Kinetics and mechanisms of the reduction of trans-dioxoruthenium (VI) by some inorganic substrates

Abstract

The kinetics of the reduction of trans-[RuVI(tmc)(O)2]²+ by Fe(II), I¯, N2H4, SO3²¯, S2O3²¯ and OH¯ have been studied in aqueous solution. The reaction with Fe²+ in acidic solution has the following stoichiometry: trans- [RuVI(tmc)(O)2]²+ + 2Fe²+ + 2H+ → trans-[RuIV(tmc)O(OH2)]²+ + 2Fe³+. The rate law is: -d[Ru(VI)]/dt = k2[Ru(VI)][Fe2+] with k2 = 27 M¯¹s¯¹ at 25.0ºC and 1.0 M ionic strength. The activation parameters are ∆H‡ = 1.1±0.3 kJ mol¯¹, ∆S‡ = -(214±20) J mol¯¹ K¯¹. A mechanism that is consistent with the rate data is as follows: [RuVI(tmc)(O2)]+ + Fe²+ + 2H+ ↔ [RuIV(tmc)(O2)]+ + Fe³+ (slow), then [Ruv(tmc)(O2)]+ + Fe²+ + 2H+ → [RuIV(tmc)O(OH2)]²+ + Fe³+ (fast). By applying the Marcus cross relation, the rate constant for the outer-sphere electron transfer between Ru(VI) and Fe²+ is calculated to be 10 M¯¹s¯¹, which is slightly lower than the experimental value. H0weve.r the low ∆H‡ and the large negative ∆S‡ is more consistent with an inner sphere mechanism. The reaction with I¯ in acidic solution has the following stoichiometry: trans-[RuVI(tmc)(O)2]²+ + 3I¯ + 2H+ → trans- [RuIV(tmc)O(OH2)]²+ + I3¯. The rate law is: -d[Ru(VI)]/dt = (ka + kb[H+])[Ru(VI)][I¯] with ka = 0.041±0.013 M¯¹ s¯¹ and kb = 18.5± 0.2 M¯²s¯² at 25.0ºC and 0.1 M ionic strength. For the ka path, ∆H‡ = 42.9∆9.4 kJ mol¯¹ and ∆S‡ = -(131±40) J mol¯¹ K¯¹, where as the kb path, ∆H‡ = 36.9∆2.9 kJ mol¯¹ and ∆S‡ = -(97±10) J mol¯¹ K¯¹. A mechanism involving oxygen atom transfer from the dioxoruthenium(VI) complex to iodide is consistent with the observed data. The kinetics of the reduction of trans-[Ru(tmc)(O)2]²+ by hydrazine was studied in solutions of pH 4 to 7. The reaction has the following stoichiometry : 2[RuVI(tmc)(O)2]²+ + N2H5+ + 3H+ → 2[RuIV(tmc)O(OH2)]²+ + N2. The rate law is: -d[Ru(VI)]/dt = k2[Ru(VI)][N2H5+], where k2 = k/(l + [H+]/K). The value of k and K at 25.0ºC and μ = 0.5 M are (3.2±0.6) x 10‾¹ M‾¹ s‾¹ and (1.9±0.06) x 10‾8 M respectively. The activation parameters for k2 at [H+] = 5.00 x 10‾5 M and μ = 0.5 M are ∆H‡ = 74.9± 5.5 kJ mol‾¹ and ∆S‡ = -(45.7±18) J mol‾¹ K‾¹. A mechanism involving hydride transfer from hydrazine to Ru(VI) is proposed. The kinetics of the reduction of trans-[Ru(tmc)(O)2]² + by sulphite has been studied in aqueous acidic solutions of pH 3.3 to 6.3. The reaction has the following stoichiometry:, [RuVI(tmc)(O)2]²+ + SO3²‾ + 2H+ → [RuIV(tmc)O(OH2)]²+ + SO4²‾. The rate law is: -d[Ru(VI)]/dt = K2[Ru(VI)][SO3²‾], where k2 = k'/(l + [H+]/K'). At 25.0ºC and μ = 1.0 M, k' = (1.5±0.4) x 10 5 M‾¹ s‾¹ and K' = (1.2±0.3) x 10‾7 M. A mechanism involving O-atom transfer is proposed. The kinetics of the reduction of trans-[Ru(tmc)(O)2]²+ by thiosulphate has been studied in the solutions of pH 4.10 to 4.85. The reaction has the following stoichiometry: [RrVI(tmc)(O)2]²+ + 2S2O3²‾ + 2H+ → [RuIV(tmc)O(OH2)]²+ + S4O6²‾. The rate law is: -d[Ru(VI)]/dt = K2[Ru(VI)][S2O3²+], where k2 = ka + kb[H+]. At 25.0ºC and μ = 1.0 M, ka = 1.0 x 10² M‾¹s‾¹ and kb = 2.4 x 10 6 M‾² s‾¹ . A mechanism involving inner-sphere one-electron transfer is proposed. The kinetics of the reduction of trans-[Ru(tmc)(O)2]²+ by hydroxide has the following stoichiometry: 4[RuVI(tmc)(O)2]²+ + 2OH‾ → 4[RuV(tmc)O2]+ + O2 + 2H+. The rate law is: -d[Ru(VI)]/dt = K2[Ru(VI)][OH‾]. A mechanism involving 0-atom transfer from Ru(VI) to OH‾ is postulated.

Date of Award	13 Oct 1995
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	T C LAU (Supervisor)