On the Border between Low-Nuclearity and One-Dimensional Solids: A Unique Interplay of 1,2,4-Triazolyl-Based {Cu II 5 (OH) 2 } Clusters and Mo VI -Oxide Matrix

A pentanuclear CuII5-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 (trans-1,4-cyclohexanediyl-4,4′-bi(1,2,4-triazole) (tr2cy)), MoVI-oxide, and CuSO4. This structural motif can be rationalized by assuming in situ generation of {Cu2Mo6O22}4– anions, which represent heteroleptic derivatives of γ-type [Mo8O26]4– further interlinked by [Cu3(OH)2]4+ cations through [N−N] bridges. The framework structure of the resulting compound [Cu5(OH)2(tr2cy)2Mo6O22]·6H2O (1) is thus built up from neutral heterometallic {Cu5(OH)2Mo6O22}n layers pillared with tetradentate tr2cy. Quantum-chemical calculations demonstrate that the exclusive site of the parent γ-[Mo8O26]4– cluster into which CuII 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 1 show strong antiferromagnetic coupling within the Cu5 chain with exchange parameters J1 = −500(40) K (−348(28) cm–1), J2 = −350(10) K (−243(7) cm–1) and g = 2.32(2), χ2 = 6.5 × 10–4. Periodic quantum-chemical calculations reproduce the antiferromagnetic character of 1 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–, O2–, and tr2cy bridging ligands