Stereochemical control of iron(II) complexes containing a diphosphine ligand with a pendant nitrogen base

A series of cis and trans Fe(II) complexes containing the diphosphine ligand PNP (where PNP is bis((diethylphosphino)methyl)methylamine) have been prepared and isolated. These include cis-[Fe(PNP)(2)(CH3CN)(2)](BF4)(2) (3), cis-[Fe(PNP)(2)(CH3CN)(CO)](BF4)(2) (4), trans-[Fe(PNP)(dmpm)(CH3CN)(2)](BF4)(2) (5), trans-Fe(PNP)(dmpm)Cl-2 (6), trans- [Fe(PNP)(dmpm)(CO)Cl](2)[FeCl4] (7), trans-HFe(PNP)(dmpm)Cl (8), and trans- [HFe(PNP)(dmpm)(CH3CN)]BPh4 (9) (where dmpm is bis(dimethylphosphino)methane). In addition, the cations trans-[HFe(PNP)(dmpm)(CO)](+) (10) and trans-[(H-2)Fe(PNP)(dmpm)(H)](+) (11) have been observed in solution. These complexes all possess a pendant base that bridges the two phosphorus atoms of the PNP ligand. A cis geometry is observed for those complexes containing two PNP ligands, whereas a trans geometry is observed for the complexes containing one PNP ligand and one dmpm ligand. The molecular structures of cis-[Fe(PNP)(2)(CH3CN)(CO)](BPh4)(2) and trans- [HFe(PNP)(dmpm)(CH3CN)]BPh4 have been confirmed by X-ray diffraction studies. Protonation of 5, 7, and 10 occurs at the nitrogen atom of the PNP ligand, and pK(a) values are reported for the corresponding protonated complexes. The formation of the dihydrogen complex [trans-[(H-2)Fe(PNP)(dmpm)(H)](+) and the protonated PNHP complex trans-[HFe(PNHP)(dmpm)(CO)](2+) demonstrate that intramolecular heterolytic cleavage of the dihydrogen ligand can be controlled by varying the nature of the ligand trans to the incipient dihydrogen ligand.