Solvent Effects on Optical and ESR Spectra of One-Electron Reduced Species of Cobalt(III) and Manganese(III) Tetraphenylporphyrins Produced by γ-Radiolysis at 77 K

Abstract

One-electron reduction by γ-ray irradiation of cobalt(III) and manganese(III) tetraphenylporphyrins (ClCo(III)TPP, ClMn(III)TPP, and IMn(III)TPP) in ethanol and MTHF(2-methyltetrahydrofuran) solutions at 77 K results in the reduction of the central metals of these porphyrins. From the optical and ESR measurements, the reduced species are found to retain the axial anion, Cl− or I−, in MTHF but not in ethanol. The absorption spectroscopic studies of these porphyrins in ethanol solutions indicate that the axial halide ion is displaced by an ethanol molecule prior to γ-ray irradiation.

Spectral, Electrochemical, and ESR Characterization of Manganese Tetraarylporphyrins Containing Four β,β′-Pyrrole Fused Butano and Benzo Groups in Nonaqueous Media

“…Although Mn­(III) porphyrins are paramagnetic with a high spin d 4 configration, they are “ESR silent” at conventional fields and frequencies due to short spin–lattice relaxation times and large zero-field splittings. − The currently investigated manganese­(III) butano- and benzoporphyrins also showed no ESR signal under the given experimental conditions. However, the Mn­(II) butano- and benzoporphyrins prepared by chemical reduction exhibit typical Mn­(II) ESR signals over a wide range of magnetic fields.…”

“…In each case, the spectra are comprised of two resonances, one located at g ⊥ = 5.6–5.8 and the other at g // = 2.0, with a six line hyperfine splitting from the 55 Mn nucleus ( I = 5/2), giving the characteristic of a high-spin (3d 5 , S = 5/2) configuration of Mn­(II) porphyrins. − …”

“…It is known that the ligand binding to the Mn­(II) porphyrin has a significant effect on the ESR properties of the compound. The four-coordinate (TMP)­Mn II (TMP = tetramesitylporphyrin) only has a distinct resonance at g // ≈ 2.0, while five-coordinate imidazole-bound Mn­(II) porphyrins have a strong resonance at g ⊥ ≈ 5.9 and a weak signal at g // ≈ 2.0. − …”

“…ESR spectra were obtained at 110 K with a Bruker 300 with 100 kHz modulation. Mn­(II) porphyrins were prepared through chemical reductions of the corresponding Mn­(III) porphyrins by sodium borohydride (NaBH 4 ) in tetrahydrofuran at room temperature …”

“…Mn(II) porphyrins were prepared through chemical reductions of the corresponding Mn(III) porphyrins by sodium borohydride (NaBH 4 ) in tetrahydrofuran at room temperature. 50 Chemicals. Dichloromethane (CH 2 Cl 2, ≥ 99.8%) was purchased from EMD Chemicals, Inc. and pyridine (Py, ≥99.9%) was purchased from Sigma−Aldrich and used as received.…”