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)2Cl]+ reacts with Zn as reducing
agent under a dihydrogen atmosphere to give the Fe­(II) hydride complex cis-[Fe­(PNP-iPr)­(CO)2H]+ 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 H2 and the coordinatively unsaturated
intermediate [Fe­(PNPH-iPr)­(CO)2]+ bearing a dearomatized pyridine moiety. This species
is able to bind and heterolytically cleave H2 to give cis-[Fe­(PNP-iPr)­(CO)2H]+. The mechanism of this reaction has been studied by DFT calculations.
The proposed mechanism was supported by deuterium labeling experiments
using D2 and the N-deuterated isotopologue of trans-[Fe­(PNP-iPr)­(CO)2Cl]+. 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)2Cl]+ was prepared,
showing no reactions with Zn and H2 under the same reaction
conditions. An alternative synthesis of cis-[Fe­(PNP-iPr)­(CO)2H]+ 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)2Cl]+ or [Fe­(PNP-iPr)­Cl2] 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)2H]+. X-ray
structures of both cis-[Fe­(PNP-iPr)­(CO)2H]+ and [Fe­(PNP-iPr)­(CO)2] are presented.