A 3D MOF with Cu₂₀/Cu₆ Clusters: Self-Assembly, CO₂ Encapsulation, Structural Features, and Magnetic Properties

Molecular design of multicopper clusters has been of increasing importance in inorganic chemistry, biomedical science, and functional materials. Herein, two new copper compounds were self-assembled from copper(II) monofluoroacetate or trifluoroacetate, elemental copper, and 1,3,5-triaza-7-phosphaadamantane (PTA). The reaction starting from copper(II) monofluoroacetate yielded a tetranuclear cluster, [Cu₄^II(μ₃-OH)₂(μ-L)₆(PTA=O)₂] (1), where L is monofluoroacetate(1-) and PTA=O is the oxide of PTA. Formation of 1 involves the oxidation of PTA and the incorporation of μ₃-OH^- ligands. In contrast, a similar reaction with copper(II) trifluoroacetate produced a unique three-dimensional metal-organic framework (3D MOF), formulated as [{Cu₂₀^II(CO₃)(μ₃-O)₂(μ₃-OH)₂₂(μ-L')₁₂}{Cu₆^I(μ₃-PTA)₆(CH₃CN)₁₂}]_n·6n(L')·3n(H₂O)·2n(CH₃CN) (2), where L' is trifluoroacetate. It comprises Cu₂₀ clusters, with an encapsulated carbonate anion fixed from atmospheric CO₂, which are bridged by {Cu₆^I(PTA)₆} units into a 3D MOF. Control experiments confirmed that carbonate originates from ambient CO₂ rather than from added carbonate salts. Both 1 and 2 were fully characterized, and their magnetic properties were investigated, revealing dominant antiferromagnetic interactions within the Cu₄ and Cu₂₀ clusters, respectively. Density functional theory (DFT) calculations confirmed the antiferromagnetic ground spin state of 1 and disclosed the stability of the Cu₂₀ core in 2. This work highlights the influence of fluorinated carboxylates and atmospheric conditions on the assembly and architecture of multicopper clusters, and extends their family to new examples.