Structural, Conformational, and Spectroscopic Studies of Primary Amine Complexes of Iron(II) Porphyrins

Three novel bis(primary amine)iron(II) porphyrins [Fe(TPP)(RNH2)2], where RNH2 = 1-butylamine, benzylamine,
and phenethylamine, have been synthesized and characterized by X-ray crystallography and IR, electronic, and
Mössbauer spectroscopy. The compounds provide unprecedented structural data for the coordination of primary
amines by iron(II) porphyrins. The Fe−Nax distances of [Fe(TPP)(1-BuNH2)2], [Fe(TPP)(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2] are 2.039(3), 2.043(3), and 2.028(2) Å, respectively. The Fe−Np distances of the three
complexes average 1.990(2) Å. The zero-field Mössbauer spectra (5−300 K) show comparable isomer shifts
(0.393(1)−0.493(1) mm/s) and quadrupole splittings (1.144(6)−1.204(3) mm/s) that are consistent with an S =
0 iron(II) assignment in each case. The bis(primary amine) complexes are structurally and spectroscopically similar
to [Fe(TPP)(Py)2] derivatives, where Py = an unsubstituted pyridine. Molecular mechanics (MM) calculations
with a force field parametrized for primary and secondary amine complexes of iron(II) porphyrins show that
stable conformations arise when the α-CH2 and NH2 protons of the coordinated ligands are staggered relative to
the Fe−Np bonds of the porphyrin core. The lowest energy conformations of the three [Fe(TPP)(RNH2)2] complexes
therefore have the ligand α-carbons positioned directly over the Fe−Np bonds of the porphyrin core. The X-ray
structure of [Fe(TPP)(PhCH2CH2NH2)2] lies close to the global minimum (φ1, φ2 = 0, 180°) on the potential
surface, while [Fe(TPP)(BzNH2)2] and [Fe(TPP)(1-BuNH2)2] show deviations that may be attributed to packing
interactions in the solid and intrinsically low barriers to axial ligand rotation (<0.5 kcal/mol). Three types of
minimum energy conformation are accessible for [Fe(TPP)(Pip)2]. The lowest energy conformation has an S4-ruffled porphyrin core. The conformation which matches the X-ray structure (Radonovich, L. J.; Bloom, A.;
Hoard, J. L. J. Am. Chem. Soc. 1972, 94, 2073−2078) is a local minimum (1.6 kcal/mol higher in energy than
the global minimum) with exact inversion symmetry. Higher in vacuo strain energy barriers (∼2.2 kcal/mol)
separate the potential minima of [Fe(TPP)(Pip)2], consistent with the increased bulk of the secondary amine axial
ligands.