Accumulative Charge Separation in a Modular Quaterpyridine Bridging Ligand Platform and Multielectron Transfer Photocatalysis of π-Linked Dinuclear Ir(III)-Re(I) Complex for CO Reduction.

Four sterically distorted quaterpyridyl () ligandbridged Ir(III)-Re(I) heterometallic complexes (, , , and ), in which the position of the coupling pyridine unit of the two 2,2'-bipyridine ligands was varied (meta ()- or para ()-position), pypy-pypy ( = and , ; = and , ; = and , ; = and , ), were prepared, along with the fully π-conjugated Ir(III)-[π linker]-Re(I) complexes (π linker = 2,2'-bipyrimidine (), ; π linker = 2,5-di(pyridin-2-yl)pyrazine (), ) to elucidate the electron mediating and accumulative charge separation properties of the bridging π-linker in a bimetallic system (photosensitizer-π linker-catalytic center). From the photophysical and electrochemical studies, it was found that the quaterpyridyl () bridging ligand (BL), in which the two planar Ir/Re metalated bipyridine (bpy) ligands were connected but slightly canted relative to each other, linking the heteroleptic Ir(III) photosensitizer, [(C^N)Ir(bpy)], and catalytic Re(I) complex, (bpy)Re(CO)Cl, minimized the energy lowering of the BL, which hampers the forward photoinduced electron transfer (PET) process from [(C^N)Ir(N^N)] to (N^N)Re(CO)Cl ( = -(0.85-0.93) V and = -(1.15-1.30) V vs SCE). This result contrasts with the fully π-delocalized bimetallic systems ( and ) that show a significant energy reduction due to the considerable π-extension and deshielding effect caused by the neighboring Lewis acidic metals (Ir and Re) on the electrochemical scale ( = -0.37 V and = -1.02 and -0.99 V vs SCE). Based on a series of anion absorption studies and spectroelectrochemical (SEC) analyses, all Ir(III)-BL-Re(I) bimetallic complexes were found to exist as dianionic form (Ir(III)-[BL]-Re(I)) after a fast reductive-quenching process in the presence of excess electron donor. In the photolysis experiment, the four complexes displayed the reasonable photochemical CO-to-CO conversion activities (TON of 366-588 for 19 h) owing to the moderated electronic coupling between two functional Ir(III) and Re(I) centers through the slightly distorted ligand, whereas and displayed negligible performances as a result of the strong electronic coupling via π-conjugation between the two functional components resulting in the energetic constraints for PET and an unwanted side reactions competing with the forward processes. These results confirm that the unit can be utilized as an efficient BL platform in π-linked bimetallic systems.