FACTORS INFLUENCING THE SITE OF ELECTROREDUCTION IN RHODIUM PORPHYRINS

The electrochemistry of rhodium(III) porphyrins containing bound phosphine, isocyanide, or carbene axial ligands was investigated by cyclic voltammetry and UV-visible spectroelectrochemistry in tetrahydrofuran (THF) and methylene chloride (CH2Cl2) containing tetrabutylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte. The investigated compounds are represented as [(TPP)Rh(L)2]PF6, (TPP)Rh(L')PF6, or (TPP)Rh(PF3)(OH), where TPP is the dianion of tetraphenylporphyrin, L = PPh3, PPh2Me, PPhMe2, and CNCH2Ph, and L' = :C(NHCH2Ph)2. The addition of one electron to these complexes leads to one of two different reduction products depending upon the temperature and the specific set of axial ligands. Some of the complexes are reversibly reduced by one electron to give a transient Rh(III) porphyrin pi anion radical while others are irreversibly reduced under the same solution conditions to give dimeric [(TPP)Rh]2. In several cases, the addition of one electron gives a Rh(II) dimer at room temperature but a Rh(III) pi anion radical at low temperature. The UV-visible data suggest that all of the investigated rhodium(III) porphyrins are initially reduced at the porphyrin pi ring system, and this is also the conclusion based on electrochemical criteria relating the potentials for oxidation and reduction of each metalloporphyrin in nonaqueous media. The absolute potential difference between E1/2 for the first room temperature oxidation of a given complex in CH2Cl2 and the first low-temperature reduction of the same species in THF (where the reaction is reversible) ranges between 2.22 and 2.32 V, suggesting that both electrode reactions involve the porphyrin pi ring system. One of the species, (TPP)Rh(PF3)(OH), undergoes a slow conversion of the electrogenerated pi anion radical to dimeric [(TPP)Rh]2, and this reaction was followed as a function of time by thin-layer UV-visible spectroelectrochemistry in THF. Exchange equilibria involving bound PPh3 and THF axial ligands were also studied in methylene chloride or tetrahydrofuran by UV-visible spectroscopy. Both [(TPP)Rh(PPh3)]+ and [(TPP)Rh(PPh3)2]+ are converted to [(TPP)Rh(PPh3)(THF)]+ in neat THF, but the addition of 1.0 equiv of PPh3 to these solutions leads to [(TPP)Rh(PPh3)2]+ as identified by UV-visible spectroscopy. The formation constant for this reaction was calculated as 103.1 using spectrophotometric methods.