Electrochemical and Spectrochemical Studies of Diphosphorylated Metalloporphyrins: Unusual Formation of Phlorin Anion

Two series of diphosphoryl-substituted porphyrins were synthesized and characterized by electrochemistry and spectroelectrochemistry in benzontrile, dichloromethane and pyridine containing 0.1 M tetra-n-butylammonium perchlorate. The investigated compounds are represented as 5,15-bis(diethoxyphosphoryl)-10,20-diphenylporphyrins and 5,15-bis-(diethoxyphosphoryl)-10,20-di(p-carbomethoxyphenyl) porphyrins (Chart). The porphyrins with non-redox active metal centers undergo two ring centered oxidations and two ring centered reductions, the latter of which is followed by a chemical reaction to give a phlorin anion. The phlorin anion is electroactive and can be reoxidized to give back the starting porphyrin or reversibly reduced to give a phlorin dianion. Each redox reaction was monitored by thin-layer UV-visible spectroelectrochemistry and an overall reduction mechanism is proposed.