title: Unusual aryl-porphyrin rotational barriers in peripherally crowded porphyrins.
abstract: Previous studies of 5,10,15,20-tetraarylporphyrins have shown that the barrier for meso aryl-porphyrin rotation (DeltaG++(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 = 4H(2+)) 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) (1)H 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 = 4H(2+) and saddle and/or ruffle conformations for M = Ni. VT NMR studies show that the effect of protonation in the TArOPPs is to increase DeltaG++(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 = 4H(2+). 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.
authors.affiliation: Department of Chemistry, University of California, Davis, California 95616, USA. medforth@chem.ucdavis.edu
authors.forename: Craig J
authors.initials: CJ
authors.lastname: Medforth
authors.forename: Raid E
authors.initials: RE
authors.lastname: Haddad
authors.forename: Cinzia M
authors.initials: CM
authors.lastname: Muzzi
authors.forename: Neal R
authors.initials: NR
authors.lastname: Dooley
authors.forename: Laurent
authors.initials: L
authors.lastname: Jaquinod
authors.forename: David C
authors.initials: DC
authors.lastname: Shyr
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Nurco
authors.forename: Marilyn M
authors.initials: MM
authors.lastname: Olmstead
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Smith
authors.forename: Jian-Guo
authors.initials: JG
authors.lastname: Ma
authors.forename: John A
authors.initials: JA
authors.lastname: Shelnutt
mesh_terms: D018360:Crystallography, X-Ray; D009682:Magnetic Resonance Spectroscopy; D008665:Metalloporphyrins / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D011166:Porphyrins / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D008665:Metalloporphyrins; D011166:Porphyrins