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

Imported from: /opt/uploadtmp/1-s2.0-S0021951726000072-main (1)_6a74e2f8c4633.pdf

Abstract Summary

This article describes a homogeneous molecular photocatalytic system for the reduction of CO₂ to CO under visible light. The catalytic components are iron(II) complexes bearing bis(pyrazolyl)phenanthroline ligands, used together with [Ru(bpy)₃]²⁺ as the photosensitizer and BIH as the sacrificial electron donor in MeCN/H₂O.

All four iron complexes were active for CO formation, and the catalytic behavior depended on the substituents on the pyrazolyl groups. Under the standard screening conditions, Fe2 gave the highest overall activity, while Fe4 gave the highest CO selectivity in the catalyst series. Under lower catalyst loading, Fe2 reached a much higher CO turnover number, and mechanistic experiments supported a homogeneous photocatalytic process in which catalyst deactivation was limited mainly by photosensitizer deterioration rather than rapid loss of the iron catalyst.

Advances and Special Progress

A key advance is the introduction of a new family of iron(II) bis(pyrazolyl)phenanthroline complexes as molecular photocatalysts for CO₂-to-CO conversion. The article presents this ligand framework as an underexplored alternative to more established iron systems based on quaterpyridine, salophen, porphyrinic, or related nitrogen-donor scaffolds.

The work also shows that systematic variation of pyrazolyl substituents changes catalytic activity and selectivity. In this series, the dimethyl-substituted complex Fe2 gave the highest activity under the main screening conditions, whereas the trifluoromethyl-substituted complex Fe4 gave the highest CO selectivity. This provides a clear structure-performance relationship within one ligand platform.

Another advance is the strong performance at low catalyst loading. When the Fe2 concentration was decreased, the turnover number for CO increased substantially, reaching 23,138, while maintaining high CO selectivity. The article also reports a short-time value for Fe2 of TON_CO 9754 after 4 h with a TOF of 2438 h⁻¹, together with a reported total quantum yield of 8.24%.

The study also contributes mechanistic insight. Spectroscopic, electrochemical, and theoretical data support ligand-centered reductions in the iron complexes, rather than simple metal-centered reduction assignments. Emission-quenching experiments show that BIH quenches the excited photosensitizer much more efficiently than the iron complexes, supporting a dominant reductive quenching pathway by the sacrificial donor. Control experiments and mercury poisoning experiments support the conclusion that the active system is homogeneous.

A further practical advance is the demonstrated compatibility with mixed aqueous acetonitrile. The article shows that 7.5–10% water is important for efficient catalysis, apparently because water assists proton-coupled electron transfer and stabilization of intermediates, while too much water decreases activity.

Additional Remarks

The chemistry is significant because photocatalytic CO₂ reduction to CO converts a thermodynamically stable small molecule into a useful carbon-containing product. CO is an important synthetic intermediate and can serve as a feedstock for downstream transformations.

At the same time, this is a sacrificial photochemical system, so its performance depends on continuous consumption of BIH rather than closed-cycle solar fuel production. The system also relies on a ruthenium photosensitizer, so although the catalyst metal is earth-abundant iron, the full photocatalytic assembly is not composed entirely of earth-abundant components.

The article shows a clear competition between CO formation and H₂ evolution. High CO selectivity is achieved, but H₂ is still formed under most conditions. Water is beneficial only within a limited range: too little water gives almost no CO, whereas too much water lowers activity, which the article relates in part to poor BIH solubility in more aqueous mixtures.

Durability is also mixed. The iron catalyst is described as robust relative to the reaction conditions, and readdition of photosensitizer restores activity, but prolonged irradiation still leads to photocatalytic slowdown because the photosensitizer deteriorates. This gives useful mechanistic insight, but also highlights a limitation of the present system.

Content of the Published Article in Detail

The molecular system consists of four iron(II) complexes, denoted Fe1-Fe4, each containing a tetradentate bis(pyrazolyl)phenanthroline ligand and two coordinated water molecules. These catalysts were tested in homogeneous solution for visible-light-driven CO₂ reduction using [Ru(bpy)₃]²⁺ as the photosensitizer and BIH as the sacrificial electron donor.

Under the main photocatalytic conditions, a borosilicate photoreactor contained catalyst, photosensitizer, and BIH in CO₂-saturated MeCN/H₂O solution. Irradiation was provided by blue light at 462 nm. Gas products were analyzed from the headspace by gas chromatography. CO and H₂ were detected, while no significant formate or CH₄ was detected.

The article first establishes important ground-state and electronic features of the catalysts. Magnetic measurements and calculations support high-spin iron(II) quintet ground states for all four complexes. DFT and TD-DFT calculations indicate that the stronger UV absorption bands are mainly ligand-centered π-π* transitions, while weaker visible-region bands are assigned to forbidden transitions. The calculations and electrochemical data together support the view that the key reductions are largely ligand-centered.

The proposed photochemical sequence begins with light absorption by [Ru(bpy)₃]²⁺. The article discusses the triplet metal-to-ligand charge-transfer excited state of the ruthenium complex. Emission quenching experiments showed that BIH quenches this excited state much more efficiently than the iron complexes. The Stern-Volmer and bimolecular quenching constants are therefore consistent with BIH acting as the dominant quencher. On this basis, the article supports a reductive quenching pathway in which excited [Ru(bpy)₃]²⁺ is quenched by BIH.

The energy-level discussion in the article further supports this interpretation. The HOMO of BIH lies above that of [Ru(bpy)₃]²⁺, making electron donation from BIH to the excited photosensitizer energetically favorable. By contrast, reductive quenching by the iron complexes is described as energetically unfavorable. The article notes that oxidative quenching of the excited ruthenium state by the iron complexes is in principle feasible from orbital energy alignment, but the photometric titrations show that BIH is the much more efficient quencher under the experimental conditions.

After reductive quenching, the reduced ruthenium species is proposed to transfer an electron to the iron catalyst. Electrochemical measurements are important here. Cyclic voltammetry showed two reduction waves for the iron complexes, and the article interprets these as predominantly ligand-centered reductions. Additional voltammetric measurements restricted to the potential window accessible to reduced [Ru(bpy)₃]⁺ showed that the first reduction process is sufficient to generate a reduced iron species that can react with CO₂. Under CO₂, the cyclic voltammograms changed substantially relative to argon, supporting reaction of the reduced catalyst with CO₂.

The article does not claim direct spectroscopic observation of all catalytic intermediates, but it proposes that single-electron reduction of the iron complex activates it toward CO₂ reduction. The discussion also states that water is essential for effective proton-coupled electron transfer and stabilization of intermediates. In the detailed discussion of solvent effects, the article explicitly states that under anhydrous conditions the absence of a proton source suppresses CO production, and that proton-coupled electron transfer steps are needed to convert coordinated CO₂ into reactive intermediates such as an Fe-COOH species, which can then evolve to CO.

Thus, the mechanistic picture supported by the data is as follows: light excites [Ru(bpy)₃]²⁺; BIH reductively quenches the excited state; the resulting reduced photosensitizer transfers electrons to the iron complex; a singly reduced iron-ligand state reacts with CO₂; water assists proton transfer and stabilizes bound intermediates; and CO is produced and released. The article does not report direct observation of CO release steps, but the product analysis confirms CO as the main carbon-containing product.

Several control experiments support this interpretation. Removing light, catalyst, photosensitizer, sacrificial donor, or CO₂ largely eliminated activity. Replacing the defined iron complex with Fe(ClO₄)₂ gave only minor CO and H₂, showing that the ligand-supported iron complex is required for high activity. Mercury poisoning experiments did not suppress catalysis, supporting a homogeneous rather than nanoparticle-based mechanism.

The article also addresses catalyst durability. Activity increased over time initially and then approached a plateau. Because BIH was present in large excess, the authors considered decomposition of the catalyst or photosensitizer. UV-Vis data showed pronounced hypochromism for the ruthenium photosensitizer upon prolonged irradiation, and adding more [Ru(bpy)₃]²⁺ after 24 h restored CO₂-to-CO conversion. These results support the conclusion that deactivation mainly arises from photosensitizer deterioration rather than rapid destruction of the iron catalyst.

Performance trends were chemically informative. Fe2 showed the highest activity under the main screening conditions. Fe4 showed the highest CO selectivity in the series, and the article relates its greater distortion and weak intramolecular H···F interactions to a possibly favorable proton-management environment around the metal center. Lower catalyst loading substantially increased turnover numbers. Water content also had a major effect: no water gave almost no CO, 7.5-10% water gave the best balance of activity and selectivity, and higher water fractions decreased performance.

Catalyst

The catalysts are molecular iron(II) complexes of the general type [Fe(bpzRphen)(H₂O)₂]X₂, where the ligand is a tetradentate bis(pyrazolyl)phenanthroline derivative. Four complexes were studied: Fe1, Fe2, Fe3, and Fe4, which differ in the substituents on the pyrazolyl groups.

These are homogeneous mononuclear iron complexes designed for photocatalytic CO₂ reduction to CO. The article describes them as high-spin iron(II) species with distorted octahedral geometries and two coordinated water ligands in trans axial positions. DFT calculations and electrochemistry support predominantly ligand-centered reduction processes.

Catalytically, all four complexes produce CO under visible light in the full photochemical system. Fe2 gives the highest activity under the standard comparison conditions, while Fe4 gives the highest CO selectivity in the series. The article states that Fe2 remains largely intact under the catalytic conditions and that the main source of deactivation is deterioration of the photosensitizer rather than collapse of the iron complex. Mercury poisoning experiments support the homogeneous nature of the catalysis.

Photosensitizer

The photosensitizer is [Ru(bpy)₃]²⁺. Its role is to absorb visible light and generate an excited state capable of initiating the electron-transfer sequence required for CO₂ reduction.

The article discusses the triplet metal-to-ligand charge-transfer excited state of this complex. Emission-quenching studies show that BIH quenches this excited state efficiently, much more strongly than the iron catalysts do. This supports a dominant reductive quenching pathway by BIH. The reduced ruthenium photosensitizer is then implicated in electron transfer to the iron catalyst.

The photosensitizer is suitable because it absorbs at the irradiation wavelength used and supports the needed photoredox steps. However, it is also identified as the main weak point in long irradiations. Prolonged irradiation causes spectral hypochromism, and readdition of [Ru(bpy)₃]²⁺ restores catalytic activity, indicating that photosensitizer deterioration is a major cause of photocatalytic slowdown.

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	50.0	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			7.5 v/v					462		987			not reported	187		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
2.	Fe2	50.0	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			7.5 v/v					462		1318			not reported	243		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
3.	Fe3	50.0	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			7.5 v/v					462		847			not reported	205		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
4.	Fe4	50.0	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			7.5 v/v					462		1265			not reported	133		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
5.	Fe2	50.0	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			7.5 v/v					462		311			not reported	51		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
6.	Fe2	50.0	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			7.5 v/v					462		1578			not reported	296		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
7.	Fe2	50.0	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			7.5 v/v					462		1593			not reported	300		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
8.	Fe2	50.0	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			0 v/v					462		2			not reported	22		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
9.	Fe2	50.0	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			10 v/v					462		1352			not reported	285		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
10.	Fe2	50.0	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			30 v/v					462		661			not reported	191		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
11.	Fe2	50.0	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			50 v/v					462		621			not reported	77		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
12.	Fe2	25.0	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			7.5 v/v					462		2086			not reported	123		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
13.	Fe2	12.5	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			7.5 v/v					462		4259			not reported	276		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
14.	Fe2	6.25	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			7.5 v/v					462		10168			not reported	862		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
15.	Fe2	3.12	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			7.5 v/v					462		23138			not reported	2177		not reported					"Molecule:" is not declared as a valid unit of measurement for this property. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "not reported" is not a number. "K" can not be assigned to a declared number type with value 308.
16.	Fe2	3.12	［Ru(bpy)3］2+	0.3	BIH	0.11			MeCN			7.5 v/v					462		9754			not reported	not reported		not reported					"not reported" is not a number. "35" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property. "K" can not be assigned to a declared number type with value 308.

Investigation-Name: inv0ExportRefresh