Coordination chemistry of calix-phosphanes: Cooperativity in the assembly of a tetragold calixarene complex

Assemblage of the tetragold complex [L .(AuCl1)(4)] from p-tertbutyl-calix[ 4]-(OCH2PPh2)(4) (L) and [AuCl(tetrahydrothiophene)] was investigated by NM R and UV/Visible spectroscopic titrations. Three steps could be resolved an d are assigned, respectively, to addition of one, two and four AuCl units t o the macrocyclic Ligand. At each stage of metallation, symmetrical species persist on the NMR time scale since the bound AuCl fragments migrate betwe en the different phosphorus centers via both intra- and intermolecular proc esses. The derived stability constants (log beta(1) = 4.4 +/- 0.2, log beta (2) = 7.0 +/- 0.7, and log beta(4) = 14.5 +/- 2.0) indicate both positive a nd negative cooperativity during build-up of the tetranuclear species. An X -ray diffraction study made with crystals obtained from an L:Au (1:2) mixtu re 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, prov ides a simple rationale by which to explain the role of cooperativity in th e overall assembly process.