Carbon Dioxide Capture and Utilization with Isomeric Forms of Bis(amino)‐Tagged Zinc Bipyrazolate Metal–Organic Frameworks

Abstract

Aiming at extending the tagged zinc bipyrazolate metal-organic frameworks (MOFs) family, the ligand 3,3'-diamino-4,4'-bipyrazole (3,3'-H₂ L) has been synthesized in good yield. The reaction with zinc(II) acetate hydrate led to the related MOF Zn(3,3'-L). The compound is isostructural with its mono(amino) analogue Zn(BPZNH₂ ) and with Zn(3,5-L), its isomeric parent built with 3,5-diamino-4,4'-bipyrazole. The textural analysis has unveiled its micro-/mesoporous nature, with a BET area of 463 m² g^-1 . Its CO₂ adsorption capacity (17.4 wt. % CO₂ at p_CO2 = 1 bar and T = 298 K) and isosteric heat of adsorption (Q_st = 24.8 kJ mol^-1 ) are comparable to that of Zn(3,5-L). Both Zn(3,3'-L) and Zn(3,5-L) have been tested as heterogeneous catalysts in the reaction of CO₂ with the epoxides epichlorohydrin and epibromohydrin to give the corresponding cyclic carbonates at T = 393 K and p_CO2 = 5 bar under solvent- and co-catalyst-free conditions. In general, the conversions recorded are higher than those found for Zn(BPZNH₂ ), proving that the insertion of an extra amino tag in the pores is beneficial for the epoxidation catalysis. The best catalytic match has been observed for the Zn(3,5-L)/epichlorohydrin couple, with 64 % conversion and a TOF of 5.3 mmol(carbonate) (mmol_Zn )^-1 h^-1 . To gain better insights on the MOF-epoxide interaction, the crystal structure of the [epibromohydrin@Zn(3,3'-L)] adduct has been solved, confirming the existence of Br⋅⋅⋅(H)-N non-bonding interactions. To our knowledge, this study represents the first structural determination of a [epibromohydrin@MOF] adduct.