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 pKa 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.