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Cyclodextrin Phosphanes as First and Second Coordination Sphere Cavitands
Author(s) -
Engeldinger Eric,
Armspach Dominique,
Matt Dominique,
Jones Peter G.
Publication year - 2003
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200304806
Subject(s) - chemistry , coordination sphere , ligand (biochemistry) , acetonitrile , crystallography , chelation , metal , cyclodextrin , stereochemistry , bite angle , medicinal chemistry , inorganic chemistry , crystal structure , denticity , organic chemistry , receptor , biochemistry
The binding properties of two α ‐cyclodextrins, each containing two C(5)‐linked “CH 2 PPh 2 ” 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 1 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 143°. Another feature of L 1 concerns its ability to function as an hemilabile ligand. Together with four methoxy groups anchored onto the primary face, the two P III centres of L 1 form a circularly arranged P 2 O 4 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 L 1 and L 2 , 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 MCl unit is maintained inside the cyclodextrin through weak Cl⋅⋅⋅H‐5 interactions. The chelate complex [Ag( L 1 )]BF 4 reacts with acetonitrile in excess to afford a mixture of two equilibrating complexes, [Ag(acetonitrile)( L 1 )]BF 4 and [Ag(acetonitrile) 2 ( L 1 )]BF 4 , 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.