Electrochemistry and surface-enhanced Raman spectroscopy (SERS) of adsorbed chromium–bipyridyl complexes†

Abstract

A series of bis-bipyridyl complexes, cis-[Cr^III(bpy)₂(L)(L′)] (L = OH, L′ = OH₂ monomer; L = L = OH dimer; L = L = Cl; and L, L′ = polyacrylic acid) were studied at silver and glassy carbon electrodes by cyclic voltammetry. On reduction, slow electron transfer reactions giving adsorbed species are evident from the cyclic voltammograms. In situ surface-enhanced Raman spectroscopy (SERS) of the aquahydroxo and dimer complexes shows a broad, intense Cr–O vibration at around 550 cm^–1. This arises as the exciting line (514.5 nm) is close to resonance with the lowest energy absorption band assigned to a ligand field transition (⁴A_2g → ⁴T_2g). At –0.4 V SERS spectra show features of both the normal Raman spectrum of [Cr(bpy)₃]³⁺ and the colloidal SERS of [Cr(bpy)₃]²⁺ and can be explained by the presence of partially-reduced adsorbed species. The electron is localized first on to the metal and then on to one of the bpy ligands as follows: [Cr^III(bpy⁰)₂X₂]ⁿ⁺ + e →[Cr^II(bpy⁰)₂X₂]^(n–1)+ and [Cr^II(bpy⁰)₂X₂]^(n–1)+ + e →[Cr^II(bpy^–)(bpy⁰)₂X₂]^(n–2)+. Although the SERS evidence suggested that the integrity of the complexes is maintained during the reaction, prolonged electrolysis leads to hydrolysis and/or substitution of the complexes. Thus [(PVP)₂-Cr(bpy)₂]³⁺ can be electrosynthesized by reduction of cis-[Cr(bpy)Cl₂]⁺ at a PVP modified glassy carbon electrode and shows a characteristic peak at –0.32 V.