Pressure-Induced Reduction of Dicyanamide by Samarium(II) in a Coordination Polymer

A samarium(II) coordination polymer, [Sm(2.2.2-cryptand)(dca)]I (dca^- = dicyanamide), has been prepared from the reaction of SmI₂ with tetrabutylammonium dicyanamide and 2.2.2-cryptand. The structure consists of [Sm(2.2.2-cryptand)]²⁺ cations bridged by dicyanamide to form corrugated 1D chains. Single crystals of this compound have been studied spectroscopically using UV-vis-NIR and Raman spectroscopy as a function of pressure to reveal a pressure-induced two-electron reduction of dicyanamide by Sm²⁺ to cyanide and cyanamide and Sm³⁺ between 5 and 8 GPa. High-pressure single-crystal diffraction studies reveal a phase change at 2.6 ± 0.1 GPa, where a polymorph exhibiting a smaller Sm²⁺···N≡C bond angle and shorter Sm²⁺···C distance was observed. These observations provide mechanistic insight into the reduction of dicyanamide into cyanide and cyanamide by pressurized samarium(II).