Multidentate mode

Indeed, these reactions proceed at 25 °C in ethanol-aqueous media in the absence of transition metal catalysts. The ease with which P-H bonds in primary phosphines can be converted to P-C bonds, as shown in Schemes 9 and 10, demonstrates the importance of primary phosphines in the design and development of novel organophosphorus compounds. In particular, functionalized hydroxymethyl phosphines have become ubiquitous in the development of water-soluble transition metal/organometallic compounds for potential applications in biphasic aqueous-organic catalysis and also in transition metal based pharmaceutical development [53-62]. Extensive investigations on the coordination chemistry of hydroxymethyl phosphines have demonstrated unique stereospe-cific and kinetic propensity of this class of water-soluble phosphines [53-62]. Representative examples outlined in Fig. 4, depict bidentate and multidentate coordination modes and the unique kinetic propensity to stabilize various oxidation states of metal centers, such as Re( V), Rh(III), Pt(II) and Au(I), in aqueous media [53 - 62]. Therefore, the importance of functionalized primary phosphines in the development of multidentate water-soluble phosphines cannot be overemphasized. 

The formation of discrete frameworks is also possible via the chelation of multidentate multidentate ligands.An example of this coordination mode is observed in the enamide [Na N(H)(=CH2)CPh (pmdeta)]2, which has a planar (NaNNaN) core containing five-coordinate sodium ions with average distances of 2.41 and 2.55 A iorendo- and cxo-cyclic [Na(NPrty)-... 

Aldridge, S., Bresner, C., Fallis, I. A., Coles, S. J., Hursthouse, M. B., Multidentate Lewis acids synthesis, structure and mode of action of a redox-based fluoride ion sensor. Chem. Commun. 2002, 740-741. 

Dendrimer-type ligand (32) serves as a lanthanide container to exhibit on-off switchable luminescence upon lanthanide complexation in response to external anions [56]. Because of the presence of two classes of coordination sites for the lanthanide cations at the inner and outer spheres, the dendrimer 32 exhibits two different binding modes to afford on-off lanthanide luminescence, in which outer complexation at the tetradentate tripod site offers the on luminescence state upon quinoline excitation whereas, inner complexation at the multidentate core site corresponds to the off luminescence state. Upon complexation of 32 with Yb(CF3 SO3 )3, the quite weak NIR luminescence from the Yb(III) center suggests that the Yb(III) ion is most probably located at the inner coordination sites and apart from the excited quinoline moieties. Nevertheless, addition of SCN anion to the 32-Yb(CF3803)3 system induced remarkable spectral changes around the quinoline absorption band and about ninefold enhancement in luminescence intensity at around 980 nm. As the intense Yb luminescence appeared upon quinoline excitation, the employed SCN anion promoted the tripod-Yb +... 

The chelate effect is distinct from multivalent binding modes that require receptor clustering [59]. Since the cell membrane is fluid, receptors are free to diffuse in two dimensions this mobility can lead to clustering of receptors. In such a system, the unliganded state of the saccharide-binding protein may not have the same distribution on the cell surface as the bound species. This clustering occurs with an entropy cost, but it does not require that the multidentate saccharide display match precisely the display of receptors on the cell surface. 

Fig. 9 Schematic views of the possible interaction modes of phosphonic acids with TCO surfaces (a) monodentate attachment (b) unidentate attachment and (c) multidentate attachment.

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