Self-assembly of chelating, bridging N, N, N donor ligands and Cu(I), Cu(II), Cd(II), Hg(II): Molecular box versus coordination polymer

Abstract

Three flexible chelating and bridging ligands, (E)-N-(pyridin-2-ylmethylene)-1-(pyridin-3-yl)methanamine (L1), 1-(pyridine-2-yl)-N-(pyridin-3-ylmethyl)methanamine (L1′), (E)-N-(pyridin-2-ylmethylene)-1-(pyridin-4-yl)methanamine (L2), have been synthesized. The reaction of these ligands with various metal salts yielded a series of compounds, namely [Cu2(L1)(PPh3)2I2]2 (1), [Cu2(L2)(PPh3)2I2]n (2), [CuL1(NO3)2]2 (3), [ZnL1(NO3)2]2 (4), [CdL1(NO3)2]2 [(5), [CdL1′ (NO3)2]2 (6) and [HgL1Cl2]n (7). The compounds were characterized by elemental analysis, spectroscopic methods (IR, UV–Vis) and X-ray crystallography. X-ray structural analysis of compound 1 reveals the formation of a centrosymmetric tetranuclear compound. The repeat unit of coordination polymer 2 comprises a Cu2I2 unit with a triphenyl phosphane substituent on Cu1 linked to an adjacent Cu2I2 unit by the ligand L2 that acts as a bridge. Compounds 3, 4, 5 and 6 exhibit a common metallo-cyclophane [M2(L)2(NO3)4] skeleton. Two M2+ cations are linked by two L1 ligands and these ligands chelate to each metal atom through the imine nitrogen (or amine nitrogen) atoms and the nitrogen atom of the 2-pyridyl ring, and bind to the other metal atom of the pair via the nitrogen atoms of the 3-pyridyl rings. Structural studies of 7 revealed the formation of a coordination polymer in which the polymer chain propagates in helical spirals along a twofold screw axis parallel to b. The metal coordination geometries and coordination of the nitrato anions to the metals are noted and discussed.