“Lock and Key” and “Induced-Fit” Host–Guest Models in Two Digold(I)-Based Metallotweezers

Abstract

Two different metallotweezers, each with two pyrene−imidazolylidene−gold(I) arms, were used as hosts for a series of planar aromatic guests. The metallotweezer with a dibenzoacridinebis(alkynyl) spacer (1) orients the two pyrene− imidazolylidene−gold(I) arms in a parallel disposition, with an interpanel distance of about 7 Å. The second metallotweezer (2) contains a carbazolylbis(alkynyl) spacer that directs the two pyrene panels in a diverging orientation. Determination of the association constants via 1 H NMR titrations demonstrates that the binding strength shown by 1 is significantly larger than that found by 2, with binding affinities as large as 10 4 M −1 (in CDCl 3 ), for the encapsulation of N,N′-dimethylnaphthalenetetracarboxydiimide with 1. The differences in the binding affinities are due to binding models associated with formation of the related host−guest complexes. While 1 operates via a "lock and key" model, in which the host does not suffer distortions upon formation of the inclusion complex, 2 operates via a guest-induced fit model. The large association constants shown by 1 with two planar guests were used for promotion of the template-directed synthesis of 1, which in the absence of an external template is produced in an equimolecular mixture with its self-aggregated congener, clippane [1 2 ]. This observation strongly suggests that the mechanically interlocked clippane is formed through a self-template-directed mechanism, while bonds are broken/formed during the synthetic protocol.

The New Di-Gold Metallotweezer Based on an Alkynylpyridine System

“…It is noteworthy that for the TNFLU ligand, no change in the chemical shift of its signals was observed. This result is totally different from the one observed in the other two clamps synthesized previously, where a large increase in the association constant was observed, about 9933 M −1 [42]. With regard to the encapsulation of metal complexes, some authors previously showed how metallotweezers containing cationic alkynylplatinum arms displayed large association constants with several neutral and anionic guest planar metal complexes, including [Pt(CˆNˆC)(CO)] and [Au(CˆNˆC)(C≡CC 6 H 4 -OCH 3 -p)].…”

“…In both cases the incorporation of the hydroxyl group to the periphery of the PAH guest produces increased in the association constant, very likely due to the stabilization produced by the hydrogen bonding interaction between the -OH group and the lone pair of the nitrogen at the pyridine linker. If we compare the binding affinities of the metallotweezer with those observed with the carbazole-connected metallotweezer [42], higher affinity is observed with the pyridine linker-based tweezer, 2, except that the rigid tweezer based on the dibenzoacridine-connected metallotweezer [42] has constants of association which are much higher than those observed in the pyridine linker-based tweezer, 2. and 6). In both cases the incorporation of the hydroxyl group to the periphery of the P guest produces increased in the association constant, very likely due to the stabiliza produced by the hydrogen bonding interaction between the -OH group and the lone of the nitrogen at the pyridine linker.…”