Spontaneous Formation of an Air‐Stable Radical upon the Direct Fusion of Diphenylmethane to a Triarylporphyrin

Abstract

The direct fusion of a diphenylmethane segment to a Ni(II) 5,10,15-triarylporphyrin with three linkages furnished an air- and moisture-stable neutral radical through unexpected and spontaneous oxidation. This radical was demetalated by treatment with H2 SO4 and CF3 CO2 H to provide the corresponding free-base radical. These porphyrin radicals are very stable owing to spin delocalization and have been fully characterized through UV/Vis/NIR absorption spectroscopy, X-ray crystallographic analysis, magnetic susceptibility measurements, electrochemical studies, laser-based ultrafast spectroscopic studies, and theoretical calculations. They were chemically oxidized and reduced to the corresponding cation and anion but did not react with hydrogen-atom donors.

Porphyrin‐Based Air‐Stable Helical Radicals

“…X‐Ray crystallographic analysis revealed the structure of rac ‐5Ar to possess a fully conjugated π‐framework involving a [6]helicene unit (Figures c,d). The molecule exists as a monomer without any π‐stacking interactions in the crystal, although several CH‐π interactions with co‐crystallizing benzenes and other molecules were observed, different from the antiparallel π‐stacked dimer of diphenylmethyl‐fused radical 4 . The sum of three angles around the central carbon is 360.0(6)°, indicating perfect sp 2 ‐hybridization.…”

“…Both radicals displayed highly reversible oxidation and reduction waves with narrow electrochemical gaps. The redox potentials of each radical showed small positive shifts by 30–60 mV as compared with those of diphenylmethyl‐fused porphyrin 4 …”

“…Recently, we reported that diphenylmethyl‐fused porphyrin radical 4 is a very stable radical under ambient conditions owing to effective spin‐delocalization over the whole π‐electronic network . Considering that 4 has a [4]helicene moiety as a sub‐structure, this molecular system may be promising as a platform to realize air‐stable chiral radicals, the spin of which is distributed over the helicene architecture.…”

“…Firstly, introduction of methyl or methoxy substituents at the 2,2′‐positions in 4 was examined to induce helical structures for the fused diphenylmethyl segment. Following our previous procedure, 2,18,20‐trichloro‐5,10,15‐tris(3,5‐di‐ tert ‐butylphenyl) Ni II porphyrin 6 was reacted with 2,2′‐substituted diphenylmethyllithiums to produce the adducts 7Me and 7OMe in 53 and 42 % yields, respectively (Scheme ). Conversion of these compounds into the corresponding helical radicals rac ‐8Me‐A and rac ‐8OMe‐A were attempted using two‐fold palladium‐catalyzed intramolecular C−H arylations and spontaneous hydrogen atom loss from the central carbon .…”

A 1,5‐Naphthyridine‐Fused Porphyrin Dimer: Intense NIR Absorption and Facile Redox Interconversion with Its Reduced Congener

“…The NICS(0) values at the fused six‐membered rings were calculated to be +11.40 ppm for 5 and −4.87 ppm for 7 . At the neighboring pyrrole rings, significantly different NICS(0) values were also confirmed for 5 (−0.82 ppm), 7 (−11.71 ppm), and 8 (−5.9 ppm) . Moreover, the ACID plots of 5 and 7 respectively exhibited counter‐clockwise and clockwise ring currents passing through the nitrogen atoms.…”