Kinetics and Mechanism of the Reduction of mer‐Tris‐picolinatoruthenium(III) by L‐Ascorbic Acid

Abstract

The kinetics of reduction of the mer-[Ru III (pic) 3 ] complex (pic -= picolinato) by ascorbic acid (AscH 2 ) leading to formation of a red ruthenium(II) species have been studied spectrophotometrically by using both conventional mixing and stopped-flow methods. The reaction was followed as a function of the reductant concentration over a wide pH range (1.0-7.4). Electron transfer proceeds by an outer-sphere mechanism involving three protolytic forms of ascorbic acid, [a]2529 AscH 2 , AscHand Asc 2-, for which specific rate constants have been determined. The Gibbs' energy of activation was found to correlate linearly with the HOMO energies of the protolytic forms of the reductant. The mer-[Ru III (pic) 3 ] complex is too sparingly soluble in water to inhibit the growth of the Escherichia coli (ATCC 8739) strain. Its cytotoxicity against non-tumorigenic cells precludes its potential use as an anticancer agent.Scheme 1. Acid/base properties of the different redox forms of lascorbic acid. [6]

Mechanistic Complications Caused by Redox Equilibration: Ascorbate Reduction of a Ruthenium(III) Complex under Low Driving Force Conditions

“…Since the reducing ability of l ‐ascorbic acid increases strongly on deprotonation (see Table ), viz. in the order AscH 2 > AscH – > Asc 2– , the reactions were studied as a function of both total AscH 2 concentration, [AscH 2 ] T , and pH. The corresponding p K a values for l ‐ascorbic acid are 4.1 and 11.3, respectively, which means that the reductant exists predominantly as the ascorbate monoanion (>97.4 %) within the studied pH range of 7.2 to 9.7.…”

“…This reaction is characterized by a high driving force (pH 5 to 6, Δ E = +0.20 to +0.23 V) such that the observed rate constant exhibited a significant acceleration with increasing pH. The reaction could be studied by conventional UV/Vis spectrophotometric detection at low pH, and with stopped‐flow techniques at higher pH, even up to the limit of such techniques at pH 7.4 The reaction showed clean pseudo‐first‐order kinetics throughout the studied pH range with no evidence for the interference of back reactions.…”

“…We have now studied the same reaction system but starting with cis ‐[Ru III Cl 2 (pic) 2 ] – that is a much weaker oxidant than mer ‐[Ru III (pic) 3 ], viz. E 1/2 for the Ru III /Ru II couple is +0.030 and +0.330 V, respectively , . Consequently, the system is characterized by a much lower driving force (pH 7 to 9, Δ E = –0.04 to +0.04 V), which allowed us to go to a much higher pH where the dianionic ascorbate ion is the main reactive species in solution, something that is usually not possible due to the very high reactivity of this anion.…”

Mechanistic information on the nitrite-controlled reduction of aquacob(III)alamin by ascorbate at physiological pH

“…Note that the abbreviation for ascorbate (Asc) refers to a mixture of the ascorbate mono-anion (HAsc − ) and ascorbate di-anion (Asc 2− ), for which the pK a2 value is 11.3. At the selected pH, the strongest reducing species is Asc 2− , it reacts five orders of magnitude faster than the HAsc − ion [17]. Changes in the UV-Vis spectrum were observed while the reaction solution changed color from red to orange.…”

Synthesis and detailed characterization of cis-dichloridobispicolinatoruthenate(III) as solid and in solution

“…We are interested in the redox behavior of Ru(III) complexes, especially their interaction with ascorbic acid/ascorbate (vitamin c), and the role of such reactions in biological systems [11]. In this study we synthesized and obtained the X-ray structures of three closely related bispicolinate ruthenium(III) salts, viz.…”