Autoreduction of Pd-Co and Pt-Co cyanogels: Exploration of cyanometalate coordination chemistry at elevated temperatures

Cyanogels are coordination polymers made from the reaction of a chlorometalate and a cyanometalate in aqueous solution, which undergo a sol-gel transition to form stable gels. At temperatures above 240°C, the cyanide ligand acts as a reducing agent and reduces the metal centers to lower oxidation states. To understand the mechanism of the autoreduction, the thermal reduction of the Pd-Co cyanogel system formed by the reaction of PdCl₄ ^2- and Co(CN)₆^3- was studied in an inert atmosphere. It was found that the reduction proceeds through two polymeric cyanide-containing intermediates, CoPd(CN)₄ and Pd(CN)₂, that form upon reduction of Co³⁺ to Co²⁺ and involves a significant rearrangement of the coordination structure. The two intermediates upon further heating reduce to metallic products, which by solid-state diffusion form a single Pd/Co alloy product. CoPd(CN)₄ was found to have a hydrated form Co(H₂O)₂Pd(CN)₄·4H ₂O with a layered structure crystallizing in an orthorhombic Pnma space group. The Pt-Co cyanogel was found to autoreduce via a similar route. CoPt(CN)₄ was confirmed as an intermediate. Understanding of the mechanism of the cyanogel autoreduction is an important step toward better understanding of opportunities that cyanogels offer in materials chemistry, as well as an expansion of the knowledge of coordination chemistry at elevated temperatures in general.