On the Border between Low-Nuclearity and One-Dimensional Solids: A Unique Interplay of 1,2,4-Triazolyl-Based {CuII5(OH)2} Clusters and MoVI-Oxide Matrix

A pentanuclear Cu-5(II)-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) (tr(2)cy)), Mo-VI-oxide, and CuSO4. This structural motif can be rationalized by assuming in situ generation of {Cu2Mo6O22}(4-) anions, which represent heteroleptic derivatives of gamma-type [Mo8O26](4-) further interlinked by [Cu-3(OH)(2)](4+) cations through [N-N] bridges. The framework structure of the resulting compound [Cu-5(OH)(2)(tr(2)cy)(2)Mo6O22]center dot 6H2O (1) is thus built up from neutral heterometallic {Cu-5(OH)(2)Mo6O22}n layers pillared with tetradentate tr(2)cy. Quantum-chemical calculations demonstrate that the exclusive site of the parent gamma-[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 1 show strong antiferromagnetic coupling within the Cu-5 chain with exchange parameters J(1) = 500(40) K (-348(28) cm(-1)), J(2) = 350(10) K (-243(7) cm(-1)) and g = 2.32(2), chi(2) = 6.5 x 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 tr(2)cy bridging ligands.