Reductive Elimination of S₂N₂ and Se₂N₂ from Heavy Main Group Dithia- and Diselenadiazoles: A DFT Evaluation

Thermal decomposition pathways for cyclotrithiazenes Ph₃EN-S₃N₃ (E = P, As) have been examined by density functional theory methods. While open-chain derivatives S-ax-Ph₃ENSNS formed by the initial release of c-N₂S can isomerize to cyclic dithiadiazoles N-ax-Ph₃ES₂N₂, subsequent reductive elimination of S₂N₂ is only kinetically and thermodynamically favored for E = As. Investigations of heavier pnictogen-based dithiadiazoles Ph₃ES₂N₂ (E = Sb, Bi) as precursors of S₂N₂ indicate that the hypervalent five-coordinate antimony derivative is too stable, but the corresponding bismuth-based heterocycle has a much lower barrier for release of S₂N₂ and more favorable overall energetics. Extension to the corresponding diselenadiazoles R₃ESe₂N₂ (E = Sb, Bi) suggests that the bismuth derivative may serve as a benign source of Se₂N₂. Exploration of heavy heteroatoms from other periodic groups indicates that Pb(IV)-based derivatives are not effective, but Te(IV)-based materials are potentially superior to the Bi(V) systems. Collectively, the results indicate that thermal release of S₂N₂ and Se₂N₂ from heavy hypervalent atom dithia- and diselenadiazoles, either in solution or solid state, may afford facile and safe synthetic routes to these fragile rings and their polymeric analogues (SN)_x and (SeN)_x. The relative energetics of the formation of the two polymers are compared through molecular dynamics (MD) calculations.