Spectral and Electrochemical Investigations on the “Tail-On” and “Tail-Off” Mechanism in Pyridine Covalently Bound Zinc(II) Porphyrins

(Tetraphenylporphyrinato)zinc(II) covalently linked with pyridine at the ortho position of one of the four phenyl rings exists as a stable pentacoordinated complex while a temperature dependent “tail-on” and “tail-off” binding mechanism is operative for the meta-substituted derivative. The axially bound pyridine does not participate in secondary chemical reactions during the electrochemical oxidation of the ortho derivative while for the meta derivative, additional chemical reactions occur between the zinc(II) porphyrin dication and the appended pyridyl entity leading to the formation of a pyrrole β-substituted isoporphyrin.