Structure Directing Influences in a Family of One-Dimensional Hybrid Cadmium Thiocyanate Halides

Organic-inorganic hybrid thiocyanates include a variety of compositions and structure types. To develop a better understanding of the interactions that control the crystal structure in this family of materials, six hybrid thiocyanate halide compounds with the general formula A₂Cd(SCN)₂X₂ (A = CH₃NH₃⁺, CH₃CH₂NH₃⁺, CH₃(CH₂)₂NH₃⁺, CH₃(CH₂)₃NH₃⁺; X = Cl^-, Br^-) have been synthesized. Single crystal X-ray diffraction shows that five of the six compounds crystallize with triclinic P1̅ symmetry, the lone exception being (CH₃(CH₂)₃NH₃)₂Cd(SCN)₂Cl₂ which adopts P2₁/c symmetry. Thermal analysis reveals decomposition temperatures between 180 and 210 °C. The intermediate hard-soft acidic nature of Cd²⁺ favors bridging thiocyanate groups and lowers the dimensionality compared to their Pb²⁺ and Sn²⁺ analogues, which adopt two-dimensional crystal structures with bridging halide ions. The presence of bridging thiocyanate groups lead to one-dimensional chains of [Cd(SCN)₂X^₂]²⁺ octahedra. Hydrogen bonding interactions between terminal halide ions and the amine headgroup of the organic cation dictate the crystal packing and play a key role in determining the thermal stability of these compounds. Diffuse reflectance measurements reveal large band gaps, ranging from 3.7 to 4.4 eV, and electronic structure calculations reveal narrow bands, with dispersions on the order of 0.5 eV. These attributes are likely to be characteristic of compounds with bridging thiocyanate linkers.