Solid-state NMR study of an open-framework aluminophosphate-oxalate hybrid

Abstract

Short-range order within a novel aluminophosphate-oxalate hybrid material with the unit cell formula CH2(NH3)CH(NH3)CH3Al4P6O20(OH)(4)(C2O4)H2O was studied by multinuclear solid-state NMR measurements. The C-13, Al-27, and P-31 MAS NMR spectra showed several resonance lines that could not be predicted by the single-crystal X-ray diffraction analysis. The underlying variety of carbon, aluminum, and phosphorus local environments was due to the nonuniform arrangement of the protonated 1,2-diaminopropane species and water molecules residing within the pores of the open-framework material. H-1-- gt C-13 CPMAS NNIR spectrum evidenced that the diaminopropane species either occupied two different positions or were present as two different isomers, which also gave rise to two slightly different deformations of one (P(3)O-4) of the three crystallographically distinct PO4 tetrahedra. One-dimensional H-1 and P-31 MAS and 2D H-1-- gt P-31 CPMAS spectra revealed that the water molecule, occupying a well-defined position within the pore with about 42% abundance, attracted H(2) from the hydroxyl group attached to the P(3) atom and involved the hydrogen atom into a strong hydrogen bond. The displacement of H(2) due to the interaction with the water molecule was determined by measuring H(2)-P(3) distances using H-1-- gt P-31 variable-contact-time CPMAS NMR experiments. As evidenced by Al-27 MAS and 3QMAS NMR spectra, the deformation of the P(3)O-4 tetrahedra by water and diaminopropane species further affected also local environments of the two proximal crystallographically inequivalent aluminum sites Al(1) and Al(2).