Directed assembly and characterization of 1D polymers based on [MII(BMA)]2+ node (M = Cu, Mn, Ni and Zn; BMA = N,N-bis (benzimidazol-2-yl-methyl)amine) with linear bridging dicyanamide and terephthalate ligands

Abstract

The directed assembly of N,N-bis(benzimidazol-2-yl-methyl)amine (BMA) with Cu(II), Mn(II), Ni(II) and Zn(II) salts based on dicyanamide (μ_1,5-dca) and terephthalate (μ-ta) linear bridging ligands, respectively, leads to four novel compounds: [Cu(BMA)(μ_1,5-dca)(ClO₄)]_n (1), {[Mn(BMA)(μ_1,5-dca)(CH₃OH)]·ClO₄·C₁₀H₉N₃O·CH₃OH}_n (2), {[Ni₂(BMA)₂(μ-ta)(μ_1,5-dca)]·ClO₄·CH₃OH·H₂O}_n (3), and {[Zn₂(BMA)₂(μ-ta)(μ_1,5-dca)]·ClO₄·CH₃OH}_n (4), which were characterized by single-crystal X-ray diffraction, elemental analysis, IR, fluorescence spectroscopy, and magnetic measurement. X-Ray analysis revealed that 1 and 2 are two infinite 1D coordination polymers, in which dca units serving as an end-to-end out-of-plane bridge bring about linear chains for 1 and zigzag chains for 2. Complexes 3 and 4 are similar, in which the metal atoms are bridged alternately by terephthalate and μ_1,5-dicyanamide ligands into 1D zigzag chains. In all these complexes, each BMA ligand adopts a tridentate chelating mode to coordinate with a transition metal forming a [M(BMA)]²⁺ node. Different rigidity bridging ligands together with the stereochemistry and supramolecular effects of benzimidazol planes may result in the dramatic structural changes from 1D to multidimensional networks for all 1–4. Fluorescent measurements established that, in solution, complex 3 displays weak blue luminescence which originates from the BMA but is significantly red-shifted and has a much lower emission intensity, compared to the free BMA ligand. Complex 4 shows stronger luminescence than 3 and still reduces luminescence efficiency compared to the free BMA ligand. The variable-temperature magnetic susceptibility measurements (2–300 K) of 1 and 3 show the dominant weak ferromagnetic interactions between the copper(II) centers with J = 3.02 cm⁻¹, zJ′ = −2.70 cm⁻¹ for 1, and the nickel(II) centers with J = 1.94 cm⁻¹, J′; = −0.38 cm⁻¹ for 3, while weak antiferromagnetic interactions between the Mn(II) centers for 2 with J = −0.27 cm⁻¹.