Spectroscopic Studies on Charge-Transfer Complexation of Iodine with Dibenzo-15-crown-5 and Benzo-12-crown-4 in Chloroform, Dichloromethane and 1,2-Dichloroethane

The formation of charge-transfer complexation between dibenzo-15-crown-5 (DB15C5) and benzo-12- crown-4 (B12C4) (Donor) and iodine is investigated spectrophotometrically in three chlorinated solvents, chloroform, dichloromethane (DCM) and 1,2-dichloroethane (DCE) solution at 25°C. The change in polarity of the solvent also doesn’t affect the stoichiometry of the complexes. Values of formation constants reflect the order of ionization potentials of the donors. The observed time dependence of the charge-transfer band and subsequent formation of I3- ion are related to the slow formation of the initially formed 1:1 Donor.I2 outer complex to an inner electron donor-acceptor (EDAr) complex, followed by fast reaction of the inner complex with iodine to form a triiodide ion, as follows:Donor I2 → Donor. I2 (outer complex), fast
Donor.I2 (outer complex) → (Donor. I)I- (inner complex), slow
(Donor . I)I- (inner complex) I2 → (Donor .I)I3-, fast
The pseudo-first-order rate constants for the transformation process were evaluated in different solvent systems. The stability constants of the resulting EDAr complexes were also evaluated and the solvent effect on their stability are discussed. The resulting complexes were isolated and characterized by FTIR and 1HNMR spectroscopy.