Synthesis, Magnetic Properties, and Structural Investigation of Mixed-Ligand Cu(II) Helical Coordination Polymers with an Amino Acid Backbone and N-Donor Propping: 1-D Helical, 2-D Hexagonal Net (hcb), and 3-D ins Topologies

Three Cu(II) coordination polymers, {[Cu(l-cysteate)(1,4-Bix)(H2O)]·H2O}n (1), {[Cu(l-aspartate)(1,4-Bix)0.5]·H2O}n (2), and {[Cu(l-aspartate)(4,4′-Bpy)0.5]H2O}n (3), have been synthesized solvothermally, employing enantiopure l-amino acids and bidentate N-donor ligands, 1,4-bis(imidazol-1-ylmethyl)benzene (1,4-Bix) and 4,4′-bipyridyl(4,4′-Bpy), to yield 1-D, 2-D, and 3-D helical coordination polymers, respectively. All the three neutral frameworks are well-characterized by various physicochemical methods (C,H,N analysis, IR, TGA) including the single crystal X-ray diffraction technique. The chiral nature of complexes 2 and 3 was established using solid state CD spectra. Structural investigation of complex 1 revealed 1-D helical chains, whereas that of complex 2 confirms a chiral and helical two-dimensional 63-hcb net, with helicity propagating in both the dimensions. Cu(II) nodes with the mixed ligands in complex 3 generate a three-dimensional framework by the pillaring of Cu(l-aspartate) 2-D helical layers by 4,4′-Bpy units, identified as a (3,4)-coordinated binodal net with (63)(65.8)-ins topology. The temperature-dependent magnetic properties of the l-aspartic acid derived for complexes 2 and 3 were studied in detail. Complex 2 showed weak antiferromagnetic behavior, which matches well with the crystallographic parameters (syn−anti conformation of carboxylate bridged Cu(II), with lowest Cu−Cu distance 5.367 Å). Similarly, complex 3, with a shorter and rigid N-donor ligand, 4,4′-Bpy, and the carboxylate group of the amino acid moiety linking the adjacent metal centers in the syn−anti conformation, also displayed a weak antiferromagnetic behavior at lower temperatures, showing the weak exchange couplings between Cu(II) centers.