Ligand-directed assembly of tetranuclear cobalt clusters: structural modulation via a lactate derivative and azacyclic linkers

The semi-rigid lactic acid derivative 5-(1-carboxyethoxy)isophthalic acid (H₃CIA) demonstrates inherent cluster-forming propensity through its carboxylate groups and ether oxygen donors. Leveraging these features, three cobalt-based coordination polymers were synthesized <i>via</i> solvothermal assembly of H₃CIA with Co(II) ions in the presence of azacyclic linkers pyrazine, 1,3-dib and 4,4’-dib: [Co₄(CIA)₂(OH)₂(pyrazine)(H₂O)]·(H₂O)(MeOH)_0.5 (<b>HU1</b>), [Co₂(CIA)(OH)(1,3- dib)]·(H₂O)₂(DMF)_0.5 (<b>HU2</b>), and [Co₂(CIA)(OH)(4,4’-dib)]·xGuest (<b>HU3</b>). Notably, all architectures incorporate tetranuclear [Co₄(μ₃-OH)₂]⁶⁺ clusters formed by means of the distinct coordination preferences of CIA^3- anion. The clusters interconnect to generate analogous two-dimensional layers with kdg net among <b>HU1-3</b>, which are further pillared by azacyclic ligands into three-dimensional frameworks. Structural variations arise from divergent linker geometries: <b>HU1</b> contains two discrete [Co₄(μ₃-OH)₂]⁶⁺ units bridged by two pyrazine ligands per cluster, while <b>HU2</b> and <b>HU3</b> feature a single [Co₄(μ₃-OH)₂]⁶⁺ unit coordinated by four 1,3-dib or 4,4’-dib ligands, respectively. Although there are obvious structural differences among complexes <b>HU1-3</b>, their frameworks can all be simplified to the same 3,8-connected tfz-d net from the perspective of topological analysis. Furthermore, optical characterization demonstrates broad absorption bands across UV-vis regions and magnetic studies confirm antiferromagnetic coupling within all three complexes. In the presence of pyrazine, 1,3-dib, and 4,4′-dib, H₃CIA assembled with Co(II) ions to result in three CPs with tetranuclear [Co₄(μ₃-OH)₂]⁶⁺ units, two-dimensional CIA-Co metal-organic layers and pillared-layer frameworks with a 3,8-connected topological network.