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.
Munro
Orde Q.
OQ
School of Chemical and Physical Sciences, University of Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa, and Department of Physics, University of the Witwatersrand, P.O. Wits 2050, Johannesburg, South Africa.
Madlala
P. Sizwe
PS
Warby
Richard A. F.
RA
Seda
Takele B.
TB
Hearne
Giovanni
G