Exploring the Scope of Macrocyclic "Shoe-last" Templates in the Mechanochemical Synthesis of RHO Topology Zeolitic Imidazolate Frameworks (ZIFs)

Abstract

The macrocyclic cavitand MeMeCH₂ is used as a template for the mechanochemical synthesis of 0.2MeMeCH₂@RHO-Zn₁₆(Cl₂Im)₃₂ (0.2MeMeCH₂@ZIF-71) and RHO-ZnBIm₂ (ZIF-11) zeolitic imidazolate frameworks (ZIFs). It is shown that MeMeCH₂ significantly accelerates the mechanochemical synthesis, providing high porosity products (BET surface areas of 1140 m²/g and 869 m²/g, respectively). Templation of RHO-topology ZIF frameworks constructed of linkers larger than benzimidazole (HBIm) was unsuccessful. It is also shown that cavitands other than MeMeCH₂-namely MeHCH₂, MeiBuCH₂, HPhCH₂, MePhCH₂, BrPhCH₂, BrC5CH₂-can serve as effective templates for the synthesis of x(cavitand)@RHO-ZnIm₂ products. The limitations on cavitand size and shape are explored in terms of their effectiveness as templates.

Keywords: mechanochemical synthesis; microporous materials; templation; zeolitic imidazolate frameworks.

Figure 1

(a) Schematic representation of the “shoe-last” templating approach. (b) Cavitand naming scheme illustrating the rccc forms of RR’Y-n (where omitted, n = 4). (c) Top-down and side views of the general MeMeCH₂@d8r motif. MeMeCH₂ is bound via eight C-H···O hydrogen bonds (green dashes). (d) Crystal structure of MeMeCH₂@RHO-Zn₁₆(Im)₃₂∙solvent, illustrating the fused empty and occupied d8rs with the empty d8r shown from the top and filled from the side (left) and vice-versa (right). Relevant θ angles of different 8r imidazoles are pointed out. (e) The crystal structure of MeMeCH₂@RHO-ZnIm₂⋅x(solvent) as a ball-and-stick representation. Half of the d8rs are occupied by a (disordered) MeMeCH₂ cavitand, shown in spacefill. The disorder is represented by green and black color on one of the cavitand molecules.

Figure 2

Exploration of the scope of the templation of the RHO topological form of zinc imidazolates, using different cavitands (see Table 1) and ligands.

Figure 3

Synthesis and characterization of 0.2MeMeCH₂@RHO-Zn₁₆(Cl₂Im)₃₂ (0.2MeMeCH₂@ZIF-71). (a) The 77 K low pressure N₂ isotherm. (b) The crystal structure of RHO-Zn(Cl₂Im)₂ (CSD code GITVIP) as viewed down the [001] axis (the d8r motif is shaded blue). (c,d) Side and top views of the d8r motif of RHO-Zn(Cl₂Im)₂ (shaded blue). (e) Powder X-Ray diffraction (PXRD) patterns of the non-templated liquid-assisted grinding (LAG)-aging reaction of ZnO and HCl₂Im after (i) 3 days and (ii) 12 days; the MeMeCH₂ templated LAG-aging reaction of ZnO and HCl₂Im after (iii) 1 day and (iv) 3 days; the templated large scale (≈1.7 g) LAG-aging reaction of ZnO and HCl₂Im (v) after 3 days, and (vi) the washed 0.2MeMeCH₂@RHO-Zn₁₆(Cl₂Im)₃₂ final product after 4 days, and (vii) the same material after activation and 77 K low pressure N₂ sorption analysis. (viii) Simulated PXRD pattern of MeMeCH₂⋅2DEF (ix) PXRD pattern of nanoparticulate ZnO. Tick marks denote predicted peak positions of the RHO-Zn(Cl₂Im)₂ (ZIF-71, GITVIP).

Figure 4

Synthesis and characterization of RHO-ZnBIm₂ (ZIF-11). (a) The 77 K low pressure N₂ isotherm. (b) The crystal structure of RHO-ZnBIm₂ (CSD code VEJZOA) as viewed down the [001] axis (the d8r motif is shaded blue). (c,d) Side and top views of the d8r motif of RHO-ZnBIm₂ (shaded blue). (e) PXRD patterns of the non-templated LAG-aging reaction of ZnO and HBIm after (i) 3 days, and (ii) 12 days; the MeMeCH₂ templated LAG-aging reaction of ZnO and HBIm after (iii) 3 days, (iv) 12 days, (v) the washed RHO-ZnBIm₂ (ZIF-11) final product after 45 days, and (vi) the same material after activation and 77 K low pressure N₂ sorption analysis. (vii) Simulated PXRD pattern of MeMeCH₂⋅2DEF. (viii) PXRD pattern of nanoparticulate ZnO. Tick marks denote predicted peak positions of RHO-ZnBIm₂ (ZIF-11, VEJZOA).

Figure 5

Experimental PXRD patterns of LAG-aging syntheses using nanoparticulate ZnO, imidazole, DEF, and selected cavitands, washed after t days. (i) cavitand = MeHCH₂, t = 16 d, (ii) cavitand = MeiBuCH₂, t = 16 d, (iii) cavitand = BrPhCH₂, t = 7 d, (iv) cavitand = BrC5CH₂, t = 5 d, (v) HPhCH_2, t = 12 d, (vi) cavitand = MePhCH₂ (rctt:rccc = 87:13), t = 5 d, (vii) cavitand = MeHCH₂-6, t = 5 d, (viii) MeHSiMe₂, t = 11 d, (ix) H(4MePh)CH₂, t = 11 d, and (xii) nanoparticulate ZnO. Simulated PXRD patterns of (x) cag-ZnIm₂ (VEJYUF) and (xi) nog-ZnIm₂ (HIFWAV). Tick marks denote predicted peak positions of 0.90MeMeCH₂@RHO-Zn₁₆Im₃₂. Stars denote small amounts of impurities that are not defined in Table 1.

Figure 6

Comparison of the crystal structures of MeHCH₂ (left; CSD code VUWMIL [56]) and MeHCH₂-6 (right; CSD code QUCWOA [55], chloroform solvate, solvent molecules omitted for clarity), illustrating the similarities of a portion of their molecular structures. (a) top view, (b) side view.