| Abstrakt: |
The binding properties of two α‐cyclodextrins, each containing two C(5)‐linked “CH2PPh2” units, L 1(A,D‐substituted) and L 2(A,C‐substituted), have been investigated. Both ligands readily form transition‐metal chelate complexes in which the metal centres are immobilised at the cavity entrance. Although diphosphane L 1displays a marked tendency to behave only as a trans‐spanning ligand, the ligand possesses a certain degree of flexibility, for example, allowing the stabilisation of a trigonal silver(I) complex in which the bite angle drops to 143°. Another feature of L 1concerns its ability to function as an hemilabile ligand. Together with four methoxy groups anchored onto the primary face, the two PIIIcentres of L 1form a circularly arranged P2O412‐electron donor set able to complex an Ag+ion in a dynamic way, each of the four oxygen atoms coordinating successively to the silver ion. Furthermore, the particular structures of L 1and L 2, characterised by the presence of PIIIunits lying close to the cavity entrance, lead upon complexation to complexes whereby the first coordination sphere is partly entrapped in the cyclodextrin. Thus, when treated with metal chlorides, both ligands systematically produce complexes in which the MCl unit is maintained inside the cyclodextrin through weak Cl⋅⋅⋅H‐5 interactions. The chelate complex [Ag(L 1)]BF4reacts with acetonitrile in excess to afford a mixture of two equilibrating complexes, [Ag(acetonitrile)(L 1)]BF4and [Ag(acetonitrile)2(L 1)]BF4, whose coordinated nitriles lie inside the cyclodextrin cavity. The inner‐cavity ligands can be substituted by a benzonitrile molecule. The present study provides the first identification of an [Ag(acetonitrile)2(phosphane)2]+ion. The unexpected stabilisation of this species probably rests on a cavity effect, the cyclodextrin walls favouring recombination of the complex after facile dissociation of the nitrile ligands. |
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