Synthesis, structure and thermal decomposition of tetra(2-pyridyl)pyrazine•I2 charge-transfer complexes

In both the solid state and in solution, 2,3,5,6-tetrakis(2'-pyridyl)pyrazine (tpp) reacts with iodine to form charge-transfer complexes. The solid state process results in the exclusive production of a mono-I-2 adduct, 3, while the solution reaction can produce both 3 and a bis-I-2 adduct, 4. The X-ray crystal structure of 4 is described and a structure for 3 is proposed based upon spectroscopic evidence and molecular orbital calculations. The thermal decompositions of 3 and 4 proceed by I-2 evolution at very different rates and lead to different polymorphs of tpp. The mechanism of this process is described in terms of solid state reaction theory.