Octadecanuclear gear wheels by self-assembly of self-assembled "double saddle"-type coordination entities: molecular "rangoli".

A series of self-assembled "double saddle"-type trinuclear complexes of [Pd3L'3L2] formulation have been synthesized by complexation of a series of cis-protected palladium(II) components with a slightly divergent "E-shaped" non-chelating tridentate ligand, 1,1'-(pyridine-3,5-diyl)bis(3-(pyridin-3-yl)urea (L). The cis-protecting agents L' employed in the study are ethylenediamine (en), tetramethylethylenediamine (tmeda), 2,2'-bipyridine (bpy), and 1,10-phenanthroline (phen), for 1, 2, 3, and 4, respectively. The crystal structures of [Pd3(tmeda)3(L)2](NO3)6 (2), [Pd3(bpy)3(L)2](NO3)6 (3), and [Pd3(phen)3(L)2](NO3)6 (4) unequivocally support the new architecture. Two of the "double saddle"-type complexes (3 and 4) are suitably crafted with π surfaces at the strategically located cis-protecting sites to facilitate intermolecular π-π interactions in the solid state. As a consequence, six units of the 3 (or 4) are assembled, by means of six-pairs of π-π stacking interactions, in a circular geometry to form an octadecanuclear molecular ring of [(Pd3L'3L2)6] composition. The overall arrangement of the rings in the crystal packing is equated with the traditional Indian art form rangoli.