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

Two conductivecoordination polymers with stable structureswere successfully synthesized, accompanied by the unique in situ reactionsof the initial phosphine-based tricarboxylate ligand. The phosphine-based tricarboxylate2-(3,3 '-dioxo-1 lambda(5)-1,1 '-(3H,3 ' H)-spirobi[2,1-benzoxaphosphol]-1-yl)benzoicacid (L) has been intenselystudied due to its unique configuration and readily oxidizable phosphoruscenter. However, the coordination of L with metals have been largelyignored in the field of coordination polymers (CPs). Herein, we reporttwo CPs, [Zn-3(L1)(2)(dib)(2)(H2O)]center dot 9H(2)O (1) and [Cu-3(L2)(2)(dib)(2)]center dot 3H(2)O (2) (H(3)L1 = tris(2-carboxyphenyl)phosphine oxide, H(3)L2= 2,2 '-phosphinico-dibenzoic acid, dib = 1,4-di(1H-imidazol-1-yl)butane), synthesized based on L. Strikingly, despitethe initial use of L, the structures of 1 and 2 reveal that L undergoes two unprecedented in situ structural changesafter coordination with the metal ion. In 1, L is transformedto the phosphine oxide derivative L1, while in 2, itis transformed to the phosphinic acid derivative L2. With the aidof the flexible ligand dib, the one-dimensional (1D) chain structureof 1 and three-dimensional (3D) supramolecular structureof 2 were constructed. For 1 and 2, especially in 1, there are abundant hydrogen bondsbetween the framework and the lattice water molecules, which are conduciveto the formation of excellent hydrophilic channels and the transportof protons. Electrochemical experimental results show that 1 and 2 exhibit high proton conductivities (sigma)of more than 10(-4) S cm(-1) overa wide temperature range of 303-353 K and low activation energies(E (a)) at 98% relative humidity. Comparedwith the sigma value of the initial ligand L (3.12 x 10(-5) S cm(-1)), the best sigma valuesof 1 and 2 are improved by 28 times and6 times, respectively, and E (a) is alsoreduced from 0.39 to 0.13 eV for 1 and 0.12 eV for 2. This is the first report of proton-conducting polymersbased on a phosphine-based carboxylate ligand, involving two in situstructural transitions of the ligand.