Structural Transitions and Flexibility during Dehydration−Rehydration Process in the MOF-type Aluminum Pyromellitate Al₂(OH)₂[C₁₀O₈H₂] (MIL-118)

Abstract

A new metal-organic framework (MOF)-type aluminum pyromellitate (MIL-118) has been hydrothermally synthesized in water at 210 °C for 24 h. The crystal structure of the as-synthesized phase (Al 2 (OH) 2 (H 2 O) 2 [C 10 O 8 H 2 ] or MIL-118A) has been solved from single-crystal analysis using synchrotron radiation with a specific microdiffraction setup at ESRF ID13 beamline. It consists of infinite chains of trans-connected aluminum-centered octahedra linked to each other through the pyromellitate ligand. The organic linker is not fully connected, and one carboxyl oxygen of two carboxylate functions is hydrogen bonded to the terminal water molecule attached to aluminum cations. Upon heating of the sample, the structure analysis (from powder X-ray diffraction data) showed that the terminal water is removed and the previously nonbonded carboxylate groups are now linked to aluminum reflecting a solid-state transition with the formation of Al-OCO bonding (Al 2 (OH) 2 [C 10 O 8 H 2 ] or MIL-118B). This dried phase is able to reversibly adsorb and desorb ambient water. This hydration process induces a second phase transition with the encapsulation of water molecules within the channels delimited by the infinite chains and the aromatic ligands. In the third hydrated phase (Al 2 (OH) 2 [C 10 O 8 H 2 ] • 2.75H 2 O or MIL-118C), whose structure was solved from powder X-ray diffraction data collected at ESRF ID31), the inorganic chains are shifted to each other with a ≈ b/6 translation along the b axis, corresponding to a tilt angle of 90°b etween the benzene rings and the chains, instead of 61°in the dried phase MIL-118B. A 36.5°rotation of the benzene rings is also observed along the c axis. The occurrence of such a flexible network (6% variation for cell volume) was fully characterized by X-ray thermodiffraction, thermogravimetric analysis, and solid-state NMR ( 27 Al, 1 H).

Solid-State NMR Spectroscopy of Metal–Organic Framework Compounds (MOFs)

“…Such techniques can, e.g., be used to determine the spatial proximity and orientation of the linkers relative to each other as well as with respect to adsorbed species (see below). Methods such as 1 H- 1 H homonuclear dipolar recoupling back-to-back (BABA) and 1 H– 1 H radio frequency dipolar recoupling (RFDR) were shown to be well suited for such purposes [63,107,108]. …”

“…The coordination environment can be detected using chemical shifts and/or quadrupolar coupling constants for metal atoms such as 27 Al [107,108,144], 71 Ga [44], 45 Sc [46,132], and 6 Li [145]. Furthermore, metal atoms outside the MOF lattice, i.e.…”

Thermally induced polymerization of binuclear [Ni2(en)2(H2O)6(pyr)]·4H2O complex

“…Water molecules, creating a network of hydrogen bonds both in (-102) plane and in between these planes, accounts for around 53% of the internal volume, according to Bader analysis, suggesting that dehydration should be accompanied by a significant structural reorganization. XRD diffractogram of the dehydrated product indicates supramolecular structure [49], with pattern similar to organic polymers [50] and significant differences compared to the initial complex ( Table 2), suggesting that polymerization occurs during dehydration of the initial complex. Structural transformations cause contraction of the unit cell and decrease in its volume, however, due to the loss of 27% of the initial mass, the density remains almost the same.…”

Synthesis, molecular and supramolecular structure of a new dinuclear aluminium(III) complex derived from 3-aminopyrazine- 2-carboxylic acid

“…A remarkable progress has been made in recent years in the development of metal -organic networks by using aluminium(III) ions and carboxylic acid ligands as building blocks [20 -23] . Few Al(III)-based metal organic frameworks with carboxylate linkers, such as those derived from terephthalic acid [24,25] , 2-aminoterephthalic acid [26] , benzene tricarboxylic acid [27,28] and benzene tetracarboxylic acid [29,30] , have been reported. Hydroxo [31,32] and methoxy [33] bridged dinuclear aluminium(III) complexes of Schiff base ligands are also known.…”