Construction of a Stable Ru-Re Hybrid System Based on Multifunctional MOF-253 for Efficient Photocatalytic CO Reduction.

Using the open N,N'-chelating sites of MOF-253 (Al(OH)(dcbpy), dcbpy = 2,2'-bipyridine-5,5'-dicarboxylic acid) to coordinate with Re(I), a linker anchored Re complex MOF-253-Re(CO)Cl active for photocatalytic CO reduction was obtained. Unlike the homogeneous bipyridine containing Re complexes which produce CO during photocatalytic CO reduction, formate was obtained as the main CO reduction product over the as-obtained MOF-253-Re(CO)Cl. The linker anchored MOF-253-Re(CO)Cl showed superior photocatalytic performance compared to its homogeneous counterpart since the usual formation of the bimolecular Re intermediate leading to the deactivation of the homogeneous Re complex was significantly inhibited in the MOF supported Re complex. To enhance its light absorption, a linker anchored Ru sensitizer was simultaneously constructed in MOF-253-Re(CO)Cl (Ru-MOF-253-Re). The total TON (TON is defined as mole of the evolved H, CO, and HCOO over per amount of Rhenium) for CO reduction (28.8 in 4 h) over the as-obtained Ru-MOF-253-Re system is comparable or even superior to most already reported Re carbonyl complexes featuring bpy ligands and the Ru-Re bimetallic supramolecular systems constructed via the covalent bond under similar reaction conditions. The enhanced photocatalytic CO reduction over the Ru-MOF-253-Re can be ascribed to the improved visible light absorption and the existence of an efficient photoinduced charge transfer from Ru sensitizer to Re catalytic center, as evidenced from the transient absorption studies. The use of MOF-253 as a metalloligand and support to assemble the Ru-Re system as well as a mediator to promote the charge transfer from Ru sensitizer to Re catalytic center resembles the construction of Ru-Re supramolecular structures using covalent bonds, but is more facile in preparation and provides more flexibility. This study demonstrates the possibility of using MOFs with open coordination sites as a platform for the construction of a stable multifunctional hybrid system for artificial photosynthesis.