The title compound, C22H28N2O4, contains two independent centrosymmetric molecules (A and B). In the previous structure determination [Xia et al. (2007 ▶). Acta Cryst. E63, o259] both A and B were modelled as neutral molecules with the H atoms of the the O—H groups included in calculated positions. In this redetermination, the transferrable H atoms were located in difference maps and freely refined, indicating that one molecule (A) crystallizes in the neutral (nonzwitterionic) form and the other in the zwitterionic form, namely 6,6′-dimethoxy-2,2′-[hexane-1,6-diylbis(nitrilodimethylidyne)]diphenol–6,6′-dimethoxy-2,2′-[hexane-1,6-diylbis(nitriliodimethylidyne)]diphenolate (1/1). This finding is supported by significant differences in the C—O(H) (A) and C—O− (B) bond lengths. In the crystal, the zwitterionic molecules (B) are involved in intermolecular N—H⋯O hydrogen bonds forming one-dimensional chains along . Each independent molecule forms an intramolecular O—H⋯N (A) or N—H⋯O (B) hydrogen bond. In molecule B, one of the –CH2– groups is disordered over two sets of sites with refined occupancies of 0.659 (8) and 0.341 (8)
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