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-tetramet hyl-3,8,12,13,17,22-hexaethylsapphyrin (SapH(3)) at all levels of protonati on. This stands in marked contrast to 5,10,15,20-tetraphenylsapphyrin (TPSH 3) where two structures, planar and inverted are known. The H-1 NMR studies provide evidence consistent with the existence of tautomeric equilibria in volving the neutral form of decaalkylsapphyrin and up to ten specific tauto meric species ({25-NH, 26-N, 27-NH, 28-N, 29-NH}, {25-NH, 26-NH, 27-N, 28-N H, 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 i n the dynamics of these equilibria, rather than dimerization effects, are i nvoked to account for the splitting of H-1 NMR resonances observed at low t emperature. H-1 NMR studies also reveal that decaalkylsapphyrin acts as a w ater and methanol binding receptor as evidenced by the upfield shift of wat er- or methanol-derived resonances. Under conditions of complexation, the w ater or methanol molecules are bound to the N-5 center of the sapphyrin mol ecule via a network of hydrogen bonds.