Control over the oxidative reactivity of metalloporphyrins. Efficient electrosynthesis of meso,meso-linked zinc porphyrin dimer.

ABSTRACT

The electrochemical oxidation of zinc(II) 5,15-p-ditolyl-10-phenylporphyrin at its first oxidation potential leads to the formation of the corresponding meso-meso porphyrin dimer as the main product. The number of electrons abstracted, the addition of the hindered base 2,6-lutidine as well as operating in DMF, instead of a CH(2)Cl(2)/CH(3)CN mixture are the key parameters to obtain high yields of the desired coupling product. Indeed, when the electrolyses are carried out in the CH(2)Cl(2)/CH(3)CN mixture, the unexpected zinc(II) 5-chloro-10,20-p-ditolyl-15-phenyl porphyrin is produced as a by-product, the chlorine atom originating from the CH(2)Cl(2) solvent. The monomer and the dimer are characterised by electrochemical analysis. The signature of the dimer is clearly distinguished on the cyclic voltammogram of the monomer on condition of the prior addition of 2,6-lutidine as a hindered base, indicating that the dimerisation process is thus strongly accelerated. Besides, unprecedented X-ray crystallographic structures of the monomer and the meso-meso dimer are presented and their respective structural parameters are compared.