Electrochemical and Spectroelectrochemical Studies of Diphosphorylated Metalloporphyrins. Generation of a Phlorin Anion Product

Abstract

Two series of diphosphoryl-substituted porphyrins were synthesized and characterized by electrochemistry and spectroelectrochemistry in nonaqueous media containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). The investigated compounds are 5,15-bis(diethoxyphosphoryl)-10,20-diphenylporphyrins (Ph)2(P(O)(OEt)2)2PorM and 5,15-bis(diethoxyphosphoryl)-10,20-di(para-carbomethoxyphenyl)porphyrins (PhCOOMe)2(P(O)(OEt)2)2PorM where M = 2H, Co(II), Ni(II), Cu(II), Zn(II), Cd(II), or Pd(II). The free-base and five metalated porphyrins with nonredox active centers undergo two ring-centered oxidations and two ring-centered reductions, the latter of which is followed by a chemical reaction of the porphyrin dianion to give an anionic phlorin product. The phlorin anion is electroactive and can be reoxidized by two electrons to give back the starting porphyrin, or it can be reversibly reduced by one electron at more negative potentials to give a phlorin dianion. The chemical conversion of the porphyrin dianion to a phlorin anion proceeds at a rate that varies with the nature of the central metal ion and the solvent. This rate is slowest in the basic solvent pyridine as compared to CH2Cl2 and PhCN, giving further evidence for the involvement of protons in the chemical reaction leading to phlorin formation. Calculations of the electronic structure were performed on the Ni(II) porphyrin dianion, and the most favorable atoms for electrophilic attack were determined to be the two phosphorylated carbon atoms. Phlorin formation was not observed after the two-electron reduction of the cobalt porphyrins due to the different oxidation state assignment of the doubly reduced species, a Co(I) π anion radical in one case and an M(II) dianion for all of the other derivatives. Each redox reaction was monitored by thin-layer UV-visible spectroelectrochemistry, and an overall mechanism for each electron transfer is proposed on the basis of these data.

Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism

“…The spectroelectrochemical measurements at an applied potential expected to produce the dianion [3] 2-show the buildup up of a species with spectroscopic features that are in agreement with those reported by Kadish and coworkers (30) for nickel phlorin anions produced via the protonation of electrogenerated Ni(II) porphyrin dianions. This spectral observation provides strong experimental evidence for the feasibility of stable phlorin formation from the nickel complexes studied in this work.…”

“…-1.9 V is applied, a very broad peak centered at 769 nm is observed (blue curve). This peak is consistent with the spectral features reported for metallophlorins (32,33), including those of nickel phlorins (30).…”

“…6A), with the most notable features being two well-resolved Q bands at 520 and 555 nm. When a potential at -1.3 V is applied, the spectrum of [3] -(red curve) is observed, featuring a lower intensity of the Q bands and the appearance of new peaks at 608 and 841 nm, which are typical for one-electron reduced nickel porphyrins (28)(29)(30). .…”

Advances in Tetrapyrrolic Chemistry over 2013-2017 of Research group Headed by Full Member of RAS A. Yu. Tsivadze: Highlights on the Occasion of his Anniversary

“…The phlorin anion is electroactive and can be reoxidized by two electrons to give back the starting porphyrin, or it can be reversibly reduced by one electron at more negative potentials to give a phlorin dianion. [15] Single-crystal X-Ray diffraction analysis was widely used for characterization of newly synthesized mesophosphorylated porphyrins to study the processes of their self-assembly in crystalline state. The insight into the crystal Figure 9.…”

“…Generation of a phlorin anion product. [15] Research in Tetrapyrrolic Chemistry Headed by A.Yu. Tsivadze packing of phosphorylated porphyrins can be achieved in terms of topological analysis of intermolecular interaction energies ( Figure 14).…”