Disproportionation of pentaammineruthenium(III)–nucleoside complexes leads to two-electron oxidation of nucleosides without involving oxygen molecules

摘要

Pentaammineruthenium(III) complexes of deoxyinosine (dIno) and xanthosine (Xao) ([Ru~(III)(NH_3)_5 (L)], L is dIno, Xao) in basic solution were studied by UV– vis spectroscopy, liquid chromatography/electrospray ionization mass spectrometry, and high-performance liquid chromatography. Both Ru~(III) complexes disproportionate to Ru~(II) and RuIV. Disproportionation followed the rate law d[Ru~(II)]/dt = (k_o + k_1[OH~-])[Ru~(III)]. k_o and k_1 of disproportionation at 25 °C were 2.1 (±0.1) 9 10~(-3) s~(-1) and 21.4 ± 3.2 M~(-1) s~(-1), respectively, for [Ru~(III)(NH_3)_5(dIno)], and 3.5 (±0.7) 9 10~(-4) s~(-1) and 59.7 ± 3.6 M~(-1) s~(-1), respectively, for [Ru~(III)(NH_3)_5(Xao)]. The [Ru~(III)(NH_3)_5 (Xao)] complex disproportionates at a faster rate than [Ru~(III)(NH_3)_5(dIno)] owing to the stronger electron-withdrawing effect of exocyclic oxygen in Xao. The activation parameters ΔH~? and ΔS~? for k_1 of [Ru~(III)(NH_3)_5(dIno)] were 80.2 ± 15.2 kJ mol~(-1) and 47.6 ± 9.8 J K~(-1) mol~(-1), respectively, indicating that the disproportionation of Ru~(III) to Ru~(II) and RuIV is favored owing to the positive entropy of activation. The final products of both complexes in basic solution under Ar were compared with those under O_2. Under both conditions [Ru(NH_3)_5(8-oxo-L)] was produced, but via different mechanisms. In both aerobic and anaerobic conditions, the deprotonation of highly positively polarized C8-H of Ru-L by OH~- initiates a two-electron redox reaction. For the next step, we propose a one-step two-electron redox reaction between L and Ru~(IV) under anaerobic conditions, which differentiates from Clarke’s mechanism of two consecutive one-electron redox reactions between L, Ru~(III), and O_2.