Geometry and Tautomerism of 26,28-Dioxasapphyrin and 26,28-Dithiasapphyrin: DFT Studies

摘要

The density functional theory (DFT) calculations have been carried out on 26,28-dioxasapphyrin (O_2SapH) and 26,28-dithiasapphyrin (S_2SapH) including all likely geometrical isomers and NH tautomers. The peculiar skeleton of sapphyrin with an inverted pyrrole ring lying opposite to the bipyrrolic unit (I) and the regular planar arrangement (P) of the macrocycle with three nitrogens and two heteroatoms pointing to the center of the macrocycle were considered. Consequently, a total of eight structures resulting from the feasible geometrical isomerism and NH tautomerism of diheterosapphyrins were studied. The optimized bond distances and angles of 26,28-diheterosapphyrins compare favorably with the relevant X-ray structures of sapphyrins and heterosapphyrin. The pyrrole bond distances decrease in the series: C_#alpha#-C_#beta#-C_(meso) > C_#alpha#-N > C_#beta#-C_#beta#, reproducing the pattern of the regular porphyrin or dicationic sapphyrins. There is an appreciable effect from the aromatic character of the macrocycle on the furan or thiophene moieties. These C_#alpha#-C_#beta# distances are longer and the C_#beta#-C_#beta# distances are shorter than those in free furan or thiophene. The relative stability of the postulated tautomeric forms decreases in the order of {25-NH, 27-N, 29-N} P25 > {25-NH, 27-N, 29-N} I25 > {25-N, 27-NH, 29-N} P27 > {25-NH, 27-N, 29-N} I27 for O_2SapH and {25-NH, 27-N, 29-N} P25 > {25-N, 27-NH, 29-N} P27 {25-NH, 27-N, 29-N} I25 > {25-NH, 27-N, 29-N} I27 for S_2SapH. The formation of the inverted 26,28-dithiasapphyrin structure is strongly energetically disfavored (ca. 30 kcal/mol). Both extreme geometries P and I are energetically accessible for 26,28-dioxasapphyrin. The localization of the NH proton on the bipyrrolic fragment is evidently preferred in each investigated case.