Axial Ligand Orientations

Np Fe Np axes in ferric octaalkyltetraphenylporphyrins in perpendicular planes.Recent DFT calculations have suggested that parallel and perpendicular axial-ligand orientations should be of essentially equal energy for both Fe and Fe , but these findings do not explain the near absence of structures of Fe porphyrinates with perpendicular ligand orientation (see also Section 4.1.2). Other DFT calculations on ferriheme complexes have also been reported. 

Scheldt, W. R., Geiger, D. K. Hayes, R. G. and Lang, G. (1983). Control of spin state in (porphinato)iron(III) complexes. Axial ligand orientation effect leading to an intermediate-spin complex. Molecular structure and physical characterization of the monoclinic form of bis(3-chloropyridine)(octaethylporphinato)iron(III) perchlorate. J. Am. Chem. Soc., 105, 2625-32. [198]... 

A number of recent studies have demonstrated the importance of axial ligand orientations in fine-tuning the physical properties of iron porphyrinate derivatives. Properties affected include structure, spin state, EPR and redox potentials. A convenient measure of the orientation is the dihedral angle formed by the coordinate plane containing oppo-... 

A final topic concerns the axial ligand orientations and an attempt to understand the reasons for the small imidazole complexes. The data in Table XVIII is taken to clearly indicate a preferred orientation of imidazole ligand(s) that are sterically unfavorable. A series of charge iterative Huckel theory calculations for a broadly representative series of complexes reveals a k bonding effect that favors eclipsed orientations of the ligand(s). The n bond is dominated by the metal pji-imidazole pit interaction. The results provide an explanation of why the orientation effect of imidazole complexes seems insensitive to metal d configuration, spin state, oxidation state, and the presence or absence of a sixth axial ligand. 

J. Am. Chem. Soc. 108, 6950 (1986)). The recent report by Strouse et al. ° of bis(substi-tuted imidazole)iron(III) derivatives confirms results reported in the body of this review concerning axial ligand orientation effects. The crystal structure of a 5-azaporphyrin derivative (Abeysekera, A. M., Grigg, R., Malone, J. F., King, T. J., Morley, J. O. J. Chem. Soc., Perkin Trans. 2, 395 (1985)) displays ir-Jt interaction classified in this review as type S. The solid state structure of Cd(TPP) (Hazell, A. Acta Crystallogr., Sect. C C42, 296 (1986)) shows interesting core conformations and intermolecular interactions. 

The seven BCHLs that comprise the BCHL-protein fall into two distinct conformational classes. INDO/s calculations for the seven individual BCHLs, based on the crystallographic data, yield absorption maxima that correlate with the conformational variations and clearly establish that skeletal variations can influence the optical properties of the chromophores. Effects of axial ligands, orientations of substituent groups and neighboring groups are also assessed. 

Figure 3. Cartoon of an EPR approach to determine the orientation of the axial ligands in the heme pocket of low-spin ferric heme proteins. The methodology consists of measurement of the X-band HYSCORE spectra and simulation of the contribution stemming from the porphyrin nitrogens, which leads to determination of the orientation of the g tensor axes in the molecular frame. Subsequent analysis of the proton HYSCORE spectra and proton combination frequencies leads to a determination of the axial ligand orientation in the g axes frame, and therefore in the molecular frame. The last step of the procedure consists of a Ml simulation of the nitrogen HYSCORE spectra. The Ml methodology is explained in the text.

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