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

Abstract

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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