USE OF NMR-SPECTROSCOPY TO DETERMINE BOND ORDERS BETWEEN BETA-PYRROLIC AND BETA'-PYRROLIC POSITIONS OF PORPHYRINS - STRUCTURAL DIFFERENCES BETWEEN FREE-BASE PORPHYRINS AND METALLOPORPHYRINS

The allylic interproton spin-spin coupling constant, 4J(CH3C = CH) (4J(Me,H)), which has previously been established as a sensitive probe of bond order, was measured in a range of free-base porphyrins and metallo-2-methylporphyrins. The measured room temperature 4J(Me,H) values in a range of substituted porphyrins ((1.02-1.45) +/- 0.03 Hz) were correlated to the pi-bond order of the four beta-beta pyrrolic linkages of the porphyrins. In 2-methyl-5,10,15,20-tetraphenylporphyrin (2) (average 4J(Me,H) 1.19 +/- 0.03 Hz), the bond order across the two beta-beta' pyrrolic bonds involved in the aromatic delocalization pathway in each of the isolated tautomers, 2a and 2b, is considerably higher than the bond order in toluene and pyrrole and corresponds to a Pauling bond order of ca. 0.76 or a self-consistent field (SCF) bond order of ca. 0.66. The bond order across the two beta-beta' pyrrolic bonds not involved in the delocalization pathway is similar to that found in "pure" double bonds, and these bonds may be regarded as essentially fully localized carbon-carbon double bonds. The room-temperature 4J(Me,H) values in methylnitroporphyrins 5-10 ranged from 1.12 to 1.45 Hz and reflect the relative tautomer populations. Ground-state structural information about bond delocalization between the beta- and beta'-pyrrolic positions of metalloporphyrins was similarly obtained from measurement of the 4J(Me,H) values for a series of 2-methylporphyrins (15-18) (M = Mg, Zn, Pd, Ni). The 4J(Me,H) values lie in the range 1.14-1.21 Hz, i.e., a Pauling bond order of ca. 0.76 which clearly rules out the possibility of a 16-atom dianion structure with localized double bonds at the four beta-beta' pyrrolic positions as has been previously been proposed for a magnesium(II) porphyrin (Spangler, D.; Maggiora, G. M.; Shipman, L. L.; Christoffersen, R. E. J. Am. Chem. Soc. 1977, 99, 7470). Bond orders were determined on two free-base methylnitroporphyrins and their corresponding zinc(II) chelates. In the free-base methylnitroporphyrins 8 and 9, the nitro group causes bond fixation such that one 18-pi tautomer predominates over the other tautomeric form. In contrast, no such bond fixation was detected in the zinc chelates, 19 and 20, respectively. These findings indicate that the bond order at all beta-beta' pyrrolic positions in metalloporphyrins is intermediate between that of aromatic and localized bonds.