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 CuII 5 -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) ( tr2 cy)), MoVI -oxide, and CuSO4 . This structural motif can be rationalized by assuming in situ generation of {Cu2 Mo6 O22 }4- anions, which represent heteroleptic derivatives of γ-type [Mo8 O26 ]4- further interlinked by [Cu3 (OH)2 ]4+ cations through [ N- N] bridges. The framework structure of the resulting compound [Cu5 (OH)2 ( tr2 cy)2 Mo6 O22 ]·6H2 O (1) is thus built up from neutral heterometallic {Cu5 (OH)2 Mo6 O22 } n layers pillared with tetradentate tr2 cy. Quantum-chemical calculations demonstrate that the exclusive site of the parent γ-[Mo8 O26 ]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 tr2 cy bridging ligands.