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

Abstract

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, this work aims 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 analogues 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 their L¹-based counterparts. Adding exogenous ligands like PPh₃ mitigated this demetallation, explaining the catalytic activity of 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.

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Introduction

In the quest for the development of eco-friendly hydrogen transfer and borrowing hydrogen synthetic processes using catalysts based on abundant metals, iron(cyclopentadienone) complexes have recently received much attention 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 non-aromatic and aromatic forms, allowing the iron to engage in a bi-electronic Fe(0)/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 electron-withdrawing substituents in para positions of the two appended aromatic rings with minimal changes to their 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 problem by replacing CO ligands with 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 monosubstituted complexes in classical borrowing hydrogen amine alkylation with alcohols and most importantly, in the development of a photo-activated multicatalytic enantioselective allylic alcohol functionalization.¹⁰ We showed that the photoirradiation of the precatalysts leads 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 improved through isocyanide tuning (Fig. 1).

Fig. 1 Iron cyclopentadienone tuning for improved reactivity under chemical and photochemical activation (top). New iron complexes merging both strategies (bottom).

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 has shown the most effective photo-activation process to date. These catalysts also feature either of 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 and dehydrogenation (for the L² ligand).⁶

Results and discussion

Catalyst 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 yields (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 darkness at room temperature. No decomposition was observed after several months of storage under these conditions (Scheme 1).

Scheme 1 Synthetic protocol to access [FeL²(CO)₂(CN-Ph-NO₂)] and [FeL³(CO)₂(CN-Ph-NO₂)].

Single crystals of [FeL²(CO)₂(CN-Ph-NO₂)] were grown by the slow diffusion of pentane into a concentrated solution of the complex in dichloromethane. Its structure and selected bond distances are shown in Fig. 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 1.78 Å 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 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.

Fig. 2 ORTEP drawing of [FeL²(CO)₂(CN-Ph-NO₂)]. 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), and C41–O(41) 1.134(3). Deposited at the Cambridge Crystallographic Data Centre under the number: CCDC 2419951.

Next, the various complexes were studied by IR, an excellent technique to evaluate the impact of the bonding environment via CO and CN stretching frequency investigation. Consistent with the mono-substitution of one CO by the isocyanide, the IR spectrum of [FeL²(CO)₂(CN-Ph-NO₂)] exhibits one vibration band at 2130 cm⁻¹ attributed to the CN stretching frequency, two absorption bands attributed to the symmetric (1997 cm⁻¹) and antisymmetric (1958 cm⁻¹) stretching frequencies of the terminal CO ligands (Fig. S1). As compared to [FeL¹(CO)₂(CN-Ph-NO₂)], the νCO and νCN of the terminal ligands vibrate at similar wavenumbers in [FeL²(CO)₂(CN-Ph-NO₂)] (Fig. S1 and Table 1). In [FeL³(CO)₂(CN-Ph-NO₂)], the νCO (1979 and 1929 cm⁻¹) and νCN (2089 cm⁻¹) of the terminal ligands vibrate at much lower wavelengths (Fig. S1 and Table 1). It suggests that replacing the cyclopentadienone L¹ by L² does not change significantly the terminal Fe–CO and Fe-isocyanide bonding which are reinforced in complexes based on the L³ type ligand. The reinforcement of the Fe–CO bonding had been observed earlier in the parent complex [FeL³(CO)₃] as compared to [FeL¹(CO)₃] and [FeL²(CO)₃] and is probably due to the higher electron-enrichment of the L³ ligand (Fig. S1).

Table 1 Experimental IR frequencies for complexes [FeL¹(CO)₂(CN-Ph-NO₂)], [FeL²(CO)₂(CN-Ph-NO₂)] and [FeL³(CO)₂(CN-Ph-NO₂)]

Iron complexes	ṽ [cm^−1]
	CN	CO (sym.)	CO (asym.)
[FeL^1(CO)2(CN-Ph-NO2)]	2117	2007	1964
[FeL^2(CO)2(CN-Ph-NO2)]	2130	1997	1958
[FeL^3(CO)2(CN-Ph-NO2)]	2089	1979	1929

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Finally, the effect of the cyclopentadienone tuning on the electronic properties 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 L₁ based complexes [FeL¹(CO)₃]⁷ and [FeL¹(CO)₂(CN-Ph-NO₂)],⁸ are displayed in Fig. 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 the L³ 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 2 Mössbauer parameters determined from simulations of the 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 ±0.02 mm s^–1, ±0.05 mm s⁻¹ and ±0.02 mm s⁻¹, on δ, ΔE_Q and Γ_fwhm, respectively

Iron complex	δ (mm s^−1)	ΔEQ (mm s^−1)	Γ (mm s^−1)
[FeL^1(CO)3]	0.06	1.46	0.29
[FeL^1(CO)2(CN-Ph-NO2)]	0.05	1.61	0.30/0.31
[FeL^2(CO)3]	0.07	1.56	0.30
[FeL^2(CO)2(CN-Ph-NO2)]	0.07	1.61	0.31
[FeL^3(CO)3]	0.11	1.57	0.36
[FeL^3(CO)2(CN-Ph-NO2)]	0.09	1.62	0.40

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Catalyst 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 with a yield of 32–47% 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

Table 3 Activity of selected iron complexes in amine alkylation under chemical activation

Entry	Iron complex	^1H NMR yield^a	Isolated yield
a Determined by ^1H NMR using dibromomethane as the internal standard.
1	[FeL^1(CO)3]	99%	nd
2	[FeL^1(CO)2(CN-Ph-NO2)]	39%	nd
3	[FeL^2(CO)3]	99%	70
4	[FeL^2(CO)2(CN-Ph-NO2)]	47%	29
5	[FeL^3(CO)3]	99%	77
6	[FeL^3(CO)2(CN-Ph-NO2)]	32%	nd

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Next, we screened the new isonitrile-substituted complexes in the condensation of dibenzoylmethane to allyl alcohol using an organocatalyst (Org) under 368 nm LED photoactivation as shown 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 conversions 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 conversion yield of 95%, 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.

Table 4 Activity of iron complexes in the photo-induced functionalization of allyl alcohol

Entry	Iron complex	Conversion^a
Reactions performed using 0.2 mmol of diketone and 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 for 86 hours.
1	[FeL^1(CO)3]	9%
2	[FeL^1(CO)2(CN-Ph-NO2)]	95%^b
3	[FeL^2(CO)3]	0%
4	[FeL^2(CO)2(CN-Ph-NO2)]	26%
5	[FeL^3(CO)3]	13%
6	[FeL^3(CO)2(CN-Ph-NO2)]	12%
7^c	[FeL^3(CO)2(CN-Ph-NO2)]	12%

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The introduction of electron-rich ligands such as L² and L³ has proved 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 catalyst 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 the absence of exogenous ligands

Initially, the absorption properties of the complexes and ligands were investigated (Fig. 3 and Table 5). The free L¹ ligand (brown line, Fig. 3) is characterized by a low absorption band at 417 nm (738 M⁻¹ cm⁻¹). The free L² ligand (blue line, Fig. 3) is characterized by an intense absorption band at 375 nm (11 240 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 (23 395 M⁻¹ cm⁻¹) and a low absorption at 509 nm (520 M⁻¹ cm⁻¹).

Fig. 3 UV-visible spectra of toluene solutions of ligands L¹ (brown line), L² (blue line), and L³ (grey line) in 2.2 × 10⁻⁴ M in toluene (top figure), tris-carbonyl complexes [FeL¹(CO)₃] (brown line), [FeL²(CO)₃] (blue line), and [FeL³(CO)₃] (grey line) at 1 × 10⁻⁴ M in toluene (middle figure) and substituted complexes [FeL¹(CO)₂(CN-Ph-NO₂)] (brown line), [FeL²(CO)₂(CN-Ph-NO₂)] (blue line), [FeL³(CO)₂(CN-Ph-NO₂)] (grey line) at 2 × 10⁻⁴ M in toluene. T = 298 K. 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 different electron densities where red corresponds to acceptor density and yellow to donor density.

Table 5 Electronic spectral data of the complexes and ligands^a

Complex/Ligand	λmax, nm (ε, M^−1 cm^−1)
a In toluene solution at 298 K. Abbreviations used: sh, shoulder; br, broad.
L^1	417 (738)
[FeL^1(CO)3]	308 (4491)
[FeL^1(CO)2(CN-Ph-NO2)]	351 (14 229), 308 sh (9466)
L^2	375 (11 240), 516 br (941)
[FeL^2(CO)3]	279 br (24 887)
[FeL^2(CO)2(CN-Ph-NO2)]	352 (12 345), 281 (45 032)
L^3	314 (23 395), 509 (520)
[FeL^3(CO)3]	284 (22 726), 350, sh, br (4121)
[FeL^3(CO)2(CN-Ph-NO2)]	284 (20 941), 379 (11 339)

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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 excitation 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 λ¹ and λ² due to a destabilization of the π donor orbitals located at the cyclopentadienone with a π* acceptor orbital which does not vary in energy. Inspired by 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 Fig. 3 (see Fig. S11 for full UV-visible spectra of the ligands).

The weak absorption band experimentally observed above 400 nm for the three ligands were computed at λ_calc = 398 nm (f = 0.03), λ_calc = 520 nm (f = 0.03) and λ_calc = 466 nm (f = 0.03) for L¹, L² and L³, respectively. In all three cases, the acceptor fragment was a π orbital delocalized on the C(2), C(3), C(4) and C(5) carbon atoms and the donor fragment was a π orbital delocalized on the C(1), O(1), C(3), C(4) atoms of the cyclopentadienone ring. The nature of this transition was similar in all ligands and its position could be controlled through cyclopentadienone substitution. Indeed these results show that the transition is shifted to higher wavelengths 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 the destabilization of π-donor orbitals located at the cyclopentadienone with a π*-acceptor orbital which does not vary in energy. For L², the intense band at 375 nm was computed at λ_calc = 342 nm (f = 0.38). The acceptor and donor fragments were 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 were computed at λ_calc = 261 nm (f = 0.44) and 275 nm (f = 0.33). In both cases, the donor fragment was a π orbital delocalized on the cyclopentadienone and fused rings while the π acceptor orbital was delocalized either on the C(2) and C(5) substituted phenyl rings or on the cyclopentadienone core. Altogether, these results show that irrespective 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 wavelengths as compared to L¹.

Next, the effect of the cyclopentadienone ring substitution on the absorption properties of the tris-carbonyl complexes was investigated. [FeL¹(CO)₃] is characterized by a stronger absorption band at 308 nm (4491 M⁻¹ 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 [FeL²(CO)₃] (blue line, Fig. 3), being characterized by a broad absorption band at a maximum located at 279 nm (24 887 M⁻¹ cm⁻¹). In the case of [FeL³(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 M⁻¹ cm⁻¹) while the strong absorption at a lower wavelength is 284 nm (22 726 M⁻¹ 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 Fig. 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 [FeL²(CO)₃] and [FeL³(CO)₃] exhibit calculated transitions with higher oscillator strengths, which explains their stronger absorption around 280 nm compared to [FeL¹(CO)₃] (see Fig. S12 and S13). The occurrence of these transitions at higher wavelengths in [FeL²(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 [FeL¹(CO)₃] to [FeL²(CO)₃] and [FeL³(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 the 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 (14 229 M⁻¹ cm⁻¹) attributed to Fe 3d to CN π* orbital transitions with significant contribution from the phenyl-NO₂ part of the ligand (brown line, Fig. 3). More importantly, light-irradiation at a wavelength corresponding to the maximum absorption of this transition led to CO dissociation which allowed establishing a correlation between the catalytic activity of this complex under irradiation and its peculiar absorption properties. The same spectroscopic behavior is observed for [FeL²(CO)₂(CN-Ph-NO₂)] (blue line, Fig. 3) and [FeL³(CO)₂(CN-Ph-NO₂)] (grey line, Fig. 3) with the appearance of strong absorption bands located 352 nm (12 345 M⁻¹ cm⁻¹) and 379 (11 339 M⁻¹ cm⁻¹) as compared to [FeL²(CO)₃] and [FeL²(CO)₃] respectively. The TD-DFT calculations performed with the same methodology for the three complexes reveal 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 [FeL¹(CO)₂(CN-Ph-NO₂)], 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 [FeL²(CO)₂(CN-Ph-NO₂)] and [FeL³(CO)₂(CN-Ph-NO₂)], 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 [FeL²(CO)₂(CN-Ph-NO₂)] or from the fused ring in the case of [FeL³(CO)₂(CN-Ph-NO₂)].

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 the complex [FeL¹(CO)₂(CN-Ph-NO₂)]. In such a 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.

Firstly, we examined the behavior of [FeL²(CO)₂(CN-Ph-NO₂)] under continuous irradiation in toluene at room temperature (Fig. 4a). 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 times did not lead to further evolution of the UV-visible spectra and stopping the irradiation did not give back the initial spectrum indicating the fast and irreversible formation of a stable species whose absorption properties look like those of the free ligand L². Surprised by these results, we examined the irradiation-dependent absorption changes of [FeL²(CO)₃] under the same conditions (Fig. 4b). 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 (Fig. 4e and f). Similar studies were conducted on [FeL³(CO)₃] and [FeL³(CO)₂(CN-Ph-NO₂)], and the results exhibited a similar trend (demetallation) (Fig. S9).

Fig. 4 Evolution of the UV-visible spectra of a solution of (a) [FeL²(CO)₂(CN-Ph-NO₂)] (2.2 × 10⁻⁵ M), (b) [FeL²(CO)₃] (2.2 × 10⁻⁵ M), (c) [FeL¹(CO)₂(CN-Ph-NO₂)] (7.3 × 10⁻⁵ M) and (d) [FeL¹(CO)₃] (7.3 × 10⁻⁵ M) in toluene at room temperature under an argon atmosphere and irradiation (368 nm ± 5 nm, 30 W). 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 ¹H NMR studies over light irradiation in CD₂Cl₂ solvent (e) [FeL²(CO)₂(CN-Ph-NO₂)], (f) [FeL²(CO)₃] and in CDCl₃ solvent (g) [FeL¹(CO)₂(CN-Ph-NO₂)] and (h) [FeL¹(CO)₃].

However, [FeL¹(CO)₂(CN-Ph-NO₂)] and [FeL¹(CO)₃] exhibit different behaviors under photoactivation (Fig. 4c and d). On the 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 remained in a large proportion. On the other hand, [FeL¹(CO)₃] was the major species after 40 minutes of irradiation and a new and unidentified species had been formed. This new species clearly differed 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 show that in the absence of a 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 as ΔG = −8 kcal mol⁻¹ while ΔG of dimerization is −5 kcal mol⁻¹ for [FeL¹(CO)₃] (Fig. 5b). Previous works have proposed such a dimer as the resting state.^4g Alternatively, a dimer where the cyclopentadienone carbonyl oxygens coordinate to the empty sites on Fe was considered (Fig. 5c). That type of compound has been isolated for the Ru analogues of these compounds.¹⁶ DFT calculations show that the formation of such a dimer species is even more likely. For both complexes ([FeL¹(CO)₂(CN-Ph-NO₂)] and [FeL¹(CO)₃]), such dimer formation is favored as ΔG = −12 kcal mol⁻¹.

Fig. 5 Putative species formed under irradiation of [FeL¹(CO)₃].

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 to either catalyst deactivation via reversion to a tris-carbonyl complex or, more critically, 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 the SI)—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 persistent) be explained? We hypothesize that the presence of an exogenous ligand (such as a 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 an exogenous ligand

A previous experiment, carried out under 368 nm irradiation, with [FeL¹(CO)₂(CN-Ph-NO₂)] in the presence of a stabilizing exogenous PPh₃ ligand, led 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 (Fig. 6) and NMR.⁸ The analysis of the structure revealed that one terminal CO had been substituted by one PPh₃ ligand. Calculations further confirmed that the substitution of one CO ligand by PPh₃ was ∼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].

Fig. 6 Irradiation of [FeL¹(CO)₂(CN-Ph-NO₂)] in the presence of the exogenous PPh₃ ligand.

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 (Fig. 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 does not lead to its photodissociation or only in small proportions.

Fig. 7 Evolution of the 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 an argon atmosphere and irradiation (368 nm ± 5 nm, 30 W). 45 scans; 1 scan every 4 seconds. Green and blue spectra correspond to the initial complex without irradiation and the final spectrum after 45 scans, respectively.

In order to identify the type of species formed we performed ¹H and ³¹P NMR experiments under the same irradiation conditions. The analysis of the ¹H NMR data (Fig. S5a) showed the complete disappearance of the signals of the initial [FeL²(CO)₂(CN-Ph-NO₂)] complex and the appearance of a new set of signals after irradiation differing from those of the free ligand L² while ³¹P NMR data (Fig. S5b) 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 the 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 SI page S9, an isolated yield of 64%) and a 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 the SI for details). Under the multicatalytic reaction, the alcohol or the diketone would not stabilize the catalyst enough to prevent its decomposition over time.

Conclusions

In conclusion, a new family of 4-nitrophenyl isonitrile-substituted iron cyclopentadienone complexes was successfully 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 superior catalytic efficiency for hydrogenation and/or dehydrogenation reactions under chemical activation. Surprisingly, the newly designed complexes exhibited significantly lower reactivity 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. The extinction coefficients of the different complexes at the irradiation wavelengths vary significantly, as shown by both our experimental UV-vis spectra and computed oscillator strengths. This variation may imply different steady-state concentrations of the photoactivated species for each complex, and differences in the quantum yield—although not measured here—could further contribute to the reactivity. While we cannot at present determine the absolute concentrations of the active species under our reaction conditions, such variations could influence the observed catalytic performances. Nevertheless, our results demonstrate that, regardless of the initial concentration of the active species, the primary cause of catalyst deactivation is ligand-dependent demetallation. This mechanistic pathway remains the central finding of this study, with implications for the design of more robust photoactivated base-metal catalysts.

Indeed, 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 catalytic conditions. In contrast, the resistance of complexes bearing 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 chemical activation, they reduced the efficiency of the corresponding complexes under photoactivation by making them more prone to demetallation. Further design of catalysts by introducing various electron-enriched cyclopentadienone ligands compatible with the photoactivation is currently underway 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.

Conflicts of interest

The authors declare no competing financial interests.

Data availability

The data supporting this article have been included as part of the SI: experimental protocols, detailed analysis, and additional experiments. See DOI: https://doi.org/10.1039/d5dt01184a.

CCDC 2419951 contains the supplementary crystallographic data for this paper.¹⁸

Acknowledgements

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) and the Labex ARCANE and CBH-EUR-GS (ANR-17-EURE-0003) are warmly acknowledged for financial support. All technical support staff from LCBM, DCM and ICMG are acknowledged for their help. Anestis Alexandridis is acknowledged for the UV reactor set-up.

References

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