Tetranuclear {CoII2CoIII2}, Octanuclear {CoII4CoIII4}, and Hexanuclear {CoIII3DyIII3} Pivalate Clusters: Synthesis, Magnetic Characterization, and Theoretical Modeling

New tetranuclear and octanuclear mixed-valent cobalt(II/III) pivalate clusters, namely, [NaCo4(O2CCMe3)6(HO2CCMe3)2(teaH)2(N3)]·2H2O (in two polymorphic modifications, 1and 1a) and [Co8(O2CCMe3)10(teaH)4(N3)](Me3CCO2)·MeCN·H2O (2) have been synthesized by ultrasonic treatment of a dinuclear cobalt(II) pivalate precursor with sodium azide and triethanolamine (teaH3) ligand in acetonitrile. The use of Dy(NO3)3·6H2O in a similar reaction led to the precipitation of a tetranuclear [NaCo4(O2CCMe3)4(teaH)2(N3)(NO3)2(H2O)2]·H2O (3) cluster and a heterometallic hexanuclear [Co3Dy3(OH)4(O2CCMe3)6(teaH)3(H2O)3](NO3)2·H2O (4) cluster. Single-crystal X-ray analysis showed that 1(1a) and 3consist of a tetranuclear pivalate/teaH3mixed-ligand cluster [CoII2CoIII2(O2CCMe3)4(teaH)2(N3)]+decorated with sodium pivalates [Na(O2CCMe3)2(HO2CCMe3)2]−(1or 1a) or sodium nitrates [Na(NO3)2]−(3) to form a square-pyramidal assembly. In 2, the cationic [Co8(O2CCMe3)10(teaH)4(N3)]+cluster comprises a mixed-valent {CoII4CoIII4} core encapsulated by an azide, 4 teaH2–alcoholamine ligands, and 10 bridging pivalates. Remarkably, in this core, the μ4-N3–ligand joins all four CoIIatoms. The heterometallic hexanuclear compound 4consists of a cationic [CoIII3DyIII3(OH)4(O2CCMe3)6(teaH)3(H2O)3]2+cluster, two NO3–anions, and a crystallization water molecule. The arrangement of metal atoms in 4can be approximated as the assembly of a smaller equilateral triangle defined by three Dy sites with a Dy···Dy distance of 3.9 Å and a larger triangle formed by Co sites [Co···Co, 6.1–6.2 Å]. The interpretation of the magnetic properties of clusters 2–4was performed in the framework of theoretical models, taking into account the structural peculiarities of clusters and their energy spectra. The behavior of clusters 2and 3containing CoIIions with orbitally nondegenerate ground states is determined by the zero-field splitting of these states and Heisenberg exchange interaction between the ions. To get a good understanding of the observed magnetic behavior of cluster 4, we take into consideration the crystal fields acting on the DyIIIions, the ferromagnetic coupling of neighboring DyIIIions, and the intercluster antiferromagnetic exchange. For all examined clusters, the developed models describe well the observed temperature dependence of the magnetic susceptibility and the field dependence of magnetization. The computational results apparently show that in cluster 4two DyIIIions with similar nearest surroundings demonstrate single-molecule-magnet (SMM) behavior, while the strong rhombicity of the ligand surrounding hinders the SMM behavior of the third DyIIIion.