Accumulative Charge Separation in a Modular Quaterpyridine Bridging Ligand Platform and Multielectron Transfer Photocatalysis of pi-Linked Dinuclear Ir(III)-Re(I) Complex for CO2 Reduction

초록

Four sterically distorted quaterpyridyl (qpy) ligand-bridged Ir-(III)-Re-(I) heterometalliccomplexes (Ir-qpy ( mm ) -Re, Ir-qpy ( mp ) -Re, Ir-qpy ( pm ) -Re, and Ir-qpy ( pp ) -Re), inwhich the position of the coupling pyridine unit of the two 2,2 '-bipyridineligands 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 pi-conjugated Ir-(III)-[pi linker]-Re-(I)complexes (pi linker = 2,2 '-bipyrimidine (bpm), Ir-bpm-Re; pi linker = 2,5-di-(pyridin-2-yl)-pyrazine(dpp), Ir-dpp-Re) to elucidate the electronmediating and accumulative charge separation properties of the bridging pi-linker in a bimetallic system (photosensitizer-pi linker-catalyticcenter). From the photophysical and electrochemical studies, it wasfound that the quaterpyridyl (qpy) bridging ligand (BL),in which the two planar Ir/Re metalated bipyridine (bpy) ligands wereconnected but slightly canted relative to each other, linking theheteroleptic Ir-(III) photosensitizer, [(C-piq<^>N)(2)Ir-III(bpy)](+), and catalytic Re-(I) complex,(bpy)-Re-I(CO)(3)Cl, minimized the energy loweringof the qpy BL, which hampers the forward photoinducedelectron transfer (PET) process from [(C-piq<^>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 pi-delocalizedbimetallic systems (Ir-bpm-Re and Ir-dpp-Re) that show a significant energy reduction due to the considerable pi-extension and deshielding effect caused by the neighboringLewis 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). Basedon a series of anion absorption studies and spectroelectrochemical(SEC) analyses, all Ir-(III)-BL-Re-(I) bimetallic complexes were foundto exist as dianionic form (Ir-(III)-[BL](2-)-Re-(I))after a fast reductive-quenching process in the presence of excesselectron donor. In the photolysis experiment, the four Ir-qpy-Re complexes displayed the reasonable photochemical CO2-to-COconversion activities (TON of 366-588 for 19 h) owing to themoderated 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 negligibleperformances as a result of the strong electronic coupling via pi-conjugationbetween the two functional components resulting in the energetic constraintsfor PET and an unwanted side reactions competing with the forwardprocesses. These results confirm that the qpy unit canbe utilized as an efficient BL platform in pi-linked bimetallicsystems. A distortion in the structure of fournew quaterpyripydyl(qpy) ligand-bridged Ir-(III)-Re-(I) complexes minimizedthe energy lowering of the pi-bridging ligand, which impededthe photoinduced electron transfer (PET) from the Ir-(III) photosensitizingunit to the Re-(I) catalytic center, exhibiting reasonable activitiesin converting CO2 to CO in a photolysis experiment withturnover numbers (TONs) of 366-480 over 19 h.

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