Proton-Mediated Electron Configuration Change in High-Spin Iron(II) Porphyrinates

Abstract

Heme proteins carry out a phenomenal range of biological processes with a common prosthetic group and a relatively small variety of axial ligands. In this paper, we report the effects on the molecular and electronic structure when the imidazole ligand is deprotonated in five-coordinate iron(II) [Fe(Por)(2-MeHIm)] derivatives. 1 Although imidazole-and imidazolate-ligated iron (II) porphyrinates both exhibit an S = 2 (quintet) state, the coordination group geometries are distinctly different with axial and equatorial bond distance differences and large changes in the displacement of iron from the porphyrin plane. The Mössbauer spectra obtained in strong magnetic fields clearly indicate a difference between the two classes in the d orbital that is doubly occupied. This change in the d-electron configuration is clearly consistent with all observed differing features and may provide control of reactivity for different classes of heme proteins.We have recently reported the synthesis, molecular and electronic structure characterization of several five-coordinate imidazole-ligated iron(II) porphyrinates. 2-4 The structural features showed a strong consistency. The displacement of the iron atom from the four nitrogen atom plane is effectively constant at 0.36 Å, although there is a range of core doming due to core conformation change.5 The axial and equatorial Fe-N bond distances of six derivatives vary over a very narrow range. Moreover, the Mössbauer spectra of nine different derivatives in small and strong applied magnetic field were found to have quadrupole splittings that are strongly temperature dependent and with a negative value of the quadrupole splitting. Values of the quadrupole splitting at 4.2 K ranged from -1.93 to -2.44 44 mm/s.We have now prepared and characterized two new fivecoordinate iron(II) complexes in which the axial ligand is 2-methylimidazolate, i.e., the deprotonated ligand. Deprotonation leads to a series of remarkable differences in properties of the new derivatives compared to the neutral imidazole complexes. The structure of the anion in [K(222)][Fe(OEP)(2-MeIm-)] is shown in Figure 1. An illustration of the full molecule is given in Figure S1 Tables S1 and S2 give full details on the fits. Tables S3-S14 give complete crystallographic details, atomic coordinates, distances and angles, anisotropic temperature factors, and fixed hydrogen atom positions for [K(222) involving the two nitrogen atoms are similar in imidazolate and different in imidazole. Second, the imidazolate ligand is known7 to be a stronger field ligand than imidazole and is also expected to be a better s donor; that these effects lead to an ∼0.09 Å axial Fe-N bond distance shortening, favoring imidazolate, is sensible. The reasons for the differences in the iron equatorial parameters are less obvious. Why should the displacement of iron(II) be much larger (0.20 Å) when imidazolate is the axial ligand than when imidazole is the axial ligand since the iron has a high-spin state in both? Concomitantly, why should the equatorial...

Just a Proton: Distinguishing the Two Electronic States of Five-Coordinate High-Spin Iron(II) Porphyrinates with Imidazole/ate Coordination

“…These differences are schematically depicted in Figure 2 for the iron(II) derivatives with imidazole and imidazol ate as the axial ligand. The values shown for imidazolate are the averages for the structures reported earlier12 and those for the imidazole averages are drawn from previously defined structures 10, 11, 1315 Although both types of species are high-spin ( S = 2), the coordination geometry differences are striking.…”

Electronic Configuration of Five-Coordinate High-Spin Pyrazole-Ligated Iron(II) Porphyrinates

“…Previous studies on imidazole- and imidazolate-ligated five-coordinate iron(II) porphyrinates have shown that these represent two distinct groups with different electronic configurations 10,11,13. Mössbauer spectra of the imidazolate-ligated and other anionic-ligated species show a large positive value of the quadrupole splitting with a small asymmetry parameter (η); they have the normal doubly occupied d xy ground state, (d xy ) 2 (d xz ) 1 (d yz ) 1 −(d z 2 ) 1 (d x 2 − y 2 ) 1 .…”

Probing Heme Vibrational Anisotropy: An Imidazole Orientation Effect?

“…This series follows the order of the importance of iron to ligand π -backbonding, with the smallest π -bonding in the imidazole complex. 75 The axial Fe–N bond distances in the three systems are 1.722, 90 2.058, 91 and 2.147 Å, 39 respectively.…”