Formation of Cyclic Selenium Imides via Acyclic Selenium Imide Chlorides in Cyclocondensation of Selenium Dichloride with tert-Butylamine

The energetics and mechanisms of cyclocondensation reactions between SeCl₂ and ^tBuNH₂ in 1:3 molar ratio are examined using density functional theory with implicit solvation in tetrahydrofuran at 193 K. The results give strong support for the proposed stepwise pathway consisting of three key steps: 1) formation of an acyclic building block ^tBuN(H)SeCl, 2) chain growth to ClSe[N(^tBu)Se]_nCl (n = 1–3, 7–9), and 3) ring closure to 1,3,5-Se₃(N^tBu)₃ (1), 1,3,5,7-Se₄(N^tBu)₄ (2), 1,3-Se₃(N^tBu)₂ (3), or 1,3,5-Se₄(N^tBu)₃ (4). The paths leading to 3 and 4 include additional redox steps and diradical intermediates to generate SeSe bonds. The proposed paths share common intermediates, which allows the rationalization of the experimental product distribution as well as changes to it upon altering the stoichiometry or the total SeCl₂ concentration. The results further show that the successive addition of [N(^tBu)Se] units quickly becomes energy neutral, decreasing the likelihood for the formation of molecules with more than eight atoms by cyclocondensation. Plausible mechanisms explaining the formation of the 15-membered ring 1,3,6,8,11,13-Se₉(N^tBu)₆ (5) involve the coupling of diradical intermediates or redox processes converting SeCl₂ to Se₂Cl₂, both of which might also play a role in the generation of SeSeSe functionalities in 1,5-Se₆(N^tBu)₂ (6).