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 2 Reduction.

Four sterically distorted quaterpyridyl ( qpy ) ligand - bridged Ir(III)-Re(I) heterometallic complexes ( Ir-qpy mm -Re , Ir-qpy mp -Re , Ir-qpy pm -Re , and Ir-qpy pp -Re ), in which the position of the coupling pyridine unit of the two 2,2'-bipyridine ligands was varied (meta ( m )- or para ( p )-position), pypy x -py x py ( x = m and m , qpy mm ; x = m and p , qpy mp ; x = p and m , qpy pm ; x = p and p , qpy pp ), were prepared, along with the fully π-conjugated Ir(III)-[π linker]-Re(I) complexes (π linker = 2,2'-bipyrimidine ( bpm ), Ir-bpm-Re ; π linker = 2,5-di(pyridin-2-yl)pyrazine ( dpp ), Ir-dpp-Re ) 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 ( qpy ) 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, [( piq C^N) 2 Ir III (bpy)] + , and catalytic Re(I) complex, (bpy)Re I (CO) 3 Cl, minimized the energy lowering of the qpy BL, which hampers the forward photoinduced electron transfer (PET) process from [( piq C^N) 2 Ir III (N^N)] + to (N^N)Re I (CO) 3 Cl ( E red1 = -(0.85-0.93) V and E red2 = -(1.15-1.30) V vs SCE). This result contrasts with the fully π-delocalized bimetallic systems ( Ir-bpm-Re and Ir-dpp-Re ) 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 ( E red1 = -0.37 V and E red2 = -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] 2- -Re(I)) after a fast reductive-quenching process in the presence of excess electron donor. In the photolysis experiment, the four Ir-qpy-Re complexes displayed the reasonable photochemical CO 2 -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 qpy ligand, whereas Ir-bpm-Re and Ir-dpp-Re 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 qpy unit can be utilized as an efficient BL platform in π-linked bimetallic systems.