Two Proton Conductive Coordination Polymers Constructed by an Unprecedented Structural Transformation of a Phosphine-Based Tricarboxylate Ligand

Abstract

The phosphine-based tricarboxylate 2-(3,3′-dioxo-1λ5-1,1′-(3H,3′H)-spirobi[2,1-benzoxaphosphol]-1-yl)benzoic acid (L) has been intensely studied due to its unique configuration and readily oxidizable phosphorus center. However, the coordination of L with metals have been largely ignored in the field of coordination polymers (CPs). Herein, we report two CPs, [Zn3(L1)2(dib)2(H2O)]·9H2O (1) and [Cu3(L2)2(dib)2]·3H2O (2) (H3L1 = tris(2-carboxyphenyl)phosphine oxide, H3L2 = 2,2′-phosphinico-dibenzoic acid, dib = 1,4-di(1H-imidazol-1-yl)butane), synthesized based on L. Strikingly, despite the initial use of L, the structures of 1 and 2 reveal that L undergoes two unprecedented in situ structural changes after coordination with the metal ion. In 1, L is transformed to the phosphine oxide derivative L1, while in 2, it is transformed to the phosphinic acid derivative L2. With the aid of the flexible ligand dib, the one-dimensional (1D) chain structure of 1 and three-dimensional (3D) supramolecular structure of 2 were constructed. For 1 and 2, especially in 1, there are abundant hydrogen bonds between the framework and the lattice water molecules, which are conducive to the formation of excellent hydrophilic channels and the transport of protons. Electrochemical experimental results show that 1 and 2 exhibit high proton conductivities (σ) of more than 10–4 S cm–1 over a wide temperature range of 303–353 K and low activation energies (E a) at 98% relative humidity. Compared with the σ value of the initial ligand L (3.12 × 10–5 S cm–1), the best σ values of 1 and 2 are improved by 28 times and 6 times, respectively, and E a is also reduced from 0.39 to 0.13 eV for 1 and 0.12 eV for 2. This is the first report of proton-conducting polymers based on a phosphine-based carboxylate ligand, involving two in situ structural transitions of the ligand.

Two Stable Bifunctional Zinc Metal–Organic Frameworks with Luminescence Detection of Antibiotics and Proton Conduction

“…In recent years, the development of multifunctional luminescent MOFs that combine fluorescence with other properties such as proton conductivity and magnetism has become an intriguing research topic. − Introducing two or more functional units into a single-phase material enables multifunctional expression and expands the range of material applications. Proton-conductive materials hold potential for use as solid-state electrolytes in clean and renewable proton exchange membrane fuel cells. , MOFs with highly tunable crystal structures and various modified pore surfaces offer significant opportunities for improving proton conduction and providing ideal crystal models to deepen our understanding of proton transfer mechanisms. , In most cases, proton transport occurs through proton hopping within a hydrogen-bond network. , Gao’s team successfully obtained a luminescent Zr-MOF with proton conductivity through molecular design and investigated the synergistic effect of pH changes and proton conductivity . Zuo et al reported a two-dimensional In­(II)-based conductive MOF with an impressive proton conductivity of up to 1.30 × 10 –2 S·cm –1 .…”