Crystal-Engineering Studies of Coordination Polymers and a Molecular-Looped Complex Containing Dipyridyl-Amide Ligands

We report herein crystal-engineering studies of coordination polymers and a molecular-looped complex containing two dipyridyl-amide ligands, 1,3-bis-pyridin-4-ylmethyl urea (L1) and N,N‘-bis-4-methylpyridyl oxalamide (L2). The reaction of Cd(OAc)₂ with L1 gives rise to [Cd(OAc)₂(L1)]_n (1), a 1-D chain through coordination to two L1 and two acetate ligands, and then the axial coordination to one urea's carbonyl group through the third L1 ligand leads 1 to form “a dimer of 1-D chains”. With a slight change in the structural backbone from L1 to L2, the reaction of L2 with Cd(OAc)₂ gives [Cd(OAc)₂(L2)(H₂O)]_n (2), a 1-D chain structure. The reaction of Cd(NO₃)₂, instead of Cd(OAc)₂, with L2 gives [Cd(NO₃)₂(L2)_3/2]_n (3), where the coordinated-anion effect on the assembly process has been observed for 2 and 3. The former forms a 1-D chain structure, and the latter, a 2-D sheet structure, depending on the coordinated anions used. [HgCl₂(L1)]_n (4) and [CuCl₂(L2)]_n (5), which are 1-D chain structures, show tetrahedral [Hg(II)] and square-planar [Cu(II)] centers, respectively. Surprisingly, 4 shows a typical amide−amide hydrogen bonding and 5 shows none. Instead, a hydrogen-bonding interaction between Cl and the amide group is observed in 5. Finally, the different structural conformation of L2 (a syn or anti form) leads to the formation of different structural motifs, coordination polymers (2, 3, and 5 with an anti form), and a macrocycle ([Pd(PPy)(L2)]₂(ClO₄)₂ (6) with a syn form, PPy = 2-phenylpyridine). Each side of the boat form of 6 (pseudo-cyclohexane) ranges from 6.12 to 6.39 Å, and the molecular loop is further hydrogen-bonded to stack into a 1-D hydrogen-bonded framework with a ladder pattern through amide−amide hydrogen bonding. Interestingly, one ClO₄^- anion is encapsulated inside the cavity through multiple CH···O interactions.