A series of rhenium tricarbonyl complexes coordinated by asymmetric diimine ligands containing a pyridine moiety bound to an oxazoline ring were synthesized, structurally and electrochemically characterized, and screened for CO reduction ability. The reported complexes are of the type Re(N-N)(CO)Cl, with N-N = 2-(pyridin-2-yl)-4,5-dihydrooxazole (1), 5-methyl-2-(pyridin-2-yl)-4,5-dihydrooxazole (2), and 5-phenyl-2-(pyridin-2-yl)-4,5-dihydrooxazole (3). The electrocatalytic reduction of CO by these complexes was observed in a variety of solvents and proceeds more quickly in acetonitrile than in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The analysis of the catalytic cycle for electrochemical CO reduction by 1 in acetonitrile using density functional theory (DFT) supports the C-O bond cleavage step being the rate-determining step (RDS) (ΔG = 27.2 kcal mol). The dependency of the turnover frequencies (TOFs) on the donor number (DN) of the solvent also supports that C-O bond cleavage is the rate-determining step. Moreover, the calculations using explicit solvent molecules indicate that the solvent dependence likely arises from a protonation-first mechanism. Unlike other complexes derived from fac-Re(bpy)(CO)Cl (I; bpy = 2,2'-bipyridine), in which one of the pyridyl moieties in the bpy ligand is replaced by another imine, no catalytic enhancement occurs during the first reduction potential. Remarkably, catalysts 1 and 2 display relative turnover frequencies, (i/i), up to 7 times larger than that of I.
The photoreduction of CO 2 by using enzyme-mimicking polymeric metallofoldamers containing Ni-thiolate cofactors was explored. Metallofoldamers consisting of folded polymers incorporated with Ni-thiolate complexes were prepared by intramolecular Ni-thiolate coordination with thiol-functionalized linear copolymers. The folded polymer backbonem ay resemble the protein framework to provide as econd coordination environment to the active sites. We showedt hat Ni-metallofoldamers were superiorlya ctive and selective for CO 2 photoreduction. At 80 8C, the turnover frequency of the Ni-metallofoldamersc ould reach 0.69 s À1 ,w hich corresponds to 2500 turnovers per hour per Ni site. Our findings are expected to provide useful guidelines to investigate artificial enzymes and to understand the role of protein frameworks in photosynthesis.New catalytic materials that can effectively capture and sustainably convert CO 2 into carbon-based fuels are of great interest. [1] In nature, an umber of metalloenzymes [e.g.,c arbon monoxide dehydrogenase (CODH) and formate dehydrogenase] are known to be highly active for variousconversion pathways in the biological metabolism of CO 2 . [2] All of these metalloenzymes contain metal-thiolates as cofactors. [1g] For example, the CODH isolated from the anaerobic bacterium Carboxydothermus hydrogenoformans can catalyze thermodynamically reversible conversionsb etween CO 2 and CO at ac omplexN i-, Fe-, and S-containing metal-thiolate site, namely,t he [Ni-Fe 4 S 4 ] Cc luster. [3] The enzymatic conversion of CO 2 has advantages, for example, high binding affinity,e xcellent product selectivity, and minimum energy input. Inspired by metalloenzymes, considerable effort has been made to synthesize mimics that can catalyzea nalogoust ransformations found in naturals ystems. [4] However,t he overall efficiency of artificial photosynthetic sys- [a] Supporting Information and the ORCID identification number(s) for the author(s) of this article can be found under: http://dx.
The atomic-level tunability of molecular structures is a compelling reason to develop homogeneous catalysts for challenging reactions such as the electrochemical reduction of carbon dioxide to valuable C1–C n products. Of particular interest is methane, the largest component of natural gas. Herein, we report a series of three isomeric rhenium tricarbonyl complexes coordinated by the asymmetric diimine ligands 2-(isoquinolin-1-yl)-4,5-dihydrooxazole (quin-1-oxa), 2-(quinolin-2-yl)-4,5-dihydrooxazole (quin-2-oxa), and 2-(isoquinolin-3-yl)-4,5-dihydrooxazole (quin-3-oxa) that catalyze the reduction of CO2 to carbon monoxide and methane, albeit the latter with a low efficiency. To our knowledge, these complexes are the first examples of rhenium(I) catalysts capable of converting carbon dioxide into methane. Re(quin-1-oxa)(CO)3Cl (1), Re(quin-2-oxa)(CO)3Cl (2), and Re(quin-3-oxa)(CO)3Cl (3) were characterized and studied using a variety of electrochemical and spectroscopic techniques. In bulk electrolysis experiments, the three complexes reduce CO2 to CO and CH4. When the controlled-potential electrolysis experiments are performed at −2.5 V (vs Fc+/0) and in the presence of the Brønsted acid 2,2,2-trifluoroethanol, methane is produced with turnover numbers that range from 1.3 to 1.8. Isotope labeling experiments using 13CO2 atmosphere produce 13CH4 (m/z = 17) confirming that methane originates from CO2 reduction. Theoretical calculations are performed to investigate the mechanistic aspects of the 8e–/8H+ reduction of CO2 to CH4. A ligand-assisted pathway is proposed to be an efficient pathway in the formation of CH4. Delocalization of the electron density on the (iso)quinoline moiety upon reduction stabilizes the key carbonyl intermediate leading to additional reactivity of this ligand. These results should aid the development of more robust catalytic systems that produce CH4 from CO2.
New cis-(1,2-azole)-aquo bis(2,2′-bipyridyl)ruthenium(II) (1,2-azole (az*H) = pzH (pyrazole), dmpzH (3,5-dimethylpyrazole), and indzH (indazole)) complexes are synthesized via chlorido abstraction from cis-[Ru(bipy)2Cl(az*H)]OTf. The latter are obtained from cis-[Ru(bipy)2Cl2] after the subsequent coordination of the 1,2-azole. All the compounds are characterized by 1H, 13C, 15N NMR spectroscopy as well as IR spectroscopy. Two chlorido complexes (pzH and indzH) and two aquo complexes (indzH and dmpzH) are also characterized by X-ray diffraction. Photophysical and electrochemical studies were carried out on all the complexes. The photophysical data support the phosphorescence of the complexes. The electrochemical behavior of all the complexes in an Ar atmosphere indicate that the oxidation processes assigned to Ru(II) → Ru(III) occurs at higher potentials in the aquo complexes. The reduction processes under Ar lead to several waves, indicating that the complexes undergo successive electron-transfer reductions that are centered in the bipy ligands. The first electron reduction is reversible. The electrochemical behavior in CO2 media is consistent with CO2 electrocatalyzed reduction, where the values of the catalytic activity [i cat(CO2)/i p(Ar)] ranged from 2.9 to 10.8. Controlled potential electrolysis of the chlorido and aquo complexes affords CO and formic acid, with the latter as the major product after 2 h. Photocatalytic experiments in MeCN with [Ru(bipy)3]Cl2 as the photosensitizer and TEOA as the electron donor, which were irradiated with >300 nm light for 24 h, led to CO and HCOOH as the main reduction products, achieving a combined turnover number (TONCO+HCOO– ) as high as 107 for 2c after 24 h of irradiation.
Inspired by the architecture of the macrocycle of heme d1, a series of synthetic mono-, di- and tri-β-oxo-substituted porphyrinoid cobalt(II) complexes were evaluated as electrocatalytic CO2 reducers, identifying complexes of...
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