Protonation of ferrous dinitrogen complexes containing a diphosphine ligand with a pendent amine

The addition of acids to ferrous dinitrogen complexes [FeX(N ₂)(P^EtN^MeP^Et)(dmpm)]⁺ (X = H, Cl, or Br; P^EtN^MeP^Et = Et ₂PCH₂N(Me)CH₂PEt₂; and dmpm = Me₂PCH₂PMe₂) gives protonation at the pendent amine of the diphosphine ligand rather than at the dinitrogen ligand. This protonation increased the ν_N2 band of the complex by 25 cm ^-1 and shifted the Fe(II/I) couple by 0.33 V to a more positive potential. A similar IR shift and a slightly smaller shift of the Fe(II/I) couple (0.23 V) was observed for the related carbonyl complex [FeH(CO)(P ^EtN^MeP^Et)(dmpm)]⁺. [FeH(P ^EtN^MeP^Et)(dmpm)]⁺ was found to bind N₂ about three times more strongly than NH₃. Computational analysis showed that coordination of N₂ to Fe(II) centers increases the basicity of N₂ (vs free N₂) by 13 and 20 pK _a units for the trans halides and hydrides, respectively. Although the iron center increases the basicity of the bound N₂ ligand, the coordinated N₂ is not sufficiently basic to be protonated. In the case of ferrous dinitrogen complexes containing a pendent methylamine, the amine site was determined to be the most basic site by 30 pK_a units compared to the N₂ ligand. The chemical reduction of these ferrous dinitrogen complexes was performed in an attempt to increase the basicity of the N₂ ligand enough to promote proton transfer from the pendent amine to the N₂ ligand. Instead of isolating a reduced Fe(0)-N₂ complex, the reduction resulted in isolation and characterization of HFe(Et ₂PC(H)N(Me)CH₂PEt₂)(P^EtN ^MeP^Et), the product of oxidative addition of the methylene C-H bond of the P^EtN^MeP^Et ligand to Fe.