Facile Formation of Stable Neutral Radicals and Cations from [22]Smaragdyrin BF<sub>2</sub> Complexes
Wei-Kang Deng, Yang Liu, Daiki Shimizu, Takayuki Tanaka, Akito Nakai, Yutao Rao, Ling Xu, Mingbo Zhou, Atsuhiro Osuka, Jianxin Song
Abstract
Abstract meso ‐Trimesityl‐substituted [20]smaragdyrin freebase was synthesized by p ‐toluenesulfonic acid catalyzed reaction of 5‐mesityldipyrromethane and 2,14‐dibromodipyrrin in an improved yield of 63 %. Unexpectedly, treatment of the [20]smaragdyrin freebase with BF 3 ⋅ OEt 2 and triethylamine (TEA) gave a stable radical species, in which the BF 2 unit is coordinated at the tripyrrin site, probably by ready release of a hydrogen atom of a [22]smaragdyrin BF 2 complex. Similar treatment of [22]smaragdyrin free base produced another [22]smaragdyrin BF 2 complex, in which the BF 2 unit is coordinated at the dipyrrin site. The tripyrrin site coordinated neutral radical was oxidized with AgSbF 6 to give a stable antiaromatic cation; this was reduced with NaBH 4 to its 22π congener, which was easily oxidized back to the neutral radical in the air and rearranged to thermodynamically stable dipyrrin site coordinated [22]smaragdyrin BF 2 complex upon treatment with BF 3 ⋅ OEt 2 and TEA. Further, the dipyrrin site coordinated [22]smaragdyrin BF 2 complex was similarly oxidized to a stable neutral radical and a stable cation in a stepwise manner. This work demonstrates a rare ability of smaragdyrin BF 2 complexes to exist in multiple redox states, particularly forming a stable neutral radical by facile release of a hydrogen atom.