Iron(II) bis(pyrazolyl)phenanthroline complexes as robust and efficient homogeneous catalysts for CO2-to-CO conversion under visible light - 2026.07.03

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Abstract Summary

This article describes a visible-light-driven molecular photocatalytic system for reducing CO₂ to CO using iron(II) bis(pyrazolyl)phenanthroline complexes as homogeneous catalysts. The catalyst series Fe1-Fe4 was combined with [Ru(bpy)₃]²⁺ as photosensitizer and BIH as sacrificial electron donor in CO₂-saturated MeCN/H₂O.

All four iron complexes were active for CO₂-to-CO photoreduction. Fe2 gave the highest overall activity in the standard comparison experiments, while Fe4 gave the highest CO selectivity in that series. Under lower catalyst loading, Fe2 reached a much higher turnover number for CO, and the system operated with substantial CO selectivity in mixed aqueous organic solvent. The article also supports a homogeneous catalytic pathway and identifies photosensitizer deterioration as a major cause of deactivation.

Advances and Special Progress

A central advance is the introduction of a new family of iron(II) bis(pyrazolyl)phenanthroline molecular catalysts for visible-light CO₂ photoreduction. The article presents this ligand platform as distinct from more commonly studied iron polypyridyl and salophen-type systems, allowing systematic comparison of substituent effects on catalytic behavior.

The work also reports strong performance at very low catalyst loading. For Fe2, decreasing the catalyst concentration increased the CO turnover number substantially, reaching 23,138 under the lowest loading explicitly reported in the main text. The article identifies this as one of the stronger performances among molecular iron-based photocatalytic CO₂-to-CO systems discussed in the paper.

Another important advance is the demonstrated compatibility with water-containing solvent mixtures. The system was studied in MeCN/H₂O, and the article shows that 7.5-10% water is essential for efficient catalysis and high CO selectivity. This is chemically significant because water both enables proton-coupled steps and competes with H₂ evolution.

The paper also provides mechanistic insight through combined photophysical, electrochemical, and computational analysis. Stern-Volmer quenching measurements, cyclic voltammetry, orbital analysis, and atmosphere-dependent electrochemistry support a mechanism in which the excited photosensitizer is quenched primarily by BIH, reduced iron species become accessible within the photosensitizer redox window, and single-electron-reduced catalyst states are implicated in CO₂ activation.

Durability was examined in a chemically informative way. The activity loss over time was linked mainly to degradation of [Ru(bpy)₃]²⁺, not primarily to catalyst destruction, and replenishing the photosensitizer restored CO production. Mercury poisoning tests further supported the conclusion that catalysis remains homogeneous rather than nanoparticle-driven.

Additional Remarks

The chemistry is significant because CO₂-to-CO photoreduction stores reducing equivalents in a useful C₁ product. CO is a valuable synthetic intermediate, but selective photochemical formation is challenging because proton reduction to H₂ competes strongly under many conditions.

This system uses an earth-abundant catalytic metal center, iron, which is an advantage from a sustainability perspective. However, the photochemical system still depends on a ruthenium photosensitizer and a sacrificial electron donor, so it is not a fully sustainable closed-cycle solar fuel system.

The article shows both strengths and limitations of sacrificial molecular photocatalysis. Strengths include clear molecular design, high CO selectivity, tunable ligand effects, and useful mechanistic observables. Limitations include reliance on BIH, sensitivity to photosensitizer degradation, and finite long-term durability under continuous irradiation.

The role of water is chemically important but also highlights practical tradeoffs. Too little water suppresses productive CO₂ reduction, while too much water lowers activity. The article attributes the decline at higher water content partly to limited BIH solubility and reduced efficiency of excited-state quenching.

The study is mechanistically informative because it distinguishes supported observations from interpretation. Product analysis, control experiments, mercury poisoning, Stern-Volmer quenching, UV/Vis changes, and cyclic voltammetry directly support the main conclusions, whereas specific catalyst intermediates in the CO₂ reduction sequence are proposed rather than directly isolated.

Content of the Published Article in Detail

The molecular photocatalytic system contains three main functional components: an iron(II) bis(pyrazolyl)phenanthroline complex as CO₂ reduction catalyst, [Ru(bpy)₃]²⁺ as visible-light photosensitizer, and BIH as sacrificial electron donor. The reaction medium is CO₂-saturated MeCN/H₂O in a sealed borosilicate photoreactor irradiated with blue light at 462 nm. Gas products were analyzed from the headspace by gas chromatography.

The iron complexes Fe1-Fe4 are mononuclear Fe(II) species with tetradentate bis(pyrazolyl)phenanthroline ligands and two water ligands. Magnetic measurements and calculations support high-spin Fe(II) ground states. The article states that all complexes are active for photocatalytic CO₂ reduction to CO, with H₂ as the main competing side product. No significant formate or CH₄ was detected.

The optical role is assigned to [Ru(bpy)₃]²⁺. Upon light absorption, the article discusses the excited metal-to-ligand charge-transfer state of the ruthenium photosensitizer. Quenching experiments and orbital-energy analysis were used to determine how this excited state interacts with BIH and the iron complexes.

The mechanistic interpretation supported by the article is that reductive quenching by BIH is dominant. Stern-Volmer experiments showed that BIH quenches the emission of [Ru(bpy)₃]²⁺ much more efficiently than Fe1-Fe4. The reported quenching constant for BIH is about one order of magnitude larger than for the iron complexes. The article therefore supports the view that BIH primarily reduces the excited photosensitizer, generating a reduced ruthenium species capable of transferring electrons onward.

The article also evaluates possible oxidative quenching by the iron catalysts. Based on the calculated energy-level alignment, the SOMO energies of Fe1-Fe4 do not favor reductive quenching of the excited photosensitizer by the iron complexes, but the SUMO levels lie below the photosensitizer LUMO, making oxidative quenching energetically feasible. Even so, the quenching data show BIH to be the dominant quencher under the studied conditions.

Electrochemical data are used to connect photophysics to catalysis. Cyclic voltammetry showed two reduction waves for the iron complexes in acetonitrile. The article interprets these reductions as predominantly ligand-centered rather than formal Fe(II)/Fe(I)/Fe(0) metal-centered reductions, based on DFT orbital composition and correlation between calculated orbital energies and electrochemical potentials. Within the potential window accessible to reduced [Ru(bpy)₃]⁺, the first reduction event is sufficiently stable to be chemically relevant.

The article states that reduced iron species formed after single-electron reduction react with CO₂. Under CO₂ atmosphere, cyclic voltammograms differed from those under Ar, with progressive changes in current response and peak shape during repeated scans. These observations support the conclusion that the reduced iron complexes interact with CO₂ to form catalytic intermediates that are not re-oxidized within the applied scan window.

The paper does not report direct spectroscopic observation of a specific bound CO₂ intermediate during photocatalysis, but it discusses catalyst reduction followed by CO₂ activation as the productive pathway. It further states that water is essential for effective proton-coupled electron transfer and stabilization of intermediates. In particular, the absence of water nearly suppresses CO formation, while 7.5-10% water strongly increases activity and selectivity.

For Fe4, the article proposes a structural feature that may contribute to selectivity: hydrogen-bonding interactions between coordinated water ligands and CF₃-substituent fluorine atoms distort the coordination environment and may help proton management near the metal center. This is presented as an interpretation linked to its high CO selectivity, not as directly observed catalytic turnover chemistry.

Control experiments strongly support the full photocatalytic assignment. Removing light, catalyst, photosensitizer, sacrificial donor, or CO₂ suppressed productive CO formation. Replacing the molecular iron complex with Fe(ClO₄)₂ gave only minor amounts of CO and H₂, showing that free Fe²⁺ is not responsible for the catalytic behavior.

The article also addresses whether the active species are homogeneous or nanoparticulate. Mercury poisoning experiments did not suppress catalytic CO formation, and this is taken as evidence against catalysis by colloidal or heterogeneous metal particles. The authors therefore conclude that the active system is homogeneous.

Catalyst lifetime was investigated indirectly through time-dependent product formation and UV/Vis monitoring. Activity rose strongly at early times and then approached a plateau. The article attributes most of this deactivation to deterioration of [Ru(bpy)₃]²⁺, supported by hypochromism in the UV/Vis spectra of the reaction mixture over time. When fresh photosensitizer was added after 24 h, CO production resumed and the turnover number increased further, supporting the idea that the catalyst remains largely intact while the photosensitizer degrades.

Overall, the data support the following chemistry in words: light excites [Ru(bpy)₃]²⁺; BIH predominantly quenches the excited state reductively; the reduced photosensitizer transfers electrons to the iron catalyst; a singly reduced iron complex is implicated in CO₂ activation; proton-coupled electron transfer steps in the presence of water lead to CO formation; H₂ is the main side product; and the catalyst functions homogeneously under the reported conditions.

Catalyst

The catalysts are a series of molecular homogeneous mononuclear iron(II) bis(pyrazolyl)phenanthroline complexes labeled Fe1-Fe4. Their formulations are reported as [Fe(bpzRphen)(H₂O)₂]X₂, where the bis(pyrazolyl)phenanthroline ligand bears different pyrazolyl substituents and X is BF₄^- or ClO₄^-.

The article describes them as high-spin Fe(II) complexes with distorted octahedral geometries. The tetradentate bis(pyrazolyl)phenanthroline ligand occupies four coordination sites, and two water ligands occupy the remaining positions. The complexes are used as CO₂ reduction catalysts in a homogeneous visible-light photocatalytic system.

Their redox behavior is reported to be mainly ligand-centered. This is relevant because electron uptake by the catalyst appears to occur on the coordinated ligand framework while still enabling catalytically relevant reduced states that react with CO₂.

Catalyst performance depends on ligand substitution. In the standard 24 h comparison at 50 μM catalyst loading, Fe2 gave the highest CO activity, while Fe4 gave the highest CO selectivity. Fe4 also showed a structurally distinctive distortion associated with possible intramolecular hydrogen-bonding interactions involving coordinated water and CF₃ substituents, which the article suggests may be linked to selectivity.

Regarding stability, the article concludes that the catalysts are robust enough to remain active in solution and that major deactivation is due mainly to photosensitizer deterioration rather than catalyst failure. Mercury poisoning tests also support the conclusion that the catalysis is homogeneous rather than nanoparticle-based.

Photosensitizer

The photosensitizer is [Ru(bpy)₃]²⁺. It serves as the visible-light absorber and initiates the photoredox sequence upon irradiation at 462 nm. The article discusses its excited metal-to-ligand charge-transfer state as the relevant photoactive state.

Photophysical measurements show that this excited state is quenched by both BIH and the iron complexes, but BIH is the more efficient quencher. The article therefore supports a mechanism in which [Ru(bpy)₃]²⁺* is quenched primarily reductively by BIH, producing a reduced ruthenium species that can transfer electrons to the iron catalyst.

The photosensitizer is chemically suitable here because its reduced form is sufficiently reducing to access the first catalyst reduction event. Electrochemical analysis explicitly compares the catalyst reduction window with the potential of the ruthenium couple.

A major limitation is stability under prolonged irradiation. The article reports photodegradation of [Ru(bpy)₃]²⁺, observed as hypochromism in UV/Vis spectra of the reaction mixture. Replenishment of the photosensitizer after 24 h restored catalytic CO production, identifying photosensitizer deterioration as a main cause of deactivation.

Investigation

	cat	cat conc [µM]	PS	PS conc [mM]	e-D	e-D conc [M]	.	.	solvent A	.	.	.	.	.	.	λexc [nm]	.	TON CO	.	.	TON H2	.	.
1.	Fe1	50	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		987			187			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
2.	Fe2	50	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		1318			243			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
3.	Fe3	50	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		847			205			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
4.	Fe4	50	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		1265			133			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
5.	Fe2	50	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		311			51			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
6.	Fe2	50	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		1578			296			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
7.	Fe2	50	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		1593			300			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
8.	Fe2	50	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		2			22			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
9.	Fe2	50	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		1352			285			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
10.	Fe2	50	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		661			191			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
11.	Fe2	50	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		621			77			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
12.	Fe2	25	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		2086			123			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
13.	Fe2	12.5	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		4259			276			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
14.	Fe2	6.25	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		10168			862			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
15.	Fe2	3.12	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		23138			2177			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.
16.	Fe2	3.12	[[[Ru(bpy)3]2+ ]]	0.3	BIH	0.11			MeCN							462		9754			3310			Property "Photosensitizer" (as page type) with input value "Ru(bpy)3]2+" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. "not reported" is not a number.

Investigation-Name: inv0ExportRefresh