| Summary: | 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 [001]. 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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