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

Treatment of either cis-[Fe(PNP)(X-2)(CO)], trans-[Fe(PNP)(X-2)(CO)], or [Fe(PNP)X-2] (X = Cl, Br; PNP are tridentate pincer-type ligands based on 2,6-diaminopyridine and 2,6-diaminopyrimidine) with I equiv of AgBF4 in the presence of CO afforded selectively octahedral iron(I complexes of the type trans-[Fe(PNP)(CO)(2)X](+). The same reaction carried out with irons-trans-[Fe-PNP-iPr)(Cl)(2)(CO)] in the absence of CO affords also trans-[Fe-Te(PNP-/Pr)(CO)(2)Cl](+) 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 basal position (A) and vice versa (B), are formed. These adopt a singlet ground state in the case of A and a triplet ground state in the case of B. The formation of trans-[Fe(PNI)(CO)(2)X](+) is kinetically controlled, with A in the singlet ground state being the key intermediate. Pathways originating from complexes with a triplet ground state are "spin-blocked" (spin forbidden) or thermodynamically disfavored.