Control of Isomerization of Pyridinethiol–Ruthenium Complexes via External Stimuli and Factors Affecting Isomerization Behavior

Pyridinethiol–ruthenium complexes [Ru(bpy)2(py)(4-pySH)](PF6)2, [Ru(bpy)(tpy)(2-pySH)](PF6)2, and [Ru(bpy)(tpy)(4-pySH)](PF6)2 (4-pySH: 4-pyridinethiol, tpy: 2,2′:6′,6′′-terpyridine, and 2-pySH: 2-pyridinethiol) were synthesized and characterized by 1H NMR, elemental analysis, X-ray photoelectron spectroscopy (XPS), and electrospray ionization-mass spectrometry. Spectral changes after the addition of acid or base by titration demonstrated that the complexes were reversibly protonated/deprotonated. Crystal-structure analysis of [Ru(bpy)(tpy)(2-pySH)](PF6)2 and XPS analysis of the complexes discussed in this study suggest that all pyridinethiol ligands coordinate in a monodentate fashion to Ru only via the sulfur atom. Regardless of the protonated or deprotonated form, the 4-pyridinethiol complexes exhibited one reversible redox couple that is attributed to RuIII/II–(4-pyridinethiol-κS or 4-pyridinethiolate-κS). The protonated form of the 2-pyridinethiol complex showed one reversible redox couple that is attributed to RuIII/II–(2-pyridinethiol-κS), whereas the deprotonated form showed irreversible redox behavior, implying linkage isomerization between RuII–(2-pyridinethiolate-κS) and RuIII–(2-pyridinethiolate-κN) through the redox reaction of RuIII/II. The reversible/irreversible redox behaviors were also confirmed by spectroelectrochemistry. Effects of ancillary ligands on this linkage isomerization are also discussed in this report.