Iron(III)-Terpyridine Complexes for CO2 Photoreduction: Effects of Ligand Substitution and Coordination Environment

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Abstract Summary

The study examines a family of molecular iron(III) terpyridine complexes (Fe1–Fe8) as homogeneous catalysts for visible-light-driven CO₂-to-CO conversion. The photocatalytic systems combine an Fe–terpyridine complex (10 µM), the organic thermally activated delayed-fluorescence dye 4CzIPN as photosensitiser, and triethanolamine (TEOA, 10 % v/v) as sacrificial electron donor in acetonitrile/TEOA (9 : 1). Under visible irradiation (>400 nm) the best performer, bis-terpyridine complex Fe6, attained a turnover number for CO (TON_CO) of 275 ± 22 after 4 h with 92 % CO selectivity, while a ferrocene-substituted analogue showed poor activity (TON_CO ≈ 2.3). Infrared spectroelectrochemistry revealed formation of Fe–CO and Fe–CO₂ intermediates, supporting a mechanistic pathway in which two sequential electron transfers to the Fe centre precede CO₂ binding and proton-assisted CO release.

Advances and Special Progress

• Demonstrates photocatalytic CO₂ reduction using only earth-abundant molecular components (Fe catalyst, organic dye photosensitiser).
• Establishes that bis-terpyridine coordination (Fe6) can outperform mono-terpyridine analogues under optimised dye/catalyst ratios.
• Provides comparative binding constants showing how 4′- and 6,6″-substitution on the terpyridine ligand tunes Fe(III) coordination and ultimately catalytic performance.
• Uses operando IR spectroelectrochemistry to detect Fe^II–CO and Fe–CO₂ adducts, lending direct support to proposed catalytic intermediates.

Additional Remarks

• The system achieves high CO selectivity (>80 %) but requires a large excess of sacrificial donor and photosensitiser; both components degrade during extended irradiation.
• Catalyst stability correlates with ligand electronics: electron-rich phenyl substituents improve activity, while a sterically hindered ferrocene-substituted ligand drastically lowers TON_CO.
• Visible-light absorption by 4CzIPN enables avoidance of precious-metal photosensitisers, yet photobleaching limits long-term durability.
• Use of neat acetonitrile with 10 % TEOA provides a homogeneous medium; water compatibility was not investigated.
• Competing H₂ evolution remained minor under the reported conditions.

Content of the Published Article in Detail

Eight iron(III) complexes were synthesised: four mono-terpyridine species (Fe1–Fe4) and their bis-terpyridine counterparts (Fe5–Fe8). Substituents at the 4′-position (H, phenyl, 4-bromophenyl, ferrocene) and at the 6,6″-positions (H or Br) modulate ligand binding strength; UV/Vis Job plots and Benesi–Hildebrand analysis showed 1 : 2 metal : ligand stoichiometry for unsubstituted and ferrocene-substituted ligands, whereas sterically hindered Br-substituted ligands formed only 1 : 1 complexes.

Cyclic voltammetry in CH₃CN revealed two reversible metal-centred reductions (Fe^III/II and Fe^II/I) followed by a ligand-centred process. Under CO₂, only a small catalytic current was observed electrochemically, indicating that photochemical rather than purely electrochemical driving is required.

Operando infrared spectroelectrochemistry in CO₂-saturated acetonitrile detected bands at ca. 1940 and 1880 cm^-1 upon the first reduction, assigned to Fe^II–CO and Fe^I–CO species, respectively. At more negative potentials (≈ –1.7 V vs Fc^+/0) new bands at ≈ 1840 cm^-1 (more reduced Fe–CO), ≈ 2070 cm^-1 (Fe–CO₂ adduct), and ≈ 2200 cm^-1 (coordinated acetonitrile) appeared. Similar spectral changes for both mono- and bis-terpyridine complexes suggest analogous catalytic cycles.

Photocatalytic experiments were conducted in CO₂-saturated CH₃CN containing 10 % v/v TEOA. 4CzIPN absorbs visible light and is reductively quenched by TEOA, forming the radical anion [4CzIPN]^•-. This strong reductant reduces Fe^III to Fe^II and then to Fe^I. The article proposes that after two reductions, either (i) one terpyridine arm decoordinates leaving an open site, or (ii) a pyridyl donor loosens while the bis-terpyridine framework remains intact. CO₂ then binds to the low-valent Fe centre, giving a metal–CO₂ species that is protonated (by water traces or TEOA) to release CO and regenerate the acetonitrile-bound Fe^II precatalyst.

Control experiments confirmed the necessity of each component: omission of the Fe complex, 4CzIPN, TEOA, light, or CO₂ led to negligible CO formation. Increasing the photosensitiser/catalyst ratio from 10 : 1 to 100 : 1 prolonged activity, indicating that dye photobleaching is a major deactivation pathway. When fresh 4CzIPN and Fe6 were added after activity ceased, catalysis resumed, confirming concurrent degradation of both species.

Catalyst

Iron(III) terpyridine complexes designated Fe1–Fe8. Each features an Fe^III centre octahedrally coordinated by either one terpyridine plus three chlorides (mono-tpy series) or two terpyridine ligands with PF₆^– counter-ions (bis-tpy series). Ligand variants include unsubstituted, 4-phenyl, 4-bromophenyl, and 4-ferrocenyl groups; two complexes also carry 6,6″-dibromo substitutions. All catalysts are molecular and homogeneous. Bis-terpyridine complex Fe6 ([Fe(Phtpy)₂](PF₆)₃) gave the highest TON_CO. IR spectroelectrochemistry confirms their ability to generate Fe–CO and Fe–CO₂ intermediates. Catalyst and photosensitiser both undergo gradual decomposition under prolonged irradiation.

Photosensitizer

4CzIPN (1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene) is an organic thermally activated delayed-fluorescence dye that absorbs throughout the visible region. In this study its excited singlet/triplet state is reductively quenched by TEOA, producing the highly reducing radical anion [4CzIPN]^•- which transfers electrons to the iron catalyst. Excess 4CzIPN extends system lifetime, but the dye photobleaches over time, limiting turnover.

Investigation

```csv cat , cat conc [µM] , PS , PS conc [mM] , e-D , e-D conc [M] , solvent A , λexc [nm] , TON CO Fe8 , 10 , 4CzIPN , 0.100 , TEOA , not reported , MeCN/TEOA (9:1) , >400 , 2.33 Fe6 , 10 , 4CzIPN , 1.00 , TEOA , not reported , MeCN/TEOA (9:1) , >400 , 275 ```