Structures of molecular and ionic compounds of ICl with pyrazine (pyz) have been established for the first time by X-ray diffraction structural analysis. In the molecular complex ICl∙pyz∙ICl (1) pyrazine serves as bidentate nonchelating ligand. The ionic complexes [Hpyz]+[ICl2]− (2) and [I2∙pyz…Hpyz∙I2]Cl (3) exhibit distinct coordination bonding patterns. The structural motif of 2 features infinite […pyz…H-pyz…]+ hydrogen bonded cationic chains and distinct [ICl2]− counterions, which interact with cationic chain not only through electrostatic anion–cation attraction but also via weak Csingle bondH⋯Cl hydrogen bonds.
In very peculiar structure of 3, the iodine molecules act as bidentate Lewis acid, bridging the hydrogen-bonded cations [pyz…Hpyz]+ and the chloride anions in the infinite, almost linear chains (…I2…pyz…Hpyz+…I2…Cl−…I2∙pyz…Hpyz+…I2…Cl−…)∞. Quantum chemical computations allowed to rationalize reactivity and establish a chemical bonding in the compounds. The dominant binding motif is governed by hydrogen bonds in [pyz…Hpyz]+ fragments and donor-acceptor halogen bonds Cl− → I–I ← N, where iodine molecules serve as bidentate Lewis acids. Unusual linear arrangement of two Lewis acids I2∙pyz and I2∙Hpyz+ around Cl− anion in 3 is facilitated due to packing effects forced by π-π staking between peripheral pyrazine rings.