On the Formation and Decomposition of the Melonate Ion in Cyanate and Thiocyanate Melts and the Crystal Structure of Potassium Melonate, K3[C6N7(NCN)3]

The synthesis of potassium melonate, K-3[C6N7(NCN)(3)], by reaction of a potassium thiocyanate melt with the polymer melon [C6N7(NH)(NH2)](n) is an established, though poorly understood, reaction. We have modified the original approach by using salt melts containing Na+ ions and/or cyanate ions to yield the respective melonate salts. These melonates, however, are not the final reaction products. We have identified them to decompose in cyanate melts to form tricyanomelaminates at higher temperatures and prolonged reaction times. This is the first selective decomposition reaction leading from heptazines to triazines. The progress of the reactions was studied by using thermal analysis, thus allowing the exact determination of reaction temperatures and weight losses. With the data at hand we are now able to gain better insight into the formation and properties of alkali melonates while establishing new synthetic routes to these compounds. We were able to isolate crystals of anhydrous potassium melonate directly from a thiocyanate melt. The structure of this compound was solved by single-crystal X-ray-diffraction. The new reaction conditions involving cyanates on the one hand avoid the release of CS2 and are no longer highly corrosive to most metallic reaction vessels and on the other hand these reagents provide new, cheap, and convenient access to melonates and tricyanomelaminates. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)