High-nuclear heterometallic oxime clusters assembled from triangular subunits: solvothermal syntheses, crystal structures and magnetic properties

Abstract

Three series of six pyrazine-2-amidoxime (H₂pzaox)-based 3d–4f clusters, {Ln₈Ni₆}, {Ln₅Ni₁₀} and {Ln₅Ni₈} (Ln = Dy and Gd), were solvothermally synthesized in the absence or presence of different coligands, and were structurally and magnetically characterized. The unusual ring-shaped {Ln₈(μ₃-OH)₄} core in the two {Ln₈Ni₆} complexes is generated by four corner-sharing triangle {Ln₃(μ₃-OH)} units, which are further connected to six outer Ni^II ions by twelve deprotonated H₂pzaox ligands in three common binding modes. By contrast, the remaining four clusters contain only two corner-sharing {Ln₃(μ₃-OH)} triangles, which interact with peripheral Ni^II ions through fourteen H₂pzaox ligands in five (for {Ln₅Ni₁₀}) and four (for {Ln₅Ni₈}) different bridging ways. Thus, the interesting core motifs observed in these clusters depend significantly on the number of the triangular {Ln₃(μ₃-OH)} subunits and their connectivity manner with the singly and doubly deprotonated pyrazine-2-amidoxime ligand. Additionally, weak ferromagnetic superexchange in the {Dy₅Ni₁₀} and {Ln₅Ni₈} clusters and antiferromagnetic coupling in {Ln₈Ni₆} and {Gd₅Ni₁₀} clusters was respectively mediated by versatile oximate bridges between the intramolecular Ln^III and Ni^II ions. Furthermore, the three Dy^III-derived aggregates exhibit slightly temperature-dependent magnetic relaxations under a zero dc field, and the three Gd^III-based clusters display large magnetic entropy changes of 23.5 J kg⁻¹ K⁻¹ for {Gd₈Ni₆}, 19.4 J kg⁻¹ K⁻¹ for {Gd₅Ni₁₀}, and 22.4 J kg⁻¹ K⁻¹ for {Ln₅Ni₈} at 4.0 K and 70 kOe. These interesting results are helpful for the understanding of oximate-based 3d–4f coordination chemistry and their structure–function relationships.