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Protonation of Ferrous Dinitrogen Complexes Containing a Diphosphine Ligand with a Pendent Amine

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journal contribution
posted on 19.02.2016, 14:55 by Zachariah M. Heiden, Shentan Chen, Michael T. Mock, William G. Dougherty, W. Scott Kassel, Roger Rousseau, R. Morris Bullock
The addition of acids to ferrous dinitrogen complexes [FeX­(N2)­(PEtNMePEt)­(dmpm)]+ (X = H, Cl, or Br; PEtNMePEt = Et2PCH2N­(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 νN2 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)­(PEtNMePEt)­(dmpm)]+. [FeH­(PEtNMePEt)­(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 pKa 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 pKa 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(Et2PC­(H)­N­(Me)­CH2PEt2)­(PEtNMePEt), the product of oxidative addition of the methylene C–H bond of the PEtNMePEt ligand to Fe.