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-->sigma charge-transfer complexes w ith iodine (I-2), in which the N ... I distance [2.532(3) Angstrom for dpq . I-2; 2.543(9) and 2.555(9) Angstrom for 4-CNpy . I-2; 2.406(7) Angstrom for 4,4'-bpy . 2I(2); 2.92(1) and 2.95(1) Angstrom for quinox . I-2] is shorter than the sum of the van der Waals radii for nitroge n (1.55 Angstrom) and iodine (1.98 Angstrom). Donation of electron den sity into the antibonding orbital of iodine weakens the I-I bond resul ting in elongation relative to the value observed in elemental iodine (2.715 Angstrom). Dpq, 4-CNpy and 4,4'-bpy form molecular adducts, whi le quinoxaline forms a polymeric species in which there are interactio ns at both ends of the I-2 molecule. The type of complex which forms d epends on the nucleophilic character of the donor (and its correspondi ng effect of the I-2 molecule) and on the lattice energy of the comple x. The strength of the N ... I interaction in each of the reported com plexes has been investigated by X-ray crystallographic analysis and vi brational spectroscopy (far-IR). All of the complexes undergo thermal decomposition involving loss of I-2, and their lattice energy, as a fu nction of thermal stability, has been explored.