Kinetically Controlled Formation of Octahedral trans-Dicarbonyl Iron(II) PNP Pincer Complexes: The Decisive Role of Spin-State Changes

Abstract

Treatment of either cis-[Fe(PNP)(X2)(CO)], trans-[Fe(PNP)(X2)(CO)], or [Fe(PNP)X2] (X = Cl, Br; PNP are tridentate pincer-type ligands based on 2,6-diaminopyridine and 2,6-diaminopyrimidine) with 1 equiv of AgBF4 in the presence of CO afforded selectively octahedral iron(II) complexes of the type trans-[Fe(PNP)(CO)2X]+. The same reaction carried out with trans-[Fe(PNP-iPr)(Cl)2(CO)] in the absence of CO affords also trans-[Fe(PNP-iPr)(CO)2Cl]+ together with unidentified paramagnetic species. This reaction involves an intermolecular CO transfer between coordinately unsaturated [Fe(PNP-iPr)(CO)(Cl)]+ intermediates. In all reactions studied, there was no evidence for the formation of cis dicarbonyl complexes. X-ray structures of representative complexes are presented. A detailed mechanism, based on DFT/B3LYP calculations, is presented, suggesting that upon irreversible removal of X− transient cationic intermediates [Fe(PNP)(CO)(X)]+ of two conformations, one with the CO in the apical and the halide in the ba...