Katrusiak Andrzej, Szafrański Marek
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznań, Poland.
J Am Chem Soc. 2006 Dec 13;128(49):15775-85. doi: 10.1021/ja0650192.
Centrosymmetric pyrazinium NH+...N bonded complexes allow their dielectric properties to be analyzed separately from the bulk ferroelectric polarization observed in the noncentrosymmetric DABCO monosalts. The method of dielectric permittivity measurements has been employed for monitoring polarization fluctuations generated by proton transfers in the NH+...N bonded linear polycations. The revealed dielectric response of [C4H5N2]+BF4- and [C4H5N2]+ClO4- cannot be reconciled with the centrosymmetric symmetry of their structures, but suggests formation of polar defects or nanoregions. Unique transformations of the pyrazine [C4H5N2]+BF4- complex between linear NH+...N hydrogen-bonded polycationic aggregates antiparallel in phase gamma, perpendicular chains in phase beta, and with NH+...N bonds broken in phase alpha have been observed. The [C4H5N2]+ClO4- and [C4H5N2]+BF4- complexes as grown at 290 K are isostructural in orthorhombic phase , space group Pbcm, with linear polycationic chains arranged antiparallel. On heating, the tetrafluoroborate and perchlorate salts each undergoes two first-order phase transitions at similar temperatures about 340-360 K, while on cooling only one phase transition has been observed. An extremely unique sequence of two phase transitions subsequently lowering the symmetry of the [C4H5N2]+BF4- crystal when temperature is increased has been evidenced: the orthorhombic phase heated above 343 K transforms into the monoclinic C2/c-symmetric phase beta, in which the NH+...N bonded linear chains assume perpendicular arrangement; and at about 353-357 K the anions and cations adopt a typical ionic-crystal packing without homonuclear NH+...N hydrogen bonds in a still lower-symmetry monoclinic P21/n-symmetric structure. The exceptional perpendicular arrangement of the linear NH+...N bonded chains in phase beta constitutes a unique system where polarization of small regions can assume various orientations within a plane, depending on the H+ sites. No phase transitions or anomalous dielectric response were observed in the NH+...O bonded [C4H5N2]+NO3- complex. The unprecedented structure-property relations of the pyrazinium complexes fully confirm the role of the NH+...N bond transformations for the dielectric response of analogous DABCO ferroelectrics.
中心对称的吡嗪鎓 NH⁺...N 键合配合物使得它们的介电性质能够与在非中心对称的 DABCO 单盐中观察到的体铁电极化分开进行分析。介电常数测量方法已被用于监测由 NH⁺...N 键合的线性聚阳离子中质子转移产生的极化波动。[C₄H₅N₂]⁺BF₄⁻和[C₄H₅N₂]⁺ClO₄⁻所显示的介电响应与其结构的中心对称不相符，但表明形成了极性缺陷或纳米区域。已观察到吡嗪[C₄H₅N₂]⁺BF₄⁻配合物在γ相中线性 NH⁺...N 氢键合的多阳离子聚集体反平行、β相中垂直链以及α相中 NH⁺...N 键断裂之间的独特转变。[C₄H₅N₂]⁺ClO₄⁻和[C₄H₅N₂]⁺BF₄⁻配合物在 290 K 生长时在正交相空间群 Pbcm 中是同构的，线性聚阳离子链反平行排列。加热时，四氟硼酸盐和高氯酸盐在约 340 - 360 K 的相似温度下各自经历两次一级相变，而冷却时仅观察到一次相变。已经证明，当温度升高时，[C₄H₅N₂]⁺BF₄⁻晶体对称性降低的独特的两次相变序列：加热到 343 K 以上的正交相转变为单斜 C2/c 对称的β相，其中 NH⁺...N 键合的线性链呈垂直排列；在约 353 - 357 K 时，阴离子和阳离子采用典型的离子晶体堆积，在对称性更低的单斜 P2₁/n 对称结构中没有同核 NH⁺...N 氢键。β相中线性 NH⁺...N 键合链的特殊垂直排列构成了一个独特的系统，其中小区域的极化可以根据 H⁺位点在平面内呈现各种取向。在 NH⁺...O 键合的[C₄H₅N₂]⁺NO₃⁻配合物中未观察到相变或异常介电响应。吡嗪鎓配合物前所未有的结构 - 性质关系充分证实了 NH⁺...N 键转变对类似 DABCO 铁电体介电响应的作用。
