Crystallographic Insights into the Synthesis and Magnetic Properties of Oxoverdazyl Radicals Functionalized by Benzoic Acid

Article abstract of DOI:10.1002/ejic.201700950

The synthesis and crystallization of two verdazyl radicals, 1,5-dimethyl-3-(4′-carboxyphenyl)-6-oxoverdazyl HI_Me and 1,5-diisopropyl-3-(4′-carboxyphenyl)-6-oxoverdazyl HI_iPr, are described. The electrochemical studies reveal that the oxidation of the two radicals is reversible, whereas their reduction is irreversible. The EPR spectrum of both radicals essentially exhibits a nine-line pattern related to the mean hyperfine interaction of the unpaired electron with the nitrogen atoms of the verdazyl cycle. The single-crystal X-ray diffraction of the intermediates towards HI_iPr allows a fine description of the cyanoborane adduct, which is the key intermediate of this synthesis. The verdazyl radicals themselves are obtained as single crystals. In the case of HI_iPr, depending on the solvent, two polymorphs are crystallized. The structure resolution reveals that, in the solid state, the organization of the verdazyl radicals is governed by both H-bonding and π–π interactions and is reminiscent of the H-bonded structures that can be present in solution. Within the 1D π stacks observed in the three compounds, the verdazyl–verdazyl distance varies from 4.88 ? in HI_Me to 7.90 ? in H^aI_iPr. This modulation of the distance strongly influences the antiferromagnetic intermolecular exchange interaction between π-stacked radicals, which goes from J = –90 cm^–1 (H = –JΣS_iS_i+1) for HI_Me to –12.96(3) cm^–1 for H^bI_iPr and –0.92 cm^–1 for H^aI_iPr.