Article pubs.acs.org/Organometallics
Phenylene- and Biphenylene-Bridged Bis-Imidazolylidenes of Palladium. Influence of the Presence of Pyrene Tags on the Catalytic Activity of the Complexes Sheila Ruiz-Botella and Eduardo Peris* Departamento de Quı ́mica Inorgánica y Orgánica, Universitat Jaume I, Av. Vicente Sos Baynat s/n, 12071 Castellón, Spain S Supporting Information *
ABSTRACT: A series of dimetallic complexes with N-heterocyclic carbene ligands with formally identical stereoelectronic properties have been obtained and fully characterized. The dimetallic complexes were bridged by bis-imidazolylidenes, with different spacers (phenylene and biphenylene). The N substituents were methyl or methylpyrene groups. The related monometallic complexes were also obtained. The catalytic properties of the complexes were tested in the acylation of aryl halides with hydrocinnamaldehyde and in the Suzuki−Miyaura coupling between aryl halides and arylboronic acids. In general, the dimetallic complexes display better activities than the monometallic analogues. The results also indicate that those complexes with pyrene groups at the N substituents display better catalytic activities than those with N-methyl groups. These observations are interpreted as a consequence of the π stacking interaction between the substrates and the pyrene groups in the pyrene-containing catalysts, which may provide some beneficial catalytic properties. The addition of a catalytic amount of pyrene to the Suzuki−Miyaura coupling reactions results in a partial inhibition of the activities of the complexes with the pyrene substituents, while the activity of the complexes with the N-methyl groups is not affected.
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INTRODUCTION N-heterocyclic carbene complexes have been recognized as being extraordinarily versatile ligands for the design of homogeneous catalysts.1 One of the major advantages of using this type of ligands is the easy access to their precursors (usually azolium salts), thus rendering an almost unlimited source of ligands with different topologies and electronic properties. One of the current research fronts in homogeneous catalysis is the combination of multiple catalytic sites into a single-frame ligand,2 because this type of system may induce cooperative effects, thus improving the activities and selectivities.3 On the other hand, polymetallic dendrimeric catalysts afford a higher nanolocal concentration of the active sites, and this sometimes leads to catalytic activities better than those shown by the monometallic analogue complexes (“dendrimer effect”).4 For the design of NHC-based multimetallic complexes, a very convenient strategy is to design polyNHC ligands in which the carbenes are geometrically isolated. A very effective example of this type of ligand is the facialopposed coordination provided by Janus-type bis-NHCs, such as those described by Bielawski and co-workers.5 We also contributed to the design of these types of ligands, by expanding the library of the family to new Janus-type bisNHCs, which allowed for the preparation of rigid polyaromatic bis-NHCs. The use of these ligands afforded a set of dimetallic complexes with variable metal-to-metal distances.6 Nonrigid poly-NHCs have also allowed the preparation of multimetallic © 2014 American Chemical Society
NHC complexes, when the carbenes are at a sufficient distance to avoid chelation.7 We have been recently interested in developing NHC-based multimetallic systems for the study of their catalytic properties.3e In a certain number of cases, we observed that the enhancement of the catalytic activities may be ascribed to π stacking interactions between the aromatic substrates and the polyaromatic ligands, rather than to the multimetallic nature of the catalysts.8 In order to shed some more light on this research, we now describe the preparation of a family of bisNHC ligands featuring different topologies. The aim of the work is to study how these small topological variations may influence the catalytic behavior of the resulting metal (in this case, palladium) complexes. The framework consists of a bisNHC bound by a phenylene (n = 1) or a biphenylene (n = 2) linker (Scheme 1). The N substituents at the NHCs are methyl and pyrene-methylene groups. These variations may allow us to determine the influence of the metal-to-metal distance on the reactivity and catalytic behavior of the complexes (by changing the spacers phenylene and biphenylene) and also the presence of a π stacking functionality (pyrene), in comparison to the situation in which a methyl group is present. The related monoNHCs have also been prepared, in order to evaluate the Received: July 25, 2014 Published: September 17, 2014 5509
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The DFT calculated structures for the two possible conformations of 2 with and without intramolecular π stacking of the pyrene groups reveal that the π stacked geometry is favored by 10.2 kcal/mol, in comparison to the situation in which the π-stacking interaction does not occur (Figure 1). This energy may be related to the energy of the π-stacking interaction in the gas phase. We believe that this noncovalent interaction may be playing a role in the stabilization of the complexes containing the bis-NHC ligands obtained from 2. The preparation of the dipalladium complexes was performed by reacting the bis-azolium salts 1−3 with palladium dichloride in 3-chloropyridine in the presence of K2CO3 (Scheme 3). The resulting products were purified by column
Scheme 1. Ligands Employed in This Work
influence of the presence of two metals in comparison to the situation in which only one metal is present.
Scheme 3. Preparation of Metal Complexes
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RESULTS AND DISCUSSION The N-Me-substituted bis-imidazolium salt 1 (Scheme 2a) was obtained according to the known literature procedure.9 The Scheme 2. Synthesis of Bis-Azolium Salts chromatography, yielding the pure compounds in yields ranging from 30 to 50%. Addition of KBr was needed in order to facilitate the preparation of the bromide-containing palladium complexes. Complexes 4−6 were characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. The NMR spectra of 4 and 6 indicate that the two metal centers are in geometrically equivalent environments. This observation is exemplified by the observance of only one resonance for the protons of the methylene linkers between the imidazolylidenes and two signals due to the protons of the backbone of the imidazolylidenes. The 13C NMR spectra show the characteristic signals due to the metalated carbene carbons at δ 147.5 and 147.6, for 4 and 6, respectively. Both the 1H and 13C NMR spectra of 5 suggest that the two metal complexes are in different coordination environments, as evidenced by the two 13 C NMR resonances for the metalated carbene carbons at 147.8 and 147.2 ppm. The molecular structure of complex 5 was unambiguously confirmed by means of X-ray diffraction studies. Figure 2 shows the molecular diagram of the complex. The molecule consists of
preparation of the pyrene-functionalized bis-azoliums 2 and 3 (Scheme 2b) was performed by reacting 1,4-bis(1H-imidazol-1yl)benzene or 4,4′-bis(1H-imidazol-1-yl)biphenyl with 1bromomethylpyrene in refluxing THF, in 82 and 94% yields, respectively. These new salts were characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. As stated above, the idea of using these bis-NHC precursors is to determine the influence of introducing small geometrical changes in the catalytic behavior of the related bis-palladium complexes. Additionally, the bis-azolium salts 2 and 3 were designed with the aim of using the π stacking abilities of the pyrene fragments to influence the interaction with the substrates, even intramolecularly. In this regard, it is interesting to point out that, while 3 is unable to intramolecularly connect its two pyrene functionalities, 2, with a shorter distance between the azolium rings, favors the intramolecular π stacking, giving rise to a “π-locked” structure, as depicted in Figure 1.
Figure 2. Molecular structure of compound 5. Hydrogen atoms and solvent (three molecules of CHCl3) have been omitted for clarity. Ellipsoids are at the 50% probability level. Selected bond distances (Å) and angles (deg): Pd(1)−C(1) 1.963(7), Pd(1)−Br(1) 2.4027(12), Pd(1)−Br(2) 2.4010(12), Pd(1)−N(3) 2.101(7); C(1)−Pd(1)−N(3) 179.3(3), C(1)−Pd(1)−Br(1) 88.7(2), C(1)−Pd(1)−Br(2) 90.5(2), N(3)−Pd(1)−Br(1) 90.6(2), N(3)−Pd(1)−Br(2) 90.1(2), Br(2)− Pd(1)−Br(1) 179.12(4).
Figure 1. Optimized structures of the π-unlocked and π-locked structures of the bis-azolium salt 2. Calculations were carried out with the M06L-DFT method. 5510
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intermolecular version of the process.8a,10,11 The reactions were carried out in DMF at 115 °C, in the presence of pyrrolidine, using a catalyst loading of 1 or 2 mol % with reference to the palladium content (0.5 or 1 mol %, respectively, with reference to the concentration of the molecules in the case of the dimetallic complexes 4−6). As can be seen from the comparative study of the activity of catalysts 4−8, it becomes evident that the pyrene-containaing catalyst 5 is the most active species under the reaction conditions used. This observation accounts for the better activity of 5 in comparison to its monometallic analogue 7 (compare Table 1, entries 1 and 4 and entries 7 and 10).
two palladium centers bound by a biphenylene-bridged bisimidazolylidene ligand. At this point it is important to mention that the two palladium centers are symmetry related, and therefore their coordination environments are identical, despite the observation of two different coordination environments evidenced by the solution NMR spectra. Each palladium atom completes its coordination sphere with two bromides in a trans conformation and a 3-chloropyridine. The relative orientation of the two palladium fragments with respect to the bis-NHC ligand is anti, with each palladium center located on one side of the plane defined by the biphenylene bridge. This conformation leads to a metal-to-metal through-space separation of 14.18 Å. The Pd−C(carbene) distance is 1.963(7) Å. The crystal packing of the molecule reveals the π−π stacking interaction between the pyrene groups of adjacent molecules (Figure 3).
Table 1. Acylation of Aryl Halides with Hydrocinnamaldehydea
Figure 3. One section of the crystal packing of complex 5, showing the π stacking between the pyrene groups. Hydrogen atoms and solvent are omitted for clarity.
In order to test the catalytic activities of the dipalladium complexes 4−6, we decided to study two palladium-catalyzed benchmark reactions, the acylation of aryl halides with hydrocinnamaldehyde and the Suzuki−Miyaura coupling between aryl halides and arylboronic acids. For comparative purposes, we also tested the activity of the monopalladium complexes 7 and 8 (Scheme 4), which formally display the
entry
cat.
Pd (%)
X
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14c
5 4 6 7 8 5 5 4 6 7 8 5 5 5
2 2 2 2 2 1 2 2 2 2 2 1 2 2
I I I I I I Br Br Br Br Br Br Cl Br
67 62 60 54 32 61 65 49 51 30 26 61 13 66
a
Conditions: molecular sieves (1 g), tetrabutylammonium bromide (16 mg, 0.05 mmol), catalyst (1−2%), aryl halide (0.5 mmol), hydrocinnamaldehyde (0.6 mmol), pyrrolidine (1 mmol), and DMF (2 mL) were placed in a 50 mL high-pressure Schlenk tube. The mixture was stirred and heated at 115 °C for 16 h. bYields calculated by GC using anisole as internal standard. cReaction carried out in the presence of a drop of mercury.
Scheme 4. Complexes 7 and 8
Complex 5 is also more active than the related pyrenecontaining dimetallic complex 4, with the shorter phenylene linker between the imidazolylidene ligand, and also more active than complex 6, with the N-Me group instead of the pyrene. These results indicate that some structural features regarding the conformation of the di-NHC ligand in 4 and 5 influence the activity of these two complexes in the reaction. It also becomes evident that the pyrene group enhances the activity of the complex, as can also be confirmed by comparing the activities of the monometallic complexes 7 and 8 (compare entries 4 and 5). We also performed a mercury drop experiment in order to discard the notion that the reaction could be heterogeneously catalyzed.12 As can be seen from a comparison of the results shown in the table (compare entries 7 and 14), the yields obtained do not vary upon addition of the drop of mercury, and therefore we believe that the heterogeneity of the process is unlikely. The same palladium complexes were tested in the Suzuki− Miyaura coupling between aryl halides and arylboronic acids. The results are given in Table 2. The reactions were carried out in toluene at 80 °C, in the presence of Cs2CO3. For all of the
same stereoelectronic properties as their dimetallic analogues. We believe that any differences in the catalytic behavior shown by the dimetallic complexes in comparison to that shown by the monometallic analogues should be ascribed to the dimetallic nature of the catalyst or to reasons related to the relative geometrical conformation of the dimetallic complexes, rather than to reasons related to the inherent stereoelectronic properties of the individual NHC ligands. The synthesis of 7 and 8 is similar to that described for the dimetallic complexes 4−6, and their full characterization may be found in the Experimental Section. The palladium-catalyzed acylation of aryl halides with aldehydes was first reported by Xiao and co-workers.10 Although this reaction is very interesting because it avoids the use of the traditional Friedel−Crafts method, a very small number of examples have been reported describing the 5511
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those with the methyl group, (ii) the dimetallic complexes show better activities than the related monometallic analogues, and (iii) between the two dimetallic complexes with the pyrene tag, that featuring a longer metal-to-metal distance (5) is the most active. If we assume that the stereoelectronic properties of the ligands present in 4−8 are similar, then the differences in the catalytic activities shown by their related palladium complexes should be ascribed to specific geometrical features or to ligandto-substrate interactions not related to the steric crowding or electron-donating character given by the ligand. For the better activities shown by the pyrene-containing complexes compared to those with methyl groups, we may think of a pyrene− substrate noncovalent interaction that may favor the interaction of the aromatic substrates with the metal, as we have previously suggested to justify the better catalytic performances of other palladium complexes with rigid polyaromatic NHC-based ligands.8 The better catalytic performance of 5 in comparison to that of 4 is intriguing, since we should have expected the reverse situation, considering the shorter metal-to-metal distance shown in the latter species. In principle, a shorter metal-to-metal distance is expected to favor the synergistic behavior of dimetallic catalysts.3a One plausible reason that may explain these significant differences in the catalytic performances of 4 and 5 may be the possibility that the phenylenebridged complex 4 gives rise to a “locked” structure with the two pyrene groups connected by π−π stacking interactions, as shown in Figure 1. For obvious geometrical restrictions, complex 5, with a longer biphenylene bridge, is unable to reach this closed situation. The intramolecular π−π stacking interaction should be favored over the intermolecular interaction at the low concentrations used in the catalytic experiments. In order to shed some light on these points, we decided to study the influence of the addition of pyrene to the Suzuki− Miyaura coupling reactions. As a π stacking additive, pyrene may have some influence on the catalytic outcome of the reaction, but this should only happen if π stacking effects play a role in the efficiency of the process. In order to have good comparative data, we performed the reactions under the same conditions used for the results shown in Table 2, but adding a catalytic amount of pyrene (10 mol % with respect to the initial amount of substrate). We also performed some experiments in which anthracene (10 mol %) was added, since we also considered anthracene as a suitable π-stacking additive. For all of the reactions that we carried out, we confirmed that pyrene (or anthracene) is not consumed or converted into any other product along the reaction course. As can be seen from the results shown in Table 3, the additions of pyrene or anthracenecene have different influences, depending on the nature of the catalyst used in the reactions. For all of the reactions catalyzed by pyrene-containing palladium complexes (4, 5, and 8), the addition of pyrene or anthracene consistently produced a decrease of the catalytic activity. This effect is more pronounced for the decrease of activity shown by the dimetallic complex 4, with the phenylene linker (compare entries 4 and 16 with entries 5, 6, and 17 in Table 3). Both of the complexes with N-methyl groups (6 and 7) were indifferent to the addition of pyrene, as seen by the same catalytic activities shown when the reactions were carried out in the presence and in the absence of pyrene. In order to confirm that all of these results are due to the higher catalytic performances of the pyrene-containing catalysts rather than any other factors, such as thermodynamic stability
Table 2. Suzuki−Miyaura Coupling between Aryl Halides and Arylboronic Acidsa
entry
cat.
X
R1
R2
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21c 22d 23e 24e
5 4 6 7 8 5 4 6 7 8 5 5 5 5 4 6 7 8 5 5 5 5 5 5
Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Cl Cl Cl
H H H H H H H H H H H COMe OMe OMe OMe OMe OMe OMe COMe Me Me H H OMe
Me Me Me Me Me H H H H H OMe Me Me H H H H H OMe H H Me Me Me
88 85 84 71 68 69 60 62 55 55 70 90 85 78 42 44 40 35 90 60 60 20 30 69
a
Reactions were carried out (unless specified otherwise) with aryl halide (0.5 mmol), arylboronic acid (0.5 mmol), Cs2CO3 (1 mmol), and [Pd] (2 mol %) in toluene (2 mL) at 80 °C for 2 h. bYields obtained by GC analysis using anisole as internal reference. cReaction carried out in the presence of a drop of mercury. dReaction carried out at 120 °C. eReaction carried out at 120 °C, 4 h.
reactions the catalyst loading was 2 mol %, based on the amount of the metal (1 mol % with reference to the concentration of the dimetallic molecules 4−6). As can be seen in the results displayed in Table 2, the dimetallic complex 5 is the most active catalyst among those that we tested. Complex 5 affords high activities in the coupling of a wide set of aryl bromides (yields in the range of 80−90%). As previously observed, 5 displays better activities than the related dipalladium complex 4, with the pyrene groups and with the phenylene linker. Also, 5 is more active than the related monometallic analogue 7, with the pyrene tag. In general, the activities of these complexes follow the trend observed for the acylation of aryl halides. Complex 5 is also moderately active in the coupling of aryl chlorides (see entries 22−24), although harsher reaction conditions were needed. In order to discard the notion that the process is heterogeneously catalyzed, we also performed an experiment using the mercury drop test, which did not show any significant differences in comparison to the experiment in the absence of the mercury drop (compare entries 20 and 21). As seen from the catalytic results shown in Tables 1 and 2, the activities of complexes 4−8 follow the same trend, from which the following conclusions can be inferred: (i) all complexes having a pyrene tag display better activities than 5512
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higher than that shown by 5 along all the reaction profile. In fact, the reaction reaches an 80% yield on the product after 20 min when catalyst 5 is used, while 1 h is needed to reach the same product yield when 6 is used. This result discards that the difference in activity between 5 and 6 may be ascribed to the different stabilities of the two catalysts. On the other hand, the addition of pyrene produces an important decrease in the activity of 5. This result is more obvious during the first 30 min of the reaction, after which the product yields are maintained lower, although just by 10−12%, indicating that the inhibiting character of pyrene is kinetic in nature. The interaction of the monometallic palladium complex 7 with pyrene was also studied by 1H NMR spectroscopy. We chose 7 because this monometallic complex provided a simpler 1 H NMR spectrum than their dimetallic analogues. The experiment was carried out by gradually adding pyrene (0− 10 equiv) to a CDCl3/CD3OD solution of 7. On comparison of the series of spectra, it is clearly observed that the signals due to the pyrene tag in 7 are significantly affected by the addition of external pyrene (all pyrene protons in 7 are shifted to lower frequencies), thus suggesting that a noncovalent interaction is produced between the pyrene tag and added pyrene.13 The selected region of the series of spectra is displayed in Figure 5, where it is clearly observed that only the signals attributed to the pyrene fragment of 7 are shifted, while the rest of the signals are unchanged.
Table 3. Suzuki−Miyaura Coupling between Phenyl Bromide and Two Arylboronic Acids, in the Presence and Absence of Pyrene and Anthracenea
entry
cat.
X
R1
R2
additive (mol %)
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
5 5 5 4 4 4 6 6 7 7 8 8 5 5 5 4 4 6 6 7 7 8 8
Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br Br
H H H H H H H H H H H H H H H H H H H H H H H
Me Me Me Me Me Me Me Me Me Me Me Me H H H H H H H H H H H
0 10 (pyrene) 10 (anthracene) 0 10 (pyrene) 10 (anthracene) 0 10 (pyrene) 0 10 (pyrene) 0 10 (pyrene) 0 10 (pyrene) 10 (anthracene) 0 10 (pyrene) 0 10 (pyrene) 0 10 (pyrene) 0 10 (pyrene)
88 80 80 85 57 55 84 85 71 59 68 68 69 53 60 60 30 62 60 55 52 55 55
a
Reactions carried out with phenyl bromide (0.5 mmol), aryl boronic acid (0.5 mmol), Cs2CO3 (1 mmol), and [Pd] (2 mol %) in toluene (2 mL) at 80 °C for 2 h. Values shown for the gray bars carried out in the presence of 0.05 mmol of pyrene (or anthracene). bYields obtained by GC analysis using anisole as internal reference.
of the catalysts, we decided to perform the reaction progress analysis of he Suzuki−Miyaura coupling of bromobenzene with 4-tolylboronic acid, using catalysts 5 and 6. We also monitored the reaction by using 5 in the presence of pyrene. The reactions were carried out using the same reaction conditions as those shown in Table 3. Figure 4 shows all three reaction profiles, where it is clearly confirmed that the catalytic activity of 5 is
Figure 5. 1H NMR spectra (300 MHz, CDCl3/CD3OD, 298 K): (a) complex 7; (b) 7 + 1 equiv of pyrene; (c) 7 + 2 equiv of pyrene; (d) 7 + 5 equiv of pyrene; (e) 7 + 7 equiv of pyrene.
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CONCLUSIONS In summary, we have prepared a series of dimetallic and monometallic complexes with N-heterocyclic carbene ligands with formally identical stereoelectronic properties. The dimetallic complexes were coordinated to bis-imidazolylidenes, with different spacers (phenylene and biphenylene), which aimed to modulated the metal-to-metal distances. The N substituents were methyl or methylpyrene groups, with the aim of studying the influence of introducing a polyaromatic hydrocarbon on the catalytic properties associated with the metal complex. The related monometallic complexes were also obtained, in order to assess the effects produced by the presence of one or two metals on the catalytic properties of the complexes. The catalytic properties of the complexes were tested in the acylation of aryl halides with hydrocinnamalde-
Figure 4. Time-course reaction profiles for the reaction of bromobenzene and 4-tolylboronic acid. Reaction conditions: phenyl bromide (0.5 mmol), 4-tolylboronic acid (0.5 mmol), Cs2CO3 (1 mmol), and [Pd] (2 mol %) in toluene (2 mL) at 80 °C. 5513
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The resulting white precipitate was collected by filtration and washed with diethyl ether. High-vacuum drying gave 2 as a white solid. Yield: 280 mg (82%). 1H NMR (500 MHz, DMSO-d6): δ 9.38 (s, 2H, NCHN), 8.44 (d, 3JHH = 9.2 Hz, 2H, CHpyrene), 8.38 (m, 6H, CHpyrene), 8.33 (d,3JHH = 9.2 Hz, 2H, CHpyrene), 8.25 (dd, 3JHH = 20.5, 8.9 Hz, 4H, CHpyrene), 8.14 (t, 3JHH = 8.2 Hz, 4H, CHpyrene), 7.88 (s, 2H, CHimidazole), 7.80 (s, 2H, CHPh), 7.39 (s, 4H, CHimidazole), 6.22 (s, 4H, CH2), 5.37 (s, 4H, CH2). 13C NMR (75 MHz, DMSO-d6): δ 136.9 (NCHN), 135.9 (Cxyly), 132.1 (Pyr), 131.2 (Pyr), 130.6 (Pyr), 129.3 (CHPh), 129.2 (Pyr), 128.8 (Pyr), 127.8 (Pyr), 127.6 (Pyr), 127.3 (Pyr), 126.6 (Pyr), 126.4 (Pyr), 125.8 (Pyr), 124.7 (Pyr), 124.2 (Pyr), 123.8 (Pyr), 123.3 (CHimidazole), 122.8 (CHimidazole), 52.0 (CH2), 50.7 (CH2). Anal. Calcd for C48H36N4Br2 (828.6342): C, 69.60; H, 4.37; N, 6.80. Found: C, C, 70.13; H, 3.90; N, 6.24. Although these results are slightly outside the range viewed as establishing analytical purity, they are provided to illustrate the best values obtained. Electrospray MS (cone 20 V) (m/z, fragment): 748.2 [M − Br]+. Dec pt: 234−236 °C. Synthesis of 3. A mixture of 4,4′-bis((1H-imidazol-1-yl)methyl)1,1′-biphenyl (209.1 mg, 0.66 mmol) and 1-(bromomethyl)pyrene (491.3 mg, 1.66 mmol) in THF (10 mL) was stirred at 75 °C for 24 h. The final suspension was cooled to room temperature, and diethyl ether (10 mL) was added. The resulting white precipitate was collected by filtration and washed with diethyl ether. High-vacuum drying gave 3 as a white solid. Yield: 500 mg (94%). 1H NMR (500 MHz, DMSO-d6): δ 9.44 (s, 2H, NCHN), 8.47 (d, 3JHH = 9.2 Hz, 2H, CHpyrene), 8.39 (m,6H, CHpyrene), 8.35 (d, 3JHH = 9.2 Hz, 2H, CHpyrene), 8.26 (dd, 3JHH = 18.0, 8.9 Hz, 4H, CHpyrene), 7.91 (s, 2, CH), 7.87 (s, 2H, CH2), 7.67 (d, 3JHH = 7.9 Hz, 4H, CHimidazole), 7.47 (d, 3 JHH = 7.9 Hz, 4H, CHimidazole), 6.26 (s,4H, CH2), 5.44 (s, 4H, CH2). 13 C NMR (75 MHz, DMSO-d6): δ 140.08 (NCHN), 136.7 (Cbiphenyl), 134.5 (Cbiphenyl), 131.8 (Pyr), 130.9 (Pyr), 130.4 (Pyr), 129.2 (CHbiphenyl), 129.1(CHbiphenyl), 128.9 (Pyr), 128.5 (Pyr), 128.4 (Pyr), 127.5 (Pyr), 127.4 (Pyr), 126.9 (Pyr), 126.3 (Pyr), 126.1 (Pyr), 125.5 (Pyr), 124.4 (Pyr), 123.9 (Pyr), 123.5 (Pyr), 123.1 (CHimidazole), 122.5 (CHimidazole), 51.9 (CH2), 50.5 (CH2). Anal.Calcd for C54H40N2Br (904.7302): C, 71.68; H, 4.46; N, 6.20. Found: C, 71.84; H, 4.40; N, 6.22. Electrospray MS (xone 20 V) (m/z, fragment): 372.3[M − 2Br]+. Dec pt: 255−258 °C. Synthesis of 4. A mixture of 2 (80.0 mg, 0.10 mmol), palladium(II) chloride (34.32 mg, 0.19 mmol), K2CO3 (80.41 mg, 0.58 mmol), and KBr (60 mg) in 3-chloropyridine (2 mL) was stirred at 100 °C for 6 h. The reaction was carried out under N2. The resulting suspension was cooled to room temperature, and the solvent was removed under vacuum. The crude product was purified by column chromatography. The pure compound 4 was eluted with dichloromethane/acetone (8/ 2) and precipitated in a dichloromethane/hexane mixture to give a yellow solid. Yield: 45 mg (30%). 1H NMR (300 MHz, CDCl3): δ 9.09 (s, 2H, CHpyrid), 8.97 (d, 3JHH = 4.2 Hz, 2H, CHpyrid), 8.53 (d, 3 JHH = 9.2 Hz, 2H, CHpyrene), 8.30−7.97 (m, 16H, CHpyrene), 7.62 (m, 2H, CHpyrid), 7.58 (s, 4H, CHPh), 7.24 (m, 1H, CHpyrid), 6.64 (s, 2H, CHimidazole), 6.50 (s, 1H, CH2), 6.45 (s, 3H, CH2), 6.35 (s, 2H, CHimidazole), 5.85 (s, 1H, CH2), 5.82 (s, 3H, CH2). 13C NMR (75 MHz, CDCl3): δ 151.8 (CHimidazole), 150.8 (CHimidazole), 147.5 (Pd− Ccarbene), 138.0 (CHpyrene), 135.8 (Cpyrid), 132.7 (Cpyrene), 132.3 (Cpyrene), 131.3 (Cpyrene), 130.92 (Cpyrene), 130.4 (CHpyrid), 130.0 (CHpyrene), 129.6 (CHpyrene), 129.2 (CHpyrene), 129.1 (CHpyrene), 128.4 (CHpyrene), 127.4 (Cpyrene), 127.2 (Cpyrene), 126.5 (CHpyrene), 125.9 (CHpyrene), 125.86 (CHpyrene), 125.30 (CHpyrene), 125.2 (Cpyrene), 125.08 (CHPh), 125.0 (CHPh), 124.7 (CPh), 123.6 (CHimidazole), 121.9 (CH imi dazol e ), 55.1 (CH 2 ), 53.9 (CH 2 ). Anal. Calcd for C58H42N6Br4Cl2Pd2CHCl3 (1563.7492): C, 45.31; H, 2.90; N, 5.37. Found: C, 44.80; H, 3.19; N, 5.18. Electrospray MS (cone 20 V) (m/z, fragment): 1346.8 [M − Br]+. Dec pt: 236−238 °C. Synthesis of 5. A mixture of 3 (80.0 mg, 0.089 mmol), palladium(II) chloride (35.5 mg, 0.2 mmol), K2CO3 (82.9 mg, 0.6 mmol), and KBr (60 mg) in 3-chloropyridine (2 mL) was stirred at 100 °C for 12 h. The reaction was carried out under N2. The resulting suspension was cooled to room temperature, and the solvent was removed under vacuum. The crude product was purified by column
hyde and in the Suzuki−Miyaura coupling between aryl halides and arylboronic acids. For these two types of reactions, the catalytic activities provided by the metal complexes follow a similar trend, in which (i) the dimetallic complex with the longer metal-to-metal distance and the pyrene substituents (5) displays the best efficiencies, (ii) the dimetallic complexes are more active than their monometallic analogues, and (iii) all complexes having a pyrene tag display better activities than the related complexes with a methyl group. We believe that the catalytic results shown by these complexes may be in part explained by considering the possibility that the pyrene substituents in the metal complexes play a role in the overall catalytic reaction, mainly due to their π stacking capabilities. In the case of the better catalytic activities provided by the biphenylene-linked complex 5, in comparison to the phenylene-linked complex 4, we tentatively propose that the shorter distance between the imidazolylidenes in 4 facilitates a situation in which the two pyrene fragments of the molecule give rise to a “π-locked” structure that increases the steric crowding about the palladium centers and therefore reduces its catalytic activity in comparison to the open situation shown by 5. For the better activities shown by the pyrenecontaining complexes in comparison to those having methyl groups, we believe that the π stacking between the aromatic substrates and the pyrene may facilitate the interaction between the substrate and the metal, and therefore the activity of the catalyst may be enhanced. This result is supported by the experimental evidence provided by the catalytic reactions carried out in the presence of catalytic amounts of pyrene, which produce the partial inhibition of the activity of the pyrene-containing palladium complexes, while the activity of the complexes with methyl substituents remains unchanged.
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EXPERIMENTAL SECTION
General Comments. All manipulations were carried out under nitrogen using standard Schlenk techniques and high vacuum. Anhydrous solvents were distilled from appropriate drying agents (SPS) and degassed prior to use by purging with dry nitrogen and kept over molecular sieves. All other reagents were used as received from commercial suppliers. NMR spectra were recorded on spectrometers operating at 300 or 500 MHz (1H NMR) and 75 and 125 MHz (13C NMR), respectively. Electrospray mass spectra (ESI-MS) were recorded employing nitrogen as drying and nebulizing gas. Synthesis and Characterization. Synthesis of 1. A mixture of 1methylimidazole (800 μL, 10 mmol) and 4,4′-bis(chloromethyl)-1,1′biphenyl (1.32 g, 5 mmol) in CH3CN (10 mL) was stirred at 90 °C for 24 h. The final suspension was cooled to room temperature, and the solvent was removed under vacuum. This solid was dissolved in methanol, and ammonium hexafluorophosphate (2.5 equiv) was added. The resulting white precipitate was subsequently washed with methanol and diethyl ether and collected by filtration. High-vacuum drying gave 1 as a white solid. Yield: 1.8 g (86%). 1H NMR (300 MHz, CD3OD): δ 9.12 (s, 2H, NCHN), 7.72 (s, 2H, CHPh), 7.69 (s, 2H,CHPh), 7.68 (t, 3JHH = 1.8 Hz, 2H, CHPh), 7.62 (t, 3JHH =1.7 Hz, 2H, CHPh), 7.57 (s, 2H, CHimidazole), 7.55 (s, 2H, CHimidazole), 5.50 (s, 4H, CH2), 3.96 (s, 6H, CH3). 13C NMR (75 MHz, CD3OD): δ 142.3 (NCHN), 138.0 (CPh), 134.8 (CPh), 130.4 (CHPh), 128.9 (CHPh), 125.3 (CH2 imidazole), 123.7(CH2 imidazole), 53.7 (CH2), 36.7 (CH3). Anal. Calcd for C22H22N4P2F12 (632.3652): C, 41.78; H, 3.5; N, 8.86. Found: C, 41.23; H, 3.72; N, 8.63. Electrospray MS (cone 20 V) (m/z, fragment): 172.4 [M − 2BF4]2+. Mp: 190−193 °C. Synthesis of 2. A mixture of 1,4-bis((1H-imidazol-1-yl)methyl)xylyl (500 mg, 2 mmol) and 1-(bromomethyl)pyrene (1.5 g, 5 mmol) in THF (10 mL) was stirred at 75 °C for 24 h. The final suspension was cooled to room temperature, and diethyl ether (10 mL) was added. 5514
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chromatography. The pure compound 5 was eluted with dichloromethane/ethyl acetate (1/1) and precipitated in a dichloromethane/ hexane mixture to give a yellow solid. Yield: 85 mg (50%). 1H NMR (500 MHz, CDCl3): δ 9.17 (s, 2H, CHpyrid), 8.99 (m, 2H, CHpyrid), 8.56 (d, 3JHH = 9.2 Hz, 2H, CHpyrene), 8.23 (t, 3JHH = 9.2 Hz, 8H, CHpyrene), 8.20−8.14 (m, 2H, CHpyrene), 8.14−8.08 (m, 4H, CHpyrene), 8.05 (t, 3JHH = 7.6 Hz, 2H, CHpyrene), 7.77 (m, 2H, CHpyrid), 7.60 (s, 8H, CHbiphenyl), 7.32 (m, 2H, CHpyrid), 6.63 (m, 2H, CHimidazole), 6.54 (s, 2H, CH2), 6.50 (s, 2H, CH2), 6.42 (m, 2H, CHimidazole), 5.94 (m, 2H, CH2), 5.89 (s, 2H, CH2). 13C NMR (75 MHz, CDCl3): δ 151.6 (CHpyrid), 151.1 (CHpyrid), 150.6 (CHpyrid), 150.0 (CHpyrid), 147.8 (Pd−Ccarbene), 147.2 (Pd−Ccarbene), 140.7 (CHpyrene), 138.0 (Cpyrid), 137.9 (Cpyrid), 134.4 (Cpyrene), 134.3 (Cpyrene), 132.6 (Cpyrene), 132.0 (Cpyrene), 131.1 (CHpyrene), 130.7 (CHpyrene), 129.8 (CHpyrene), 129.6 (CHpyrid), 129.5 (CHpyrid), 129.0 (CHpyrene), 128.9 (CHpyrene), 128.8 (CHpyrene), 128.1 (CHpyrene), 127.6 (Cpyrene), 127.2 (CHpyrene), 127.1 (CHbiphenyl), 127.0 (CHbiphenyl), 126.2 (Cpyrene), 125.7 (Cpyrene), 125.1 (Cbiphenyl), 125.0 (Cbiphenyl), 124.9 (CHpyrid), 124.8 (CHpyrid), 124.5 (CHpyrene), 123.3 (CHbiphenyl), 123.2 (CHbiphenyl), 121.7 (CHimidazole), 121.3 (CHimidazole), 54.9(CH2), 53.7 (CH2). Anal. Calcd for C64H48N6Cl2Br4Pd2 (1522.4834): C, 51.10; H, 3.22; N, 5.58. Found: C, 51.01; H, 2.82; N, 5.54. Dec pt: 225−227 °C. Synthesis of 6. A mixture of 6 (80 mg, 0.19 mmol), palladium(II) chloride (70.6 mg, 0.39 mmol), K2CO3 (161.7 mg, 1.16 mmol), and KBr (100 mg) in 3-chloropyridine (2 mL) was stirred at 100 °C for 8 h. The reaction was carried out under N2. The resulting suspension was cooled to room temperature, and the solvent was removed under vacuum. The crude product was purified by column chromatography. The pure compound 6 was eluted with dichloromethane/acetone (9/ 1) and precipitated in a dichloromethane/hexane mixture to give a yellow solid. Yield: 85 mg (41%). 1H NMR (300 MHz, CDCl3): δ 9.09 (d, 3JHH = 2.3 Hz, 2H, CHpyrid), 8.99 (dd, 3JHH = 5.5, 1.2 Hz, 2H, CHpyrid), 7.81−7.68 (m, 2H, CHpyrid), 7.59 (q, 3JHH = 8.5 Hz, 8H, CHbiphenyl), 7.30 (dd, 3JHH = 8.2, 5.5 Hz, 2H, CHpyrid), 6.92 (d, 3JHH = 2.0 Hz, 2H, CH), 6.76 (d, 3JHH = 2.0 Hz, 2H, CH), 5.80 (s, 4H, CH2), 4.14 (d, 3JHH = 9.5 Hz, 6H, CH3). 13C NMR (75 MHz, CDCl3): δ 151.8 (CHpyrid), 150.7 (CHpyrid), 147.6 (Pd−Ccarbene), 140.9 (Cpyrid), 138.1 (CHpyrid), 134.5 (Cbiphenyl), 132.7 (Cbiphenyl), 129.7 (CHbiphenyl), 127.7 (CHbiphenyl), 125.0 (CHpyrid), 123.9 (CHimidazole), 121.6 (CH i mi dazo le ), 54.8 (CH 2 ), 38.7 (CH 3 ). Anal. Calcd for C32H30N6Br4Cl2Pd2·2CHCl3 (1340.7381): C, 30.46; H, 2.41; N, 6.27. Found: C, 30.60; H, 2.81; N, 6.02. Electrospray MS (cone 20 V) (m/z, fragment): 907.8 [M − Br − C5H4NCl]+. Mp: 165−170 °C. Synthesis of 7. A mixture of 1-benzyl-3-(pyren-2-ylmethyl)-1Himidazol-3-ium bromide (80.0 mg, 0.18 mmol), palladium(II) chloride (32.2 mg, 0.18 mmol), K2CO3 (75.2 mg, 0.54 mmol), and KBr (60 mg) in 3-chloropyridine (2 mL) was stirred at 100 °C for 6 h. The reaction was carried out under N2. The resulting suspension was cooled to room temperature, and the solvent was removed under vacuum. The crude product was purified by column chromatography. The pure compound 8 was eluted with dichloromethane/acetone (9/ 1) and precipitated in a dichloromethane/hexane mixture to give a yellow solid. Yield: 83 mg (61%). 1H NMR (300 MHz, CDCl3): δ 9.16 (d, 3JHH = 2.3 Hz, 1H, CHpyrid), 9.09 (m, 1H, CHpyrid), 8.54 (d, 3 JHH = 9.3 Hz, 1H, CHpyrene), 8.23−8.02 (m, 8H, CHpyrene), 7.78 (s, 1H, CHpyrid), 7.75 (d, 3JHH = 1.3 Hz, 1H, CHpyrid), 7.54 (d, 3JHH = 6.5 Hz, 2H, CHPh), 7.44−7.28 (m, 4H, 3CHph and CHpyrid), 6.56 (d, 3JHH = 2.0 Hz, 1H, CHimidazole), 6.48 (s, 2H, CH2), 6.36 (d, 3JHH = 2.1 Hz, 1H, CHimidazole), 5.85 (s, 2H, CH2). 13C NMR (75 MHz, CDCl3): δ 151.9 (CH pyrid), 150.8 (CHpyrid ), 147.3 (Pd−Ccarbene), 138.1 (CHpyrene), 135.2 (CHpyrid), 132.8 (Cpyrene), 132.3 (Cpyrene), 131.3 (Cpyrene), 130.9 (Cpyrene), 130.1 (Cpyrene), 129.3 (CHpyrid), 129.2 (CHpyrene), 129.1 (CHpyrene), 129.1 (CHpyrene), 128.7 (CH pyrene), 128.4 (CHpyrene), 127.4 (CH pyrene), 127.2 (Cpyrene), 126.4 (CHpyrene), 125.9 (CHpyrene), 125.9 (CHpyrid), 125.2 (Cpyrene), 125.1 (CHPh), 125.0 (CHPh), 124.7 (CPh), 123.6 (CHPh), 121.8 (CHimidazole), 121.5 (CH i mi dazo le ), 55.5 (CH 2 ), 53.9 (CH 2 ). Anal. Calcd for C32H24N3Br2ClPd (752.2341): C, 51.10; H, 3.22; N, 5.58. Found: C, 51.93; H 2.60; N, 5.31. Although these results are slightly outside the range viewed as establishing analytical purity, they are provided to
illustrate the best values obtained. Electrospray MS (cone 20 V) (m/z, fragment): 672.1 [M − Br]+. Dec pt: 153−155 °C. Synthesis of 8. A mixture of 1-benzyl-3-methyl-1H-imidazol-3-ium bromide (80 mg, 0.31 mmol), palladium(II) chloride (56.3 mg, 0.31 mmol), K2CO3 (129.5 mg, 0.93 mmol), and KBr (100 mg) in 3chloropyridine (2 mL) was stirred at 100 °C for 8 h. The reaction was carried out under N2. The resulting suspension was cooled to room temperature, and the solvent was removed under vacuum. The compound 7 was washed with hexane and collected by filtration. Yield 85 mg (51%). 1H NMR (300 MHz, CDCl3): δ 9.09 (d, 3JHH = 2.2 Hz, 1H, CHpyrid), 8.99 (d, 3JHH = 5.4 Hz, 1H, CHpyrid), 7.76 (d, J = 8.2 Hz, 1H, CHpyrid), 7.50 (d, J = 6.3 Hz, 2H, CHPh), 7.38 (d, J = 7.0 Hz, 3H, CHPh), 7.34−7.29 (m, 1H, CHpyrid), 6.90 (s, 1H, CHimidazole), 6.70 (d, 3 JHH = 1.8 Hz, 1H, CHimidazole), 5.76 (s, 2H, CH2), 4.14 (d, 3JHH = 9.4 Hz, 3H, CH3). 13C NMR (75 MHz, CDCl3): δ 151.6 (CHpyrid), 150.6 (CHpyrid), 147.6 (Pd−Ccarbene), 137.9 (CHpyrid), 135.0 (Cpyrid), 132.6 (CPh), 129.1 (CHPh), 128.9 (CHPh), 128.5 (CHPh), 124.8 (CHpyrid), 123.6 (CHimidazole), 121.4 (CHimidazole), 55.0 (CH2), 38.5 (CH3). Anal. Calcd for C16H16N3Br2ClPd (551.9993): C, 34.81; H, 2.92; N, 7.61. Found: C, 34.7; H, 3.4; N, 7.1. Electrospray MS (cone 20 V) (m/z, fragment): 531.1 [M − Br − C5H4NCl]+. Mp: 145−150 °C. General Procedure for the Acylation of Aryl Halides with Hydrocinnamaldehyde. MS (1 g), tetrabutylammonium bromide (16 mg, 0.05 mmol), catalyst (1−2%), aryl halide (0.5 mmol), hydrocinnamaldehyde (0.6 mmol), pyrrolidine (1 mmol), and DMF (2 mL) were placed in a 50 mL high-pressure Schlenk tube. The mixture was stirred and heated at 115 °C for 16 h. The yields of the reactions were calculated by GC using anisole as internal standard. General Procedure for Palladium-Catalyzed Suzuki− Miyaura Coupling. In air, a Schlenk tube was charged with the catalyst (2%), aryl halide (0.5 mmol), the corresponding phenylboronic acid (0.6 mmol), and Cs2CO3 (0.326 g, 1 mmol). The Schlenk tube was sealed with a septum and purged with nitrogen three times. Toluene (2 mL) was added. The mixture was stirred and heated at 80 °C for 2 h. The yields of the reaction were calculated by GC using anisole as the internal standard. X-ray Diffraction Studies. Crystals suitable for an X-ray study of complex 6 were obtained by slow diffusion of hexane into a concentrated solution of the complex in dichloromethane. The crystal was kept at 200.05(10) K during data collection. Using Olex2,14 the structure was solved with the ShelXS15 structure solution program using direct methods and refined with the ShelXL15 refinement package using least-squares minimization. Crystal Data for C70H49Br4Cl19.5N6Pd2: Mr = 2197.86, triclinic, space group P1̅ (No. 2), a = 13.8715(3) Å, b = 13.9886(3) Å, c = 14.2318(3) Å, α = 64.449(2)°, β = 61.369(2)°, γ = 62.9117(19)°, V = 2076.20(9) Å3, Z = 1, T = 220 K, μ(Mo Kα) = 3.029 mm−1, Dcaldc =1.758 g/cm3, 41128 reflections measured (5.686° ≤ 2θ ≤ 51.978°), 8149 unique reflections (Rint = 0.0403, Rσ = 0.0255) which were used in all calculations. The final R1 value was 0.0807 (I > 2σ(I)), and the wR2 value was 0.2849 (all data). Computational Studies. The calculations were carried out in Gaussian 0916 using the M06-L functional,17 tight optimizations, and the ultrafine integration grid (a pruned (99590) grid), also using the TZVP basis set18,19 together with the density fitting basis set TZVPFit. The optimized geometries were verified to have no negative frequencies by frequency calculations, which also provided the reported enthalpies.
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ASSOCIATED CONTENT
S Supporting Information *
Text, figures, tables, and CIF and xyz files giving information regarding the preparation and characterization of the new complexes, including high resolution mass spectrometry, NMR spectra, and DFT atom coordinates of computationally studied species and X-ray crystallographic details. This material is available free of charge via the Internet at http://pubs.acs.org. 5515
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(13) (a) Zhang, W.-Y.; Han, Y.-F.; Weng, L.-H.; Jin, G.-X. Organometallics 2014, 33, 3091−3095. (b) Nagarajaprakash, R.; Divya, D.; Ramakrishna, B.; Manimaran, B. Organometallics 2014, 33, 1367−1373. (c) Mishra, A.; Jeong, Y. J.; Jo, J.-H.; Kang, S. C.; Kim, H.; Chi, K.-W. Organometallics 2014, 33, 1144−1151. (d) Vajpayee, V.; Song, Y. H.; Jung, Y. J.; Kang, S. C.; Kim, H.; Kim, I. S.; Wang, M.; Cook, T. R.; Stang, P. J.; Chi, K.-W. Dalton Trans. 2012, 41, 3046− 3052. (e) Mishra, A.; Jung, H.; Park, J. W.; Kim, H. K.; Kim, H.; Stang, P. J.; Chi, K.-W. Organometallics 2012, 31, 3519−3526. (14) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. J. Appl. Crystallogr. 2009, 42, 339−341. (15) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09; Gaussian, Inc., Wallingford, CT, 2009. (17) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 194101− 194118. (18) Schafer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829−5835. (19) Gusev, D. G. Organometallics 2013, 32, 4239−4243.
AUTHOR INFORMATION
Corresponding Author
*E.P.: e-mail,
[email protected]; fax, (+)34 964 387522. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Ministerio de Economia y Competitividad of Spain (CTQ2011-24055/ BQU). The authors are grateful to the Serveis Centrals d’Instrumentació Cientı ́fica (SCIC) of the Universitat Jaume I for providing us with all characterization techniques. We are very grateful to Prof. Dmitri Gusev from the Wilfrid Laurier University for the DFT calculations included in this work.
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