The Squaric Acid Derivatives C8O4S2 and C8O4Se2 – Crystal Structures, Explosive Thermal Behavior and the Preparation of Carbon Suboxide Selenide OC3Se by Flash Vacuum Pyrolysis

2,7-Diselenatricyclo[6.2.0.0³.⁶]deca-1,3-diene-4,5,9,10-tetraone, C₈O₄Se₂, was prepared from 1,2-diselenosquarate and squaric acid dichloride. Its crystal structure and the structure of the already known sulfur analogue C₈O₄S₂ were determined (C₈O₄S₂: orthorhombic, Pca2₁, a = 1413.64(2), b = 599.850(9), c = 968.8(1) pm; C₈O₄Se₂: orthorhombic, Pnnm, a = 415.46(2), b = 894.29(5), c = 1160.14(7) pm). The structures are not isotypic and show a different packing of the molecules whose symmetry deviate only slightly from D₂ₕ. In the four-membered C₄ rings the C-C bonds represent one single bond, one double bond and two slightly shortened single bonds. The C₄ rings are thus to be considered as cyclobutene-dione fragments. The vigorous exothermic decomposition of the compounds that occurs on heating to 220 to 240 °C shows that both are energetic materials. The explosions are accompanied by a heat evolution of −192 kJ/mol for C₈O₄S₂ and −224 kJ/mol for C₈O₄Se₂. Performing the decomposition of C₈O₄S₂ in a closed autoclave leaves a residue of the composition “C₆S” which was examined by transmission electron microscopy techniques and shown to consist mainly of amorphous carbon. This thermal behaviour is limiting the utilization of C₈O₄S₂ and C₈O₄Se₂ as precursors for the syntheses of OC₃S and the yet unknown OC₃Se via FVP. The formation of OC₃S could be proven by the reaction of the trapped, slightly yellow product (evaporation at 200 °C, pyrolysis at 500 °C, trapping at −196 °C) with aniline which yielded thiomalonic acid dianilide, of which the crystal structure was determined (monoclinic, C2/c, a = 2814.8(16), b = 1201.7(8), c = 809.2(4) pm, β = 91.88(4)°, V = 2736(3) ・ 10⁶ pm³). The mass spectrum of C₈O₄Se₂ shows the strongest signal for OC₃Se+, and FVP experiments (evaporation at 220 °C, pyrolysis at 650 °C, trapping at −75 °C) yielded small amounts of a bright yellow material which rapidly converted into a black polymer.