Synergistic interaction of Re complex and amine functionalized multiple ligands in metal-organic frameworks for...

A metal-organic framework (MOF) is composed of secondary building units (SBUs) of metal ions and organic ligands to link each SBU. Moreover, the photosynthetic synthesis of a valuable CO chemical from carbon dioxide (CO₂) represents an important class of appealing methods. Herein, we find that a molecular photocatalyst with high selectivity and activity can be designed via a fine balance in the proximity of Re complex (ReI(CO)₃(BPYDC)(Cl), BPYDC = 2,2'-bipyridine-5,5'-dicarboxylate) and -NH₂ functionalized multiple ligands composing a MOF photocatalyst, denoted as Re-MOF-NH₂. These ligands in Re-MOF-NH₂ has been confirmed by infrared, UV-visible, and ¹H nuclear magnetic resonance spectra. Moreover, we show from extended X-ray absorption fine structure and in-situ infrared spectra that the bond corresponding to Re-CO upon introduction of -NH₂ functional groups is divided into asymmetric bonds of 1.4 Å and 2.3 Å along with different CO₂ vibrations, thus making the configuration of carbonyl groups in a Re metal complex become asymmetric in addition to aiding formation of CO₂ intermediates within Re-MOF-NH₂. Indeed, both of the uneven electron distribution in asymmetric carbonyl groups for Re-CO and the intermolecular stabilization of carbamate intermediates are proven to give the approximately 3-fold increase in photocatalytic activity for conversion of CO₂ into CO.