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Review

Heterobinuclear Metallocomplexes as Photocatalysts in Organic Synthesis

by
Violetta A. Ionova
1,
Anton S. Abel
1,*,
Alexei D. Averin
1 and
Irina P. Beletskaya
1,2,*
1
Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1-3, Moscow 119991, Russia
2
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky Pr. 31, Moscow 119071, Russia
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(4), 768; https://doi.org/10.3390/catal13040768
Submission received: 15 March 2023 / Revised: 11 April 2023 / Accepted: 14 April 2023 / Published: 18 April 2023
(This article belongs to the Section Photocatalysis)
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Abstract

:
Photocatalytic processes under visible light have constantly been finding more and more applications in organic synthesis as they allow a wide range of transformations to proceed under mild conditions. The combination of photoredox catalysis with metal complex catalysis gives an opportunity to employ the advantages of these two methodologies. Covalent bonding of photocatalyst and metal complex catalyst using bridging ligands increases the efficiency of the electron and energy transfer between these two parts of the catalyst, leading to more efficient and selective catalytic systems. Up to now, after numerous investigations of the photocatalytic reduction of CO2 and hydrogen generation, such a strategy was firmly established to substantially increase the catalyst’s activity. This review is aimed at the achievements and perspectives in the field of design and application of heterobinuclear metal complexes as photocatalysts in organic synthesis.

Graphical Abstract

1. Introduction

The 21st Century witnessed the active introduction of photoredox catalysis in synthetic chemistry using visible light (400–700 nm); this trend provided new possibilities to the organic chemistry of free radicals [1,2]. A wide range of available photocatalysts with different RedOx potentials in ground and excited states allows the selective and efficient generation of free radicals under the action of visible light. It opens the way to various reactions, from simple processes such as aerobic oxidation of amines, thiols and alcohols, to multi-component processes such as radical arylation, cycloaddition, halogenation, difunctionalization of alkenes, etc. [3,4,5,6]. New synthetic approaches to a great variety of molecules, including sophisticated natural compounds, are provided [7].
In recent years, metallaphotoredox catalysis, which employs mutual application of the photoredox catalysis and catalysis with transition metal complexes under the action of visible light [8,9], has been actively developed. This allows us to combine mild reaction conditions and visible light as the energy source (main features of the photoredox catalysis) with the key advantages of transition metal catalysis such as chemo-, regio- and enantioselectivity [9,10,11].
There are many examples of metal-catalyzed reactions which proceed under visible light irradiation [12], with the intermediate complex in the excited state, which favors the reductive elimination step. Often the ability of such complexes to absorb visible light is insufficient, and the use of high-energy ultraviolet irradiation may lead to undesirable side photochemical reactions. Due to the application of photoactive Ru(II) or Ir(III) complexes [7,13] or dyes, such as Eosin Y and BODIPY [6], which are characterized by high molar extinction coefficients in the range of 400–600 nm, metallaphotoredox catalytic reactions can be performed under visible light irradiation. Significant progress in this field has occurred due to the use of various inorganic conductors (TiO2, CdS, MoS2, quantum dots, C3N4, etc.) as photocatalysts [14,15,16,17]. Varying their structure and doping with various components provides high activity in various photocatalyzed redox processes. Moreover, being heterogeneous, these catalysts can be separated from the reaction mixture and reused, as well as used in flow reactors [18,19].
Metal complexes as components of the metallaphotoredox catalytic systems usually incorporate nickel, copper, palladium and gold, and in rare cases, cobalt and some other metals [9]. A key step in the catalytic transformations with such systems is a single electron transfer (SET) or energy transfer (EnT) between photocatalytic and metal complex parts. It is quite logical to ensure the covalent binding of these two components using bridging ligands whose structure should provide their best synergism (Scheme 1).
To date, energy and electron transfer processes in di- and polynuclear complexes have been extensively studied using various spectral and electrochemical methods, as well as DFT calculations [20,21,22,23,24,25,26,27,28], allowing us to find out appropriate complexes for catalytic purposes. During the last two decades, great progress was achieved in the application of supramolecular catalysts in the CO2 photoreduction [29,30,31] and hydrogen photogeneration [32,33,34], which is helpful for the fine-tuning of the catalytic properties of binuclear complexes. As for metallaphotoredox catalysis, it has been rapidly developing during the last 7 years, and the application of binuclear complexes is still limited. In the present review, we address known examples of heterobinuclear metallocomplexes as photocatalysts in order to demonstrate the challenges and perspectives of their use in organic synthesis.

2. Heterobinuclear Photocatalysts

As our main interest is the application of heterobinuclear photoactive metal complexes in the synthesis of organic compounds, their use in the photocatalytic CO2 reduction [29,30,31] and hydrogen photogeneration [32,33,34] could be judged to be beyond the frames of this review. Moreover, each of these reactions was dealt with in excellent reviews covering achievements and tendencies in this field. However, the efficiency of the covalent bonding of the photoactive Ru(II) or Ir(III) complex with the catalytic metallocomplex center was first described for these processes. In the frames of these reactions, various photophysical and physicochemical studies of electron and energy transfer were carried out, and the main synthetic approaches to the polyazine heterobinuclear complexes were elaborated. As a result, the rapid development of the use of such complexes in organic synthesis was strongly supported by these investigations. Thus, we suppose to consider selected catalytic systems employed in the photocatalytic CO2 reduction and hydrogen photogeneration to underline a great variety of known bridging ligands and supramolecular photocatalysts and to show the possibilities of their adjustment to a certain catalytic process.
Classical ditopic polyazine ligands are given in Figure 1; they differ by the conjugation between two coordination sites, flexibility, and distance between metallocenters.

2.1. Heteropolynuclear Complexes for Hydrogen Photogeneration

Hydrogen is considered to be one of the most important ecologically clean sources of energy, which is to become a key component in the transition to “green” energetics and sustainable development. The use of solar light is one of the techniques to produce environmentally friendly hydrogen, and this fact stimulates the search for hydrogen photogeneration catalysts. Photocatalyst absorbs visible light assuring its transition to an excited state and provides the electron transfer to the metal complex catalyst (usually Pd, Pt or Rh). Ru(II) polypyridine complexes are thought to be most perspective as photoactive components as they possess a unique combination of spectral, photophysical and electrochemical properties [35]. The transfer of two electrons from the photocatalyst to the metal catalytic center provides the formation of one molecule of dihydrogen. Creating favorable conditions for the electron transfer process is a way to increase the efficiency of the catalytic system [34]. Triethylamine is used as a reductant, and the reactions are carried out in acetonitrile-water media.
The general mechanism of hydrogen photogeneration catalyzed by the Ru-Pd binuclear complex is depicted in Scheme 2. Under visible light irradiation, the electron is promoted from Ru(II)-center to π*-orbitals of the bridging ligand (Metal Ligand Charge Transfer, MLCT). Then, Ru(III) accepts the electron from the sacrificial reductant (triethylamine), followed by the electron transfer from the bridging ligand to Pd(II) and proton reduction [34].
The main characteristic showing the activity and robustness of the catalyst is its TON. Recently, it has been shown [36] that even a simple spatial approach of the photoactive and catalytic components using supramolecular interactions of the type “host–guest” increases the efficiency of the catalyst in this process. Covalent bonding of the photocatalyst with the metal catalytic center provides a further increase in the catalyst activity [33,34]; nowadays, a good number of such catalytic systems have been reported [37]. During the last 15 years, extended studies of bi- and polynuclear complexes were conducted, demonstrating the efficiency of this strategy [32,33,34]. The most representative examples will be discussed below.
Systematic theoretical and experimental investigation of Ru-Pd and Ru-Pt binuclear complexes in the photogeneration of dihydrogen is described in the papers by Vos and Rau. The studies reported in [38,39,40,41] dealt with binuclear complexes for hydrogen production with 2,5-bis(pyridin-2-yl)pyrazine (RuPd-1 and RuPd-2), 2,2′:5′,2′′-terpyridine (RuPd-3, RuPt-1 and RuPt-2) and 2,2′:6′,2′′-terpyridine (RuPd-4) as bridging ligands (Figure 2). It was revealed that the RuPd-1 complex was not catalytically active, and only the presence of electron-withdrawing substituents in the RuPd-2 complex allowed TON 400 in 18 h. Among the cyclometallated complexes RuPd-3 and RuPd-4, only the first one was catalytically active (TON 138, lower than for the RuPd-2 complex). To note, during the first 8 h its TON was higher than that found for the mixed catalytic systems (Ru-complex/Pd(MeCN)2Cl2). Platinum complexes RuPt-1 and RuPt-2 with TONs 80 and 650, respectively, turned out to be more reactive than corresponding Ru-Pd complexes.
Pt complexes RuPt-3 and RuPt-4 with 2,3-bis(pyridin-2-yl)pyrazine as a bridging ligand (Figure 3) were inefficient as catalysts of hydrogen generation, and only RuPt-5 provided TON 44 for 18 h [42].
Brewer and coworkers studied RuRh-1–RuRh-4 complexes (Figure 3) with Rh(III) in the catalytic center. RuRh-2, containing terpyridine as a peripheral ligand, demonstrated better activity in DMF than RuRh-1 complex with 2,2′-bipyridines (TONs 58 and 33, respectively) [43]. Studying the trinuclear complexes of RuRh-3 type (Figure 3) revealed that the hydroxo complex was the most active in DMF [44]. Further development of the catalysts of this type led to a water-soluble complex RuRh-4 with sulfo groups, which provided TON 120 for 18 h in an aquatic medium with the ascorbate buffer [45].
The same researchers studied Ru-Pt complexes RuPt-6–RuPt-8 (Figure 4) [46]. The authors explored the influence of π-extended bridging ligands and the increased number of photoactive centers in the structure of supramolecular catalysts on their efficiency. RuPt-8 complex comprising three photoactive Ru(II) centers was shown to be less efficient than the RuPt-7 complex (TONs 110 and 230, respectively), authors accounted it to a steric factor. The RuPt-6 complex provides TON 130 and is equally efficient as the RuPt-7 complex. Thus, the approach using π-extended bridging ligands seems to be reasonable and perspective.
There are many examples of supramolecular photocatalysts in which bridging ligands possess azine moieties organized in one heteroaromatic system. A classical ligand of this type is tetrapyridophenazine and its derivatives. Rau et al. [47,48,49], in order to optimize the catalytic center and peripheral ligands, studied RuPd-5, RuPt-9 and RuPt-10 complexes (Figure 5). These catalysts provided the following TONs in the photogeneration of hydrogen: 238 (18 h), 7 (18 h), and 279 (70 h), respectively. The RuPt-10 complex is notable for its high stability under reaction conditions, which helped to increase the reaction time up to 70 h and also to increase TON. Mengele and Raw [50] were the first to propose a promising RuRh-5 complex, which is also characterized by increased stability. Palladacycle-containing RuPd-6 complex was studied in [51]. A comparison of RuPd-5 and RuPd-6 complexes revealed that the latter is less efficient but retains activity better over long time spans (25 h). The authors also studied the formation of colloidal Pt and Pd in the course of the reaction and found out that Pt complexes are much less prone to this process. The RuPt-11 NHC complex was studied in [52], and it demonstrated higher activity than the RuPt-9 complex; moreover, the efficiency of RuPt-11 can be increased in the presence of iodides due to in situ ligand exchange.
Another studied type of supramolecular photocatalysts consists of systems without π-conjugation between azine ligands. The Pd and Pt binuclear complexes RuPd-7, RuPd-8, RuPt-12 and RuPt-13 (Figure 6) were studied in [53]; these complexes contain 2,2′:5′,3′′:6′′,2′′′-quaterpyridine as a bridging ligand. RuPd-8 with electron-withdrawing substituents demonstrated the best TON 489 in 18 h, which is higher than for the majority of Pd complexes with other bridging ligands. RuPt-14 complex featuring bis(1,10-phenanthroline) ligand (Figure 6) was found to be insufficiently robust under the reaction conditions, and the formation of hydrogen was mainly due to the Pt colloid formed during the decomposition of this complex.
The trinuclear complex RuRh-6 (Figure 6) with a flexible linker was studied in [54]. It was shown to be active in hydrogen photogeneration in a water solution and provided good TON. The covalent bonding of the photocatalyst with the Rh(III) complex provides not only a higher TON compared to mixed catalytic systems but also leads to better catalyst stability under the reaction conditions.
The work by Elias et al. [55] was dedicated to a comparison of the supramolecular catalysts IrCo-1–IrCo-6 (Figure 7) and the elucidation of the influence of the ligands’ nature in the photoactive Ir(III) complexes on the catalysts’ activity. The position of the linker in 2,2′-bipyridine core, providing better interaction of Ir(III) and Co(III) complexes on the catalyst efficacy, was also studied.
These complexes can be considered a clear example of the advantage of a supramolecular photocatalyst over a mixed catalytic system. Thus, under the same conditions, supramolecular photocatalysts IrCo-1–IrCo-4 provide 6–11 times higher TON and up to 2 times higher TOF (Figure 7) than the mixture of Co(III) complex and the corresponding photocatalyst. It was found that the catalyst’s activity depends on the wavelength of the irradiation. Blue light (452 nm) was efficient for IrCo-6 and IrCo-2 complexes, and yellow and red light provided best the TONs for IrCo-3 and IrCo-6, in spite of the fact that the absorbance of light is governed by the nature of the additional ligands and not of the bridging ligand. Authors suppose that the use of π-extended ligands allowed the reaction under irradiation with a red light of lower energy.
Recently, Rau et al., have compared RuRh-5, RuRh-7 and RuRh-8 complexes (Figure 8), differing by the structure of the bridging ligand. Photocatalytic formate-driven reduction of nicotinamide (3-pyridinecarboxamide, NAD) into NADH (1,4-dihydropyridine-3-carboxamide) was employed as a model reaction. A key step of the process was the two-electron reduction leading to the Rh(I) complex, which is provided by the photoinduced electron transfer from the Ru complex. For each of these supramolecular catalysts, similar TONs were observed when conducting the reaction under thermal conditions without irradiation. Under irradiation, the RuRh-8 catalyst showed the best activity at 25 °C, as well as at other temperatures, while RuRh-7 and RuRh-8 catalysts demonstrated similar TONs and TOFs at 45 °C for 90 min. A series of spectral investigations disclosed that the key feature of an efficient catalytic system is the high rate of the first electron transfer between photoactive and catalytic centers.
Thus, the above-mentioned examples clearly demonstrate wide opportunities for catalytic system optimization by selecting the structure of the bridging ligand depending on the chemical nature of the photoactive and catalytic centers, as well as peripheral ligands of these centers.

2.2. Heterobinuclear Complexes for Photocaytalytic CO2 Reduction

The reduction of CO2 into CO is a perspective process for diminishing CO2 content in the atmosphere and further use of carbon monoxide as a C1 synthone in organic transformations. The use of visible light is considered to be an ecologically friendly energy source. The general principle of CO2 photoreduction using supramolecular photocatalysts is shown in Scheme 3 [20]. The catalyst consists of the photosensitizer (PS) and metal complex catalyst (Cat). Under irradiation, the photosensitizer transforms into an excited state which can donate an electron to the catalyst (oxidative quenching) or accept electrons from the reductant (reductive quenching). The reductive quenching is also followed by the electron transfer to the catalyst. In the first way, the supramolecular catalyst can relax to the ground state via the next electron transfer, so the second way is considered to be preferable.
In the elaboration of photocatalysts for CO2 reduction, one needs to ensure the stability of the catalyst and high TONs, and on the other hand, a good selectivity of the catalyst is also demanded in order to prevent the formation of hydrogen, which is a side reaction in this process.
The following compounds are used as sacrificial electron donors in this transformation: sodium ascorbate, 1-benzyl-1,4-dihydronicotinamide (BNAH), 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) and triathanolamine (TEOA). The group of Kimura was the first to propose RuNi-1–RuNi-3 binuclear complexes (Figure 9) on the basis of photoactive Ru(II) complexes and Ni(II)-cyclam complex bound by a flexible linker for CO2 reduction [57,58,59]. The authors demonstrated better selectivity of the binuclear catalysts compared to mixed the catalytic system; however, their TONs were low. RuNi-4, RuCo-1 and RuCo-2 complexes containing bis(1,10-phenanthroline) bridging ligands were synthesized by Komatsuzaki et al. [60], but they also demonstrated low TONs and insufficient CO/H2 selectivity.
A veritable progress in the creation of supramolecular photocatalysts for the selective reduction of CO2 into CO was achieved by Ishitani et al. [29,31]. In 2005, they proposed supramolecular photocatalytic systems containing a Ru(II) photoactive complex and Re(I) complex [61]. Several catalytic systems, RuRe-1–RuRe-5, were studied, which differ by the linker type, orientation, and the position of substituents in the photoactive complex (Figure 10).
The RuRe-2 complex with a flexible linker and electron donor substituents was shown to provide the best activity and selectivity in this series, and only this complex outperforms other mixed catalytic systems in TON value. It was also disclosed that the combination of several Re(I) complexes and one Ru(II) complex in one molecule helps to further increase TON.
Successive studies had the goal of revealing optimal combinations of the ligands in the Re(I) complex, substituents in the Ru(II) complex and the structure of bridging ligands. The investigation of RuRe-6–RuRe-8 complexes (Figure 11) showed that an increase in the length of the linker between Re(I) and Ru(II) complexes from C2 (RuRe-6) to C4 and C6 led to a decrease in the catalytic activity of the complexes [62]. Bian et al., compared RuRe-6 and RuRe-9 complexes and demonstrated that with the same linker length, π-conjugation resulted in a twofold diminishing of TON [63]. Authors assumed that this was due to the lowering of the π* orbital energy of the Re(I) complex, which led to poorer catalytic activity. This fact is consistent with the results obtained earlier for RuRe-4 and RuRe-5 complexes which also contain π-conjugated bridging ligands [61]. Thus, in some cases, conjugation can disfavor the electron transfer.
The works [64,65,66] of Ishitani et al., studied in detail the processes of photoinduced electron transfer in RuRe-10–RuRe16 complexes (Figure 12) and their catalytic activity in the CO2 reduction reaction. The introduction of the electron-withdrawing oxygen in the structure of bridging ligand allows an increase in the catalytic activity of the RuRe-10 complex compared to RuRe-12 and RuRe-13 complexes. On contrary, sulfur-containing bridge led to a low photostability of the complex RuRe-11. The analysis of the electron transfer rate revealed its increase in the series RuRe-15 < RuRe-14 < RuRe-10 < RuRe-13 < RuRe-16. However, the highest TON was achieved for RuRe-10, while for RuRe-16 it was two times lower (253 and 123, respectively) [67,68]. It is obvious that the variation of the geometry of the bridging ligand allows for a fine-tuning of the electron interactions of photoactive and catalytic centers.
The catalytic activity of RuRe-17 and RuRe-18 (Figure 13) binuclear complexes with a short sulfur-containing linker in CO2 photoreduction was investigated in [69]. The TONs of these catalysts were humble compared to the above-mentioned complexes; nevertheless, the RuRe-18 catalyst with an electron-withdrawing SO2 linker, allowing photoinduced electron transfer from Ru(II) complex, showed superiority over mixed catalytic systems. Both catalysts were found to be unstable (as was RuRe-11 complex) due to C-S bond cleavage.
The binuclear complex RuRe-19 was synthesized using «chemistry on complex» approach via a classical CuAAC reaction [70]. The authors could not obtain TON better than that of the mixed catalytic system. They supposed that it was due to a delocalization of the first electron, which was produced by a photoexcited Ru(II) complex, at π* orbitals of the bridging ligand. Nevertheless, the two-electron reduction of Re was observed, which is helpful for further the development of such catalysts in other reactions.
Various possibilities of immobilization of binuclear complexes on solid supports are being investigated, both for the creation of photoelectrocatalytic systems and to increase catalysts’ efficiency [71,72]. For example, recently, a hybrid polymer RuRe-20 with tethered Ru(II) and Re(I) complexes has been described [73], which demonstrated different activity in CO2 reduction into CO depending on the ratio of Re and Ru in the polymer. The most active and best TONs were found for the polymer with an Re:Ru ratio of 1:19.
The advantage of supramolecular RuRe photocatalysts over a mixed catalytic system in the CO2 reduction can be clearly seen by comparing the activity of the RuRe-21 complex and a mixture of complexes Ru(4dmb)32+ and fac-Re(4dmb)(CO)3Cl (Figure 14). The binuclear complex provides a twofold higher TON, a 2.5-fold higher quantum yield of the reaction, as well as a higher selectivity of CO formation.
Ishitani et al., described supramolecular photocatalysts RuMn-1 and RuMn-2 (Figure 15) in which Re is substituted by a cheaper Mn [74]. The RuMn-1 complex is more active than RuMn-2 and the corresponding mixed catalytic system. Unlike with RuRe complexes, selectivity of CO2 reduction is lower, and the main product is HCOOH.
It is worth noting a significant progress during the last years in the chemistry of Re(I) complexes, covalently linked with the photosensitizers, on the basis of porphyrins [60,75,76,77,78,79,80,81,82]. These compounds demonstrated excellent activity in CO2 reduction but this chemistry is beyond the frames of the present review.

2.3. Heterobinuclear Photocatalysts in Organic Synthesis

The examples described above demonstrate a great variety of available bridging ligands and complexes on their basis, which opens wide the possibilities of fine-tuning the catalytic systems. The nature and rigidity/flexibility of bridging ligands and the distance between photoactive and catalytic centers provide means to direct the electron and energy transfer. RedOx potential of the photocatalyst can be tuned by the appropriate choice of the substituents in the additional ligands of Ru or Ir; it also allows optimization of the selectivity and rate of the catalytic process. Application of this experience for creating efficient catalysts for visible light-driven reactions seems to be very promising.

2.3.1. Dimerization of α-Methylstyrene

One of the first reactions of the photocatalytic C-C bond formation using binuclear photocatalysts is a selective radical-free coupling of α-methylstyrene proposed by Inagaki and Akita [83] (Scheme 4).
Binuclear complexes RuPd-9–RuPd-22 with 2,2-bipyrimidine derivatives as bridging ligands were studied as catalysts in this process (Figure 16).
Authors thoroughly studied the influence of the substituents in the bridging ligand and in 2,2′-bipyridine in the Ru(II) complex on the catalyst selectivity in view of either enhancing the yield of the dimer or promoting polymerization [83,84,85,86]. The introduction of electron donor substituents in 2,2′-bipyridine and electron-withdrawing substituents (such as Br) in the bridging ligand led to an increased activity of RuPd-9–RuPd-16 catalysts. The presence of four naphthyl substituents in the photocatalyst increases the lifetime of the excited state of the catalyst and enhances the polymerization rate (RuPd-20–RuPd-22 catalysts), while the RuPd-19 complex favors the formation of the dimer. The mechanism of the reaction was studied in details using DFT calculations [87] which supported the proposed reaction mechanism (Scheme 5). The Ru(II) complex acts as a light energy harvesting unit, providing the monomer intrusion in the chain and the elimination of the products from the complex. To note, without irradiation α-methylstyrene does not participate in this reaction.
The obtained data were used to develop the catalyst for photopolymerization IrPd-1 (Figure 16) [88]. This catalyst contains the Ir(III) complex as a photoactive moiety. It also provides radical-free polymerization, and it was studied in the copolymerization reaction of styrene with fluorinated vinyl ethers. Under irradiation, styrene participates in the catalytic polymerization while in the darkness polymerization of vinyl ethers takes place. Thus, by switching light on and off the authors succeeded in the controllable copolymerization of styrene with ethers.
Recently, the same group has proposed CuPd-1 and CuPd-2 photocatalysts (Figure 16) for polymerization employing the same principle [89]. Cu(I) complexes with 2,9-disubstituted 1,10-phenanthrolines are used as light-harvesting units which are thought to be a cheap alternative to Ru(II) complexes [90].

2.3.2. Other Pd-Catalyzed Reactions

Cross-coupling reactions catalyzed by metal complexes became a versatile and veritable method of organic synthesis for C-C bond formation [91,92,93,94]. Complexes and intermediates are transferred into excited state under irradiation, their reaction ability increases and this is employed for catalyzing various processes [12]. However, the reagents are often not able to efficiently absorb visible light which imposes limitations on this approach and demands the application of photosensitizers. In 2007, the photoactivation of Pd-catalyzed Cu-free Sonogashira coupling was demonstrated in the presence of the Ru(bpy)32+ complex [95]. It was shown later that the covalent binding of (Phen)PdCl2 with BODIPY chromophore results in a higher activity of the catalyst in the copper-free Sonogashira coupling [96].
The activity of RuPd-23 complex (Figure 17) containing 2,2-bipyrimidine as bridging ligand was studied in the Suzuki-Miyaura reaction [97] (Scheme 6). This binuclear complex showed TON two times higher than a mixed catalytic system (Ru(bpy)2(bpm)2+/Pd(bpy)Cl2) under visible light (Table 1, entries 1–4). In the absence of irradiation, no difference in activity was noted. In spite of low TON and humble product yields, the authors demonstrated the principal superiority of binuclear photocatalyst over the mixed catalytic system. The covalently linked photosensitizer is assumed to generate the Pd(0) complex which participates in the catalytic cycle.
Iridium-containing binuclear complexes IrPd-2–IrPd-4 (Figure 17) also were studied in the Suzuki–Miyaura reaction (Table 1, entries 6–8) [98], these complexes also comprise 2,2′-bipyrimidine as a bridging ligand. After optimizing the reaction conditions, the authors established that the best yields in the reaction were provided by the IrPd-4 complex with π-extended ligands. The authors also studied RuPd-23 catalyst under adjusted conditions and proved its ability to provide good yields comparable with those obtained with Ir complexes (Table 1, entry 5). The catalyst activity decreases twofold in the darkness (Table 1, entry 9), which supports the photocatalytic character of the reaction. The addition of the phosphine ligand improves the result probably by stabilizing the catalytically active Pd(0) complex. The authors also showed the lower yield of the mixed catalytic system (Table 1, entry 10).
The complex IrPd-4 was studied using 12 different substrates and provided good to high yields of the products (78–93%).
The authors did not study the reaction mechanism and catalytically active species in detail. The proposed mechanism is given in Scheme 7. It is assumed that the energy transfer from the excited iridium complex to palladium results in its reduction into Pd(0) which participates in the catalytic cycles. The catalyst in an excited state promotes oxidative addition and reductive elimination resulting in the activity enhancement under irradiation with visible light.
Binuclear complex IrPd-5 (Figure 17) contains iridium and palladacycle [99]. This catalyst was studied with a wide range of substrates (Table 1, Entries 11–15), the authors showed a decrease in the products yields by several times in the darkness as well as in the case of using mixed catalytic system (analogues of the photocatalyst and Pd complex). This catalyst was also studied in the Cu-free Sonogashira coupling (Scheme 8). Under optimized conditions, high yields of the products were obtained using a representative scope of substrates.
Without irradiation, the reaction was shown not to proceed. It is to be noted that the use of the mixed catalytic system Ir(ppy)2L/[Pd(ppy)Cl]2/PPh3 led to a quantitative reduction of ArBr into ArH under visible light. Thus, binding the photoactive component and the catalytic center in one complex gives possibility not only of increasing the catalyst activity but also of improving the reaction selectivity, changing its direction.
A recent work [100] describes metallaphotoredox catalyzed CH-allenylation of N-aryl tetrahydroisoquinolines in the presence of RuP-1/Pd(Oac)2, IrP-1/Pd(Oac)2 and IrP-2/Pd(Oac)2 catalytic systems (Scheme 9).
In this reaction Ru(II) or Ir(III) photoactive complex acts not as a photosensitizer like in the cross-coupling reactions but rather is employed in the generation of the radical from tetrahydroisoquinoline via single electron transfer (Scheme 10), thus, serving as a photoredox catalyst. This radical is trapped in the coordination sphere of palladium followed by the elimination of the acetate anion. After the second intramolecular single electron transfer from photocatalyst to palladium reductive elimination takes place affording the target product.
The authors studied in situ formed binuclear complexes, starting from RuP-1, IrP-1 and IrP-2 complexes as a result of Pd coordination to phosphine unit. In this case, bridging ligand is composed of the derivative of Sphos (dicyclohexyl (2′,6′-dimethoxybiphenyl-2-yl)phosphine) ligand with 2,2′-bipyridine moiety, which participates in the formation of the photoactive complex.
The authors demonstrated that mixed catalytic systems with Ru(bpy)32+, as well as with [Ir(ppy)2(dtbbpy)]+, afforded the target product as low as 38% yield and byproduct 40% yield (Scheme 11). Among binuclear catalytic systems, the best conversion and selectivity was shown by the RuP-1/[Pd] catalyst (71% yield of target product), while IrP-1/[Pd] was somewhat less efficient and IrP-2/[Pd] was the least active and selective in this reaction. The authors demonstrated a wide scope of the reaction, target compounds were obtained in 39–73% yields. Thus, the combination of the photoactive complex and metal catalyst increases not only the activity but also the selectivity of the catalytic system.

2.3.3. Oxidation Reactions

One of the important directions of photoredox catalysis development under visible light is the elaboration of the catalytic systems capable of selective oxidation of organic substrates under mild conditions at low catalyst loadings. Binuclear photoactive complexes have been extensively studied as photocatalysts for water splitting with oxygen production (so called artificial photosynthesis) [101,102,103]. The catalytic cycle of such oxidation processes is given in Scheme 12. Photoactive moiety of the complex in an excited state transmits an electron to an outer oxidant (or to anode) followed by intramolecular single electron transfer which leads to the oxidation of the metal complex catalytic center. Two such transfers produce an M=O oxo complex which can either release molecular oxygen or oxidize organic substrates.
Polynuclear complex Ru2Ru was studied as a photocatalyst for selective oxidation of benzylic alcohols into ketones [104] (Scheme 13). Co(III) salt is used as oxidant.
A photoactive moiety of Ru2Ru catalyst contains two Ru(tpy)2 complexes, and metal catalytic center features Ru(tpy)(bpy) complex (tpy = 2,2′,6′,2′′-terpyridine) with a coordinated water molecule (Figure 18). The authors could not achieve high yields of ketones although selectivity of oxidation was remarkable.
The binuclear complex RuFe with a flexible linker was described in [105]. Its photophysical characteristics were thoroughly studied and the formation of Fe(IV) oxocomplex was established, which can oxidize triphenylhposphine to phosphinoxide. The authors did not study the catalytic properties of this complex, but underline its good potential as a catalyst of oxidation processes.
An important reaction used in organic synthesis is selective oxidation of organic sulfides into corresponding sulfoxides. Due to mild reaction conditions and its high selectivity, photoredox catalysis seems to be the perspective for this process. The oxidation of sulfides into sulfoxides was described for a variety of homogeneous and heterogeneous photocatalysts [106]. Several mechanisms of this process were proposed [107,108], one of them assuming the generation of a singlet oxygen as a result of energy transfer from the photocatalyst in excited state to the molecule of oxygen. The second mechanism includes single electron transfer from the organic sulfide to photocatalyst. In the presence of copper complexes, the oxygen molecule is activated via coordination with Cu(I) which favors the oxidation of the organic substrate.
The binuclear complex RuCu-1 (Figure 19), in which photoactive Ru(II) complex is bound with Cu(I) complex via a flexible linker, was synthesized and studied in [109]. The authors supposed that this complex can catalyze the oxidation of organic sulfides into sulfoxides according to different mechanisms (Scheme 14), but the key step is inevitably the participation of dioxygen copper complex in the oxidation. The formation of such complexes needs the presence of the Cu(I) complex, which is generated in a series of SET between the Cu(II) complex and the photocatalyst using an external reducing agent.
The proposed catalytic system (RuCu-1) is characterized by high activity and selectivity in the oxidation of sulfides and triphenylphosphine (Scheme 15). Triethanolamine (TEOA) is used as an external reductant. Without photocatalyst, the copper complex was shown to be catalytically inactive and the activity of a mixed catalytic system is 4 times lower. Moreover, the results are essentially the same as those obtained with Ru(bpy)32+ without the copper complex. Thus, it is the covalent binding of the photoactive and metal complexes which provides high catalytic activity. The authors reached TON 300 at 60% conversion with 100% selectivity.
Aerobic oxidation of indene in the presence of RuCu-1 under analogous conditions led to a corresponding cis-diol in 37% yield with 100% conversion of starting indene. Oxidation of cyclohexene and cis-cyclooctene gave desirable α,β-unsaturated ketones in 57 and 42% yields, respectively, with 100% selectivity (Scheme 15). The authors propose this catalytic system as a promising alternative to classical oxidants.
Complex RuCu-2 (Figure 19) with a more rigid bridging ligand and electron-withdrawing substituents in the photoactive Ru(II) complex was described in [110] for the aerobic oxidation of organic sulfides. The majority of substrates provided high yields of the oxidation products (Scheme 16) in the presence of as low as 0.005 mol.% catalysts, which is equivalent to TON ca. 18,000, while in a special experiment TON was increased to an incredibly high 32,000. Despite the fact that the reaction time is significantly increased due to the low loading of the catalyst, it demonstrates a high stability of the catalyst under the reaction conditions.

2.3.4. Catalysis by Ni, Mn and Au Containing Complexes

One of the most important achievements in merging photoredox and transition metal catalysis is the possibility of employing cheaper catalysts on the basis of Ni complexes in various reactions of carbon–carbon and carbon–element bonds formation [11,111]. Among other reactions, it is the case of cross-coupling reactions of aryl halides with various nucleophiles. Single electron transfers between nickel complexes and photocatalysts allow to generate in the frames of one catalytic cycle both Ni(0), assuring oxidative addition, and Ni(III), providing efficient reductive elimination (Scheme 17). It would be quite expectable that the combination of Ni complex and photoredox active component in one compound increases the activity of the catalytic system.
In a recent work [112], a series of binuclear Ir-Ni complexes with different bridging ligands has been studied in the catalytic coupling of aryl and vinyl bromides (Scheme 18) with carboxylic acids. Binuclear complexes were generated in situ from IrN-1–IrN-5 complexes (Figure 20) and Ni(II) complex.
The authors have found that only the IrN-2/Ni(PPh3)2Cl2 catalytic system provides high yields of the coupling products with aryl and vinyl bromides (Scheme 18). It is important that the target compound can be obtained only with IrN-1/Ni(PPh3)2Cl2 or IrN-2/Ni(PPh3)2Cl2 systems, other complexes as well as mixture of complexes were inactive under the same reaction conditions. Thus, the presence of bridging ligand in the compound and its nature are important for the reaction to occur.
The same catalytic system was shown to be applicable to Ni-catalyzed reactions of C-S and C-N coupling (Scheme 19).
The binuclear complex RuMn-3 with 2,2′-bipyrimidine as a bridging ligand [113] was employed as a co-catalyst of the copper-catalyzed azide-alkyne [3+2] cycloaddition (Scheme 20).
The photoactive complex does not participate directly in the catalysis of this process but rather acts as a photoredox catalyst (Scheme 21). Under irradiation with visible light, the binuclear complex provides the reduction of Cu(II) into Cu(I) with triethylamine via several intra- and intermolecular single electron transfers, and Cu(I) catalyzes the reaction.
The reaction did not proceed without photocatalyst under described conditions. Moreover, the use of only the Mn complex or application of only the photocatalyst were inefficient, and mixed catalytic system Ru(bpy)32+/Mn(bpm)(CO)3Br provided only 46% yield in a model reaction (versus 96% in the case of RuMn-3). Thus, only binuclear complex provides required RedOx potential of the catalyst.
A novel catalytic system for the arylation of diethylphosphite with diazonium salts (Scheme 22) has been put forward recently in [114]. Isomeric binuclear complexes RuAu-1 and RuAu-2 were used as catalysts which were obtained according to different approaches (sequential complexation for RuAu-2 and “chemistry on complex” in the case of RuAu-1). Both complexes demonstrated similar catalytic activity.
The reaction proceeds according to a radical mechanism with the formation of aryl radical as a result of SET reduction of the diazonium salt (Scheme 23). Authors studied the influence of irradiation, nature of catalyst, and the covalent binding of Ru and Au complexes on the reaction yield.
The product was formed in all cases when the Au complex was present in the reaction mixture, the yield of the aryl phosphonate ranged from 33 to 64% due to an easy diazonium salt reduction into corresponding aryl radical. Nevertheless, the best yield (74%) was obtained under irradiation in the presence of RuAu-2 binuclear complex.

3. Conclusions and Prospect

This review covers the application of heterobinuclear complexes as photocatalysts in various reactions. Numerous investigations carried out up to date demonstrate that the covalent binding via the bridging ligand of the photoactive polypyridine complex of Ru(II) or Ir(III) with the catalytically active metal complex favors photoinduced electron or energy transfer between these two parts of the catalytic system. Thus, various heterobinuclear complexes were shown to be efficient for the photocatalytic reduction of CO2 and hydrogen photogeneration. Variations in the bridging ligand properties, such as flexibility, π-conjugation, and distance between two metal centers open way to improve these photocatalytic processes. During the last 10 years, metallaphotoredox catalysis, which combines catalysis by transition metal complexes and photoredox catalysis, was extensively developed. Now a great number of catalytic reactions of this type have been introduced to organic synthesis employing catalysts featuring the complexes of copper, nickel, gold, manganese, palladium, and other transition metals. The results discussed in this review demonstrate that the concept of covalent binding of the photoactive complex and metal catalytic center widely studied in H2 generation and CO2 reduction processes also can be successfully applied to organic synthesis as it substantially increase the catalyst efficiency and selectivity compared with mixed catalytic systems. However, the mechanisms of these catalytic transformations are not thoroughly studied and are yet to be elucidated. One may confidently state that further introduction of heterobinuclear photocatalysts in the area of metallaphotoredox catalysis on a wider scale will open new vistas for various reactions driven by visible light.

Author Contributions

Conceptualization, A.S.A. and I.P.B.; writing—original draft preparation, V.A.I. and A.S.A.; writing—review and editing, A.S.A., A.D.A. and I.P.B.; funding acquisition, A.S.A. and I.P.B. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Russian Science Foundation (grant no. 22-73-00031).

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 00768 sch001 550
Scheme 1. Schematic diagram of covalently bonded photocatalyst.
Scheme 1. Schematic diagram of covalently bonded photocatalyst.
Catalysts 13 00768 sch001
Catalysts 13 00768 g001 550
Figure 1. Commonly used polyazine bridging ligands.
Figure 1. Commonly used polyazine bridging ligands.
Catalysts 13 00768 g001
Catalysts 13 00768 sch002 550
Scheme 2. General mechanism of hydrogen photogeneration [34]. BL–bridging ligand.
Scheme 2. General mechanism of hydrogen photogeneration [34]. BL–bridging ligand.
Catalysts 13 00768 sch002
Catalysts 13 00768 g002 550
Figure 2. Ru-Pd and Ru-Pt complexes with 2,5-bis(pyridin-2-yl)pyrazine or 2,2′:5′,2′′-terpyridine for hydrogen photogeneration.
Figure 2. Ru-Pd and Ru-Pt complexes with 2,5-bis(pyridin-2-yl)pyrazine or 2,2′:5′,2′′-terpyridine for hydrogen photogeneration.
Catalysts 13 00768 g002
Catalysts 13 00768 g003 550
Figure 3. Heteropolynuclear 2,3-bis(pyridin-2-yl)pyrazine complexes for hydrogen photogeration.
Figure 3. Heteropolynuclear 2,3-bis(pyridin-2-yl)pyrazine complexes for hydrogen photogeration.
Catalysts 13 00768 g003
Catalysts 13 00768 g004 550
Figure 4. Heteronuclear Ru-Pt complexes with 2,3-bis(pyridin-2-yl)pyrazine and 2,3-bis(pyridin-2-yl)quinoxaline.
Figure 4. Heteronuclear Ru-Pt complexes with 2,3-bis(pyridin-2-yl)pyrazine and 2,3-bis(pyridin-2-yl)quinoxaline.
Catalysts 13 00768 g004
Catalysts 13 00768 g005 550
Figure 5. Examples catalysts based on tetrapyridophenazine and imidazophenanthroline.
Figure 5. Examples catalysts based on tetrapyridophenazine and imidazophenanthroline.
Catalysts 13 00768 g005
Catalysts 13 00768 g006 550
Figure 6. Catalysts based on linked 1,10-phenanthroline and 2,2′-bipyridine.
Figure 6. Catalysts based on linked 1,10-phenanthroline and 2,2′-bipyridine.
Catalysts 13 00768 g006
Catalysts 13 00768 g007 550
Figure 7. Heterobinuclear Ir-Co complexes based on terpyridine ligands and similar mixed catalytic system.
Figure 7. Heterobinuclear Ir-Co complexes based on terpyridine ligands and similar mixed catalytic system.
Catalysts 13 00768 g007
Catalysts 13 00768 g008 550
Figure 8. Heteronuclear Ru-Rh complexes based studied in [56].
Figure 8. Heteronuclear Ru-Rh complexes based studied in [56].
Catalysts 13 00768 g008
Catalysts 13 00768 sch003 550
Scheme 3. General mechanisms of CO2 photoreduction [29]. PS—Photosensitizer, Cat—Catalyst.
Scheme 3. General mechanisms of CO2 photoreduction [29]. PS—Photosensitizer, Cat—Catalyst.
Catalysts 13 00768 sch003
Catalysts 13 00768 g009 550
Figure 9. Ru-Ni and Ru-Co heteronuclear photocatalysts.
Figure 9. Ru-Ni and Ru-Co heteronuclear photocatalysts.
Catalysts 13 00768 g009
Catalysts 13 00768 g010 550
Figure 10. Ru-Re heteronuclear photocatalysts developed in [61].
Figure 10. Ru-Re heteronuclear photocatalysts developed in [61].
Catalysts 13 00768 g010
Catalysts 13 00768 g011 550
Figure 11. Ru-Re heteronuclear photocatalysts with various aliphatic linkers.
Figure 11. Ru-Re heteronuclear photocatalysts with various aliphatic linkers.
Catalysts 13 00768 g011
Catalysts 13 00768 g012 550
Figure 12. Ru-Re heteronuclear photocatalysts with various flexible linkers.
Figure 12. Ru-Re heteronuclear photocatalysts with various flexible linkers.
Catalysts 13 00768 g012
Catalysts 13 00768 g013 550
Figure 13. Ru-Re heteronuclear photocatalysts with various linkers.
Figure 13. Ru-Re heteronuclear photocatalysts with various linkers.
Catalysts 13 00768 g013
Catalysts 13 00768 g014 550
Figure 14. Comparison of the efficiency of the mixed catalytic system (Ru(4dmb)32+ + fac-Re(4dmb)(CO)3Cl) and binuclear catalyst RuRe21.
Figure 14. Comparison of the efficiency of the mixed catalytic system (Ru(4dmb)32+ + fac-Re(4dmb)(CO)3Cl) and binuclear catalyst RuRe21.
Catalysts 13 00768 g014
Catalysts 13 00768 g015 550
Figure 15. Ru-Mn heteronuclear catalysts for CO2 reduction.
Figure 15. Ru-Mn heteronuclear catalysts for CO2 reduction.
Catalysts 13 00768 g015
Catalysts 13 00768 sch004 550
Scheme 4. Photocatalytic dimerization of α-methylstyrene.
Scheme 4. Photocatalytic dimerization of α-methylstyrene.
Catalysts 13 00768 sch004
Catalysts 13 00768 g016 550
Figure 16. Binuclear photocatalysts for dimerization and polymerization of styrenes.
Figure 16. Binuclear photocatalysts for dimerization and polymerization of styrenes.
Catalysts 13 00768 g016
Catalysts 13 00768 sch005 550
Scheme 5. Proposed mechanism of photocatalytic dimerization of α-methylstyrene.
Scheme 5. Proposed mechanism of photocatalytic dimerization of α-methylstyrene.
Catalysts 13 00768 sch005
Catalysts 13 00768 g017 550
Figure 17. Binuclear photocatalysts for Suzuki-Miyaura and Sonogashira coupling.
Figure 17. Binuclear photocatalysts for Suzuki-Miyaura and Sonogashira coupling.
Catalysts 13 00768 g017
Catalysts 13 00768 sch006 550
Scheme 6. Photocatalytic Suzuki-Miyaura coupling.
Scheme 6. Photocatalytic Suzuki-Miyaura coupling.
Catalysts 13 00768 sch006
Catalysts 13 00768 sch007 550
Scheme 7. Proposed mechanism of photocatalytic Suzuki-Miyaura coupling.
Scheme 7. Proposed mechanism of photocatalytic Suzuki-Miyaura coupling.
Catalysts 13 00768 sch007
Catalysts 13 00768 sch008 550
Scheme 8. Photocatalytic Sonogashira coupling.
Scheme 8. Photocatalytic Sonogashira coupling.
Catalysts 13 00768 sch008
Catalysts 13 00768 sch009 550
Scheme 9. Photocatalyst-directed Pd-catalyzed allenylation of N-aryl tetrahydroisoquinolines.
Scheme 9. Photocatalyst-directed Pd-catalyzed allenylation of N-aryl tetrahydroisoquinolines.
Catalysts 13 00768 sch009
Catalysts 13 00768 sch010 550
Scheme 10. Proposed mechanism of photoredox-catalyzed allenylation.
Scheme 10. Proposed mechanism of photoredox-catalyzed allenylation.
Catalysts 13 00768 sch010
Catalysts 13 00768 sch011 550
Scheme 11. Comparison of the mixed catalytic system and supramolecular catalyst in photoredox-catalyzed allenylation.
Scheme 11. Comparison of the mixed catalytic system and supramolecular catalyst in photoredox-catalyzed allenylation.
Catalysts 13 00768 sch011
Catalysts 13 00768 sch012 550
Scheme 12. Mechanism of photocatalysed two-electron oxidation.
Scheme 12. Mechanism of photocatalysed two-electron oxidation.
Catalysts 13 00768 sch012
Catalysts 13 00768 sch013 550
Scheme 13. Photocatalysed alcohol oxidation.
Scheme 13. Photocatalysed alcohol oxidation.
Catalysts 13 00768 sch013
Catalysts 13 00768 g018 550
Figure 18. Ru-Ru and Ru-Fe photocatalysts for oxidation reactions.
Figure 18. Ru-Ru and Ru-Fe photocatalysts for oxidation reactions.
Catalysts 13 00768 g018
Catalysts 13 00768 g019 550
Figure 19. Binuclear photocatalysts for organic sulfides oxidation.
Figure 19. Binuclear photocatalysts for organic sulfides oxidation.
Catalysts 13 00768 g019
Catalysts 13 00768 sch014 550
Scheme 14. Mechanisms of organic sulfides oxidation photocatalyzed with binuclear complexes.
Scheme 14. Mechanisms of organic sulfides oxidation photocatalyzed with binuclear complexes.
Catalysts 13 00768 sch014
Catalysts 13 00768 sch015 550
Scheme 15. RuCu-1 in photocatalyzed oxidation reactions.
Scheme 15. RuCu-1 in photocatalyzed oxidation reactions.
Catalysts 13 00768 sch015
Catalysts 13 00768 sch016 550
Scheme 16. Oxidation of organic sulfides to sulfoxides with RuCu-2 catalyst.
Scheme 16. Oxidation of organic sulfides to sulfoxides with RuCu-2 catalyst.
Catalysts 13 00768 sch016
Catalysts 13 00768 sch017 550
Scheme 17. Proposed mechanism of Ni-photocatalyzed carbon–heteroatom coupling.
Scheme 17. Proposed mechanism of Ni-photocatalyzed carbon–heteroatom coupling.
Catalysts 13 00768 sch017
Catalysts 13 00768 g020 550
Figure 20. Ir-complexes for generation Ir-Ni-complexes in situ.
Figure 20. Ir-complexes for generation Ir-Ni-complexes in situ.
Catalysts 13 00768 g020
Catalysts 13 00768 sch018 550
Scheme 18. Cooperative Ir-Ni-photocatalysis for coupling of aryl- and vinyl bromides with carboxylic acids.
Scheme 18. Cooperative Ir-Ni-photocatalysis for coupling of aryl- and vinyl bromides with carboxylic acids.
Catalysts 13 00768 sch018
Catalysts 13 00768 sch019 550
Scheme 19. C-S and C-N coupling catalyzed with Ir-Ni binuclear complex.
Scheme 19. C-S and C-N coupling catalyzed with Ir-Ni binuclear complex.
Catalysts 13 00768 sch019
Catalysts 13 00768 sch020 550
Scheme 20. Visible light-assisted copper-catalyzed azide-alkyne [3+2] cycloaddition.
Scheme 20. Visible light-assisted copper-catalyzed azide-alkyne [3+2] cycloaddition.
Catalysts 13 00768 sch020
Catalysts 13 00768 sch021 550
Scheme 21. Proposed mechanism of visible-light-assisted CuAAC.
Scheme 21. Proposed mechanism of visible-light-assisted CuAAC.
Catalysts 13 00768 sch021
Catalysts 13 00768 sch022 550
Scheme 22. Arylation of diethylphosphite with diazonium salts catalyzed by heterobimetallic Ru-Au complexes.
Scheme 22. Arylation of diethylphosphite with diazonium salts catalyzed by heterobimetallic Ru-Au complexes.
Catalysts 13 00768 sch022
Catalysts 13 00768 sch023 550
Scheme 23. Proposed mechanism of metallaphotoredox-catalyzed arylation of phosphites with diazonium salts.
Scheme 23. Proposed mechanism of metallaphotoredox-catalyzed arylation of phosphites with diazonium salts.
Catalysts 13 00768 sch023
Table
Table 1. Photoaccelerated energy transfer catalysis of Suzuki–Miyaura coupling with binuclear complexes.
Table 1. Photoaccelerated energy transfer catalysis of Suzuki–Miyaura coupling with binuclear complexes.
EntryRR’ConditionsYield, %Reference
1HHRuPd-23/PPh3, (1/2 mol.%)
K2CO3, EtOH, r.t.,
visible light		10[97]
2HHRuPd-23/PPh3, (1/2 mol.%)
K2CO3, EtOH, r.t., dark		6[97]
3HHRu(bpy)2(bpm)2+/ Pd(bpy)Cl2/PPh3 (1/1/2 mol.%)
K2CO3, EtOH, r.t.,
visible light		6[97]
4HHPd(bpy)Cl2/PPh3 (1/2 mol.%)
K2CO3, EtOH, r.t.,
visible light		5[97]
5MeHRuPd-23/PPh3, (2.5/5 mol.%)
Cs2CO3, DCM-EtOH, r.t.,
blue LED		80[98]
6MeHIrPd-2/PPh3, (2.5/5 mol.%)
Cs2CO3, DCM-EtOH, r.t.,
blue LED		86[98]
7MeHIrPd-3/PPh3, (2.5/5 mol.%)
Cs2CO3, DCM-EtOH, r.t.,
blue LED		54[98]
8MeHIrPd-4/PPh3, (2.5/5 mol.%))
Cs2CO3, DCM-EtOH, r.t.,
blue LED		93[98]
9MeHIrPd-4/PPh3, (2.5/5 mol.%))
Cs2CO3, DCM-EtOH, r.t.,
dark		40[98]
10MeHIr(pq)2(bpy)+/Pd(bpm)Cl2/ PPh3
(2.5/2.5/5 mol.%)
Cs2CO3, DCM-EtOH, r.t.,
blue LED		75[98]
11MeC(O)MeIrPd-5/PPh3, (1/2 mol.%)
Cs2CO3, MeOH, 30 °C,
blue LED		99[99]
12MeC(O)MeIr(ppy)2L/[Pd(ppy)Cl]2/PPh3
(1/1/2 mol.%)
Cs2CO3, MeOH, 30 °C,
blue LED		35[99]
13MeMeIrPd-5/PPh3, (1/2 mol.%)
Cs2CO3, MeOH, 30 °C,
blue LED		98[99]
14CNMeIrPd-5/PPh3, (1/2 mol.%)
Cs2CO3, MeOH, 30 °C,
blue LED		98[99]
15OMeMeIrPd-5/PPh3, (1/2 mol.%)
Cs2CO3, MeOH, 30 °C,
blue LED		78[99]
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Ionova, V.A.; Abel, A.S.; Averin, A.D.; Beletskaya, I.P. Heterobinuclear Metallocomplexes as Photocatalysts in Organic Synthesis. Catalysts 2023, 13, 768. https://doi.org/10.3390/catal13040768

AMA Style

Ionova VA, Abel AS, Averin AD, Beletskaya IP. Heterobinuclear Metallocomplexes as Photocatalysts in Organic Synthesis. Catalysts. 2023; 13(4):768. https://doi.org/10.3390/catal13040768

Chicago/Turabian Style

Ionova, Violetta A., Anton S. Abel, Alexei D. Averin, and Irina P. Beletskaya. 2023. "Heterobinuclear Metallocomplexes as Photocatalysts in Organic Synthesis" Catalysts 13, no. 4: 768. https://doi.org/10.3390/catal13040768

APA Style

Ionova, V. A., Abel, A. S., Averin, A. D., & Beletskaya, I. P. (2023). Heterobinuclear Metallocomplexes as Photocatalysts in Organic Synthesis. Catalysts, 13(4), 768. https://doi.org/10.3390/catal13040768

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