Overall reaction mechanism of photocatalytic CO2 reduction on a Re(i)-complex catalyst unit of a Ru(ii)–Re(i) supramolecular photocatalyst

Rhenium(i) complexes fac-[ReI(diimine)(CO)3(L)]n+ are mostly used and evaluated as photocatalysts and catalysts in both photochemical and electrochemical systems for CO2 reduction. However, the selective reduction mechanism of CO2 to CO is unclear, although numerous mechanistic studies have been reported. A Ru(ii)–Re(i) supramolecular photocatalyst with fac-[ReI(diimine)(CO)3{OC(O)OCH2CH2NR2}] (R = C2H4OH) as a catalyst unit (RuC2Re) exhibits very high efficiency, selectivity, and durability of CO formation in photocatalytic CO2 reduction reactions. In this work, the reaction mechanism of photocatalytic CO2 reduction using RuC2Re is fully clarified. Time-resolved IR (TR-IR) measurements using rapid-scan FT-IR spectroscopy with laser flash photolysis verify the formation of RuC2Re(COOH) with a carboxylic acid unit, i.e., fac-[ReI(diimine)(CO)3(COOH)], in the photocatalytic reaction solution. Additionally, this important intermediate is detected in an actual photocatalytic reaction using steady state irradiation. Kinetics analysis of the TR-IR spectra and DFT calculations demonstrated the reaction mechanism of the conversion of the one-electron reduced species of RuC2Re with a fac-[ReI(diimine˙−)(CO)3{OC(O)OCH2CH2NR2}]− unit, which was produced via the photochemical reduction of RuC2Re by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), to RuC2Re(COOH). The kinetics of the recovery processes of the starting complex RuC2Re from RuC2Re(COOH) accompanying the release of CO and OH− was also clarified. As a side reaction of RuC2Re(COOH), a long-lived carboxylate–ester complex with a fac-[ReI(diimine)(CO)3(COOC2H4NR2)] unit, which was produced by the nucleophilic attack of TEOA to one of the carbonyl ligands of RuC2Re(CO) with a fac-[ReI(diimine)(CO)4]+ unit, was formed during the photocatalytic reaction. This complex works not only as a precursor in another minor CO formation process but also as an external photosensitiser that photochemically reduces the other complexes i.e., RuC2Re, RuC2Re(COOH), and the intermediate that is reductively converted to RuC2Re(COOH).


Materials
Prior to its use in the measurements, DMSO was dried over 4 Å molecular sieves at room temperature before being distilled in the presence of calcium hydride at reduced pressure.Super dehydrated DMSO purchased from Fujifilm Wako Pure Chemical Corporation was used for the TR-IR measurements.TEOA was distilled under reduced pressure (<133 Pa) prior to its use.After distillation, both solvents were stored under an Ar atmosphere.Et 4 NBF 4 was dried under a vacuum overnight at 100 °C prior to its use.All other reagents were of reagent-grade quality and were used without further purification.2][3][4] fac-Re(dmb)(CO) 3 (COOH) (Re(COOH)) was prepared using the method for the synthesis of fac-Re(bpy)(CO) 3 (COOH) described in previous studies, except that [Re(dmb)(CO) 4 ](OTf) was used as a starting material. 5,6 2Re: RuC2Re(MeCN) was dissolved in DMSO, and the resulting solution was kept at room temperature in the dark under an Ar atmosphere for 4.5 h for changing the MeCN ligand of all of the added Re complexes to the DMSO ligand.After this time, TEOA was added into the solution (DMSO : TEOA = 5 : 1 v/v), which was kept under an Ar atmosphere in the dark at room temperature for a period of more than 4 h.The resulting solution was bubbled with CO 2 to give a DMSO-TEOA (5 : 1 v/v) solution containing RuC2Re.In the experiment under 13 CO 2 atmosphere, RuC2Re with 13 C-labeled carbonate ester bonds was prepared by bubbling with 13 CO 2 instead of ordinary CO 2 .

Rapid scan time-resolved FT-IR measurement
The rapid-scan time-resolved FTIR spectra of the CO 2 saturated DMSO-TEOA (5:1 v/v) solution containing RuC2Re (1.0 mM) and BIH (0.1 M) in an IR cell with an optical path length of 0.1 mm were recorded on a Bruker Vertex 80 spectrophotometer (resolution: 4 cm -1 ) with a scan velocity of 320 kHz and the acquisition mode of the single-sided forward and backward at 298 K.A Ge filter to cut infrared light at >2200 cm -1 (Andover, 4.50ILP-25) was placed in the infrared beam path.Two scans that consist of one lap of forward and backward scans require 42 ms with an additional interval of 6 ms.The visible pump pulse was obtained by generating the second harmonic (532 nm) of the Q-switched Nd:YAG laser [Quanta-Ray INDI-40-10; ∼7 ns full width at half-maximum (FWFM), ∼50 mJ cm -2 pulse -1 ].One 2-scan measurement before a single flash and then forty 2-scan, fifteen 8-scan, eleven 210-scan, and twelve 840-scan measurements after the flash were carried out.The laser flash interval was 10 min.

Quantum chemical calculations
Quantum chemical calculations based on DFT were performed using the Gaussian 16 package. 7Geometric optimization and frequency calculations were performed in DMSO (PCM,  = 46.7)using the ωB97XD function, Lanl2DZ basis set for Re with an added f polarization function, 8 and 6-311G+(d,p) for the other elements.The free energy in the solution was calculated based on the electronic energy and vibrational frequency obtained from the calculations under this condition with a correction of +1.9 kcal/mol for the change in the reference states from 1 atm to 1 M. 9 The calculated vibrational frequencies were scaled by a scaling factor of 0.9618 to account for the anharmonicity in the comparison of the experimental values (Table S1).The stationary points were verified by frequency analysis and the transition states were connected to minima using intrinsic reaction coordinate (IRC) computations.(red) and BI + (blue) obtained by UHPLC analysis.The black dash line is the fitting result.Although BI + that is a twoelectron oxidized and deprotonated species of BIH was also eluted at 3.25 min, the chromatograms shown in Fig. 5a should not be affected by the simultaneous elution of BI + because it has no absorption at 460 nm as shown by its spectrum in blue.After mixing, the UV-Vis absorption was changed owing to the CO 2 capture reaction (forward reaction of eq.S2) with an isosbestic point at 400 nm.Although the complex solution before mixing also contains a DMSO complex RuC2Re(DMSO), the ligand exchange reaction (eq.S1) occurred for a time scale of a few hours and could be ignored on the time scale of this experiment.The CO 2 concentration change could be ignored because of the higher concentration of CO 2 (0.060 M; half of the saturation concentration) after mixing compared with the concentration of the complex.The CO 2 saturation concentration in the DMSO-TEOA (5:1 v/v) mixed solution was determined by a previously reported titration method. 10The CO 2 dissociation reaction (backward reaction of eq.S2) could also be ignored because of the higher equilibrium constant of eq.S2. [10][11][12] Thus, the UV-Vis spectra shown in Figure S21a could be analyzed using a simple pseudo-first-order kinetics model (the apparent rate constant is 0.060k ins ) and the bimolecular rate constant k ins was determined to be 44 M −1 s −1 .

Figure S3 .Figure S4 .
Figure S3.TR-IR spectra of an ordinary CO 2 (red) or 13 CO 2 (blue) saturated DMSO-TEOA (5:1 v/v) solution containing RuC2Re (1.0 mM) and BIH (0.1 M) at (a) 21 ms, (b) 3.0 s and (c, d) 5.0 min after the laser flash.These spectra were normalized by absorbance at 2019 cm −1 at 21 ms after pulsed excitation.A total of 12 loops of spectra using two samples were averaged for the final data.

Figure S5 .Figure S6 .
Figure S5.(a,b) Kinetics traces (dots) of TR-IR spectra from 21 ms to 4.0 s at characteristic wavelengths with their fits obtained by global analysis using a two-component global sequential routine (black line).(c) Evolutionassociated spectra (EAS) generated by global analysis of TR-IR spectra from 21 ms to 4 s after laser flash using a two component.

Figure S7 .Figure S8 .
Figure S7.Difference spectra of a CO 2 saturated DMSO-TEOA (5:1 v/v) solution containing RuC2Re (1.0 mM) and BIH (0.1 M) between FT-IR spectra per 10 laser flash irradiations and FT-IR spectrum before irradiations.The spectra on the high wavenumber side (b) were aligned to zero absorbance at 2050 cm −1 .

Figure S9 .Figure S10 .
Figure S9.Kinetic traces (dots) of different FT-IR spectra of a CO 2 saturated DMSO-TEOA (5:1 v/v) solution containing RuC2Re (2 mM) and BIH (0.1 M) after (a) 1 min, (b) 2 min and (c) 4 min steady-state light irradiation (  = 480 nm) with their fits obtained by global analysis in 1800-2100 cm −1 using a single exponential function and one constant (black line) at characteristic wavelengths.The time in the figure is the time since light irradiation was stopped.

Figure S12 .
Figure S12.(a) UV-Vis spectra after mixing a DMSO-TEOA (5:2 v/v) solution with the same volume of a DMSO solution containing RuC2Re(CO) (0.1 mM).(b) Temporal change (blue) of Abs.at 375 nm with their fitting (red) using a single exponential function and one constant.

Figure S15 .
Figure S15.Light intensity dependence of the concentration of RuC2Re(CO-TEOA) after 5 min irradiation.An Ar purged DMSO-TEOA (5:1 v/v) solution containing RuC2Re(CO-TEOA) (0.5 mM) and BIH (0.1 M) was irradiated with a 530 nm LED light.The x-axis values are the input power of the apparatus.Owing to equipment limitations, the absolute light intensity cannot be calculated.

Figure S16 .
Figure S16.TR-IR spectra of a CO 2 saturated DMSO-TEOA (5:1 v/v) solution containing RuC2Re (1.0 mM), BIH (0.1 M) and NH 4 PF 6 (20 mM) (a) from 21 ms to 2.0 s and (b) from 2.0 s to 5.0 min after pulsed excitation at 532 nm.A total of 25 loops of spectra were averaged for the final data.

Figure .
Figure.S21 (a) UV-Vis spectra after mixing an Ar purged DMSO-TEOA (5:1 v/v) solution containing RuC2Re(DMSO) and RuC2Re(TEOA) complexes (the structures are shown in the reactions below; total concentration of these solvent complex is 0.01 mM) with the same volume of a CO 2 saturated (0.12 M) DMSO-TEOA (5:1 v/v) solution.(b) Temporal change (blue) of Abs.at 360 nm (purple) and 450 nm (blue) with their fitting (red) using a single exponential function and one constant.