The electrochemistry of rhodium(III) porphyrins containing bound phosp
hine, isocyanide, or carbene axial ligands was investigated by cyclic
voltammetry and UV-visible spectroelectrochemistry in tetrahydrofuran
(THF) and methylene chloride (CH2Cl2) containing tetrabutylammonium he
xafluorophosphate (TBAPF6) as supporting electrolyte. The investigated
compounds are represented as [(TPP)Rh(L)2]PF6, (TPP)Rh(L')PF6, or (TP
P)Rh(PF3)(OH), where TPP is the dianion of tetraphenylporphyrin, L = P
Ph3, PPh2Me, PPhMe2, and CNCH2Ph, and L' = :C(NHCH2Ph)2. The addition
of one electron to these complexes leads to one of two different reduc
tion products depending upon the temperature and the specific set of a
xial ligands. Some of the complexes are reversibly reduced by one elec
tron to give a transient Rh(III) porphyrin pi anion radical while othe
rs are irreversibly reduced under the same solution conditions to give
dimeric [(TPP)Rh]2. In several cases, the addition of one electron gi
ves 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 invest
igated rhodium(III) porphyrins are initially reduced at the porphyrin
pi ring system, and this is also the conclusion based on electrochemic
al criteria relating the potentials for oxidation and reduction of eac
h metalloporphyrin in nonaqueous media. The absolute potential differe
nce between E1/2 for the first room temperature oxidation of a given c
omplex in CH2Cl2 and the first low-temperature reduction of the same s
pecies in THF (where the reaction is reversible) ranges between 2.22 a
nd 2.32 V, suggesting that both electrode reactions involve the porphy
rin 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 methyl
ene 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 solutio
ns leads to [(TPP)Rh(PPh3)2]+ as identified by UV-visible spectroscopy
. The formation constant for this reaction was calculated as 103.1 usi
ng spectrophotometric methods.