TY - JOUR
T1 - Ion pairing in tetraphenylporphyrin oxidation
T2 - A semiquantitative study
AU - Sun, Haoran
AU - Biffinger, Justin C.
AU - DiMagno, Stephen G.
PY - 2005/10/7
Y1 - 2005/10/7
N2 - In 1,2-difluorobenzene (DFB), electrolyte conductivity measurements and cyclic voltammetric titration on the traditional benchmark tetraphenylporphyrin, H2tpp, permit the first estimate of ion pair association constants for singly- and doubly-oxidized free-base porphyrins. From ion titration cyclic voltammetry and digital simulation, measured association constants for H 2tpp+X- were 65, 120, 210, 520 and 730 M -1, for X- = PF6-, ClO 4-, NTf2-, BF4 - and OTf-, respectively, relative to the association constant for the H2tpp+TFPB- complex. By similar methods it was found that the association constants for the corresponding dication, H2tpp2+, were at least 3.0 × 104 M-1 (PF6-), 2.5 × 10 6 M-1 (ClO4-), 5.2 × 10 5 M-1 (NTf2-), 1.9 × 10 6 M-1 (BF4-) and 2.7 × 10 6 M-1 (OTf-). We demonstrate that differences in association constants allow the formal potential of the second oxidation of H2tpp to be shifted by more than 800 mV simply by varying the solvent and electrolyte. In addition, calculated electrostatic potential energy maps for porphyrin dications suggest that exposure of the core N - H groups is responsible for the change in ordering of anion affinities that occurs upon oxidation of H2tpp+.
AB - In 1,2-difluorobenzene (DFB), electrolyte conductivity measurements and cyclic voltammetric titration on the traditional benchmark tetraphenylporphyrin, H2tpp, permit the first estimate of ion pair association constants for singly- and doubly-oxidized free-base porphyrins. From ion titration cyclic voltammetry and digital simulation, measured association constants for H 2tpp+X- were 65, 120, 210, 520 and 730 M -1, for X- = PF6-, ClO 4-, NTf2-, BF4 - and OTf-, respectively, relative to the association constant for the H2tpp+TFPB- complex. By similar methods it was found that the association constants for the corresponding dication, H2tpp2+, were at least 3.0 × 104 M-1 (PF6-), 2.5 × 10 6 M-1 (ClO4-), 5.2 × 10 5 M-1 (NTf2-), 1.9 × 10 6 M-1 (BF4-) and 2.7 × 10 6 M-1 (OTf-). We demonstrate that differences in association constants allow the formal potential of the second oxidation of H2tpp to be shifted by more than 800 mV simply by varying the solvent and electrolyte. In addition, calculated electrostatic potential energy maps for porphyrin dications suggest that exposure of the core N - H groups is responsible for the change in ordering of anion affinities that occurs upon oxidation of H2tpp+.
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U2 - 10.1039/b506109a
DO - 10.1039/b506109a
M3 - Article
C2 - 16172638
AN - SCOPUS:26844432269
SP - 3148
EP - 3154
JO - Dalton Transactions
JF - Dalton Transactions
SN - 1477-9226
IS - 19
ER -