Photocatalytic Reduction of CO2 to CO in Aqueous Solution under Red-Light Irradiation by a Zn-Porphyrin-Sensitized Mn(I) Catalyst

This work demonstrates photocatalytic CO2 reduction by a noble-metal-free photosensitizer-catalyst system in aqueous solution under red-light irradiation. A water-soluble Mn(I) tricarbonyl diimine complex, [MnBr(4,4′-{Et2O3PCH2}2-2,2′-bipyridyl)(CO)3] (1), has been fully characterized, including single-crystal X-ray crystallography, and shown to reduce CO2 to CO following photosensitization by tetra(N-methyl-4-pyridyl)porphyrin Zn(II) tetrachloride [Zn(TMPyP)]Cl4 (2) under 625 nm irradiation. This is the first example of 2 employed as a photosensitizer for CO2 reduction. The incorporation of −P(O)(OEt)2 groups, decoupled from the core of the catalyst by a −CH2– spacer, afforded water solubility without compromising the electronic properties of the catalyst. The photostability of the active Mn(I) catalyst over prolonged periods of irradiation with red light was confirmed by 1H and 13C{1H} NMR spectroscopy. This first report on Mn(I) species as a homogeneous photocatalyst, working in water and under red light, illustrates further future prospects of intrinsically photounstable Mn(I) complexes as solar-driven catalysts in an aqueous environment.


4,4'-dicarboxy-2,2′-bipyridine
4,4′-dimethyl-2,2′-bipyridine (2.5 g 13.5 mmol) was dissolved in of 98% H 2 SO 4 (100 cm -3 ). To this solution, potassium dichromate (12 g) was added in portions whilst stirring, ensuring the temperature remained between 40-80 o C. The reaction mixture was then cooled in an ice bath, and the resulting precipitate isolated by vacuum filtration. The precipitate was then refluxed for 16 h in 50% HNO 3 (50 cm 3 ), then poured over ice and subsequently diluted to with water (400 cm 3 ) and left to warm to room temperature. The resulting white precipitate was then filtered off, washed with water and acetone and dried under a vacuum to yield the product (2.29 g, 91%). 1 H NMR (400 MHz, d 6 4,4′-dicarboxy-2,2′-bipyridine (1.05 g, 4.12 mmol) was suspended in ethanol (40 cm 3 ) then 98% H 2 SO 4 (2 cm 3 ) was added. The mixture was refluxed at 85 °C under inert atmosphere for 24 hours to give a pale pink solution. Removal of the solvent under reduced pressure left a pale pink oil, which was mixed with water (20 cm 3 ) and extracted with chloroform (3 x 50 cm 3 ). The combined organic fractions were dried over anhydrous MgSO 4 . The mixture was then filtered, and the volume reduced under reduced pressure to approximately 20 cm 3 . Addition of methanol (20 cm 3 ) resulted in formation of a pale pink precipitate, which was isolated by vacuum filtration and then dried under high vacuum to yield the product (964 mg, 76%).

4,4′-bis(hydroxymethyl)-2,2′-bipyridine
4,4′-bis(ethoxyester)-2,2′-bipyridine (750 mg, 2.4 mmol) was suspended in ethanol (50 cm 3 ) followed by the addition of sodium borohydride (2 g, 53 mmol). The mixture was refluxed at 65 °C under inert atmosphere for 3 hours. A gel was observed forming on the surface of the reaction mixture after approximately 1 hour, which was dissolved by addition of additional ethanol (25 cm 3 ). After cooling to room temperature, a saturated aqueous solution of NH 4 Cl (100 cm 3 ) was added to the mixture and stirred for 15 min. The ethanol was removed under reduced pressure and the resulting white solid was dissolved in the minimum volume of water (ca. 150 cm 3 ). The solution was extracted with ethyl acetate (5 x 50 cm 3 ), then the combined organic fractions were dried over anhydrous MgSO 4 . The solvent was removed under reduced pressure, yielding the product as a pale pink solid (306 mg, 50%).

4,4′-bis(Et 2 O 3 PCH 2 )-2,2′-bipyridine
A solution of diethyl phosphite (15 cm 3 ) in chloroform (10 cm 3 ) was purged with N 2 for 30 min. This mixture was transferred to a flask containing 4,4′-bis(bromomethyl)-2,2′-bipyridine (842 mg, 2.46 mmol) and subsequently refluxed at 85 °C for 6 hours. The reaction mixture was allowed to cool to room temperature, and then the solvent and excess diethyl phosphite were removed under reduced pressure to yield the crude product as a pale pink solid. The crude product was purified by column chromatography (SiO 2 , 80:20 ethyl acetate: methanol) to yield the purified product as a pale yellow oil. The oil was left overnight, resulting in crystallisation of the product as an off-white solid (942 mg, 64%).

UV-vis spectroscopy
Visible absorption spectroscopy of the manganese complex shows successful coordination of the bipyridyl ligand to the complex, as evidenced by the * and MLCT electronic absorption bands. The UV-vis spectrum showed that it would be possible to irradiate the CO 2 reducing systems at 625 nm with no light absorption from the catalyst. Therefore, photolysis could be prevented through the use of porphyrins as photosensitisers.

FT-IR spectroscopy
FT-IR spectra of the analyte complex in DCM provide evidence for the proposed structure. It was possible to identify vibrational modes attributed to the CO, CH and PO bond vibrations.    3 Br] in d 6 -dimethylsulphoxide.

C NMR:
The methylene group in this complex appears as a doublet due to the coupling between the 31 P and 13 C nuclei. The small peak at =31 is the CH 3 resonance of acetone. All other signals are attributed to the analyte complex, not all quaternary carbons were observed.

P NMR
A single resonance was observed in the proton decoupled 31 P spectrum. The resonance appears to be a singlet with a small shoulder. This was likely due to convolution with a doublet resulting from 31 P-13 C coupling. The doublet is much less intense than the singlet peak due to the low abundance of 13 C compared to 31 P.

[Mn(phos-bpy)(CO) 3 Br] -2 nd reduction
Due to the large E p,a -E p,c separation of the second reduction process, it was not possible to isolate the redox process from the 1 st reduction potential, thus the scan rate dependence of the second reduction was obtained from CV data in the range 0 to -2.5 V vs. Fc/Fc + .

Figure S11 -Cyclic voltammograms of the 2 nd reduction of [Mn(phos-bpy)(CO) 3 Br] in anhydrous acetonitrile, under N 2 atmosphere at various scan rates, as shown on the graph.
[Mn(phos-bpy)(CO) 3 Br] -Plots of peak current against square root of the scan rate

X-Ray crystallographic data
1 was crystallised by diffusion of Et 2 O vapour into a solution of the complex in dichloromethane (DCM) to yield yellow crystals of the complex. The studied crystal with dimensions 0.5 x 0.5 x 0.4 mm was found to be triclinic with the P-1 space group. The unit cell contained two molecules of 1, and no solvent co-crystallised with the complex. The complex formed the expected facial isomer, consistent with previously reported [Mn(L 2 )(CO) 3 (X)] complexes. In the unit cell, the two molecules are offset and rotated 180° from one another, with the axial plane of the Mn centre pointed toward the bipyridyl π-system of the other complex. This geometry minimised the steric interaction of the four phosphonate ester groups by separating them by the largest possible distance. The space group of the crystal, P-1 (No. 2) was the same as the previously reported X-ray structure for [Mn(4,4′-{Et 2 O 3 PCH 2 } 2 -bpy)(CO) 3 Br]. 4 However, differences in the orientation of the two complexes in the unit cell structure were found, where the unit cell volume of the crystal data reported in this work was smaller than the previously reported crystal by 34.1 Å 3 .

Crystal structure refinement procedure
Data were corrected for absorption using empirical methods (SADABS) 5 based upon symmetry equivalent reflections combined with measurements at different azimuthal angles. 6 The crystal structures were solved and refined against F2 values using ShelXT 7 for solution and ShelXL 8 for refinement accessed via the Olex2 program. 9 Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions with idealized geometries and then refined by employing a riding model and isotropic displacement parameters.

Estimation of the apparent quantum yield of photocatalysis
The quantum yield of photocatalysis was estimated as the ratio of the number of moles of photons emitted from the LED diode to the number of moles of carbon monoxide formed from the catalytic reaction. The photon energy of the 625 nm light was 3.18x10 -19 J, thus if the power density is 308 mW cm -2 and the light is focussed to a 4 cm 2 area, the photon number per second is 3.87x10 18 s -1 . Conversion to moles of photons per hour yields 1.79x10 -9 mol hr -1 . The estimated quantum yields are shown in the table below.
-Estimation of the apparent quantum yield of photocatalysis based on the number of moles of product formed per unit time divided by the number of photons incident on the reaction mixture.

Reductive quenching
It was found that the reductive quenching mechanism was not feasible, as the initial reduction of [ZnTMPyP]Cl 4 by ascorbic acid was thermodynamically unfavourable.