Equivalence of NH₄⁺, NH₂NH₃⁺, and OHNH₃⁺ in directing the noncentrosymmetric diamondoid network of O-H···O^- hydrogen bonds in dihydrogen cyclohexane tricarboxylate

Abstract

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⁺ (η = 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 P) 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.