Experimental and theoretical study on 3,5‐(oxo/thioxo) derivatives of 2,7‐dimethyl‐1,2,4‐triazepines–iodine molecular complexes

Intermolecular charge‐transfer (CT) spectra of 3‐thioxo‐5‐oxo‐, 5‐thioxo‐3‐oxo‐, and 3,5‐dithioxo‐ derivatives of 2,7‐dimethyl‐[1,2,4]‐triazepine 1:1 molecular complexes with molecular iodine were studied in the UV‐visible region. Equilibrium constants and free energy changes of the formed complexes were determined in solution. Ab initio calculations at HF/LANL2DZ* and MP2/LANL2DZ* were carried out to establish the nature of the complexation site, to determine the complex structures, and to examine the basicity of these compounds toward molecular iodine. The 3,5‐dithioxo‐2,7‐dimethyl‐[1,2,4]‐triazepine is the most basic one toward molecular iodine. In all cases, the complexation takes place at the heteroatom attached to position 3 of the triazepine. Hence, although in general, thiocarbonyls are stronger bases than carbonyls in the gas phase, 5‐thioxo‐2,7‐dimethyl‐[1,2,4]‐triazepin‐3‐one behaves as an oxygen base towards I2. Experimental free energies in solution and gas‐phase computational values are linearly correlated.