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Issue 10, 1999
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NH tautomerization of 2,7,18,23-tetramethyl-3,8,12,13,17,22-hexaethylsapphyrin


A planar, pyrrole-in macrocyclic geometry is favored for 2,7,18,23-tetramethyl-3,8,12,13,17,22-hexaethylsapphyrin (SapH3) at all levels of protonation. This stands in marked contrast to 5,10,15,20-tetraphenylsapphyrin (TPSH3) where two structures, planar and inverted are known. The 1H NMR studies provide evidence consistent with the existence of tautomeric equilibria involving the neutral form of decaalkylsapphyrin and up to ten specific tautomeric species ({25-NH, 26-N, 27-NH, 28-N, 29-NH}, {25-NH, 26-NH, 27-N, 28-NH, 29-N}, {25-NH, 26-N, 27-NH, 28-NH, 29-N}, {25-N, 26-NH, 27-NH, 28-N, 29-NH}, {25-NH, 26-NH, 27-NH, 28-N, 29-N}, {25-N, 26-N, 27-NH, 28-NH, 29-NH}, {25-NH, 26-NH, 27-N, 28-N, 29-NH}, {25-NH, 26-N, 27-N, 28-NH, 29-NH}, {25-N, 26-NH, 27-NH, 28-NH, 29-N}, {25-N, 26-NH, 27-N, 28-NH, 29-NH}). Changes in the dynamics of these equilibria, rather than dimerization effects, are invoked to account for the splitting of 1H NMR resonances observed at low temperature. 1H NMR studies also reveal that decaalkylsapphyrin acts as a water and methanol binding receptor as evidenced by the upfield shift of water- or methanol-derived resonances. Under conditions of complexation, the water or methanol molecules are bound to the N5 center of the sapphyrin molecule via a network of hydrogen bonds.

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Article type: Paper
DOI: 10.1039/A901983I
Citation: J. Chem. Soc., Perkin Trans. 2, 1999, 2189-2195
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    NH tautomerization of 2,7,18,23-tetramethyl-3,8,12,13,17,22-hexaethylsapphyrin

    K. Rachlewicz, L. Latos-Grażyński, A. Gebauer, A. Vivian and J. L. Sessler, J. Chem. Soc., Perkin Trans. 2, 1999, 2189
    DOI: 10.1039/A901983I

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