Influence of the central metal ion in controlling the self-assembly and magnetic properties of 2D coordination polymers derived from [(NiL) ₂M]²⁺ nodes (M = Ni, Zn and Cd) (H₂L = salen-type di-Schiff base) and dicyanamide spacers

Abstract

Three new 2D coordination polymers (CPs) ²∞[(NiL)₂Ni(μ_1,5-N(CN)₂)₂]_n (1), ²∞[(NiL) ₂Cd(μ_1,5-N(CN) ₂)₂]_n (2) and ²∞[(NiL) ₂Zn(μ_1,5-N(CN) ₂)₂]_n (3) have been synthesized by reacting a [NiL] “metalloligand” (where H₂L = N,N′-bis(salicylidene)-1,3-propanediamine) with three different metal(II) (Ni, Cd and Zn) perchlorates and sodium dicyanamide, with identical molar ratios of the reactants. All three products have been characterized by IR and UV-vis spectroscopies, elemental analyses, powder and single crystal X-ray diffraction and variable temperature magnetic measurements. The isomorphous compounds 1 and 2 consist of similar [(NiL) ₂M(μ_1,5-N(CN) ₂)] (M = Ni for 1 and Cd for 2) angular trinuclear units in which two terminal “metalloligands” [NiL] coordinate to the central nickel(II) (in 1) or cadmium(II) (in 2) ion through phenoxido oxygen atoms. The μ_1,5-bridging dicyanamido spacers connect the central Ni(II) or Cd(II) of one node to terminal Ni(II) of two different nodes giving rise to 2D CPs. Compound 3 also contains trinuclear units with the same formula as those of 1 and 2: [(NiL)₂M(μ_1,5-N(CN) ₂)] (M = Zn in 3). The main differences are that these units are linear in 3 and the dicyanamide spacers link only the nickel atoms of neighbouring nodes. As in 1 and 2, these trinuclear units are connected to four other units via four μ_1,5-bridging dicyanamido ligands, giving rise to 2D CP with a similar topology: a uninodal 4-connected underlying net with the sql (Shubnikov tetragonal plane net) topology and (4⁴•6²) point symbol. The magnetic properties show the presence of moderate intra-trimer antiferromagnetic interactions in 1 (J = −12.9 cm⁻¹) and weak antiferromagnetic interactions between the terminal Ni(II) ions in 2 (J = −2.4 cm⁻¹). In 3 the Ni(II) ions are well isolated by the central Zn(II) ion and accordingly, only a very weak antiferromagnetic interaction through the single μ_1,5-bridging dicyanamido ligands is observed (J = −0.44 cm⁻¹, D = −3.9 cm⁻¹).