Kinetic and thermodynamic properties of chromium(III) complexes containing pyrazine radical ligands

Article abstract of DOI:10.1021/ic50207a009

Green complexes of 1:1 stoichiometry are formed when solutions of chromous ion and pyrazine are mixed; complexation is accompanied by proton binding at the remote nitrogen site on the ring. The association constant for monoprotonated pyrazine binding is K₁ = 1.5 × 10⁴ M^-1 at 20.6°C, μ = 0.10 M. The association - dissociation kinetics were studied by using combined stopped-flow relaxation techniques; a single relaxation curve was obtained for which τ^-1 = k_f[Cr(II)][pyrazine] + k_r; the upper limit for coordination by monoprotonated pyrazine, k₁ ≤ 3.6 × 10⁶ M^-1 s^-1 at 28°C, μ = 0.10 M, was about 10²-fold slower than anticipated for rate-limiting substitution on chromous ion. In the presence of excess pyrazine the green complex disappears in a biphasic manner; the fast bleaching reaction gave rise to a 69-line EPR signal, identified as the dihydropyrazine radical cation. The reaction was described by the equilibrium (where pyz = pyrazine) Cr(pyzH)³⁺ + pyzH⁺ ?_k-2^k2 pyzH₂⁺ + Cr^III(pyz)³⁺ for which K₂ = 1.3, k₂ ? 14 M^-1 s^-1, and k_-2 ? 10 M^-1 s^-1 at 20.6°C, μ = 0.10 M. A transitory species with spectral features nearly identical with the Cr(pyzH)³⁺ ion was formed when solutions containing chromous and pentaammine-(pyrazine)cobalt(III) ions were mixed; the colored species decayed rapidly (t_1/2 ? 100 ms). Application of temperature-jump perturbation immediately after mixing yielded a single relaxation curve which could not be fitted to a simple mechanism. Evidence favoring formulation of the green species as chromium(III)-bound ligand radicals is discussed.