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

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.