Iron(II) complexes featuring κ3- and κ2-bound PNP pincer ligands - the significance of sterics

Treatment of anhydrous FeX2 (X = Cl, Br) with 2 equiv. of the sterically little demanding N,N'-bisphosphino-2,6-diaminopyridine based PNP ligands - featuring Ph, biphenol (BIPOL), Me, Et, nPr, and nBu substituents at the phosphorus sites and H, Me, and Ph substituents at the N-linkers - afforded diamagnetic cationic octahedral complexes of the general formula [Fe(kappa(3)-P,N,P-PNP)(kappa(2)-P, N-PNP)X](+) featuring a kappa(2)- P,N bound PNP ligand. With the sterically more encumbered N-methylated ligand PNPMe-Ph the related complex [Fe(kappa(3)-P,N,P-PNPMe-Ph)(kappa(2)-P,N-PNHMe-Ph)Cl](+) rather than [Fe(kappa(3)-P,N,P-PNPMe-Ph)Cl-2] was formed. This reaction was accompanied by P-N bond cleavage, thereby forming the kappa(2)-P, N-bound N-diphenylphosphino-N,N'-methyl-2,6-diaminopyridine ligand. In contrast, with the N-phenylated ligands PNPPh-Et and PNPPh-nPr, despite small Et and nPr substituents at the phosphorus sites, complexes [Fe(kappa(3)-P,N,P-PNPPh-Et)Cl-2] and [Fe(kappa(3)-P,N,P-PNPPh-nPr)Cl-2] were formed, revealing that sterics can be also controlled by substituent variations at the amino N-sites. Depending on the solvent, complexes featuring kappa(2)-P,N-bound ligands undergo facile rearrangement reactions to give dicationic complexes of the type [Fe(kappa(3)-P,N,P-PNP)(2)](2+) where both PNP ligands are bound in a kappa(3)-P,N,P-fashion. In the presence of either Ag+ or Na+ salts as halide scavengers this reaction takes place within a few minutes. The pendant PR2 arm of the kappa(3)-kappa(2)-complexes is readily oxidized to the corresponding phosphine sulfides upon treatment with elemental sulfur. This was exemplarily shown for [Fe(kappa(3)-P,N,P-PNP-nPr)(kappa(2)-P,N-PNS-nPr)Cl](+). Halide abstraction afforded the dicationic bis-chelated octahedral Fe(II) complex [Fe(kappa(3)-P,N,P-PNP)(2)](2+) together with the free SNP ligand rather than [Fe(kappa(3)-P,N,P-PNP-nPr)(kappa(3)-S,P,N-PNS-nPr)](2+).