Iron(II) bis(pyrazolyl)phenanthroline complexes as robust and efficient homogeneous catalysts for CO2-to-CO conversion under visible light - 2026.08.27-2

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Abstract Summary

This work describes a homogeneous molecular photocatalytic system for the visible-light reduction of CO₂ to CO using iron(II) bis(pyrazolyl)phenanthroline complexes as catalysts. The system uses an iron catalyst, [Ru(bpy)₃]²⁺ as photosensitizer, BIH as sacrificial electron donor, and mixed MeCN/H₂O solvent under blue-light irradiation.

All four iron complexes were active for CO formation. The best overall catalyst in the main comparison was Fe2, which reached a reported TON_CO of 1318 with 84% CO selectivity under standard conditions. Under lower catalyst loading, Fe2 reached a much higher TON_CO of 23,138 with CO selectivity up to 91%. Fe4 gave the highest CO selectivity in the catalyst series under the standard comparison, reaching 91%. The study also reports electrochemical, spectroscopic, and theoretical evidence supporting ligand-centered reduction and a homogeneous catalytic process.

Advances and Special Progress

A key advance is the introduction of a new iron(II) catalyst family based on bis(pyrazolyl)phenanthroline ligands for photocatalytic CO₂-to-CO conversion. The article presents this ligand platform as previously underexplored for this reaction, while showing that it can support active and selective homogeneous photocatalysis under visible light.

The study also shows strong catalytic performance at low catalyst loading. Fe2 reached a reported TON_CO of 23,138 at 3.12 μM catalyst concentration, which the article identifies as among the higher values reported for Fe-based homogeneous photocatalysts. High CO selectivity was maintained across conditions, including up to 91% under low loading.

Another reported advance is solvent compatibility with added water. The system operates in MeCN/H₂O mixtures, and the article identifies 7.5-10% water as especially beneficial for catalytic activity and selectivity. Water is described as essential for effective proton-coupled electron transfer and stabilization of intermediates.

The work also provides mechanistic insight. Electrochemical and DFT results support predominantly ligand-centered reduction rather than simple metal-centered reduction. Emission quenching experiments show that BIH quenches the excited photosensitizer much more efficiently than the Fe complexes. Mercury poisoning experiments and control reactions support a homogeneous catalytic system, while catalyst deactivation is attributed mainly to photosensitizer deterioration rather than immediate catalyst failure.

Additional Remarks

The chemistry is significant because photochemical CO₂ reduction converts a thermodynamically stable molecule into CO, a useful carbon feedstock. The article emphasizes this as a route to value-added products under mild conditions, while also showing how ligand design can tune catalyst activity and selectivity.

At the same time, this remains a sacrificial photochemical system. It depends on BIH as a sacrificial electron donor and on [Ru(bpy)₃]²⁺ as a noble-metal photosensitizer. This means the system is chemically informative and mechanistically useful, but not fully earth-abundant as a complete light-harvesting platform.

The reaction competes with H₂ evolution, and product selectivity depends strongly on catalyst structure, water content, and catalyst loading. Water is beneficial only within a limited range. Too little water suppresses CO formation, while too much water lowers activity, which the article relates in part to poor BIH solubility in more aqueous media.

Durability is mixed. The iron catalyst is described as robust and homogeneous, but the overall system still deactivates over time, mainly because the photosensitizer undergoes photodegradation. Thus, the work demonstrates strong molecular catalyst performance while also illustrating a common limitation of sacrificial photocatalytic CO₂ reduction systems.

Content of the Published Article in Detail

The article studies four molecular iron(II) complexes, Fe1-Fe4, each containing a tetradentate bis(pyrazolyl)phenanthroline ligand and two coordinated water molecules in a distorted octahedral environment. The pyrazolyl substituents were varied to probe structure-activity relationships: H in Fe1, methyl in Fe2, phenyl in Fe3, and CF₃ in Fe4. All complexes were characterized as high-spin Fe(II) species.

The photocatalytic experiments used a borosilicate photoreactor containing catalyst, [Ru(bpy)₃]²⁺ as photosensitizer, and BIH as sacrificial electron donor in a CO₂-saturated MeCN/H₂O solution. Irradiation was performed with blue light at 462 nm. Gas products in the headspace were analyzed by gas chromatography. CO and H₂ were the detected gaseous products, while no significant formate or CH₄ was detected.

The article distinguishes clearly between observed data and mechanistic interpretation. UV-Vis spectroscopy and TD-DFT calculations show that the strong absorptions of the iron complexes in the ultraviolet region are mainly ligand-centered π-π* transitions. Weak bands near 450-480 nm are assigned to forbidden transitions. These data do not indicate that the iron complexes act as the primary light absorbers under the catalytic conditions; instead, [Ru(bpy)₃]²⁺ serves as the light-harvesting photosensitizer.

A mechanistic picture is proposed from energy-level analysis, fluorescence quenching, electrochemistry, and control experiments. After visible-light absorption, [Ru(bpy)₃]²⁺ reaches its excited state. The article proposes that BIH is the dominant quencher of this excited photosensitizer through a reductive quenching pathway. This is supported by Stern-Volmer measurements showing a much larger quenching constant for BIH than for the Fe complexes. The reported quenching rate constant for BIH is near the diffusion-controlled limit, whereas the iron complexes quench less efficiently.

The reduced photosensitizer is then proposed to transfer an electron to the iron complex. The article's DFT analysis indicates that the low-lying acceptor orbitals of Fe1-Fe4 are largely ligand-based. Cyclic voltammetry shows two reduction waves for the Fe catalysts, and the article interprets these as predominantly ligand-centered reductions rather than simple formal Fe(II)/Fe(I)/Fe(0) reductions. This interpretation is supported by the calculated frontier orbitals, which contain limited iron contribution in the relevant reduced states.

Electrochemical studies under a restricted potential window show that the first one-electron-reduced species are relatively stable. Under CO₂ atmosphere, the voltammograms differ from those under Ar, which the article interprets as evidence that reduced iron species react with CO₂ to form catalytic intermediates not re-oxidized within the scanned window. The article therefore supports the idea that one-electron reduction of the catalyst is sufficient to activate it toward CO₂ reduction under conditions accessible to the reduced photosensitizer.

The article does not report direct spectroscopic observation of a bound CO₂ adduct or a metal-carboxyl intermediate during catalysis. Instead, it proposes that proton-coupled electron transfer is important after reduction and CO₂ binding. Water is described as essential in this role. In anhydrous MeCN, CO formation is almost completely suppressed. Upon adding 7.5-10% water, both activity and CO selectivity increase sharply. The article explains this by the need for a proton source and by hydrogen-bond stabilization of key intermediates involved in converting coordinated CO₂ into CO-releasing species.

CO formation is therefore described as proceeding through reduced iron-ligand states that react with CO₂, followed by proton-coupled steps leading to CO release. H₂ evolution is the main competing side reaction. The balance between CO formation and H₂ production depends on ligand substitution, catalyst loading, and water content. Fe4, which contains electron-withdrawing CF₃ groups, shows especially high CO selectivity, and the article notes weak intramolecular H···F interactions in the calculated structure that may assist proton management; this is presented cautiously as a possible contributor.

Several control experiments support the photocatalytic assignment. Omitting light, catalyst, photosensitizer, BIH, or CO₂ suppresses product formation. Using Fe(ClO₄)₂ instead of the defined molecular catalyst gives only minor activity. A mercury poisoning experiment gives essentially unchanged CO production, which supports a homogeneous rather than nanoparticle-mediated process. Long-term experiments show that activity levels off over time. UV-Vis monitoring of the reaction mixture and catalyst-replenishment tests support the interpretation that system deactivation arises mainly from photosensitizer degradation rather than rapid destruction of the Fe catalyst.

Catalyst

The catalysts are molecular homogeneous iron(II) complexes designated Fe1-Fe4. Each contains a bis(pyrazolyl)phenanthroline ligand framework and is formulated as [Fe(bpzRphen)(H₂O)₂]X₂, where the pyrazolyl substituent R is varied across the series and X is BF₄ or ClO₄.

These are mononuclear Fe(II) complexes with high-spin quintet ground states and distorted octahedral geometries according to magnetic measurements and DFT calculations. The tetradentate ligand binds through nitrogen donors, while two water ligands occupy axial positions. The catalysts function as the CO₂-reduction components of the photocatalytic system rather than as the primary light absorbers.

A notable feature is that the relevant reductions are described as mainly ligand-centered. This is important because it shapes how the reduced catalyst is generated and how it reacts with CO₂. Fe2 gave the highest overall activity in the standard catalytic comparison, while Fe4 gave the highest CO selectivity. The catalysts are described as robust and homogeneous, with system deactivation attributed mainly to photosensitizer deterioration, although some catalyst decomposition under reaction conditions is not excluded.

Photosensitizer

The photosensitizer is [Ru(bpy)₃]²⁺. It is a molecular ruthenium polypyridyl photosensitizer that serves as the visible-light absorber in the catalytic system. Irradiation was performed with blue light centered at 462 nm, matching the excitation of the Ru complex.

Its excited state is involved in the photoredox cycle. The article's energy-level analysis and emission quenching experiments support reductive quenching of the excited photosensitizer by BIH as the dominant pathway. Oxidative quenching by the iron complexes is considered energetically feasible from orbital alignment, but experimentally BIH is shown to be the much more efficient quencher.

The reduced photosensitizer is then proposed to transfer electrons to the Fe catalyst. The article also identifies photosensitizer deterioration as the main cause of catalytic deactivation over extended irradiation. UV-Vis measurements show hypochromism on prolonged irradiation, and adding fresh [Ru(bpy)₃]²⁺ after 24 h restores CO production.

Investigation

	cat	cat conc [µM]	PS	PS conc [mM]	e-D	e-D conc [M]	.	.	solvent A	.	.	.	additives	.	.	.	.	λexc [nm]	.	TON CO	.	.	TON CH4	TON H2	.	TON HCOOH	.	.
1.	Fe1	50	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		987			not reported	187		not reported			"not reported" is not a number.
2.	Fe2	50	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		1318			not reported	243		not reported			"not reported" is not a number.
3.	Fe3	50	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		847			not reported	205		not reported			"not reported" is not a number.
4.	Fe4	50	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		1265			not reported	133		not reported			"not reported" is not a number.
5.	Fe2	50	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		311			not reported	51		not reported			"not reported" is not a number.
6.	Fe2	50	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		1578			not reported	296		not reported			"not reported" is not a number.
7.	Fe2	50	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		1593			not reported	300		not reported			"not reported" is not a number.
8.	Fe2	50	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		2			not reported	22		not reported			"not reported" is not a number.
9.	Fe2	50	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		1352			not reported	285		not reported			"not reported" is not a number.
10.	Fe2	50	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		661			not reported	191		not reported			"not reported" is not a number.
11.	Fe2	50	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		621			not reported	77		not reported			"not reported" is not a number.
12.	Fe2	25	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		2086			not reported	123		not reported			"not reported" is not a number.
13.	Fe2	12.5	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		4259			not reported	276		not reported			"not reported" is not a number.
14.	Fe2	6.25	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		10168			not reported	862		not reported			"not reported" is not a number.
15.	Fe2	3.12	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN				not reported					462		23138			not reported	2177		not reported			"not reported" is not a number.

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