A coupled dual hydrogen bridge system—solution dynamics and crystal structure of 2-dimethylamino-4,6-bis(2-hydroxyphenyl)-1,3,5-triazine

Abstract

Detailed NMR analysis (CD₂Cl₂, 183 K) shows the two intramolecular hydrogen bridges of 2-dimethylamino-4,6-bis(2-hydroxyphenyl)-1,3,5-triazine to be directed either towards N-1 and N-3 (S conformer; both hydrogen bonds to an aza nitrogen ortho to the N(CH₃)₂ group) or towards N-1/N-3, respectively, and N-5 (A conformer; one ortho and one para hydrogen bond each), with the ratio S∶A = 1∶2.6₅. The conformational equilibrium has been probed by dynamic NMR spectroscopy. For 180° rotation of one 2-hydroxyphenyl moiety, i.e. the mutual interconversion of A and S, ΔG^‡ ≅ 47 kJ mol⁻¹ is determined by complete line shape analysis (range 213–243 K). The activation barrier for simultaneous rotation of both 2-hydroxyphenyl groups is higher by only ≅ 0.5 kJ mol⁻¹; this process thence contributes substantially to the overall dynamic process. For the crystalline state, X-ray diffraction shows only the A conformer. The C–O–H bond angles appear compressed towards 90° for both the ortho and the para bridge. In the para bridge, O(15)–H(15)⋯N(5), the bonding hydrogen is located close to symmetrically between the donor O and the acceptor N (bond angle ≈165°).