Heterolytic Cleavage of Dihydrogen by an Iron(II) PNP Pincer Complex via Metal-Ligand Cooperation.

The bis-carbonyl Fe(II) complex [Fe(PNP-Pr)(CO)Cl] reacts with Zn as reducing agent under a dihydrogen atmosphere to give the Fe(II) hydride complex [Fe(PNP-Pr)(CO)H] in 97% isolated yield. A crucial step in this reaction seems to be the reduction of the acidic NH protons of the PNP-Pr ligand to afford H and the coordinatively unsaturated intermediate [Fe(PNP-Pr)(CO)] bearing a dearomatized pyridine moiety. This species is able to bind and heterolytically cleave H to give [Fe(PNP-Pr)(CO)H]. The mechanism of this reaction has been studied by DFT calculations. The proposed mechanism was supported by deuterium labeling experiments using D and the N-deuterated isotopologue of [Fe(PNP-Pr)(CO)Cl]. While in the first case deuterium was partially incorporated into both N and Fe sites, in the latter case no reaction took place. In addition, the N-methylated complex [Fe(PNP-Pr)(CO)Cl] was prepared, showing no reactions with Zn and H under the same reaction conditions. An alternative synthesis of [Fe(PNP-Pr)(CO)H] was developed utilizing the Fe(0) complex [Fe(PNP-Pr)(CO)]. This compound is obtained in high yield by treatment of either [Fe(PNP-Pr)(CO)Cl] or [Fe(PNP-Pr)Cl] with an excess of NaHg or a stoichiometric amount of KC in the presence of carbon monoxide. Protonation of [Fe(PNP-Pr)(CO)] with HBF gave the hydride complex [Fe(PNP-Pr)(CO)H]. X-ray structures of both -[Fe(PNP-Pr)(CO)H] and [Fe(PNP-Pr)(CO)] are presented.