# Merging cyclopentadienone tuning and CO to isonitrile substitution to develop photo-activated iron cyclopentadienone catalysts

Clémence Gounon, Gaëtan Quintil, Jacques Pécaut, Martin Clémancey, Ragnar Björnsson, Adrien Quintard, Amélie Kochem

► To cite this version:

Clémence Gounon, Gaëtan Quintil, Jacques Pécaut, Martin Clémancey, Ragnar Björnsson, et al. Merging cyclopentadienone tuning and CO to isonitrile substitution to develop photo-activated iron cyclopentadienone catalysts. Dalton Transactions, 2025, 54 (37), pp.14111-14122. 10.1039/d5dt01184a. hal-05302251

HAL Id: hal-05302251

https://hal.science/hal-05302251v1

Submitted on 5 Nov 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
ARTICLE

# Merging cyclopentadienone tuning and CO to isonitrile substitution to develop photo-activated iron cyclopentadienone catalysts.

Received 00th January 20xx,
Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

Clémence Gounon,ª Gaëtan Quintil,ª,ᵇ Jacques Pécaut,ᶜ Martin Clémancey,ᵇ Ragnar Bjornsson,ᵇ Adrien Quintard,*,ª Amélie Kochem*ᵇ

This study explores the development and reactivity of novel iron cyclopentadienone complexes incorporating isonitrile ligands for photo-activated borrowing hydrogen (BH) catalysis. By merging strategies of cyclopentadienone tuning with isonitrile functionalization, the work aimed to enhance the efficiency of photoactivation processes. New complexes featuring 4-nitrophenyl isonitrile ligands combined with electron-rich cyclopentadienones (L² and L³) were synthesized and characterized through X-ray crystallography, IR, and Mössbauer spectroscopy. Despite promising structural and electronic properties, these complexes showed reduced catalytic activity compared to classical Knölker-type analogs under photoactivation, particularly in allylic alcohol functionalization and BH amine alkylation. Mechanistic studies revealed that the diminished reactivity stemmed from light-induced demetallation of the L² and L³-based complexes, contrasting with the stability of L¹-based counterparts. Adding exogenous ligands like PPh₃ mitigated this demetallation, explaining the reminiscence of catalytic activity in L² and L³ based complexes. The findings highlight the interplay of ligand design, light activation, and catalytic performance, providing a basis for advancing sustainable photocatalysis.

## INTRODUCTION

In the quest of the development of eco-friendly hydrogen transfer and borrowing hydrogen synthetic processes using catalysts based on abundant metals, iron(cyclopentadienone) complexes have received much attention recently due to several key features including their cheap and easy synthesis, handling and reactivity.¹ This reactivity can compete with noble-metal catalysts thanks to the use of redox active cyclopentadienone ligands switching between a non-aromatic and aromatic form allowing the iron to engage in a bi-electronic Fe(O)/Fe(II) switch that operates in hydrogen transfer reactions.² This crucial cooperation between the metal and the ligand allows the catalyst to abstract H₂ from a donor and to transfer it to a polar double bond such as carbonyl or imine. Over time, the versatility of these catalysts in terms of reactivity has continually increased, with applications in reactions such as reductions of carbonyls and imines, oxidations of alcohols, and borrowing hydrogen reactions, as examples.³ Improving the catalyst efficiency via complex structure tuning mainly focused on cyclopentadienone ring modifications,⁴ with limited studies systematically investigating the underlying effects of these modifications.⁵ This is primarily due to structural alterations of

the cyclopentadienone core, which often impact both its steric and electronic properties, creating challenges in developing structure-activity relationships. Recently, Funk and co-workers studied a series of (3,4-diphenylcyclopentadienone)iron carbonyl compounds with electron-donating and –withdrawing substituents in para positions of the two appended aromatic rings with minimal changes to its overall size.⁶ Thanks to the exploration of their reactivity under chemical activation and elegant mechanistic studies, including kinetic isotope effect experiments and modifications to substrate electronics, they demonstrated that catalysts with electron-rich cyclopentadienone ligands reacted more quickly than those without, in both transfer hydrogenations and dehydrogenations. They could notably isolate a trimethylamine-ligated(cyclopentadienone)iron dicarbonyl compound proposed as a resting state for both hydrogenation and dehydrogenation as suggested by previous studies.⁷ In comparison, fewer studies have been conducted on the modification of the iron-tricarbonyl part of the catalyst. This can be explained in part due to the fact that strong field CO ligands are required for the catalytic activity as they increase the d-orbital splitting and stabilize a low-spin iron configuration. Our group recently tackled this problematic by replacing CO ligands by electronically similar but tunable isonitrile ligands. We developed a family of various isonitrile-substituted iron cyclopentadienone complexes and investigated their reactivity.⁸,⁹ We have notably established the successful application of the monosubstituted complexes in classical borrowing hydrogen amine alkylation with alcohols and most importantly, in the development of a photo-activated

<footnote>ª Univ. Grenoble Alpes, CNRS, DCM, 38000 Grenoble, France.</footnote>

<footnote>ᵇ Université Grenoble Alpes, CNRS, CEA, LCBM (UMR 5249), F-38000 Grenoble, France.</footnote>

<footnote>ᶜ Univ. Grenoble Alpes, CEA, CNRS, IRIG, SYMMES F-38000 Grenoble, UMR 5819.</footnote>

<footnote>Supplementary Information available: Experimental protocols, detailed analysis, additional experiments. See DOI: 10.1039/x0xx00000x</footnote>
multicatalytic enantioselective allylic alcohol functionalization.¹⁰ We showed that the photoirradiation of the precatalysts lead to CO dissociation and the generation of a new active species [FeL¹(CO)(CN-R)] and that the efficiency of the photoactivation process could be directly triggered through isocyanide tuning.

<figure><img src="image_2.png" /><figcaption>**Figure 1:** Iron cyclopentadienone tuning for improved reactivity under a) chemical activation and b) photochemical activation (top). New iron complex merging both strategy (bottom).</figcaption></figure>

From these results, we hypothesized that catalyst efficiency could be enhanced by merging both strategies, e.g. cyclopentadienone and isocyanide tuning. In this work, we report the development and structural analysis of new complexes that incorporate the substituted isocyanide CN-Ph-NO₂ which had shown the most effective photo-activation process to date. These catalysts also feature either the electron-rich L² and L³ ligands, identified by Funk, Renaud and collaborators as having the best catalytic activity for hydrogenation (for L² and L³ ligands)¹¹ as well as for dehydrogenation (for L² ligand).⁶

## RESULTS AND DISCUSSION

### Catalysts Preparation and Structural Analysis.

The complexes [FeL¹(CO)₃],¹²,¹³ [FeL₂(CO)₃],¹⁴ [FeL₃(CO)₃]¹¹ and [FeL₁(CO)₂(CN-Ph-NO₂)]⁸ were prepared according to published procedures. The new complexes [FeL²(CO)₂(CN-Ph-NO₂)] and [FeL³(CO)₂(CN-Ph-NO₂)] were obtained in high yield (62 and 46 %, respectively) by reacting the corresponding tris carbonyl complexes in the presence of a slight excess of isocyanide and Me₃NO at room temperature in pentane. The complexes were air stable solids stored under dark at room temperature. No

decomposition was observed after several months of storage under these conditions.

<figure><img src="image_3.png" /><figcaption>**Scheme 1:** Synthetic protocol to access [FeL²(CO)₂(CN-Ph-NO₂)] and [FeL³(CO)₂(CN-Ph-NO₂)].</figcaption></figure>

Single crystals of [FeL²(CO)₂(CN-Ph-NO₂)] were grown by slow diffusion of pentane into a concentrated solution of the complex in dichloromethane. Its structure and selected bond distances are depicted in Figure 2. The X-ray crystal structures of the parent complex [FeL²(CO)₃]¹⁴ and derivatives in which one CO has been substituted either by one trimethylamine⁶ or one NHC carbene¹⁴ ligand were previously described. In all these structures, the four aromatic rings are tilted out of the plane of the cyclopentadienone. Moreover, the replacement of one CO by a strong donor and weaker π-acceptor such as trimethylamine or NHC ligand induced a contraction of the remaining two Fe–CO bond distances (from an average of 1.81 Å in tris-carbonyl complex to 1.78Å in trimethylamine monosubstituted complex to 1.76 Å in the NHC monosubstituted complex). Such contraction was also observed in [FeL¹(CO)₂(CN-Ar)] complexes where one CO ligand was replaced by an isonitrile ligand, with the extent of contraction influenced by the type of isonitrile functionalization.⁸ For example, electron-poor isonitriles such as CN-Ph-NO₂ promoted greater π-backbonding, resulting in reduced contraction of the remaining Fe–CO bond. In [FeL²(CO)₂(CN-Ph-NO₂)], the average of the two Fe–CO bond distances is of 1.79 Å, similar to its parent tris-carbonyl complex. It shows the propensity of the CN-Ph-NO₂ ligand to mimic the CO properties as a good σ donor and π acceptor ligand.
<figure><img src="image_4.png" /><figcaption>**Figure 2:** ORTEP drawing of $[FeL^2(CO)_2(CN-Ph-NO_2)]$. Displacement ellipsoids are at the 30% probability level. H-atoms have been omitted for clarity. Selected bond lengths (Å): Fe(1)-C(1) 2.395(2), Fe(1)-C(2) 2.139(2), Fe(1)-C(3) 2.085(2), Fe(1)-C(4) 2.092(2), Fe(1)-C(5) 2.103(2), Fe(1)-C(31) 1.843(2), Fe(1)-C(41) 1.788(2), Fe(1)-C(42) 1.791(3), C(1)-O(1) 1.227(3), C(31)-N(31), 1.158(3), C(42)-O(42) 1.134(4), C41-O(41) 1.134(3). Deposited on Cambridge database under the number: CCDC 2419951</figcaption></figure>

Next, the various complexes were studied by IR, an excellent technique to evaluate the impact of bonding environment via CO and CN stretching frequencies investigation. Consistent with the mono-substitution of one CO by the isocyanide, the IR spectrum of $[FeL^2(CO)_2(CN-Ph-NO_2)]$ exhibits one vibration band at $2130 \text{ cm}^{-1}$ attributed to the CN stretching frequency, two absorption bands attributed to the symmetric ($1997 \text{ cm}^{-1}$) and antisymmetric ($1958 \text{ cm}^{-1}$) stretching frequency of the terminal CO ligands (Figure S1). As compared to $[FeL^1(CO)_2(CN-Ph-NO_2)]$, the vCO and vCN of the terminal ligands vibrate at similar wavenumber in $[FeL^2(CO)_2(CN-Ph-NO_2)]$ (Figure S1, Table 1). In $[FeL^3(CO)_2(CN-Ph-NO_2)]$, the vCO (1979 and 1929 $\text{cm}^{-1}$) and vCN (2089 $\text{cm}^{-1}$) of the terminal ligands vibrate at much lower wavelength (Figure S1, Table 1). It suggests that replacing the cyclopentadienone $L^1$ by $L^2$ doesn't change significantly the terminal Fe-CO and Fe-isocyanide bonding which are reinforced in complexes based on the $L^3$ type ligand. The reinforcement of the Fe-CO bonding was already observed in the parent complex $[FeL^3(CO)_3]$ as compared to $[FeL^1(CO)_3]$ and $[FeL^2(CO)_3]$ and is probably due to the higher electron-enrichment of the $L^3$ ligand (Figure S1).

<table><caption>**Table 1:** Experimental IR Frequencies for Complexes $[FeL^1(CO)_2(CN-Ph-NO_2)]$, $[FeL^2(CO)_2(CN-Ph-NO_2)]$ and $[FeL^3(CO)_2(CN-Ph-NO_2)]$.</caption><thead><tr><th rowspan="2">Iron complexes</th><th rowspan="2">CN</th><th colspan="2">v̄ [cm<sup>-1</sup>]</th></tr><tr><th>CO (sym.)</th><th>CO (asym.)</th></tr></thead><tbody><tr><td>[FeL<sup>1</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>2117</td><td>2007</td><td>1964</td></tr><tr><td>[FeL<sup>2</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>2130</td><td>1997</td><td>1958</td></tr></tbody></table>

<table><tr><td>[FeL<sup>3</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>2089</td><td>1979</td><td>1929</td></tr></table>

Finally, the effect of the cyclopentadienone tuning on the electronic of the iron center in the series of tris-carbonyl and monosubstituted complexes was investigated by Mössbauer spectroscopy. Their spectra recorded at 80 K, complemented by that of the $L_1$ based complexes $[FeL^1(CO)_3]$⁷ and $[FeL^1(CO)_2(CN-Ph-NO_2)]$⁸ are displayed in Figure S2 and S3. All the complexes are characterized by a single doublet whose nuclear parameters are listed in Table 2. They all exhibit similar isomer shifts and quadrupolar splittings, except for complexes based on $L^3$ type ligand, which exhibit a slightly higher isomer shift. These results show that the cyclopentadienone tuning has limited electronic impact on the iron center.

<table><caption>**Table 2:** Mössbauer parameters determined from simulations of spectra of tris-carbonyl and mono-substituted complexes. Only the absolute value of the quadrupole splitting can be experimentally determined from zero-field Mössbauer spectra. Uncertainties are $\pm 0.02 \text{ mm s}^{-1}$, $\pm 0.05 \text{ mm s}^{-1}$ and $\pm 0.02 \text{ mm s}^{-1}$, on $\delta$, $\Delta E_Q$ and $\Gamma_{fwhm}$, respectively.</caption><thead><tr><th>Iron complex</th><th>δ<br>(mm.s<sup>-1</sup>)</th><th>ΔE<sub>Q</sub><br>(mm.s<sup>-1</sup>)</th><th>Γ<br>(mm.s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>[FeL<sup>1</sup>(CO)<sub>3</sub>]</td><td>0.06</td><td>1.46</td><td>0.29</td></tr><tr><td>[FeL<sup>1</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>0.05</td><td>1.61</td><td>0.30/0.31</td></tr><tr><td>[FeL<sup>2</sup>(CO)<sub>3</sub>]</td><td>0.07</td><td>1.56</td><td>0.30</td></tr><tr><td>[FeL<sup>2</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>0.07</td><td>1.61</td><td>0.31</td></tr><tr><td>[FeL<sup>3</sup>(CO)<sub>3</sub>]</td><td>0.11</td><td>1.57</td><td>0.36</td></tr><tr><td>[FeL<sup>3</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>0.09</td><td>1.62</td><td>0.40</td></tr></tbody></table>

### Catalysts Activity

The first trials with the newly designed catalysts in borrowing hydrogen reactions, particularly in the traditional amine alkylation using alcohols, validated their catalytic activity under chemical activation (Table 3). Using 5 mol% of the iron complexes, conversion to the alkylated amine occurred in 32-47% yield using the mono-substituted isonitrile complexes (entries 2, 4 and 5), while all the tris carbonyl complexes provided a quantitative yield under the same conditions (entries 1, 3 and 6). The lower catalytic activity of the isocyanide-substituted complexes is not surprising and can be correlated to their more challenging chemical activation compared to their tris-carbonyl counterparts.⁸

<figure><img src="image_5.png" /><figcaption>**Table 3:** Selected iron complexes activity in amine alkylation under chemical activation.</figcaption></figure>
<table><thead><tr><th>Entry</th><th>Iron complex</th><th><sup>1</sup>H NMR yield<sup>a)</sup></th><th>Isolated yield</th></tr></thead><tbody><tr><td>1</td><td>[FeL<sup>1</sup>(CO)<sub>3</sub>]</td><td>99%</td><td>nd</td></tr><tr><td>2</td><td>[FeL<sup>1</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>39%</td><td>nd</td></tr><tr><td>3</td><td>[FeL<sup>2</sup>(CO)<sub>3</sub>]</td><td>99%</td><td>70</td></tr><tr><td>4</td><td>[FeL<sup>2</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>47%</td><td>29</td></tr><tr><td>5</td><td>[FeL<sup>3</sup>(CO)<sub>3</sub>]</td><td>99%</td><td>77</td></tr><tr><td>6</td><td>[FeL<sup>3</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>32%</td><td>nd</td></tr></tbody></table>

[a] Determined by ¹H NMR using dibromomethane as internal standard.

Next, we screened the new isonitrile-substituted complexes in the condensation of dibenzoylmethane to allyl alcohol using an organocatalyst (Org) and 368 nm LED photoactivation as represented in Table 4. Their catalytic efficiency was compared to the complexes [FeL¹(CO)₃] and [FeL¹(CO)₂(CN-Ph-NO₂)] previously investigated.⁸ Not surprisingly, the tris-carbonyl complexes exhibit poor, if not negligible, catalytic activity, with conversion of 0% and 13% for [FeL²(CO)₃] and [FeL³(CO)₃], respectively. This is consistent with previous observations for [FeL¹(CO)³].³b,⁸ Indeed, while this catalyst demonstrated good efficiency under chemical activation, its performance decreased under UV activation, highlighting the importance of selecting the appropriate catalyst based on the type of activation used. However, the catalytic activity of the corresponding monosubstituted complexes is more unexpected. While the introduction of the isocyanide CN-Ph-NO₂ in [FeL¹(CO)₂(CN-Ph-NO₂)] drastically increased catalytic activity under photoactivation, achieving a 95% conversion yield, the efficiency of the new complexes [FeL²(CO)₂(CN-Ph-NO₂)] and [FeL³(CO)₂(CN-Ph-NO₂)] remains low, with conversion yields of 26% and 12%, respectively (Table 2, entry 3 and 6). Extending the reaction time did not improve the catalytic efficiency (Table 4, entry 8), indicating a potential degradation of the active catalyst over time.

<figure><img src="image_6.png" /><figcaption>**Table 4:** Iron complexes activity in the photo-induced functionalization of allyl alcohol.</figcaption></figure>

<table><thead><tr><th>Entry</th><th>Iron complex</th><th>Conversion<sup>[a)</sup></th></tr></thead><tbody><tr><td>1</td><td>[FeL<sup>1</sup>(CO)<sub>3</sub>]</td><td>9%</td></tr><tr><td>2</td><td>[FeL<sup>1</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>95%<sup>[b)</sup></td></tr><tr><td>3</td><td>[FeL<sup>2</sup>(CO)<sub>3</sub>]</td><td>0%</td></tr><tr><td>4</td><td>[FeL<sup>2</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>26%</td></tr><tr><td>6</td><td>[FeL<sup>3</sup>(CO)<sub>3</sub>]</td><td>13%</td></tr><tr><td>7</td><td>[FeL<sup>3</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>12%</td></tr><tr><td>8<sup>[c)</sup></td><td>[FeL<sup>3</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>12%</td></tr></tbody></table>

Reactions performed using 0.2 mmol of diketone, 0.4 mmol of allyl alcohol in 0.325 mL of toluene over 43 hours. [a] Determined by ¹H NMR. [b] 88:12 *er* determined by SFC. [c] Reaction performed under 86 hours.

The introduction of electron-rich ligands such as L² and L³ has proven beneficial for the catalytic activity of the corresponding tris-carbonyl complexes under chemical activation. However, these results demonstrate that these ligands are detrimental to the reactivity of photoactivated complexes. At this stage, many fundamental questions arise concerning the structure/activity relationship. What explains this difference in reactivity? Are the activation kinetics of the precatalysts different depending on their structure or the type of activation used? Is the nature and quantity of active species formed different depending on the catalyst or the type of activation used? And can it be directly correlated to catalysts efficiency? To attempt to answer these questions, we first studied the behavior of the catalysts under light-irradiation.

**Behavior and species formed under light-irradiation of the precatalyst in absence of exogenous ligand.**

At a first glance, the absorption properties of the complexes and ligands were investigated (Figure 3 and Table 5). The free L¹ ligand (brown line, Figure 3), is characterized by a low absorption band at 417 nm (738 M⁻¹·cm⁻¹). The free L² ligand (blue line, Figure 3), is characterized by an intense absorption band at 375 nm (11240 M⁻¹·cm⁻¹) and a band of lowest intensity at 516 nm (941 M⁻¹·cm⁻¹) while the diaryl L³ ligand is characterized by an intense absorption band at 314 nm (23395 M⁻¹·cm⁻¹) and a low absorption at 509 nm (520 M⁻¹·cm⁻¹).

The absorption properties of tetraarylcyclopentadienones were previously investigated.¹⁵ All of them exhibited at least three electronic transitions with an absorption maximum changed upon substitution of the para positions of the phenyl rings. Thanks to computational chemistry, Hughes et al. attributed each of these transitions to ligand-to-ligand charge transfer excitations involving the π molecular orbitals (MOs) of the cyclopentadienone core (π to π* transitions). They notably showed that the introduction of electron-donating substituents
on the para position of the phenyl rings induced a lowering of the transition energies $\lambda^1$ and $\lambda^2$ due to destabilization of $\pi$ donor orbitals located at the cyclopentadienone with a $\pi^*$ acceptor orbital which doesn't vary in energy. Inspired from

these works, we performed time-dependent DFT (TD-DFT) calculations. Difference density plots for the computed transitions of L¹, L² and L³ are shown in Figure 3 (see Figure S11 for full UV-visible spectra of the ligands)

<figure><img src="image_7.png" /><figcaption>**Figure 3:** UV-visible spectra of toluene solutions of ligands L¹ (brown line), L² (blue line), L³ (grey line) at 2.2 $10^{-4}$ M in toluene (top figure), tris-carbonyl complexes [FeL¹(CO)₃] (brown line), [FeL²(CO)₃] (blue line), [FeL³(CO)₃] (grey line) at 1 $10^{-4}$ M in toluene (middle figure)</figcaption></figure>
and substituted complexes $[FeL^1(CO)_2(CN-Ph-NO_2)]$ (brown line), $[FeL^2(CO)_2(CN-Ph-NO_2)]$ (blue line), $[FeL^3(CO)_2(CN-Ph-NO_2)]$ (grey line) at $2 \times 10^{-4}$ M in toluene. T = 298 K. Captions: TD-DFT assignment of the electronic transitions of the ligands and complexes. Calculated wavelengths and oscillator strengths are noted in square brackets. Also shown are difference electron densities where red corresponds to acceptor density and yellow to donor density.

The weak absorption band experimentally observed above 400 nm for the three ligands are computed at $\lambda_{calc} = 398$ nm (f = 0.03), $\lambda_{calc} = 520$ nm (f = 0.03) and $\lambda_{calc} = 466$ nm (f = 0.03) for L¹, L² and L³, respectively. In all three cases, the acceptor fragment is a π orbital delocalized on the C(2), C(3), C(4) and C(5) carbon atoms and the donor fragment is a π orbital delocalized on the C(1), O(1), C(3), C(4) atoms of the cyclopentadienone ring. The nature of this transition is similar in all ligands and its position can be controlled through cyclopentadienone substitution. Indeed these results show that the transition is shifted at higher wavelength via the introduction of electron-donating substituents on the C(3) and C(4) positions of the cyclopentadienone ring. Hughes et al. previously established that the lowering of this transition energy is due to destabilization of π-donor orbitals located at the cyclopentadienone with a π*-acceptor orbital which doesn't vary in energy. For L², the intense band at 375 nm is computed at $\lambda_{calc} = 342$ nm (f = 0.38). The acceptor and donor fragments are π orbitals delocalized over the entire cyclopentadienone core and on the 2- and 5-phenyl rings, respectively. For L³, the main transitions contributing to the strong absorption observed at 314 nm are computed at $\lambda_{calc} = 261$ nm (f = 0.44) and 275 nm (f = 0.33). In both cases, the donor fragment is a π orbital delocalized on the cyclopentadienone and fused rings while the π acceptor orbital is delocalized either on the C(2) and C(5) substituted phenyl rings or on the cyclopentadienone core. All together, these results show that irrespectively of the substitution pattern, the ligands exhibit a weak absorption band involving π orbitals delocalized only on the cyclopentadienone core. In L² and L³, the substitution of the C(3) and C(4) positions by either an electron-enriched fused ring or phenyl rings substituted in para positions by electron-donating substituents leads to the lowering of the energy of this transition and also to the appearance of strong absorption bands at lower wavelength as compared to L¹.

Next, the effect of the cyclopentadienone ring substitution on the absorption properties of the tris-carbonyl complexes was investigated. $[FeL^1(CO)_3]$ is characterized by a stronger absorption band at 308 nm ($4 \times 491 \text{ M}^{-1}\text{cm}^{-1}$) as compared to the free L¹ ligand while the coordination of the $Fe^{(0)}(CO)_3$ moiety to the L² ligand leads to a partial loss of the transition of high intensity of the free ligand with $[FeL^2(CO)_3]$ (blue line, Figure 3), being characterized by a broad absorption band at a maximum located at 279 nm ($24 \times 887 \text{ M}^{-1}\text{cm}^{-1}$). In the case of $[FeL^3(CO)_3]$, the coordination of the $Fe^{(0)}(CO)_3$ moiety to the ligand induces the appearance of an absorption band at 350 nm ($4121 \text{ M}^{-1}\text{cm}^{-1}$) while the strong absorption at lower wavelength is 284 nm ($22 \times 726 \text{ M}^{-1}\text{cm}^{-1}$).

The exact nature of the orbitals involved in the UV-vis transitions of tris-carbonyl complexes was investigated using TD-DFT calculations. Difference density plots for the computed transitions responsible for the absorption of the complexes above 280 nm are shown in Figure 3. For all complexes, the main

electronic excitations contributing to the absorption above 280 nm are attributed to metal-to-ligand charge transfer (MLCT), with the donor orbital primarily localized on the iron center and the acceptor π* orbitals mostly delocalized over the cyclopentadienone core. The complexes $[FeL^2(CO)_3]$ and $[FeL^3(CO)_3]$ exhibit calculated transitions with higher oscillator strengths, which explains their stronger absorption around 280 nm compared to $[FeL^1(CO)_3]$ (see Figures S12 and S13). The occurrence of these transitions at higher wavelengths in $[FeL^2(CO)_3]$ leads to the broadening of its MLCT absorption. It's interesting to see that the red shift of the MLCT by going from $[FeL^1(CO)_3]$ to $[FeL^2(CO)_3]$ and $[FeL^3(CO)_3]$ is well reproduced by the calculations and attributed to the substitution of the cyclopentadienone core by the corresponding ligands.

Studies performed in our group had shown that with L¹ based complex the substitution of one CO by one isocyanide (CN-Ph-NO₂) led to the appearance of a new absorption band located at 351 nm ($14 \times 229 \text{ M}^{-1}\text{cm}^{-1}$) attributed to Fe 3d to CN π* orbital transitions with significant contribution from the phenyl-NO₂ part of the ligand (brown line, Figure 3). More importantly, light-irradiation at a wavelength corresponding to the maximum absorption of this transition led to CO dissociation which allowed to establish a correlation between the catalytic activity of this complex under irradiation and its peculiar absorption properties. The same spectroscopic behavior is observed for $[FeL^2(CO)_2(CN-Ph-NO_2)]$ (blue line, Figure 3) and $[FeL^3(CO)_2(CN-Ph-NO_2)]$ (grey line, Figure 3) with the appearance of a strong absorption band located 352 nm ($12 \times 345 \text{ M}^{-1}\text{cm}^{-1}$) and 379 ($11 \times 339 \text{ M}^{-1}\text{cm}^{-1}$) as compared to $[FeL^2(CO)_3]$ and $[FeL^3(CO)_3]$ respectively. TD-DFT calculations performed with the same methodology for the three complexes reveals that one main transition is responsible for the strong absorption experimentally observed. In all three complexes, the acceptor fragment is similar and corresponds to a CN π* orbital with a significant contribution from the phenyl-NO₂ part of the ligand. However, the nature of the donor fragment differs considerably. In $[FeL^1(CO)_2(CN-Ph-NO_2)]$, the donor corresponds to Fe 3d orbitals, with a contribution from the π orbital located on the carbonyl group of the cyclopentadienone. In contrast, in $[FeL^2(CO)_2(CN-Ph-NO_2)]$ and $[FeL^3(CO)_2(CN-Ph-NO_2)]$, the donor is primarily composed of π orbitals delocalized over the entire cyclopentadienone, with a significant contribution from the 2- and 5-phenyl rings in $[FeL^2(CO)_2(CN-Ph-NO_2)]$ or from the fused ring in the case of $[FeL^3(CO)_2(CN-Ph-NO_2)]$.

<table><caption>Table 5: Electronic Spectral Data of the Complexes and Ligand.ª</caption><thead><tr><td>Complex/Ligand</td><td>λ<sub>max</sub>, nm (ε, M<sup>-1</sup>·cm<sup>-1</sup>)</td></tr></thead><tbody><tr><td>L<sup>1</sup></td><td>417 (738)</td></tr><tr><td>[FeL<sup>1</sup>(CO)<sub>3</sub>]</td><td>308 (4 491)</td></tr><tr><td>[FeL<sup>1</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>351 (14 229), 308 sh (9466)</td></tr><tr><td>L<sup>2</sup></td><td>375 (11 240), 516 br (941)</td></tr><tr><td>[FeL<sup>2</sup>(CO)<sub>3</sub>]</td><td>279 br (24 887)</td></tr></tbody></table>
<table><tr><td>[FeL<sup>2</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>352 (12 345), 281 (45 032)</td></tr><tr><td>L<sup>3</sup></td><td>314 (23 395), 509 (520)</td></tr><tr><td>[FeL<sup>3</sup>(CO)<sub>3</sub>]</td><td>284 (22 726), 350, sh, br (4121)</td></tr></table>

<table><tr><td>[FeL<sup>3</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)]</td><td>284 (20 941), 379 (11 339)</td></tr></table>

<sup>a</sup>In toluene solution at 298 K. Abbreviations used: sh, shoulder; br, broad.

<figure><img src="image_8.png" /><figcaption>**Figure 4:** Evolution of UV-visible spectra of a solution of a) [FeL<sup>2</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)] (2.2 10<sup>-5</sup> M), b) [FeL<sup>2</sup>(CO)<sub>3</sub>] (2.2 10<sup>-5</sup> M), c) [FeL<sup>1</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)] (7.3 10<sup>-5</sup> M) and d) [FeL<sup>1</sup>(CO)<sub>3</sub>] (7.3 10<sup>-5</sup> M) in toluene at room temperature under argon atmosphere and irradiation (368 nm ± 5 nm, 30W). 45 scans; 1 scan each 4 seconds. Green and blue spectra correspond to the initial complex without irradiation and the final spectra after 45 scans, respectively. Time-dependent <sup>1</sup>H NMR studies over light irradiation in CD<sub>2</sub>Cl<sub>2</sub> solvent e) [FeL<sup>2</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)], f) [FeL<sup>2</sup>(CO)<sub>3</sub>] and in CDCl<sub>3</sub> solvent g) [FeL<sup>1</sup>(CO)<sub>2</sub>(CN-Ph-NO<sub>2</sub>)] and h) [FeL<sup>1</sup>(CO)<sub>3</sub>].</figcaption></figure>
The absorption properties of the isocyanide complexes suggest that they possess the necessary characteristics to induce the photo-dissociation of one carbonyl ligand and the generation of the catalytically active species, as previously observed in complex [FeL¹(CO)₂(CN-Ph-NO₂)]$. In such case, what can explain the poorer catalytic activity of the new complexes [FeL²(CO)₂(CN-Ph-NO₂)] and [FeL³(CO)₂(CN-Ph-NO₂)]? To answer this question, light-irradiation experiments were performed over time.

First, we examined the behavior of [FeL²(CO)₂(CN-Ph-NO₂)] under continuous irradiation in toluene at room temperature (Figure 4-a). A gradual and fast decrease of the absorption maximum at 359 nm and a concomitant increase of absorption bands at 284 nm and 525 nm were observed upon keeping the complex in solution under continuous irradiation at 368 nm over time (3 minutes total). Irradiation over longer time didn't lead to further evolution of the UV-visible spectra and stopping the irradiation did not gave back the initial spectrum indicating the fast and irreversible formation of a stable species whose absorption properties looks like the free ligand L². Surprised by these results, we examined the irradiation-dependent absorption changes of [FeL²(CO)₃] under the same conditions (Figure 4-b). Here again we observed a fast, gradual and irreversible evolution of the initial spectrum with the appearance of two absorption bands at 370 nm and 509 nm and a decrease of the absorption at 326 nm (2.3 minutes total of continuous irradiation). The resemblance to the absorption spectrum of the free ligand and the presence of isobestic points suggest that irradiation leads to demetallation of the complex. Time-dependent ¹H NMR studies were performed under the same irradiation conditions and confirmed the fast and irreversible demetallation of both complexes [FeL²(CO)₃] and [FeL²(CO)₂(CN-Ph-NO₂)] under irradiation (Figure 4-e and -f). Similar studies were conducted on [FeL³(CO)₃] and [FeL³(CO)₂(CN-Ph-NO₂)] and exhibited similar trend (demetallation) (Figure S9).

However, [FeL¹(CO)₂(CN-Ph-NO₂)] and [FeL¹(CO)₃] exhibit different behavior under photoactivation (Figure 4-c and d). On one hand, after 40 minutes under continuous irradiation of the complex [FeL¹(CO)₂(CN-Ph-NO₂)] the signal of the corresponding ligand was observed but the initial complex remains in large proportion. On the other hand, [FeL¹(CO)₃] remains the major species after 40 minutes of irradiation and a new and unidentified species is formed. This new species clearly differs from the free L¹ ligand. Several attempts to isolate it failed and it was putatively attributed to the formation of an iron dimer complex. Indeed, DFT calculations shows that in absence of substrate, the formation of a dimer species from a monomer after CO loss is favored. For the [FeL¹(CO)₂(CN-Ph-NO₂)] case, dimer formation is favored by ΔG =-8 kcal mol⁻¹ while ΔG of dimerization is -5 kcal mol⁻¹ for [FeL¹(CO)₃] (Figure 5-b). Previous work proposing such dimer as resting state.⁴⁸ Alternatively, a dimer where the cyclopentadienone carbonyl oxygens coordinate to the empty sites on the Fe was considered (Figure 5-c). That type of compound has been isolated for the Ru analogues of these compounds.¹⁶ DFT calculations shows

that the formation of such dimer species is even more likely. For both complexes ([FeL¹(CO)₂(CN-Ph-NO₂)] and [FeL¹(CO)₃]), such dimer formation is favored by ΔG =-12 kcal mol⁻¹.

<figure><img src="image_9.png" /><figcaption>**Figure 5.** Putative species formed under irradiation of [FeL¹(CO)₃].</figcaption></figure>

Altogether, these results show that complexes based on the L¹ type ligand are much more resistant to demetallation under irradiation at 365 nm explaining their excellent catalytic activity under photo-activation as compared to complexes based on L² and L³ ligands. Bütikofer and Chen have shown that ligand exchange may occur in carbonyl-bridged dimers, leading either to catalyst deactivation via reversion to a tris-carbonyl complex or, more critically, to demetallation and irreversible catalyst deactivation.¹⁷ Although demetallation via dimer formation could not be experimentally confirmed for L² and L³ based complexes—due to the inability to trap intermediates and the complete inhibition of the reaction upon addition of a chemical trap (see Supporting Information)—this mechanism remains a plausible hypothesis. If the photo-irradiation of the complexes leads to their demetallation, how can the observed catalytic activity (low but still persistant) be explained? We hypothesize that the presence of exogenous ligand (such as substrate) could alter this process and prevent the demetallation to different extents depending on the cyclopentadienone ligand. To investigate this hypothesis the photo-irradiation of the complexes was performed in the presence of a stabilizing PPh₃ ligand (σ donor, π acceptor).

## Behavior and species formed under irradiation of the complexes in the presence of exogenous ligand

A previous experiment, carried out by irradiating at 368 nm [FeL¹(CO)₂(CN-Ph-NO₂)] in the presence of a stabilizing exogenous PPh₃ ligand, lead to the formation of a mixture of unreacted [FeL¹(CO)₂(CN-Ph-NO₂)] (27%) and a new species [FeL¹(CO)(PPh₃)(CN-Ph-NO₂)] in 60% isolated yield which could be characterized by XRD (Figure 6) and NMR.⁸ Analysis of the structure reveals that one terminal CO had been substituted by one PPh₃ ligand. Calculations further confirm that the substitution of one CO ligand by PPh₃ is ~4 kcal mol⁻¹ more favorable (ΔG) than substitution of the isonitrile ligand. These
results thus strongly suggest the decoordination of one terminal
CO to occur under irradiation and the generation of an activated
species whose structure differs from the classical [FeL(CO)₂].

<figure><img src="image_10.png" /><figcaption>**Figure 6:** Irradiation of [FeL¹(CO)₂(CN-Ph-NO₂)] in the presence of exogenous PPh₃ ligand.</figcaption></figure>

Similar experiments were conducted in the case of
[FeL²(CO)₂(CN-Ph-NO₂)] . Examination of the time-dependent
UV-visible spectra of an irradiated solution of [FeL²(CO)₂(CN-Ph-
NO₂)] revealed a different trend in the presence of 2 equivalents
of PPh₃ with a decrease of the absorption band at 352 nm and
an increase of the band at 317 nm without a clear isosbestic
point (Figure 7). The absence of a clear isobestic point suggests
the formation of more than one type of new species, most likely
substituted complexes with PPh₃. The spectroscopic fingerprint
of the free ligand L² was not observed in this case
demonstrating that the irradiation doesn't lead to its
photodissociation or only in small proportions.

<figure><img src="image_11.png" /><figcaption>**Figure 7:** Evolution of UV-visible spectra of a solution of [FeL²(CO)₂(CN-Ph-NO₂)] at 2.2 10⁻⁵ M in toluene in the presence of 2 equivalents of PPh₃ at room temperature under argon atmosphere and irradiation (368 nm ± 5 nm, 30W). 45 scans; 1 scan each 4 seconds. Green and blue spectra correspond to the initial complex without irradiation and the final spectrum after 45 scans, respectively.</figcaption></figure>

In order to identify the type of species formed we performed ¹H and ³¹P NMR experiments under the same irradiation conditions. Analysis of the ¹H NMR data (Figure S5-a) showed the complete disappearance of the signals of the initial [FeL²(CO)₂(CN-Ph-NO₂)] complex and the appearance of new set of signals after irradiation differing from those of the free ligand L² while ³¹P NMR data's (Figure S5-b) revealed the consumption of free PPh₃ ligand at -5.6 ppm and the appearance of two new signals at 59.0 ppm (major species) and 61.5 ppm (minor species). Purification of this mixture by chromatography column under air and analysis of the isolated species led to the doubtless attribution of the ³¹P NMR signal at 59.0 ppm to the formation of the substituted complex [FeL²(CO)(PPh₃)(CN-Ph-NO₂)] (see supporting information page S9, isolated yield of 64%) and portion of the free cyclopentadienone ligand which was not identified in the case of the irradiation of [FeL¹(CO)(PPh₃)(CN-Ph-NO₂)]. These studies clearly show that the presence of an exogenous strongly stabilizing PPh₃ ligand mitigates the demetallation of the complex [FeL²(CO)₂(CN-Ph-NO₂)], albeit at the expense of the reactivity (see SI for details). Under the multicatalytic reaction, the alcohol or the diketone would not stabilize enough the catalyst to prevent its decomposition over time

Conclusions

In conclusion, a new family of 4-nitrophenyl isonitrile-
substituted iron cyclopentadienone complexes was successful-
ly prepared. Their design involved combining bis-carbonyl-
isonitrile-iron, identified as the most promising moiety for
efficient photocatalytic BH activity, with various electron-
enriched cyclopentadienones, which have demonstrated supe-
rior catalytic efficiency for hydrogenation and/or dehydrogena-
tion reactions under chemical activation. Surprisingly, the newly
designed complexes exhibited significantly lower reac-tivity
than the classical Knölker complex substituted with 4-
nitrophenyl isonitrile.

Characterization of their physical properties, combined with DFT calculations, provided insights into their behavior under photo-irradiation and helped establish correlations with their reactivity. Prolonged photo-irradiation of the tris-carbonyl or isocyanide-substituted complexes bearing L2 and L3-type ligands clearly indicated complete demetallation. This phenomenon is mitigated by the presence of an exogenous ligand, which could explain the partial reactivity observed under cata-lytic conditions. In contrast, the resistance of complexes bear-ing the L1-type ligand to photo-bleaching clearly accounts for their much higher activity compared to the others.

This underscores the importance of selecting isonitrile and
cyclopentadienone ligands that are compatible with the chosen
activation mode to achieve optimal reactivity. Indeed, while
electron-enriched ligands provided better reactivity compared
to iron complexes based on the classical L1 ligand under chem-
ical activation, they reduced the efficiency of the correspond-
ing complexes under photoactivation by making them more
prone to demetallation. Further design of catalysts by introduc-
ing various electron-enriched cyclopentadienone ligands com-
compatible with the photoactivation are currently undergoing in our laboratory. Given the significance of this class of catalysts and their activation via photochemical pathways, we hope that these studies will guide readers in designing catalysts tailored to their chosen activation method.

## Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

## Corresponding Author

Adrien Quintard
Univ. Grenoble Alpes, CNRS, DCM, 38000 Grenoble, France
orcid.org/0000-0003-0193-6524

Email: adrien.quintard@univ-grenoble-alpes.fr

Amélie Kochem
Université Grenoble Alpes, CNRS, CEA, LCBM (UMR 5249), F-38000 Grenoble, France.

Email: amelie.kochem@cea.fr

## Present Addresses

Clémence Gounon
Université Grenoble Alpes, CNRS, CEA, LCBM (UMR 5249), F-38000 Grenoble, France

Gaëtan Quintil
Univ. Grenoble Alpes, CNRS, DCM, 38000 Grenoble, France

Jacques Pécaut
Univ. Grenoble Alpes, CEA, CNRS, IRIG, SYMMES F-38000 Grenoble, UMR 5819

Martin Clémancey
Université Grenoble Alpes, CNRS, CEA, LCBM (UMR 5249), F-38000 Grenoble, France

Ragnar Bjornsson
Université Grenoble Alpes, CNRS, CEA, LCBM (UMR 5249), F-38000 Grenoble, France

## Conflicts of interest

The authors declare no competing financial interests.

## Funding sources

The Centre National de la Recherche Scientifique (CNRS), Université Grenoble-Alpes, the Commissariat à l'énergie Atomique, the Agence Nationale de la Recherche (ANR-23-CE07-0043) the Labex ARCANE and CBH-EUR-GS (ANR-17-EURE-0003) are warmly acknowledged for financial support.

## Data availability

A data availability statement (DAS) is required to be submitted alongside all articles. Please read our [full guidance on data availability statements](#) for more details and examples of suitable statements you can use.

## Acknowledgements

All technical support staff from LCBM, DCM and ICMG are acknowledged for their help. Anestis Alexandridis is acknowledged for the UV reactor set-up.

## Notes and references

1. a) Quintard, A.; Rodriguez, J. Iron Cyclopentadienone Complexes: Discovery, Properties, and Catalytic Reactivity. *Angew. Chem. Int. Ed.* **2014**, *53*, 4044–4055. b) Pignataro, L.; Gennari, C. Recent Catalytic Applications of (Cyclopentadienone) Iron Complexes. *Eur. J. Org. Chem.* **2020**, *2020*, 3192–3205. c) Akter, M.; Anbarasan, P. (Cyclopentadienone) Iron Complexes: Synthesis, Mechanism and Applications in Organic Synthesis. *Chem. Asian J.* **2021**, *16*, 1703–1724.

2. For initial discoveries in hydrogenation and hydrogen transfer, see: a) Casey, C. P.; Guan, H. An Efficient and Chemoselective Iron Catalyst for the Hydrogenation of Ketones. *J. Am. Chem. Soc.* **2007**, *129*, 5816 – 5817; b) Casey, C. P.; Guan, H. Cyclopentadienone Iron Alcohol Complexes: Synthesis, Reactivity, and Implications for the Mechanism of Iron-Catalyzed Hydrogenation of Aldehydes. *J. Am. Chem. Soc.* **2009**, *131*, 2499 – 2507.

3. For a selection of borrowing hydrogen reactions using such iron complexes, see: a) Quintard, A.; Constantieux, T.; Rodriguez, J. An Iron/Amine-Catalyzed Cascade Process for the Enantioselective Functionalization of Allylic Alcohols. *Angew. Chem. Int. Ed.* **2013**, *52*, 12883-12887; b) Roudier, M.; Constantieux, T.; Quintard, A.; Rodriguez, J. Enantioselective Cascade Formal Reductive Insertion of Allylic Alcohols into the C(O)-C Bond of 1,3-Diketones: Ready Access to Synthetically Valuable 3-Alkylpentanol Units. *Org. Lett.* **2014**, *16*, 2802-2805; c) Yan, T.; Feringa, B. L.; Barta, K. Iron catalysed direct alkylation of amines with alcohols. *Nature Communications*, **2014**, *5*, 5602-5608; d) Elangovan, S.; Sortais, J.-B.; Beller, M.; Darcel, C. Iron-Catalyzed α-Alylation of Ketones with Alcohols. *Angew. Chem. Int. Ed.* **2015**, *54*, 14483-14486; e) Pan, H.-J.; Nga, T. W.; Zhao, Y. Iron-catalyzed amination of alcohols assisted by Lewis acid. *Chem. Commun.* **2015**, *51*, 11907-11910; f) Rawlings, A. J.; Diorazio, L. J.; Wills, M. C-N Bond Formation between Alcohols and Amines Using an Iron Cyclopentadienone Catalyst. *Org. Lett.* **2015**, *17*, 1086–1089; g) Roudier, M.; Constantieux, T.; Quintard, A.; Rodriguez, J. Triple Iron/Copper/Iminium Activation for the Efficient Redox Neutral Catalytic Enantioselective Functionalization of Allylic Alcohols. *ACS Catal.* **2016**, *6*, 5236-5244; h) Seck, C.; Diagne Mbaye, M.; Gaillard, S.; Renaud, J.-L. Bifunctional Iron Complexes Catalyzed Alkylation of Indoles. *Adv. Synth. Catal.* **2018**, *360*, 4640-4645; i) Lator, A.; Gaillard, S.; Poater, A.; Renaud, J.-L. Well-Defined Phosphine-Free Iron-Catalyzed N-Ethylation and N-Methylation of Amines with Ethanol and Methanol. *Org. Lett.* **2018**, *20*, 5985–5990; j) Polidano, K.; Allen, B. D. W.; Williams, J. M. J.; Morrill, L. C. Iron-Catalyzed Methylation Using the Borrowing Hydrogen Approach. *ACS Catal.* **2018**, *8*, 6440-6445; k) Quintard, A.; Roudier, M.; Rodriguez, J. Multicatalytic Enantioselective Borrowing Hydrogen δ-Lactonization Strategy from β-Keto Esters and Allylic Alcohols. *Synthesis.* **2018**, *50*, 785-792; l) Latham, D. E.; Polidano, K.; Williams, J. M. J.; Morrill, L. C. One-Pot Conversion of Allylic Alcohols to α-Methyl Ketones via Iron-Catalyzed Isomerization–Methylation. *Org. Lett.* **2019**, *21*, 7914-7918; m) Bettoni, L.; Gaillard, S.; Renaud, J.-L. Iron-Catalyzed β-Alkylation of Alcohols. *Org. Lett.* **2019**, *21*, 8404–8408; n) Bettoni, L.; Gaillard, S.; Renaud, J.-L. Iron-Catalyzed α-Alkylation of Ketones with Secondary Alcohols: Access to
β-Disubstituted Carbonyl Compounds. *Org. Lett.* **2020**, *22*, 2064–2069; o) Shao, N.; Rodriguez, J.; Quintard, A. Catalysis Driven Six-Step Synthesis of Apratoxol A Key Polyketide Fragment. *Org. Lett.* **2022**, *24*, 6537; p) Wu, J.; Tongdee, A.; Cordier, M.; Darcel, C. Selective Iron Catalyzed Synthesis of N-alkylated Indolines and Indoles. *Chem. Eur. J.* **2022**, *28*, e202201809; q) Alexandridis, A.; Rancon, T.; Halliday, A.; Kochem, A.; Quintard, A. Iron- and Organo-Catalyzed Borrowing Hydrogen for the Stereoselective Construction of Tetrahydropyrans. *Org. Lett.*, **2024**, *26*, 5788–5793.

4 For the initial studies on these complexes, see: a) Knölker, H-J.; Heber, J.; Mahler, C. H. Transition Metal-Diene Complexes in Organic Synthesis, Part 14.1 Regioselective Iron-Mediated [2+2+1] Cycloadditions of Alkynes and Carbon Monoxide: Synthesis of Substituted Cyclopentadienones. *Synlett* **1992**, *12*, 1002–1004; b) Pearson, A. J.; Shively, Jr. R. J.; Dubbert, R. A. Iron Carbonyl Promoted Conversion of alpha,omega-Diynes to (Cyclopentadienone) iron Complexes. *Organometallics* **1992**, *11*, 4096-4104; c) Knölker, H.-J.; Heber, J. Transition Metal-Diene Complexes in Organic Synthesis, Part 18.1 Iron- Mediated [2+2+1] Cycloadditions of Diynes and Carbon Monoxide: Selective Demetalation Reactions. *Synlett* **1993**, *12*, 924–926; For later applications in catalysis: d) Moyer, S. A.; Funk, T. W. Air-stable iron catalyst for the Oppenauer-type oxidation of alcohols. *Tetrahedron Letters*, **2010**, *51*, 5430–5433; e) Johnson, T. C.; Clarkson, G. J.; Wills, M. (Cyclopentadienone)iron Shvo Complexes: Synthesis and Applications to Hydrogen Transfer Reactions. *Organometallics* **2011**, *30*, 1859–1868; f) Mérel, D. S.; Elie, M.; Lohier, J-F.; Gaillard, S. Renaud, J-L. Bifunctional Iron Complexes: Efficient Catalysts for C=O and C=N Reduction in Water. *ChemCatChem* **2013**, *5*, 2939 – 2945; g) Moulin, S.; Dentel, H.; Pagnoux-Ozherelyeva, A.; Gaillard, S.; Poater, A.; Cavallo, L.; Lohier, J-F.; Renaud, J-L. Bifunctional (Cyclopentadienone)Iron–Tricarbonyl Complexes: Synthesis, Computational Studies and Application in Reductive Amination. *Chem. Eur. J.* **2013**, *19*, 17881–17890; h) Thai, T-T.; Mérel, D. S.; Poater, A.; Gaillard, S.; Renaud, J-L. Highly Active Phosphine-Free Bifunctional Iron Complex for Hydrogenation of Bicarbonate and Reductive Amination. *Chem. Eur. J.* **2015**, *21*, 7066-7070; i) Gajewski, P.; Renom-Carrasco, M. ; Facchini, S. V. ; Pignataro, L. ; Lefort, L. ; de Vries, J. G. ; Ferraccioli, R. ; Piarulli, U. ; Gennari, C. Synthesis of (R)-BINOL-Derived (Cyclopentadienone)iron Complexes and Their Application in the Catalytic Asymmetric Hydrogenation of Ketones. *Eur. J. Org. Chem.* **2015**, 5526-5536; j) Hodgkinson, R.; Del Grosso, A.; Clarkson, G.; Wills, M. Iron cyclopentadienone complexes derived from C2-symmetric bis-propargylic alcohols; preparation and applications to catalysis. *Dalton Trans.*, **2016**, *45*, 3992–4005; k) Vailati Facchini, S.; Neudörfl, J-M.; Pignataro, L.; Cettolin, M.; Gennari, C.; Berkessel, A.; Piarullia, U. Synthesis of [bis(hexamethylene)cyclopentadienone]iron tricarbonyl and its application to catalytic reductions of C=O. *ChemCatChem.* **2017**, *9*, 1461-1468; l) Del Grosso, A.; Chamberlain, A. E.; Clarkson, G. J.; Wills, M. Synthesis and applications to catalysis of novel cyclopentadienone iron tricarbonyl complexes. *Dalton Trans.*, **2018**, *47*, 1451–1470; m) van Slagmaat, C. A. M. R.; Chou, K. C.; Morick, L.; Hadavi, D.; Blom, B.; De Wildeman, S. M. A. *Catalysts* **2019**, *9*, 790-813: n) Fusi, G. M.; Gandini, T.; Gazzola, S.; Grell, T.; Colombo, V.; Pignataro, L.; Piarulli, U. Design, Synthesis and Catalytic Activity of (Cyclopentadienone)iron Complexes Containing a Stereogenic Plane and a Stereogenic Axis. *Chem. Eur. J.* **2023**, *29*, e202302533: o) Bütikofer, A.; Chen, P. Cyclopentadienone Iron Complex-Catalyzed Hydrogenation of Ketones: An Operando Spectrometric Study Using Pressurized Sample Infusion-Electrospray Ionization-Mass Spectrometry.

*Organometallics* **2022**, *41*, 2349–2364; p) Hackl, L.; Ho, L. P.; Bockhardt, D.; Bannenberg, T.; Tamm, M. Tetraaminocyclopentadienone Iron Complexes as Hydrogenation Catalysts. *Organometallics* **2022**, *41*, 836–851; q) Gaignard Gaillard, Q.; Bettoni, L.; Paris, D.; Lequertier, C.; Lohier, J-F.; Gaillard, S.; Renaud, J-L. Synthesis of Triaminocyclopentadienyl Iron(II) Tricarbonyl Complexes and Application to the Synthesis of Aminophosphines. *Adv. Synth. Catal.* **2023**, *365*, 3704-3712; r) Manna, S.; Peters, J.; Bermejo-López, A.; Himo, F.; Bäckvall, J-E. Mechanistic Studies on Iron-Catalyzed Dehydrogenation of Amines Involving Cyclopentadienone Iron Complexes, Evidence for Stepwise Hydride and Proton Transfer. *ACS Catal.* **2023**, *13*, 8477–8484; s) Goel, B.; Neugebauer, H.; VanderWeide, A. I. ; Sánchez, P. ; Lalancette, R. A. ; Grimme, S.; Hansen, A.; Prokopchuk, D. E. Essential Roles of Cp Ring Activation and Coordinated Solvent During Electrocatalytic H₂ Production with Fe(CpN₃) Complexes. *ACS Catal.* **2023**, *13*, 13650–13662.

For a selection, see: a) Berkessel, A.; Reichau, S.; von der Höh, A.; Leconte, N.; Neudörfl, J.-M. Light-Induced Enantioselective Hydrogenation Using Chiral Derivatives of Casey's Iron-Cyclopentadienone Catalyst. *Organometallics* **2011**, *30*, 3880-3887; b) Plank, T. N.; Drake, J. L.; Kim, D. K.; Funk, T.W. Air-Stable, Nitrile-Ligated (Cyclopentadienone)iron Dicarbonyl Compounds as Transfer Reduction and Oxidation Catalysts. *Adv. Synth. Catal.* **2012**, *354*, 597–601; c) Elangovan, S.; Quintero-Duque, S. ; Dorcet, V. ; Roisnel, T. ; Norel, L. ; Darcel, C. ; Sortais, J-B. Knölker-Type Iron Complexes Bearing an N-Heterocyclic Carbene Ligand: Synthesis, Characterization, and Catalytic Dehydration of Primary Amides. *Organometallics* **2015**, *34*, 4521–4528; d) Bütikofer, A.; Chen, P. Zwitterionic Halido Cyclopentadienone Iron Complexes and Their Catalytic Performance in Hydrogenation Reactions. *Inorg. Chem.* **2023**, *62*, 4188–4196.

See ref 4 and : Werley, B. K.; Hou, X.; Bertonazzi, E. P.; Chianese, A.; Funk, T. W. Substituent Effects and Mechanistic Insights on the Catalytic Activities of (Tetraaryl)cyclopentadienone)iron Carbonyl Compounds in Transfer Hydrogenations and Dehydrogenations. *Organometallics*, **2023**, *42*, 3053–3065.

Yagoub, I.; Clémancey, M.; Bayle, P-A.; Quintard, A.; Delattre, G.; Blondin, G.; Kochem, A. Mössbauer spectroscopic and computational investigation of an iron cyclopentadienone complex. *Inorg. Chem.* **2021**, *60*, 11192–11199.

Quintil, G.; Diebold, L.; Fadel, G.; Pécaut, J.; Philouze, C.; Clémancey, M.; Blondin, G.; Jornsson, R.; Quintard, A.; Kochem, A. CO to Isonitrile Substitution in Iron Cyclopentadienone Complexes: A Class of Active Iron Catalysts for Borrowing Hydrogen Strategies. *ACS Catalysis*, **2024**, *14*, 7795-7805.

For the preparation of tris(isonitriles) complexes, see: Bütikofer A.; Chen, P. Cyclopentadienone triisocyanide iron complexes: general synthesis and crystal structures of tris(2,6-dimethylphenyl isocyanide)(η⁴-tetraphenylcyclopentadienone) iron and tris(naphthalen-2-yl isocyanide)(η⁴-tetraphenylcyclopentadienone)iron acetone hemisolvate. *Acta Cryst.* **2023**, E79, 626–632.

For examples of photo-induced, iron cyclopentadienone complexes activation, see: See ref 5, 8) and a) Knölker, H-J.; Goesmann, H.; Klauss, R. A Novel Method for the Demetalation of Tricarbonyliron–Diene Complexes by a Photolytically Induced Ligand Exchange Reaction with Acetonitrile. *Angew. Chem. Int. Ed.* **1999**, *38*, 702-705; b) Emayavaramban, B.; Chakraborty, P.; Dahiya, P.; Sundararaju, B. Iron-Catalyzed α-Methylation of Ketones Using Methanol as the C1 Source under Photoirradiation. *Org. Lett.* **2022**, *24*, 6219-6223; c) Abdallah, M-S.; Joly, N.; Gaillard, S.; Poater, A.; Renaud, J-L. Blue-Light-Induced Iron-Catalyzed α-Alkylation of
Ketones. *Org. Lett.* **2022**, *24*, 5584-5589; d) Waheed, M.; Alsharif, M. A.; Issa Alahmdi, M.; Mukhtar, S.; Parveen, H. Visible Light Promoted Iron-Catalyzed One-Pot Synthesis of 2-Arylimino-2H-Chromenes from 2-Hydroxybenzyl Alcohols and β-Ketothioamides at Room Temperature. *Eur. J. Org. Chem.* **2023**, e202300136; e) Waheed, M.; Alsharif, M. A.; Alahmdi, M. I.; Mukhtar, S.; Parveen, H. Visible Light Promoted Iron-Catalyzed One-Pot Synthesis of 2-Arylimino-2H-Chromenes from 2-Hydroxybenzyl Alcohols and β-Ketothioamides at Room Temperature. *Eur. J. Org. Chem.* **2023**, e202300136. f) Song, D.; Chen, R.; Huang, W.; Yang, P.; Chen, L.; Zhang, J.; Ling, F.; Zhong, W. Room-Temperature Synthesis of α-Alkylated Indolin-2-ones via a Photo Assisted Iron Catalyzed Borrowing Hydrogen Reaction. *Adv. Synth. Catal.* **2024**, *366*, 1893–1898.

11 Lator, A.; Gaillard, S.; Poater, A.; Renaud, J. Iron-Catalyzed Chemoselective Reduction of α,β-Unsaturated Ketones. *Chem.* – *Eur. J. Chem.* **2018**, *24* (22), 5770–5774.

12 Knölker, H.-J.; Heber, J.; Mahler, C. H. Transition Metal-Diene Complexes in Organic Synthesis, Part 14. ¹ Regioselective Iron-Mediated [2+2+1] Cycloadditions of Alkynes and Carbon Monoxide: Synthesis of Substituted Cyclopentadienones. *Synlett* **1992**, *1992* (12), 1002–1004.

13 Knölker, H.-J.; Heber, J. Transition Metal-Diene Complexes in Organic Synthesis, Part 18. ¹ Iron-Mediated [2+2+1] Cycloadditions of Diynes and Carbon Monoxide: Selective Demetalation Reactions. *Synlett* **1993**, *1993* (12), 924–926.

14 Cingolani, A.; Cesari, C.; Zacchini, S.; Zanotti, V.; Cassani, M. C.; Mazzoni, R. Straightforward Synthesis of Iron Cyclopentadienone N-Heterocyclic Carbene Complexes. *Dalton Trans.* **2015**, *44* (44), 19063–19067.

15 Potter, R. G.; Hughes, T. S. Predicting the UV–Vis Spectra of Tetraarylcyclopentadienones: Using DFT Molecular Orbital Energies to Model Electronic Transitions of Organic Materials. *J. Org. Chem.* **2008**, *73* (8), 2995–3004.

16 Mays, M. J.; Morris, M. J.; Raithby, P. R.; Shvo, Y.; Czarkie, D. X-Ray Structure, Reactivity and Catalytic Properties of a (Cyclopentadienone)Ruthenium Dimer, [(C₄Ph₄CO)(CO)₂Ru]₂. *Organometallics* **1989**, *8* (5), 1162–1167.

17 Bütikofer, A and Chen, P. *Organometallics* **2022**, *41*, 16, 2349–2364.