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

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Abstract Summary

The article describes a family of molecular high-spin Fe(II) complexes bearing bis(pyrazolyl)-1,10-phenanthroline ligands that act as homogeneous catalysts for visible-light-driven reduction of CO₂ to CO. When combined with the photosensitiser [Ru(bpy)₃]²⁺ and the sacrificial electron donor BIH in CO₂-saturated CH₃CN/H₂O, the best catalyst, Fe2, reaches turnover numbers for CO (TON₍CO₎) up to 23 138 with CO selectivity up to 94 %. Photocatalytic activity is observed under blue LED irradiation (λ = 462 nm) and proceeds through a homogeneous mechanism supported by control and poisoning experiments.

Advances and Special Progress

• Development of a new ligand platform (bis(pyrazolyl)-phenanthroline) that maintains high activity while allowing systematic electronic tuning through substituents (H, Me, Ph, CF₃).
• High intrinsic activity of the Me-substituted complex Fe2: TON₍CO₎ = 9 754 after 4 h at 3.12 µM catalyst loading (TOF ≈ 2 438 h⁻¹).
• High CO selectivity (up to 94 %) maintained in the presence of 7.5–10 % v/v water, demonstrating tolerance to protic additives.
• Detailed optimisation studies (catalyst loading, water content, irradiation time) that disentangle factors controlling activity and selectivity.
• Mechanistic insight from Stern–Volmer studies showing dominant reductive quenching of the [Ru(bpy)₃]²⁺ ³MLCT state by BIH, whereas the iron complexes can participate in slower oxidative quenching pathways.
• Electrochemical evidence that the first ligand-centred reduction of the Fe(II) complexes is accessible to the reduced photosensitiser, enabling catalytic turnover without requiring highly negative potentials.

Additional Remarks

Although the catalysts are based on earth-abundant iron, the system still relies on the noble-metal photosensitiser [Ru(bpy)₃]²⁺ and a large excess of the organic reductant BIH. Catalyst deactivation is limited, but photodegradation of the photosensitiser eventually curtails long-term performance; refreshing the Ru complex restores activity. Hydrogen evolution remains a minor competing pathway, becoming more pronounced at higher catalyst loadings or higher water content. The requirement for sacrificial BIH and partial water content limits immediate scalability but provides a well-defined platform for mechanistic studies and rational ligand development.

Content of the Published Article in Detail

Four iron complexes were synthesised:

• Fe1 = [Fe(bpzphen)(H₂O)₂](BF₄)₂
• Fe2 = [Fe(bpzMe₂phen)(H₂O)₂](BF₄)₂
• Fe3 = [Fe(bpzPhphen)(H₂O)₂](ClO₄)₂
• Fe4 = [Fe(bpzCF₃phen)(H₂O)₂](ClO₄)₂

All adopt distorted octahedral geometries with the tetradentate bis(pyrazolyl)-phenanthroline ligand in the equatorial plane and two trans H₂O ligands axially. Magnetic measurements and CAM-B3LYP calculations point to high-spin quintet ground states (S = 2).

Photocatalytic set-up

The standard reaction mixture (total volume 4 mL) contained catalyst (typically 50 µM), [Ru(bpy)₃]²⁺ (0.3 mM), BIH (0.11 M), and MeCN/H₂O (92.5:7.5 v/v) saturated with CO₂. The sealed borosilicate reactor was irradiated with 462 nm LEDs at 308 K, and gaseous products were quantified by headspace GC with TCD/FID detection. Formate and methane were monitored by ¹H NMR and were not detected in significant amounts.

Control experiments established that light, catalyst, photosensitiser, sacrificial donor, and CO₂ are all essential; omission of any component or substitution of the molecular catalyst with Fe(ClO₄)₂ suppressed CO formation. Hg(0) poisoning did not affect activity, confirming a homogeneous mechanism.

Proposed electron-transfer sequence

1. Absorption of 462 nm light populates the ³MLCT state of [Ru(bpy)₃]²⁺. 2. Reductive quenching by BIH (k_q ≈ 1.6 × 10¹⁰ M⁻¹ s⁻¹) generates [Ru(bpy)₃]⁺ and BIH•⁺/BI· radicals. 3. The one-electron-reduced photosensitiser transfers an electron to the iron catalyst, accessing the first ligand-centred reduced state observed in cyclic voltammetry at −1.32 to −1.52 V vs Fc⁺/Fc (within reach of [Ru(bpy)₃]⁺). 4. The reduced iron complex binds and activates CO₂; DFT and water-dependence suggest proton-coupled electron transfer, leading to an Fe–COOH intermediate that releases CO. 5. A second PCET regenerates the high-spin Fe(II) resting state, closing the catalytic cycle.

Stern–Volmer data show slower oxidative quenching of the photosensitiser by the iron complexes (k_q ≈ 2 × 10⁹ M⁻¹ s⁻¹), which may constitute a minor parallel pathway.

Supporting evidence

• Quenching studies quantified K_SV and k_q for BIH and all four Fe complexes.
• Cyclic voltammetry under CO₂ displayed current changes consistent with catalytic engagement after the first reduction.
• Time-course experiments showed a rapid rise in TON during the first 24 h, followed by plateauing attributed to photosensitiser bleaching. Adding fresh [Ru(bpy)₃]²⁺ restored activity.
• Water-dependence experiments indicated an optimum of 7.5–10 % v/v H₂O: lower water suppressed PCET, whereas higher water decreased BIH solubility and increased H₂ evolution.

Catalyst

The catalysts are mononuclear Fe(II) complexes of the formula [Fe(bis(pyrazolyl)-phenanthroline)(H₂O)₂]²⁺. The tetradentate N₄ ligand creates a relatively weak ligand field, favouring a high-spin d⁶ configuration that facilitates ligand-centred reductions at modest potentials. Changing the pyrazolyl substituents tunes the reduction potentials and photocatalytic performance: electron-donating Me groups (Fe2) lower the first reduction potential and give the highest TON₍CO₎, whereas electron-withdrawing CF₃ groups (Fe4) enhance CO selectivity. The complexes remain largely intact during catalysis; deactivation stems mainly from photosensitiser decomposition rather than catalyst breakdown.

Photosensitizer

[Ru(bpy)₃]²⁺ is a classical Ru(II) polypyridyl complex that absorbs blue light to give a long-lived ³MLCT state (τ ≈ 890 ns). In this system it undergoes efficient reductive quenching by BIH to form [Ru(bpy)₃]⁺, which possesses sufficient reducing power to deliver an electron to the Fe catalyst. Prolonged irradiation leads to hypochromism and loss of Ru absorption features, implicating photobleaching as the primary limitation on sustained turnover. Re-addition of [Ru(bpy)₃]²⁺ restores full catalytic activity, supporting its role as the sacrificial light absorber rather than as a pre-catalyst or co-catalyst.

Investigation

```csv cat , cat conc [µM] , PS , PS conc [mM] , e-D , e-D conc [M] , solvent A , λexc [nm] , TON CO Fe1 , 50 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN/H2O (7.5 % v/v) , 462 , 987 Fe2 , 50 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN/H2O (7.5 % v/v) , 462 , 1318 Fe3 , 50 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN/H2O (7.5 % v/v) , 462 , 847 Fe4 , 50 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN/H2O (7.5 % v/v) , 462 , 1265 Fe2 , 50 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN/H2O (7.5 % v/v) , 462 , 311 Fe2 , 50 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN (dry) , 462 , 2 Fe2 , 50 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN/H2O (10 % v/v) , 462 , 1352 Fe2 , 50 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN/H2O (30 % v/v) , 462 , 661 Fe2 , 50 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN/H2O (50 % v/v) , 462 , 621 Fe2 , 25 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN/H2O (7.5 % v/v) , 462 , 2086 Fe2 , 12.5 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN/H2O (7.5 % v/v) , 462 , 4259 Fe2 , 6.25 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN/H2O (7.5 % v/v) , 462 , 10168 Fe2 , 3.12 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN/H2O (7.5 % v/v) , 462 , 23138 Fe2 , 3.12 , [Ru(bpy)3]2+ , 0.3 , BIH , 0.11 , CH3CN/H2O (7.5 % v/v) , 462 , 9754 ```