Photoluminescent Lanthanide−Organic 2D Networks: A Combined Synchrotron Powder X-ray Diffraction and Solid-State NMR Study

Abstract

A series of two-dimensional lanthanide−organic materials (LnOFs), [Ln(H3NMP)]·1.5H2O [Ln3+ = La3+ (1), Pr3+ (2), Nd3+ (3), Sm3+ (4), or Eu3+ (5); H3NMP3- is a residue of nitrilotris(methylenephosphonic acid)], has been isolated as microcrystalline powders from hydrothermal synthesis and characterized by high-resolution laboratory and synchrotron powder X-ray diffraction (PXRD), solid-state NMR, FTIR and FT Raman spectroscopies, CHN elemental analysis, thermogravimetry, scanning electron microscopy, and energy dispersive analysis of X-ray spectroscopy. The crystal structure of [Pr(H3NMP)]·1.5H2O (2) has been solved from a combined study of ab initio methods using high-resolution PXRD data and high-resolution solid-state NMR techniques performed on [La(H3NMP)]·1.5H2O (1) (13C, 15N, 31P CPMAS and 2D 1H−1H/31P HOMCOR/HETCOR). The structure contains a single lanthanide center which does not have water molecules in its first coordination sphere. This Ln3+ center acts as the node of a neutral undulated two-dimensional network, ∞2[Pr(H3NMP)], having a (4,4) topology, which close packs along the [100] direction of the unit cell (adjacent layers are related by inversion). Water molecules of crystallization occupy the interlayer spaces, and a one-dimensional water cluster (spiral chain topology) is confined to the channels formed by the packing of adjacent layers. Removal of these water molecules (investigated by variable-temperature PXRD) leads to a new crystalline phase with a smaller interlayer space. The material partially reabsorbs water from the surrounding environment, originating the parent phase. This dehydration/rehydration process has also been monitored by photoluminescence (PL) spectroscopy, revealing that the presence of water in the interlayer spaces does not affect much the PL properties of [Eu(H3NMP)]·1.5H2O (5).