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

Abstract

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(κ³-P,N,P-PNP)(κ²-P,N-PNP)X]⁺ featuring a κ²-P,N bound PNP ligand. With the sterically more encumbered N-methylated ligand PNP^Me-Ph the related complex [Fe(κ³-P,N,P-PNP^Me-Ph)(κ²-P,N-PN^HMe-Ph)Cl]⁺ rather than [Fe(κ³-P,N,P-PNP^Me-Ph)Cl₂] was formed. This reaction was accompanied by P–N bond cleavage, thereby forming the κ²-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(κ³-P,N,P-PNP^Ph-Et)Cl₂] and [Fe(κ³-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 κ²-P,N-bound ligands undergo facile rearrangement reactions to give dicationic complexes of the type [Fe(κ³-P,N,P-PNP)₂]²⁺ where both PNP ligands are bound in a κ³-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 κ³-κ²-complexes is readily oxidized to the corresponding phosphine sulfides upon treatment with elemental sulfur. This was exemplarily shown for [Fe(κ³-P,N,P-PNP-nPr)(κ²-P,N-PNS-nPr)Cl]⁺. Halide abstraction afforded the dicationic bis-chelated octahedral Fe(II) complex [Fe(κ³-P,N,P-PNP)₂]²⁺ together with the free SNP ligand rather than [Fe(κ³-P,N,P-PNP-nPr)(κ³-S,P,N-PNS-nPr)]²⁺.