Imidazolium Dicarboxylate Based Metal-Organic Frameworks Obtained by Solvo-Ionothermal Reaction

N-Heterocyclic carbenes and their precursors in functionalised porous materials

Though N-heterocyclic carbenes (NHCs) have emerged as diverse and powerful discrete functional molecules in pharmaceutics, nanotechnology, and catalysis over decades, the heterogenization of NHCs and their precursors for broader applications in porous materials, like metal-organic frameworks (MOFs), porous coordination polymers (PCPs), covalent-organic frameworks (COFs), porous organic polymers (POPs), and porous organometallic cages (POMCs) was not extensively studied until the last ten years. By de novo or post-synthetic modification (PSM) methods, myriads of NHCs and their precursors containing building blocks were designed and integrated into MOFs, PCPs, COFs, POPs and POMCs to form various structures and porosities. Functionalisation with NHCs and their precursors significantly expands the scope of the potential applications of porous materials by tuning the pore surface chemical/physical properties, providing active sites for binding guest molecules and substrates and realizing recyclability. In this review, we summarise and discuss the recent progress on the synthetic methods, structural features, and promising applications of NHCs and their precursors in functionalised porous materials. At the end, a brief perspective on the encouraging future prospects and challenges in this contemporary field is presented. This review will serve as a guide for researchers to design and synthesize more novel porous materials functionalised with NHCs and their precursors.

Synthesis and structural characterization of homochiral coordination polymers with imidazole-based monocarboxylate ligands

Chiral Na[(S)-L^R] (R = Me, 1a; ⁱPr, 1b; CH₂ⁱPr, 1c, and (S)-^secBu, 1d) and Na[(R)-L^R] (R = Me, 1a') compounds were synthesised following standard procedures. New compounds 1d and 1a' were analytically and spectroscopically characterised. 1a and 1c were structurally identified by single-crystal X-ray diffraction methods as homochiral 2D coordination polymers, {Na(H₂O)[(S)-L^Me]}_n and {Na[(S)-L^CH₂iPr]}_n, respectively. Both (S)-2alkyl,2-(1H-imidazol-1-yl)acetate anions displayed unprecedented coordination modes in these coordination polymers: μ₃κ²OκO' for 1a and μ₄κ²Oκ²O' for 1c. Enantiomeric species 1a', {Na(H₂O)[(R)-L^Me]}_n, showed the same X-ray powder diffractogram (XRPD) as 1a, in agreement with a similar crystal structure. DFT calculations on the [L^R]^- anions confirmed their coordination capabilities as ditopic linkers. In fact, the reaction of Na[L^R] with several metal salts yielded the following coordination polymers: {Ag[(S)-L^Me]}_n, 2a, {Ag[(R)-L^Me]}_n, 2a', {Cu[(S)-L^R]₂}_n (R = Me, 3a; ⁱPr, 3b), {Cu[(R)-L^Me]}_n, 3a', {Zn[(S)-L^R]₂}_n (R = Me, 4a; ⁱPr, 4b; (S)-^secBu, 4d) and {Zn[(R)-L^Me]₂}_n, 4a'. For the known compounds 3a and 4a, this procedure is a new synthetic route that avoided high temperature reaction conditions. New complexes 2, 3a', b, and 4a', b, d were characterised by elemental analysis, infrared and XRPD methods and complex 2a by single-crystal X-ray diffraction. This complex is also a two-dimensional coordination polymer in which the [(S)-L^Me]^- anion acts as a μ₄κN,κ²O,κ²O' bridging ligand. Compounds 1-4a' are the first examples of homochiral coordination polymers with imidazole-monocarboxylate ligands based on non-natural amino acids. Preliminary studies on the metal-catalysed preparation of chiral α-aminophosphonates were carried out but, unfortunately, no enantioselectivity was observed.

Magnetic and luminescent coordination networks based on imidazolium salts and lanthanides for sensitive ratiometric thermometry

The synthesis and characterization of six new lanthanide networks [Ln(L)(ox)(H₂O)] with Ln = Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺ and Yb³⁺ is reported. They were synthesized by solvo-ionothermal reaction of lanthanide nitrate Ln(NO₃)₃·xH₂O with the 1,3-bis(carboxymethyl)imidazolium [HL] ligand and oxalic acid (H₂ox) in a water/ethanol solution. The crystal structure of these compounds has been solved on single crystals and the magnetic and luminescent properties have been investigated relying on intrinsic properties of the lanthanide ions. The synthetic strategy has been extended to mixed lanthanide networks leading to four isostructural networks of formula [Tb_{1- x} Eu _x (L)(ox)(H₂O)] with x = 0.01, 0.03, 0.05 and 0.10. These materials were assessed as luminescent ratiometric thermometers based on the emission intensities of ligand, Tb³⁺ and Eu³⁺. The best sensitivities were obtained using the ratio between the emission intensities of Eu³⁺ (⁵D₀→⁷F₂ transition) and of the ligand as the thermometric parameter. [Tb_0.97Eu_0.03(L)(ox)(H₂O)] was found to be one of the best thermometers among lanthanide-bearing coordination polymers and metal-organic frameworks, operative in the physiological range with a maximum sensitivity of 1.38%·K^-1 at 340 K.

Synthesis, structural characterization and luminescence properties of 1-carboxymethyl-3-ethylimidazolium chloride

A microcrystalline carboxyl-functionalized imidazolium chloride, namely 1-carboxymethyl-3-ethylimidazolium chloride, C₇H₁₁N₂O₂⁺·Cl^-, has been synthesized and characterized by elemental analysis, attenuated total reflectance Fourier transform IR spectroscopy (ATR-FT-IR), single-crystal X-ray diffraction, thermal analysis (TGA/DSC), and photoluminescence spectroscopy. In the crystal structure, cations and anions are linked by C-H...Cl and C-H...O hydrogen bonds to create a helix along the [010] direction. Adjacent helical chains are further interconnected through O-H...Cl and C-H...O hydrogen bonds to form a (10-1) layer. Finally, neighboring layers are joined together via C-H...Cl contacts to generate a three-dimensional supramolecular architecture. Thermal analyses reveal that the compound melts at 449.7 K and is stable up to 560.0 K under a dynamic air atmosphere. Photoluminescence measurements show that the compound exhibits a blue fluorescence and a green phosphorescence associated with spin-allowed (¹π←¹π*) and spin-forbidden (¹π←³π*) transitions, respectively. The average luminescence lifetime was determined to be 1.40 ns for the short-lived (¹π←¹π*) transition and 105 ms for the long-lived (¹π←³π*) transition.