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

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.