mn(i) PNP pincer hydride complexes were shown for the first time to catalyse the hydrogenation of CO₂ to formate reaching quantitative yields and high TONs.

The synthesis and application of [Fe(PNP^Me-iPr)(CO)(H)(Br)] and [Fe(PNP^Me-iPr)(H)₂(CO)] as catalysts for the homogeneous hydrogenation of aldehydes is described. These systems were found to be among the most efficient catalysts for this process reported to date and constitute rare examples of a catalytic process which allows selective reduction of aldehydes in the presence of ketones and other reducible functionalities. In some cases, TONs and TOFs of up to 80000 and 20000 h^–1, respectively, were reached. On the basis of stoichiometric experiments and computational studies, a mechanism which proceeds via a trans-dihydride intermediate is proposed. The structure of the hydride complexes was also confirmed by X-ray crystallography.

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(PNP^Me-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.