American Chemical Society, Organometallics, 15(32), p. 4114-4121, 2013
DOI: 10.1021/om400241x
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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(PNP(H)-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(PNP(Me)-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.