On the Border between Low-Nuclearity and One-Dimensional Solids: A Unique Interplay of 1,2,4-Triazolyl-Based {Cu^II₅(OH)₂} Clusters and Mo^VI-Oxide Matrix

A pentanuclear Cu^II₅-hydroxo cluster possessing an unusual linear-shaped configuration was formed and crystallized under hydrothermal conditions as a result of the unique cooperation of bridging 1,2,4-triazole ligand (<i>trans</i>-1,4-cyclohexanediyl-4,4′-bi­(1,2,4-triazole) (<i>tr</i>₂<i>cy</i>)), Mo^VI-oxide, and CuSO₄. This structural motif can be rationalized by assuming <i>in situ</i> generation of {Cu₂Mo₆O₂₂}^4– anions, which represent heteroleptic derivatives of γ-type [Mo₈O₂₆]^4– further interlinked by [Cu₃(OH)₂]⁴⁺ cations through [<i>N</i>−<i>N</i>] bridges. The framework structure of the resulting compound [Cu₅(OH)₂(<i>tr</i>₂<i>cy</i>)₂Mo₆O₂₂]·6H₂O (<b>1</b>) is thus built up from neutral heterometallic {Cu₅(OH)₂Mo₆O₂₂}_<i>n</i> layers pillared with tetradentate <i>tr</i>₂<i>cy</i>. Quantum-chemical calculations demonstrate that the exclusive site of the parent γ-[Mo₈O₂₆]^4– cluster into which Cu^II inserts corresponds with the site that has the lowest defect (“MoO₂ vacancy”) formation energy, demonstrating how the local metal-polyoxomolybdate chemistry can express itself in the final crystal structure. Magnetic susceptibility measurements of <b>1</b> show strong antiferromagnetic coupling within the Cu₅ chain with exchange parameters <i>J</i>₁ = −500(40) K (−348(28) cm^–1), <i>J</i>₂ = −350(10) K (−243(7) cm^–1) and <i>g</i> = 2.32(2), χ² = 6.5 × 10^–4. Periodic quantum-chemical calculations reproduce the antiferromagnetic character of <b>1</b> and connect it with an effective ligand-mediated spin coupling mechanism that comes about from the favorable structural arrangement between the Cu centers and the OH^–, O^2–, and <i>tr</i>₂<i>cy</i> bridging ligands