Sticking and patching: tuning and anchoring cyclometallated ruthenium(ii) complexes

Abstract

A series of [Ru(bpy)₂(C^N)][PF₆] (HC^N = 2-phenylpyridine derivative) complexes functionalized in the cyclometallating C^N phenyl ring with F, Me, OMe, CO₂Me, S^tBu, SO₂Me (ligands H1–H6) or in the C^N pyridine ring with 4-CO₂Me or 4-C₆H₄P(O)(OEt)₂ substituents (ligands H7 or H9) have been prepared and characterized; representative crystal structures confirm the structural features of the complexes. When the C^N ligand contains a CO₂H substituent (ligand H₂8), deprotonation in addition to cyclometallation occurs to give a neutral, zwitter-ionic complex [Ru(bpy)₂(8)]. The synthesis of the cationic complexes requires addition of a silver(I) salt (AgPF₆ or AgBF₄) to abstract Cl⁻ from cis-[Ru(bpy)₂Cl₂] and ¹H NMR spectroscopic data are consistent with interactions between Ag⁺ and the coordinated C^N ligand in [Ru(bpy)₂(C^N)]⁺. The absorption spectra of [Ru(bpy)₂(C^N)][PF₆] (C^N = 1–6) are similar, but the introduction of the anchoring domains in [Ru(bpy)₂(C^N)][PF₆] with C^N = 7 or 9 enhances the absorption response; the greatest influence is observed in [Ru(bpy)₂(9)]⁺ with the introduction of the 4-C₆H₄P(O)(OEt)₂ substituent. Trends in emission and electrochemical behaviours of the complexes are interpreted in terms of the influence of the electronic properties of the C^N ligand substituents on the energies of the HOMO which is localized on the C^N ligand and Ru centre. This study provides an optimized synthetic route to the phosphonate ester derivative [Ru(bpy)₂(9)][PF₆], designed to anchor to a semiconductor surface; this complex also exhibits the most favourable absorption properties among the complexes studied.