Heteroleptic ruthenium complexes

During the last two to three years the advent of heteroleptic ruthenium complexes furnished with an antenna function has raised the performance of the DSSC to a new level. Two examples of these dyes are ClOl and Z991 (Figure 3.16). 

The low efficiencies could be due to lack of intimate contact (interface) between the sensitizer (which is hydrophilic) and the spirobifluorene (which is hydrophobic). Moreover, the surface charge also plays a significant role in the regeneration of the dye by the electrolyte.98 In an effort to reduce the charge of the sensitizer and improve the interfacial properties between the surface-bound sensitizer and the spirobifluorene hole-carrier, amphiphilic heteroleptic ruthenium(II) complexes ((48)-(53)) have been used as sensitizers. These complexes show excellent stability and good interfacial properties with hole-transport materials, resulting in improved efficiencies for the solar cells. 

The attachment of bipyridine moieties also enables the synthesis of homo- and heteroleptic ruthenium(II)trisbipyridyl complexes with interesting photochemical properties [56]. 

In addition to described heterogeneous systems a homogeneous photocatalytic C02 reduction was tested using heteroleptic rhenium complexes [99, 100] and supramolecular ruthenium and rhenium bi- and tetranuclear complexes their excited states were quenched by 1-benzyl-1,4-dihydronicotinamide (BNAH) and C02 was reduced by the electron donor intermediate species [101]. 

K. Maruszewki, D.P. Strommen, K. Handrich, and J.R. Kincaid, Synthesis and Spectroscopic Properties of Zeolite-entrapped Bis-heteroleptic Ruthenium(II) Polypyridine Complexes. Inorg. Chem., 1991, 30, 4579 4582. 

Along with the distances associated with proton transfer, the distance-dependence of electron transfer on PCET reactions of phenols has also been of considerable interest. To study the effect of distance on the rates of phenol oxidation, Wenger and coworkers synthesized a series of heteroleptic ruthenium polypyridine complexes where the distance from the metal to the distal phenol was varied as a function of the... 

Scheme 3 shows the details of the synthetic strategy adopted for the preparation of heteroleptic cis- and trans-complexes. Reaction of dichloro(p-cymene)ruthenium(II) dimer in ethanol solution at reflux temperature with 4,4,-dicarboxy-2.2 -bipyridine (L) resulted the pure mononuclear complex [Ru(cymene)ClL]Cl. In this step, the coordination of substituted bipyridine ligand to the ruthenium center takes place with cleavage of the doubly chloride-bridged structure of the dimeric starting material. The presence of three pyridine proton environments in the NMR spectrum is consistent with the symmetry seen in the solid-state crystal structure (Figure 24). 

A dendrimer with metal complexes both in the core and in the branches was described by Balzani et al. The luminescent, heteroleptic (having different ligands), dendritic polypyridine-ruthenium or polypyridine-osmium complex can be prepared both divergently and convergently [130] (cf. Section 2.5.2). 

Synthetic routes to homoleptic and heteroleptic tris(diimine)ruthenium(II) complexes incorporating bidentate imine ligands and used, particularly, as photosensitizers in transformations of solar energy to chemical or electric energy 04CCR(248)1329. 

Keywords Dyes / Solar cells / Ruthenium / Heteroleptic complexes / Charge recombination / Electron transfer... 

In this communication we would like to report the synthesis and characterization of a new heteroleptic rutheni-um(II) complex with one of the bipyridine ligands replaced by a more conjugated ligand such as the 5,6-dimethyl-l,10-phenanthroline. Scheme 1 illustrates the molecular structure of the ruthenium(II) heteroleptic complex. Moreover, we have also carried out a study of the interfacial charge-transfer kinetics of the molecule when anchored onto the surface of nanocrystalline Ti02 semiconductor particles. 

Figure 7 Transient resonance Raman spectra of homoleptic and heteroleptic complexes of ruthenium(II) with bipyridine and bip5razine (adapted from ref. 9).

Two tris(heteroleptic) Ru(II) complexes bearing styryl subunits, (4,4 -dimethyl-2,2 -bipyridine)(4,4 -di-tert-butyl-2,2 -bipyridine)(4,4 -bis[ -(p-methylcarboxy-styryl)]-2,2 -bipyridine)mthenium(II) hexafluorophosphate, [Ru(dmbpy)(dtbbpy)(p-COOMe-styryl-bpy)](PF6)2 and (4,4 -dimethyl-2,2 -bipyridine)(4,4 -di- nonyl-2,2 -bipyridine)(4,4 -bis[ -(p-methylcar-boxy-styryl)]-2,2 -bipyridine) ruthenium(II) hexafluorophosphate, [Ru(dmbpy)(dnbpy)(p-COOMe-styryl-bpy)](PF6)2 have been synthesized by Myahkostupov and Castellano, and characterized by the use of NMR /hh couplings including those across four bonds have been determined for the complexes and compared with those measured for the free ligands. 

For this triad, only iron (Table 4.17) gives homoleptic complexes containing (tM — C bonds. The remaining elements form heteroleptic compounds. The stability of phosphine complexes of the type M(aryl)2 (PR3)2 decreases considerably according to the series Ni>Co>Fe. Ruthenium and osmium complexes characteristically activate the C —H bond in coordinated ligands and form compounds with (tM—C bonds as a result of oxidative addition. The following reactions serve as examples ... 

---