Acid dissociation reaction of palladium-4-(2-pyridylazo)-resorcinol complexes

Palladium(II) formed four kinds of stable 1:1 chelates with 4-(2-pyridylazo)-esorcinol (PAR or H₂R): below pH 4. a green chelate [Pd(H₂R)H₂O]²⁺ε= 1.75·10⁴ at 440 nm and ε=8.63·10³ at 580 nm); at pH 4-7 two kinds of red helate [Pd( HR)H₂O]⁺ (ε= 2.68·10⁴ at 515 nm) and [Pd(R)H₂0]⁰ (ε=2.89·10⁴ at 525 nm); and at pH 8-11 a red chelate [Pd(R)OH]^- (ε=3.25·10⁴ at 520 nm). The acid dissociation constants of the chelates were estimated as K_a1=[Pd(HR)H₂O⁺] [H⁺]/[Pd(H₂R)H₂O²⁺]=10^-4.35±0.05 and K_a2=[Pd(R)H₂O]· [[H⁺]/[Pd(HR)H₂O⁺]=10-^5.38±0.05 in aqueous 50% dioxane solution at 25°, μ=0.1 (NaClO₄). At pH 6-11, the red palladium(II)-PAR chelates react with tetradecyldimethylbenzylammonium chloride to form an ion-association complex which is easily extracted into chloroform and has maximal absorbance at 540 nm with a molar absorptivity of 3.29·10⁴.