# Unraveling Photo-Driven H2 Production with a Bio-Inspired NiFe Complex: Revealing an Unexpected Hydrogen Atom Source

Irene Suarez-Antuna, Noémie Lalaoui, Emilien Chaigne-Tarlotin, Florian

Molton, Frédérique Loiseau, Carole Duboc

► To cite this version:

Irene Suarez-Antuna, Noémie Lalaoui, Emilien Chaigne-Tarlotin, Florian Molton, Frédérique Loiseau, et al. Unraveling Photo-Driven H2 Production with a Bio-Inspired NiFe Complex: Revealing an Unexpected Hydrogen Atom Source. Journal of the American Chemical Society, 2025, 147 (25), pp.21451-21458. 10.1021/jacs.5c01329. hal-05295965

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# Unraveling Photo-Driven H₂ Production with a Bio-Inspired NiFe Complex: Revealing an Unexpected Hydrogen Atom Source.

Irene Suarez-Antuna, Noémie Lalaoui,* Emilien Chaigne-Tarlotin, Florian Molton, Frédérique Loiseau, and Carole Duboc*

Department of Molecular Chemistry (DCM), UMR 5250, Université Grenoble Alpes (UGA), 38000 Grenoble, France
*Bio-inspired chemistry • NiFe hydrogenase • Photocatalysis • H₂ production • BIH role

**ABSTRACT:** Developing efficient photocatalytic systems for sustainable H₂ production is a crucial step toward reducing dependence on fossil fuels. While extensive research over the past two decades has focused on bio-inspired FeFe catalysts, NiFe-based systems remain largely unexplored. This study addresses that gap by introducing the first heterodinuclear NiFe complex capable of catalyzing H₂ production under photo-assisted conditions. The catalyst demonstrates remarkable performance, achieving a turnover frequency (TOF) of 2750 h⁻¹ and a turnover number (TON) of up to 41200. A comprehensive mechanistic investigation, supported by EPR and labeling experiments, reveals that the photo-driven catalytic pathway differs from the electrocatalytic mechanism. Under photo-assisted conditions, BIH not only serves as a sacrificial electron donor, but its oxidized form BIH⁺, also acts as a hydrogen atom donor, directly contributing to the high activity of this system. These findings highlight the dual role of BIH in photo-assisted catalysis, extending beyond its specific application in H₂ production to other reactions of interest, such as CO₂ reduction.

## Introduction

The design of photocatalytic processes for H₂ production has emerged as a promising solution to the global demand for sustainable energy.¹⁻² This approach goes beyond traditional fossil fuel-based energy production by utilizing sunlight as a renewable energy source to drive the hydrogen evolution reaction (HER) and generate H₂. As an environmentally friendly energy carrier, H₂ emits only water when used, significantly reducing environmental impact.³ In addition,

this approach converts solar energy directly into chemical energy, providing an efficient means of energy storage.⁴ The bio-inspired approach has attracted significant interest in the search for molecular HER catalysts because this strategy focuses on catalysts composed of abundant, environmentally friendly, and inexpensive 3d metal ions.⁵⁻⁷ Inspiration is drawn from hydrogenases, metalloenzymes known for their exceptional activity in catalyzing the reversible reduction of protons into H₂.⁸⁻⁹ The [NiFe] and [FeFe]

<figure><img src="image_2.png" /><figcaption>[NiFe] Models</figcaption></figure>

<figure><img src="image_3.png" /></figure>

**Scheme 1.** Schematic representation of the active site of the [NiFe] hydrogenase (PDB: 5XLE), the trinuclear NiFe₂ complex²⁰,²¹ and the dinuclear NiRu precatalyst²² with their respective performance for photocatalytic H₂ production, as well as the NiFe complex described in this work.

hydrogenases possess a dinuclear {MFeS₂} core (M = Ni or Fe, respectively) in their active sites, with CO, CN⁻, and cysteine residues as additional ligands to complete the first coordination sphere (see Scheme 1 for the three-dimensional structure of the [NiFe] hydrogenase and its active site).

Over the past two decades, significant effort has been dedicated to engineering catalysts for photo-assisted HER processes that mimic key features of the active site of FeFe hydrogenase.⁹⁻¹³ While notable progress has been achieved with FeFe-based HER photocatalytic systems, those
inspired by the NiFe hydrogenase remain scarce, likely due to the synthetic challenges raised by its heterodinuclear active site (see scheme 1).¹⁴⁻¹⁸ Heterodinuclearity, however, is recognized as essential in catalysis, with synergistic cooperation between two distinct metals enhancing performance, particularly for small molecule activation.¹⁹ To date, only two systems have been described, and no dinuclear NiFe photocatalytic systems are known: a trinuclear NiFe₂ complex²⁰⁻²¹ and a dinuclear NiRu precatalyst that becomes tetranuclear during the catalytic cycle.²²

A recent alternative strategy has been to design artificial metalloenzymes²³⁻²⁴ or de novo metalloproteins²⁵⁻²⁶ containing mononuclear tetrathiolate-based Ni complexes or peptide scaffolds for arranging dinuclear NiNi complexes.²⁷ These HER catalysts have provided insights into the formation of Ni¹ and Ni¹¹ species under photochemical conditions, akin to the generation of the first reduced species, Ni¹Fe¹¹ (NiL), in the NiFe hydrogenase.²⁸

A key aspect of photo-driven reduction reactions is the use of a sacrificial electron donor (SED), with BIH (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]-imidazole) commonly utilized in HER and CO₂ reduction (CO₂R) catalysis in organic solvents.²⁹⁻³⁰ While the reactivity of BIH and its derivatives, BIH⁺ and BI⁻, is well documented,³¹ their roles in photo-assisted catalytic processes are mainly described as a SED (where BIH, along with the photo-activated photosensitizer, PS*, undergoes reductive quenching to generate BIH⁺, Eq.1) or as a reducing agent (BI⁺, formed via the deprotonation of BIH⁺ in the presence of a base, Eq.2).³² Though BIH⁺ has been shown to act as a hydrogen atom donor, directly forming BI⁺ (Eq.3),³² to the best of our knowledge, this pathway has not been demonstrated in any proposed HER or CO₂R mechanisms.³³

$$PS^* + BIH \rightarrow PS^- + BIH^+ \qquad \text{Eq.1}$$

$$BIH^+ + B^- \rightarrow BI^- + BH, \text{ with } B^- \text{ a base} \qquad \text{Eq.2}$$

$$BIH^+ \rightarrow BI^+ + H^+ \qquad \text{Eq.3}$$

In this context, we investigate the ability of the previously described bio-inspired NiFe complex, denoted NiFeCp for [NiFeL<sup>N2S2</sup>CpCO]⁺ (where (<sup>N2S2</sup>)<sup>-</sup> = 2,2'-bipyridine-6,6'-diyl(bis(1,1-diphenylethanethiolate)) and Cp = cyclopentadienyl; see Scheme 1) to act as a photo-assisted HER catalyst. This NiFeCp complex not only shares structural similarities with the active site of NiFe hydrogenase but also exhibits robust HER electrocatalytic activity in both MeCN (with a second-order rate constant of 2.5 × 10⁴ M⁻¹ s⁻¹ and a turnover frequency, TOF, of 250 s⁻¹ at 10 mM [H⁺]<sup>34</sup> and aqueous solutions (TOF of 200 s⁻¹ at pH 3, and 7.2 × 10⁶ turnover numbers, TON, in 10 hours).³⁵ After defining optimized photo-assisted catalytic conditions to achieve high HER performance, a detailed mechanistic investigation was conducted using EPR experiments to identify intermediate species, complemented by labelling experiments. Overall, we demonstrated that under photo-assisted conditions, the mechanism differs from the electrocatalytic pathway, with BIH acting not only as a sacrificial electron donor but also its oxidized form BIH⁺ as a hydrogen atom donor.

## Results and Discussion

**Redox and electrocatalytic HER properties of NiFeCp in DMF.** Experiments were conducted in DMF to address solubility issues of the different chemical components in the photo-assisted process. As a preliminary step, the redox and electrocatalytic HER properties of the NiFeCp complex were investigated in this solvent. The cyclic voltammogram (CV) recorded in DMF exhibits two reversible, diffusion-controlled one-electron reduction waves at E₁/₂ = -1.29 and -1.90 V vs Fc⁺/Fc (Figure S2B). Based on prior studies in MeCN³⁴ and the identical redox behavior observed here, the first reduction wave is attributed to the reduction of Ni¹¹Fe¹¹Cp to Ni¹Fe¹¹Cp, while the second wave is ascribed to the reduction of Ni¹Fe¹¹Cp to L·Ni¹Fe¹¹Cp. Upon the addition of Et₃NHBF₄, a HER catalytic process develops at Ecat/₂ = -1.90 V vs. Fc⁺/Fc, indicating that H₂ production proceeds via the two-electron reduction of NiFeCp, as previously observed in MeCN (Figure S3). The catalytic cycle, previously elucidated through experimental data³⁴ and DFT calculations,³⁶ follows an E[ECEC] mechanism in MeCN (Figure S4). Entry into the catalytic cycle corresponds to the one-electron reduced species Ni¹Fe¹¹Cp, which is then reduced and protonated to generate a hydride species that is subsequently reduced and protonated to produce H₂.

**The light-induced catalytic activity of NiFeCp.** The photoproduction of H₂ with the NiFeCp catalyst was first evaluated using Ir(ppy)₃ (ppy = 2-phenylpyridyl) as the photosensitizer (PS). The reduced [Ir(ppy)₃] presents a suitable redox potential (Ir⁰⁻¹, E₁/₂ = -2.57 V vs. Fc⁺/Fc)³⁷ to

<figure><img src="image_4.png" /><figcaption>**Figure 1.** H₂ produced in µM after 1h of irradiation in the absence of light, Et₃NHBF₄, BIH, Ir(ppy)₃ and NiFeCp; with [NiL<sup>N2S2</sup>] or/and FeCp as the catalyst, and in the absence and presence of Hg. Conditions Ir(ppy)₃ (250 µM), BIH (0.1 M), Et₃NHBF₄ (0.1 M), NiFeCp (1 µM). *H₂ produced with only the PS, BIH, and H⁺ under similar conditions is subtracted.</figcaption></figure>
generate the active L·Ni¹Fe¹¹Cp species. This approach also
enables a direct comparison of the reactivity of NiFeCp with
the previously reported NiRu complex, which was studied
under irradiation with Ir(ppy)₃ as PS, BIH as SED, and
Et₃NHBF₄ as the proton source.²²

When NiFeCp (10 µM) is added to a DMF solution contain-
ing Ir(ppy)₃ (1 mM), BIH (0.1 M), and Et₃NHBF₄ (0.1 M) and
subjected to irradiation, the system demonstrates HER abil-
ity, with the amount of H₂ produced quantified by gas chro-
matography (Figure S5). Under these conditions, the pro-
duction of H₂ was estimated to correspond to 1800 TON
within 4 hours. Notably, the absence of BIH, Ir(ppy)₃,
Et₃NHBF₄, or light results in an undetectable amount of H₂.
When NiFeCp is removed, a certain amount of H₂ is quanti-
fied and subtracted in all other experiments. This H₂ evolu-
tion is attributed to the hydride-donating capability of BIH,
which can engage in a heterolytic reaction with the proton
source, Et₃NHBF₄, to yield H₂. Ref. J. Am. Chem. Soc. 2024,
146, 39, 26830–26843. These control experiments highlight
the critical importance of each component in the system.
Furthermore, when Hg is added to the solution, the perfor-
mance of the system remains unaffected (Figures 1 & S6),
indicating the absence of nanoparticle formation with cata-
lytic HER activities.

To rule out the possibility of H₂ production catalyzed by a
complex originating from the degradation of NiFeCp, it was
replaced by the mononuclear [NiL²S²] complex, the mono-
nuclear [FeCpCO(MeCN)₂] complex (FeCp), and a mixture of
both of them (knowing that the heterodinuclear NiFeCp is
not formed under these conditions). In all these three cases,
no HER activity is observed, reinforcing that the heterodi-
nuclear nature of NiFeCp is crucial for efficient H₂ produc-
tion (Figure 1). The impact of the concentration of each
component was systematically assessed to define the opti-
mal photocatalytic conditions (Figure S7). The concentra-
tions of the photosensitizer and the sacrificial electron do-
nor showed negligible effects on H₂ production, while vary-
ing proton concentrations significantly influenced the pho-
tocatalytic HER performance of the system, with an optimal
concentration of 0.1 M. At 0.01 M, only trace amounts of H₂
can be detected, while at 0.5 M, H₂ production begins to de-
crease (30% less). Such a trend is commonly observed, par-
ticularly in the case of FeFe hydrogenase mimics.³⁸ Besides,
NiFeCp shows instability at high acid concentrations (Fig-
ure S19), which likely accounts for the observed decrease in
HER catalytic activity under these conditions. A current
trend in recent literature has focused on reducing catalyst
concentrations in photocatalytic reactions to micromolar or
nanomolar concentrations to gain insight into how a cata-
lyst performs in environments where catalytic sites are well
isolated.²⁵,³⁹⁻⁴⁰ Decreasing the concentration of NiFeCp from
10 to 1 µM under similar conditions (Figures S7D and S8)
impacts the catalytic activity. An enhancement is observed
at lower concentrations, with a threefold increase in TON
when the concentration is divided by ten. This catalytic be-
havior is commonly observed in homogeneous photo-as-
sisted catalytic HER processes. Using the optimized condi-
tions identified in this study (1 µM NiFeCp, 250 µM Ir(ppy)₃,
0.1 M BIH, and 0.1 M acid) 2750 TON is achieved in 1 hour,
corresponding to a TOF value of 2750 h⁻¹ (approximately
0.76 s⁻¹).

In the optimized system, H₂ evolution stopped after 7

<figure><img src="image_5.png" /></figure>

**Figure 2.** Time-dependent H₂-evolution and TON, under the following conditions: NiFeCp (1 µM), Ir(ppy)₃ (250 µM), BIH (0.1 M) and Et₃NHBF₄ (0.1 M) in degassed dry DMF. *H₂* produced with only PS, BIH and H⁺ under similar conditions is sub- hours of irradiation, corresponding to 9850 TON (Figure 2). To understand the cause of this deactivation, we conducted experiments in which one or more components were added after the reaction ceased. Through various trials, we found that reintroducing the acid into the solution after 7 hours allowed the system to maintain its performance. It is im- portant to note that adding NiFeCp after 7 hours does not lead to H₂ production (Figure S9), further emphasizing that the shutdown of the system is not due to catalyst degrada- tion. After 16 hours, both the acid and the SED needed to be reintroduced for the system to restart. This indicates that NiFeCp can achieve 35500 TON in 21 hours without show- ing any signs of degradation. Initial investigations into the stability of NiFeCp in the presence of water show promising results. Specifically, with a water concentration of 10% in DMF, over 70% of H₂ production is retained (Figure S10). The observed decrease in HER activity is likely due, in part, to the limited stability of BIH⁺ in water, which impacts the formation of the key hydride intermediate.

**Mechanistic investigation.** *Photophysical properties.*
Time-resolved photoluminescence experiments were con-
ducted to investigate the electron transfer processes be-
tween Ir(ppy)₃, BIH, and NiFeCp. The photoluminescence
spectrum of a DMF solution of Ir(ppy)₃, recorded upon exci-
tation at 410 nm, displays a large band associated with
³MLCT deactivation at 510 nm, with a lifetime of 1430 ns, in
agreement with the data reported in the literature.²², ⁴¹ To
decipher whether the excited [Ir(ppy)₃]* or reduced
[Ir(ppy)₃]⁻ species directly reduced NiFeCp, luminescence
quenching experiments were performed in the presence of
either NiFeCp or BIH (Figures S12 and S13, respectively).
We found the electron transfer rate constants from
[Ir(ppy)₃]* to NiFeCp to be *kq*(*NiFeCp*) = 5.0x10⁹ M⁻¹S⁻¹, and the
quenching constant of [Ir(ppy)₃]* by BIH to be kq(BIH) = 1.03x10⁷ M⁻¹s⁻¹. Under the photo-assisted catalytic conditions ([NiFeCp] = 1-10 µM and [BIH] = 0.1 M), reductive quenching is expected to occur, with a rate constant of kq(BIH)x[BIH] = 1.03x10⁶ s⁻¹, compared to oxidative quenching, which has a rate constant of kq(NiFeCp)x[NiFeCp] = 5.0x10³-5.0x10⁴ s⁻¹.

Impact of the photosensitizer and the sacrificial electron
donor. Ir(ppy)₃ was selected as PS based on the Ir⁰/⁻¹ redox
potential (E₁/₂ = -2.57 V vs. Fc⁺/Fc), allowing the key inter-
mediate L·Ni'Fe''Cp to be generated. To investigate whether
this species is also involved under these photo-assisted cat-
alytic conditions, the less reducing [Ru(bpy)₃]²⁺ complex
(Ru²⁺/⁺, E₁/₂ = -1.71 V vs. Fc⁺/Fc)³第七 was evaluated as an al-
ternative PS. Unexpectedly, the performance of the system

improves when [Ru(bpy)₃]²⁺ replaces Ir(ppy)₃, achieving
4000 TON compared to 2750 TON after 1 hour of irradiation
(Figure S14). These results suggest that the system may op-
erate via a mechanism distinct from that which occurs un-
der electrocatalytic conditions (see below). The stability of
the photo-assisted catalytic system with [Ru(bpy)₃]²⁺ was
evaluated. A plateau is reached in a shorter time than with
Ir(ppy)₃, specifically after 3 hours. During this period, the
color of the solution changes from orange to pale yellow, in-
dicating the degradation of [Ru(bpy)₃]²⁺, which aligns with
previously reported observations.⁴² To investigate the
cause of the deactivation process, three experiments were
conducted with the re-addition of either Et₃NHBF₄, NiFeCp,
or [Ru(bpy)₃]²⁺ to the catalytic system after 4 hours of irra-
diation. The catalytic activity for H₂ evolution was restored
only upon

<figure><img src="image_6.png" /><figcaption>**Scheme 2.** Proposed mechanism for the photo-assisted HER process catalyzed by NiFeCp.</figcaption></figure>

the addition of [Ru(bpy)₃]²⁺ to the deactivated system (Fig-
ure S14), confirming that the interruption of H₂ evolution
was due to the deactivation of the PS. This behavior con-
trasts with the Ir(ppy)₃ system, where the limiting factor
was acid depletion (see Figure 2).

To evaluate the role of the SED, BIH was replaced with
BNAH (1-Methyl-1H-imidazole-4-carboxylic acid), known
to deliver only one electron and one proton.⁴³ In a system

containing 1 µM NiFeCp, 250 µM Ir(ppy)₃, 0.1 M acid, and
0.1 M BNAH, only a negligible amount of H₂ was produced
after 1 hour of irradiation, suggesting that BIH acts as more
than just an electron donor in this system. The absence of
any photocatalytic H₂ production when N,N,N',N''-tetra-
ethylethylenediamine (TEOA) and triethylamine (TEA) are
used instead of BIH further supports this conclusion (see
experimental details in the SI).
*Generation of Ni¹Fe¹¹Cp under irradiation.* EPR spectroscopy is a powerful tool for detecting paramagnetic intermediates and provides valuable mechanistic insight under photocatalytic conditions. In particular, operando EPR experiments can be used to probe transient species in photocatalytic systems as previously shown with a bio-inspired FeFe dyad. (REF) The generation of the one-electron reduced species Ni¹Fe¹¹Cp under irradiation in the presence of [Ru(bpy)₃]²⁺ or Ir(ppy)₃ and BIH or BNAH was evidenced by EPR spectroscopy. As an example, after 5 seconds of irradiation of a DMF solution containing Ni¹¹Fe¹¹Cp (100 µM), Ir(ppy)₃ (250 µM), and BNAH (0.1 M), the characteristic EPR signal of this *S* = ½ species is observed (Figure S15) with a well-resolved axial signal at g|| = 2.168 and g⊥ = 2.060.³⁴ We also noted that Ni¹Fe¹¹Cp is more stable when BNAH is employed as a SED, indicating that Ni¹Fe¹¹Cp does not react with the produced BNAH⁺·.

*Role of BIH in the catalytic cycle.* Since HER occurs with [Ru(bpy)₃]²⁺, and H₂ is not produced when BNAH is used, a mechanism different from the electrocatalytic catalytic cycle is followed (Figure S4). We thus envisaged that BIH⁺·, generated from the reductive quenching of the photosensitizer, can provide the equivalent of a hydrogen radical, “H·” (Eq.3). Based on this hypothesis, the mechanism described

in Scheme 2 can be proposed. In the presence of the reduced species Ni¹Fe¹¹Cp, formed via the reduction of Ni¹¹Fe¹¹Cp by the PS⁻, the hydride species Ni¹¹(H)Fe¹¹Cp can thus be directly generated. This hydride species can then undergo further reduction through the photo-assisted production of PS⁻, resulting in the formation of a second hydride species, L·Ni¹¹(H)Fe¹¹Cp. In the presence of H⁺, this intermediate generates H₂ and Ni¹Fe¹¹Cp. As in the case of the electrocatalytic mechanism, the second protonation likely occurs on one of the thiolates before the formation of H₂ (see Figure S4). Since the reduction of Ni¹¹(H)Fe¹¹Cp to L·Ni¹¹(H)Fe¹¹Cp is predicted by DFT calculations to occur at *E*calc= - 1.43 V vs. Fc⁺/Fc, it can be reduced by either [Ir(ppy)₃]⁻ or [Ru(bpy)₃]⁺.³⁴,³⁶ This hypothesis was tested by monitoring the reaction in the absence of protons using EPR. First, we investigated the reaction of Ni¹¹Fe¹¹Cp with [Ru(bpy)₃]²⁺ and BIH (Figures 3, S16-17, and Table S3). After a few seconds of irradiation, the EPR spectrum reveals the signature of Ni¹Fe¹¹Cp, corresponding to 50% of the total amount of the starting complex. After one minute, the main product becomes a radical attributed to BI·. The generation of BI· is expected to occur in the absence of protons and arises from the known evolution of unreacted product BIH⁺· (Eq.2). When the

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 3.** A) Experimental and simulated EPR spectra recorded on a DMF solution of NiFeCp (100µM), [Ru(bpy)₃]²⁺ (250µM) and BIH (0.1M) after 1 s of irradiation, and B) Distribution ratios of the different species present in solution after various times of irradiation, estimated from the deconvolution of the experimental data with simulated spectra (used parameters in Table S3). A small amount (less than 2%) of an unknown radical-based species is also observed (named radical X).</figcaption></figure>

same reaction is monitored by ¹H NMR, Ni¹¹(H)Fe¹¹Cp is not observed as it is reduced into L·Ni¹¹(H)Fe¹¹Cp, which rapidly decomposes in the absence of protons. The same monitoring was conducted using Ir(ppy)₃ as the photosensitizer (Figures 4 and Table S4), generating different or similar products but with distinct amounts, thus highlighting other competitive pathways. The two main products observed within one minute are Ni¹Fe¹¹Cp and the previously characterized reduced mononuclear [NiL²S²]⁻ species.⁴⁴ The

presence of [NiL²S²]⁻ indicates that the heterodinuclear Ni-FeCp complex undergoes decomposition under these conditions. The stronger reducing ability of [Ir(ppy)₃]⁻ compared to [Ru(bpy)₃]⁺, allows [Ir(ppy)₃]⁻ to reduce [NiL²S²] (E₁/₂ = -1.82 V vs. Fc⁺/Fc), while [Ru(bpy)₃]⁺ cannot.⁴⁵ A small proportion of BIH⁺· and BI· is also detected. Note also that no paramagnetic species are observed by EPR spectroscopy under catalytic conditions, i.e., in the presence of acid.
<figure><img src="image_9.png" /><figcaption>**Figure 4.** A) Experimental and simulated EPR spectra recording on a DMF solution of NiFeCp (100µM), Ir(ppy)₃ (250µM) and BIH (0.1M) after 1 min of irradiation, and B) Distribution ratios of the different species present in solution after various times of irradiation, estimated from the deconvolution of the experimental data with simulated spectra (used parameters in Table S4).</figcaption></figure>

Labeling experiments were also conducted to determine the origin of the two hydrogen atoms. A d<sup>7</sup>-DMF solution containing Ni<sup>II</sup>Fe<sup>II</sup>Cp (100 µM), [Ru(bpy)<sub>3</sub>]<sup>2+</sup> (250 µM), BIH (0.1 M), and deuterated trifluoroacetic acid (TFAD, 0.05 M) was irradiated for 20 minutes. The resulting <sup>1</sup>H NMR spectrum shows a triplet at 4.52 ppm (J = 40 Hz), indicating the production of HD (Figure 5). In contrast, when unlabelled trifluoroacetic acid is used, a single peak at 4.55 ppm is observed, consistent with the production of H<sub>2</sub> as the sole product. This experiment clearly demonstrates that one hydrogen atom comes from BIH, while the second originates from the acid source. This mechanism rationalizes the lack of reactivity when BNAH replaces BIH, as BNAH cannot

<figure><img src="image_10.png" /><figcaption>**Figure 5.** <sup>1</sup>HNMR spectra in deaerated d<sup>7</sup>-DMF solution containing NiFeCp (100 µM), [Ru(bpy)<sub>3</sub>]<sup>2+</sup> (250 µM) and BIH (0.1 M) with 0.05 M TFA-D before irradiation (red), and after 20 min of irradiation with 0.05 M TFA-H (green), 0.05 M TFA-D (blue), and 0.05 M MTFA-D (purple) in the absence of catalyst (a small amount of HD is observed corresponding to the uncatalyzed HER activity, see Figure 1)</figcaption></figure>

provide an additional electron, or when experiments are conducted without a proton source.

## Conclusion

This study successfully generated a paramagnetic Ni<sup>II</sup>Fe<sup>II</sup> species under photo-assisted conditions, which is relevant to the NiL state of the NiFe hydrogenase. This state exists in equilibrium with the NiC state, a Ni<sup>III</sup>(H)Fe<sup>II</sup> species.<sup>8</sup> Recent approaches have focused on characterizing paramagnetic mononuclear Ni<sup>II</sup> and Ni<sup>III</sup> species in thiolate-rich environments,<sup>23-26</sup> as well as their interconversion between Ni<sup>II</sup> and Ni<sup>III</sup>-hydride species,<sup>46</sup> as key intermediates in HER catalysis. Here, we demonstrated that the Ni<sup>II</sup>Fe<sup>II</sup>Cp intermediate reacts with BIH<sup>++</sup> to directly generate a Ni<sup>II</sup>(H)Fe<sup>II</sup>, serving as a model for the NiR state of the NiFe hydrogenase (Scheme 2).

The HER reactivity of NiFeCp is highly dependent on experimental conditions. The mononuclear [NiL<sup>N2S2</sup>], while active for HER under electrocatalytic conditions,<sup>34</sup> fails to generate H<sub>2</sub> under photo-assisted conditions. This aspect underscores the crucial role of heterodinuclearity in NiFeCp for efficient photo-assisted HER catalysis. Furthermore, since BIH<sup>++</sup> serves as a hydrogen atom donor, the NiFeCp complex avoids the need for a two-electron reduction before reacting with protons (Scheme 2). This decreases the necessity for highly reducing conditions ([Ru(bpy)<sub>3</sub>]<sup>2+/+</sup> redox system, E<sub>1/2</sub> = -1.71 V vs F<sup>+</sup>/F<sup>37</sup> compared to electrocatalysis (E<sub>cat/2</sub> = -1.90 V vs F<sup>+</sup>/F<sup>3</sup>).

Our in-depth mechanistic investigation provides the first direct evidence that BIH<sup>++</sup> can serve as a hydrogen donor atom in this system, in addition to the established role of BIH as a sacrificial electron donor and proton donor in photocatalysis performed in organic solvents. We showed that
one electron is provided by $[Ru(bpy)₃]⁺$, while the second and one proton are supplied by BIH⁺⁺, with the acid providing the second proton. This scenario is likely not unique to this system and warrants consideration in other photo-assisted catalytic processes.

In summary, this study marks a significant milestone, presenting the first example of photo-assisted H₂ production using a bio-inspired NiFe catalyst, which exhibits remarkable performance in both kinetics and stability. It serves as a proof of concept demonstrating the viability of such a bio-inspired catalytic system. However, future efforts will focus on replacing the noble-metal photosensitizers with more earth-abundant alternatives to improve the sustainability of the approach further. The catalytic mechanism was clarified through *operando* EPR spectroscopy and labeling experiments, unveiling a new role for BIH being the precursor of BIH⁺⁺ acting as hydrogen atom donor. These findings offer valuable insights for future studies of photo-driven multi-electron and multi-proton reduction processes, such as HER and CO₂R, to pave the way for further advancements in this field.

## ASSOCIATED CONTENT

The supplementary information provides an experimental section together with additional data and experiments. The authors have cited additional references within the Supporting Information<sup>21-22, 34, 36, 47-56</sup> This material is available free of charge via the Internet at http://pubs.acs.org.

## AUTHOR INFORMATION

### Corresponding Author

* E-mail for N.L.: noemie.lalaoui@univ-grenoble-alpes.fr

* E-mail for C.D.: carole.duboc@univ-grenoble-alpes.fr

### Author Contributions

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

### Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENT

The authors gratefully acknowledge research support for this work by the French National Agency for Research in the framework of the "Investissements d'avenir" program (ANR-15-IDEX-02), the Labex ARCANE, the CDP-DefiCO2 and CBH-EUR-GS (ANR-17-EURE-0003).

## REFERENCES

1. S.-L. Meng, C. Ye, X.-B. Li, C.-H. Tung, L.-Z. Wu, Photochemistry Journey to Multielectron and Multiproton Chemical Transformation. *J. Am. Chem. Soc.* **2022**, *144*, 16219-16231

2. A. J. Esswein, D. G. Nocera, Hydrogen Production by Molecular Photocatalysis. *Chem. Rev.* **2007**, *107*, 4022-4047

3. S. van Renssen, The hydrogen solution? *Nat. Clim. Change* **2020**, *10*, 799-801

4. N. Armaroli, V. Balzani, The Hydrogen Issue. *ChemSusChem* **2011**, *4*, 21-36

5. F. Cheng, O. Pavliuk, S. Hardt, L. A. Hunt, B. Cai, T. Kubart, L. Hammarström, N. Plumeré, G. Berggren, H. Tian, Embedding biocatalysts in a redox polymer enhances the performance of dye-sensitized photocathodes in bias-free photoelectrochemical water splitting. *Nature Commun.* **2024**, *15*, 3202

6. Y. Liu, C. Pulignani, S. Webb, S. J. Cobb, S. Rodríguez-Jiménez, D. Kim, R. D. Milton, E. Reisner, Electrostatic [FeFe]-hydrogenase-carbon nitride assemblies for efficient solar hydrogen production. *Chem. Sci.* **2024**, *15*, 6088-6094

7. M. G. Allan, T. Pichon, J. A. McCune, C. Cavazza, A. Le Goff, M. F. Kühnel, Augmenting the Performance of Hydrogenase for Aerobic Photocatalytic Hydrogen Evolution via Solvent Tuning. *Angew. Chem. Int. Ed.* **2023**, *62*, e202219176

8. W. Lubitz, H. Ogata, O. Rüdiger, E. Reijerse, Hydrogenases. *Chem. Rev.* **2014**, *114*, 4081-4148

9. J. T. Kleinhaus, F. Wittkamp, S. Yadav, D. Siegmund, U.-P. Apfel, [FeFe]-Hydrogenases: maturation and reactivity of enzymatic systems and overview of biomimetic models. *Chem. Soc. Rev.* **2021**, *50*, 1668-1784

10. S. Gao, W. Fan, Y. Liu, D. Jiang, Q. Duan, Artificial water-soluble systems inspired by [FeFe]-hydrogenases for electro- and photocatalytic hydrogen production. *Int. J. Hydrog. Energy* **2020**, *45*, 4305-4327

11. J. Amaro-Gahete, M. V. Pavliuk, H. Tian, D. Esquivel, F. J. Romero-Salguero, S. Ott, Catalytic systems mimicking the [FeFe]-hydrogenase active site for visible-light-driven hydrogen production. *Coord. Chem. Rev.* **2021**, *448*, 214172

12. P. Buday, C. Kasahara, E. Hofmeister, D. Kowalczyk, M. K. Farh, S. Riediger, M. Schulz, M. Wächtler, S. Furukawa, M. Saito, D. Ziegenbalg, S. Gräfe, P. Bäuerle, S. Kupfer, B. Dietzek-Ivanšić, W. Weigand, Activating a [FeFe] Hydrogenase Mimic for Hydrogen Evolution under Visible Light. *Angew. Chem. Int. Ed.* **2022**, *61*, e202202079

13. R. Gupta, A. K. Pal, Bioinspired photo-driven hydrogen evolution systems based on hydrogenases and their mimics. *Sustain. Energy Fuels* **2024**, *8*, 4709-4751

14. D. Basu, T. S. Bailey, N. Lalaoui, C. P. Richers, T. J. Woods, T. B. Rauchfuss, F. Arrigoni, G. Zampella, Synthetic Designs and Structural Investigations of Biomimetic Ni-Fe Thiolates. *Inorg. Chem.* **2019**, *58*, 2430-2443

15. D. Schilter, J. M. Camara, M. T. Huynh, S. Hammes-Schiffer, T. B. Rauchfuss, Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides. *Chem. Rev.* **2016**, *116*, 8693-8749

16. S. Ogo, K. Ichikawa, T. Kishima, T. Matsumoto, H. Nakai, K. Kusaka, T. Ohhara, A Functional [NiFe]Hydrogenase Mimic That Catalyzes Electron and Hydride Transfer from H2. *Science* **2013**, *339*, 682-684

17. K. Weber, T. Krämer, H. S. Shafaat, T. Weyhermüller, E. Bill, M. van Gastel, F. Neese, W. Lubitz, A Functional [NiFe]-Hydrogenase Model Compound That Undergoes Biologically Relevant Reversible Thiolate Protonation. *J. Am. Chem. Soc.* **2012**, *134*, 20745-20755

18. C. Tard, C. J. Pickett, Structural and Functional Analogues of the Active Sites of the [Fe]-, [NiFe]-, and [FeFe]-Hydrogenases. *Chem. Rev.* **2009**, *109*, 2245-2274

19. A. C. Ghosh, C. Duboc, M. Gennari, Synergy between metals for small molecule activation: Enzymes and bio-inspired complexes. *Coord. Chem. Rev.* **2021**, *428*,

20. P. A. Summers, J. A. Calladine, F. Ghiotto, J. Dawson, X.-Z. Sun, M. L. Hamilton, M. Towrie, E. S. Davies, J. McMaster, M. W. George, M. Schröder, Synthesis and Photophysical Study of a [NiFe] Hydrogenase Biomimetic Compound Covalently
Linked to a Re-diimine Photosensitizer. *Inorg. Chem.* **2016**, *55*, 527-536

21. P. A. Summers, J. Dawson, F. Ghiotto, M. W. D. Hanson-Heine, K. Q. Vuong, E. Stephen Davies, X.-Z. Sun, N. A. Besley, J. McMaster, M. W. George, M. Schröder, Photochemical Dihydrogen Production Using an Analogue of the Active Site of [NiFe] Hydrogenase. *Inorg. Chem.* **2014**, *53*, 4430-4439

22. X.-Z. Wang, S.-L. Meng, H. Xiao, K. Feng, Y. Wang, J.-X. Jian, X.-B. Li, C.-H. Tung, L.-Z. Wu, Identifying a Real Catalyst of [NiFe]-Hydrogenase Mimic for Exceptional H2 Photogeneration. *Angew. Chem. Int. Ed.* **2020**, *59*, 18400-18404

23. A. E. Wertz, S. C. Marguet, C. Turro, H. S. Shafaat, Targeted Modulation of Photocatalytic Hydrogen Evolution Activity by Nickel-Substituted Rubredoxin through Functionalized Ruthenium Phototriggers. *Inorg. Chem.* **2024**, *63*, 20438-20447

24. J. W. Slater, S. C. Marguet, H. A. Monaco, H. S. Shafaat, Going beyond Structure: Nickel-Substituted Rubredoxin as a Mechanistic Model for the [NiFe] Hydrogenases. *J. Am. Chem. Soc.* **2018**, *140*, 10250-10262

25. P. Prasad, L. A. Hunt, A. E. Pall, M. Ranasinghe, A. E. Williams, T. L. Stemmler, B. Demeler, N. I. Hammer, S. Chakraborty, Photocatalytic Hydrogen Evolution by a De Novo Designed Metalloprotein that Undergoes Ni-Mediated Oligomerization Shift. *Chem. Eur. J.* **2023**, *29*, e202202902

26. S. Malayam Parambath, A. E. Williams, L. A. Hunt, D. Selvan, N. I. Hammer, S. Chakraborty, A De Novo-Designed Artificial Metallopeptide Hydrogenase: Insights into Photochemical Processes and the Role of Protonated Cys. *ChemSusChem* **2021**, *14*, 2237-2246

27. J. Timm, D. H. Pike, J. A. Mancini, A. M. Tyrishkin, S. Poudel, J. A. Siess, P. M. Molinaro, J. J. McCann, K. M. Waldie, R. L. Koder, P. G. Falkowski, V. Nanda, Design of a minimal di-nickel hydrogenase peptide. *Sci. Adv.* **2023**, *9*, eabq1990

28. A. F. T. Waffo, C. Lorent, S. Katz, J. Schoknecht, O. Lenz, I. Zebger, G. Caserta, Structural Determinants of the Catalytic Nia-L Intermediate of [NiFe]-Hydrogenase. *J. Am. Chem. Soc.* **2023**, *145*, 13674-13685

29. J. Zhang, P. She, Q. Xu, F. Tian, H. Rao, J.-S. Qin, J. Bonin, M. Robert, Efficient Visible-Light-Driven Carbon Dioxide Reduction using a Bioinspired Nickel Molecular Catalyst. *ChemSusChem* **2024**, *17*, e202301892

30. H. Takeda, H. Kamiyama, K. Okamoto, M. Irimajiri, T. Mizutani, K. Koike, A. Sekine, O. Ishitani, Highly Efficient and Robust Photocatalytic Systems for CO2 Reduction Consisting of a Cu(I) Photosensitizer and Mn(I) Catalysts. *J. Am. Chem. Soc.* **2018**, *140*, 17241-17254

31. X.-Q. Zhu, M.-T. Zhang, A. Yu, C.-H. Wang, J.-P. Cheng, Hydride, Hydrogen Atom, Proton, and Electron Transfer Driving Forces of Various Five-Membered Heterocyclic Organic Hydrides and Their Reaction Intermediates in Acetonitrile. *J. Am. Chem. Soc.* **2008**, *130*, 2501-2516

32. F. Draper, S. DiLuzio, H. J. Sayre, L. N. Pham, M. L. Coote, E. H. Doeven, P. S. Francis, T. U. Connell, Maximizing Photon-to-Electron Conversion for Atom Efficient Photoredox Catalysis. *J. Am. Chem. Sci.* **2024**, *146*, 26830-26843

33. R. Miyajima, T. Kiuchi, Y. Ooe, H. Iwamoto, S.-y. Takizawa, E. Hasegawa, Electron and hydrogen atom donor photocatalysts in situ generated from benzimidazolium salts and hydride reagents. *J. Photochem. Photobiol. A* **2023**, *16*, 100195

34. D. Brazzolotto, M. Gennari, N. Queyriaux, T. R. Simmons, J. Pécaut, S. Demeshko, F. Meyer, M. Orio, V. Artero, C. Duboc, Nickel-centred proton reduction catalysis in a model of [NiFe] hydrogenase. *Nat. Chem.* **2016**, *8*, 1054-1060

35. M. E. Ahmed, S. Chattopadhyay, L. K. Wang, D. Brazzolotto, D. Pramanik, D. Aldakov, J. Fize, A. Morozan, M. Gennari, C. Duboc, A. Dey, V. Artero, Hydrogen Evolution from Aqueous Solutions Mediated by a Heterogenized NiFe - Hydrogenase Model: Low pH Enables Catalysis through an Enzyme-Relevant Mechanism. *Angew. Chem. Int. Ed.* **2018**, *57*, 16001-16004

36. H. Tang, M. B. Hall, Biomimetics of [NiFe]-Hydrogenase: Nickel- or Iron-Centered Proton Reduction Catalysis? *J. Am. Chem. Soc.* **2017**, *139*, 18065-18070

37. C. K. Prier, D. A. Rankic, D. W. C. MacMillan, Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis. *Chem. Rev.* **2013**, *113*, 5322-5363

38. Y. Na, M. Wang, J. Pan, P. Zhang, B. Åkermark, L. Sun, Visible Light-Driven Electron Transfer and Hydrogen Generation Catalyzed by Bioinspired [2Fe2S] Complexes. *Inorg. Chem.* **2008**, *47*, 2805-2810

39. H. Shirley, X. Su, H. Sanjanwala, K. Talukdar, J. W. Jurss, J. H. Delcamp, Durable Solar-Powered Systems with Ni-Catalysts for Conversion of CO2 or CO to CH4. *J. Am. Chem. Soc.* **2019**, *141*, 6617-6622

40. B. B. Zhang, L. C. Sun, Artificial photosynthesis: opportunities and challenges of molecular catalysts. *Chem. Soc. Rev.* **2019**, *48*, 2216-2264

41. K. K.-W. Lo, Luminescent Rhenium(I) and Iridium(III) Polypyridine Complexes as Biological Probes, Imaging Reagents, and Photocytotoxic Agents. *Acc. Chem. Res.* **2015**, *48*, 2985-2995

42. E. T. Luis, H. Iranmanesh, J. E. Beves, Photosubstitution reactions in ruthenium(II) trisdiimine complexes: Implications for photoredox catalysis. *Polyhedron* **2019**, *160*, 1-9

43. Y. Tamaki, T. Morimoto, K. Koike, O. Ishitani, Photocatalytic CO<sub>2</sub><sub>sub</sub> reduction with high turnover frequency and selectivity of formic acid formation using Ru(II) multinuclear complexes. *PNAS* **2012**, *109*, 15673-15678

44. M. Gennari, M. Orio, J. Pécaut, E. Bothe, F. Neese, M.-N. Collomb, C. Duboc, Influence of Mixed Thiolate/Thioether versus Dithiolate Coordination on the Accessibility of the Uncommon +I and +III Oxidation States for the Nickel Ion: An Experimental and Computational Study. *Inorg. Chem.* **2011**, *50*, 3707-3716

45. D. Brazzolotto, L. K. Wang, H. Tang, M. Gennari, N. Queyriaux, C. Philouze, S. Demeshko, F. Meyer, M. Orio, V. Artero, M. B. Hall, C. Duboc, Tuning Reactivity of Bioinspired NiFe-Hydrogenase Models by Ligand Design and Modeling the CO Inhibition Process. *ACS Catal.* **2018**, *8*, 10658-10667

46. S. Chakrabarti, S. Sinha, G. N. Tran, H. Na, L. M. Mirica, Characterization of paramagnetic states in an organometallic nickel hydrogen evolution electrocatalyst. *Nature Commun.* **2023**, *14*, 905

47. S. Fu, Y. Liu, Y. Ding, X. Du, F. Song, R. Xiang, B. Ma, A mononuclear cobalt complex with an organic ligand acting as a precatalyst for efficient visible light-driven water oxidation. *Chem. Commun.* **2014**, *50*, 2167-2169

48. X. Li, Y. Zhang, H. Chen, J. Sun, F. Feng, Protein Nanocages for Delivery and Release of Luminescent Ruthenium(II) Polypyridyl Complexes. *ACS Appl. Mater. Interf.* **2016**, *8*, 22756-22761

49. K. A. Teegardin, J. D. Weaver, Preparation of Fac - Tris(2-Phenylpyridinato) Iridium(III). *Organic Synth.* **2018**, *95*, 29-45
50. P. M. Stanley, F. Sixt, J. Warnan, Decoupled Solar Energy Storage and Dark Photocatalysis in a 3D Metal-Organic Framework. *Adv. Mater.* **2023**, *35*, 2207280

51. R. Guo, X. Qi, H. Xiang, P. Geaneotes, R. Wang, P. Liu, Y.-M. Wang, Stereodivergent Alkyne Hydrofluorination Using Protic Tetrafluoroborates as Tunable Reagents. *Angew. Chem. Int. Ed.* **2020**, *59*, 16651-16660

52. M. H. Gehlen, The centenary of the Stern-Volmer equation of fluorescence quenching: From the single line plot to the SV quenching map. *J. Photochem. Photobiol. C* **2020**, *42*, 100338

53. D. Ziegenbalg, A. Pannwitz, S. Rau, B. Dietzek-Ivanšić, C. Streb, Comparative Evaluation of Light-Driven Catalysis: A Framework for Standardized Reporting of Data. *Angew. Chem. Int. Ed.* **2022**, *61*, e202114106

54. G. Rothenberg, In *Catalysis: Concepts and Green Applications*, WILEY-VCH Verlag GmbH & Co. KGaA: 2008.

55. H. Yan, J. Yang, G. Ma, G. Wu, X. Zong, Z. Lei, J. Shi, C. Li, Visible-light-driven hydrogen production with extremely high quantum efficiency on Pt-PdS/CdS photocatalyst. *J. Catal.* **2009**, *266*, 165-168

56. E. N. Musa, A. K. Yadav, K. T. Smith, M. S. Jung, W. F. Stickle, P. Eschbach, X. Ji, K. C. Stylianou, Boosting Photocatalytic Hydrogen Production by MOF-Derived Metal Oxide Heterojunctions with a 10.0 % Apparent Quantum Yield. *Angew. Chem. Int. Ed.* **2024**, *63*, e202405681
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TOF 2750 W⁻¹

TON 41200 (2.5h)