Substituent effects of the meso-aryl (Ar) groups on the 1H and 13C NMR chemical shifts in a series of low-spin highly saddled iron(III) octaethyltetraarylporphyrinates, [Fe(OETArP)L2] +, where axial ligands (L) are imidazole (Hlm) and tert-butylisocyanide (tBuNC), have been examined to reveal the nature of the interactions between metal and porphyrin orbitale. As for the bis(Hlm) complexes, the crystal and molecular structures have been determined by X-ray crystallography. These complexes have shown a nearly pure saddled structure in the crystal, which is further confirmed by the normal-coordinate structural decomposition method. The substituent effects on the CH2 proton as well as meso and CH2 carbon shifts are fairly small in the bis(Hlm) complexes. Since these complexes adopt the (dxy)2(d xz, dyz)3 ground state as revealed by the electron paramagnetic resonance (EPR) spectra, the unpaired electron in one of the metal dπ orbitals is delocalized to the porphyrin ring by the interactions with the porphyrin 3eg-like orbitale. A fairly small substituent effect is understandable because the 3eg-like orbitale have zero coefficients at the meso-carbon atoms. In contrast, a sizable substituent effect is observed when the axial Hlm is replaced by tBuNC. The Hammett plots exhibit a large negative slope, -220 ppm, for the meso-carbon signals as compared with the corresponding value, +5.4 ppm, in the bis(Hlm) complexes. Since the bis(tBuNC) complexes adopt the (dxz, dyz)4(dxy)1 ground state as revealed by the EPR spectra, the result strongly indicates that the half-filled dxy orbital interacts with the specific porphyrin orbitale that have large coefficients on the meso-carbon atoms. Thus, we have concluded that the major metal-porphyrin orbital interaction in low-spin saddle-shaped complexes with the (dxz, dyz) 4(dxy)1 ground state should take place between the d xy and a2u-like orbital rather than between the d xy and a1u-like orbital, though the latter interaction is symmetry-allowed in saddled D2d complexes. Fairly weak spin delocalization to the meso-carbon atoms in the complexes with electron-withdrawing groups is then ascribed to the decrease in spin population in the dxy orbital due to a smaller energy gap between the d xy and dπ orbitale. In fact, the energy levels of the dxy and dπ orbitals are completely reversed in the complex carrying a strongly electron-withdrawing substituent, the 3,5-bis(trifluoromethyl)phenyl group, which results in the formation of the low-spin complex with an unprecedented (dxy)2(d xz, dyz)3 ground state despite the coordination of tBuNC.
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