Keto-Enolic Equilibria of an Isatin-Schiff Base Copper(II) Complex and its Reactivity toward Carbohydrate Oxidation

An interesting isatin-Schiff base copper(II) complex, [Cu(isapn)](ClO4)2 where isapn= N,N′-[bis-(3,3′-indolin-2-one)]-1,3-diiminepropane, was prepared and characterized by different techniques, both in the solid state and in solution, and its reactivity toward carbohydrate oxidation was verified. The positive ion electrospray mass spectrum detects the complex as an isotopologue cluster of singly charged intact isatin-copper(I) ions of m/z 395 (for 65Cu) with an isotopic pattern identical to that calculated for C19H16CuN4O2+. Tandem mass spectrometry reveals an interesting and structurally diagnostic collision-induced dissociation behavior for this ionized complex, which is dominated by the cleavage of the N—(CH2)3—N propylene bridge. In aqueous solution, this complex undergoes a peculiar keto-enolic equilibrium, verified at different pH's by spectroscopic methods (u.v.–vis. and e.p.r.), with a corresponding pKa determined as 9.5. The e.p.r. parameter ratio g‖/A‖ for this complex, in frozen MeOH/H2O (4:1, v/v) solution at 77 K, changes from 188 cm in acidic medium (pH 2.5–3.0) to 118 cm in basic medium (pH 11), indicating a significant structural change from a distorted tetrahedral to a more tetragonal geometry around the copper ion. This compound was shown to catalyze the oxidation of hexoses (glucose, fructose and galactose), in alkaline media, via reactive oxygen species, which were detected by using specific enzymes, and by e.p.r. spin trapping. The reaction was monitored at (25.0 ± 0.1)°C by the consumption of oxygen, and showed first-order dependence on catalyst, followed by a saturation effect. First-order kinetics with respect to [OH−] concentration was also observed, indicating that enolization of the substrate as well as the metal-catalyzed enediol oxidation are the rate-determining steps.