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)Cl₂ (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-[(H₂)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-[(H₂)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.