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

Three series of six pyrazine-2-amidoxime (H(2)pzaox)-based 3d-4f clusters, {Ln(8)Ni(6)}, {Ln(5)Ni(10)} and {Ln(5)Ni(8)} (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(mu(3)-OH)(4)} core in the two {Ln(8)Ni(6)} complexes is generated by four corner-sharing triangle {Ln(3)(mu(3)-OH)} units, which are further connected to six outer NiII ions by twelve deprotonated H(2)pzaox ligands in three common binding modes. By contrast, the remaining four clusters contain only two corner-sharing {Ln(3)(mu(3)-OH)} triangles, which interact with peripheral NiII ions through fourteen H(2)pzaox ligands in five (for {Ln(5)Ni(10)}) and four (for {Ln(5)Ni(8)}) different bridging ways. Thus, the interesting core motifs observed in these clusters depend significantly on the number of the triangular {Ln(3)(mu(3)-OH)} subunits and their connectivity manner with the singly and doubly deprotonated pyrazine-2-amidoxime ligand. Additionally, weak ferromagnetic superexchange in the {Dy5Ni10} and {Ln(5)Ni(8)} clusters and antiferromagnetic coupling in {Ln(8)Ni(6)} and {Gd5Ni10} clusters was respectively mediated by versatile oximate bridges between the intramolecular Ln(III) and Ni-II 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 {Ln(5)Ni(8)} 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.