Solid State Structure, and Optical and Magnetic Properties, of Free Base Tetra(4-pyridyl)porphyrin {H2T(4-Py)P}•- Radical Anions

Abstract

A crystalline {cryptand[2.2.2.](K⁺)}{H₂T(4-Py)P^•-}·C₆H₄Cl₂ (1) salt with tetra(4-pyridyl)porphyrin radical anions was obtained, enabling the effect of reduction on a metal-free porphyrin macrocycle to be studied. In contrast to pristine H₂T(4-Py)P, the H₂T(4-Py)P^•- radical anions have altered C-C(meso) bonds due to partial loss of aromaticity from the porphyrin macrocycle. Short and long bonds have average lengths of 1.396(3) and 1.426(3) Å, which thus differ by 0.03 Å. Reduction affects the positions of the Soret and Q-bands of porphyrin observed in the spectrum of 1 at 439 and 512, 583, and 614 nm, and new bands of the radical anion appear at 684, 755, and 900 nm. The H₂T(4-Py)P^•- radical anions have a spin state of S = 1/2 and a magnetic moment of 1.64 μ_B at 300 K. Salt 1 shows a narrow asymmetric EPR signal fitted with two Lorentzian lines, with g_⊥ = 2.0031 and a line width (ΔH) of 0.186 mT, and g_∥ = 2.0019 (ΔH = 0.284 mT) at 295 K, and this signal splits into three components below 39 K. Salt 1 shows antiferromagnetic spin coupling with a Weiss temperature of -2 K.