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

The bis-carbonyl Fe(II) complex trans-[Fe(PNP-iPr)(CO)(2)Cl](+) reacts with Zn as reducing agent under a dihdrogen atmosphere to give the Fe(II) hydride complex cis-[Fe(PNP-iPr)(CO)(2)H](+) in 97% isolated yield. A crucial step in this reaction seems to be the reduction of the acidic NH protons of the PNP-iPr ligand to afford H-2 and the coordinatively unsaturated intermediate [Fe(PNPH-iPr)(CO)(2)](+) bearing a dearomatized pyridine moiety. This species is able to bind and heterolytically cleave H-2 to give cis-[Fe(PNP-iPr)(CO)(2)H](+). The mechanism of this reaction has been studied by DFT calculations. The proposed mechanism was supported by deuterium labeling experiments using D-2 and the N-deuterated isotopologue of trans-[Fe(PNP-iPr)(CO)(2)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 trans-[Fe(PNPMe-iPr)(CO)(2)Cl](+) was prepared, showing no reactions with Zn and H-2 under the same reaction conditions. An alternative synthesis of cis-[Fe(PNP-iPr)(CO)(2)H](+) was developed utilizing the Fe(0) complex [Fe(PNP-iPr)(CO)(2)]. This compound is obtained in high yield by treatment of either trans-[Fe(PNP-iPr)(CO)(2)Cl](+) or [Fe(PNP-iPr)Cl-2] with an excess of NaHg or a stoichiometric amount of KC8 in the presence of carbon monoxide. Protonation of [Fe(PNP-iPr)(CO)(2)] with HBF4 gave the hydride complex cis-[Fe(PNP-iPr)(CO)(2)H](+). X-ray structures of both cis-[Fe(PNP-iPr)(CO)(2)H](+) and [Fe(PNP-iPr)(CO)(2)] are presented.