Recently, the research group led by Weimin Xuan at the College of Chemistry and Chemical Engineering has achieved an important breakthrough in the high-order assembly of giant polyoxometalates and proton conduction. The related research was published in the Journal of the American Chemical Society (JACS) under the title "Dimensional Evolution from a Giant Molybdenum-Red Cage-like {Mo200} to 1D Chains Enabling Ultrahigh Proton Conduction".
Giant cage-like polyoxometalates (POMs) with confined cavities and open windows not only provide abundant metal and oxygen sites but also serve as excellent ion-transport containers, holding great potential in energy-conversion fields such as proton conduction. However, how to use these bulky hollow cage-like clusters as building blocks to assemble into high-dimensional, long-range-ordered continuous networks across steric hindrance has remained a highly challenging problem in POM assembly chemistry. Meanwhile, increasing the dimensionality helps enhance the continuity of proton-transport channels, thereby opening a viable pathway for developing superprotonic conductors.
To address the above synthetic challenge, the team led by Specially Appointed Researcher Weimin Xuan successfully synthesized an unprecedented 204-nuclearity hollow nanocage, compound 1 {Mo8}0.5@{Mo200}, together with its one-dimensional chain derivative, compound 2 [{Mo8}0.5@{Mo198}{Mo8}]n, by introducing phenylphosphonic acid as a structure-directing agent. During the assembly of compound 1, the organophosphonate ligands induced the formation of novel V-shaped {Mo5L} and {Mo5L}* building blocks. Owing to the non-equivalence of these two units and the synergistic templating effect of the {Mo8} guest inside the cavity, the symmetry of the entire giant framework is broken from the ideal pseudo-D5d to C2h, forming a "drum-shaped" molecular cage that deviates from the conventional fullerene-like topology. The structure possesses a huge internal cavity of approximately 2.2 nm × 1.5 nm, belonging to the extremely rare "molybdenum-red" family.
Figure 1. Structure of the giant cage-like molybdenum-red cluster {Mo8}0.5@{Mo200}.
By tuning the synthetic conditions, the two sets of {Mo5L}* units in the equatorial region of compound 1 undergo adaptive coordination adjustments, detaching two {Mo1} tail units and converting the {Mo200} cage framework into a {Mo198} intermediate with open connection sites. Subsequently, the {Mo8} cluster, acting as a two-connecting inorganic ligand, undergoes precise directional assembly with the {Mo198} nodes, thereby driving the transition of the structure from 0D discrete cages to 1D chain architectures. This work, for the first time, uses giant clusters as nodes to construct multidimensional frameworks through a "cluster-connected-to-cluster" strategy, breaking through the limitations of conventional dimensional evolution of clusters and providing a new paradigm for the design of high-order topologies.
Figure 2. Structure of the one-dimensional chain [{Mo8}0.5@{Mo198}{Mo8}]n.
Figure 3. Proton conductivities and activation energies of compounds 1 and 2, and in situ infrared spectra.
Benefiting from the abundant surface oxygen sites, the huge internal confined cavity, and the open windows at the top and bottom ends, the aforementioned structures seamlessly connect the internally enriched water molecules with the external environment, spontaneously forming a multiscale hydrated hydrogen-bonding network connecting the interior and exterior. AC impedance measurements show that under 80 °C and 98% relative humidity, the discrete cage-like compound 1 exhibits a high proton conductivity of 8.28 × 10⁻² S cm⁻¹. In the one-dimensional continuous chain structure, numerous extended Mo–O–Mo bridging bonds effectively stitch together the physical gaps between clusters, significantly lowering the energy barrier for proton hopping and boosting the proton conductivity of compound 2 to an ultrahigh 1.28 × 10⁻¹ S cm⁻¹, approximately 1.5 times that of compound 1. This performance ranks among the best POM-based solid-state proton conductors. This work not only greatly enriches the structural library of giant molybdenum-red clusters, but also lays a solid theoretical and experimental foundation for the rational design and development of high-performance solid-state proton conductor materials and next-generation clean-energy conversion devices.
Specially Appointed Researcher Weimin Xuan of Donghua University, Professor Yongge Wei of Tsinghua University, and Professors Leroy Cronin and De-Liang Long of the University of Glasgow are the co-corresponding authors, with Donghua University as the first corresponding affiliation.
Paper link: https://doi.org/10.1021/jacs.6c06499
