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, <b>L</b><b><sup>1</sup></b><sup></sup>, only indirect evidence for some involvement of a lactamized form of its Ni(II) complex has been obtained. Crystal structure determinations for [Cu(H<sub>2</sub><b>L</b><b><sup>1</sup></b><sup></sup>)](NO<sub>3</sub>)<sub>3.5</sub>Cl<sub>0.5</sub>·2.5H<sub>2</sub>O and [Ni(H<b>L</b><b><sup>1</sup></b><sup></sup>)]Cl<sub>3</sub>·3H<sub>2</sub>O show distorted octahedral coordination of all six endocyclic N-donor atoms in both cases. For related diaminosarcophagine derivatives with either two (1,8; <b>L</b><b><sup>2</sup></b><sup></sup>) or three (1,1,8; <b>L</b><b><sup>3</sup></b><sup></sup>) 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 [H<sub>2</sub><b>L</b><b><sup>3</sup></b><sup></sup>][ZnCl<sub>4</sub>]·6H<sub>2</sub>O. The lactamized ligands appear to strongly favor square planar four-coordination, and the Co(II) complex of <b>L</b><b><sup>2</sup></b><sup></sup> 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.