Charge transfer complexes of diquat and paraquat with halide anions

Abstract

The association of methylviologen, 1,1′-dimethyl-4,4′-bipyridynium (PQ 2+ ), 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 (K 1 ,) 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 (DQ 2+ ), with first association constants 3.3 (Cl − ), 4.1 (Br − ) and 4.6 (I − ) M −1 . First and second (K 2 ) association constants were also determined by a potentiometric technique using ion selective electrodes: K 1 = 2.0 M −1 ; K 2 = 0.4 M −1 for the system PQ 2+ –Cl − and K 1 = 5.3 M −1 ; K 2 = 0.3 M −1 for DQ 2+ Br − .