Radical Anions of Free‐Base Tetraphenyl‐ and Tetrakis(pentafluorophenyl)porphyrins: Effect of Substituents on the Properties and Charge Disproportionation in {Cryptand[2.2.2](Cs+)}(H2TPP·–)

Abstract

The new method for the reduction of free‐base porphyrins has been developed and crystalline {cryptand[2.2.2](Cs+)}{H2TPP·–} (1) and {cryptand[2.2.2](Cs+)}{H2T(F5Ph)P·–}·C6H5CH3 (2) salts containing porphyrin radical anions [TPP is tetraphenylporphyrin, T(F5Ph)P is tetrakis(pentafluorophenyl)porphyrin] have been obtained. The Cs+ cations partially come out of the cryptand cage in {cryptand[2.2.2](Cs+)} approaching close to negatively charged porphyrin surface. As a result, one porphyrin radical anion forms close contacts with two Cs+ cations with the Cs···P(plane) distances of 2.83–2.86 Å for 1 and 3.42 Å for 2 but another radical anion is positioned rather far from these cations. The salts have effective magnetic moments of 1.63–1.67 µB at 300 K indicating the contribution of one S = 1/2 spin per radical anion. Salt 2 shows paramagnetic behaviour indicating the preservation of H2T(F5Ph)P·– down to 2 K. Magnetic moment of 1 is preserved down 240 K and below this temperature decrease of magnetic moment is observed. This can be attributed to charge disproportionation accompanied by electron transfer between H2TPP·– with different cationic surrounding and the formation of diamagnetic H2TPP0 and H2TPP2–. This transfer is a thermally activated process with estimated energy of 200 ± 2 K. The effects of freezing of the disproportionation at low temperatures and shift of the equilibrium to paramagnetic H2TPP·– in magnetic field have been found. EPR spectra of 1 and 2 are also presented and discussed.