Preorganization enhanced coordination

Enhanced coordination by preorganization of binding sites on NPs can also be used to enable guest binding by ligands that are unable to attract the target species under certain conditions in their free molecular form in solution. As Hsu and coworkers have shown, introdnction... 

II. ENHANCED COORDINATION BY PREORGANIZATION. SURFACE CHELATE EFFECT AND SIGNALING... 

CyDs were used as scaffolds for making metallocavitands [7]. Two phosphine units can be highly preorganized on the CyD rim and a metal center can be rigidly held above the entrance of the opened CyD cavity [189-193]. These metallocavitands are expected to lead to the development of new metal catalysts with enhanced selectiv-ities and activities (CyD catalysis is presented in Chapter 4), in particular because of the protection against undesired side reactions provided by substrate inclusion into the CyD cavity and by stabilization of unusual coordination modes. 

Despite the vast number of outstanding examples of enzymatic catalysis that rely on the collaboration of two or more vicinal metal centers hitherto disclosed, the design and development of efficient binuclear organometallic complexes able to enhance the performance of mononuclear catalyst by means of an intermetallic cooperative process remains widely unexplored [22-24]. In fact, the formation of bi- or polynuclear complexes has been often described as a catalyst deactivation pathway [25-30]. However, the availability of more electron density at the active site, extra coordination positions, and the possibility to develop more preorganized systems that allow for (enantio)selective reactions shows great promise for an improved catalytic performance [9]. 

For alkali metal cation transport across bulk chloroform membranes by lariat ether amides based upon dibenzo-16-crown-5 rings, preorganization of the coordinating side arm over the polyether cavity enhances both the efficiency and selectivity of Na transport. For the corresponding lariat ether thioamides, efficient transport of Ag", but not the alkali metal cations, is observed. In this case, preorganization of the side arm is detrimental which indicates predominant coordination of the soft Ag" with the soft sulfur atom of the thioamide group. 

This thermodynamic effect of the enhancement of classical recognition features is basically due to entropic factors related to the preorganization of the coordination sites on the surface that reduce the conformational flexibility of the receptors and increase their effective concentration at the surface. This results in a remarkable improvement of the recognition characteristics. From a molecular viewpoint, surface functionalization creates a multidentate coordination environment that displays a statistical advantage in relation to coordination when compared to the molecular receptor. This effect has also been called the surface chelate effect and is not a unique feature of surfaces functionalized with binding sites. It has also been reported for coordinating dendrimers that show a positive dendritic effect, which is related to... 

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