Unusual aryl-porphyrin rotational barriers in peripherally crowded porphyrins.

Journal Article · Sun Jun 01 04:00:00 EDT 2003 · Proposed for publication in Inorganic Chemistry.

OSTI ID:990090

Shyr, David C ^[1]; Dooley, Neal R ^[1]; Haddad, Raid Edward ^[2]; Shelnutt, John Allen; Muzzi, Cinzia M ^[1]; Ma, Jian-Guo ^[2]; Olmstead, Marilyn M ^[1]; Jaquinod, Laurent A ^[1]; Nurco, Daniel J ^[1]; Medforth, Craig John; Smith, Kevin M ^[3]

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1. University of California, Davis, CA
2. University of New Mexico, Albuquerque, NM
3. Louisiana State University, Baton Rouge, LA

Previous studies of 5,10,15,20-tetraarylporphyrins have shown that the barrier for meso aryl-porphyrin rotation ({Delta}G{sub 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{sup 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) {sup 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{sup 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 {Delta}G{sub 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{sup 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.

• Research Organization: Sandia National Laboratories
• Sponsoring Organization: USDOE
• DOE Contract Number: AC04-94AL85000
• OSTI ID: 990090
• Report Number(s): SAND2003-2364J
• Journal Information: Proposed for publication in Inorganic Chemistry., Journal Name: Proposed for publication in Inorganic Chemistry. Journal Issue: 7 Vol. 42; ISSN 0020-1669; ISSN INOCAJ
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
CRYSTALLOGRAPHY
PORPHYRINS
ROTATION
SPECTROSCOPY