A series of helical coordination polymers based on two racemic bis(pyridylmethylene) propane-1,2-diamine ligands: Relationship of conformations, structures and properties

Helical coordination polymers have recently gained more concerns owing to their unique architectures and potential applications to date. In this work, self-assembly of different d10 metal salts and two racemic ligands with one chiral carbon generates ten complexes, namely, [M2(L2)Cl4]n [M = Zn (1 and 2) or Cd (3)], [Cd(L2)(SO4)]n (4), [Cd(L2)(X)(H2O)]n·nX [X = ClO4 (5) or NO3 (6)], [Ag(L1)]n·nClO4 (7), [Ag(L1)]n·nNO3·nH2O (8), [Ag(L2)]n·nClO4 (9) and [Ag(H2L2)]n·3nNO3·2nH2O (10) (L1 = N,N’-bis(pyridin-4-ylmethyl)propane-1,2-diamine, L2 = N,N’-bis(pyridin-3-ylmethyl)propane-1,2-diamine). Structural analyses indicate that except for the wavelike chain structure of complex 10 originated from the protonated H2L2+ ligand, the other complexes 1-9 all contain helical chain or helical layer structures with the pitches falling in the range of 7.97~19.76 A. Especially, complexes 3 and 4 present different 3D sqc and uog nets, which construct from (Cd-Cl)n helical layers and (Cd-L2)n helical chains, respectively. Complexes 7 and 8 exhibit the same (4,4) layer incorporating homo-helical chains. These results show that diverse conformations of two racemic ligands are highly responsible for the formation of helical architectures and even some chiral structures. The chirality of complexes 4, 5, 7 and 8 is further demonstrated by the circular dichroism (CD) spectroscopy. Luminescent investigation reveals that the Zn(II) and Cd(II) based complexes 1-6 exhibit blue emission maxima varying from 417 to 483 nm in the solid state at room temperature. Moreover, complexes 4 and 6 show average luminescence lifetimes of 8.87 (4) and 24.28 ns (6).