Metallo-supramolecular self-assembly: The case of triangle-square equilibria

For the efficient self-assembly of metallo-supramolecular complexes, not only reversibility is required but also two other properties have to be controlled as well: (i) The right binding sites need to be programmed into the building blocks at the appropriate positions. (ii) The building blocks must be rigid enough to support the geometrical arrangement and to avoid the unfavorable entropy effects connected with the conformational fixation of flexible molecules. A series of different bis-pyridyl ligands is reported which self-assemble with (dppp)M(OTf)(2) complexes (dppp = 1,3-bis-(diphenylphosphino)propane; M = Pd-II, Pt-II) to yield squares and/or triangles as the products. Enthalpic contributions (higher strain in the triangle) and entropic contributions (higher number of triangles from the same building blocks) determine the equilibrium. The effects of concentration, temperature, and solvent properties on the equilibrium have been studied. To characterize the complexes under study, a combination of H-1, P-31, and diffusion-ordered NMR spectroscopy, electrospray-ionization Fourier-transform ion-cyclotron-resonance mass spectrometry, and X-ray crystallography is needed. Variable-temperature NMR spectroscopy provides evidence for fast ligand-exchange processes occurring for the Pd complexes, while the Pt complexes exchange ligands much more slowly.