Stability

The stability of Rubpy complexes is a result of a number of factors including:

1. the large electronic stabilization found in these octahedral low spin d6 second row transition metal complexes (while the symmetry of the complexes in Fig. 7 is not Oh, to a first approximation an octahedral coordination environment adequately describes many of the electronic properties);

2. the considerable covalent character of the bond as reflected in the similar electronegativities for Ru (% 2.2) and N (% 3.0), and

3. the multiple-bond character associated with n bond formation between the full Ru t2g orbitals and the n-accepting orbitals on the polypyridyl ligands.

These factors and the associated kinetic inertness of the low spin d6 electronic configuration make these complexes exceptionally stable with respect to ligand loss or decomposition.

Polypyridyl Ligands
Fig. 7. Chemical structures of three commonly used Ru(II) polypyridyl complexes.

[Ru(bpy)3]2+ and [Ru(phen)3]2+ are modestly sensitive to photochemical degradation via photolabilization of the ligands upon excitation of the MLCT band.58,59 The 3MLCT state that is utilized in most light-to-energy conversion schemes, is populated with essentially a 100% quantum yield.60 At room temperature, however, thermal population of ruthenium-based d-d states, only ~ 43 kJ/mol higher in energy than the 3MLCT state, can occur resulting in decomposition via ligand dissociation.58,59 Quantum yields for photochemical decomposition are generally small (^ ~ 0.04) and highly dependent on the solvent conditions and temperature.58 Aqueous solutions of varying ionic strength and acidity show quantum decomposition yields between 0.3% to 0.001% at 343 K.59 Dichloro methane solutions are considerably more reactive with decomposition quantum yields between 6 and 1 % at 298 K with the nature of the counterion playing an important role as small coordinating anions, such as halides, increasing the decomposition yield.58 In dichloromethane, the increased decomposition is attributed to more significant ion pairing in this non-polar solvent, which leads to more effective trapping of the 5-coordinate intermediate, and stabilization of the product by anation. Photoracemization of optically pure A-[Ru(bpy)3]2+ in aqueous solutions was also observed with a similar activation energy to the photode-composition reaction and is presumed to occur via the same 5-coordinate interme-diate.61

Photochemical decomposition of the ruthenium polypyridyl chromophore has not been significant obstacle to their use in energy conversion schemes because in most cases the 3MLCT state is rapidly quenched and the resulting photoproduct is stable with respect to ligand dissociation. For example, addition of the oxidative quencher, methylviologen (MV2+), greatly reduces the photodecomposition of [Ru(bpy)3]2+* as the MV2+ rapidly reacts with the 3MLCT state by electron transfer yielding [Ru(bpy)3]3+ and MV+.62

[Ru(bpy)3]3+ and [Ru(bpy)3]+ are commonly produced as photoproducts in the photochemistry of [Ru(bpy)3]2+ and, as noted, are potent oxidants and reductants, respectively. Barring a redox reaction, these complexes show good stability towards both substitution and racemization,63 however Ru(III) complex is not indefinitely stable in aqueous solutions. Over time spontaneous reduction of [Ru(bpy)3]3+ to [Ru(bpy)3]2+ is observed with some sacrificial degradation of the bpy ligands

(~ 10%).64 Reductions are generally considered to be ligand-based and up to three reductions are often possible. Both the monocationic [Ru(bpy)3]+ 65 and the neutral Ru(bpy)3 66 have been isolated and structurally characterized by X-ray diffraction.

The presence O2 either from the atmosphere or as a product in water oxidation suggests its reactions with [Ru(bpy)3]2+* bear close attention. Dioxygen is a well-known quencher of [Ru(bpy)3]2+* and acts either by energy transfer or oxidative electron transfer quenching to yield singlet O2 or superoxide radical O2-, respectively.67-69 As the 3MLCT state is quickly quenched, dioxygen typically protects the complex from photodecomposition via ligand loss.58 However, when the O2 and [Ru(bpy)3]2+ are not free to diffuse apart as in viscous solution or zeolite matrices, the activated singlet O2 or other reactive oxygen species can oxidize the diimine ligands irreversibly and thus damage the complex.62,70

Elaborate structures including those with up to three different bidentate ligands71 are readily prepared due to the substitutional inertness of the Ru(II) ion. A number of synthetic procedures exist to build such complexes with most dealing with different strategies to coordinate select diimine or triimine ligands in a specific order. Some useful and often used starting complexes include Ru(bpy)2Cl2,72 {Ru(bpy)Cl3}x,73 Ru(bpy)(CO)2Cl2,71,74 and Ru(terpy)Cl3.75 For most of these syntheses, it is trivial to substitute another diimine for the bpy ligand or triimine for the terpy ligand.

The tris-diimine and bis-triimine complexes, once formed, are tolerant of any number of ligand-based functional groups and the transformations typical of these functional groups can be performed on the metal complex with virtually no effect on the structural core integrity and stereochemistry. For example, the base catalyzed cross-coupling reactions of arylhalides with Ru(II) complexes, bearing di- or triimine ligands with peripheral phenols or hydroxymethyl (benzylic alcohol) substituents, gives the corresponding ethers in high yield and no reported degradation of the Ru complex.76-79

Palladium-catalyzed cross-coupling of bromoaryl-containing complexes with ethynylaryl-containing complexes is readily accomplished without degradation or racemization.80-82 Barton and coworkers,83,84 MacDonnell and coworkers85-88 and others89,90 have shown that the condensation of Ru(II) coordinated 1,10-phenanthroline-5,6-diones with ortho-phenylenediamines occurs without degradation of the complex or affecting the ruthenium stereochemistry. Electroploymerization of Ru(II) complexes bearing vinylbpy ligands has been demonstrated by Meyers and coworkers.91 One illustrative example of the robustness of such complexes is shown in Figure 8. The coordinatively saturated Ru(II)trisphenanthroline complex can be oxidized to the tris-phenanthroline-5,6-dione complex in high yield (ca. 80 %) under extremely harsh chemical conditions.88,92 The oxidation occurs in a mixture of concentrated sulfuric acid, nitric acid and NaBr at reflux, yet not only is the basic structural core retained, but if enantiomerically pure starting material is used, e.g., A-[Ru(phen)3]2+, the quinone product is also optically pure. This reaction has been carried out on larger dendritic assemblies of [Ru(phen)3]2+ units with similar success indicating the stability of this structural core to acidic and oxidizing conditions.85

The shapes of the three complexes in Figure 7 have important consequences in their use as sensitizers in multi-component assemblies. The tris-bpy and tris-phen complexes have three-fold symmetry (D3 point group) while the bis-terpy complex

Water Oxidation Ruthenium
Fig. 8. Oxidation of [Ru(phen)3]2+ with retention of stereochemistry.

has two-fold symmetry (Dh point group). The stereochemical issues related to the tris complexes complicates their use in multi-component assemblies. As can be seen, the two tris complexes lack a convienent attachment site to build highly symmetric multi-component assemblies. Furthermore, these complexes are chiral (existing as A and A enantiomers) and the incorporation of multiple chiral centers in any supramo-lecular or covalent assembly leads to diastereomers which are very difficult to isolate pure. A number of research groups have addressed the stereochemical issues that arise when multiple ruthenium tris-diimine centers are covalently joined.81,86,93-97 The stereochemistry of the terpy complexes conversely is well-suited to appending other components along the major two-fold axis such that stereochemically pure, linear arrays are prepared in a relatively straightforward manner.98

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