Short-chain basket handle porphyrins: synthesis and characterisation

Abstract

Basket handle porphyrins containing short p- or m-phenylenedimethylenedioxy chains covalently linked at the ortho position of phenyl groups of 5,10,15,20-tetraphenylporphyrin have been synthesised. Isomer I of the para derivative (H₂L^Ia) and isomers II and III(H₂L^IIb and H₂L^IIIb) of the meta derivative have been characterised by various spectroscopic methods. Electronic and ¹H NMR spectral studies indicate significant distortion of the porphyrin skeleton in these derivatives. Protonation of the free-base derivatives results in a small blue shift of the Q bands, attributed to the lack of conjugation between the phenyl groups and the porphyrin plane because of restricted rotation of the porphyrin-phenyl bond. The bridging phenylenedimethylenedioxy group does not interfare with the axial ligation of metal derivatives (Co²⁺ and Zn²⁺) of H₂L^IIb and H₂L^IIIb. Electrochemical studies indicate easier oxidations and more difficult reductions for the free-base derivatives relative to the corresponding unstrapped derivative, attributed in the former case to the loss of coplanarity due to distortion and in the latter to destabilisation of the anions and dianions due to the lack of solvation. Redox potential data for [CoL^IIb] indicate that the metal centred molecular orbital A₁(d_z²) is situated above the filled A_2u orbital of the porphyrin ring.