Bimetallic nanosized solids with acid and redox properties for catalytic activation of C–C and C–H bonds

A conceptually new formation of bimetallic nanosized solids with acid and redox properties for catalytic C–C and C–H activation is presented.


Introduction
Metal triimides M n+ Tf n [M: metal cation, Tf: À N(SO 2 CF 3 ) 2 ] are strong Lewis acids due to the high delocalization of the negative charge in the triimide anion, which confers high mobility to associated cations and protons. Contrary to other so acids with highly delocalized anions (i.e. tetrauoroborates and hex-aurometalates), triimide salts are relatively stable and safeto-handle 1 and, for instance, LiNTf 2 is commercially employed as an electrolyte in batteries. These properties make triimide salts powerful Lewis acid catalysts to activate C-C bonds and, if the redox properties of the metal cation are tuned with ligands, also very active catalysts to oxidize reluctant C-H bonds. 2 However, both acid/redox functions are typically mutually exclusive and their concomitant use in a single reaction is hampered. Besides that, triimide salts are expensive and difficult to recover when in solution, which limits their use basically to a laboratory scale. Thus, it is clear that the synthesis of a recoverable solid triimide catalyst, insoluble in common solvents, and with its acid/redox sites operative at the same time, would solve those problems. However, to our knowledge, no solid triimide has been reported yet. 3,4 Here we present a new preparation concept that allows the formation of a family of triimide solids with very strong acidity and readily available redox metal sites. The material consists of metal (Fe 3+ , Cu 2+ , Yb 3+ or Bi 3+ ) m-oxide triimides on Ag nanoparticles (Ag NPs) concomitantly formed under ambient conditions, aer adding thiophenol to a solution of the corresponding metal triimide and Ag + . The mechanism follows a simple redox-coupled sequence to furnish self-supported solids that behave as efficient acid, redox and acid/redox heterogeneous catalysts for different C-H and C-C activation reactions.

Results and discussion
2.1. Synthesis, characterization and structure of the solid Fe 2 O(NTf 2 ) 5 @AgNPs Fig. 1 shows the preparation of the solid material. The procedure consists of mixing Fe 3+ triimide [Fe(NTf 2 ) 3 ] with Ag tri-imide (AgNTf 2 , 0.1-1.0 equivalent) in 1,4-dioxane at room temperature, and then adding one equivalent of thiophenol (PhSH) at once. A so yellow solid precipitates in up to 90% yield in 2 gram scale for Ag : Fe molar ratio ¼ 0.5. Analysis using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) gives 5.8 wt% of Fe and 6.2 wt% of Ag for the solid prepared with an Ag : Fe molar ratio ¼ 0.5, which accounts for the amounts of Fe and Ag atoms initially added. Elemental analysis (CHNS) of the solid gives a sulfur-to-nitrogen ratio of 2 : 1, that corresponds exactly to triimide anions, and that in principle discards the presence of thiophenol in the structure. Analysis of the organic molecules present in the solid aer dissolution in HNTf 2 , extraction with diethyl ether and quantication using gas chromatography-mass spectrometry (GC-MS) with n-dodecane as an external standard, conrms the lack of thiols and shows the presence of $15 wt% of 1,4dioxane. Thermogravimetric (TG) analysis ( Fig. S1 in ESI †) conrms the amount of 1,4-dioxane present (16 wt%, loss at $100 C) and also shows that the material contains 4 wt% of water and two types of triimide anions, that desorb at $200 C (10 wt%) and 300 C (47 wt%). With all these data in hand, we can calculate the initial formula Ag 0.5 Fe(NTf 2 ) 2.5 $(dioxane) 2-3 $H 2 O 1-2 for the solid material. Fig. 2 shows transmission electron microscopy (TEM) photographs of the material (Ag loading 5.1 wt%, 0.5 equivalents of starting Ag) and well-dispersed, crystalline Ag NPs can be seen. Electron-dispersive X-ray (EDX) analysis indicates that the analyzed area of the NP is mainly Ag (98.5%), and mapping of Ag and Fe atoms in the whole micrograph shows that, while all Ag atoms are concentrated in the NP, Fe atoms are dispersed throughout the micrograph. On average, the material presents the expected molecular Ag : Fe mol ratio (0.5) and the photographs suggest that the Ag NPs are somehow embedded within a matrix of Fe.
The classical analytical test for Fe 2+ with ICl does not give any trace of Fe 2+ in the solid aer dissolution with HNTf 2 . In contrast, the analytical test with SnCl 2 gave Fe 3+ as the only iron species present in the solid. The diffuse reectance ultraviolet-visible (RD-UV vis) spectrum of the solid shown in Fig. 3 (top-le) conrms the absence of d-d transition bands for low spin Fe 2+ complexes, that should appear at around 600 nm. These results indicate that Fe 3+ is the main iron species present in the solid, with less than 0.1-0.01% of Fe 2+ . Notice that the RD-UV vis spectrum ts well with the sum of the individual absorption spectra of independently-prepared Fe(NTf 2 ) 3 and AgNPs (see ESI †). Fig. 3 (top-right) shows the Fourier transform-infrared (FT-IR) spectrum of the material with the presence of two different triimide peaks (see Table S1 in ESI † for a complete list of bands and discussion), in accordance with the TG analysis. The triimide anions are coordinated to Fe cations either through the oxygen or nitrogen atoms of the triimide groups. 5 The IR spectrum also reveals that a m-hydroxo or m-oxo bridge between two Fe atoms is present. 6 These results indicate that the solid is formed by oxo-bridged Fe 2 O(NTf 2 ) 5 species. Fig. 3 (middle) shows the 13 C nuclear magnetic resonancemagic angle spinning (NMR-MAS) spectrum of the solid, with the expected peaks for 1,4-dioxane ($69 ppm) and triimide ($120 ppm). Since no differentiation by 13 C NMR can be expected for different triimides, we prepared a sample of isotopically-labelled 15 N-solid, i.e. Fe 2 O( 15 N-Tf 2 ) 5 @AgNPs, by using Fe( 15 NTf 2 ) 3 and Ag 15 NTf 2 as starting metal salts during the synthetic procedure depicted in Fig. 1 (see Fig. S2 in ESI † for  details). The corresponding 15 N MAS-NMR spectrum in Fig. 3 shows two well-resolved 15 N NMR peaks, one main peak that corresponds to triimide anions bound to one Fe 3+ atom (À144 ppm), and a second peak upshied À63 ppm that integrates for 1 4 of the former. This result is in good agreement with the existence of two different triimide anions as indicated by FT-IR and TG measurements. Moreover, the 4 : 1 ratio ts with the m-hydroxo or m-oxo bridged Fe 2 O(NTf 2 ) 5 species.
To obtain direct evidence for the presence of the dimeric species, an exhaustive extraction of the material was performed in different solvents, and we found that treatment of the solid in hot diethyl ether for 3 days gives a solution that contains one single compound in amounts large enough to be analysed using electrospray ionization mass spectrometry (ESI-MS). The mass obtained for this compound is 1660.9 Daltons, as shown in Fig. 3, which can be assigned to . Matrix-assisted laserdesorption/ionization coupled to a time-of-ight mass spectrometer (MALDI-TOF-MS) conrms that no heavier species up to 40 000 m/z are present in this solution. These results indicate that the solid contains signicant amounts of Fe 2 O(NTf 2 ) 5 species.
Electronic paramagnetic resonance (EPR) measurements of the solid showed a lack of EPR signals for Fe in the solid (Fig. S3 in ESI †) which can only be explained by either very strong antiferromagnetic coupling between two Fe 3+ atoms, or because the Fe centers are low spin Fe 2+ . 7 The latter can be rejected since the material does not present any Fe 2+ according to the analytical tests and RD-UV vis spectroscopy (vide supra). Thus, the fact that the material is diamagnetic at room temperature (EPR silent and no alteration in the NMR shis) can only be explained by a strong antiferromagnetic coupling between two oxo-bridged Fe 3+ atoms. Indeed, a semi-quantitative empirical correlation gives a J ¼ 1.753 Â 10 12 e À12 : 663R, where R is half of the shortest exchange pathway, equivalent to the Fe-O distance, and agrees well with O-bridging between two Fe 3+ atoms. The existence of a m-hydroxo or m-oxo bridge between two Fe 3+ atoms explains the 5 triimides in 4 : 1 ratio observed in the TG, FT-IR and 15 N MAS-NMR techniques: 4 triimides bound to a single Fe atom (two per Fe atom) and the h one bridging the two Fe 3+ atoms, which close the hexagonal coordination sphere of each Fe 3+ in the dimer Fe 2 (NTf 2 ) 5 O(H), as shown in Fig. 4. Notice that, electronically, the Fe 3+ dimer with 5 triimides is compensated with the m-hydroxo bridge, however, a m-oxo bridge could also exist if the H + remains associated to 1,4dioxane or water molecules.
With all these data in hand, we can say that the structure of the yellow solid obtained is formed by well-dispersed Ag nanoparticles embedded in a matrix of O, triimide-dibridged Fe 3+ dimers, named hereaer as Fe 2 (NTf 2 ) 5 O@AgNPs and depicted in Fig. 1. The reasonable dispersion of Ag NPs in the solid must be related to the very fast (<1 min) formation of the material, which prevents further agglomeration, giving nally Ag NPs without the aid of any ligand or additional support. 8 2.2. Catalytic results of Fe 2 O(NTf 2 ) 5 @AgNPs 2.2.1. Acid-catalysed C-C activation. Taking into account the potential strong acidity of the solid, its catalytic activity was tested for reactions that require an acidity value H 0 < À12, i.e. reactions that can only be performed with concentrated H 2 SO 4 or stronger acids. The catalytic solid material was that directly obtained from the synthetic procedure described above (i.e. precipitated, washed in hexane and dried under vacuum), and no grinding or sieving was needed to achieve reproducible results. Fig. 5 shows the results obtained for the different acidcatalysed reactions.
The head-to-tail dimerization of styrenes (Fig. 5A) is a useful reaction to obtain branched styrenes in a 100% atom-economical manner, 9 and it has been catalyzed previously with, among others, Pd triate complexes, 10 a combination of metal complexes such as Pd/In triate/phosphines 11 or Co/Zn, 12 and with Fe(NTf 2 ) 3 , 13 all of them in homogenous solution. Thus, a simple solid catalyst for this reaction remains a challenge. Indeed, only the linear (not branched) dimerization of styrenes has been reported with a heterogeneous catalyst, i.e. Ru supported on CeO 2 , and using formaldehyde or ethanol as promoters to give a 63% yield of 1,4-diaryl-1-butenes. 14 Fe 2 (NTf 2 ) 5 O@AgNPs catalyzes the regioselective head-to-tail dimerization of styrenes 1a-c to 2a-c in yields up to 98%. The hot ltration test for styrene 1a showed that the catalytic activity completely stops when the solid catalyst is ltered off, which conrms the heterogeneous nature of the catalysis.  The Markovnikov hydration of alkynes (Fig. 5B) was earlier catalysed by Hg salts at the industrial level 15 and, due to toxicity issues, alternative catalysts based on transition metals are currently being explored, mainly gold. [16][17][18] It is difficult to nd efficient catalysts that operate under mild conditions beyond noble metals, 7 and systems based on Brönsted acids, either homogeneous or heterogeneous, require high wt% loadings, harsh reaction conditions (>150 C) or very particular modications of the catalytic system (additives, singular reaction media such as microemulsions, and surface modications in solids). 19-21 Fe 2 O(NTf 2 ) 5 @AgNPs catalyzes regioselectively the hydration of alkynes 3a-d to ketones 4a-d without any additive in up to 97% yield.
The addition of methyl acetoacetate to styrenes (Fig. 5C) was originally reported with Fe complexes in stoichiometric amounts 22 and further developed with noble metal catalysts, 23 until Fe could be employed catalytically. 24 It is not easy to nd a heterogeneous catalyst for this reaction. Fe 2 O(NTf 2 ) 5 @AgNPs is able to catalyze the reaction between styrene 1c and methyl acetoacetate 5 to give the product 6 in 90% yield.
The hydrodeoxygenation reaction (Fig. 5D) is of interest in the biorening industry to obtain alkanes from oxygen-rich biomass derived chemicals. 25 For that, a catalytic combination of a soluble very acidic metal salt, such as a triate, and a solid hydrogenation catalyst, such as Pt/C, is employed. 26 Thus, it would be desirable to have both functions on a solid, either as a single solid or as a composite. A catalytic composite of Pt on TiO 2 /C has been reported 27 and operates at temperatures of $300 C. Here, a composite of Fe 2 O(NTf 2 ) 5 @AgNPs and Pt/C catalyzes the hydrodeoxygenation of cyclohexanol 7 to cyclohexane 8 in 98% yield at 150 C.
In summary, Fe 2 O(NTf 2 ) 5 @AgNPs catalyzes the synthetically useful, acid strength-demanding reactions shown above in very good yields, with similar catalytic activity to the state-of-the-art soluble catalysts (Table S2 †). For the sake of comparison, the dimerization of styrene 1a and the hydration of phenylacetylene 3a were also carried out with representative strong solid acid catalysts such as Naon™, 28 H-USY zeolite, 29 and sulfated zirconia, 30 under the indicated conditions (Fig. S4 †). The results showed that only Naon™ improves on the catalytic activity of Fe 2 (NTf 2 ) 5 O@AgNPs, while H-USY and sulfated zirconia are much poorer acid catalysts, which places Fe 2 -O(NTf 2 ) 5 @AgNPs among the most effective solid acid catalysts reported.

Redox-catalysed C-H activation.
Selective oxidation of C-H bonds with environmentally benign reagents such as O 2 or H 2 O 2 under mild reaction conditions is one of the main challenges in organic synthesis. 2 Nature makes use of enzymes with non-heme O-bridged bisiron centers, 31 and successful biomimetic lines of research have been reported with O-bridged bisiron complex catalysts having low-coordinating ligands for C-H activation and C-C cleavage. [32][33][34][35] Given the resemblance between these adducts 32,33 and the Fe 3+ dimer present in Fe 2 -O(NTf 2 ) 5 @AgNPs (see Fig. 4 above), we tested Fe 2 O(NTf 2 ) 5 @-AgNPs as a catalyst for the oxidation of alkanes under similar conditions to those reported, but without the need of adding AcOH to activate the iron catalyst. 34,35 The results in Fig. 6 show that Fe 2 O(NTf 2 ) 5 @AgNPs converts linear and cyclic alkanes to a mixture of alcohols and ketones 9a-d, in reasonable conversions aer a few minutes at room temperature, with good selectivity towards sterically accessible methylene groups. The solid catalyst is suitable not only for methylene oxidation but also for the Baeyer-Villiger oxidation of cyclic ketones to the corresponding cyclic esters 10a-c, 36 under the same reaction conditions.

Acid/redox-catalysed C-C and C-H activation.
The performance of redox catalysis in acidic media gives access to otherwise elusive reactions. Since Fe 2 O(NTf 2 ) 5 @AgNPs is operative in acid and redox reactions, separately, the catalytic activity of Fe 2 O(NTf 2 ) 5 @AgNPs was tested for reactions that  require both functions present (see Table S2 † for comparison with state-of-the-art catalysts). The results are shown in Fig. 7.
The Markovnikov hydrothiolation of styrenes is catalyzed by a Fe 3+ /Fe 2+ manifold mechanism with the participation of protons in the redox catalytic cycle. 37 The results obtained (Fig. 7A) show that Fe 2 O(NTf 2 ) 5 @AgNPs is as active as the stateof-the-art soluble catalysts, 38 with good yields for the representative products 12a-d. Besides, the solid catalyst could be reused up to 9 times aer simple ltration, without a signicant loss of its inherent catalytic activity (Fig. S5 †).
The selective demethylation of N,N-dimethylanilines is a biomimetic reaction catalyzed by iron complexes in the presence of strong oxidants, such as PhIO. 39 Nature makes use of O 2 as the terminal oxidant, thus the synthetic use of O 2 as an environmentally-friendly oxidant for this reaction would be a signicant advance. Recently, it has been reported that this reaction can moderately occur under aerobic conditions ($40% combined yield with trimers and formamides) if a metal triate salt, i.e. Zn 2+ , Ba 2+ or Y 3+ , is used in combination with the Fe 3+ complex to form a triate bridge between the two metal cations. 40 It was envisioned that the solid triimide composed of Fe dimers could mimic this system and catalyze the demethylation of N,N-dimethylanilines under similar aerobic conditions (Fig. 7B). Indeed, the reaction worked well with Fe 2 O(NTf 2 ) 5 @AgNPs, with similar yields and better selectivity for the products than with the soluble metal complex since, presumably, the higher acidity of the triimide solid overrides undesired biphenyl formation by the non-acidic aryl/Fe electron transfer. 40 The possibility of engaging the hydration of the aryl alkyl alkyne 15 and the methylene oxidation reaction in cascade 41 was also tested, with Fe 2 O(NTf 2 ) 5 @AgNPs as a single catalyst for both processes (Fig. 7C). The result shows that the solid tri-imide indeed preserves its redox catalytic activity aer acting as an acid catalyst, and the 1,5-diketone 16 is thus formed in one-pot. Notice that the regioselective formation of the rst carbonyl group by acid-catalyzed hydration directs the later oxidation of the methylene group.
2.2.4. Making the Ag NPs accessible to reactants. When the solid with an Ag : Fe ratio ¼ 0.5 was tested for reactions typically catalyzed by Ag NPs of $10 nm, including the epoxidation of styrene, 42 the aerobic dehydrogenation of alcohols 43 and the synthesis of azocompounds from anilines (see Fig. S6 in ESI †), 44 no conversion was found for any of these reactions. Blank experiments with independently synthesized Ag NPs of $10 nm showed the expected reactivity for the Ag catalyst. [42][43][44] Therefore, the lack of catalytic activity of the Ag nanoparticles in the Fe 2 -O(NTf 2 ) 5 @AgNPs solid suggests that the contact between Ag NPs and the reactants may be hampered in some way. To further assess if the Ag NPs in the solid were accessible to the reactant molecules, in situ low-temperature FT-IR experiments with carbon monoxide (CO), as a probe molecule, were carried out (Fig. S7 in ESI †). The results show that there is no interaction between CO and Ag 0 or Ag d+ atoms, 45 which unequivocally indicates that the Ag atoms in the solid are not accessible to external chemicals. The lack of interaction between CO and Fe 3+ was expected due to the low-coordinating nature of the triimide anions and reects the high acidity of the Fe 3+ atoms in the solid. Considering the amount and size of Ag NPs in the material for Ag : Fe ratios # 0.5 and the number of Fe atoms, a simple calculation shows that the number of Fe dimers is at least 10 4 higher than the number of Ag nanoparticles. Thus, there are enough Fe 2 O(NTf 2 ) 5 molecules to embed all the NPs. In fact, only 5% of Fe dimers are enough to cover the whole surface of the Ag NPs. It appears then that the Fe 3+ matrix acts as a chemical shield for the Ag NPs, conferring them protection against air, water or any other external atmosphere. For instance, the material (Ag : Fe mol ratio ¼ 0.5) is stable aer heating at 200 C for 1 day under an H 2 atmosphere of 10 bars and no agglomeration of Ag NPs was observed aer this treatment.
However, it should be in principle possible to make accessible the Ag NPs in the solid by just increasing the Ag to Fe 3+ ratio, in such a way that the Fe 3+ triimide dimers will not completely cover the Ag NPs. Fig. 8 shows that, indeed, the solid material has clearly visible $10 nm Ag NPs on the surface when the synthesis is performed with an Ag : Fe mol ratio ¼ 1. The resultant material with accessible Ag NPs is active for the classical Ag-catalyzed reaction of aniline to give azobenzene. 44 The solid catalyst now gives a 20% conversion with a turnover number (TOF) relative to the exposed Ag atoms of 100 (see Fig. S6 in ESI †). Comparatively, the Ag : Fe mol ratio ¼ 0.5 solid is completely inactive and commercially available Ag/C gives only 7% conversion. These results conrm that the accessibility of the reactants to the Ag NPs in the solid material can be regulated by varying the relative amount of metals during the synthesis.
In summary, the results shown in Section 2.2 provide a picture of the rather unique acid/redox catalytic behavior of the Fe 2 O(NTf 2 ) 5 @AgNPs solid.

Mechanism of formation of Fe 2 O(NTf 2 ) 5 @AgNPs
A possible mechanism for the formation of the Fe 2 O(NTf 2 ) 5 @ AgNPs solid is depicted in Fig. 9. This mechanism is supported by reactivity tests and isotopic experiments, and it consists of the one-electron reduction of Fe(NTf 2 ) 3 to Fe(NTf 2 ) 2 by PhSH, forming a coordinatively unsaturated Fe 2+ species that re- oxidizes at expense of reducing Ag + to Ag NPs or, alternatively, with air.
The one-electron reduction of Fe(NTf 2 ) 3 to Fe(NTf 2 ) is supported by the quantitative transformation of the one-electron reductant PhSH to diphenyl disulde Ph 2 S 2 , observed using GC-MS and liquid NMR measurements of the solution, when PhSH is added to a mixture of Fe(NTf 2 ) 3 and AgNTf 2 , to form the yellow solid. In contrast, if PhSH is added to a solution of Fe(NTf 2 ) 3 without AgNTf 2 present in the medium, the solid does not form despite Ph 2 S 2 being quantitatively produced. Furthermore, if PhSH is added to a solution of AgNTf 2 without Fe(NTf 2 ) 3 present, no reaction occurs. These results indicate that the one-electron reduction occurs to form Fe 2+ , in agreement with the well-known single electron transfer (SET) process between PhSH and Fe 3+ salts at room temperature. 37 Other potential reductants for Fe 3+ such as isopropanol and Ce 3+ failed to form the solid.
The yield of solid decreases signicantly if an excess of PhSH with respect to metal triimide is employed during the synthesis (90%, 65%, and 28% for 1, 3 and 10 equivalents of PhSH, respectively), due to the formation of stable Ag + -thiolate complexes. When Au, Pd and Pt salts were tested instead of Ag no solid formation occurred. These results, in principle, discard the possibility that thiophenol acts as a "seed" for nanoparticle formation, and the SET transfer reaction seems to be predominant in the mechanism.
The second step in the mechanism of formation of the bimetallic solid is a redox reaction between the in situ formed Fe 2+ and Ag + , to give Fe 3+ and Ag NPs, as expected from their respective redox potentials. Accordingly, the pH of the solution is made very acidic (<1) by the released triimidic acid, and the protons may stay either coordinated to the oxygen atoms of the O-bridges or to the dioxane molecules.
The third and last step is the formation of the m-oxo or m-hydroxo bridge between two Fe 3+ atoms. To shed light on this step, we used an atmosphere of 18 O 2 , and the corresponding FT-IR spectrum of the yellow solid showed the formation of an Fe-18 O-Fe bridge according to the slight shi of the corresponding IR band with respect to the non-labelled yellow solid (see Fig. 3). 6 Additionally, we found that H 2 18  O observed using GC-MS. These results support that the oxygen atom for the O-bridged Fe 3+ dimer comes from water molecules present in the medium. If so, the bridge should also be produced under an inert atmosphere provided that water is in the medium. Indeed, a 65% yield of solid was obtained under N 2 when 0.5 equivalents of Ag were used. It must be noticed, however, that the formation of the bridge is less efficient in the absence of O 2 since a 90% yield is obtained when the process is run aerobically, which is explained by the better oxidation of Fe 2+ to Fe 3+ when molecular oxygen is present.
All the steps described above occur in less than 1 minute at room temperature, and the formation of a coordinatively unsaturated Fe(NTf 2 ) 2 species seems to play a key role in the formation of the solid, accommodating the bridging NTf 2 and O species. In agreement with this, when a sample of independently prepared Fe 2+ (NTf 2 ) 2 was used as the starting material to form the solid, 13 with the whole hexacoordination sphere saturated (2 bridging triimides and 4 bisoxo-coordinated terminal triimides, 2 per Fe 2+ atom), no solid was formed. This result evidences the need for in situ reducing the Fe 3+ triimide.

Extension of the work to other metallic systems: synthesis of Cu, Bi and Yb-triimide@AgNPs
According to the mechanism in Fig. 9, it might be expected that other metal cations could participate in the proposed redox sequence, as long as the corresponding metal tri-imide can suffer the SET reduction with PhSH and the generated reduced metal cation is re-oxidized back with O 2 and/or Ag + . If so, a family of ligand-free, self-supported bimetallic solids could be available following the preparation method described here. Aer examining the redox potentials of tabulated metal cations and testing those suitable, we found that the triimides of Cu 2+ , Yb 3+ and Bi 3+ were able to form the corresponding solids in moderate to good yields (84% for copper, 75% for bismuth and 63% for ytterbium) under the same reaction conditions as for Fe 2 O(NTf 2 ) 5 @ AgNPs. Other cations such as Ru 3+ or Co 3+ failed to produce the corresponding solids. Fig. 10 shows the characterization of the solids using FT-IR and TEM. The results suggest that, in principle, these solids have a similar structure to Fe 2 O(NTf 2 ) 5 @AgNPs, i.e. they are formed by well-dispersed Ag NPs embedded within a metal triimide matrix. In the case of Cu, the Ag NPs ourish to the surface (see le microphotograph in Fig. 10).
The Cu and Bi materials were tested as catalysts in the vinylation of 1,3-diphenylpropargyl alcohol [46][47][48] and in the hydrothiolation of styrenes, 37 respectively. Fig. 11 shows that products 18 and 12d were obtained in good yields, similar to those previously reported.  9 Proposed mechanism for the formation of Fe 2 O(NTf 2 ) 5 @AgNPs. Fig. 10 Top: FT-IR spectra of the solids made by the same procedure as Fe 2 O(NTf 2 ) 5 @AgNPs but with Cu(NTf 2 ) 2 , Bi(NTf 2 ) 3 and Yb(NTf 2 ) 3 as starting materials, from left to right, respectively. Bottom: transmission electron microscopy (TEM) photographs with the corresponding histograms for, at least, 5 different photos.