2.9: Ruthenium

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The chemiluminescence involving tris(2,2'-bipyridyl)ruthenium(II), [Ru(bpy)₃]²⁺, is most interesting. It involves the oxidation of [Ru(bpy)₃]²⁺ to [Ru(bpy)₃]³⁺, which is followed by reduction with an analyte species to produce an emission of light, thus:

\[[Ru(bpy)_3]^{2+} \rightarrow [Ru(bpy)_3]^{3+} + e^- \tag{Oxidation} \]

\[[Ru(bpy)_3]^{3+} + e^- \rightarrow [Ru(bpy)_3]^{2+*} \tag{Reduction by analyte} \]

\[[Ru(bpy)_3]^{2+*} \rightarrow [Ru(bpy)_3]^{2+} + \underbrace{h\nu}_{\text{620 nm}} \tag{Chemiluminescence} \]

800px-Delta-ruthenium-tris(bipyridine)-cation-3D-balls.png

Structure of [Ru(bpy)₃]²⁺: The arrangement of the three 2,2'-bipyridine ligands about the central ruthenium atom in the complex ion tris(2,2'-bipyridyl)ruthenium(II); the nitrogen atoms occupy the corners of an octahedron. (Public Domain; Benjah-bmm27)

Analytical usefulness depends on the emission of light of a measurable intensity that is clearly indicative of the analyte concentration. Chemiluminescence intensity depends on the efficiency and mechanism of the reduction step (eqn. B9.2). Common to all analytical applications of ruthenium chemiluminescence is the production of the oxidant [Ru(bipy)3]3+ (eqn. B9.1), which has been obtained by a variety of methods - chemical, photochemical and electrochemical oxidation including in situ electrogenerated chemiluminescence. Each of these generation methods has been discussed in a comprehensive review by Barnett and co-workers^[1]. Chemical generation of [Ru(bpy)₃]³⁺ has been achieved by a range of reagents such as cerium(IV) sulphate, lead dioxide and potassium permanganate.

The chemiluminescence reactions between primary, secondary or tertiary amines and [Ru(bpy)₃]²⁺are very sensitive and have been widely applied to the determination of various analytes containing an amine functionality. The chemistry of electrogenerated chemiluminescence activity of tertiary amines with [Ru(bpy)₃]²⁺ and their chemiluminescence reaction mechanism have been reviewed by Knight and Greenway^[2] and that of the chemiluminescence reaction between secondary amine and tertiary amine arising from hydrolyzed and unhydrolyzed β-lactam antibiotics, respectively, has been reported by Liang et al.. More recently, there have been several reports dealing with the detection and determination of drugs by using the [Ru(bpy)₃]²⁺/potassium permanganate system. These included tetracyclines, cinnamic acid, enalapril maleate and metoclopramide hydrochloride