Metal-Assisted Salphen Organic Frameworks (MaSOFs) with Trinuclear Metal Units for Synergic Gas Sorption

Abstract

Metal-assisted salphen organic frameworks (MaSOFs) are known to possess high affinities to CO2 due to Lewis acidic metal sites and are therefore able to selectively adsorb CO2 over CH4 or N2. By aligning two metal centers in a carefully designed geometry, a “single molecular trap” (SMT) effect is generated, resulting in an interaction of two metal centers with one molecule CO2 by synergic effects. A condensation of a rigid triptycene based trissalicylaldehyde with tetrammino benzene is used to realize these metal alignments into MaSOFs. Characterization of the discrete trinuclear complexes proves that the chosen geometry is nearly optimal for synergic CO2 adsorption. The corresponding MaSOFs show high selectivities of CO2 against CH4 with a selectivity S IAST (according to the Ideal Adsorbed Solution Theory) of up to 13 and a selectivity of S IAST up to 70 against N2, which are also reflected by isosteric heat of adsorptions (Q st) of up to 35 kJ/mol. Density functional theory (DFT) calculations support the hypothesis by geometry optimized models and furthermore show a positive cooperative effect by an energy gain of ∼14 kJ/mol during the adsorption of CO2 in the second binding pocket of the trinuclear metal–salphen compared to a monomolecular adsorption.

Metal Salen- and Salphen-Containing Organic Polymers: Synthesis and Applications

“…As it has been mentioned above, the sheets of the 2D salphen and salen COFs pack in an eclipsed fashion, thus in theory making the metal centers inaccessible for substrates of any kind. To avoid such a packing in combination with an ideal close arrangement of salphen metal centers, we synthesized Ni 3 -and Cu 3 -MaSOFs (Scheme 11) 115 based on a triptycene trissalicylaldehyde. 116 Indeed, by single-crystal X-ray diffraction (SCXRD) structure analyses of discrete trinuclear model complexes (Scheme 11, right), metal-to-metal distances of d M-M ¼ 6.7 Å were found, which according to Zhang should be ideal to bind CO 2 synergically.…”

“…This value is only outperformed by salphen networks designed for gas sorption (Table 2). 118c, 115,123,127 For example, one of the first porous MaSOFs, MaSOF-2, has already been reported to have an uptake of 2.23 mmol/g (9.8 wt.%) of CO 2 at 273 K and 1 bar. 123 In 2018 the Mastalerz group introduced the next generation of MaSOFs based on a triptycene hexakissalicylaldehyde.…”

“…127 For all MaSOFs in these series, high Henry-and ideal adsorbed solution theory (IAST) selectivities of CO 2 against CH 4 or N 2 were determined with Pt-MaSOF 100 having the highest selectivity for CO 2 /N 2 of S IAST ¼ 86 at 0.1 bar at 273 K. 127 This value is one of the highest reported for all polysalphens, higher than those found in the MsMOP-M series (S IDEAL up to 46) 118c and only outperformed by Ni 3 -MaSOF as (S IAST ¼ 89 (CO 2 /N 2 ) at 0.1 bar; see also discussion below). 115 The next evolutionary step in the construction of MaSOFs for gas sorption was the development of the M 3 -MaSOFs (M ¼ Cu and Ni). 115 Based on previous results in the area of MOFs, 117,137 the idea was to generate networks that contain trinuclear metal units, where the metals are in close proximity to synergically bind CO 2 .…”

“…115 The next evolutionary step in the construction of MaSOFs for gas sorption was the development of the M 3 -MaSOFs (M ¼ Cu and Ni). 115 Based on previous results in the area of MOFs, 117,137 the idea was to generate networks that contain trinuclear metal units, where the metals are in close proximity to synergically bind CO 2 . Indeed, model complexes of these trinuclear units revealed metal-metal distances (d ¼ 6.65-6.74 Å) that should be ideal for CO 2 sorption (see above; Scheme 11, Table 2, and Figure 6).…”

“…Indeed, model complexes of these trinuclear units revealed metal-metal distances (d ¼ 6.65-6.74 Å) that should be ideal for CO 2 sorption (see above; Scheme 11, Table 2, and Figure 6). 115 For Ni 3 -MaSOF as , an isosteric heat of adsorption of Q st ¼ 35.6 kJ/mol ( Figure 6 and Figure 7) was measured, which is comparable to the values of some of the bestperforming MOFs such as Ni-MOF-74 (Q st ¼ 37-42 kJ/mol), 138 HKUST-1 (Q st ¼ 15-35 kJ/mol), 139 or MIL-120 (Q st ¼ 38 kJ/mol). 140 The resulting selectivities are S IAST ¼ 16 (CO 2 /CH 4 ) and S IAST ¼ 89 (CO 2 /N 2 ) for Ni 3 -MaSOF as at 0.1 bar and 273 K (Table 2 and Figure 7).…”