Synthesis and electronic coupling studies of cyclometalated diruthenium complexes bridged by 3,3′,5,5′-tetrakis(benzimidazol-2-yl)-biphenyl

Three cyclometalated diruthenium complexes bridged by 3,3′,5,5′-tetrakis(benzimidazol-2-yl)biphenyl (H-tbibp) and capped with different terminal ligands have been synthesized and examined. In addition, two monoruthenium complexes with H-tbibp have been prepared for the purpose of comparison studies. The degree of Ru-Ru electronic coupling of these diruthenium complexes has been investigated by electrochemical and intervalence charge-transfer (IVCT) analyses. These results suggest that when the same or similar terminal ligands are used, the strength of H-tbibp in mediating the Ru-Ru coupling is enhanced with respect to that of the previously reported bridging ligand 3,3′,5,5′-tetrakis( N -methylbenzimidazol-2-yl)biphenyl, but it is slightly inferior to that of the classical bridging ligand 3,3′,5,5′-tetrakis(pyrid-2-yl)biphenyl. This trend is also supported by CNS analyses based on the hole-superexchange mechanism. In addition, DFT calculations have been performed to probe the spin density distributions of the singly-oxidized diruthenium complexes with H-tbibp and TDDFT calculations are used to reproduce the IVCT transitions. Electrochemical and intervalence charge-transfer studies suggest that when the same or similar terminal ligands are used, the strength of the bridging ligand in mediating the Ru-Ru coupling decreases in the order of tpbp > H-tbibp > Me-tbibp.