The Synthesis and Structure of Encapsulating Ligands: Properties of Bicyclic Hexamines

Abstract

Template syntheses based on tris (ethane-1,2-diamine)cobalt(III) lead to cobalt(III) complexes of cage hexamines of the ' sarcophagine ' type ( sarcophagine = sar = 3,6,10,13,16,19- hexaazabicyclo [6.6.6] icosane ) rapidly and in high yield. Reduction of these species to their cobalt(II) forms enables the ligands to be removed in concentrated acids at elevated temperatures, and in hot aqueous solutions containing excess cyanide ion. The free sarcophagine and 1,8-diaminosarcophagine [(NH₂)₂sar 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, pK₁ = 11.95, pK₂ = 10.33, pK₃ = 7.17, pK₄ ≈ 0 for sarcophagine , and pK₁ = 11.44, pK₂ = 9.64, pK₃ = 6.49, pK₄ = 5.48, pK₅ ≈ 0 for diaminosarcophagine , with very similar values being found for triflate medium. Crystal structure determinations for both free bases, the chloride, sulfate, perchlorate and nitrate salts of diamsar , the complex of zinc chloride with sar, and the magnesium nitrate complex with diamsar show remarkably small variations in the cavity defined by the bicyclic ligands, though relatively subtle bond length and bond angle changes can be rationalized in terms of the effects of proton and metal ion binding. Exhaustive methylation 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 sar itself. All the ligands rapidly form metal ion complexes of generally exceptional kinetic and thermodynamic stability.