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

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 (Ea) 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 Ea 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.