Thermal autoreduction of cyanogels: Mechanism and application to the synthesis of transition metal alloys and nanoalloys

Cyanogels are inorganic coordination polymers containing transition metal centers bridged by cyanide ligands, which upon formation in aqueous solution undergo a sol-gel transition forming a hydrated gel. Upon heating, cyanogels undergo a reduction by the cyanide ligands leading to the formation of metallic products. Most previous work focused on the Pd/Co model.; This work extends the preparation of metal alloys from cyanogels to various transition metal binary and ternary systems and designs ways to control the composition of the final alloy product. New methods to synthesize nanoparticles are developed, leading to composite materials of nanoparticles of transition metal alloys in silicate-based matrices. The methods are applied to the synthesis of nanoparticles of several ferromagnetic alloys. A superparamagnetic transition is demonstrated in these materials. Cyanogels are also found to absorb microwave radiation by direct dielectric heating of the coordination polymer and to convert the absorbed energy to heat efficiently enough to induce autoreduction to metallic products. The microwave processing thus becomes an ultra-fast method to convert cyanogels to metals.; In order to better understand the autoreduction chemistry of cyanogels, the conversion of the Pd/Co model cyanogel is studied in greater detail. It is 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+ by a cyanide ligand accompanied by 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. Understanding of the mechanism of the autoreduction of cyanogels is not only an important step towards better understanding of opportunities that cyanogels offer in materials chemistry, but also an expansion of the general knowledge of coordination chemistry at elevated temperatures.