Anion complexation by neutral Lewis acidic host molecules containing tin

Abstract

A family of synthetic host molecules have been prepared, ranging in structural complexity from spiro-annelated monocycles 16 to symmetric bicycles 12 and ditopic tricycles 14. The neutral host molecules contain either two or four Lewis acidic tin atoms functionalized with sulfur or halogen substituents. The halogenated hosts 12 and 14 were restricted to binding halide ions identical to the halogen substituent due to exchange while the sulfur substituents proved more robust, and hosts 16 were treated with a variety of anions. The complexation of anions was studied in both the solution and solid state by [^119]Sn NMR spectroscopy and X-ray crystallography. Variable temperature NMR spectra for hosts 12 illustrated dynamic exchange processes wherein the guest anion migrated into and out of the cavity (complexation-decomplexation) or was in rapid equilibration between the two tin atoms. Structural isomerism in the larger bicyclic hosts 12 e,f (in-out) was also found to be temperature as well as guest concentration dependent. Line shape simulations of the spectra with a two or three site exchange model provided kinetic and thermodynamic information about the complexation phenomena. Principally, the limits of exchange were governed by the size complementarity of the hosts' cavity and the anion volume; high specificity translated into slow rates of decomplexation for hosts 12b with fluoride and 12d with chloride. Equilibrium constants were calculated from the kinetic results and the Hildebrand-Benesi method. Stoichiometries of the various complexes were determined by the method of continuous variations or by direct observation of the complex species. The formation of 1:1 (guest:host) complexes was indicative of cooperative binding by the multidentate host compounds. Two crystalline complexes (host 12b*F and 12d*Cl) were isolated and analyzed by X-ray crystallography, which showed unequivocally that the anion was encrypted by the host molecule. The structure of the symmetrical fluoride complex was a bis-hemistannate (two trigonal bipyramidal tin atoms) while the solid state structure of the chloride complex was a stannane-stannate hybrid (one tetrahedral tin and one trigonal bipyramidal tin).