Small Crown-Ether Complexes as Molecular Models for Dihydrogen Adsorption in Undercoordinated Extraframework Cations in Zeolites

Abstract

1:1 metal complexes of small crown-ethers are structurally similar to extraframework sites in metal-exchanged zeolites. Using ab initio calculations, we show that adsorbed molecular hydrogen follows the same trends in adsorption energies and vibrational frequencies at both types of metal sites. Unlike zeolites, crown-ethers can be characterized in the gas phase, which opens new possibilities for understanding the bonding of dihydrogen at undercoordinated metal sites to help guide the rational design of porous materials for hydrogen isotope separation. Because more strongly binding adsorbates affect the geometry of the hosts, the similarity of crown-ethers and zeolites with regard to the vibrational spectra of the adsorbed molecule seems to be limited to H₂.

B12X11(H2)−: exploring the limits of isotopologue selectivity of hydrogen adsorption

“…Previous studies of undercoordinated metal sites show a substantial (albeit sublinear) increase of the adsorption zero-point energy with the adsorption energy in paddlewheel MOFs 16 and zeolites 17 (see also Fig. 2c ).…”

“…The dissociation behavior of H 2 at B 12 X 11 − is, however, unlike what has been observed in the case of undercoordinated metal sites. In a previous publication we have predicted particularly strong H 2 interaction at zeolitic Au + sites 17 and observed strong elongation of the H–H bond to the point of dissociation using local coupled-cluster calculations. However, dissociation was homolytic in that case and attempts to create structures with H + /H − polarization failed (geometry optimization reverted to structures with equivalent H atoms).…”

“…For all X investigated, the H–H bond is elongated by more than 9 pm, that is at least 3 pm more than for H 2 coordinated at (C 4 H 8 O 2 )Cu + (C 4 H 8 O 2 = 1,4-dioxane; calculated value from ref. 17 ). A comparison of the geometric parameters, which are listed in Table 1 , reveals that X = F is an outlier with a much longer H–H bond length and a shorter B 0 –H 2 distance, which has consequences for the thermodynamic properties and isotopologue selectivity as discussed below.…”

“…2b , the frequency shift of the H–H stretch vibration correlates almost linearly with the H–H distance, not only for different B 12 X 11 − , but also when expanding the dataset using data from different (C 4 H 8 O 2 )M + calculated in ref. 17 . Near-linearity holds true even across different classes of compounds and breaks down only at extreme elongations (H 2 at B 12 F 11 − and C 4 H 8 O 2 Au + ; the latter is not shown in Fig.…”

“…The partial positive charge at the undercoordinated boron atom would be expected to lead to strong H 2 attraction in a way similar to undercoordinated metal cations. 17 The significant interaction between the undercoordinated boron site and noble gas atoms reported by experiment and theory 21–24 suggests that this system will also strongly bind dihydrogen, and that gas-phase experiments addressing this interaction should be feasible. A particularly attractive feature of these systems is the prospect of modifying the substituents X in order to form a cavity that sterically confines the bound dihydrogen to further increase isotopologue selectivity.…”