Tetranuclear {Co^II₂Co^III₂}, Octanuclear {Co^II₄Co^III₄}, and Hexanuclear {Co^III₃Dy^III₃} Pivalate Clusters: Synthesis, Magnetic Characterization, and Theoretical Modeling

Abstract

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, 1 and 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 3 consist of a tetranuclear pivalate/teaH₃ mixed-ligand cluster [Co^II₂Co^III₂(O₂CCMe₃)₄(teaH)₂(N₃)]⁺ decorated with sodium pivalates [Na(O₂CCMe₃)₂(HO₂CCMe₃)₂]^- (1 or 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 {Co^II₄Co^III₄} core encapsulated by an azide, 4 teaH^2- alcoholamine ligands, and 10 bridging pivalates. Remarkably, in this core, the μ₄-N₃^- ligand joins all four Co^II atoms. The heterometallic hexanuclear compound 4 consists of a cationic [Co^III₃Dy^III₃(OH)₄(O₂CCMe₃)₆(teaH)₃(H₂O)₃]²⁺ cluster, two NO₃^- anions, and a crystallization water molecule. The arrangement of metal atoms in 4 can 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 2-4 was performed in the framework of theoretical models, taking into account the structural peculiarities of clusters and their energy spectra. The behavior of clusters 2 and 3 containing Co^II ions 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 Dy^III ions, the ferromagnetic coupling of neighboring Dy^III ions, 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 4 two Dy^III ions 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 Dy^III ion.