Dynamic self-assembly of an M<inf>3</inf>L<inf>6</inf> molecular triangle and an M<inf>4</inf>L<inf>8</inf> tetrahedron from naked Pd^II ions and bis(3-pyridyl)-substituted arenes

The pyridine-appended non-chelating bidentate ligands 1,4-bis(3-pyridyl) benzene (1) and 4,4′-bis(3-pyridyl) biphenyl (2) were complexed with a naked PdII ion for the construction of molecular cage compounds. Prior to these experiments, the complexation of the ligands with cis-[Pd(en)(NO3)2] was also examined, because self-assemblies from the cis-protected PdII ion were expected to be simple motifs that constitute the assemblies from naked PdII ion. The structures of the self-assembled compounds resulting from 1 and [Pd(en)(NO 3)2] depended on the solvent employed. In aqueous solution, an M2L2 trenchlike compound was obtained. In dimethyl sulfoxide, however, a mixture of the M2L2 trench and an M3L3 macrocycle was found in equilibrium, the dynamic nature of which was confirmed by the concentration-dependent nature of the species. At higher concentration, an M4L4 macrocycle was mostly observed. The complexation of 1 with naked PdII ions was expected to produce novel structures that are combinations of the M nLn type frameworks. A peculiar tetrahedral M 4L8 assembly was obtained quantitatively from 1 and Pd(NO3)2, rather than the smallest possible M 3L6 double-walled triangle. Interestingly, the use of Pd(CF3SO3)2 resulted in the sole formation of the latter structure. Thus, the anion is important as a template in the formation of these assemblies. Ligand 2, which contains an extra p-phenylene unit compared to 1, behaved in a similar manner when treated with [Pd(en)(NO3)2], but showed subtle differences with naked PdII ions. With Pd(NO3)2, 2 gave mostly a tetrahedron along with a double-walled triangle. With Pd(CF3SO 3)2, this longer ligand formed a double-walled triangle with a negligible amount of tetrahedra. A single discrete assembly of a perfect tetrahedron was obtained from 2 and PdII ions by choosing p-tosylate as a counterion. © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.