Charge-transfer adducts vs iodine(I) complexes: dual role of halogen bonding in reactions of diiodine with N-donor bases.

Abstract

The interaction of diiodine with quinuclidine (QN) and 4-dimethylaminopyridine (DMAP) in solutions with 1:1 molar ratio of reactants at room temperature produced (in essentially quantitative yields) pure charge-transfer QN·I2 adducts and iodine(I) salt [DMAP-I-DMAP]I3, respectively. In comparison, the quantitative formation of pure iodine (I) salt [QN-I-QN]I5 was observed for the room-temperature reactions of QN with a 50% excess of I2, and the charge-transfer adducts of I2 with DMAP (and other pyridines) were formed when reactions were carried out at low temperatures. Computational analysis related the switch from the formation of charge-transfer adducts to iodine(I) complexes in these systems to the strength of the halogen bonding of diiodine to the N-donor bases. It shows that while the halogen bonded adducts represent critical intermediates in the formation of iodine(I) complexes, exceedingly strong halogen bonding between diiodine and the base prevent any subsequent transformations. In other words, while halogen bonding usually facilitates electron and halogen transfer, the halogen-bonded complexes may serve as "black holes" hindering any follow-up processes if this intermolecular interaction is too strong.