Carboxymethylated Cage Amines:  Coordination and Lactamization

Depending upon the position and degree of substitution, carboxymethyl derivatives of cage amines of the “sarcophagine” (3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) type vary considerably in the stability of their lactamized forms. For 1,8-diamino-3-carboxymethylsarcophagine, L¹, only indirect evidence for some involvement of a lactamized form of its Ni(II) complex has been obtained. Crystal structure determinations for [Cu(H₂L¹)](NO₃)_3.5Cl_0.5·2.5H₂O and [Ni(HL¹)]Cl₃·3H₂O show distorted octahedral coordination of all six endocyclic N-donor atoms in both cases. For related diaminosarcophagine derivatives with either two (1,8; L²) or three (1,1,8; L³) carboxymethyl substituents on the exocyclic N atoms, crystallographic studies have shown a dilactam form for the ligands in their Ni(II) and Cu(II) complexes which is of almost identical conformation to that of the diprotonated “free” ligand in [H₂L³][ZnCl₄]·6H₂O. The lactamized ligands appear to strongly favor square planar four-coordination, and the Co(II) complex of L² shows a remarkable lack of reactivity toward oxygen. Kinetic studies indicate that the hydrolytic stability of the lactam rings is comparable to that of uncoordinated analogues.