The role of alkali metal cations in the construction of heterometallic NiII polymeric cyclopropane-1,1-dicarboxylates

Abstract

In this work, new Ni^II coordination compounds with cyclopropane-1,1-dicarboxylic acid (H₂cpdc) dianions were synthesized: {[Na₂Ni(cpdc)₂(H₂O)₆]·4H₂O}_n (1), {[K₆Ni₃(cpdc)₆(H₂O)₁₀]·8H₂O}_n (2), {[Rb₆Ni₃(cpdc)₆(H₂O)₁₂]·5.5H₂O}_n (3), and [Cs₂Ni(cpdc)₂(H₂O)₈]_n (4). All complexes contained mononuclear bischelate fragments {Ni^II(cpdc)₂(H₂O)₂}²⁻ bound by alkali atoms in a 1D coordination polymer for 1, 2D for 2 and 4, and 3D for 3. In the topological representation of coordination bonds and ionic interactions, the Ni center represents the 4-c node of underlying (simplified) nets for all the four compounds. Na⁺, K⁺, Rb⁺, and Cs⁺ provide coordination numbers 4, 5&7&8, 6&7&8, and 6 in the underlying nets. The periodicity is higher for compounds with higher coordination numbers of large cations and a less amount of terminal aqua ligands. In the topological representation of ionic interactions, the fragment {Ni(cpdc)₂(H₂O)₂}²⁻ plays the role of 4-c (1–3) or 2-c (4) node of the underlying nets. H-bonds produce highly connected 3D supramolecular networks. The compounds were characterized by single-crystal X-ray diffraction. The DC magnetic data and ab initio calculations revealed that Ni²⁺ ions in 2 and 3 possess a positive D value and an “easy-plane” anisotropy type.