Chiral Half-Sandwich Arene Complexes

Planar Chirality Chirality at the metal created by the coordination of an arene ligand bearing nonequivalent substituents at ortho- or wet -positions. Hence, coordination of the arene to the metal ion through either arene face gives rise to different enantiomeric forms of the complex [18]. In particular, planar chirality is typical of ferrocene derivatives, although half-sandwich arene complexes bearing this type of chirality have also been described [19]. 

Figure 3.2 General structures of some chiral complexes (e.g., M = Pt(ll)) with ligand-organometallic therapeutic agents, (a) Metallo- centered chirality, (d) Octahedral cyclometa-cenes (e.g., M = Fe(ll)). (b) Half-sandwich arene lated complexes (e.g., M = Ir(lll)). complexes (e.g., M = Ru(ll), Os(ll)). (c) Carbene...

Dicationic tj -arene Ru half-sandwich complexes have also undergone development as Ru Lewis acids. Reaction of the readily available [Ru( -arene)Cl2]2 with bidentate ligands affords [Ru( -arene)(L-L )Cl][Cl], and halide removal with a silver salt then yields the corresponding dicationic Lewis acid. With chiral dissymmetric... 

The amino acid complexes [(q -arene)Ru(aa)Cl], where aa = N,0-chelated amino acid, undergo rapid epimerization even at temperatures below 0 °C [66,67]. Similar lability and epimerization has been found for sahcylaldiminato complexes [68]. In contrast, [(ri -p-cymene)Ru(imine)Cl] complexes are configurationally stable unless heated to high temperatures in polar solvents [69]. Configurationally-stable half sandwich Ru(If) arene complexes are ofmuch interest as potential asymmetric catalysts (e.g. for Diels-Alder and Mukaiyama reactions) [63]. The chloride ligand is usually removed in order to allow the complex to act as a Lewis add catalyst and exchange ofcoordinated water on [(r -l,3,5-trimethylbenzene)Ru(pyridyloxazoline-N,N )(H20)] is slow on the NMR timescale in acetone-water [70]. Chiral chloro arene Ru(II) catalysts for asymmetric transfer hydrogenation can be activated in situ by treatment with KOH in 2-propanol [71]. 

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