THE SYNTHESIS AND STRUCTURE OF ENCAPSULATING LIGANDS - PROPERTIES OF BICYCLIC HEXAMINES

Template syntheses based on tris(ethane-1,2-diamine)cobalt(III) lead t o cobalt(III) complexes of cage hexamines of the 'sarcophagine' type ( sarcophagine = sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) rap idly and in high yield. Reduction of these species to their cobalt(II) forms enables the ligands to be removed in concentrated acids at elev ated temperatures, and in hot aqueous solutions containing excess cyan ide ion. The free sarcophagine and 1,8-diaminosarcophagine [(NH2)(2)sa r or diamsar] ligands are strong bases, accepting up to four and five protons, respectively, in aqueous solution. In chloride medium, I = 1. 0, at 298 K, pK(1) = 11.95, pK(2) = 10.33, pK(3) = 7.17, pK(4) approxi mate to 0 for sarcophagine, and pK(1) = 11.44, pK(2) = 9.64, pK(3) = 6 .49, pK(4) = 5.48, pK(5) approximate to 0 for diaminosarcophagine, wit h very similar values being found for triflate medium. Crystal structu re determinations for both free bases, the chloride, sulfate, perchlor ate and nitrate salts of diamsar, the complex of zinc chloride with sa r, and the magnesium nitrate complex with diamsar show remarkably smal l variations in the cavity defined by the bicyclic ligands, though rel atively subtle bond length and bond angle changes can be rationalized in terms of the effects of proton and metal ion binding. Exhaustive me thylation of sarcophagine produces the highly lipophilic, hexatertiary base hexamethylsarcophagine, which, in the solid state, adopts quite different conformations and nitrogen-atom configurations to those of s ar itself. All the ligands rapidly form metal ion complexes of general ly exceptional kinetic and thermodynamic stability.