Structure and bonding in molecular and ionic complexes of iodine monochloride with pyrazine

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 CH⋯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.