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 K_app = k_f [bpm]/k_b. Both the forward (k_f) and back (k_b) reactions are pH dependent in the range 3.5 to 8.6. At pH 3.5 and 8.6, k_f has the values of 1.6 × 10⁸ and 4.4 × 10⁷ dm³ mol^–1 s^–1 and k_b 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)₅Cr^II(bpm)^2+ k₂⇄_k–2 (H₂O)₄Cr^II(bpm)^2+k₃⇄_k–3 (H₂O)₄Cr^III(bpm^˙–)²⁺ where [(H₂O)₅Cr^II(bpm)]²⁺ and [(H₂O)₄Cr^II(bpm)]²⁺ represent steady-state intermediates present in stoichiometrically negligible concentrations. On the 0.1 ms timescale, [(H₂O)₄Cr^III(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.