Monodentate ligands ligand families

FIGURE 14.2 Monodentate ligand families based on BINOL. 

It is, thus, important that the ruthenium(II) complexes that are to be used as building blocks of the future machines contain sterically hindering chelates so as to force the coordination sphere of the metal to be distorted from the perfect octahedral geometry. We will discuss the photochemical reactivity of rotaxanes and catenanes of this family as well as non-interlocking systems like scorpionates since the lability of bulky monodentate ligands could also lead to useful photosubstitution reactions. 

Phosphorus is one of the best soft donor atoms for Cu thus many Cu -phosphine complexes have been reported, including ones with other hgands that by themselves would be unlikely to sufficiently stabihze Cu. By far, most of the known complexes involve monodentate phosphine ligands. As one goes down the nitrogen family, progressively fewer complexes have been characterized, such that substantially less arsine-coordinated complexes have been reported and there are only a handful of stibine species. 

A real breakthrough of the application of monodentate ligands has taken place recently. A family of BINOL-based monodentate phosphoramidites (11) is synthesized and is used in various homogeneous catal3d ic reactions. The amine moiety of the phosphoramidate was varied from 2-methyl-benzylamine, possessing another chiral center, to simple secondary amines such as piperidine and morpholine (12). 

Monodentate ligands possessing only a planar chirality are relatively rare examples in asymmetric catalysis. Two types of such planar-chiral monophosphines based on chiral T -Cr[arene]-templates, synthesized by Nelson and Hilfiker, as well as a family of stable benzoferrocenyl ligands, recently reported by Fang and co-workers, are illustrated in Figure 29.5. 

Thiolates (RS ) represent an extensive family of ligands, and include chelating forms. Thiolates are known to act as monodentate donors, but often act in a bridging role. There is a clear biological interest, through participation of thiolates (cysteine residues) as donors in many metalloproteins both as terminal S donors and bridging ligands in, for example, Fe S clusters. 

The studies summarized in this review demonstrate the broad utility of the QUINAPHOS hgand family for a variety of reactions in combination with different metals. Both monodentate and bidentate ligands derived from 1,2-dihydro-quinoline ensure high levels of enantioselectivity and/or activity, demonstrating the suitability of this chiral backbone. 

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