# Tris(bipyridine)ruthenium(II) chloride

Tris(bipyridine)ruthenium(II) chloride is the chloride salt coordination complex with the formula [Ru(bpy)$_3$]Cl$_2$. This polypyridine complex is a red crystalline salt obtained as the hexahydrate, although all of the properties of interest are in the cation [Ru(bpy)$_3$]$^{2+}$, which has received much attention because of its distinctive optical properties. The chlorides can be replaced with other anions, such as PF$_6$$^−$.

## Synthesis and structure

This salt is prepared by treating an 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. [Ru(bpy)$_3$]$^{2+}$ is octahedral, containing a central low spin d$^6$ Ru(II) ion and three bidentate bpy ligands. The Ru-N distances are 2.053(2), shorter than the Ru-N distances for [Ru(bpy)$_3$]$^{3+}$. The complex is chiral, with D$_3$ symmetry. It has been resolved into its enantiomers. In its lowest lying triplet excited state the molecule is thought to attain lower C$_2$ symmetry, as the excited electron is localized primarily on a single bipyridyl ligand.

## Photochemistry of [Ru(bpy)$_3$]$^{2+}$

[Ru(bpy)$_3$]$^{2+}$ absorbs ultraviolet and visible light. Aqueous solutions of [Ru(bpy)$_3$]Cl$_2$ are orange due to a strong MLCT absorption at 452 ± 3 nm (extinction coefficient of 14,600 M$^{−1}$cm$^{−1}$). Further absorption bands are found at 285 nm corresponding to ligand centered π$^*$← π transitions and a weak transition around 350 nm (d-d transition). Light absorption results in formation of an excited state have a relatively long lifetime of 890 ns in acetonitrile and 650 ns in water. The excited state relaxes to the ground state by emission of a photon or non-radiative relaxation. The quantum yield is 2.8% in air-saturated water at 298 K and the emission maximum wavelength is 620 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 forbidden and therefore often slow.
Like all molecular excited states, the triplet excited state of [Ru(bpy)$_3$]$^{2+}$ has both stronger oxidizing and reducing properties than its ground state. This situation arises because the excited state can be described as an Ru$^{3+}$ complex containing a bpy$^{•−}$ radical anion as a ligand. Thus, the photochemical properties of [Ru(bpy)$_3$]$^{2+}$ are reminiscent of the photosynthetic assembly, which also involves separation of an electron and a hole.
[Ru(bpy)$_3$]$^{2+}$ has been examined as a photosensitizer for both the oxidation and reduction of water. Upon absorbing a photon, [Ru(bpy)$_3$]$^{2+}$ converts to the aforementioned triplet state, denoted [Ru(bpy)$_3$]$^{2+}$*. This species transfers an electron, located on one bpy ligand, to a sacrificial oxidant such as peroxodisulfate (S$_2$O$_8$$^{2−}$). The resulting [Ru(bpy)$_3$]$^{3+}$ is a powerful oxidant and oxidizes water into O$_2$ and protons via a catalyst. Alternatively, the reducing power of [Ru(bpy)$_3$]$^{2+}$* can be 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 of [Ru(bpy)$_3$]$^{2+}$ are numerous. Such complexes are widely discussed for applications in biodiagnostics, photovoltaics and organic light-emitting diode, but no derivative has been commercialized. Application of [Ru(bpy)$_3$]$^{2+}$ and its derivatives to fabrication of optical chemical sensors is arguably one of the most successful areas so far.

## [Ru(bpy)$_3$]$^{2+}$ and photoredox catalysis

Photoredox catalysis exploits [Ru(bpy)$_3$]$^{2+}$ as a sensitizer as a strategy for organic synthesis. Many analogues of [Ru(bpy)$_3$]$^{2+}$ are employed as well. These transformations exploit the redox properties of [Ru(bpy)$_3$]$^{2+}$* and its reductively quenched derivative [Ru(bpy)$_3$]$^+$.

## Safety

Metal bipyridine as well as related phenanthroline complexes are generally bioactive, as they can act as intercalating agents.