Azacyanocarbon chemistry. Comparison of the hexacyano-3,4-diazahexa-1,5-dienediide [C10N8]2- with the corresponding radical anion [C10N8]˙-: electrochemical and EPR studies, crystallographic and electronic structures

Abstract

Electrochemical, spectrochemical and structural characterization of the tetraalkylammonium salts of the 1,1,2,5,6,6-hexacyano-3,4-diazahexa-1,5-dienediide anion [(R₄N)₂(C₁₀N₈): R = Et 1a; R = Bu 1b] are reported. The dianion displays two one-electron reversible oxidation processes at E′⁰₁ = -0.10 V [(C₁₀N₈)˙^-/(C₁₀ N₈)^2-] and E′⁰₂ = +0.59 V [(C₁₀N₈)/(C₁₀N₈)˙ ^-] (ref. Fc⁺/Fc). Solutions of the radical anion have been prepared by bulk electrolysis and fully characterized by EPR spectroscopy. Chemical oxidation of 1a by iodine in dichloromethane affords this radical anion as its tetraethylammonium salt (Et₄N)(C₁₀N₈) 3a. (C₁₀N₈)^2- and (C₁₀N₈)˙^- have unequivocally different IR and visible spectra. Single crystal X-ray diffraction studies of 1b and 3a have been carried out. In 1b, the dianion is planar within ±0.03 Å and has a crystallographically imposed symmetry but the point group is approximately C_2h while in 3a the (C₁₀N₈)˙^- unit, which has no crystallographically imposed symmetry, is only roughly planar. In both cases, the two –C(CN)C(CN)₂ groups are in trans positions relative to the N–N bond. The crystal structure of 3a clearly shows that the anion exist as dimer units [(C₁₀N₈)₂]^2-. This is in agreement with the EPR spectra and the molecular orbital study carried out at the semi-empirical AM1 level which both indicate a thermally populated triplet excited state.