Dimeric metallo-organic supramolecular quinoxaline stacking synthons

Condensation of ortho-phenylene diamines with an unsymmetrical 2,6'-bipyridyl-2"-pyridyl alpha-diketone (2) affords helical ligands 3a and 3b that self-assemble in the presence of copper(i) to produce the helical, dimeric metallo-organic complexes [3a(2)Cu(2)][PF6](2) and [3b(2)Cu(2)][PF6](2). These complexes have a flat, double-decker-like structure that results from atopomeric twisting in the ligand scaffolding, as confirmed by X-ray crystal structural analysis on [3a(2)Cu(2)][PF6](2). This study also indicates a close parallel between the distances involved in inter- and intramolecular stacking phenomena. Detailed NMR spectroscopic analysis of both complexes indicates that their dications possess very closely related solution-state structures. Achieving this self-assembly program is an important step towards realizing dimerically self-assembled metallo-organic materials that have a discotic alignment.