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)2X2]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.