Crystal Structure of 4,4′-Bi(3-(1-methylethoxy)-3-cyclobutene-1,2-dione)

Abstract

Bisquaric acid, a hydrolysed form of 4,4¢-bi(3-(1methylethoxy)-3-cyclobutene-1,2-dione) (title compound, Fig. 1), reported by Liebeskind et al.1 possesses unique characteristics in the single-crystal phase, such as a dielectric response2 and an increase in the p-resonance,3 due to proton migration via quasi-one-dimensional hydrogen bonding. These characteristics originate from the concerted dynamics of the p-electrons and proton involved in strong hydrogen bonding, which has been confirmed by crystal structure, 13C-NMR, and DFT analyses. Most of the discussion emphasises the effect of hydrogen bonding. Further, the property originating from a bisquaric framework is not well considered. Accordingly, we analysed the single-crystal structure of the title compound in which the hydrogen bonding is quenched so as to investigate the p-electron system in the molecular framework. The title compound and its single crystal were prepared by Liebeskind’s method.1 The single crystal was mounted on a glass capillary, transferred to a Bruker AXS SMART APEX diffractometer equipped with a CCD area detector and Mo Ka (l = 0.71073 A) radiation, and centred in the beam at room temperature (293 K). The structure was solved and refined with SHELXS-97 and SHELXL-2014/7 (ref. 4), respectively, by the direct method before being expanded by Fourier techniques. A least-squares refinement was first carried out with all nonhydrogen atoms refined isotropically, with a subsequent assignment of the anisotropic thermal parameters to these atoms to achieve convergence. Hydrogen atoms were located by difference Fourier mapping, and all parameters were refined. All isotropic displacements of the hydrogen atoms were greater than the equivalent isotropic displacements of their connecting atoms. Crystal data are given in Table 1, as well as an ORTEP view is illustrated by ORTEP-3 (ref. 5). The atom numbering scheme is shown in Fig. 2. PLATON software6 suggested the presence of very weak intramolecular (C5–H5·O2) and intermolecular (C5– H5·O1i) hydrogen bonds. However, these are non-classical hydrogen bonds, and hence are insignificant for conjugated p-electron systems due to the low probability of proton migration. Table 2 summarises the lengths of the weak hydrogen bonds and C–C bonds of four membered ring and linker bond. The length of the C1–C4 bond is similar to that of the ethylene carbon–carbon double bond (1.33 A),7 while the cyclic linker C1–C1a and the adjacent C1–C2 bond lengths are similar to the sp2–sp2 bond length (1.47 A).7 On the other hand, the C2–C3 and C3–C4 bonds are shorter than the carbon–carbon Crystal Structure of 4,4¢-Bi(3-(1-methylethoxy)-3-cyclobutene-1,2-dione)