Ligands modulated the variable binuclear Cd2-SBUs and structures of four layered coordination frameworks†
Four layered coordination polymers, namely, [Cd2(fuma)2(phen)2] (1), [Cd2(BDC)2(phen)2]·2DMF (2), [Cd2(NDC)2(phen)2]·0.5H2O (3), and [Cd2(NDC)2(bipy)2]·2DMF (4) have been synthesized under solvo-thermal conditions (fuma = fumaric acid, H2BDC = 1,4-benzenedicarboxylic acid, H2NDC = 1,4-naphthalenedicarboxylic acid, phen = 1,10-phenanthroline, bipy = 2,2′-bipyridine), which are composed of asymmetric/symmetric binuclear Cd2(RCOO)4(phen)2 or Cd2(RCOO)4(bipy)2 clusters as secondary building units (SBUs) linked by fuma2−, BDC2−, and NDC2−. Owing to the different steric hindrances of the di-carboxylates, the variation in Cd2(RCOO)4(phen)2 SBUs showed an intriguing structural modulation, which was due to the carboxylates adopting dissimilar coordination fashions, while both the terminal phen co-ligands changed from cis- to trans-position. Thus, this further induced twist angles (three neighbouring SBUs connected by the di-carboxylic acid) in the single layer of 1–3 varying from 109.7°, 132.3° to 180° (viewed along the a-axis) and correspondingly, three Cd2-SBU-based CPs exhibited structural evolution from an undulating to a lamellar layer. When the co-ligand bipy replaced phen, 4 also formed a lamellar layer framework based on the paddle-wheel Cd2(RCOO)4(bipy)2 SBUs with bipy perpendicular to the layer rather than the slanted phen in 3. The luminescence properties of 1–4 have also been explored, and their fluorescence emission mechanism of ligand-to-ligand charge transfer (LLCT) are confirmed by the time-dependent density functional theory (TDDFT) calculations.