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

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 NiII 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 NiII 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 LnIII and NiII ions. Furthermore, the three DyIII-derived aggregates exhibit slightly temperature-dependent magnetic relaxations under a zero dc field, and the three GdIII-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.