4 years ago

Crystallographic insights into the synthesis and magnetic properties of oxoverdazyl radicals functionalized by benzoic acid.

Cyrille Train, Ghénadie Novitchi, Varun Kumar, Vincent Maurel, Sergiu Shova
The synthesis and crystallization of two verdazyl radicals, 1,5-dimethyl-3-(4'-carboxyphenyl)-6-oxoverdazyl HIMe and 1,5-diisopropyl-3-(4'-carboxyphenyl)-6-oxoverdazyl HIi-Pr, 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 HIi-Pr allows a fine description of the cyanoborane adduct which is the key intermediate of this synthesis. The verdazyl radicals themselves were obtained as single crystals. In the case of HIi-Pr, depending on the solvent, two polymorphs were 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 interaction 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 Å in HIMe to 7.90 Å in HaIi-Pr. This modulation of the distance strongly influences the antiferromagnetic intermolecular exchange interaction between π-stacked radicals which goes from J=-90 cm-1 (H=-JΣSiSi+1) for HIMe to -12.96(3) cm-1 for HbIi-Pr and -0.92 cm-1 for HaIi-Pr.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/ejic.201700950

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