Ligand Substituent Effects on Ruthenium (III/II) Redox Properties: An Advanced Inorganic Laboratory Experiment

Abstract

A convenient inorganic laboratory exercise is presented that exposes the student to the influences of ligand substituents on a metal center’s redox properties. Students prepare a series of compounds with the general formula [Ru(bpy)2(R1,R2mal)](PF6), where bpy is 2,2′-bipyridine and R1,R2mal is an anionic β-diketone ligand with various R1 and R2 substituents. Each complex is prepared in a single synthetic step, and the Ru(III)/Ru(II) couple is characterized by cyclic voltammetry. The potential of the Ru(III)/Ru(II) couple shifts upon varying R1 and R2 from 0.64 to 1.10 volts vs. SSCE in the following order: (R1 = R2 = Me) < (R1 = R2 = Ph) < (R1 = Me, R2 = CF3) < (R1 = R2 = CF3). The inductive effects of the R substituents on the Ru(III)/Ru(II) couple are consistent with their electron-donating and electron-withdrawing properties. Hammett constants for the substituents provide an approximate measure of these effects. A linear relationship is observed when the Hammett constants are plotted against the Ru(III)/Ru(II) potential. This laboratory exercise applies a versatile electroanalytical method, cyclic voltammetry, to measure the substituent effects on a metal center’s electron density. Student results of the syntheses and redox characterizations of a series of [Ru(bpy)2(R1,R2mal)]+ complexes are presented and discussed.