Mechanical-Bond-Protected, Air-Stable Radicals

Abstract

Radical templation centered around a heterotrisradical tricationic inclusion complex DB^•+⊂DAPQT^2(•+), assembled from an equimolar mixture of a disubstituted 4,4'-bipyridinium radical cation (DB^•+) and an asymmetric cyclophane bisradical dication (DAPQT^2(•+)), affords a symmetric [2]catenane (SC·7PF₆) and an asymmetric [2]catenane (AC·7PF₆) on reaction of the 1:1 complex with diazapyrene and bipyridine, respectively. Both these highly charged [2]catenanes have been isolated as air-stable monoradicals and characterized by EPR spectroscopy. X-ray crystallography suggests that the unpaired electrons are delocalized in each case across two inner 4,4'-bipyridinium (BIPY²⁺) units forming a mixed-valence (BIPY₂)^•3+ state inside both [2]catenanes, an observation which is in good agreement with spin-density calculations using density functional theory. Electrochemical studies indicate that by replacing the BIPY²⁺ units in homo[2]catenane HC^•7+-composed of two mechanically interlocked cyclobis(paraquat-p-phenylene) rings-with "zero", one, and two more highly conjugated diazapyrenium dication (DAP²⁺) units, respectively, a consecutive series of five, six, and seven redox states can be accessed in the resulting SC·7PF₆ (0, 4+, 6+, 7+, and 8+), HC·7PF₆ (0, 2+, 4+, 6+, 7+, and 8+), and AC·7PF₆ (0, 1+, 2+, 4+, 6+, 7+, and 8+), respectively. These unique [2]catenanes present a promising prototype for the fabrication of high-density data memories.