Ligand Substituent Effects on Ruthenium(III/II) Redox Properties

A convenient inorganic lab exercise is presented which exposes the student to the influence ligand substituents can have 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 diketone ligand with various R1 and R2 substituents. Each complex is prepared in a single synthetic step and the Ru(III/II) couple is characterized by cyclic voltammetry. The potential of the Ru(III/II) couple shifts with varying R1 and R2 across a range 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/II) couple are consistent with their electron donating/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/II) potential. This lab exercise demonstrates the influence ligand substituents can have on a metal center’s electron density and it applies a versatile electroanalytical method, cyclic voltammetry, to measure the substituent effects.