Charge transfer complexes of diquat and paraquat with halide anions

Abstract

The association of methylviologen, 1,1′-dimethyl-4,4′-bipyridynium (PQ2+), with halide and pseudo-halide ions was studied in water at 25 °C by ultraviolet spectrophotometry. A charge transfer band could be observed in the region around 300 nm. From difference absorption measurements the first association constant (K1,) was obtained as 1.7 (Cl−), 3.3 (Br−), 3.9 (I−), 0.7 (CN−), and 2.0(SCN−) M−1. Similar results were obtained for the dication, 6,7-dihydrodipyrido[1,2-a:2′,1′-c]pyrazinium (DQ2+), with first association constants 3.3 (Cl−), 4.1 (Br−) and 4.6 (I−) M−1. First and second (K2) association constants were also determined by a potentiometric technique using ion selective electrodes: K1 = 2.0 M−1; K2 = 0.4 M−1 for the system PQ2+–Cl− and K1 = 5.3 M−1; K2 = 0.3 M−1 for DQ2+Br−.