Inverted meso-aryl porphyrins with heteroatoms; Characterization of thia, selena, and oxa N-confused porphyrins

Synthesis and characterization of inverted porphyrins containing S, Se, and O are reported. A simple 3 +1 MacDonald-type condensation using modified t ripyrrane containing-the N-confused ring and diols -afforded various N-conf used porphyrins 6a-f in 19-30% yield. The single-crystal X-ray structure of 6b shows a ruffled conformation with tilt angles of 21.11 degrees and 31.2 3 degrees for the N-confused ring and the adjacent pyrrole ring III, respec tively, revealing its severe nonplanarity. Significant changes in C-alpha-C Bbeta, C-beta-C-beta, and C-alpha-X bond lengths areobserved in 6b relative to free thiophene and pyrrole, suggesting the altered delocalization pathw ay in the modified N-confused porphyrins. The two. molecules in the unit ce ll show a cyclophane-type noncovalent dimer with a face to face orientation of two N-confused pyrrole rings as a result of the presence of weak N-H . . .N and C-H . . .N intermolecular hydrogen bonds involving pyrrole-NH, the N atom of the N-confused ring, and the C atom of the pyrrole ring. A detai led H-1 and C-13 NMR study by 1D and 2D methods allowed assignments of all the peaks in the free base and protonated forms. NMR studies reveal the pre sence of three different tautomeric forms in solution for 6c in CDCl3 at lo w temperature. UV-visible studies reveal absorption band shifts upon hetero atom substitution, and the magnitudes of these shifts are dependent on the nature of the heteroatom In all cases both monoprotonated and diprotonated species have been identified, and on addition of acid, the first proton goe s to the outer N2 atom of the N-confused ring.