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

Abstract

The bis-carbonyl Fe(II) complex <i>trans-</i>[Fe(PNP-<i>i</i>Pr)(CO)₂Cl]⁺ reacts with Zn as reducing agent under a dihydrogen atmosphere to give the Fe(II) hydride complex <i>cis-</i>[Fe(PNP-<i>i</i>Pr)(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-<i>i</i>Pr ligand to afford H₂ and the coordinatively unsaturated intermediate [Fe(PNP^H-<i>i</i>Pr)(CO)₂]⁺ bearing a dearomatized pyridine moiety. This species is able to bind and heterolytically cleave H₂ to give <i>cis-</i>[Fe(PNP-<i>i</i>Pr)(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 <i>trans-</i>[Fe(PNP-<i>i</i>Pr)(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 <i>trans-</i>[Fe(PNP^Me-<i>i</i>Pr)(CO)₂Cl]⁺ was prepared, showing no reactions with Zn and H₂ under the same reaction conditions. An alternative synthesis of <i>cis-</i>[Fe(PNP-<i>i</i>Pr)(CO)₂H]⁺ was developed utilizing the Fe(0) complex [Fe(PNP-<i>i</i>Pr)(CO)₂]. This compound is obtained in high yield by treatment of either <i>trans-</i>[Fe(PNP-<i>i</i>Pr)(CO)₂Cl]⁺ or [Fe(PNP-<i>i</i>Pr)Cl₂] with an excess of NaHg or a stoichiometric amount of KC₈ in the presence of carbon monoxide. Protonation of [Fe(PNP-<i>i</i>Pr)(CO)₂] with HBF₄ gave the hydride complex <i>cis-</i>[Fe(PNP-<i>i</i>Pr)(CO)₂H]⁺. X-ray structures of both <i>cis</i>-[Fe(PNP-<i>i</i>Pr)(CO)₂H]⁺ and [Fe(PNP-<i>i</i>Pr)(CO)₂] are presented.