Coordination Chemistry of Calix-Phosphanes: Cooperativity in the Assembly of a Tetragold Calixarene Complex

摘要

Assemblage of the tetragold complex [L·(AuCl)_4] from p-tert-butyl-calix [4]-(OCH_2PPh_2)_4 (L) and [AuCl (tetrahydrothiophene)] was investigated by NMR and UV/Visible spectroscopic titrations. Three steps could be resolved and are as-signed, respectively, to addition of one, two and four AuCi units to the macrocyclic ligand. At each stage of metallation, symmetrical species persist on the NMR time scale since the bound AuCl fragments migrate between the different phosphorus centers via both intra- and intermolecular processes, The derived stability constants (logβ_1 = 4.4 ± 0.2, logβ_2 = 7.0 ± 0.7, and logβ_4 = 14.5 ± 2.0) indicate both positive and negative cooperativity during build-up of the tetranuclear species. An X-ray diffraction study made with crystals obtained from an L:Au (1:2) mixture revealed the formation of an unusual binuclear complex whose solid state structure comprises a polymeric chain of [L·AuCl] units linked by pseudo-tetrahedral Au~+ ions, each generic unit containing a pendant linear PAuCl fragment. This polymeric species, with its two distinct binding modes, provides a simple rationale by which to explain the role of cooperativity in the overall assembly process.