SOLVATION AND AXIAL LIGATION PROPERTIES OF (2,3,7,8,12,13,17,18-OCTABROMO-5,10,15,20-TETRAPHENYLPORPHYRINATO)ZINC(II)

The solvation of (2,3,7,8,1 2,13,17,18-octabromo-5,10,1 5,20-tetraphenylporphyrinato)zinc(II) [Zn(obtpp)], in twelve different solvents results in large red shifts of the B and Q bands of the porphyrin accompanied by enhanced absorbance ratios of the Q bands. These observations are ascribed to the destabilisation of the highest occupied molecular orbital a2u of the porphyrin arising from a flow of charge from the axial ligand to the porphyrin ring through the zinc(II) ion. The binding constants of adducts of [Zn(obtpp)] with neutral bases have been found to be an order of magnitude greater than those observed for the corresponding adducts of (5,10,15,20-tetraphenylporphyrinato)zinc and vary in the order piperidine > imidazole > pyridine > 3-methylpyridine > pyridine-3-carbaldehyde. The enhanced binding constants and large spectral shifts are interpreted in terms of the electrophilicity of [Zn(obtpp)] induced by the electron-withdrawing bromine substituents in the porphyrin core. The structure of [Zn(obtpp)(PrCN)2] has been determined; it reveals six-co-ordinated zinc(II) with two long Zn-N distances [2.51(4), 2.59(3) angstrom]. The porphyrin is non-planar and displays a saddle-shaped conformation.