Rotation and orientation of axially coordinated imidazoles in low-spin ferric porphyrin complexes

Electronic ground states of low-spin iron(III) porphyrinoids

“…The result can be explained in terms of a smooth change of electronic ground state from (d xy ) 2 (d xz , d yz ) 3 to (d xz , d yz ) 4 (d xy ) 1 caused by the ruffling of the porphyrin core [36]. The difference in chemical shifts of the meso-C signals in a series of [Fe(TMP)L 2 ] + can be explained similarly; they are 40 ppm (L = 1-MeIm), 80 and 150 ppm (L = 2-MeIm), 128 ppm (L = BzIm), and 321 ppm (L = 2-MeBzIm) at 221 K [55,56]; the meso-C signal splits into two signals in [Fe(TMP)(2-MeIm) 2 ] + due to the slow rotation of the coordinated 2-MeIm ligands around the Fe-N bond on the NMR timescale. Granted that the field strengths of these imidazole ligands are different, the major reason for the downfield shifts of the pyrrole-H and meso-C signals on going from the less bulky to bulky imidazole ligands should be ascribed to the change in electronic ground state from (d xy ) 2 (d xz , d yz ) 3 to (d xz , d yz ) 4 (d xy ) 1 , which is caused by the ruffling of the porphyrin core due to the steric repulsion between the axial ligands and the porphyrin core.…”

Effect of sterically inhibited axial azaferrocene ligands on the physical properties of iron(III) porphyrins. Crystal structures of bis(azaferrocene) complexes of iron(III) and cobalt(III) porphyrinates

“…Recently Nakamura et al 69 have reported sterically hindered porphyrin complexes which although manifesting perpendicular arrangements of the ligands gave g z values that are more typical of rhombic spectra. It thus appears from this work and that of Nakamura et al 69 that what is important is not the absolute value of g z but the overall shape of the EPR spectrum.…”

“…Recently Nakamura et al 69 have reported sterically hindered porphyrin complexes which although manifesting perpendicular arrangements of the ligands gave g z values that are more typical of rhombic spectra. It thus appears from this work and that of Nakamura et al 69 that what is important is not the absolute value of g z but the overall shape of the EPR spectrum. Thus there is a region of overlap in g z between values of 3.0 and 3.3 where both rhombic and HALS spectra can occur depending on the nature of the axial ligands and/or steric effects on the porphyrin.…”

Protecting the triplet excited state in sterically congested platinum porphyrin

“…Although porphyrin 1, 18 as well as its iron and iridium complexes, 19,20 has been reported, no synthetic details (conditions and yields) or photophysical characterization was disclosed.…”