Equivalence of NH₄⁺, NH₂NH₃⁺, and OHNH₃⁺ in Directing the Noncentrosymmetric Diamondoid Network of O−H···O^- Hydrogen Bonds in Dihydrogen Cyclohexane Tricarboxylate

The assembly of hexagonal and diamond network architectures from functionalized tectons of trigonal and tetrahedral symmetry, respectively, is an important activity in crystal engineering. We report a novel supramolecular transformation for the synthesis of diamond network structures from the trigonal molecule, 1,3-cis,5-cis-cyclohexanetricarboxylic acid (H3CTA). Crystal structures of some salts of the trigonal anion, H2CTA-, with tetrahedral counterions is analyzed in H2CTA-·NH4+ 1, H2CTA-·MeNH3+ 2, H2CTA-·EtNH3+ 3, H2CTA-·NH2NH3+ 4, and H2CTA-·OHNH3+ 5. The trigonal anion functions as a tetrahedral self-complementary node in the presence of NH4+ counterion (salt 1) via two COOH donors and COO- as a double hydrogen-bond acceptor. The triply interpenetrated diamondoid network of O−H···O- hydrogen bonds in 1 is reproduced in isostructural 3D nets of 4 and 5 by substituting NH4+ by NH2NH3+ and OHNH3+ (Π = 0.025, 0.027). The SHG activity of noncentrosymmetric diamondoid solids 1, 4, and 5 (space group Cc) is comparable to that of the nonlinear optical (NLO) material potassium dihydrogen phosphate (KDP) (0.3 × urea). However, salts 2 and 3 (space groups P21/c and P1̄) have hexagonal and square grid layers of H2CTA- anions because the ammonium cation in these structures is devoid of the fourth strong hydrogen-bond donor group to extend crystal growth to the 3D diamond network. Thus, RNH3+ counterions may be used to control the anionic network of the H2CTA- molecule based on a tetrahedral node in 1, 4, and 5, a trigonal node in 2, and a square node in 3. The function of cyclohexane tricarboxylate as a four-connected node, shown for the first time in a trigonal molecule, is in contrast to the usual role of the trimesate anion as a three-connected node in molecular complexes.以三角对称与四面体对称的功能化构造单元分别组装六方网络与金刚石网络结构，是晶体工程领域的重要研究方向。本文报道了一种新颖的超分子转化策略，可利用三角分子1,3-顺式,5-顺式-环己烷三羧酸（1,3-cis,5-cis-cyclohexanetricarboxylic acid，H3CTA）合成金刚石网络结构。我们对三角阴离子H2CTA-与四面体抗衡离子形成的五类盐的晶体结构进行了分析，所涉盐类包括H2CTA-·NH4+（化合物1）、H2CTA-·MeNH3+（化合物2）、H2CTA-·EtNH3+（化合物3）、H2CTA-·NH2NH3+（化合物4）以及H2CTA-·OHNH3+（化合物5）。在NH4+抗衡离子（化合物1）存在的条件下，该三角阴离子可作为四连接自互补节点，通过两个COOH质子供体与COO-作为双重氢键受体实现相互作用。化合物1中由O−H···O-氢键构筑的三重互穿金刚石型网络，可通过将NH4+分别替换为NH2NH3+与OHNH3+，在化合物4与5的同构三维网络中得到重现（结构参数Π分别为0.025、0.027）。空间群为Cc的非中心对称金刚石型固体1、4与5，其二次谐波生成（SHG）活性可与非线性光学（NLO）材料磷酸二氢钾（KDP）相媲美（强度为尿素的0.3倍）。而化合物2与3的空间群分别为P21/c与P-1，其晶体结构中仅存在H2CTA-阴离子形成的六方网格层与方格网格层，这是由于此类结构中的铵阳离子缺少第四个强氢键供体基团，无法将晶体生长拓展至三维金刚石网络。由此可见，可通过调控RNH3+型抗衡离子的种类，实现对H2CTA-阴离子网络结构的精准调控：在化合物1、4与5中，H2CTA-为四连接节点；在化合物2中为三角节点；在化合物3中则为方格节点。本研究首次在三角分子中实现了环己烷三羧酸根作为四连接节点的功能，这与苯均三酸根阴离子在分子配合物中通常作为三连接节点的常规角色形成鲜明对比。