Synthesis and proton conducting ability of a crystalline 3-D vanadoborate with [V6B20] architectures

Abstract

A new crystalline three-dimensional (3-D) vanadoborate (H₂en)₄[(VO)₆(B₁₀O₂₂H₄)₂(H₂O)]·4H₂O (1, H₂en = NH₃CH₂CH₂NH₃), based on the lantern-type [(VO)₆(B₁₀O₂₂H₄)₂(H₂O)] cluster (denoted as [V₆B₂₀]), was successfully synthesized employing a hydrothermal reaction. In the structure, H₂en²⁺ cations and H₂O molecules are accommodated in the intercluster space of the framework and interact with the [V₆B₂₀] polyanions via hydrogen bonds (N–H⋯O and O–H⋯O), through which the respective molecular species are integrated into 3-D architectures. The presence of adequate guest ions and molecules (i.e., H₂en²⁺ and H₂O), and abundant H-bonding networks in the resulting channels imparts humidity- and temperature-dependent proton conducting ability to compound 1. Strikingly, at 333 K and 100% RH, compound 1 exhibits a good proton conductivity of 8.82 × 10⁻⁴ S cm⁻¹, with the magnitude comparable to that of other excellent MOF-based proton conductors. A high activation energy of 0.48 eV for 1 indicates that the proton migration in this compound is mainly governed by vehicle mechanism, combined with the synergic Grotthuss mechanism.