Characterization of novel redox-active triruthenium(III) complexes with a disulfidealkylligand. Anion-dependent redox behaviour of monolayer assemblies on gold

Abstract

New oxo-centred acetate-bridged triruthenium(III) complexes with a long alkyl-chain disulfide ligand, (NC₅H₄)CH₂NHC(O)(CH₂)_nSS(CH₂)_nC(O)NHCH₂(C₅H₄N) (C₅H₄N represents a 4-pyridyl residue, n = 10; abbreviated as C10PY), have been prepared as a redox-active metal cluster adsorbate on gold electrodes: [Ru₃(O)(CH₃CO₂)₆(mpy)₂(C10PY)]ClO₄5a (mpy = 4-methylpyridine), [Ru₃(O)(CH₃CO₂)₆(mpy)₂(C10PY)]CF₃SO₃5b. The preparation, spectroscopic and electrochemical properties of 5a,5b are presented and are compared with those of triruthenium(III) analogues of a shorter alkyl-chain disulfide ligand (n = 2; abbreviated as C2PY), e.g. [Ru₃(O)(CH₃CO₂)₆(mpy)₂(C2PY)]ClO₄3a and [Ru₃(O)(CH₃CO₂)₆(MeIm)₂(C2PY)]ClO₄4a (MeIm = 1-methylimidazole). Characterization of all the compounds was accomplished using ¹H and ¹H–¹H COSY NMR, UV-vis and infrared spectroscopy, elemental analysis, fast-atom bombardment (FAB) mass spectrometry and cyclic voltammetry. Compounds 3a, 4a and 5a,5b in 0.1 M [n-Bu₄N]PF₆–CH₃CN exhibit three {Ru₃(O)} cluster core-based one-electron redox waves which are all reversible in the applied potential range between +1.2 and –2.0 V vs. Ag–AgCl. The redox potentials are dependent on the basicity of the terminal ligands (mpy and MeIm), but are insensitive to the length of the methylene chains of the disulfide ligand (C10PY and C2PY). Self-assembly of 5b on a gold electrode results in the formation of electrochemically stable monolayers which exhibit one-electron redox wave corresponding to the Ru₃ redox process III,III,III–II,III,III in aqueous media containing 0.1 M supporting electrolytes. The redox potentials and the shape of the voltammetric waves of the surface-attached triruthenium complexes are found to be largely dependent upon the nature of anions used (ClO₄⁻, CF₃SO₃⁻, NO₃⁻, PF₆⁻ and SO₄²⁻) and also upon the basicity of terminal ligands on the {Ru₃(O)} cluster moiety.