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)}{HT(4-Py)P}·CHCl (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 HT(4-Py)P, the HT(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 HT(4-Py)P radical anions have a spin state of S = 1/2 and a magnetic moment of 1.64 μ 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.

Molecular Structure, Optical and Magnetic Properties of Dianionic Free‐Base Tetrapyrazinoporphyrazine Macrocycle

“…It is seen that changes in lengths of the C‐N im bonds are opposite within one pyrrolopyrazine unit to those of the C−C bonds in the pyrrole ring and the C‐N pyr bonds (Figure a). Earlier it was found that single reduction of phthalocyanine, corrole and porphyrin macrocycles is also accompanied by alternation of the C‐N im or C‐C meso bonds (the difference between short and long bonds is 0.02‐0.04 Å) . However, alternation of bonds is essentially more pronounced for the dianionic macrocycles such as H 2 TPyzPz 2‐ .…”

“…For example, it has been shown that under partial oxidation of phthalocyanine macrocycle in H 2 Pc, M II Pc or the [M III (CN) 2 Pc] ‐ anions, conductivity is realized through the π‐system of the macrocycle . In spite of that oxidation chemistry of macrocycles has been developed for a long time, reduced macrocyclic compounds have been obtained only recently and their crystal structures, optical and magnetic properties are not studied well . Selection of appropriate reductants is required to obtain compounds with radical anions of macrocycles.…”

“…7 Substituted porphyrins and corroles (Cor) have even more negative reduction potentials: −1.23 V ( vs SCE in CH 2 Cl 2 ) for H 2 TPP (TPP is tetraphenylporphyrin) 13 and from −0.98 to −1.36 V ( vs SCE in benzonitrile) for substituted H 3 TRCor, where R can be different aryl substituents. Their reduced species can be obtained if very strong reductants such as decamethylchromocene or potassium graphite (KC 8 ) are used ,. It was shown for triphenylcorrole that the reduction is accompanied by the deprotonation of the macrocycle and the formation of the radical H 2 TPCor •2‐ dianions .…”

“…Thus, reduction of H 2 TPyzPz leads to the noticeable shortening of the N–H bonds in the center of the macrocycle. Earlier it was shown that the formation of the H 2 Pc •‐ radical anions is also accompanied by shortening of the N–H bonds …”

Double‐Decker Paramagnetic {(K)(H3Hhp)2}⋅2−Radical Dianions Comprising Two [30]Trithia‐2,3,5,10,12,13,15,20,22,23,25,30‐Dodecaazahexaphyrins and a Potassium Ion

“…[6] It is aw eake lectron acceptor, whosef irst redox potential is between those of free-base tetraphenyl-or tetra(4-pyridyl)porphyrin, H 2 T(4-Py)P (E red is À1.23 to À1.26 Vi nC H 2 Cl 2 vs. SCE) [9] and phthalocyanine (E red is À0.72 V in DMF vs. SCE). [10] Changes in the electron configuration of the macrocycles at their partial or one-electrono xidation [11] or reduction [12][13][14][15][16] produce an unpairede lectron, which can participate in magnetic interactions or high conductivity.R ecently,c onvenient methods for the preparation of radical anions or dianions of freebase macrocycles-phthalocyanine, naphthalocyanine,a nd porphyrin-have been developed. [12][13][14][15] However,t he preparation of crystalline anionic complexes with free-base macrocycles with ad efined crystal structure is still rare.…”