Structure at 200 and 298 K and X-ray investigations of the phase transition at 242 K of [NH₂(CH₃)₂]₃Sb₂Cl₉ (DMACA)

[NH 2 (CH 3 ) 2 ] 3 Sb 2 Cl 9 (dimethylammonium nonachlorodiantimonate, DMACA) has, at 200 K, a monoclinic Pc space group, with a = 9.470 (3), b = 9.034 (3), c = 14.080 (4) Å, β = 95.81 (3)°, V = 1198.4 (4) Å 3 , Z = 2 [ R = 0.024, wR = 0.025 for 4613 independent reflections with F > 4 σ ( F )]. At 298 K DMACA has P 2 1 / c space group with a = 9.686 (3), b = 9.037 (3), c = 14.066 (4) Å, β = 95.57 (3)°, V = 1225.3 (5) Å 3 , Z = 2 [ R = 0.034, wR = 0.035 for 2736 reflections with F > 4 σ ( F )]. The anionic sublattice of DMACA consists of polyanionic (Sb 2 Cl 9 3− ), layers. In the low-temperature phase there are three crystallographically non-equivalent dimethylammonium cations in the crystal structure. One of the cations is located inside the polyanionic layers, two others – one ordered and one disordered – between the polyanionic layers. In the room-temperature phase there are two non-equivalent cations – both disordered – in the crystal structure. Temperature dependencies of lattice parameters between 200 and 300 K were determined. The occurrence of a second-order phase transition at T = 242 K was confirmed. The dependence of lengths of Sb—Cl contacts on the presence and strength of N—H...CI hydrogen bonds was discussed. It was found that lengths of Sb—Cl bonds may differ from each other by as much as 0.3 Å, because of the presence of N—H...Cl hydrogen bonds. These differences were attributed to distortion of the lone-electron pair on antimony(Ill).

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