Two coordination polymers via one-pot synthesis and crystallization based on semirigid tripodal carboxylic acid

Technical Summary: Two Coordination Polymers via One-Pot Synthesis Based on Semirigid Tripodal Carboxylic Acid

Problem and Context
Coordination polymers (CPs) and metal-organic frameworks (MOFs) are of significant interest due to their versatile structures and potential applications in photochemistry, magnetism, catalysis, and adsorption. A critical challenge in this field is understanding how organic ligands influence framework topology, particularly regarding space length, flexibility, and steric hindrance. While rigid tricarboxylic acids are widely utilized, semi-rigid tricarboxylic acids have received less attention despite their potential to rotate and adapt to metal ion coordination geometries. Specifically, 5-(4'-carboxylphenoxy)isophthalic acid (H3cpia), a semi-rigid tripodal carboxyl ligand, has limited reported derivatives. The study addresses the need to systematically explore the structural outcomes of combining H3cpia with auxiliary N-donor ligands to generate novel topologies.

Methodology
The researchers employed a one-pot solvothermal synthesis strategy to construct two new three-dimensional cadmium-based coordination polymers.

• Reagents: The synthesis utilized Cd(NO3)2·4H2O as the metal source, the semi-rigid tripodal acid H3cpia (3,5,4'-tricarboxydiphenyl ether), and the auxiliary ligand 4,4'-bis(imidazol-1-ylmethyl)-biphenyl (bimb).
• Synthesis Conditions: The components were dispersed in a DMF/H2O (2:1) solvent system and heated in a Teflon-lined stainless-steel reactor. Two distinct temperature regimes were used: 90°C and 110°C for 48 hours. Notably, the reaction yielded a mixture of two distinct crystalline phases (Complex 1 and Complex 2) rather than a single pure product, with a combined yield of approximately 60%.
• Characterization: The resulting colorless bulk crystals were analyzed using single-crystal X-ray diffraction (SCXRD) to determine atomic structures. Additional characterization included elemental analysis, Fourier-transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), and solid-state fluorescence measurements. The SQUEEZE program was utilized to account for disordered solvent molecules in the crystal structures.

Key Contributions and Results
The study successfully synthesized and structurally characterized two new 3D coordination polymers:

1. {[Cd3(cpia)2(trans-bimb)3]}n (Complex 1):

   • Structure: Crystallizes in the monoclinic space group C2/c. The asymmetric unit contains half a Cd(II) ion, a cpia3- anion, and half a bimb ligand.
   • Coordination Geometry: Cd1 adopts a distorted trigonal bipyramidal geometry (CdO3N2), while Cd2 is distorted octahedral (CdO4N2).
   • Ligand Conformation: The bimb ligands adopt a trans configuration.
   • Topology: The trans-bimb ligands link Cd(II) ions into meso-helical chains. These chains are bridged by cpia3- anions to form a 2D planar structure, which further assembles into a 3D framework. The topology is simplified as a {6.82}2 {64.8.10}2 {85.10} network.

2. {[Cd3(cpia)2(cis-bimb)3·(H2O)]}n (Complex 2):

   • Structure: Crystallizes in the orthorhombic space group Pna21. The asymmetric unit is more complex, containing three independent Cd(II) cations, two cpia3- anions, three bimb ligands, and one lattice water molecule.
   • Coordination Geometry: Cd1 and Cd2 exhibit twisted triangular bipyramidal geometries (CdO3N2), while Cd3 is seven-coordinated in a distorted pentagonal bipyramidal arrangement.
   • Ligand Conformation: All bimb ligands adopt a cis configuration.
   • Topology: The cis-bimb ligands bridge distinct cadmium centers (Cd1 and Cd2) to form discrete [Cd(bimb)]2 rings. Cd1 and Cd2 are also bridged by carboxylates to form [Cd(COO)2]n chains. These elements connect via cpia3- ions to form a 3D framework.

Comparative Analysis and Properties

• Conformational Influence: The primary finding is that the conformational difference of the bimb ligand (trans in 1 vs. cis in 2) profoundly dictates the overall structural features, leading to different topologies and coordination environments.
• Spectroscopic Confirmation: FTIR spectra confirmed distinct coordination modes for the carboxylate groups in the two complexes, evidenced by shifts in asymmetric and symmetric stretching frequencies (1589/1384 cm⁻¹ for 1 vs. 1603/1382 cm⁻¹ for 2).
• Stability and Purity: TGA indicated that both complexes possess good thermal stability, with structural collapse occurring around 370°C. PXRD patterns of the bulk material matched simulated patterns derived from single-crystal data, confirming phase purity and the absence of conformational changes during powder preparation.
• Fluorescence: Both complexes exhibited solid-state fluorescence. Complex 1 showed emission at ~355 nm with a shoulder at 550 nm (excitation 346 nm), while Complex 2 emitted at 355 nm (excitation 369 nm). The authors attribute these properties to ligand-to-metal charge transfer (LMCT), with variations arising from the distinct coordination environments of the Cd(II) centers.

Significance and Claims
The paper claims that the combination of the semi-rigid H3cpia and the flexible bimb ligand is an effective strategy for generating new crystal structures and topologies. The authors emphasize that the "kink" flexibility of the H3cpia ligand and the configurational variability of the bimb ligand (switching between trans and cis) are the key factors enabling the formation of these two distinct frameworks from a single reaction mixture. The study highlights the importance of ligand conformation in directing the assembly of coordination polymers.

The authors conclude modestly, noting that while the synthesis was successful, the two polymers were obtained as a mixture. Future work is explicitly stated to focus on completely separating these two coordination polymers to further explore their optical properties, rather than claiming immediate industrial applications.