Modeling Metal Influence on the Gate Opening in ZIF-8 Materials

Zeolitic imidazolate frameworks (ZIFs) undergo pressure-induced phase transitions that are peculiar to each ZIF. The phase transition is associated with the rotation of the imidazolate, and it is accompanied by an increase of their pore openings, affecting ZIFs performance in separation processes. This phenomenon is known as the gate opening or the swing effect. Here we report the metal dependence of the ambient pressure and high pressure (HP) phases of ZIF-8(M) with M = Mg, Fe, and Zn, determined by using periodic Kohn-Sham density functional models. We show that the substitution of Zn with Mg or Fe has a big influence on the gate opening energy, which significantly decreases, an opposite trend than what was previously reported upon functionalization of the linker. The lowest energy phase of ZIF-8(Fe) is different than for ZIF-8(Mg) and ZIF-8(Zn), and its structure is significantly closer to the HP phase. Multireference wave function methods have been used to study the electronic structure of ZIF-8(Fe), confirming the metal center to be high spin (S = 2) divalent iron in antiferromagnetic coupling. The high-spin nature of the iron species coupled with a band gap in the visible light range makes ZIF-8(Fe) an interesting material for catalysis and photocatalysis.

Automated summary

Zeolitic imidazolate frameworks (ZIFs) undergo pressure-induced phase transitions that are peculiar to each ZIF, and the metal dependence of the ambient pressure and high pressure phases of ZIF-8(M) with Mg, Fe, and Zn has been studied. The substitution of Zn with Mg or Fe influences the gate opening energy significantly, with ZIF-8(Fe) showing a different lowest energy phase structure compared to ZIF-8(Mg) and ZIF-8(Zn), which is closer to the high pressure phase. Additionally, ZIF-8(Fe) is confirmed to have high-spin divalent iron in antiferromagnetic coupling, making it a promising material for catalysis and photocatalysis due to its band gap in the visible light range.