Molecular quaterpyridine-based metal complexes for small molecule activation: water splitting and CO2 reduction.

Artificial photosynthesis is considered as one of the most promising strategies for solar-to-fuel conversion through sunlight-driven water splitting and CO2 reduction. This tutorial describes recent developments in the use of metal quaterpyridine complexes as electrocatalyts and photocatalysts for artificial photosynthesis.

An Iron Quaterpyridine Complex as Precursor for the Electrocatalytic Reduction of CO2 to Methane.

A Fe quaterpyridine complex was used as a molecular precursor for the electrochemical reduction of CO2 to CH4 in acetonitrile in the presence of triethanolamine. CH4 was produced with a faradaic yield of approximately 2.1 % at 25 °C and 1 atm pressure of CO2 as reactant. Controlled potential electrolysis coupled to ex situ X-ray photoelectron spectroscopy and X-ray absorption spectroscopy of the electrode surface revealed the formation of metallic iron covered by iron oxides as species responsible for catalysis.

A Carbon Nitride/Fe Quaterpyridine Catalytic System for Photostimulated CO2-to-CO Conversion with Visible Light.

Efficient and selective photostimulated CO2-to-CO reduction by a photocatalytic system consisting of an iron-complex catalyst and a mesoporous graphitic carbon nitride (mpg-C3N4) redox photosensitizer is reported for the first time. Irradiation in the visible region (λ ≥ 400 nm) of an CH3CN/triethanolamine (4:1, v/v) solution containing [Fe(qpy)(H2O)2]2+ (qpy = 2,2':6',2'':6'',2''-quaterpyridine) and mpg-C3N4 resulted in CO evolution with 97% selectivity, a turnover number of 155, and an apparent quantum yield of ca. 4.2%. This hybrid catalytic system, comprising only earth abundant elements, opens new perspectives for solar fuels production using CO2 as a renewable feedstock.

Photocatalytic Conversion of CO2 to CO by a Copper(II) Quaterpyridine Complex.

The invention of efficient systems for the photocatalytic reduction of CO2 comprising earth-abundant metal catalysts is a promising approach for the production of solar fuels. One bottleneck is to design highly selective and robust molecular complexes that are able to transform the CO2 gas. The CuII quaterpyridine complex [Cu(qpy)]2+ (1) is found to be a highly efficient and selective catalyst for visible-light driven CO2 reduction in CH3 CN using [Ru(bpy)3 ]2+ (bpy: bipyridine) as photosensitizer and BIH/TEOA (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole/triethanolamine) as sacrificial reductant. The photocatalytic reaction is greatly enhanced by the presence of H2 O (1-4 % v/v), and a turnover number of >12 400 for CO production can be achieved with 97 % selectivity, which is among the highest of molecular 3d CO2 reduction catalysts. Results from Hg poisoning and dynamic light scattering experiments suggest that this photocatalyst is homogenous. To the best of our knowledge, 1 is the first example of molecular Cu-based catalyst for the photoreduction of CO2 .

Local Proton Source in Electrocatalytic CO2 Reduction with [Mn(bpy-R)(CO)3 Br] Complexes.

The electrochemical behavior of fac-[Mn(pdbpy)(CO)3 Br] (pdbpy=4-phenyl-6-(phenyl-2,6-diol)-2,2'-bipyridine) (1) in acetonitrile under Ar, and its catalytic performances for CO2 reduction with added water, 2,2,2-trifluoroethanol (TFE), and phenol are discussed in detail. Preparative-scale electrolysis experiments, carried out at -1.5 V versus the standard calomel electrode (SCE) in CO2 -saturated acetonitrile, reveal that the process selectivity is extremely sensitive to the acid strength, producing CO and formate in different faradaic yields. A detailed spectroelectrochemical (IR and UV/Vis) study under Ar and CO2 atmospheres shows that 1 undergoes fast solvolysis; however, dimer formation in acetonitrile is suppressed, resulting in an atypical reduction mechanism in comparison with other reported MnI catalysts. Spectroscopic evidence of Mn hydride formation supports the existence of different electrocatalytic CO2 reduction pathways. Furthermore, a comparative investigation performed on the new fac-[Mn(ptbpy)(CO)3 Br] (ptbpy=4-phenyl-6-(phenyl-3,4,5-triol)-2,2'-bipyridine) catalyst (2), bearing a bipyridyl derivative with OH groups in different positions to those in 1, provides complementary information about the role that the local proton source plays during the electrochemical reduction of CO2 .

Highly Efficient and Selective Photocatalytic CO2 Reduction by Iron and Cobalt Quaterpyridine Complexes.

The design of highly efficient and selective photocatalytic systems for CO2 reduction that are based on nonexpensive materials is a great challenge for chemists. The photocatalytic reduction of CO2 by [Co(qpy)(OH2)2](2+) (1) (qpy = 2,2':6',2″:6″,2‴-quaterpyridine) and [Fe(qpy)(OH2)2](2+) (2) have been investigated. With Ru(bpy)3(2+) as the photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the sacrificial reductant in CH3CN/triethanolamine solution under visible-light excitation (blue light-emitting diode), a turnover number (TON) for CO as high as 2660 with 98% selectivity can be achieved for the cobalt catalyst. In the case of the iron catalyst, the TON was >3000 with up to 95% selectivity. More significantly, when Ru(bpy)3(2+) was replaced by the organic dye sensitizer purpurin, TONs of 790 and 1365 were achieved in N,N-dimethylformamide for the cobalt and iron catalysts, respectively.

A local proton source in a [Mn(bpy-R)(CO)3Br]-type redox catalyst enables CO2 reduction even in the absence of Brønsted acids.

The effect of a local proton source on the activity of a bromotricarbonyl Mn redox catalyst for CO2 reduction has been investigated. The electrochemical behaviour of the novel complex [fac-Mn(dhbpy)(CO)3Br] (dhbpy = 4-phenyl-6-(1,3-dihydroxybenzen-2-yl) 2,2'-bipyridine), containing two acidic OH groups in the proximity of the metal centre, under a CO2 atmosphere showed a sustained catalysis in homogeneous solution even in the absence of Brønsted acids.