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(κ(3)-P,N,P-PNP)(κ(2)-P,N-PNP)X](+) featuring a κ(2)-P,N bound PNP ligand. With the sterically more encumbered N-methylated ligand PNP(Me)-Ph the related complex [Fe(κ(3)-P,N,P-PNP(Me)-Ph)(κ(2)-P,N-PN(HMe)-Ph)Cl](+) rather than [Fe(κ(3)-P,N,P-PNP(Me)-Ph)Cl2] was formed. This reaction was accompanied by P-N bond cleavage, thereby forming the κ(2)-P,N-bound N-diphenylphosphino-N,N'-methyl-2,6-diaminopyridine ligand. In contrast, with the N-phenylated ligands PNP(Ph)-Et and PNP(Ph)-nPr, despite small Et and nPr substituents at the phosphorus sites, complexes [Fe(κ(3)-P,N,P-PNP(Ph)-Et)Cl2] and [Fe(κ(3)-P,N,P-PNP(Ph)-nPr)Cl2] were formed, revealing that sterics can be also controlled by substituent variations at the amino N-sites. Depending on the solvent, complexes featuring κ(2)-P,N-bound ligands undergo facile rearrangement reactions to give dicationic complexes of the type [Fe(κ(3)-P,N,P-PNP)2](2+) where both PNP ligands are bound in a κ(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 κ(3)-κ(2)-complexes is readily oxidized to the corresponding phosphine sulfides upon treatment with elemental sulfur. This was exemplarily shown for [Fe(κ(3)-P,N,P-PNP-nPr)(κ(2)-P,N-PNS-nPr)Cl](+). Halide abstraction afforded the dicationic bis-chelated octahedral Fe(II) complex [Fe(κ(3)-P,N,P-PNP)2](2+) together with the free SNP ligand rather than [Fe(κ(3)-P,N,P-PNP-nPr)(κ(3)-S,P,N-PNS-nPr)](2+).