Ruthenium tris(bipyridine) chloride

Ruthenium(II) tris(bipyridine) dichloride is the chemical compound with the formula [Ru(bipy)₃]Cl₂. This salt is obtained as the hexahydrate, although all of the properties of interest are in the cation [Ru(bipy)₃]²⁺, which has received much attention because of its distinctive optical properties. The chlorides can be replaced with other anions, such as PF₆⁻.

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Synthesis and structure

This orange crystalline salt is prepared by treating aqueous solution of ruthenium trichloride with 2,2'-bipyridine. In this conversion, Ru(III) is reduced to Ru(II), and hypophosphorous acid is typically added as a reducing agent.^[1]. [Ru(bipy)₃]²⁺ is an octahedral coordination complex. The complex is chiral, with D₃ symmetry. It has been resolved into its enantiomers, which are kinetically stable.

Photochemistry of [Ru(bipy)₃]²⁺

Distinctively, [Ru(bipy)₃]²⁺ absorbs UV light and visible light to give an excited state with a comparatively long lifetime of 600 nanoseconds. The excited state relaxes to the ground state by emission of a photon at the wavelength of 600 nm. The long lifetime of the excited state is attributed to the fact that it is triplet, whereas the ground state is a singlet state and in part due to the fact that the structure of the molecule allows for charge separation. Singlet-triplet transitions are often slow.

The triplet excited state has both oxidizing and reducing properties. This unusual situation arises because the excited state can be described as an Ru³⁺ complex containing a bipy^- ligand. Thus, the photochemical properties of [Ru(bipy)₃]²⁺ are reminiscent of the photosynthetic assembly, which also involves separation of an electron and a hole.^[2]

[Ru(bipy)₃]²⁺ has been examined as a photosensitiser for both the oxidation and reduction of water. Upon absorbing a photon, [Ru(bipy)₃]²⁺ converts to the aforementioned triplet state, denoted [Ru(bipy)₃]²⁺*. This species transfers an electron, located on one bipy ligand, to a sacrificial oxidant such as [persulfate|S₂O₈]^2-. The resulting [Ru(bipy)₃]³⁺ is a powerful oxidant and oxidizes water into O₂ and protons via a metal oxide catalyst.^[3] Alternatively, the reducing power of [Ru(bipy)₃]²⁺* can harnessed to reduce methylviologen, a recyclable carrier of electrons, which in turn reduces protons at a platinum catalyst. For this process to be catalytic, a sacrificial reductant, such as EDTA^4- or triethanolamine is provided to return the Ru(III) back to Ru(II).

Derivatives

The family of derivatives of [Ru(bipy)₃]²⁺ is enormous. These complexes are widely used in biodiagnostics, photovoltaics and organic light-emitting diode.

Safety

Metal bipyridine as well as related phenanthroline complexes are generally bioactive.

References

1. ^ Broomhead, J. A.; Young, C. G. “Tris(2,2’-bipyridine)Ruthenium(II) Dichloride Hexahydrate” Inorganic Syntheses, 1990, volume 28, ISBN 0-471-52619-3
2. ^ A. J. Bard and M. A. Fox (1995). "Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen". Acc. Chem. Res. 28 (3): 141-145. doi:10.1021/ar00051a007.
3. ^ M. Hara, C. C. Waraksa, J. T. Lean, B. A. Lewis and T. E. Mallouk (2000). "Photocatalytic Water Oxidation in a Buffered Tris(2,2'-bipyridyl)ruthenium Complex-Colloidal IrO2 System". J. Phys. Chem. A 104 (22): 5275-5280. doi:10.1021/jp000321x.