Conformational analysis and synthetic approaches to polydentate perhydro-diazepine ligands for the complexation of gallium(III)

Synthetic approaches are reported to polydentate ligands based on 6-phenyl-6-amino-perhydro-1,4-diazepine. The synthetic route devised averts ring-opening reactions, allowing the exocyclic N-substituent to be introduced separately and involves a nitro-Mannich condensation, prior to chemoselective RANEY (R) nickel reduction. Comparison of the solid-state structures of four synthetic intermediates reveals that the seven-membered ring adopts a preferred twist-chair conformer in the solid state. Solution state NMR experiments highlight a conformational preference for the bulky aryl groups to adopt an equatorial site, pre-disposing the ligand to metal binding, by adoption of a conformation that creates a facial array of the ligand nitrogen atoms. This ligand conformation averts the formation of less stable metal complexes with differing ligation modes, notably in the binding of Ga3+ to related ligands, where a C-methyl substituent replaces the phenyl group at the quaternary centre.