The synthesis and spectroscopic characterisation of chiral meso-tetraarylmetalloporphyrins bearing meso-pentafluorophenyl groups

Abstract

The successful synthesis of 5,10,15,20-tetrakis[(R,R)-2,6-bis(1-phenylbutoxy)phenyl]porphyrin, with (R)-1-phenylbutoxy substituents on each of the eight ortho-positions, by a 2+2 approach viameso-[(R,R)-2,6-bis(1-phenylbutoxy)phenyl]dipyrromethane rather than a one-pot condensation of (R,R)-2,6-bis(1-phenylbutoxy)benzaldehyde with pyrrole, is described. The synthesis has also been modified using meso-(pentafluorophenyl)dipyrromethane to prepare four further chiral porphyrins containing one, two (cis and trans) and three pentafluorophenyls in place of the bis(phenylbutoxy)phenyl groups. The cross-coupling of the two dipyrromethanes with pentafluorobenzaldehyde gave as one of the products the unexpected cis-disubstituted 5,10-bis(pentafluorophenyl)-15,20-bis[(R,R)-2,6-bis(1-phenylbutoxy)phenyl]porphyrin. It seems likely that the formation of the latter compound involves the acid-catalysed reversion of the dipyrromethane synthesis. Both faces of each of the porphyrins are chiral and equivalent in this way the wasteful formation and time-consuming separation of atropisomers is avoided. Four iron(III) and one manganese(III) complex of these porphyrins have been prepared. The ¹H and ¹⁹F NMR spectra of the series of porphyrin ligands reveal some interesting structure- and symmetry-dependent splitting patterns and trends which are used to confirm the identities of the compounds. In particular, the ¹H NMR couplings of the β-pyrrole hydrogens are very diagnostic of the substitution patterns of the meso-aryl groups on the porphyrin ring.