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. 2025 Feb 19;1(5):214–225. doi: 10.1021/aps.4c00026

Tuning the Photocatalytic CO2 Reduction through para-Substituents in Bipyridyl Rhenium Complexes

Andressa V Müller †,‡,*, Wendel M Wierzba †, Luis G A do Nascimento §, Javier J Concepcion ‡, Sofia Nikolaou §, Dmitry E Polyansky ‡, Andre S Polo †,*
PMCID: PMC12478854  PMID: 41031222

Abstract

Six fac-[Re­(NN)­(CO)3Cl] complexes, where NN is a 2,2′-bipyridine ligand with systematically varied electron-donating or -withdrawing groups at the 4,4′ positions, were explored as photocatalysts for CO2 reduction to CO. The light absorption, redox potentials, and excited-state dynamics of the complexes, as well as the reactivity and stability of key catalysis intermediates, were correlated with the Hammett constants (σp) of the substituents, revealing their impact on the photocatalytic activity. Electron-donating substituents, such as −OCH3 and −CH3, resulted in slower excited-state quenching by sacrificial electron donors in comparison to the unsubstituted complex, but their corresponding one-electron-reduced species (OERS) reacted quickly with CO2. On the other hand, electron-withdrawing substituents, such as −Br, −COOH, and −CO2CH3, resulted in more favorable and faster reductive quenching, but at the cost of slower reactivity of the OERS. A comparison of the catalytic activity of the complexes employing two different sacrificial electron donors displayed how the interplay of these factors affects the CO2 reduction performance and how photocatalysis can be controlled by strategic manipulation of both the ligand environment and the reaction conditions.

Keywords: CO2 photoreduction, rhenium(I) polypyridyl photocatalysts, photocatalysis, artificial photosynthesis, Hammett constants, solar energy

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Introduction

Rhenium­(I) tricarbonyl complexes, fac-[Re­(NN)­(CO)3X]n+, where NN is a bidentate polypyridyl ligand and X is the axial ligand, have emerged as promising candidates for photocatalytic CO2 reduction, offering a versatile platform for exploring the effects of ligand modifications on catalytic activity. − The photocatalytic CO2 reduction cycle, as exemplified in Figure , is initiated with the absorption of light by the fac-[Re­(NN)­(CO)3Cl] complex, resulting in the formation of an excited state of the photocatalyst, denoted as fac-[Re­(NN)­(CO)3Cl]*. This often involves a metal-to-ligand charge transfer (MLCT), where an electron is excited from a rhenium d-orbital to a π* orbital of the bidentate ligand (NN). , From a practical point of view, light absorption by the complexes in the visible region of the electromagnetic spectrum is highly desirable since this region accounts for over 40% of sunlight, while UV radiation comprises only 8%. The following step is the reductive quenching of the excited fac-[Re­(NN)­(CO)3Cl]* by a sacrificial electron donor, forming an one-electron reduced species (OERS), fac-[Re­(CO)3(NN)­(Cl)]−. Typically, the OERS has two key roles. First, it acts as a precursor for the “17-electron species”, fac-[Re­(CO)3(NN)], an important intermediate that reacts with CO2 and yields a CO2 adduct. This critical step in the photochemical CO2 reduction involves the dissociation of the axial ligand from the OERS. Second, the OERS provides a second electron to the CO2 adduct, resulting in the two-electron reduction of the substrate and release of CO. ,, At this point, the starting photocatalyst may be regenerated by recoordination of the axial ligand or coordinating a solvent molecule.

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General photocatalytic reaction mechanism for CO2 reduction using fac-[Re­(NN)­(CO)3Cl] photocatalysts, in which NN is a substituted 2,2′-bipyridine. Adapted from G. Sahara et al. Copyright 2015 American Chemical Society.

Changes in the axial or NN ligands have a crucial role in modulating the electronic structure of the complexes, thus influencing their absorption spectra and excited-state dynamics, and profoundly impacting the stability and reactivity of the complexes during the catalysis. The photocatalytic ability of fac-[Re­(bpy)­(CO)3X]n+, where bpy = 2,2′-bypiridine, has been shown to be strongly affected by the ligand X. While fac-[Re­(bpy)­(CO)3L], L = Cl–, exhibited a quantum yield of CO formation (ΦCO) of 16%, in the cases of L = CH3CN, PPh3, pyridine or CN–, the photocatalytic activities of the complexes were relatively low (CH3CN: ΦCO = 4%, PPh3: ΦCO = 5%, pyridine: ΦCO = 3%, CN–: ΦCO = 0%) because of the negative influence of these ligands on the reactivity of the OERS. On the other hand, higher efficiencies were obtained for L = NCS– (ΦCO = 30%) and P­(OEt)3 (ΦCO = 38%) due to a combination of favorable photophysical properties, ability of dissociation of the L ligand and reactivity of reduced species.

Modifications in the polypyridyl ligand also significantly impact the photocatalytic activity. The attachment of N-heterocyclic moieties (pyrrole, indole, or carbazole) to the 1,10-phenanthroline (phen) moiety of fac-[Re­(phen)­(CO)3Cl] photocatalysts resulted in up to a 10-fold increase in the TONCO values in comparison to the unsubstituted parent complex. The outstanding photocatalytic efficiency of this series of complexes was attributed to the aromatic substituents, which enhanced visible-light absorption, improved the kinetics and thermodynamics of each photocatalytic step, prevented dimerization reactions that deactivate the photocatalysts, stabilized reduced intermediates, and facilitated an alternative CO2 reduction mechanism that involves primarily a two-electron reduced intermediate.

A wide range of other diimine ligands NN has also been explored aiming to achieve higher efficiencies in the photocatalytic conversion of CO2 to CO, such as 2,2′-biquinolines, 2,2′-bipyrimidines, dipyridoquinoxalines or dipyridophenazines. The ligand variations resulted in complexes with a diverse array of physicochemical properties, although no direct correlation between catalytic activity and spectroscopic or electrochemical properties was established. The photocatalytic efficiency can also be modulated by incorporating pendant groups into the NN ligand, often within the second coordination sphere of the metal complex. , These groups can interact with catalytic intermediates or transition states, affecting reaction rates, efficiency, and stability of molecular catalysts, and reducing activation barriers.

Importantly, the presence of electron donating or withdrawing substituents at the NN ligand is known to significantly affect the spectroscopic, electrochemical, photophysical, and photochemical properties of the Re metal complexes that can directly impact the photocatalysis. − However, although some substitutions at the 4,4’ positions of the 2,2′-bypiridine ligand were explored and resulted in different photocatalytic efficiencies in comparison to the complex with an unsubstituted ligand, fac-[Re­(bpy)­(CO)3X]n+, these were reported separately and different reaction conditions were employed, , complicating their direct comparison. This work aims to address this gap by investigating six different Re­(I) tricarbonyl complexes, shown in Figure , each with systematically varied electron donating or withdrawing groups at the 4,4′ positions of the bpy ligand. The properties of the complexes relevant to the CO2 reduction cycle were directly correlated with the Hammett constants σp of the substituents. The CO2 reduction performance by the six photocatalysts was evaluated employing consistent reaction conditions and utilizing two different sacrificial electron donors, triethanolamine (TEOA) or 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo­[d]­imidazole (BIH), and differences were associated with the overall contribution of the substituent groups. This work is an important step for understanding the factors that govern the catalytic efficiency and could provide insights into optimizing the design of Re-based photocatalysts for enhanced CO2 reduction performance.

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Structures of the fac-[Re­(NN)­(CO)3Cl] photocatalysts investigated in this study and respective abbreviations.

Results and Discussion

During each step of the catalytic cycle, several factors influence the efficiency of CO2 reduction by the fac-[Re­(NN)­(CO)3Cl] complexes, such as the ability of the photocatalyst to absorb visible light and generate the excited state efficiently, the electron transfer efficiency from the sacrificial donors to the excited photocatalyst, the stability of key intermediates, and the binding affinity and activation capability of the reduced rhenium species toward CO2. In the following sections, some of these properties were correlated with the Hammett constants σp of the NN substituents. By optimizing these factors through a rational design of the polypyridyl ligands, the efficiency of CO2 photoreduction can be significantly enhanced, contributing to the development of effective methods for solar fuel production.

Characterization of Photocatalysts: Spectroscopy, Electrochemistry, and Photophysics

The fac-[Re­(NN)­(CO)3Cl] compounds were characterized in terms of their light absorption, electrochemical properties and photophysical behavior to assess their potential for CO2 photoreduction using TEOA or BIH as the sacrificial electron donors.

The UV–vis electronic spectra of the six fac-[Re­(NN)­(CO)3Cl] compounds studied herein, as shown in Figure (top), exhibited intense ILNN π → π* intraligand absorptions between 190 and 350 nm, and low-energy Re­(I) MLCT dπRe → π*NN bands with maxima (λmax) between 355 and 413 nm. , The λmax values are summarized in Table . Compared to the parent compound with the unsubstituted bipyridine (Re–bpy), the λmax of the MLCT bands displayed a blue shift when electron donating groups (−OCH3 and –CH3) were present at the 4,4’ positions of the NN ligand. The electron donating substituents increase the energy of the π*NN acceptor orbitals to a greater degree than they do the dπRe orbitals and results in a higher energy separation between these orbitals. Conversely, a bathochromic shift was observed for compounds having electron withdrawing substituents (−Br, –COOH and –CO2CH3). A linear correlation (R2 = 0.98902) between the λmax of the MLCT bands and the paraσ Hammett constant of the substituent groups (σp) was observed, as shown in Figure (bottom).

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(Top) Electronic spectra of the fac-[Re­(NN)­(CO)3Cl] photocatalysts in acetonitrile and (bottom) correlation between their MLCT maxima wavelengths with the Hammett constant (σp) of the substituent at the 4,4′ positions.

1. Wavelengths of Maximum Absorption (λmax) and Molar Extinction Coefficients (ε) of the fac-[Re­(NN)­(CO)3Cl] Complexes in Acetonitrile.

Substituent λmax (nm) (ε (104 L mol–1 cm–1))
OCH3 283 (1.6), 355 (0.42)
CH3 288 (1.5), 364 (0.35)
H 291 (1.5), 372 (0.34)
Br 294 (1.9), 390 (0.39)
COOH 311 (1.6), 410 (0.44)
CO2CH3 311 (1.6), 413 (0.44)
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Cyclic and differential pulse voltammetry techniques were employed to investigate the electrochemical properties of the fac-[Re­(NN)­(CO)3Cl] complexes, Figure (top). Oxidative processes were observed between +1.50 and +2.10 V versus normal hydrogen electrode (NHE). The irreversible waves at less positive potentials are assigned to the metal-centered one-electron oxidation of the fac-[ReI(NN)­(CO)3Cl] compounds to fac-[ReII(NN)­(CO)3Cl]+. This one-electron oxidized species undergoes rapid Cl– loss in acetonitrile and yields the solvento-complex fac-[ReI(CO)3(NN)­(CH3CN)]+, which is then reversibly oxidized at more positive potentials and is responsible for the second one-electron oxidation wave. Reductive waves were observed at potentials more negative than −0.68 V versus NHE. The quasi-reversible first reduction process is centered on the NN ligand and yields the radical anion fac-[ReI(NN•)­(CO)3Cl]−, while the irreversible second reduction is metal centered (Re+/0) and results in the loss of the Cl– ligand, yielding the pentacoordinate anion [Re0(NN•)­(CO)3]−. − The potentials are summarized in Table .

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(Top) Cyclic voltammogram (solid lines) and differential pulse voltammogram (dashed lines) of the fac-[Re­(NN)­(CO)3Cl] compounds (1 mM) in 0.1 mol L–1 TBAPF6/acetonitrile electrolyte (v(CV) = 100 mV s–1; v(DPV) = 10 mV s–1; T = 298 K) and (bottom) correlation between the 1st oxidation and 1st reduction potentials with the Hammett constant σp of the substituent at the 4,4′ positions.

2. Electrochemical and Photophysical Properties of the fac-[Re­(NN)­(CO)3Cl] Complexes in Acetonitrile.

Substituent σp E 1/2 (V vs NHE) λPL (nm) ϕPL (× 10–3) τPL (ns) ΔG ES (eV) E°(Re+*/0) (V vs NHE)
OCH3 –0.27 –1.23 +1.50 619 3.6 18.7 2.63 1.40
CH3 –0.17 –1.20 +1.54 627 8.0 36.1 2.62 1.42
H 0 –1.11 +1.57 634 5.7 29.7 2.57 1.46
Br 0.23 –0.87 +1.62 680 1.0 5.4 2.43 1.56
COOH 0.45 –0.73 +1.63 713 1.8 32.7/8.8 2.33 1.60
CO2CH3 0.45 –0.68 +1.64 725 1.1 12.8 2.28 1.60
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The decay was fit as a double-exponential with equal contributions of each component.

The electron donating or withdrawing ability of the substituents directly affected the redox potentials of the compounds, as previously reported for similar Re tricabonyl diimine complexes. ,,, The stronger electron withdrawing effect of the −Br, −COOH and −CO2CH3 substituents resulted in more positive potentials than those of Re–bpy, reflecting the easier reduction and more difficult oxidation of electron deficient compounds. In contrast to the electron-withdrawing substituents, the complexes with electron-donating substituents, −CH3 and −OCH3, displayed reduction potentials more negative than those of the unsubstituted Re–bpy. Linear Hammett relationships for the reduction potentials were observed, as shown in Figure (bottom) for the first reduction (ReI–NN→ ReI–NN•, with R2 = 0.98466) and the first oxidation (ReI/II, with R2 = 0.97545). The difference in the first reduction potentials between the most electron-withdrawing −CO2CH3 (σp = +0.45) and electron donating −OCH3 (σp = −0.27) was approximately 550 mV.

Excitation at the lowest-energy absorption bands of the fac-[Re­(NN)­(CO)3Cl] compounds at room temperature resulted in photoluminescence (PL) from their lowest-lying 3MLCT excited state. The PL bands, shown in Figure -a (top), were broad and nonstructured, typical of similar Re­(I) polypyridyl complexes. ,,, The spectra show the substantial effect that the different substituents at the bipyridine ligand have on the spectral properties of the complexes. The emission maxima (λPL), summarized in Table , displayed a systematic bathochromic shift of approximately 110 nm going from the most electron-donating to the most electron-withdrawing substituent. The latter accommodates more efficiently the charge in the MLCT excited state. The energy of the PL bands also displayed a linear correlation with the Hammett constants of the substituent groups, as shown in Figure -a (bottom). A schematic energy level diagram for the fac-[Re­(NN)­CO3Cl] compounds is shown in Figure b. On the other hand, the PL lifetimes (τPL) and quantum yields (ΦPL) increased in the order Re–(Br)­bpy < Re–(CO2CH3)­bpy < Re–(OCH3)­bpy < Re–(COOH)­bpy < Re–bpy < Re–(CH3)­bpy, as shown in Table and Figure S7. This trend does not track the Hammett constants of the substituent groups, since heavy-atom effect and hydrogen bonding may also play a role in the deactivation of the excited state of the complexes. − Furthermore, the involvement of other excited states with energies close to that of the 3MLCT can play a role in the deactivation process, particularly when the 3MLCT state is higher in energy.

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(a) (Top) Steady-state photoluminescence spectra of the indicated fac-[Re­(NN)­(CO)3Cl] compounds in deaerated acetonitrile (λexc= 375 nm, T = 298 K) and (bottom) correlation between the energy of the PL maxima and the Hammett constant (σp) of the substituent at the 4,4′ positions. (b) Schematic energy level diagram for the fac-[Re­(NN)­CO3Cl] compounds. The dashed lines correspond to the E°(Re+*/0) energy level.

The reductive quenching of the 3MLCTRe → NN state of the fac-[Re­(NN)­(CO)3Cl] compounds by sacrificial electron donors was investigated by measuring the photoluminescence intensity of the complexes in deaerated acetonitrile as a function of the concentration of TEOA or BIH, as shown in Figure S8 and S9. All the compounds investigated have enough reducing power in their excited states to be quenched by either TEOA or BIH. The driving forces for this reaction, ΔG red, correlated well with the Hammett constants of the substituent groups, Table and Figure a. The quenching data were analyzed using the Stern–Volmer equation, Eq. S1, revealing linear relationships , between ∫I0/∫I (where I0 and I are the PL intensities in the absence and presence of the quencher, respectively) and the quencher concentration, as shown in Figure b and Figure S10. When TEOA was employed as the quencher, the Stern–Volmer constants K SV spanned from 0.17 to 1.53 L mol–1, partially following the trend of σp. Dividing K SV by the respective PL lifetimes in the absence of TEOA provided the bimolecular quenching rate constants (k q) on the order of 107 L mol–1 s–1, consistent with the reductive quenching of the 3MLCTRe → NN excited state of Re­(I) tricarbonyl polypyridyl complexes by tertiary amines. ,, The ln­(k q) values increased with the electron-withdrawing effect of the substituent group at the bpy ligand, Figure c. The quenching fractions (ηq) of the excited photocatalysts by TEOA (1.26 mol L–1) were calculated using Eq. S2, representing the proportion of reduced molecules generated from the excited states. The ηq values ranged from 0.18 to 0.66; the yields less than a unit indicate that the formation of the OERS is not optimal, especially for the photocatalysts having electron-donating substituents.

3. Driving Force (ΔG red) for the Reductive Quenching of the 3MLCTRe → NN State of fac-[Re­(NN)­(CO)3Cl] by TEOA or BIH, Stern–Volmer Constants (K SV), Reductive Quenching Rate Constants (k q), and Quenching Fractions (ηq) in Deaerated Acetonitrile.

    ΔG red (kcal mol–1)
 K SV (L mol–1)
 k q (107 L mol–1 s–1)
 ηq
Substituent σp TEOA BIH TEOA BIH TEOA BIH TEOA BIH
OCH3 –0.27 –8.2 –19.0 0.17 146 0.91 781 0.18 0.99
CH3 –0.17 –8.6 –19.5 0.38 272 1.1 753 0.32 1.00
H 0 –9.6 –20.4 0.51 242 1.7 815 0.39 0.99
Br 0.23 –12.0 –22.8 0.47 144 8.3 2530 0.37 1.00
COOH 0.45 –13.0 –23.8 1.53 217 8.1 1140 0.66 1.00
CO2CH3 0.45 –12.9 –23.7 0.57 104 4.5 813 0.42 0.99
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Calculated for [TEOA] = 1.26 mol L–1.

b

Calculated for [BIH] = 0.1 mol L–1.

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(a) Correlations between the driving force for the reductive quenching of the excited state of fac-[Re­(NN)­(CO)3Cl] with the indicated substituents by TEOA (hollow circles) or BIH (solid circles) and the respective Hammett constants. (b) Stern–Volmer plots of the reductive quenching of the 3MLCTRe → NN state of the fac-[Re­(NN)­(CO)3Cl] complexes by TEOA in deaerated acetonitrile. (c) Relationships between ln­(k q) and Hammett constants (σp) of the complexes having the indicated substituents.

The reductive quenching of the excited state of the fac-[Re­(NN)­(CO)3Cl] compounds by BIH is thermodynamically more favorable than by TEOA by 10.8 kcal mol–1. Consequently, the K SV values spanned from 104 to 272 L mol–1, with respective k q constants ranging from 7.5 × 109 L mol–1 s–1 to 5.5 × 1010 L mol–1 s–1, being at least 2 orders of magnitude faster than with TEOA. The quenching fractions ([BIH] = 0.1 mol L–1) were unitary for all the complexes, independently of the substituent group, suggesting that during photocatalysis, the quenching step does not limit the efficiency of CO2-to-CO conversion when BIH is present.

Formation and Stability of Reduced Re Species

The reductive quenching process yields the corresponding OERS of the photocatalyst, , fac-[Re­(NN•)­(CO)3Cl]−. The photogeneration of the OERS was examined by irradiating with monochromatic light (λirr = 365 nm) solutions of the fac-[Re­(NN)­(CO)3Cl] complexes in a DMF-TEOA (5:1 v/v) mixture under an argon or CO2 atmosphere. Their absorption spectra were recorded over time during irradiation, as shown in Figure a for Re–(CH3)­bpy and Figures S11 to S15 for the other complexes. Upon irradiation, broad absorption bands appeared between 450 and 600 nm, with maxima summarized on Table , which are diagnostic of the radical anionic form of the bpy ligand (NN•–). , The rates of formation of the OERS were strongly influenced by the substituent on the NN ligand. The fastest rates were observed for the formation of the OERS of Re–bpy and Re–(CH3)­bpy, while all the other substituent groups, independently of having electron donating or withdrawing features, resulted in slower formation rates, as shown in Figure S22a.

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(Top) Electronic absorption spectra recorded during the irradiation of Re–(CH3)­bpy in 5:1 v/v DMF–TEOA (λirr = 365 nm; I 0 = 1.0 × 10–8 einstein s–1). (Bottom) Normalized absorbance changes (λ = 519 nm) measured during the dark decay of the OERS of Re–(CH3)­bpy under Ar and CO2 atmospheres.

4. Wavelengths of Maximum Absorption for the Lowest-Energy Bands of the OERS of the fac-[Re­(NN)­(CO)3Cl] Complexes (λmax OERS), Half-Lifetimes (τ1/2) for the Photoconversion of the Starting Compounds into the Corresponding OERS under an Ar Atmosphere, and Decay Half-Lifetimes of the OERS in 5:1 DMF–TEOA Solutions under an Ar or CO2 Atmosphere .

      τ1/2 (s)
Substituent σp λmax OERS (nm) Formation (Ar) Decay (Ar) Decay (CO2)
OCH3 –0.27 500 41 ± 3 1.9 ± 0.1 1.1 ± 0.3
CH3 –0.17 519 4.8 ± 0.2 2.0 ± 0.3 0.8 ± 0.1
H 0 505 4.4 ± 0.3 2.2 ± 0.3 1.6 ± 0.3
Br 0.23 507 36 ± 4 450 ± 38 99 ± 21
COOH 0.45 495; 522 18 ± 3 4.3 ± 0.3 1.3 ± 0.3
CO2CH3 0.45 485; 520 451 ± 32 1584 ± 40 1309 ± 58
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λirr = 365 nm; I 0 = 1.0 × 10–8 einstein s–1.

After formation of the OERS in the catalytic cycle, it can either release the axial ligand to form the “17-electron species”, fac-[Re­(CO)3(NN)], which reacts with CO2 to form an adduct, or supply to the CO2 adduct the second electron necessary for the formation and release of CO. While the latter role requires the OERS to be sufficiently stable, it has to be reactive enough to allow formation of the CO2 adduct. In this context, the stability and reactivity of the photochemically generated OERS were investigated as follows. Solutions of the fac-[Re­(NN)­(CO)3Cl] photocatalysts in a DMF-TEOA (5:1 v/v) mixture were irradiated with monochromatic light (λirr = 365 nm, I0 = 1.0 × 10–8 einstein s–1) under an argon or CO2 atmosphere until the absorption spectra stopped to change. Immediately after interrupting irradiation, the absorbance changes at the lowest-energy absorption maxima of the OERS were recorded in the dark, as shown in Figure b for Re–(CH3)­bpy and Figures S17 to S21 for the other five complexes. The corresponding rate constants are summarized in Table . The rates of decay of the OERS of the complexes Re–bpy, Re–(OCH3)­bpy, and Re–(CH3)­bpy were identical within the experimental error, and approximately twice as fast under CO2 than under argon. Decay rates were slower for the OERS of complexes with electron-withdrawing substituents, but not necessarily following the trend of their Hammett constants, τ1/2 Re–(COOH)­bpy < τ1/2 Re–(Br)­bpy < τ1/2 Re–(CO2CH3)­bpy, Figure S22b. Notably, while the decay of the OERS of Re–(COOH)­bpy and Re–(Br)­bpy was at least three times faster under CO2 than under argon, the presence of CO2 did not enhance significantly the decay OERS of Re–(CO2CH3)­bpy, indicating poor reactivity that can negatively affect catalysis.

Photocatalytic CO2 Reduction

Characterization of the properties and reactivity of the six complexes shown in Figure demonstrated that they are suitable candidates for promoting the photocatalytic reduction of CO2 to CO. Photoreduction of carbon dioxide was investigated by irradiating (λirr = 405 nm) 0.5 mM solutions of the fac-[Re­(NN)­(CO)3Cl] complexes in a 1:5 mixture of TEOA/DMF and quantifying the amount of CO produced as a function of time. All the complexes, except for Re–(CO2CH3)­bpy, efficiently converted CO2 into CO upon irradiation, with significant variations in the turnover numbers (TONCO) and turnover frequencies (TOFCO) depending on the substituent of the NN ligand, as shown in Figure a and Table . Upon increasing the electron-donating effect of the substituent, the photocatalytic efficiency decreased in comparison to the unsubstituted photocatalyst, TONCO Re–bpy > Re–(CH3)­bpy > Re–(OCH3)­bpy, Figure . This decrease can be directly linked to the lower quenching efficiencies of the excited state of the complexes by TEOA under photocatalytic conditions, Table . Furthermore, the considerably lower TOFCO for Re–(OCH3)­bpy reflects the slower formation of its OERS, as shown in Table . Meanwhile, the presence of progressively stronger electron-withdrawing substituents also led to a decrease in the photocatalytic efficiency, TONCO Re–bpy > Re–(Br)­bpy > Re–(COOH)­bpy > Re–(CO2CH3)­bpy. While in these cases the quenching efficiencies are equal or higher than the value measured for Re–bpy, the slower formation of the OERS and rapid deactivation of the photocatalyst (evidenced by the faster plateauing of the curves in Figure a) are probably the dominating factors in reducing their performance. Hydrogen bonding can also significantly affect the catalytic ability due to different degrees of stabilization of key intermediates, − which might be the case for Re–(COOH)­bpy, which was reported to be poorly active in electrochemical CO2 reduction. Re–(CO2CH3)­bpy did not photocatalyzed the reduction of CO2 to CO as a result of very slow accumulation of the OERS, and poor reactivity of the OERS with CO2.

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Turnover numbers (TONCO) for the formation of CO catalyzed by the indicated fac-[Re­(NN)­(CO)3Cl] photocatalysts. The measurements were performed in DMF–TEOA 5:1 v/v solutions saturated with CO2 and containing 0.5 mmol L–1 of the photocatalysts, in the (a) absence or (b) presence of BIH (λirr = 405 nm).

5. Photocatalytic Activity of the Indicated fac-[Re­(NN)­(CO)3Cl] Photocatalysts for CO2 Reduction in DMF–TEOA (5:1 v/v) under a CO2 Atmosphere, λ = 405 nm.

    No BIH
 BIH
Substituent σp TON TOF (h–1) TON TOF (h–1)
OCH3 –0.27 4.7 ± 0.9 0.8 ± 0.1 83 ± 2 2.3 ± 0.2
CH3 –0.17 11 ± 1 1.5 ± 0.3 81 ± 4 3.3 ± 0.2
H 0 23.8 ± 0.1 1.7 ± 0.3 81 ± 3 2.7 ± 0.3
Br 0.23 17.7 ± 0.5 1.4 ± 0.1 83 ± 3 2.07 ± 0.01
COOH 0.45 5.5 ± 0.5 1.95 ± 0.07 37 ± 1 1.98 ± 0.02
CO2CH3 0.45 0 0 0 0
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Dependence on the maximum turnover numbers for CO formation and Hammett constants of the substituents (σp) of the fac-[Re­(NN)­(CO)3Cl] photocatalysts, measured in DMF–TEOA (5:1 v/v) under a CO2 atmosphere in the absence (hollow circles) or presence (solid circles) of 0.1 M BIH, λ = 405 nm.

It is important to note that the maximum TONCO achieved was 23.8 for Re–bpy, consistent with previous reports under similar conditions. It was demonstrated that the TONCO can be increased by employing a sacrificial electron donor with higher reducing power. TEOA is oxidized at 1.04 V vs NHE, and it quenches only approximately 40% of the excited state of Re–bpy at >1 M concentration. BIH has a much stronger reducing power, being oxidized at 0.57 V vs NHE. Besides, in the presence of a base such as TEOA, the one-electron oxidized radical BI• is formed rapidly, which is also a very strong reducing agent (−1.8 V vs NHE) and can donate a second electron. From Table , it is clear that even 0.1 M BIH can completely quench the excited state of all the complexes investigated herein and this step should not limit the catalysis. Therefore, to further evaluate the photocatalytic performance of each complex, 0.1 M BIH was added to the previously mentioned solutions. As shown in Figure b and Table , there was a significative enhancement of the TONCO and TOFCO in comparison to the experiments without addition of the benzimidazole hydride. For example, the TONCO of Re–bpy displayed a 3-fold improvement, and Re–(OCH3)­bpy exhibited a 17-fold increase. Interestingly, Re–bpy, Re–(CH3)­bpy, Re–(OCH3)­bpy and Re–(Br)­bpy exhibited the same TONCO values, Figure , despite the very different intensity of electron-donation/withdrawing effect of these substituents. A difference was observed in the TOFCO values for these complexes, that were smaller for Re–(OCH3)­bpy and Re–(Br)­bpy due to the slower formation of the OERS, as shown in Table . Similar to what was observed in the absence of BIH, Re–(COOH)­bpy was deactivated faster and exhibited inferior performance than the other photocatalysts.

Overall, electron-donating substituents, such as – CH3 and–OCH3, shifted the MLCT absorption maxima to higher energies, made the complexes more difficult to be reduced, and resulted in slower quenching rate constants in comparison to Re–bpy, but their corresponding OERS reacted quickly with CO2. On the other hand, electron-withdrawing substituents shifted the absorption maxima to the visible region, which is desirable for sunlight energy conversion, and resulted in a more favorable and fast reduction of the excited state of the complexes by sacrificial electron donors, but at the cost of slower reactivity of the OERS. When TEOA was employed as the sacrificial electron donor, the unsubstituted Re–bpy exhibited the highest efficiency of the series, with a TONCO = 23.8, while the complex with electron-withdrawing –COOH groups exhibited the highest TOFCO = 1.95 h–1, despite of its rapid deactivation. The catalytic performance was increased by employing BIH, a stronger reductant, as the sacrificial electron donor. Under these conditions, the identity of the substituent did not affect the TONCO for most cases, being identical for Re–bpy, Re–(CH3)­bpy, Re–(OCH3)­bpy and Re–(Br)­bpy, with an average TONCO of 82. The highest TOFCO of 3.3 h–1 was achieved by Re–(CH3)­bpy. The comparison between the performance of the different photocatalysts under these two distinct experimental conditions allowed to rationalize the interplay between the diverse processes that drive photocatalysis, and demonstrated that an efficient quenching can neglect, to some extent, a less efficient reactivity of OERS.

Conclusions

It is essential to understand how structural modifications impact the CO2 reduction to CO performance to develop robust and practical molecular photocatalysts for this process. A systematic study of the effect of the 4,4′-bpy substituents in a series of Re­(I) tricarbonyl photocatalysts allowed the correlation of Hammett constants with their spectroscopic, electrochemical, photophysical, and photochemical properties relevant to catalysis. The photocatalytic experiments provided clear insights into how the different substituents affect key steps involved in the CO2 reduction cycle, such as reductive quenching, formation and reactivity of intermediates, and decomposition of the catalysts. These individual steps, when combined, have a significant impact on the overall catalytic performance. Moreover, by changing the reductive power of the sacrificial electron donor, it was possible to modulate the contributions of these steps, offering an additional degree of control over the photocatalysis. This highlights the potential to optimize the CO2 photoreduction performance through strategic manipulation of both the ligand environment and reaction conditions.

Experimental Section

Materials

The following reagents and materials were used as received from the indicated commercial suppliers: 2,2′-bipyridine (Vetec, 98%), 4,4′-dimethoxy-2,2′-bipyridine (Oakwood Chemical, 98%), 4,4′-dimethyl-2,2′-bipyridine (Strem, 99%), 4,4′-dibromo-2,2′-bipyridine (Ark Pharm, 98%), [Re­(CO)5Cl] (Aldrich, 98%), tetra-n-butylammonium hexafluorophosphate (TBAPF6; Fluka, ≥ 99.0%), ferrocene (Aldrich, 98%), Na2SO4 (Synth, anhydrous, 99.0%), K2Cr2O7 (Synth, 99.5%), 2-phenylbenzimidazole (Oakwood, 97%), potassium carbonate (K2CO3; Acros Organics, Anhydrous, 98%), sodium borohydride (NaBH4; Aldrich, 98%), triethanolamine (TEOA; Sigma-Aldrich, ≥ 99%), HCl (Synth, 36.5%), H2SO4 (Merck, 95–97%), NaOH (Sigma-Aldrich, > 98%), iodomethane (Sigma-Aldrich, 99%), acetonitrile (Sigma-Aldrich, HPLC grade, ≥ 99.9%), CD3CN (Aldrich, ≥ 99.8%), ethanol (Merck, LiChrosolv, ≥ 99.9%), methanol (Synth, 99.8%), diethyl ether (Synth, 98.0%), xylene (Synth, 98.5%), n-pentane (Synth, 99%), dichloromethane (Synth, 99.5%), hexanes (Synth, 98%), argon gas (Airgas, > 99.998%), CO2 gas (White Martins, > 99.8%). N,N-dimethylformamide (DMF; Merck, ≥ 99.8%) was dried over activated 4 Å molecular sieves prior to use. K3[Fe­(C2O4)3]·3H2O was available from a previous study.

General Methods

1H NMR spectra of mM CD3CN solutions were recorded at 298 K on a 500 MHz Varian spectrometer, with chemical shifts referenced to residual solvent proton signals. A Thermo Scientific Flash EA 1112 elemental analyzer was employed for the determination of weight percentage content of carbon, hydrogen and nitrogen of the solid complexes. UV–vis absorption measurements were performed using an Agilent 8453 diode-array spectrophotometer. Steady-state photoluminescence spectra were measured in quartz cuvettes using a Cary Eclipse spectrofluorometer. Photoluminescence lifetimes were measured using a PicoQuant FluoTime 300 spectrometer. The excitation source was a 375 nm diode laser (LDH–P-C-375B, 40 MHz repetition rate, 52 ps pulse width, PicoQuant GmbH) driven by a PDL 820 computer controller. Before the photoluminescence measurements, the solutions were bubbled with argon gas for 30 min. Electrochemical measurements were conducted using a μAutolab III potentiostat/galvanostat (Autolab) and a standard three-electrode cell, equipped with a glassy carbon working electrode (Metrohm), a Pt rod counter electrode (Metrohm), and an Ag wire as a pseudoreference electrode. Solutions for analysis were prepared by dissolving the complexes (1 mM) in a 0.1 mol L–1 TBAPF6/CH3CN electrolyte, and bubbling Ar gas for at least 15 min before the measurements. An argon blanket was maintained over the solutions during the experiments. The ferrocenium/ferrocene (Fc+/Fc) redox couple was used as the internal standard. All potentials were converted to the NHE scale using the Fc+/Fc half-wave potential (+630 mV versus NHE in acetonitrile).

Synthesis

2,2′-Bipyridine-4,4′-dicarboxylic acid, (COOH)­bpy

A procedure previously reported in the literature was employed to synthesize the ligand 2,2′-bipyridine-4,4′-dicarboxylic acid. Yield: 93%. Analysis (calcd, found for C12H8N2O4): C (59.02, 58.74), H (3.30, 3.24), N (11.47, 11.08). NMR 1H (D2O/NaOD, 500 MHz, δ/ppm): 8.61 (d, 2H, J = 4.9 Hz); 8.19 (s, 2H); 7.73 (d, 2H, J = 4.9 Hz)).

4,4′-Dimethoxycarbonyl-2,2′-bipyridine, (CO2CH3)­bpy

The ligand 4,4′-dimethoxycarbonyl-2,2′-bipyridine was synthesized according to a reported procedure. Yield: 86%. Analysis (calcd, found for C14H12N2O4): C (61.72, 61.66), H (4.41, 4.36), N (10.28, 9.92). NMR 1H (CDCl3, 500 MHz, δ/ppm): 8.96 (s, 2H), 8.86 (d, 2H, J = 4.9 Hz), 7.90 (dd, 2H, J = 4.9 and 1.6 Hz), 3.99 (s, 6H).

fac-[Re­(NN)­(CO)3Cl] Complexes

The complexes fac-[Re­(NN)­(CO)3Cl], where NN = 2,2′-bipyridine (bpy), 4,4′-dimethoxy-2,2′-bipyridine ((OCH3)­bpy), 4,4′-dimethyl-2,2′-bipyridine ((CH3)­bpy), 4,4′-dibromo-2,2′-bipyridine ((Br)­bpy), or 4,4′-dimethoxycarbonyl-2,2′-bipyridine ((CO2CH3)­bpy) were prepared based on reported methods for the synthesis of other Re­(I) diimine tricarbonyl complexes. ,, To a round-bottom flask, [Re­(CO)5Cl] and 1.5 equiv of the corresponding NN ligand were suspended in xylene. The mixture was refluxed for 2–6 h under an argon atmosphere. The reaction mixture was filtered hot, and the obtained solid was washed with diethyl ether. The product was recrystallized from dichloromethane with slow addition of n-pentane.

The complex fac-[Re­((COOH)­bpy)­(CO)3Cl] was prepared following a modified literature procedure. To a round-bottom flask, [Re­(CO)5Cl] and 1.2 equiv of (COOH)­bpy were suspended in ethanol. The mixture was refluxed for 7 h under an argon atmosphere. The reaction mixture was filtered hot, and the obtained solid was washed with diethyl ether. The product was recrystallized from acetone with slow addition of hexane.

Re-bpy (Yield: 77%. Analysis (calcd, found for C13H8ClN2O3Re): C (33.81, 34.24), H (1.75, 1.73), N (6.07, 5.73). NMR 1H (CD3CN, 500 MHz, δ/ppm): 9.02 (d, 2H, J = 5,5 Hz), 8.43 (d, 2H, J = 8.1 Hz), 8.20 (dt, 2H, J = 8.1 and 1.3 Hz), 7.63 (dt, 2H, J = 5.5 and 1.3 Hz)).

Re-(OCH3)­bpy (Yield: 80%. Analysis (calcd, found for C15H12ClN2O5Re): C (34.52, 34.81, H (2.32, 2.46), N (5.37, 5.39). NMR 1H (CD3CN, 500 MHz, δ/ppm): 8.77 (d, 2H, J = 6.5 Hz), 7.88 (d, 2H, J = 2.7 Hz), 7.13 (dd, 2H, J = 6.5 and 2.7 Hz), 4.04 (s, 6H)).

Re-(CH3)­bpy (Yield: 81%. Analysis (calcd, found for C15H12ClN2O3Re): C (36.77, 37.37), H (2.47, 2.47), N (5.72, 5.49). NMR 1H (CD3CN, 500 MHz, δ/ppm): 8.83 (d, 2H, J = 5.6 Hz), 8.27 (s, 2H), 7.44 (d, 2H, J = 5.6 Hz), 2.56 (s, 6H)).

Re-(Br)­bpy (Yield: 77%. Analysis (calcd, found for C13H6BrClN2O3Re): C (25.20, 25.61), H (0.98, 0.97), N (4.52, 4.36). NMR 1H (CD3CN, 500 MHz, δ/ppm): 8.49 (d, 2H, J = 5.9 Hz), 8.33 (d, 2H, J = 2.0 Hz), 7.49 (dd, 2H, J = 5.9 and 2.0 Hz)).

Re-(COOH)­bpy (Yield: 71%. Analysis (calcd, found for C15H8ClN2O7Re): C (32.76, 32.29), H (1.47, 1.65), N (5.09, 4.78). NMR 1H (CD3CN, 500 MHz, δ/ppm): 9.16 (d, 2H, J = 5.6 Hz), 9.02 (s, 2H), 8.06 (dd, 2H, J = 5.6 and 1.6 Hz)).

Re-(CO2CH3)­bpy (Yield: 71%. Analysis (calcd, found for C17H12ClN2O7Re): C (35.33, 35.49), H (2.09, 2.29), N (4.85, 4.69). NMR 1H (CD3CN, 500 MHz, δ/ppm): 9.20 (d, 2H, J = 5.8 Hz), 8.96 (s, 2H), 8.07 (dd, 2H, J = 5.8 and 1.7 Hz), 4.02 (s, 6H)).

1,3-Dimethyl-2-phenyl-2,3-dihydro-1H-benzo­[d]­imidazole, BIH

BIH (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo­[d]­imidazole) was prepared with modifications on the reported procedure. To a round-bottom flask, 50 mL of methanol, 9.0 g of 2-phenylbenzimidazole (47 mmol) 6.4 g of K2CO3 (47 mmol), and 11.6 mL of iodomethane (188 mmol) were added, and the mixture was heated at 50 °C for 18 h. The suspension was allowed to cool down to room temperature, the solid portion was filtered off, washed with methanol multiple times, and dried under vacuum to yield 9.4 g (27 mmol) of 2-phenyl-1,3-dimethylbenzimidazolium iodide (yield = 57%). 1H NMR (400 MHz, DMSO-d 6) δ 8.14 (dd, J = 6.3, 3.1 Hz, 2H), 7.92 (dd, J = 8.3, 1.6 Hz, 2H), 7.88–7.73 (m, 5H), 3.90 (s, 6H). The solid was dissolved in 225 mL of methanol and cooled in an ice bath, followed by the slow addition of 5.1 g of sodium borohydride (135 mmol) in small portions. After stirring at room temperature for approximately 30 min, the resulting white solid was collected by filtration and rinsed with water. The final product was recrystallized from a 5:1 ethanol/water mixture, dried under vacuum and stored inside an inert atmosphere glovebox until further use. Yield: 7.0 g (31 mmol, 66%). 1H NMR (400 MHz, DMSO-d 6) δ 7.55 (dd, J = 6.4, 1.9 Hz, 2H), 7.45 (dd, J = 5.1, 2.1 Hz, 3H), 6.62 (dd, J = 5.4, 3.1 Hz, 2H), 6.45 (dd, J = 5.5, 3.2 Hz, 2H), 4.87 (s, 1H), 2.48 (s, 6H).

Photocatalytic Studies

The photocatalytic CO2 reduction experiments and quantification of gaseous products were performed in a custom-built system previously described. , Each complex (0.5 mM) was dissolved in a 1:5 mixture of triethanolamine/DMF to a total volume of 10 mL. Alternatively, 0.1 M BIH was also added to the solutions. The mixtures were transferred to a 25 mL Pyrex glass bottle equipped with a homemade GL-25 threaded screw cap, which has three embedded 1/4–28 flat bottom HPLC fitting ports for an airtight connection of 1/16” OD gas tubes: one for the inlet of CO2 gas, on for exhaust outlet, and one connecting to the gas chromatograph (GC) sampling loop, through actuated solenoid valves (Bio-Chem Fluidics). An orbital shaker was used to continuously mix the solutions. The solutions were irradiated through the bottom of the bottle using a 405 nm LED. Under the conditions employed herein, all photons were absorbed by the photocatalysts. The experimental setup was automated using a custom LabVIEW software that controlled the LED, the opening and closing of the actuated valves, and GC data collection. During a typical experiment, before the photolysis, CO2 was initially bubbled in the solution for 20 min through the gas inlet port and exited through the exhaust port. The solution was then irradiated for 5 min, after when the LED was switched off and the sample was allowed to equilibrate for 7 min. This time was necessary to equilibrate the gaseous products between the solution and the headspace. The GC loop was evacuated for 3 min before automatic injection of the headspace gas. The quantification of the gaseous products was performed using an Agilent 6890 N gas chromatograph having a HP-PLOT Molesieve column (30 m × 0.32 mm × 1.5 μm). A TCD detector was used to detect any O2, N2, or H2 present in the sample, while a FID detector was employed to detect CO, which was previously converted to CH4 on a methanizer. The purging was repeated for 10 min before the next irradiation cycle to remove all the gaseous products that were generated previously. The following irradiation times were increased through an arithmetic progression.

The formation and stability of the one-electron reduced species of the photocatalysts were investigated by irradiating solutions of the photocatalysts in 5:1 v/v DMF–TEOA at 365 nm using an Oriel 200 W high-pressure Hg­(Xe) arc lamp (model 6291), mounted in a Newport lamp house (model 67005) with a Newport universal power supply (model 69907). A suitable interference filter was employed to select the irradiation wavelength, and a shutter and a chronometer were used to manually control the irradiation time. The light intensity was maintained at 1.0 × 10–8 einstein s–1, as measured by chemical actinometry. ,, An Agilent 8453 spectrophotometer was placed perpendicularly to the irradiation source to measure the electronic spectra of the samples over of time.

Supplementary Material

af4c00026_si_001.pdf (1.1MB, pdf)

Acknowledgments

A.V.M., A.S.P, L.G.A.N., and S.N. acknowledge the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for the financial support under grant numbers 2019/23277-4, 2016/24020-9, 2022/16402-0, and 2022/03478-8. W.M.W. acknowledges Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for the financial support. S.N. also acknowledges the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support under grant number 305761/2021-8. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Solar Photochemistry Program, under contract DE-SC0012704.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/aps.4c00026.

  • 1H NMR of photocatalysts, time-dependent photoluminescence intensities, photoluminescence spectra in the presence of different concentrations of sacrificial electron donors, electronic spectra measured during formation of OERS, decay profiles of OERS (PDF)

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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