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

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{{DOI|doi=10.1016/j.jcat.2026.116673}}

== Abstract Summary ==
This article describes a homogeneous molecular photocatalytic system for the visible-light reduction of CO<sub>2</sub> to CO using iron(II) bis(pyrazolyl)phenanthroline complexes as catalysts. The system combines an Fe catalyst, [Ru(bpy)<sub>3</sub>]<sup>2+</sup> as photosensitizer, and BIH as sacrificial electron donor in CO<sub>2</sub>-saturated MeCN/H<sub>2</sub>O.

All four iron complexes were active for CO formation. The best overall catalyst under the standard screening conditions was Fe2, which gave a turnover number for CO of 1318 with 84% CO selectivity. Under lower catalyst loading, Fe2 reached a much higher CO turnover number of 23,138 with up to 91% CO selectivity. The study also shows that modest amounts of water are essential for efficient catalysis, and that the catalytic system operates homogeneously, with loss of activity attributed mainly to photosensitizer deterioration rather than catalyst decomposition.

== Advances and Special Progress ==
The article presents several advances that are explicitly supported by the reported data:

* '''New catalyst family for photocatalytic CO<sub>2</sub> reduction''': The work introduces Fe(II) complexes bearing bis(pyrazolyl)phenanthroline ligands as molecular catalysts for visible-light-driven CO<sub>2</sub>-to-CO conversion. The article identifies this ligand platform as previously underexplored for this reaction.
* '''Use of an earth-abundant catalytic metal''': The catalytic center is iron, rather than a precious-metal catalyst.
* '''High activity at low catalyst loading''': Fe2 achieved a reported TON<sub>CO</sub> of 23,138 at 3.12 μM catalyst loading.
* '''High CO selectivity''': Fe4 reached 91% CO selectivity under the catalyst comparison conditions, and Fe2 reached up to 94% CO selectivity at reduced catalyst loading.
* '''Water-compatible mixed-solvent operation''': Efficient catalysis was observed in MeCN/H<sub>2</sub>O mixtures, with 7.5-10% water identified as especially effective.
* '''Mechanistic support from multiple methods''': The article combines UV/Vis spectroscopy, DFT and TD-DFT calculations, cyclic voltammetry, control experiments, mercury poisoning tests, and Stern-Volmer emission quenching analysis to support its mechanistic interpretation.
* '''Evidence for homogeneous catalysis and catalyst robustness''': Mercury poisoning experiments did not suppress activity, and additional experiments indicated that the main deactivation pathway is deterioration of the photosensitizer rather than rapid destruction of the Fe catalyst.

== Additional Remarks ==
Photocatalytic reduction of CO<sub>2</sub> to CO is chemically important because CO is a useful carbon-containing product and can serve as a precursor in further synthesis. This study uses a sacrificial photochemical system, which is useful for mechanistic study and catalyst evaluation but still depends on a separate sacrificial electron donor.

The catalytic system also relies on a ruthenium photosensitizer, so the full photocatalytic assembly is not composed entirely of earth-abundant components, even though the catalyst itself is iron-based.

The work shows a clear competition between CO<sub>2</sub> reduction and H<sub>2</sub> evolution. Product selectivity depends strongly on catalyst structure, catalyst loading, and water content. Water is beneficial up to a point, but too much water lowers activity, which the article attributes in part to poor BIH solubility in more aqueous media.

The article presents useful mechanistic evidence, but the mechanism remains an interpretation based on spectroscopy, electrochemistry, and control studies rather than direct observation of every catalytic intermediate.

== Content of the Published Article in Detail ==
The molecular photocatalytic system contains three essential functional components:

* an iron complex catalyst from the Fe1-Fe4 series,
* [Ru(bpy)<sub>3</sub>]<sup>2+</sup> as photosensitizer,
* BIH as sacrificial electron donor.

The photocatalytic reactions were carried out in a borosilicate photoreactor containing a CO<sub>2</sub>-saturated MeCN/H<sub>2</sub>O solution. The standard reaction mixture used 50 μM catalyst, 0.3 mM [Ru(bpy)<sub>3</sub>]<sup>2+</sup>, and 0.11 M BIH in 4.0 mL solvent, with irradiation at 462 nm. CO and H<sub>2</sub> formed in the headspace were quantified by gas chromatography. The article states that no significant amounts of formate or CH<sub>4</sub> were detected.

=== Molecular components and catalyst series ===
The Fe catalysts are iron(II) complexes supported by tetradentate bis(pyrazolyl)phenanthroline ligands, with two water ligands in the coordination sphere. The ligand substituents were systematically varied:

* Fe1: unsubstituted pyrazolyl units
* Fe2: 3,5-dimethyl-substituted pyrazolyl units
* Fe3: diphenyl-substituted pyrazolyl units
* Fe4: CF<sub>3</sub>-substituted pyrazolyl units

The article reports that all complexes are high-spin Fe(II) species with distorted octahedral geometries. DFT calculations support a quintet ground state for all four complexes.

=== Optical and electronic properties ===
UV/Vis spectra showed strong absorptions in the ultraviolet region, assigned mainly to ligand-centered π-π* transitions. Much weaker bands at longer wavelength were assigned to forbidden transitions. TD-DFT calculations supported these assignments.

The article emphasizes that the optical features do not indicate that the Fe complexes themselves are acting as the main light absorbers in the catalytic system. Instead, the ruthenium complex serves as the photosensitizer.

Electrochemical measurements showed two reduction waves for the Fe complexes. The article discusses whether these could be metal-centered or ligand-centered reductions, and concludes from DFT orbital analysis that the reductions are primarily ligand-centered. The reduced states are described as species such as [Fe(L•−)(H<sub>2</sub>O)<sub>2</sub>]<sup>+</sup> and [Fe(L<sup>2•−</sup>)(H<sub>2</sub>O)<sub>2</sub>]<sup>0</sup>.

=== Photocatalytic behavior and component necessity ===
Control experiments showed that no significant CO or H<sub>2</sub> formation occurred in the absence of light, catalyst, photosensitizer, sacrificial electron donor, or under Ar instead of CO<sub>2</sub>. Replacing the defined Fe catalyst with Fe(ClO<sub>4</sub>)<sub>2</sub> gave only minor amounts of CO and H<sub>2</sub>, which supports the need for the molecularly defined catalyst.

These observations establish that productive photocatalysis requires the full multicomponent system and that free Fe<sup>2+</sup> ions alone do not account for the reported activity.

=== Quenching and initial photochemical steps ===
The article uses DFT energy alignment and Stern-Volmer emission quenching experiments to discuss the first electron-transfer steps.

After light absorption by [Ru(bpy)<sub>3</sub>]<sup>2+</sup>, the article considers quenching of the excited state by BIH and by the Fe complexes. The quenching experiments show that both BIH and the Fe complexes can quench the emission of the ruthenium photosensitizer, but BIH is much more efficient. The reported Stern-Volmer and quenching-rate data support BIH as the dominant quencher under the reaction conditions.

Based on the orbital energy analysis, the article proposes that:

* '''reductive quenching''' of the excited [Ru(bpy)<sub>3</sub>]<sup>2+</sup> by BIH is energetically favorable,
* '''oxidative quenching''' by the Fe complexes is also energetically possible in principle,
* but the quenching measurements show that BIH is the major excited-state quencher.

Thus, the data support a mechanism in which the ruthenium photosensitizer is primarily reductively quenched by BIH.

=== Role of BIH ===
BIH acts as the sacrificial electron donor. The article shows that no photocatalytic CO or H<sub>2</sub> production occurs without BIH. Increasing BIH concentration increases photocatalytic performance up to 0.11 M under the tested conditions, while further increase to 0.165 M does not substantially improve activity.

In mechanistic terms, BIH is presented as the reagent that reduces the excited photosensitizer, thereby enabling downstream electron transfer to the Fe catalyst.

=== Electron transfer to the Fe catalyst ===
The article proposes that reduced [Ru(bpy)<sub>3</sub>]<sup>+</sup> can reduce the Fe catalyst. Cyclic voltammetry was used to examine catalyst behavior in the potential region accessible to the reduced photosensitizer. Within that range, the first reduction event of the Fe complexes is sufficiently accessible, and under CO<sub>2</sub> atmosphere the voltammetric behavior changes in a way consistent with reaction of the reduced iron species with CO<sub>2</sub>.

The article therefore concludes that single-electron reduction of the Fe bis(pyrazolyl)phenanthroline complexes is sufficient to activate the catalyst toward CO<sub>2</sub> reduction under the photocatalytic conditions.

=== CO2 activation and CO formation ===
Direct observation of a catalytic CO<sub>2</sub>-bound intermediate is not reported. However, the article discusses the reduction chemistry in terms of reduced Fe-ligand states reacting with CO<sub>2</sub>. It also states that proton-coupled electron transfer is essential for catalysis and specifically mentions reactive intermediates such as Fe-COOH in the discussion of the role of water.

Thus, the mechanistic interpretation supported by the article is that:

# light excitation occurs at [Ru(bpy)<sub>3</sub>]<sup>2+</sup>,
# BIH reductively quenches the excited photosensitizer,
# the resulting reduced photosensitizer transfers an electron to the Fe catalyst,
# the reduced Fe catalyst reacts with CO<sub>2</sub>,
# proton-coupled electron transfer steps convert the CO<sub>2</sub>-derived intermediate toward CO formation,
# CO is released, and H<sub>2</sub> evolution competes as a side reaction.

The article does not report direct spectroscopic detection of the Fe-COOH intermediate; this is presented as mechanistic interpretation rather than direct observation.

=== Role of water and proton transfer ===
Water strongly affects catalytic performance. In anhydrous MeCN, CO production was essentially suppressed. Addition of 7.5-10% water caused a large increase in both activity and CO selectivity. The article interprets this as evidence that water is important for proton-coupled electron transfer and for stabilization of catalytic intermediates, including by hydrogen bonding.

At higher water fractions, activity decreases. The article attributes this partly to reduced BIH availability because BIH is poorly soluble in more aqueous media. Therefore, water is beneficial but only within a limited composition range.

=== Product distribution ===
The major desired product is CO. H<sub>2</sub> is the principal side product. The article states that no significant CH<sub>4</sub> or formate was detected.

Under the standard 24 h catalyst comparison conditions, Fe2 gave the highest CO turnover number, while Fe4 gave the highest CO selectivity. At lower catalyst concentration, Fe2 gave much larger TON values while maintaining high CO selectivity.

=== Evidence for homogeneous catalysis and deactivation pathway ===
The article reports mercury poisoning tests. Since activity was essentially unchanged in the presence of mercury, the authors interpret this as evidence against catalysis by iron nanoparticles or colloidal metal species.

The article also examined catalyst longevity indirectly. Activity increased early and then approached a plateau. Because BIH was present in excess, the article argues that the main cause of deactivation is not BIH depletion. UV/Vis observations indicated deterioration of the ruthenium photosensitizer upon prolonged irradiation. Adding more photosensitizer after 24 h restored additional CO production. These results support the conclusion that the Fe catalyst is relatively robust and that photosensitizer degradation is the main deactivation process under the reported conditions.

== Catalyst ==
The catalysts are a series of homogeneous molecular iron(II) complexes, Fe1-Fe4, formulated as [Fe(bpzRphen)(H<sub>2</sub>O)<sub>2</sub>]X<sub>2</sub>, where the ligand is a tetradentate bis(pyrazolyl)phenanthroline derivative and X is BF<sub>4</sub><sup>−</sup> or ClO<sub>4</sub><sup>−</sup> depending on the complex.

Chemically relevant features explicitly reported in the article are:

* '''metal center''': Fe(II)
* '''catalyst type''': homogeneous molecular catalyst
* '''coordination environment''': distorted octahedral geometry with tetradentate bis(pyrazolyl)phenanthroline ligand and two water ligands
* '''spin state''': high-spin quintet ground state
* '''redox behavior''': reductions are primarily ligand-centered according to electrochemical and DFT analysis
* '''catalytic role''': reduction of CO<sub>2</sub> to CO under visible light in combination with an external photosensitizer and sacrificial electron donor
* '''structure-performance relationship''':
** Fe2 gave the highest CO turnover number under the catalyst screening conditions
** Fe4 gave the highest CO selectivity in that screening series
* '''stability''': the catalytic system was interpreted as homogeneous and reasonably robust; deactivation was mainly associated with photosensitizer deterioration rather than rapid loss of the Fe catalyst

Fe2 is the best-performing catalyst in the article under most optimized conditions. Fe4 is notable for high CO selectivity. The article also notes that weak hydrogen-bonding interactions involving the CF<sub>3</sub>-substituted ligand of Fe4 may contribute to its enhanced CO selectivity.

== Photosensitizer ==
The photosensitizer is [Ru(bpy)<sub>3</sub>]<sup>2+</sup>.

The article explicitly supports the following points:

* '''identity''': [Ru(bpy)<sub>3</sub>]<sup>2+</sup>
* '''class''': ruthenium polypyridyl photosensitizer
* '''role''': absorbs visible light and initiates the photoredox sequence
* '''excited-state behavior''': its emission is quenched by BIH and by the Fe catalysts
* '''dominant quenching pathway''': BIH is the dominant quencher according to Stern-Volmer analysis and quenching-rate constants
* '''mechanistic function''': after reductive quenching by BIH, the reduced photosensitizer is proposed to transfer an electron to the Fe catalyst
* '''stability limitation''': prolonged irradiation causes deterioration of the photosensitizer, and this is identified as the main deactivation pathway of the overall photocatalytic system

The article therefore presents [Ru(bpy)<sub>3</sub>]<sup>2+</sup> as an effective visible-light absorber and redox mediator, but also as the least durable component under long irradiation times.

== Investigation ==
{{#experimentlist:|form=Photocatalytic_CO2_conversion_experiments|name=inv0}}

[[Category: Homogeneous photocatalytic CO2 conversion]]
[[Category: Photocatalytic CO2 conversion]]
[[Category: CO2 conversion]]
[[Category: Photocatalytic CO2 conversion to CO]]
[[Category: CO conversion]]