Metal-porphyrin orbital interactions in highly saddled low-spin iron(iii) porphyrin complexes

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 (d_xy)²(d _xz, d_yz)³ ground state as revealed by the electron paramagnetic resonance (EPR) spectra, the unpaired electron in one of the metal d_π orbitals is delocalized to the porphyrin ring by the interactions with the porphyrin 3e_g-like orbitale. A fairly small substituent effect is understandable because the 3e_g-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 (d_xz, d_yz)⁴(d_xy)¹ ground state as revealed by the EPR spectra, the result strongly indicates that the half-filled d_xy 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 (d_xz, d_yz) ⁴(dxy)¹ ground state should take place between the d _xy and a_2u-like orbital rather than between the d _xy and a_1u-like orbital, though the latter interaction is symmetry-allowed in saddled D_2d 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 d_xy orbital due to a smaller energy gap between the d _xy and d_π orbitale. In fact, the energy levels of the d_xy and d_π 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 (d_xy)²(d _xz, d_yz)³ ground state despite the coordination of ^tBuNC.