Reticular chemistry approach to explore the catalytic CO2-epoxide cycloaddition reaction over tetrahedral coordination Lewis acidic sites in a Rutile-type Zinc-phosphonocarboxylate framework

The problem with metal–organic frameworks (MOFs) as CO2-epoxide cycloaddition catalysts is the ambiguous catalytic sites. No system can precisely modify the structures and thus optimize the catalytic performance. Reticular chemistry is recognized as an efficient approach to systematically control the chemical and structural features as well as the properties of porous, crystalline materials. In this work, a Zn-phosphonocarboxylate framework (ZnPC-2) was employed as a prototype structure to investigate the structure–property relationship (SPR). ZnPC-2 is a porous rutile-type framework constructed by 6-connected Zn3(PO3)2(CO2)4 secondary building unit (Zn3-SBU) and tritopic 5-phosphonobenzene-1,3-dicarboxylic acid (H4pbdc). ZnPC-2 showed the ability to catalyze the CO2 cycloaddition reactions and Zn3-SBU was proved as efficient catalytic site. Guided by reticular chemistry, a series of iso-reticular frameworks, including [email protected] (Hpyr = protonated pyrrolidine, Hpyr is fixed by hydrogen bonding interactions to participate the channel into segments); MZnPC-2 (M = Mg, Mn, Fe, Co, Cu; Zn(II) atoms on Zn3-SBU were partially replaced by M) and ZnPC-2-im (im = imidazole, im coordinates to the apical Zn(II) on Zn3-SBU), were designed and successfully synthesized to explore the precise active site. The catalytic performances over these iso-structures together with density functional theory calculations revealed that the dynamical transformation of the apical Zn (II) on Zn3-SBU from tetrahedral geometry to five coordinated model played the pivotal role during the catalytic process.