Syntheses and reactions of iron(II) complexes containing diphosphine ligands with pendant nitrogen bases

A series of new iron(II)-hydride complexes that contain diphosphine ligands with pendant amine bases of the formula cis-[HFeL(PNP)(2)](+), where PNP = Et2PCH2NMeCH2PEt2 and L = CH3CN (3), CO (4), P(OEt)(3) (5), have been synthesized and characterized. Protonations of the pendant bases in the PNP complexes have been characterized, and for selected complexes, pK(a) values have been determined. The acidity of the PNHP ligand depends significantly on the electronic properties of the Fe center to which it is bound, ranging from <7.0 to 12.7, depending on the nature of the coligands present. Unlike the previously studied hydride complexes [HNi(PNP)(2)](+) and trans- [HFe(CH3CN)(PNP)(dmpm)](+) (where dmpm is bis(dimethylphosphino)methane), the new hydride complexes reported here do not show rapid intramolecular exchange between the protonated base of the diphosphine and the hydride ligand. This is attributed to steric interactions between ethyl substituents on the PNP ligands.