Cyclodextrin phosphanes as first and second coordination sphere cavitands

The binding properties of two a-cyclodextrins, each containing two C(5)-linked CH2PPh2 units, L1 (A,D-substituted) and L2 (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 L1 displays 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 143degrees. Another feature of L1 concerns its ability to function as an hemilabile ligand. Together with four methoxy groups anchored onto the primary face, the two Put centres of L1 form a circularly arranged P2O4 12-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 L1 and L2, characterised by the presence of P-III units 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 M-Cl unit is maintained inside the cyclodextrin through weak (ClH)-H-...-5 interactions. The chelate complex [Ag(L1)]BF4 reacts with acetonitrile in excess to afford a mixture of two equilibrating complexes, [Ag(acetonitrile)(L1)BF4 and [Ag(acetonitrile),(L1)]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.