Construction of (3,6)-connected polyoxometalate-based metal–organic frameworks (POMOFs) from triangular carboxylate and dimerized Zn4-ε-Keggin

Abstract

Using the methodology of extension of reduced transition metal-grafted ε-Keggin polyoxoanions with two types of terphenyl-based tricarboxylates of H₃L¹ (3,5′,3′′-position substitution) and H₃L² (4,5′,4′′-position substitution) we isolated two (3,6)-connected 3D polyoxometalate-based metal–organic frameworks, [TBA]₃[H₃PMo₁₂O₄₀][Zn₄L²] (1, YZU-105), and [TPA]₃[H₃PMo₁₂O₄₀][Zn₄L¹]·0.5H₂O (2, YZU-106) (H₃L¹ = [1,1′;3′,1′′-terphenyl]-3,5′,3′′-tricarboxylic acid; H₃L² = [1,1′;3′,1′′-terphenyl]-4,5′,4′′-tricarboxylic acid; TBA = tetrabutylammonium; TPA = tetrapropylammonium). In both compounds, the building block was the dimerized form of Zn₄-{ε-H₃PMo₁₂O₄₀}. Such dimerization left six anchoring points for each dimer and, as a result, a 6-connected node was formed. Compounds 1 and 2 exhibited topologies of (4·8⁵)₃(4·8²)₆ and (6⁵·10)₃(6³)₆, respectively. This work illustrates that use of tri-carboxylate substitutions in different positions (3,5′,3′′-position/4,5′,4′′-position) in tripodal terphenyl-based ligands allows different extents of twisting of the peripheral aromatic ring with respect to the central ring, thereby giving rise to different extending directions and symmetries.