Reaction of aquachromium(II) ions with 2,2′-bipyrimidine. A pulse radiolysis study
Abstract
Chromium(II) ion was generated from Cr(NH3)63+ and hydrated electrons in the presence of 2,2′-bipyrimidine (bpm). The reaction of Cr(NH3)63+ with e–(aq) has k = 4.9 × 1010 dm3 mol–1 s–1 at 25 °C. The Cr(NH3)62+ so produced loses all the ammonia ligands with k > 105 s–1. A product complex with an absorption maximum at around 350 nm was formed in the reaction of chromium(II) ion and bpm. In the presence of a moderate excess of bpm an equilibrium is established on the 0.1 ms timescale with Kapp = kf [bpm]/kb. Both the forward (kf) and back (kb) reactions are pH dependent in the range 3.5 to 8.6. At pH 3.5 and 8.6, kf has the values of 1.6 × 108 and 4.4 × 107 dm3 mol–1 s–1 and kb of 4.3 × 104 and 3.2 × 103 s–1, respectively. The UV spectra of the reaction product(s) indicate that the absorbing product contains chromium(III) and bipyrimidine radical ligand. The first acid dissociation constant of chromium(II) ion was determined as 1.4 × 10–5 mol dm–3. At pH ⩽ 3.5, where all chromium species are predominantly in the aqua form, the proposed reaction scheme is (H2O)6Cr2+ + bpm k1⇄k–1 (H2O)5CrII(bpm)2+ k2⇄k–2 (H2O)4CrII(bpm)2+k3⇄k–3 (H2O)4CrIII(bpm˙–)2+ where [(H2O)5CrII(bpm)]2+ and [(H2O)4CrII(bpm)]2+ represent steady-state intermediates present in stoichiometrically negligible concentrations. On the 0.1 ms timescale, [(H2O)4CrIII(bpm˙–)]2+ is stable but undergoes changes at longer times. The protonated bpm molecule, H+bpm, is reduced by 1-hydroxy-1-methylethyl radicals, k = 5 × 109 dm3 mol–1 s–1, yielding a diprotonated bpm radical, H2bpm˙+. No reaction was observed between Cr(H2O)63+ and H2bpm˙+ in pulse radiolysis experiments, which places the rate constant for this reaction at <2 × 106 dm3 mol–1 s–1.