Unusual Aryl−Porphyrin Rotational Barriers in Peripherally Crowded Porphyrins

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 a
large metal (M = Zn) and for a dication (M = 4H2+) versus a free base porphyrin (M = 2H). This has been
attributed to changes in the nonplanar distortion of the porphyrin ring and the deformability of the macrocycle
caused 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 of
DArPs indicate differences in the nonplanar conformation of the macrocycle as a function of M, with saddle
conformations 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 opposite
of the effect seen for the TArPs, and MM calculations also predict a strikingly high barrier for the TArOPPs when
M = 4H2+. These and other findings suggest that the aryl−porphyrin rotational barriers in the DArPs are closely
linked to the deformability of the macrocycle along a nonplanar distortion mode which moves the substituent being
rotated out of the porphyrin plane.