Binuclear and mononuclear active sites of porous tin-organic frameworks for selective epimerization of D-glucose into D-mannose

• Four tin-organic frameworks with different aromatic linkers (Ar) were synthesized. • Ar 4 Sn, (Ar 3 Sn) 2 O, n Bu x Ar (4-x) Sn, and SnO 2 active sites were detected. • Ar 4 Sn and (Ar 3 Sn) 2 O represent the active sites for the epimerization. • Bimetallic (Ar 3 Sn) 2 O sites exhibit the highest catalytic activity. • n Bu x Ar (4-x) Sn and SnO 2 are responsible for minor co-formation of D-fructose. Porous tin-organic frameworks (Sn-OFs) are composed of Lewis acid Sn 4+ sites connected via organic linkers by tin-carbon covalent bonds. Although high activity of Sn-OFs for selective epimerization of D-glucose into D-mannose has been reported, the roles of different active sites of the Sn-OFs in catalysis remain under debate. Herein, four Sn-OFs with different aromatic linkers were synthesized and characterized using elemental analysis, low-temperature N 2 adsorption, and SEM, along with ATR-IR, 1 H, 13 C, and 119 Sn MAS NMR, XANES and EXAFS spectroscopies. Comparative studies with organotin model molecules revealed tetraaryltin Ar 4 Sn as major structural element of the Sn-OFs with additional sites including dimers Ar 3 Sn O SnAr 3 , alkylaryltin sites n Bu x Ar (4-x) Sn, and SnO 2 . Catalytic tests of the respective organotin molecules suggest Ar 4 Sn and Ar 3 Sn O SnAr 3 as active sites for the epimerization, whereas the activity of the latter binuclear sites appears to be superior. In contrast, n Bu x Ar (4-x) Sn alkylaryltin sites and SnO 2 contributed to the formation of side products, converting D-glucose into D-fructose. The Sn-OFs with symmetrical linkers are more structured materials exhibiting higher catalytic activity than the Sn-OFs with asymmetrical linkers. These findings clarify the roles of various Sn sites in Sn-OFs, advancing understanding of their catalytic behavior.