Conformational Solvatomorphism in a [2]Catenane

The combination of a flexible pyridine-appended ligand (L1) with 4,4′-bipyridine (L2) and cis-protected Pd(II) units, Pd(L′)(NO3)2, in water yielded a concentration-dependent equilibrium mixture of the corresponding [2]catenanes [Pd2(L′)2(L1)(L2)]2(NO3)8 and their constituent macrocycles [Pd2(L′)2(L1)(L2)](NO3)4. The cis-protecting units (L′) used were ethylenediamine (en), tetramethylethylenediamine (tmeda), 2,2′-bipyridine (bpy), and 1,10-phenanthroline (phen). Crystallization of the [2]catenanes via slow evaporation of their CH3CN/H2O (1:1) solutions yielded a single crystalline form in the case of [Pd2(2,2′-bpy)2(L1)(L2)]2(NO3)8 and two concomitant crystalline forms (solvates) in the case of [Pd2(phen)2(L1)(L2)]2(NO3)8. The formation of the [2]catenane is facilitated by noncovalent interactions such as N–H···O, C–H···O, and C–H···π contacts and π···π stacking. The solvates of [Pd2(phen)2(L1)(L2)]2(NO3)8 revealed differences in the orientation of the amide carbonyl group and conformational differences in the ligands and phen units in the macrocycles and may thus be termed conformational solvatomorphs. The molecular packing in all the structures was found to be driven by intermolecular π···π stacking interactions between the aromatic rings in their cis-protected Pd(II) units. However, because of the awkward shape of the molecules, the closest 2,2′-bpy and phen units in all the structures overlap differently rather than the commonly observed "head-to-tail" or "head-to-head" modes.