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N-alkylation |
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A series of N-methylated porphyrins has been synthesized and their crystal structures obtained illustrating the relative effects of varying degrees of N-Methylation. (2b) The series includes the following porphyrins:
A = 1,3 di-N-methyl OETPP2+
B = tri-N-methyl OETPP+
C = tetra-N-methyl OETPP2+
D = mono methyl OBrTPP
E = 1,2-di-methyl OBrTPP
For comparison purposes the OETPP and OBrTPP dications are also included in this analysis:
F = H4OETPP2+
G = H4OBrTPP2+
All of these structures demonstrate non-planar saddled conformations with very large deviations from planarity. These deviations from planarity are the result of two main structural features, sterically bulky peripheral and core substitution of the porphyrin macrocycles. Specific structural measurements are given in the table shown below. The distortions observed for these N-methylated structures show much greater deviations from planarity than do the corresponding free base structures of the N-unsubstituted analogues to these porphyrins. The crystal structures of dication A, monocation B, dication C, and D exhibit extreme tilts of their pyrroles bearing N-methyl substituents, particularly for pyrroles bearing N-methyl substituents opposing pyrroles bearing N-methyl substituents or hydrogen. The pyrrole tilts observed for these porphyrins are substantially greater then the those observed in any of the pyrroles of dication F or dication G. The crystal structure of E demonstrates a situation where both pairs of opposing pyrroles have one nitrogen bearing a lone pair of electrons and hence steric congestion in the porphyrin core is minimized. This results in smaller N-methyl bearing pyrrole tilts than are observed for dication G or for the other N-methylated porphyrins herein presented. These data clearly illustrate that extreme pyrrole distortions in porphyrin macrocycles are observed when N-methyl substituents are present on opposing pyrroles; second to this the greatest distortions are observed when a pyrrole bearing an N-methyl substituent opposes a pyrrole bearing a hydrogen. Pyrroles bearing N-methyl substituents opposing pyrroles bearing lone pairs of electrons demonstrate the smallest distortions of the porphyrin macrocycles for any of the N-methylated porphyrins.
| 1 | 2 | 3 | 4 | |
| A | 0.613 | 1.49(2) | 35.7(6) | N1 (H) |
| 52.7(4) | N2 (CH3) | |||
| 36.1(4) | N3 (H) | |||
| 52.1(2) | N4 (CH3) | |||
| B | 0.587 | 1.417(5) | 37.3(1) | N1 (CH3) |
| 48.2(2) | N2 (CH3) | |||
| 31.2(2) | N3 (:) | |||
| 50.8(2) | N4 (CH3) | |||
| C | 0.608 | 1.380(4) | 46.7(1) | N1 (CH3) |
| 45.0(2) | N2 (CH3) | |||
| 44.3(2) | N3 (CH3) | |||
| 43.8(1) | N4 (CH3) | |||
| D | 0.571 | 1.434(9) | 35.0(4) | N1 (:) |
| 51.6(5) | N2 (CH3) | |||
| 35.0(4) | N3 (:) | |||
| 31.3(5) | N4 (H) | |||
| E #1 | 0.584 | 1.42(2) | 40.8(5) | N1 (CH3) |
| 50.1(4) | N2 (CH3) | |||
| 34.7(4) | N3 (:) | |||
| 40.0(5) | N4 (:) | |||
| E #2 | 0.583 | 1.35(2) | 40.1(5) | N5 (0.41 CH3) |
| 43.4(5) | N6 (CH3) | |||
| 41.7(7) | N7 (0.59 CH3) | |||
| 37.8(7) | N8 (:) | |||
| F | 0.612 | 1.381 | 40.2 | N1 (H) |
| 39 | N2 (H) | |||
| 40.1 | N3 (H) | |||
| 37.6 | N4 (H) | |||
| G | 0.689 | 1.541 | 45.2 | N1 (H) |
| 48.1 | N2 (H) | |||
| 45.6 | N3 (H) | |||
| 48.1 | N4 (H) |
Table definitions: 1 = mean deviation of the 24 core atoms from the porphyrin mean plane, 2 = max. Cb deviation from the porphyrin mean plane, 3 = pyrrole tilts to the porphyrin mean planes, 4 = the N substituent for the respective pyrrole tilt angle, (CH3) = methyl, (H) = hydrogen, (:) = electron lone pair.
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