2,7-Di-tert-butyl-9,9′(10H,10′H)-spirobiacridine-10,10′-dioxyl as a ground triplet biradical: The role of tert-butylation

The synthesis of a doubly tert-butylated spirobiradical, 2,7-di-tert-butyl-9,9'(10H,10'H)-spirobiacridine-10,10'-dioxyl is reported. The X-ray diffraction study clarified that the two dihydroacridine skeletons were fused at the sp(3)-carbon atom affording a cruciform structure. The frozen-solution electron spin resonance spectrum showed a typical zero-field-splitting pattern. The magnetic study revealed the presence of an intramolecular ferromagnetic interaction with 2J/k(B) = +22.3(5) K. The density-functional-theory calculation results are entirely consistent with the experimental results. After comparison with the known unsubstituted spirobiradical, we can conclude that tert-butylation is a useful method to reduce the intermolecular interaction without breaking the degeneracy of the singly occupied molecular orbitals. (C) 2020 Elsevier Ltd. All rights reserved.