Structural diversity of transition-metal complexes derived from N-propionic acid functionalized 1,4,7-triazacyclononane: From enchanting cluster motifs to unprecedented homometallic polymeric networks

Abstract

Self-assembly between perchlorate salts of Cu(II), Zn(II) and Cd(II) with tri-substituted N-propionic acid functionalized 1,4,7-triazacyclononane (H₃3L₁1) or the mono-substituted one (HL₂2) results in the formation of four novel complexes [Cu₅(H₂O)₆(L₁1)₂](ClO₄)₄ (1), [Zn₉(L₁1)₄(H₂O)₁₈](ClO₄)₆·9H₂O (2), [Cd₂(H₂2L₁1)₂](ClO₄)₂ (3) and [Cd(L₂2)]ClO₄·CH₃OH (4) with different aggregation modes. All these complexes were structurally characterized by solid state X-ray diffraction measurement. Complex 1 belongs to a homometallic 2D polymer, where each [Cu(L₁1)]⁻ complex entity acts as a three-connected node joined to three Cu(II) ions through μ₂ bridged propionate arms to afford a puckered honeycomb framework. In complex 2, two face-to-face arranged [Zn(L₁1)]⁻ moieties clip three Zn(II) ions by virtue of pendant propionate arms generating a macrotricyclic metallacryptate. Two such metallacryptates are fused via co-sharing one lateral Zn(II) ion into an unprecedented nonazinc cluster with a double-cage architecture. Complex 3 exhibits a discrete centrosymmetric dinuclear unit [Cd₂(H₂2L₁1)₂]²⁺ consisting of two pendant propionate groups linked to a [Cd₂O₂] core. Complex 4 reveals Cd(II) centers are associated through the unique propionate bridge of the L₂2⁻ ligand leading to an infinite 1D zig–zag polymeric chain of [Cd(L₂2)]⁺ cations. Deduced from the structural investigations, the linkage patterns of the selected macrocyclic ligands towards metal centers may be exquisitely tuned according to the type of metals and the number of the coordinating pendant arms present in the macrocyclic ligands, and then the architectural variation of these metal–organic coordination species is achieved. In addition, magnetic studies for complex 1 show a weak Cu⋯Cu antiferromagnetic coupling in the 2D honeycomb layer with a Weiss temperature of −4.31 K.