Ligands modulated the variable binuclear Cd2-SBUs and structures of four layered coordination frameworks

Abstract

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.