High Water-Assisted Proton Conductivities of Two Cadmium(II) Complexes Constructed from Zwitterionic Ligands

Abstract

Finding more metal complexes with outstanding water stability and high proton conductivity still has important research significance for the energy field. Herein, two highly proton-conductive complexes, one hydrogen-bonded supramolecular framework (HSF) [Cd(CBIA) 2 (H 2 O) 4 ]•2H 2 O (1) and one coordination polymer (CP), {[Cd 2 (CBIA) 2 (4,4′-bipy) 2 (H 2 O) 2 ]• (CBIA)•(OH)•2H 2 O} n (2) (4,4′-bipy = 4,4′-bipyridine), were triumphantly assembled using a zwitterionic organic compound, 2-(1-(carboxymethyl)-1H-benzo[d]imidazol-3-ium-3-yl)acetate (HCBIA).In the structure of HSF 1, there are several coordination and lattice H 2 O units except for the two monodentate CBIA − anions. CP 2 with a one-dimensional (1D) cylindrical structure has free CBIA − units and free H 2 O units located in the cavity. Thanks to the ability of the uncoordinated carboxyl groups and coordination/ lattice water molecules to construct the rich H-bonding networks, both complexes exhibit super-high proton conductivities, reaching 5.09 × 10 −3 and 3.41 × 10 −3 S cm −1 under 100 °C/98% relative humidity (RH), respectively. Based on the exploration of crystal structure data, combined with the calculated activation energy, and adsorption/desorption plots of nitrogen and water vapor, the causes and differences in proton conductivity of the two complexes, especially the proton-conductive mechanism, are compared and analyzed. This study again confirms that the zwitterionic ligands can exert important effects on forming organo-inorganic hybrid materials with high proton conductivity.

Two Dual-Function Zr/Hf-MOFs as High-Performance Proton Conductors and Amines Impedance Sensors

“…One way to improve the stability of MOFs is to adopt high-valence metals and carboxylic acid ligands. , Note that compared with the low valence metal cations, high-valence metal cations can construct strong coordination bonds with the same ligand. , For example, the first Zr-MOF with excellent chemical and water stability was first reported by Cavka Group in 2008 . Since then, research interest in such Zr-MOFs has grown rapidly, therefore the family of Zr-MOFs grown fast including the UiO, PCN, NU, and, DUT series. − In the field of proton conduction, such MOFs have captured the attention of researchers because of their remarkable structural stability including water stability and chemical stability. Zr-MOFs covering UiO-66 series MOFs, UiO-67 series MOFs, phosphonate series MOFs, phenolic hydroxyl series MOFs, etc.…”

Photochromic Ln-Phosphonates Assembled by an Imidazole Derivative: Construction, Crystal Structures, and Light-Enhanced Proton Conductivity

“…Coordination polymers (CPs) assembled with metal ions/clusters and organic ligands are a class of crystalline complexes and have attracted great attention in sensing detection, separation, and adsorption due to their designable and tunable structures and functional porosity. − Recently, CPs have become a new candidate material for proton conduction as the hydrogen-bond networks could be formed through introducing typical ligands with hydrophilic units or proton carriers. − In this way, high-conductivity CPs are constructed with good proton transport paths. Most importantly, the visualization of the structure of CPs provides an opportunity to explore the proton conduction mechanism and pathway, which makes it possible to analyze the structure–property relationship in depth.…”

Ultra-High Proton Conductivity iHOF Based on Guanidinium Arylphosphonate for Proton Exchange Membrane Fuel Cells

“…Crystalline materials such as metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) have been widely used in various fields due to their high crystallinity, functional designability, and tunability. − Due to the ability to provide and accommodate a variety of proton carriers, MOFs and COFs crystalline materials have proven to be good proton conductors. − In addition, hydrogen-bonded organic frameworks (HOFs) are crystalline materials with intermolecular hydrogen bonds formed by self-assembly, and the rich hydrogen bonding network provides a natural transport pathway for protons. , Owing to the weak hydrogen bonding interactions and reversibility, HOFs materials show unique advantages such as mild preparation conditions, good solution processability, and easy regeneration and purification, which make them good candidates for PEMs. − Traditional single-component HOFs form weak hydrogen bonds through their functional groups, resulting in the poor thermal and chemical stability. − However, their stability can be enhanced by adding extra intermolecular interactions like π–π stacking and van der Waals forces, as well as introducing interpenetration, chemical cross-linking, and charge-assisted hydrogen bonding, among others. Two-component ionic hydrogen-bonded organic frameworks (iHOFs) also known as charge-assisted hydrogen bonding networks exhibit good stability due to the introduction of multiple hydrogen bonds, which are further enhanced by electrostatic interactions. − …”