P. Bhyrappa et al., SOLVATION AND AXIAL LIGATION PROPERTIES OF ABROMO-5,10,15,20-TETRAPHENYLPORPHYRINATO)ZINC(II), Journal of the Chemical Society. Dalton transactions, (12), 1993, pp. 1901-1906
The solvation of (2,3,7,8,1 2,13,17,18-octabromo-5,10,1 5,20-tetraphen
ylporphyrinato)zinc(II) [Zn(obtpp)], in twelve different solvents resu
lts in large red shifts of the B and Q bands of the porphyrin accompan
ied by enhanced absorbance ratios of the Q bands. These observations a
re ascribed to the destabilisation of the highest occupied molecular o
rbital a2u of the porphyrin arising from a flow of charge from the axi
al 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 corre
sponding adducts of (5,10,15,20-tetraphenylporphyrinato)zinc and vary
in the order piperidine > imidazole > pyridine > 3-methylpyridine > py
ridine-3-carbaldehyde. The enhanced binding constants and large spectr
al shifts are interpreted in terms of the electrophilicity of [Zn(obtp
p)] induced by the electron-withdrawing bromine substituents in the po
rphyrin 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 s
addle-shaped conformation.