Molecular Iodine Stabilization in an Extended N···I–I···N Assembly

The adduct [bis(quinoxaline)-2,2 ,3,3 -disulfide·I2] (Q2S2·I2) (1) can be easily synthesised from the reaction of Q2S2 and I2 in CH2Cl2 or, in the absence of any solvent, through diffusion of I2 vapours at 60 °C. X-ray diffraction analysis shows the presence of an extended N···I–I···N assembly in which each I2 molecule links a Q2S2 molecule at both ends through a nitrogen atom to form a polymeric species; the d(I–I) and d(N–I) bond lengths confirm a very weak nitrogen–iodine interaction at the base of the N···I–I···N assembly. DFT calculations provide optimised distances for the N···I and I–I bonds and explanation for the zigzag chain formation: the mPW1PW functional and the B3LYP hybrid functional with a variety of basis sets for the I atomic species [CRENBL, LANL2DZ, LANL2DZ(d,p), LANL08(d), SBKJC, SBKJC polarised- LFK and Stuttgart RLC] have been tested. Compound 1 proved stable up to nearly 100 °C, and the stability is to be mainly attributed to the lattice energy of its polymeric structure then to donor–acceptor stabilisation. The facile insertion of molecular iodine into the Q2S2 network makes this compound an interesting iodine sponge, suitable for I2 storage; moreover, Q2S2 can easily collect and release I2(g) by a temperature- controlled process (60 and 97 °C, respectively).