| Abstract
| - Variable temperature 1H NMR studies reveal unusual barriers for aryl−porphyrin rotation (ΔG‡ROT) in the peripherally crowded porphyrin 1: ΔG‡ROT is similar for 1 and 2 (M = Ni) (55 ± 2 vs 49 ± 3 kJ mol-1) but very different for 1 and 2 (M = 4H2+) (91 ± 3 vs 47 ± 2 kJ mol-1). X-ray structures and molecular mechanics calculations suggest that ΔG‡ROT in 1 is strongly influenced by the ability of the porphyrin macrocycle to deform and move the aryl group being rotated out of the porphyrin plane.
- Previous studies of 5,10,15,20-tetraarylporphyrins have shown that the barrier for meso aryl−porphyrin rotation(ΔG‡ROT) varies as a function of the core substituent M and is lower for a small metal (M = Ni) compared to alarge metal (M = Zn) and for a dication (M = 4H2+) versus a free base porphyrin (M = 2H). This has beenattributed to changes in the nonplanar distortion of the porphyrin ring and the deformability of the macrocyclecaused by the core substituent. In the present work, X-ray crystallography, molecular mechanics (MM) calculations,and variable temperature (VT) 1H NMR spectroscopy are used to examine the relationship between the aryl−porphyrin rotational barrier and the core substituent M in some novel 2,3,5,7,8,10,12,13,15,17,18,20-dodecaarylporphyrins (DArPs), and specifically in some 5,10,15,20-tetraaryl-2,3,7,8,12,13,17,18-octaphenylporphyrins (TArOPPs),where steric crowding of the peripheral groups always results in a very nonplanar macrocycle. X-ray structures ofDArPs indicate differences in the nonplanar conformation of the macrocycle as a function of M, with saddleconformations being observed for M = Zn, 2H or M = 4H2+ and saddle and/or ruffle conformations for M = Ni.VT NMR studies show that the effect of protonation in the TArOPPs is to increase ΔG‡ROT, which is the oppositeof the effect seen for the TArPs, and MM calculations also predict a strikingly high barrier for the TArOPPs whenM = 4H2+. These and other findings suggest that the aryl−porphyrin rotational barriers in the DArPs are closelylinked to the deformability of the macrocycle along a nonplanar distortion mode which moves the substituent beingrotated out of the porphyrin plane.
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