Geometry and tautomerism of sapphyrin—DFT studies

Abstract

The structure and total energy have been investigated, applying the density functional theory (DFT), for idealized sapphyrin derivatives, created by replacements of meso-phenyl groups of 5,10,15,20-tetraphenylsapphyrin or β-alkyl groups of decaalkylsapphyrin with hydrogens or two methyl groups, respectively. The following species have been considered: the dication of plain sapphyrin (SapH 5 2+) 12,13-dimethylsapphyrin (12,13-DMSapH 3), 12,13-dimethylsapphyrin dication (12,13-DMSapH 5 2+), and the 10,15-dimethylsapphyrin dication (10,15-DMSapH 5 2+). To address the issue of tautomerization the calculations have been carried out for the six feasible distribution patterns of NH protons among the five nitrogen centers of the planar sapphyrin P 0 i ( i=1–6). The B3LYP/6-31G optimized bond lengths and angles of sapphyrin or (dimethylated sapphyrin) skeletons are in satisfactory agreement with X-ray crystallographic values determined for decaalkylsapphyrin dications. The calculated total energies, using the B3LYP/6-31G**//3-21G approach demonstrate that the plain sapphyrin and 12,13-dimethylsapphyrin dications favor the planar geometry of the macrocycle. Localization of the methyl groups at the 10,15- meso positions of sapphyrin reversed the order of stability leading to the energy preference for the inverted structure. Thus, this steric factor related to the meso substitution seems to be instrumental in the C pyrrolic ring inversion. The DFT calculation also provided the relative stability frame for the complex tautomeric equilibria of SapH 3 which involve six different forms. Their relative stability decreases in the range: {25-NH, 26-N, 27-NH, 28-NH, 29-N} P 0 3>({25-NH, 26-N, 27-NH, 28-N, 29-NH} P 0 1>{25-NH, 26-NH, 27-N, 28-NH, 29-N} P 0 5>{25-N, 26-NH, 27-NH, 28-NH, 29-N} P 0 2>{25-NH, 26-N, 27-N, 28-NH, 29-NH} P 0 6>{25-N, 26-N, 27-NH, 28-NH, 29-NH} P 0 4. The stability order can be qualitatively related to the number and the nature of the cis NH interactions present in the sapphyrin core.