Double Pancake Versus Long Chalcogen-Chalcogen Bonds in Six-Membered C,N,S-Heterocycles

The double "pancake" bonding in the dimers of the six-membered heterocycles 1,3-dithia-2,4,6-triazine (4) and 1,3-dithia-2,4-diazine (16) were investigated by means of high-level quantum chemical calculations (B3LYP and CCSD(T)). It was found that the S-S dimers, 20 a and 27, are not the most stable isomers, but the dimers showing short S-N (21 a) and S-C (25, 28) bonds. An investigation of the 5-phenyl-1,3-dithia-2,4,6-triazine (4 b) yields that the syn dimer with two S-S bonds (2.57 Å) is the most stable one. In this dimer, the phenyl groups are placed on top of each other. The additional dispersion energy of the phenyl rings causes a stabilization of the syn-S-S (C2v -like) isomer. As a result, two weak albeit relevant single S-S bonds (2.57 Å) are predicted. These findings contradict the recently published concept of double "pancake" bonding in the dimer 4 b2 .