A new strategy to obtain tetranuclear cobalt(ii) metal–organic frameworks based on the [Co4(μ3-OH)2] cluster: synthesis, structures and properties

Abstract

Three metal–organic frameworks (MOFs) {[Co₄(OH)₂(adc)₆(H₂O)₅][Co₂(OH)(btrb)]₂·8H₂O}_n (1), {[Co₄(OH)₂(btrb)(nip)₃(H₂O)₃]·4H₂O}_n (2·4H₂O) and {[Co₄(OH)₂(btrb)(btc)₂(H₂O)₂]·9H₂O}_n (3·9H₂O) (btrb = 1,4-bis(1,2,4-triazol-4-ylmethyl)benzene, H₂adc = 1,3-adamantanedicarboxylic acid, H₂nip = 5-nitroisophthalic acid and H₃btc = 1,3,5-benzenetricarboxylic acid) were synthesized by a hydrothermal method. 1 shows an unusual (3,4)-connected 2D network with a point symbol of (4·6²)₂(4²·6²·8²) based on [Co₄(μ₃-OH)₂] and [Co₂(μ₃-OH)] units. 2 is a 6-connected pcu topology based on the [Co₄(μ₃-OH)₂] unit. 3 is a rare (3,8)-connected 3D framework with a rare point symbol of (4·5·6)₂(4²·5⁶·6¹⁶·7²·8²) based on the [Co₄(μ₃-OH)₂] unit. The magnetic measurements reveal that 1–3 exhibit antiferromagnetic interactions between the adjacent cobalt(II) centers. 1–3 are good photocatalysts for the degradation of methylene blue (MB). The 1,2-position nitrogen atoms of the 4-substituted 1,2,4-triazole ligand act as strong σ-donators to form a quite stable coordination quadrangle with two metal ions. The btrb ligand containing two 4-substituted 1,2,4-triazole rings is favorable to form tetranuclear metal units. By the regulation of the multicarboxylate ligands, the connection number of [Co₄(μ₃-OH)₂] clusters increases from 4 in 1 to 6 in 2 and then 8 in 3. This work presents a novel but effective method to design and regulate the connection number (4, 6 and 8 connected in this work) of MOFs with 4-substituted 1,2,4-triazole and rigid multicarboxylate ligands.