Elongation of phenoxide C - O bonds due to formation of multifold hydrogen bonds: Statistical, experimental, and theoretical studies

Statistical studies using the Cambridge Structural Database have revealed that there are several elongated phenoxide C - O bonds. They are characterized by the formation of 3-fold (or occasionally 2-fold) hydrogen bonds to the phenoxide oxygen atoms, and their mean bond length extends up to 1.320 Å, which is quite different from the theoretically predicted carbon - oxygen bond length of C₆H₅O^- (1.26 Å). Elongated phenoxide C - O bonds associated with the formation of 3-fold hydrogen bonds were also observed in the X-ray structures of proton-transfer complexes (2X - O^-)(TEAH⁺)s derived from 5′-X-substituted 5,5′′-dimethyl-1,1′:3′,1′′-terphenyl-2, 2′,2′′-triols (2X - OHs, where X = NO₂, CN, COOCH₃, Cl, F, H, and CH₃) and triethylamine (TEA). By comparing the X-ray structures, C - O bond elongation was found to be only slightly affected by an electron-withdrawing substituent at the para position (X). This along with strong bathochromic shifts of N - H(⋯O^-) and O - H(···O^-) stretching vibrations in the IR spectra indicates that the elongated C - O bonds in (2X - O^-) (TEAH⁺)s essentially have single-bond character. This is further confirmed by molecular orbital calculations on a model complex, showing that the negatively charged phenoxide oxygen atom is no longer conjugated to the central benzene ring, and the NICS values of the three benzene rings are virtually identical. However, C - O bond elongation in (2X - O^-)(TEAH ⁺)s was considerably influenced by a change in the hydrogen-bond geometry. This also suggests that hydrogen bonds significantly affect phenoxide C - O bond elongation.