Proton-Conducting 3D Polyoxoniobate Supramolecular Frameworks via Exclusive Guanidinium and Synergistic 4p–4f Metal-Oxo Clusters

A strategy focusing on framework stability and proton-conducting hydrogen-bonding networks in polyoxoniobates (PONbs) has been implemented, wherein protonated organic amines act as exclusive counter cations, synergistically enhanced by embedded 4p–4f metal-oxo clusters. This approach yields a new family of three-dimensional (3D) supramolecular frameworks, H 6 (CN 3 H 6 ) 8 [As 4 Ln 4 O 12 (H 2 O) 4 (CO 3 ) 4 (Nb 5 O 14 ) 2 ]·34H 2 O ( 1-Ln, Ln = Sm, Eu, Tb, Dy, Ho, Er, Yb), representing the first reported PONbs featuring a “double-tetrahedral, coaxially nested” 4p–4f metal-oxo cluster (As 4 Ln 4 ). Impedance measurements show that 1-Eu exhibits a proton conductivity of 1.21 × 10 –2 S·cm –1 at 85 °C and 98% RH. Water vapor adsorption reaches saturation at P / P 0 = 0.946 with a maximum uptake of 296.6 cm 3 ·g –1 . In addition, 1-Eu and 1-Tb display characteristic lanthanide-centered luminescence.