Geometry and tautomerism of 26,28-dioxasapphyrin and 26,28-dithiasapphyrin: DFT studies

The density functional theory (DFT) calculations have been carried out on 2 6,28-dioxasapphyrin (O(2)SapH) and 26,28-dithiasapphyrin (S(2)SapH) includi ng all likely geometrical isomers and NH tautomers. The peculiar skeleton o f sapphyrin with an inverted pyrrole ring lying opposite to the bipyrrolic unit (I) and the regular planar arrangement (P) of the macrocycle with thre e nitrogens and two heteroatoms pointing to the center of the macrocycle we re considered. Consequently, a total of eight structures resulting from the feasible geometrical isomerism and NH tautomerism of diheterosapphyrins we re studied. The optimized bond distances and angles of 26,28-diheterosapphy rins compare favorably with the relevant X-ray structures of sapphyrins and heterosapphyrin. The pyrrole bond distances decrease in the series: C-alph a-C-beta > C alpha-C-meso > C-alpha-N > C-beta-C-beta, reproducing the patt ern of the regular porphyrin or dicationic sapphyrins. There is an apprecia ble effect from the aromatic character of the macrocycle on the furan or th iophene moieties. These C-alpha-C-beta distanes are longer and the C-beta-C -beta distances are shorter than those in foe furan or thiophene. The relat ive 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-N, 27-NH, 29-N} I27 for O(2)Saph and {25-NH, 27-N, 29-N} P25 > {25-N , 27-NH, 29-N P27 >> (25-NH, 27-N, 29-N) I25 > (25-N, 27-NH, 29-N) I27 for S(2)SapH. The formation of the inverted 26,28-dithiasapphyrin structure is strongly energetically disfavored (ca. 30 kcal/mol). Both extreme geometrie s P and I are energetically accessible for 26,28-dioxasapphyrin. The locali zation of the NH proton on the bipyrrolic fragment is evidently preferred i n each investigated case.