Crystal structures and phase transitions of imidazolium hypodiphosphates

Abstract

Two imidazolium hypodiphosphates, (C3H5N2)(H3P2O6) (I) and (C3H5N2)2(H2P2O6) (II), have been synthesized and structurally characterized. In both metal-free organic–inorganic hybrids (I) and (II), the hypodiphosphate mono- and dianions, (H3P2O6)− and (H2P2O6)2−, form hydrogen-bonded frameworks of different types, to which the organic cations are linked via N—H...O and C—H...O hydrogen bonds. The purity of the compounds was confirmed by powder X-ray diffraction. Differential scanning calorimetry of compound (I) revealed two structural phase transitions: continuous at 311.8 K [cooling/heating; from high-temperature phase (HTP) to room-temperature phase (RTP)] and a discontinuous one at 287.9/289.2 K [RTP → low-temperature phase (LTP)]. Compound (I) is characterized in a wide temperature range by single-crystal and powder X-ray diffraction methods. Crystal structures of high- and low-temperature phases are determined, which show orthorhombic (HTP, Pnna, No. 52) → monoclinic (LTP, P21/n11, No. 14, a-axis doubled) structural change on cooling with an intermediate incommensurately modulated phase (RTP). Dynamic properties of polycrystalline (I) were studied by means of dielectric spectroscopy. The dielectric behaviour is explained by the motion of imidazolium cations.

Multifunctionality of the [C2mim][Ln(fod)4] series (Ln = Nd–Tm except Pm): magnetic, luminescence and thermochemical studies

“…The nature of the phase transformation that occurs when passing from one solid phase to another solid phase cannot be inferred only by DSC analysis. However, some examples exist where this kind of solid-solid phase-exchange in organic imidazolium 30 or organometallic imidazolium salts 31 has already been described, polymorphism being one of the possible causes. Additional studies will be necessary to clarify the nature of these solid-solid transitions.…”

Structural variety and dehydration in 3-aminopyridine–hypodiphosphoric acid–water system

“…Up to now, only few examples of structural studies of organic hypodiphosphates are known, which are 2-aminopyridiunium salts and co-crystals, 31 tetraalkylammonium ionic co-crystals, 32 imidazolium 33 and adeninium 34 salts. Worth mentioning is the dehydration in tetrabutylammonium salt crystal, which took place as single-crystal-to-single-crystal process and resulted in conformational transformation of the H 3 P 2 O 6 − anion from staggered to perfectly eclipsed form.…”

“…[20][21][22][23][24][25][26][27][28] The analyses of the Cambridge Structural Database (CSD; 29 salts without metal ions) revealed that the vast majority of the deposited structures had pyridine nitrogen atom protonated, and very rarely the amino group was protonated. 30 Up to now, only few examples of structural studies of organic hypodiphosphates are known, which are 2-aminopyridiunium salts and co-crystals, 31 tetraalkylammonium ionic co-crystals, 32 imidazolium 33 and adeninium 34 salts. Worth mentioning is the dehydration in tetrabutylammonium salt crystal, which took place as singlecrystal-to-single-crystal process and resulted in conformational transformation of the H 3 P 2 O 6 − anion from staggered to perfectly eclipsed form.…”

“…This work continues exploration of the supramolecular chemistry, thermal stability and spectroscopic characterization of hypodiphosphates. [31][32][33][34] A series of 3-aminopyridine-containing compounds is presented (Scheme 1). Two of the hybrid crystals: (1) and ( 3) contain water molecules.…”

Crystal engineering and structural diversity of 2-aminopyridinium hypodiphosphates obtained by crystallization and dehydration

“…Moreover, the tendency of hypodiphosphates to form networks stabilized by strong O–H⋯O hydrogen bonds, composed exclusively of hypodiphosphate anions and/or acid molecules, has been clearly indicated. 20–24 Hypodiphosphate substructures are stabilized by the formation of one, two or three hydrogen bond bridges between anions or acid molecules. In most of the known crystals of organic hypodiphosphates, two hydrogen bonds link adjacent anions giving rise to motifs as shown in Fig.…”