Electrochemical Studies on cis-[CrIII(bipy)2(SCN)2]I3 (Where bipy Denotes 2,2′-Bipyridine) in Acetonitrile

Abstract

The molar conductivities (Λ) of solutions of bis(2,2′-bipyridine)bis(thiocyanate)chromium(III) triiodide [CrIII(bipy)2(SCN)2]I3 (where bipy denotes 2,2′-bipyridine, C10H8N2), [ $\mathrm{A}^{+}\mathrm{I}_{3}^{-}$ ], were measured in acetonitrile (ACN) at the temperatures 294.15, 299.15, and 305.15 K. In addition, cyclic voltammograms (CVs) of [ $\mathrm{A}^{+}\mathrm{I}_{3}^{-}$ ] were recorded on platinum, gold, and glassy carbon working electrodes in ACN, using n-tetrabutylammonium hexafluorophosphate (NBu4PF6) as the supporting electrolyte, at scan rates (v) ranging from 0.05 to 0.12 V⋅s−1. Furthermore, electrochemical impedance spectroscopic (EIS) measurements were carried out in the frequency range 50 Hz<f<50 kHz using these three working electrodes. The measured molar conductivities (Λ) demonstrate that [ $\mathrm{A}^{+}\mathrm{I}_{3}^{-}$ ] behaves as uni-univalent electrolyte in ACN over the investigated temperature range. The Λ values were analyzed by means of the Lee-Wheaton conductivity equation in order to estimate the limiting molar conductivities (Λ o), as well as the thermodynamic association constants (K A), at each experimental temperature for formation of [A+ $\mathrm{I}_{3}^{-}$ ] ion-pairs. The limiting ionic conductivities ( $\lambda_{\pm}^{\mathrm{o}}$ ), the diffusion coefficients at infinite dilution (D ±), as well as the Stokes’ radii (r St) were determined for both A+ and $\mathrm{I}_{3}^{-}$ ions. The thermodynamic parameters for the ionic association process, i.e. the Gibbs energy ( $\Delta G_{\mathrm{A}}^{\mathrm{o}}$ ), enthalpy ( $\Delta H_{\mathrm{A}}^{\mathrm{o}}$ ), and entropy ( $\Delta S_{\mathrm{A}}^{\mathrm{o}}$ ), were also determined. The mobility and diffusivity of the A+ ion increase linearly with increasing temperature because the solvent medium becomes less viscous as the temperature increases. The K A values indicate that significant ion association occurs that is not influenced by temperature changes. The ion-pair formation process is exothermic ( $\Delta H_{\mathrm{A}}^{\mathrm{o}}<0$ ), leading to the generation of additional entropy ( $\Delta S_{\mathrm{A}}^{\mathrm{o}}>0$ ). As a result, the Gibbs energy $\Delta G_{\mathrm{A}}^{\mathrm{o}}$ is negative ( $\Delta G_{\mathrm{A}}^{\mathrm{o}}<0$ ) and the formation of $[\mathrm{A}^{+}\mathrm{I}_{3}^{-}]$ becomes favorable. CV studies on $[\mathrm{A}^{+}\mathrm{I}_{3}^{-}]$ solutions indicated that the redox pair Cr3+/2+ appears to be quasi-reversible on a glassy carbon electrode but is completely irreversible on platinum and gold electrodes. EIS experiments confirm that, among these three electrodes, the glassy carbon working electrode has the smallest resistance to electron transfer.