Electronic Configuration Assignment and the Importance of Low-Lying Excited States in High-Spin Imidazole-Ligated Iron(II) Porphyrinates

The synthesis and characterization of six new high-spin deoxymyoglobin models (imidazole(tetraarylporphyrinato)iron(II)) are described. These have been intensively studied by temperature-dependent Mössbauer spectroscopy from 295 to 4.2 K. All complexes show a strong temperature dependence for the quadrupole splitting consistent with low-lying excited states of the same or lower multiplicity. An analysis of the data obtained in applied magnetic fields leads to the assignment of the sign of the quadrupole splitting. All model compounds as well as those of deoxymyoglobin and deoxyhemoglobin, previously studied, have a negative sign for the quadrupole splitting. Although not previously predicted, this experimental observation leads to the assignment of the ground-state electronic configuration for all high-spin <i>imidazole-ligated</i> iron(II) porphyrinates as (d<i><sub>xz</sub></i>)<sup>2</sup>(d<i><sub>yz</sub></i>)<sup>1</sup>(d<i><sub>xy</sub></i>)<sup>1</sup>(d<i><sub>z</sub></i><sup><sub>2</sub></sup>)<sup>1</sup>(d<i><sub>x</sub></i><sup><sub>2</sub></sup><sub>-</sub><i><sub>y</sub></i><sup><sub>2</sub></sup>)<sup>1</sup>. This is a distinctly different ground-state electronic configuration from other high-spin iron(II) porphyrinates; differences in structural details for the two classes of high-spin complexes are also discussed. The apparent anomaly of differing signs for the zero-field splitting constant between previously studied model complexes and the heme proteins is addressed; the difference appears to result from the fact that the assumptions used in the spin Hamiltonian approach that has been applied to these complexes are not adequately satisfied. Structures of four of the new five-coordinate species have been determined. Core conformations in these derivatives show variation, but these and previously studied compounds reveal a limited number of conformational patterns. The bond lengths and other geometrical parameters such as porphyrin core size and iron out-of-plane displacement support a high-spin state assignment for the iron(II).