Structural Transformation from a Discrete Cu^II₄ Cluster to Two Extended Cu^II₄ + Cu^II₁ Chain-Based Three-Dimensional Frameworks by Changing the Spacer Functionality: Synthesis, Crystal Structures, and Magnetic Properties

Three unique [Cu₄(μ₃-OH)₂]⁶⁺ core-based magnetic complexes, [Cu₄(amtrz)₂(μ₃-OH)₂(nb)₆]·2H₂O (1), {[Cu₅(H₂O)₆(trz)₂(μ₃-OH)₂(btc)₂]·0.6H₂O}_n (2), and {[Cu₅(H₂O)₂(Htrz)₂(μ₃-OH)₂(btec)₂]·0.125H₂O}_n (3) (amtrz = 4-amino-1,2,4-triazole, Htrz = 1,2,4-triazole, Hnb = 4-nitrobenzoic acid, H₃btc = 1,2,3-benzenetricarboxylic acid, and H₄btec = 1,2,4,5-benzenetetracarboxylic acid), were synthesized by varying the spacer functionality of the mixed ligands. Significantly resulting from the cooperative cocoordination of the metal ions with polytopic triazolyl and carboxylate groups, a structural transformation from a discrete Cu^II₄ cluster for 1 to two alternate Cu^II₄ + Cu^II₁ chain-based three-dimensional frameworks for 2 and 3 was achieved. Moreover, the connectivity manner of the structural subunits in 2 and 3 depends strongly on the number and position of the carboxylate group attached on the phenyl backbone. The nearest-neighbor interactions within the Cu^II₄ cluster and between Cu^II₄ cluster and Cu^II₁ core mediated by the mixed short bridges were quantitatively calculated and compared based on the established magneto-structural relationships. Strong antiferromagnetic coupling interactions up to −206(4) cm^–1 were observed in the isolated Cu^II₄ cluster due to the favorable spatial orientation of the mixed multiple heterobridges toward the spin carriers.