Controlled modification of the inorganic and organic bricks in an Al-based MOF by direct and post-synthetic synthesis routes

Abstract

Four new porous CAU-1 derivatives CAU-1–NH₂ ([Al₄(OH)₂(OCH₃)₄(BDC–NH₂)₃]·xH₂O, BDC–NH₂²⁻ = aminoterephthalate), CAU-1–NH₂(OH) ([Al₄(OH)₆(BDC–NH₂)₃]·xH₂O), CAU-1–NHCH₃ ([Al₄(OH)₂(OCH₃)₄(BDC–NHCH₃)₃]·xH₂O) and CAU-1–NHCOCH₃ ([Al₄(OH)₂(OCH₃)₄(BDC–NHCOCH₃)₃]·xH₂O) all containing an octameric [Al₈(OH)_4+y(OCH₃)_8−y]¹²⁺ cluster, with y = 0–8, have been obtained by MW-assisted synthesis and post-synthetic modification. The inorganic as well as the organic unit can be modified. Heteronuclear ¹H–¹⁵N, ¹H–¹³C and homonuclear ¹H–¹H connectivities determined by solid-state NMR spectroscopy prove the methylation of the NH₂ groups when conventional heating is used. Varying reaction times and temperatures allow controlling the degree of methylation of the amino groups. Short reaction times lead to non-methylated CAU-1 (CAU-1–NH₂), while longer reaction times result in CAU-1–NHCH₃. CAU-1–NH₂ can be modified chemically by using acetic anhydride, and the acetamide derivative CAU-1–NHCOCH₃ is obtained. Thermal treatment permits us to change the composition of the Al-containing unit. Methoxy groups are gradually exchanged by hydroxy groups at 190 °C in air. Solid-state NMR spectra unequivocally demonstrate the presence of the amino groups, as well as the successful post-synthetic modification. Furthermore ¹H–¹H correlation spectra using homonuclear decoupling allow the orientation of the NHCOCH₃ groups within the pores to be unravelled. The influence of time and temperature on the synthesis of CAU-1 was studied by X-ray powder diffraction, elemental analyses, and ¹H liquid-state NMR and IR spectroscopy.