A new strategy to obtain tetranuclear cobalt(II) metal-organic frameworks based on the [Co-4(mu(3)-OH)(2)] cluster: synthesis, structures and properties

Three metal-organic frameworks (MOFs) {[Co-4(OH)(2)(adc)(6)(H2O)(5)][Co-2(OH)(btrb)](2)center dot 8H(2)O}(n) (1), {[Co-4(OH)(2)(btrb)(nip)(3)(H2O)(3)]center dot 4H(2)O}(n) (2 center dot 4H(2)O) and {[Co-4(OH)(2)(btrb)(btc)(2)(H2O)(2)]center dot 9H(2)O}(n) (3 center dot 9H(2)O) (btrb = 1,4-bis(1,2,4-triazol-4-ylmethyl)benzene, H(2)adc = 1,3-adamantanedicarboxylic acid, H(2)nip = 5-nitroisophthalic acid and H(3)btc = 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 center dot 6(2))(2)(4(2)center dot 6(2)center dot 8(2)) based on [Co-4(mu(3)-OH)(2)] and [Co-2(mu(3)-OH)] units. 2 is a 6-connected pcu topology based on the [Co-4(mu(3)-OH)(2)] unit. 3 is a rare (3,8)-connected 3D framework with a rare point symbol of (4 center dot 5 center dot 6)(2)(4(2)center dot 5(6)center dot 6(16)center dot 7(2)center dot 8(2)) based on the [Co-4(mu(3)-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 sigma-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 [Co-4(mu(3)-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.