THE STRUCTURE OF 3,4-DIAZA-1,6,6aλ⁴−TRITHIAPENTALENES-A COMBINED EXPERIMENTAL AND THEORETICAL STUDY

Abstract

The X-ray structure of 2,5-bis(piperidyl)-3,4-diaza-1,6,6aλ⁴−trithiapentalene (1, R = piperidyl) is reported and compared with the equilibrium geometries calculated for the diamino-substituted compound by density functional theory (B3LYP exchange-correlation functional). The molecular geometry and electronic structure of the 3,4-diaza-1,6,6aλ⁴−trithi-apentalene parent compound (1, R = H) has been examined in more detail in conjunction with the nitrogen-free 1,6,6aλ⁴−trithiapentalene (2, R = H). Whereas Hartree-Fock-calculations predict 1 and 2 as valence isomers in equilibrium, DFT(B3LYP) as well as ab inirio MP2 calculations result in a single minimum structures of C_2v−symmetry. Substitution of methine groups by aza nitrogen in the 3- and 4-positions and introduction of amino groups in the 2- and 5-positions of 1,6,6aλ⁴ -trithiapentalene (2, R = H) stabilize the heterocyclic compound by formation of a polymethine-type substructure. According to DFT calculations the C_2v−symmetry is retained in passing from 1 (R = H) to the amino-substituted compound (1, R = NH₂). The nucleus-independent chemical shifts (NICS) of the five-membered rings of 3.4-diaza-1,6,6aλ⁴ -trithiapentalene and of its nitrogen-free congener exhibit aromatic bond delocalization.

Key Words:

• 1,6,6aλ⁴− trithiapentalene
• 3.4-diaza-1,6,6aλ⁴−trithiapentalene
• X-ray structure
• NMR chemical shift
• Aromaticity
• ab initio quantum chemistry
• density functional theory
• topological analysis
• natural population analysis
• natural resonance theory
• homodesmic reaction