Multifunctional lanthanum tetraphosphonates: Flexible, ultramicroporous and proton-conducting hybrid frameworks

Abstract

A new flexible ultramicroporous solid, La(H₅DTMP)·7H₂O (1), has been crystallized at room temperature using the tetraphosphonic acid H₈DTMP, hexamethylenediamine-N,N,N′,N′-tetrakis(methylenephosphonic acid). Its crystal structure, solved by synchrotron powder X-ray diffraction, is characterised by a 3D pillared open-framework containing 1D channels filled with water. Upon dehydration, a new related crystalline phase, La(H₅DTMP) (2) is formed. Partial rehydration of 2 led to La(H₅DTMP)·2H₂O (3). These new phases contain highly corrugated layers showing different degrees of conformational flexibility of the long organic chain. The combination of the structural study and the gas adsorption characterization (N₂ and CO₂) suggests an ultramicroporous flexible framework. NO isotherms are indicative of a strong irreversible adsorption of NO within the pores. Impedance data indicates that 1 is a proton-conductor with a conductivity of 8 × 10⁻³ S cm⁻¹ at 297 K and 98% of relative humidity, and an activation energy of 0.25 eV.