Half-sandwich ruthenium -arene complexes

Fig. 12. (a) General structure of the half-sandwich, piano-stool ruthenium—arene complexes (b) X and Y are commonly occupied by a bidentate ligand L giving a monofunctional complex (c) tethering of a monodentate ligand to the arene results in a bifunctional complex. 

Fernandez R, Melchart M, Habtemariam A, Parsons S, Sadler PJ (2004) Use of chelating ligands to tune the reactive site of half-sandwich ruthenium(II)-arene anticancer complexes. Chem Eur J 10 5173-5179... 

Bis(arene) complexes of ruthenium (99), first prepared in relatively low yield by the Fischer-Hafiier synthesis (see Fischer-Hafner Synthesis) (MCb/AlCb/arene) are now readily available through solvento cations [(arene)RuS] + (S = acetone, nitromethane), which in turn are made from the dimeric half-sandwich halides [(arene)RuCl2]2 (Section 6.3) with Ag+ with the appropriate arene as solvent (Scheme 23). This method allows a whole range of functionalities to be incorporated to at least one of the arenes. The dications (99) + are of relatively high solvolytic stability and are decomposed by DMSO only to form [(arene)Ru(DMSO)3] +. 

We reported the first crystal structure of a half-sandwich arene ruthenium(II)-enzyme complex (Fig. 21) (90). The crystal structure showed a Ru(r 6-p-cym)X2 half-sandwich fragment... 

It is clear from the preceding section that the field of tethered arene-metal complexes is dominated by ruthenium and by arene-phosphines as ligands. In part, this situation has arisen because of the current surge of interest in the catalytic properties of ruthenium complexes in organic synthesis.85,86 Moreover, the tethered arene complexes are usually air-stable, crystalline solids with a well-defined, half-sandwich molecular geometry that, in principle, can lock the configuration at the metal centre. These compounds should, therefore, be ideal both for the study of the stereospecificity of reactions at the metal centre and for stereospecific catalysis. 

Clarke has proposed that the activity of Ru(III) complexes, which are usually relatively inert towards ligand substitution, is dependent on in vivo reduction to more labile Ru(II) complexes [19, 24]. With this in mind we have explored the activity of Ru(II) complexes. We discovered that Ru(II) aminophosphine complexes were cytotoxic to cancer cells [25], but they had poor aqueous solubility and were difficult to isolate and purify in large quantities. Since arenes are known to stabilize ruthenium in its +2 oxidation state, we have investigated the potential of Ru(II) arene complexes as anticancer agents and their associated aqueous chemistry. We have found that half-sandwich Ru(II) mono-arene complexes often possess good aqueous solubility (an advantage for clinical use) and that the Ru(II)-arene bonds are very stable. 

Very recently, Sadler et al. have prepared a half-sandwich arene ruthenium(ll) derivative of crystallized lysozyme by slow diffusion of the complex [(ri -p-cymene)-RuCl2(H20)] in tetragonal lysozyme crystals (80). High resolution X-ray crystallography of the modified crystals unexpectedly revealed that a single arene ruthenium entity happened to be coordinated to the enzyme by interaction with the imidazole ring borne by the H1S15 residue. This is the first example of this kind of protein structure reported in the literature. 

The first examples for photocatalyzed ROMP (PROMP) using ruthenium initiators were introduced by Miihlebach et al. [33] in 1995. The catalysts systems were based on three different types r 6-arene half-sandwich complexes 51, sandwich... 

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