Metallaphotoredox catalysis

• Comment: The version as of July 6th reads as an advert for this type of catalyst. This is not how we write articles, they need to be neutral. Please rewrite. Ldm1954 (talk) 14:13, 6 July 2026 (UTC)

---

In organic chemistry metallaphotoredox catalysis is a form of catalysis born from the merge of transition metal catalysis and photoredox catalysis. By combining the powerful capability of transition metal in bond formation with the broad utility of photoinduced electron- (PET) and energy-transfer (PEnT) processes a wide range of new reactivities has been unlocked, providing activation pathways that complement traditional catalytic methods.

History

Transition metal catalysis has been recognized as a reliable and potent way to construct complex molecules from simple and readily accessible starting materials since its discovery in the last century. Nevertheless, strategies for modulating metal reactivity have remained fairly stagnant. In particular, the greatest progress has been achieved in the design and synthesis of ligands, which have enabled the development of new methodologies of high synthetic interest. The manipulation of the metal's oxidation state, on the other hand, has remained less mature, and still relied on the use of stoichiometric reducing or oxidizing agents that substantially alter the reaction conditions. More recently, chemists have begun to exploit the great potential of light as a means of supplying energy to reactions, combining it with the versatile light-absorbing capacity of organic molecules. In the early 2000s, the first organic dyes and organometallic complexes were employed as mediators in reactions proceeding through single electron transfer (SET) and energy transfer (ET) mechanisms, giving rise to a new discipline in chemistry known as photoredox catalysis.

The potentiality of this new approach enabled the development of methodologies combining transition metal catalysis with photoredox catalysis. In this way, oxidation states that were previously difficult to access could be achieved through the simple use of light. The first pioneering work in this field was developed by the Sanford group in 2011, who were able to synthesize biaryls under mild conditions by combining palladium catalysis with visible-light photocatalysis. Some years later MacMillan group extended this approach developing a method to make carbon-carbon bonds exploiting an iridium photocatalyst in combination with nickel(II) as a metal source.

C-C coupling of aryl bromides with decarboxylative-generated radicals and catalytic cycle developed by MacMillan

From these initial studies based on palladium and nickel catalysis, many others metals have been employed in the next years, using heavy metals-based photocatalysts. however soon the use of organic dyes started to be investigated to replace the expensive and toxic ruthenium and iridium photosensitizers for more suistainable methodologies.

Mechanistic principles

At its core, metallaphotoredox catalysis merges two catalytic cycles. A photoredox catalyst absorbs visible light and enters an excited state capable of transferring either an electron or energy to another species. Meanwhile, a transition-metal catalyst, controls bond-forming steps. The interaction between these cycles allows highly reactive radical intermediates to be generated under comparatively gentle conditions and then captured by the metal catalyst before unwanted side reactions occur.

Quenching

Main article: Quenching (fluorescence)

The process of transferring energy from an excited molecule to an another specie decreasing its fluorescence is called quenching.

Quenching processes can be classified into two types: static and dynamic. In static quenching, a complex forms between the quencher and the excited molecule, which results in the suppression of emission, as the complex is unable to emit light. In dynamic quenching, an excited molecule interacts with a quencher present in solution, causing relaxation to the ground state. This interaction typically leads to excitation, oxidation, or reduction of the quencher. Usually, the excited quencher is not able to emit light, which results in a decrease in fluorescence or phosphorescence intensity.

Given these processes, catalytic cycles can be designed to involve either an "oxidative quenching cycle" or a "reductive quenching cycle".

Generic oxidative and reductive quenching cycles

Since most of the time the energy transfer mechanism is a collisional phenomena, is crucial to use a photocatalyst with a long excited-state lifetime. In particular the reactive species must be able to exist long enough to interact with the quencher. For organic photocatalysis, the excited-state lifetime typically needs to be on the order of at least 10⁻⁸ seconds. Triplet states, which typically have lifetimes in the range of microseconds, are preferred over singlet states because they can maintain their excited state for longer durations. This is the primary reason why phosphorescent molecules are favored over fluorescent ones.

Stern-Volmer experiments

Main article: Stern–Volmer relationship

A wide range of substances can act as fluorescence quenchers. Due to the variety of quenching agents available, it is possible to identify specific fluorophore-quencher combinations for targeted applications. However, it is important to note that not all fluorophores are quenched by every known quencher.

When a quencher is present, an additional decay pathway is introduced for the fluorophore with a rate that depends on the concentration of the quencher. By analyzing a Stern-Volmer plot, it is possible to determine the excited-state lifetime of a fluorophore and assess the effectiveness of different quenchers.

Choose of the photocatalyst

Selecting an appropriate photocatalyst is a non-trivial task, as multiple parameters must be considered and accurately determined. Some characteristics, such as the maximum absorption and emission wavelengths or the molar extinction coefficient, are well known and straightforward to assess. However, other critical properties, including fluorescence and phosphorescence lifetimes and quantum yield, require careful evaluation.

Beyond these fundamental properties, additional parameters specifically relevant to photocatalysis must also be defined. When an energy transfer (EnT) mechanism is involved, it is particularly important to determine the excited-state energies of S₁ (E^S1_0,0) and T₁ (E^T1_0,0), which correspond to the energy levels of the non-vibrational excited singlet (S₁) and triplet (T₁) states relative to the ground state (S₀). These values can be obtained from the intersection of the absorption and emission spectra of the photocatalyst and are usually expressed in electronvolts (eV).

In contrast, when a photoinduced electron transfer (PET) mechanism is operative, both the ground and excited-state redox potentials play a crucial role in defining the oxidation and reduction capabilities of the photocatalyst. The ground-state redox potential can be experimentally determined via cyclic voltammetry, while the excited-state potential is estimated using the E_0,0 values for S₁ and T₁ through the following equations:

$E(P{C}^{*}/P{C}^{\bullet -})=E(PC/P{C}^{\bullet -})+E_{0,0}$

$E(P{C}^{\bullet +}/P{C}^{*})=E(P{C}^{\bullet +}/PC)-E_{0,0}$

These values are expressed in volts (V) and are typically referred to a saturated calomel electrode (SCE). Considering all these parameters is essential for assessing the feasibility of a given photocatalytic reaction.

Photocatalysts

The most widely used photocatalysts are ruthenium and iridium polypyridyl complexes due to their numerous advantages, including:

• The heavy atom effect, which facilitates access to the triplet excited state, leading to longer lifetimes;
• The ability to absorb visible light;
• Ligand modularity, which allows fine-tuning of their properties.

However, the use of these metal-based dyes is limited by the drawbacks associated with noble metals, which are not only expensive but also scarce and often toxic. Their availability is subject to fluctuations in price, often influenced by geopolitical factors. For this reason, in recent years, significant research efforts have been directed toward replacing these complexes with organic dyes, which are not only significantly more cost-effective but also exhibit comparable excited-state lifetimes.

Some of the most used and commercially available noble metal complexes (A) and dyes (B) used in photocatalysis

A particularly promising class of organic photocatalysts that has gained attention in recent years is donor-acceptor (D-A) cyanoarenes. Initially developed for OLED applications, these compounds were pioneered by Adachi in 2012 with the synthesis of 4CzIPN. More recently, Zhang and later Zeitler successfully synthesized various derivatives, leveraging the modular nature of these molecules. By strategically combining different electron-acceptor cores with electron-donor units, a wide range of tunable properties and photocatalytic performances can be achieved. Due to steric effects and the nature of the substituents, the HOMO and LUMO orbitals of these compounds are primarily localized on the donor and acceptor moieties, respectively. As a direct consequence, the energy gap between the singlet (S1) and triplet (T1) states is reduced, increasing the likelihood of ISC. At the same time, this structural feature also favors thermally activated delayed fluorescence (TADF), allowing these molecules to undergo relaxation via delayed fluorescence.

Some of most used D-A cyanoarenes

Nickel catalysis

Nickel is undoubtedly one of the most widely used metals in cross-coupling reactions as a substitute for palladium, benefiting from its ability to undergo rapid oxidative addition to alkyl electrophiles while being less susceptible to β-hydride elimination with aliphatic ligands. Recently, photoredox catalysis has enabled significant advancements in this field, broadening the scope of coupling partners to simpler and more cost-effective reagents.

For instance, Molander and co-workers further demonstrated that alkyltrifluoroborates can be converted into carbon-centered radicals via single-electron transfer (SET) using a photocatalyst. This approach circumvents the traditionally slow transmetalation step required for alkylboron reagents.

Photocatalyzed C-C coupling between alkyltrifluoroborates and aryl bromides.

A particularly challenging aspect of transition metal catalysis has been the direct functionalization of C–H bonds in cross-coupling reactions. In this context, photoredox catalysis mediated by visible light has introduced new approaches that, compared to previous methodologies, offer greater simplicity and practicality. These homolytic C–H bond cleavage strategies enable the direct use of feedstock chemicals as reagents for cross-coupling, eliminating the need for stoichiometric prefunctionalization. The first example of this reactivity was reported by Doyle and MacMillan, who demonstrated an oxidation-deprotonation mechanism for generating α-amino radicals. They found that exposing an aniline derivative bearing α-amino C–H bonds to photoredox conditions led to a SET-induced formation of an amine radical cation. This intermediate readily underwent deprotonation, furnishing a stable α-amino radical, which was shown to be a competent open-shell nucleophile in dual-catalytic cross-coupling reactions.

Light mediated α-amino C-H bond activation

Nickel has been extensively employed not only in the construction of all-carbon frameworks but also in the formation of carbon–heteroatom bonds. The conditions reported by MacMillan for C–O cross-coupling demonstrate that, in the absence of light, nickel complexes are unable to promote this type of transformation. The oxidation of Ni(II) to Ni(III) is required to trigger the reductive elimination step of the catalytic cycle, thus driving the reaction toward the desired ether product. A similar procedure has been developed for C–N coupling, serving as an alternative to the Pd-catalyzed Buchwald–Hartwig amination.

Carbon-heteroatom cross coupling protocols

Subsequent mechanistic investigations led to a revised understanding of these transformations. Nocera proposed a dual-cycle mechanism, wherein the dark Ni(I)/Ni(III) catalytic cycle is responsible for the actual coupling reaction. A second photoredox cycle is required to continuously regenerate the active Ni(I) species by reducing the Ni(II) pre-catalyst.

Catalytic mechanism proposed by Nocera

Cobalt catalysis

In synthetic chemistry, the exploration of cobalt as a light-activatable transition metal for organic transformations has gained attention only in recent years. Previously, cobalt was primarily employed to mimic natural systems for the reductive splitting of water. The first example of cobalt-based photochemistry was reported by the Wu group in 2014. In this study, the authors described a cobalt-photocatalyzed cross-coupling, inspired by a prior ruthenium-based system for the coupling of indoles with tetrahydroisoquinolines (THIQs). The reaction uses Co(dmgH)₂Cl₂ in combination with Eosin Y as a photocatalyst. Upon excitation to its triplet state, Eosin Y is quenched by the N-aryl-THIQ substrate, generating a radical cation. This species subsequently reduces the cobalt catalyst from Co(III) to Co(II), which, upon proton loss, forms an iminium ion capable of reacting as an electrophile with the nucleophilic indole.

Cobalt-photocatalyzed cross-coupling

Copper catalysis

Copper has been widely employed as a catalyst in organic transformations due to its abundance in the Earth's crust and its low toxicity. However, several challenges remain, particularly concerning the substrate activation mechanism, which often requires high temperatures and is limited to nucleophiles capable of efficiently coordinating or transmetallating with the metal center. The advent of photocatalysis has significantly facilitated radical generation, and the combination of highly reactive radical intermediates with copper's strong ability to interact with radicals has expanded its applications in carbon–carbon and carbon–heteroatom bond-forming reactions. Sanford's group reported a notable application of copper photoredox catalysis for a trifluoromethylation protocol. This method provides a mild and efficient catalytic system for the cross-coupling of phenyl boronic acids with perfluoroalkyl iodides. The reaction proceeds via the reduction of CF₃I by a Ru(I) complex, generating a trifluoromethyl radical. This radical is then trapped by Cu(II) through an oxidative addition step, forming a Cu(III) species. Subsequent transmetallation with the boronic acid, followed by reductive elimination, yields the final product. Finally, both the ruthenium photocatalyst and the copper catalyst are regenerated via a single-electron transfer mechanism, closing both catalytic cycles.

Copper photocatalyzed trifluoromethylation of boronic acids

See also

• Catalysis
• Photocatalysis
• Redox

References

This article "Metallaphotoredox catalysis" is from Wikipedia. The list of its authors can be seen in its historical and/or the page Edithistory:Metallaphotoredox catalysis. Articles copied from Draft Namespace on Wikipedia could be seen on the Draft Namespace of Wikipedia and not main one.

---