Rigid Bipodal Ligand as Molecular Building Block to Construct New Metal-organic Frameworks

In a previous work of our group, four Coordination Networks (CNs) of Co (II), Cd (II), Er (III) and Gd (III) were obtained using the rigid ligand of 4,4'-ethynylenbenzoate. 1,2 The carboxylate ligand adopted different coordination modes which together with the preferred geometries of the metal ions formed the 1D, 2D and 3D CNs containing varied interpenetrations and rare topologies. As part of our ongoing research on rigid CNs, we prepared the rigid ligand H2L1 with two triple bonds in the spacer to be included in our library of ligands.3–5 Five steps of reactions have been performed to obtain H2L1 ligand: the first one is an esterification to afford 1, the second is a Sonogashira coupling to obtain 2; the third step is a hydrodesililation to afford 3; the fourth step is an Eglinton homo-coupling to afford 4; finally the hydrolysis of 4 gives rise two coordination sites, see scheme below. The H2L1 and 1, 2, 3 and 4 have been characterized by NMR 1H, IV, TGA and DSC. Also, 4 and H2L1 had their photophysical properties evaluated in the solid state. H2L1 showed strong emission at 492, 462 and 433 nm when excited at 367, 340, 316 nm due to the  conjugation. The H2L1 is being used as a building block in many synthetic trials for new MOFs (Metal-organic Framework) and CNs materials of d and f block elements for CO2 capture and photoluminescent studies.