Synthesis, structure and thermal decomposition of nitrogen–iodine charge-transfer complexes

Dipyridylquinoxaline (dpq), 4-cyanopyridine (4-CNpy), 4,4′-bipyridine (4,4′-bpy) and quinoxaline form n→σ* charge-transfer complexes with iodine (I2), in which the N· · ·I distance [2.532(3) Å for dpq·I2; 2.543(9) and 2.555(9) Å for 4-CNpy·I2; 2.406(7) Å for 4,4′-bpy·2I2; 2.92(1) and 2.95(1) Å for quinox·I2] is shorter than the sum of the van der Waals radii for nitrogen (1.55 Å) and iodine (1.98 Å). Donation of electron density into the antibonding orbital of iodine weakens the I–I bond resulting in elongation relative to the value observed in elemental iodine (2.715 Å). Dpq, 4-CNpy and 4,4′-bpy form molecular adducts, while quinoxaline forms a polymeric species in which there are interactions at both ends of the I2 molecule. The type of complex which forms depends on the nucleophilic character of the donor (and its corresponding effect of the I2 molecule) and on the lattice energy of the complex. The strength of the N· · ·I interaction in each of the reported complexes has been investigated by X-ray crystallographic analysis and vibrational spectroscopy (far-IR). All of the complexes undergo thermal decomposition involving loss of I2, and their lattice energy, as a function of thermal stability, has been explored.