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

Three novel bis(primary amine)iron(II) porphyrins [Fe(TPP)(RNH(2))(2)], where RNH(2) = 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-N(ax) distances of [Fe(TPP)(1-BuNH(2))(2)], [Fe(TPP)(BzNH(2))(2)], and [Fe(TPP)(PhCH(2)CH(2)NH(2))(2)] are 2.039(3), 2.043(3), and 2.028(2) Å, respectively. The Fe-N(p) 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 alpha-CH(2) and NH(2) protons of the coordinated ligands are staggered relative to the Fe-N(p) bonds of the porphyrin core. The lowest energy conformations of the three [Fe(TPP)(RNH(2))(2)] complexes therefore have the ligand alpha-carbons positioned directly over the Fe-N(p) bonds of the porphyrin core. The X-ray structure of [Fe(TPP)(PhCH(2)CH(2)NH(2))(2)] lies close to the global minimum (phi(1), phi(2) = 0, 180 degrees ) on the potential surface, while [Fe(TPP)(BzNH(2))(2)] and [Fe(TPP)(1-BuNH(2))(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 S(4)-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 ( approximately 2.2 kcal/mol) separate the potential minima of [Fe(TPP)(Pip)(2)], consistent with the increased bulk of the secondary amine axial ligands.