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Ru(ii)-Re(i) binuclear photocatalysts connected by -CH2XCH2- (X = O, S, CH2) for CO2 reduction.

ABSTRACT: We developed Ru(ii)-Re(i) supramolecular photocatalysts in which each metal complex unit is connected by a -CH2XCH2- (X = O, S, CH2) chain. The photocatalyst with X = O exhibited the best photocatalytic efficiency for CO2 reduction in the reported systems using a NAD(P)H model compound as an electron donor because the introduced oxygen atom strengthened the oxidation power of the Ru photosensitizer unit in the excited state and accelerated electron transfer from the one-electron-reduced Ru photosensitizer unit to the Re catalyst unit. In contrast, the catalytic ability of the photocatalyst with X = S rapidly decreased during irradiation because the supramolecular structure split into mononuclear complexes. A detailed mechanism for the efficient photocatalytic reaction involving these supramolecular photocatalysts was investigated for the first time.

SUBMITTER: Kato E

PROVIDER: S-EPMC5729417 | biostudies-literature | 2015 May

REPOSITORIES: biostudies-literature

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Ru(ii)-Re(i) binuclear photocatalysts connected by -CH2XCH2- (X = O, S, CH2) for CO2 reduction.

Kato Eishiro E Takeda Hiroyuki H Koike Kazuhide K Ohkubo Kei K Ishitani Osamu O

Chemical science 20150302 5

We developed Ru(ii)-Re(i) supramolecular photocatalysts in which each metal complex unit is connected by a -CH2XCH2- (X = O, S, CH2) chain. The photocatalyst with X = O exhibited the best photocatalytic efficiency for CO2 reduction in the reported systems using a NAD(P)H model compound as an electron donor because the introduced oxygen atom strengthened the oxidation power of the Ru photosensitizer unit in the excited state and accelerated elect ...[more]

PMID: 29403638

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Project description:We have designed and synthesized a new tris-chelating polypyridine ligand (bpy3Ph) suitable to be used as a bridging ligand (BL) for constructing various supramolecular photocatalysts. This BL is a phenylene ring with three ethylene chains at 1, 3, and 5 positions, of which the other terminals are connected to 2,2'-bipyridine moieties. The ligand bpy3Ph has been used to prepare, according to a multi-step synthetic protocol, trinuclear supramolecular photocatalysts containing different metal subunits. In particular, the compounds Ru2Re and RuRe2 have been prepared, containing different ratios of components based on Ru(dmb)3 2+-type and Re(dmb)(CO)3Cl-type units (dmb = 4,4'-dimethyl-2,2'-bipyridine), which can play the roles of photosensitizers and catalyst units for photocatalytic CO2 reduction, respectively. The trinuclear model Ru3 and mononuclear and dinuclear Ru and Ru2 precursor metal complexes, containing free chelating sites, have also been synthesized using the same bridging ligand. The absorption spectra, redox behaviour and photophysical properties of the new species indicate that there is no strong electronic interaction among the Ru and Re units. The trinuclear complexes Ru2Re and RuRe2 could photocatalyze CO2 reduction to CO with high selectivity (up to 97%), high efficiency (Φ COs of 28% and 25%, respectively: BIH as a reductant), and high durability (TONCOs of 5232 and 6038, respectively: BIH as a reductant) which are the largest TONs for CO2 reduction using supramolecular photocatalysts in homogeneous solutions. The absence of negligible accumulation of the mono-reduced form of the photosensitizer indicates fast electron transfer to the catalyst unit(s) through the relatively large bridging ligand and is proposed to contribute to the outstanding photocatalytic properties of the new species, including their durability. The relevant photocatalytic behaviour of the new systems indicates new avenues for the design of extended bridging ligands capable of efficiently and functionally integrating photosensitizers and catalysts towards the preparation of new, larger supramolecular photocatalysts for selective CO2 reduction.

Project description:Supramolecular photocatalysts in which Ru(ii) photosensitizer and Re(i) catalyst units are connected to each other by an ethylene linker are among the best known, most effective and durable photocatalytic systems for CO2 reduction. In this paper we report, for the first time, time-resolved infrared (TRIR) spectra of three of these binuclear complexes to uncover why the catalysts function so efficiently. Selective excitation of the Ru unit with a 532 nm laser pulse induces slow intramolecular electron transfer from the 3MLCT excited state of the Ru unit to the Re unit, with rate constants of (1.0-1.1) × 104 s-1 as a major component and (3.5-4.3) × 106 s-1 as a minor component, in acetonitrile. The produced charge-separated state has a long lifetime, with charge recombination rate constants of only (6.5-8.4) × 104 s-1. Thus, although it has a large driving force (-?G0CR ? 2.6 eV), this process is in the Marcus inverted region. On the other hand, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH), reductive quenching of the excited Ru unit proceeds much faster (kq[BNAH (0.2 M)] = (3.5-3.8) × 106 s-1) than the abovementioned intramolecular oxidative quenching, producing the one-electron-reduced species (OERS) of the Ru unit. Nanosecond TRIR data clearly show that intramolecular electron transfer from the OERS of the Ru unit to the Re unit (kET > 2 × 107 s-1) is much faster than from the excited state of the Ru unit, and that it is also faster than the reductive quenching process of the excited Ru unit by BNAH. To measure the exact value of kET, picosecond TRIR spectroscopy and a stronger reductant were used. Thus, in the case of the binuclear complex with tri(p-fluorophenyl)phosphine ligands (RuRe(FPh)), for which intramolecular electron transfer is expected to be the fastest among the three binuclear complexes, in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), kET was measured as kET = (1.4 ± 0.1) × 109 s-1. This clearly shows that intramolecular electron transfer in these RuRe binuclear supramolecular photocatalysts is not the rate-determining process in the photocatalytic reduction of CO2, which is one of the main reasons why they work so efficiently.

Dataset Information

Ru(ii)-Re(i) binuclear photocatalysts connected by -CH2XCH2- (X = O, S, CH2) for CO2 reduction.

Publications

Ru(ii)-Re(i) binuclear photocatalysts connected by -CH<sub>2</sub>XCH<sub>2</sub>- (X = O, S, CH<sub>2</sub>) for CO<sub>2</sub> reduction.

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