Divergent Synthesis of Porous Tetraphenylmethane Dendrimers - The

Article pubs.acs.org/joc

Divergent Synthesis of Porous Tetraphenylmethane Dendrimers Julio I. Urzúa† and Mercedes Torneiro* Departamento de Química Orgánica, Facultade de Química, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain S Supporting Information *

ABSTRACT: Tetraphenylmethane-ethynylene-based shape-persistent dendrimers are a new class of nanoobjects with an intriguing 3D architecture. We report an efficient divergent strategy for their synthesis based on the Sonogashira Pd-catalyzed coupling of terminal alkynes with aryl iodides. As repeat unit, we prepared a tetraphenylmethane derivative bearing a terminal alkyne and three triazene moieties. Coupling of this building block to tetrakis(p-iodophenyl)methane afforded, after triazene activation, a dodecaiodo-terminated first generation dendrimer, which was transformed by another Sonogashira coupling into a methoxy-terminated second generation dendrimer with persistent globular shape and well-defined cavities. This work also unveils new aspects of triazene chemistry, i.e., the unprecedented efficient generation of an azo compound by mixing of a triazene with phenol.

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quantum yield.12 The 2G dendrimer (1, Figure 1a,b), containing 68 aromatic rings, has a protein-like 3D persistent globular structure with a hydrophobic interior, a polar surface, and well-defined inner cavities. The molecular architectures of these dendrimers are highly modular and, therefore, suitable to variations at the core and periphery and also at the skeleton by introducing other linear linkers to enlarge or functionalize the cavities. By virtue of their privileged spatial architecture and versatility, tetraphenylmethane shape-persistent dendrimers, which are soluble and surface-functionalizable analogs of microporous polymers (Figure 1c),5c can have potential applications as scaffolds for gas separation and storage, nanomachines, artificial catalytic centers, sensors, and multichromophoric luminescent materials, among others. As part of a broader research program on porous materials, we report the first divergent synthesis of ethynylene-bridged tetraphenylmethane shape-persistent dendrimers, with the aim of providing alternative synthetic routes that can be tailored for the modification of the compounds for different applications. For the iterative bonding of tetraphenylmethane building blocks through ethynylene linkers we have relied on the powerful Sonogashira Pd-catalyzed halide-alkyne coupling chemistry, amply demonstrated for a variety of oligomeric and macrocyclic phenylene ethynylene systems,13 as for the convergent aproach.12

INTRODUCTION Rigid tetrahedral molecular scaffolds have found increasing applications as building blocks for advanced functional materials.1 Tetraphenylmethane is a synthetically accessible and simple tetrahedral structure that has been widely used, particularly in the field of porous materials. The aromatic rings scale the geometry of the sp2 central carbon to give a threedimensional radially stiff structure whose utility to build porous materials was demonstrated for crystalline reticular MOFs2 and COFs,3 for molecular cages4 and amorphous microporous networks such as hyper-cross-linked polymers and related materials.5 Dendrimers are ideally the molecularly perfect monodisperse versions of hyper-cross-linked polymers.6 Considered in many aspects as protein mimics, their applications are well-documented.6b,c,7 Nevertheless, dendrimers lack the 3D structural precision of proteins. Common dendrimers are very flexible and exhibit a densely packed interior with backfolding of terminal groups as described by the dense core model.8 Whereas numerous shape-persistent π-conjugated dendrimers have been described,9,10 the use of rigid 3D building blocks to control tighter the topology of dendrimers both at local and global levels is an underdeveloped area of research. Recently, we and others have developed shape-persistent dendrimers with tetrahedral nodes, synthesized by convergent approaches. Garcia-Garibay et al. described a second generation (2G) dendrimer made of trityl groups linked by ethynylene spacers around a phenylene core and studied its behavior as a molecular rotor in the solid state to find fast rotational motion of all 25 aromatic rings as a consequence of the low packing density in the crystalline structure or crystal fluidity.11 Our laboratory has reported dendrimers made of tetraphenylmethane units bonded by ethynylene linkers, which showed intrinsic fluorescence with a narrow emission band and high © 2017 American Chemical Society

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RESULTS AND DISCUSSION The divergent route starts from the core, and we had to decide if the corresponding building block was bearing the alkyne or the halide functionality. The main side-reaction in the standard Sonogashira coupling is the Glaser-type dimerization of the Received: September 12, 2017 Published: November 14, 2017 13231

DOI: 10.1021/acs.joc.7b02302 J. Org. Chem. 2017, 82, 13231−13238

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The Journal of Organic Chemistry

Figure 1. Structure of dendrimer 1 (a), its 3D model (b), and analogous microporous polymers (c).

Scheme 1. Divergent Route to Dendrimer 1

precursors, need to be carried out in the growing dendron throughout the whole synthesis. Another advantage is that the potential Glasser side-products would arise from dimerization of small molecules (3 or the terminal tetraphenylmethane units), what would facilitate its separation from the macromolecular dendrimers. The first stage in the divergent synthesis of 1 was the preparation of the branching unit 3. In the retrosynthetic analysis (Scheme 2) we planned to prepare this building block from a (pseudo)halide 5 using Pd-coupling chemistry to install the alkyne moiety. As a suitable starting material to access 5 we

alkyne counterpart; therefore, it seemed convenient to avoid the presence throughout the synthesis of compounds with multiple terminal alkyne functionalities, which could give rise to many cross-linked side-products. Taking also into account the superior reactivity of iodides in comparison to other (pseudo)halides, we selected as starting material the readily available tetraiodide 214 (Scheme 1). This led us to install the alkyne moiety at the focal point of the branching unit 3, which also bears three triazene functional groups. Aryl triazenes are effective precursors of iodoarenes that have been successfully used in the synthesis of several arylacetylene systems.15 Therefore, we envisaged that coupling 4 equiv of 3 to the core 2 would give, after triazene replacement, the iodoterminated first generation (1G) dendrimer 4 (Scheme 1). This compound could itself work as substrate of a second Sonogashira coupling with 3 or other ethynyltetraphenylmethane derivatives to give 2G dendrimers, such as 1, selected as model compound for developing the divergent methodology. This route is very attractive for the preparation of dendrimers with different peripheries, as the building block bearing the terminal functionality is introduced in the last step. In the convergent route the diversification at the periphery is not straightforward because the terminal groups, or their

Scheme 2. Retrosynthesis of Building Block 3

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The Journal of Organic Chemistry

yield as the azo compound 11, which was also prepared in low yield from the triamine 6 using a standard procedure. To the best of our knowledge, the diazenylation of phenolic compounds with triazenes is unprecedented. Examples of azo transfer with aryl triazenes are scarce and related to the acidinduced intramolecular rearrangement of diaryl triazenes to aminoaryl azoderivatives,21 intramolecular triazene-alkyne coupling,22 or the recently reported intermolecular diazenylation of β-dicarbonyl compounds23 and of N-heterocyclic compounds.24 A plausible and simple mechanism for the transformation of 9 to 11 involves the decomposition of the triazene groups to diazonium salts induced by phenol acting as a Brönsted acid, followed by diazo coupling with phenol or the resulting phenoxide ion. The excellent yield of the reaction is noteworthy, specially taking into account that this process must occur three times per molecule. Next, we carried out the Friedel−Crafts reaction with 11, by treatment with phenol and H2SO4 as catalyst, to obtain the tetraphenylmethane derivative 12 in 90% yield. The transformation of 9 into 12 was also carried out without isolation of 11 with the same yield of the two-step process (82%). In the next step, we cleaved the azo moieties of 12 by hydrogenolysis to give the triamino tetraphenylmethane derivative 8 in 75% yield. Therefore, the 3-step route from 6 via 9 and 12 is an effective pathway to 8. We proceeded with the synthesis by reinstalling the three triazene groups in 8 using standard conditions to give the phenolic compound 13 (28%), that was transformed into the triflate 5a in good yield by treatment with Tf2O and Et3N (Scheme 5). Then, the Sonogashira coupling of 5a with trimethylsilylacetylene using Pd(PPh3)4, PPh3, and CuI in piperidine, in the presence of 4 Å molecular sieves, afforded the protected branched unit 14 (87%), which was desilylated in basic media to furnish the key building block 3 almost quantitatively. With the branching unit 3 in hand, we prepared the core unit 2 by a slightly modified known procedure,14 to be ready to address the synthesis of the first generation dendrimers. The Pd-catalyzed coupling of 2 and 3 proceeded smoothly using the same conditions as above to furnish in 72% yield the desired triazene-terminated 1G dendrimer 15 as a very polar compound that was purified by gel filtration. Triazene to iodide conversion under standard conditions (MeI, 120 °C) afforded the poorly soluble iodo-terminated 1G dendrimer 4 (68%). Dendrimers 15 and 4 were characterized by 1H and 13C NMR. Whereas in our previous studies we found that the analogous methoxy-terminated 1G and 2G dendrimers could be conveniently characterized by MALDI-TOF MS,12 the triazene and iodo-terminal functionalities seem to hamper the ability of 15 and 4 to fly under MALDI experiments. Finally, dendrimer growth was accomplished by Sonogashira reaction of 4 with the terminal unit 16, in our hands from previous studies.12 Using the same Pd catalytic system as in the preceding couplings we obtained the target fluorescent 2G dendrimer 1 as a white solid in 77% yield after chromatographic purification. Its Rf, GPC retention time, NMR, and MS data were in full concordance to those of the compound previously obtained by convergent synthesis.12 No side-products (e.g., partially coupled compounds bearing terminal iodo or triazene groups) were detected by careful examination of the spectra. In summary, we have developed a practical divergent strategy for the synthesis of shape-persistent tetraphenylmethane dendrimers, soluble counterparts of microporous polymers. During the course of this synthesis we discovered an

identified the triamine 6, lacking one aromatic ring which could be introduced by Friedel−Crafts reaction, a standard method for synthesis of tetraphenylmethane derivatives. However, our attempts to carry out the Friedel−Crafts reaction of phenol with 6 were unsuccessful (see the SI for failed approaches toward 5). Therefore, we decided to prepare intermediate 8 by amination of the known triiodide 7, readily prepared from 616 (Scheme 3). We performed the amination of Scheme 3. Synthesis of Intermediate 8

7 by the method of Wu and Darcel,17 with ammonia in the presence of Fe2O3, CuI, and NaOH, that gave the desired compound 8 in 21% yield after 8 days.18 The poor yield in the last transformation prompted us to assay an alternative synthetic route. We considered the possibility of installing first the triazene functional groups in 6 and then carrying out the acid-catalyzed Friedel−Crafts reaction with phenol to build up the tetraphenylmethane skeleton, although we were aware of the high risk of triazene decomposition in the acidic reaction media. The tris(triazene) 9 was prepared in good yield from 6 via the diazonium salt that was trapped with piperidine (Scheme 4) and, alternatively, from Scheme 4. Alternative Syntheses of 8

the known triazene 10,19 by treatment with nBuLi, and subsequent reaction of the resulting organolithium species with dimethylcarbonate (93% yield).20 To our surprise, when we dissolved the tris(triazene) 9 in phenol without any added acid we observed by TLC analysis the gradual formation of three products of increasing polarity. After 3 h at 80 °C, only the more polar one was observed, identified upon isolation in 92% 13233


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The Journal of Organic Chemistry Scheme 5. Completion of the Synthesis of Dendrimer 1

(DEPT) was used to assign carbon types. Low resolution mass spectra were acquired on a Hewlett-Packard HP5988A electron impact (EI) quadrupole mass spectrometer. High resolution mass spectra were acquired on an Autospec Micromass electron impact (EI) spectrometer. MALDI-TOF mass spectra were recorded on a Bruker Ultraflex III TOF/TOF spectrometer using dithranol/AgTFA as the matrix. Analytical-scale SEC (GPC) was performed at room temperature in a PL-GPC 50 Integrated GPC/SEC System (Agilent) with a PLgel 3 μm MIXED-E (100−30000 Da) column calibrated by polystyrene standards and a differential refractometer as detector, with THF as eluent (1.0 mL/min). No unexpected or unusually high safety hazards were encountered during the experimental work. 4-(tris(4-Iodophenyl)methyl)phenol (7).16 A solution of 6 (500 mg, 1.64 mmol) in a mixture of H2SO4 (4 mL, 96%) and H2O (10 mL) was cooled to 0 °C. After 10 min a solution of NaNO2 (3.8 mL, 3.0 M) was added, and the mixture was stirred at 0 °C during 1 h. A solution of KI (11 mL, 3.0 M) was added, and the reaction mixture was allowed to reach rt, and then it was warmed at 80 °C during 2 h. The mixture was cooled to rt and extracted with EtOAc (20 mL). The organic phase was washed with a saturated solution of Na2S2O3 (3 × 10 mL), dried, and concentrated, and the residue was purified by flash chromatography (5% EtOAc/hexanes) to afford tris(4-iodophenyl)methanol16 as a yellowish solid [1.2 g, 1.88 mmol, 57%, Rf = 0.34 (10% CH2Cl2/hexanes)]. 1H NMR (CDCl3, 250 MHz): 7.61 (6H, d, J = 8.4 Hz, Ar), 6.96 (6H, d, J = 8.4 Hz, Ar), 2.75 (1H, s, OH). 13C NMR (CDCl3, 63 MHz): 145.3 (C), 137.1 (CH), 129.6 (CH), 93.6 (C), 81.1 (C). A mixture of tris(4-iodophenyl)methanol (600 mg, 0.94 mmol) and phenol (1.33 g, 14.1 mmol) was heated at 80 °C until obtaining an homogeneous mixture. Then H2SO4 (8 drops) was added, and the mixture was heated at 80 °C for 5 h. A solution of NaOH (20 mL, 10%) was added, and the mixture was extracted with EtOAc (3 × 30 mL). The combined organic phase was dried and concentrated, and the residue was purified by flash chromatography (10% EtOAc/hexanes) to give 716 as a yellowish solid [614 mg, 0.86 mmol, 91%, Rf= 0.47 (20% EtOAc/hexanes), mp 264−268 °C]. 1H NMR (CDCl3, 250 MHz): 7.57 (6H, d, J = 8.5 Hz, ArI), 6.97 (2H, d, J = 8.7 Hz, ArOH), 6.90 (6H, d, J = 8.6 Hz, ArI), 6.72 (2H, d, J = 8.7 Hz, ArOH), 5.00 (1H, broad s, OH). 13C NMR (CDCl3, 63 MHz):

unprecedented reaction of a triazene compound, i.e., the diazenylation of phenol with the tris(triazene) intermediate 9. The process occurs smoothly and in excellent yield without any catalyst. This conceptually simple and surprisingly unexplored reaction sheds light on the utility of triazenes as stable masked diazonium salts, and their use in diazenylation reactions, a field recently in development.23,24 Studies are being pursued in our laboratory to apply the divergent method described herein to the synthesis of dendrimers with modified peripheries. Efforts are also being undertaken to take advantage of the unique geometrical characteristics of tetraphenylmethane-based dendrimers for specific applications, whose results will be published in due course.

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EXPERIMENTAL SECTION

General Methods and Materials. All reagents were used as purchased from commercial sources without further purification, except CuI that was purified according to the literature.25 Solvents were dried using standard techniques prior to use. Tetrahydrofuran (THF) was distilled from Na/benzophenone, Et3N and methylene chloride were distilled from calcium hydride, MeOH from Mg/I2, and piperidine from phosphorus pentoxide. Reactions involving oxygen or moisture sensitive compounds were carried out under a dry argon (L50) atmosphere using oven-dried glassware. Reaction temperatures refer to external bath temperatures. Organic extracts were dried over anhydrous Na2SO4, filtered, and concentrated using a rotary evaporator. Reactions were monitored by thin-layer chromatography using aluminum-backed silica gel plates (0.2 mm thickness); the chromatograms were visualized with ultraviolet light (254 nm). Flash chromatography was performed with silica gel (230−400 mesh). Preparative-scale size-exclusion chromatography (SEC) was performed with BioRad Biobeads SX-1 using a glass column (4 × 150 cm) under gravity flow. 1 H and 13C NMR spectra were recorded on a Bruker WM-250 (5.87 T, 250 MHz to 1H and 63 MHz to 13C) NMR spectrometer using the residual proton or carbon signal of the deuterated solvent as an internal standard. Distortionless enhancement by polarization transfer 13234


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The Journal of Organic Chemistry 154.0 (C), 145.9 (C), 137.6 (C), 137.0 (CH), 132.9 (CH), 132.1 (CH), 114.9 (CH), 92.3 (C), 63.7 (C). 4-(tris(4-Aminophenyl)methyl)phenol (8) from 7. To a suspension of 7 (500 mg, 0.70 mmol) in EtOH (5 mL) in a Schlenk tube equipped with a stirring bar were added Fe2O3 (34 mg, 0.21 mmol), CuI (39 mg, 0.21 mmol), NaOH (168 mg, 4.20 mmol), and NH3 (ac) (0.53 mL, 30%) under argon. The Schlenk tube was sealed with a Teflon screw cap, and the reaction mixture was stirred at 90 °C during 8 days and then allowed to reach rt. The mixture was extracted with EtOAc (20 mL), and the organic phase was washed with brine (3 × 10 mL), dried, and concentrated. The residue was purified by flash chromatography (20−70% EtOAc/hexanes) to give 8 as a violet solid [56 mg, 0.15 mmol, 21%, Rf = 0.1 (EtOAc)]. 1H NMR (DMSO-d6, 250 MHz): 6.79 (2H, d, J = 8.3 Hz, ArOH), 6.64 (6H, d, J = 8.2 Hz, ArNH2), 6.56 (2H, d, J = 8.3 Hz, ArOH), 6.37 (6H, d, J = 8.2 Hz, ArNH2), 4.84 (6H, s, NH2), 4.12 (1H, broad s, OH). 13C NMR (DMSO-d6, 63 MHz): 154.7 (C), 145.9 (C), 139.0 (C), 135.7 (C), 131.6 (CH), 131.1 (CH), 113.8 (CH), 112.8 (CH), 61.3 (C). MS (EI, m/z, %): 381.3 (M+, 53), 289.2 ([M-C6H5O]+, 100). HRMS (EI) calcd. For C25H23N3O+ (M+) 381.1841, found 381.1834. (E)-1-((4-Iodophenyl)diazenyl)piperidine (10).19 A solution of HCl (2.7 mL, 10%) was slowly added to a suspension of 4-iodoaniline (200 mg, 0.91 mmol) in acetone (10 mL) at 0 °C. After 10 min, a solution of NaNO2 (0.6 mL, 3.0 M) was slowly added, and the mixture was stirred at 0 °C during 1 h to prepare the corresponding triazonium salt. In another flask, a solution of K2CO3 (2.3 mL, 2.0 M) was added to a solution of piperidine (0.6 mL, 9.13 mmol) in acetone (5 mL) and THF (5 mL) at 0 °C. After 10 min, the solution of the triazonium salt was added via cannula at 0 °C. The reaction mixture was stirred allowing to reach rt during 12 h, and then it was concentrated. The residue was dissolved in CH2Cl2 (15 mL) and washed with brine (3 × 10 mL). The organic phase was dried and concentrated, and the residue was purified by flash chromatography to give 1019 as a yellow solid [279 mg, 0.89 mmol, 97%, Rf = 0.85 (hexanes)]. 1H NMR (CDCl3, 250 MHz): 7.61 (2H, d, J = 8.7 Hz, Ar), 7.18 (2H, d, J = 8.7 Hz, Ar), 3.75 (4H, s, CH2−N), 1.66 (6H, s, CH2). 13C NMR (CDCl3, 63 MHz): 150.4 (C), 137.6 (CH), 122.4 (CH), 89.6 (C), 48.1 [broad, N(CH2)2], 25.2 (CH2), 24.2 (CH2). tris(4-(Piperidin-1-yldiazenyl)phenyl)methanol (9) from 10. A solution of 10 (2.5 g, 7.93 mmol) in THF (15 mL) was cooled to −78 °C and treated with nBuLi (3.2 mL, 7.93 mmol, 2.5 M in hexanes) under argon and stirred for 1 h. Dimethyl carbonate (0.21 mL, 2.56 mmol) was slowly added, and the reaction mixture was allowed to reach rt during 12 h. Then it was concentrated, and the residue was dissolved in CH2Cl2 (10 mL) and washed with brine (3 × 15 mL). The organic phase was dried and concentrated, and the residue was purified by flash chromatography (20% EtOAc/hexanes) to afford 9 as an orange solid [1.42 g, 2.39 mmol, 93%, Rf = 0.46 (20% EtOAc/ hexanes), mp >200 °C (dec 190−193 °C, color change to dark gray)]. 1 H NMR (CDCl3, 250 MHz): 7.19 (12H, AB, J = 8.6 Hz, Ar), 3.63 (12H, m, CH2−N), 1.55 (18H, s, CH2). 13C NMR (CDCl3, 63 MHz): 150.2 (C), 144.9 (C), 129.0 (CH), 120.3 (CH), 82.2 (C), 48.6 [broad, N(CH2)2], 25.7 (CH2), 24.8 (CH2). MS (EI+, m/z, %): 593.5 (M+, 65), 481.4 ([M-C5H10N3]+, 43), 258.2 ([M-C15H29N9]+, 100). HRMS (EI) calcd. for C34H43N9O+ (M+) 593.3591, found 593.3589. tris(4-(Piperidin-1-yldiazenyl)phenyl)methanol (9) from 6. A solution of HCl (5.5 mL, 10%) was slowly added to a solution of 6 (500 mg, 1.64 mmol) in MeOH (10 mL) at 0 °C. After 10 min, a solution of NaNO2 (3.3 mL, 3.0 M) was added, and the mixture was stirred at 0 °C during 1 h to prepare the corresponding tris(triazonium) salt. In another flask, a solution of K2CO3 (3.6 mL, 2.0 M) was added to a solution of piperidine (1.6 mL, 16.4 mmol) in MeOH (10 mL) at 0 °C. After 10 min, the solution of the tris(triazonium) salt was added via cannula at 0 °C. The reaction mixture was stirred allowing to reach rt during 12 h, and then it was concentrated. The residue was dissolved in EtOAc (20 mL) and washed with brine (3 × 15 mL). The organic phase was dried and concentrated, and the residue was purified by flash chromatography (10−20% EtOAc/hexanes) to give 9 as an orange solid [778 mg, 1.33

mmol, 80%, Rf = 0.46 (20% EtOAc/hexanes)]. Its spectroscopic data are identical to those from the previous experiment. tris(4-((4-Hydroxyphenyl)diazenyl)phenyl)methanol (11) from 9. A mixture of 9 (500 mg, 0.84 mmol) and phenol (1.5 g, 16.8 mmol) was heated at 80 °C under argon during 3 h. A mixture of EtOAc/ hexanes (50 mL, 10%) was added, and after removing the sobrenatant with a pipet, the resulting precipitate was triturated with EtOAc/ hexanes (5 × 50 mL, 10%). The solid residue was purified by flash chromatography (50% EtOAc/hexanes) to afford 11 as an orange solid [480 mg, 0.77 mmol, 92%, Rf = 0.44 (60% EtOAc/hexanes), mp >200 °C (dec 148−152 °C, color change to dark gray)]. 1H NMR (CD3OD, 250 MHz): 7.79 (12H, m, Ar), 7.45 (6H, d, J = 8.6 Hz, Ar), 6.90 (6H, d, J = 8.9 Hz, ArOH), 4.99 (4H, broad s, OH). 13C NMR (CD3OD, 63 MHz): 161.7 (C), 152.6 (C), 149.9 (C), 147.1 (C), 129.6 (CH), 125.7 (CH), 122.6 (CH), 116.3 (CH), 82.1 (C). MS (EI, m/z, %) 620.5 (M+, 5), 604.5 ([M−OH]+, 13), 499.4 ([M-C6H5N2O]+, 12), 94.1 (100). HRMS (EI) calcd. for C37H28N6O4+ (M+) 620.2172, found 620.2167. tris(4-((4-Hydroxyphenyl)diazenyl)phenyl)methanol (11) from 6. A solution of NaNO2 (1.9 mL, 3.0 M) was added to a solution of 6 (500 mg, 1.64 mmol) in a mixture of H2SO4 (2 mL, 96%) and H2O (5 mL), previously cooled at 0 °C during 30 min, and the mixture was stirred at 0 °C during 1 h. Then, a solution of phenol (771 mg, 8.2 mmol), NaOH (328 mg, 8.20 mmol), and Na2CO3 (869 mg, 8.20 mmol) in H2O (8 mL) was added and the stirring was continued during 3 h at 0 °C. HCl (37%) was added until pH 4 and the mixture was extracted with EtOAc (20 mL). The organic phase was washed with brine (2 × 10 mL), dried, and concentrated. The residue was purified by flash chromatography (20−40% EtOAc/hexanes) to give 11 as an orange solid [153 mg, 0.25 mmol, 15%, Rf = 0.44 (60% EtOAc/hexanes)]. Its spectroscopic data are identical to those from the previous experiment. 4-(tris(4-((4-Hydroxyphenyl)diazenyl)phenyl)methyl)phenol (12) from 11. A mixture of 11 (250 mg, 0.40 mmol) and phenol (752 mg, 8.0 mmol) was heated a 80 °C under argon until obtaining a homogeneous mixture. Then H2SO4 (6 drops) was added, and the mixture was heated at 80 °C during 12 h. A mixture of EtOAc/hexanes (50 mL, 10%) was added, and after removing the sobrenatant with a pipet, the resulting precipitate was triturated with EtOAc/hexanes (5 × 50 mL, 10%). The solid residue was purified by flash chromatography (50% EtOAc/hexanes) to afford 12 as an orange solid [250 mg, 0.36 mmol, 90%, Rf = 0.47 (60% EtOAc/hexanes), mp 185−188 °C]. 1H NMR (CD3OD, 250 MHz): 7.71 (6H, d, J = 8.9 Hz, Ar), 7.61 (6H, d, J = 8.7 Hz, ArOH), 7.27 (6H, d, J = 8.9 Hz, Ar), 6.99 (2H, d, J = 8.7 Hz, ArOH), 6.83 (6H, d, J = 8.7 Hz, ArOH), 6.66 (2H, d, J = 8.7 Hz, ArOH), 5.00 (4H, broad s, OH). 13C NMR (CD3OD, 63 MHz): 161.9 (C), 156.6 (C), 151.9 (C), 150.2 (C), 147.4 (C), 138.1 (C), 133.1 (CH), 132.6 (CH), 126.0 (CH), 122.7 (CH), 116.6 (CH), 115.6 (CH), 65.5 (C). MS (EI, m/z, %): 696.4 (M+, 2), 499.3 ([MC12H9N2O]+, 2), 422.3 (49), 121.1 (59), 93.1 (100). HRMS (EI) calcd. for C43H32N6O4+ (M+) 696.2485, found 696.2482. 4-(tris(4-((4-Hydroxyphenyl)diazenyl)phenyl)methyl)phenol (12) from 9. A mixture of 9 (760 mg, 1.28 mmol) and phenol (2.4 g, 25.6 mmol) was heated at 80 °C under argon until obtaining a homogeneous mixture. Then H2SO4 (6 drops) was added, and the mixture was heated at 80 °C during 12 h. A mixture of EtOAc/hexanes (50 mL, 10%) was added, and after removing the sobrenatant with a pipet, the resulting precipitate was triturated with EtOAc/hexanes (5 × 50 mL, 10%). The solid residue was purified by flash chromatography (50% EtOAc/hexanes) to afford 12 as an orange solid [735 mg, 1.05 mmol, 82%, Rf = 0.47 (60% EtOAc/hexanes)]. Its spectroscopic data are identical to those from the previous experiment. 4-(tris(4-Aminophenyl)methyl)phenol (8) from 12. Pd/C (100 mg, 10%) was added to a solution of 12 (730 mg, 1.05 mmol) in MeOH (20 mL), previously desoxigenated and purgued with argon. The resulting suspension was stirred under a H2 atmosphere (balloom) for 12 h. The reaction mixture was filtered through Celite and concentrated. The residue was puirified by flash chromatography (70% EtOAc/hexanes) to give 8 as a violet solid [300 mg, 0.79 mmol, 13235


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The Journal of Organic Chemistry 75%, Rf = 0.1 (EtOAc)]. Its spectroscopic data are identical to those from the previous experiment. 4-(tris(4-(Piperidin-1-yldiazenyl)phenyl)methyl)phenol (13). A solution of HCl (1.8 mL, 10%) was slowly added to a solution of 8 (220 mg, 0.58 mmol) in MeOH (5 mL) at 0 °C. After 10 min, a solution of NaNO2 (1.2 mL, 3.0 M) was added, and the mixture was stirred at 0 °C during 1 h to prepare the corresponding tris(triazonium) salt. In another flask, a solution of K2CO3 (1.7 mL, 2.0 M) was added to a solution of piperidine (0.56 mL, 5.77 mmol) in MeOH (10 mL) at 0 °C. After 10 min, the solution of the tris(triazonium) salt was added via cannula at 0 °C. The reaction mixture was allowed to warm to rt and stirred during 12 h, and then it was concentrated. The residue was dissolved in CH2Cl2 (20 mL) and washed with brine (3 × 15 mL). The organic phase was dried and concentrated, and the residue was purified by flash chromatography (12% EtOAc/hexanes) to give 13 as a brown solid [110 mg, 0.16 mmol, 28%, Rf = 0.46 (hexanes)]. 1H NMR (CDCl3, 250 MHz): 7.33 (6H, d, J = 8.5 Hz, Ar), 7.22 (6H, d, J = 8.5 Hz, Ar), 7.12 (2H, d, J = 8.5 Hz, ArOH), 6.70 (2H, d, J = 8.5 Hz, ArOH), 5.3 (1H, broad s, OH), 3.79 (12H, m, CH2−N), 1.73 (18H, m, CH2). 13C NMR (CDCl3, 63 MHz): 153.4 (C), 148.4 (C), 144.6 (C), 139.4, (C) 132.3 (CH), 131.6 (CH), 119.3 (CH), 114.1 (CH), 63.3 (C), 48.1 [broad, N(CH2)2], 25.2 (CH2), 24.3 (CH2). MS (EI, m/z, %) 669.0 (M+, 12), 558 (24), 446 (25), 334 (80), 258 (79). HRMS (EI) calcd. for C40H47N9O+ (M+) 669.3904, found 669.3903. 4-(tris(4-((E)-piperidin-1-yldiazenyl)phenyl)methyl)phenyl trifluoromethanesulfonate (5a). Et3N (0.045 mL, 0.32 mmol) was added to a solution of 13 (70 mg, 0.10 mmol) in CH2Cl2 (25 mL) under argon. The solution was cooled to −78 °C, and after 10 min Tf2O (0.018 mL, 0.12 mmol) was added. The reaction mixture was allowed to reach rt during 12 h. Then it was washed with a saturated solution of NH4Cl (20 mL), and the aqueous phase was extracted with CH2Cl2 (3 × 10 mL). The combined organic phase was dried and concentrated and the residue was purified by flash chromatography (10% EtOAc/hexanes) to give 5a as a brownish solid [76 mg, 0.095 mmol, 90%, Rf = 0.48 (20% EtOAc/hexanes)]. 1H NMR (CDCl3, 250 MHz): 7.32 (8H, m, Ar), 7.15 (8H, m, Ar), 3.76 (12H, m, CH2−N), 1.69 (18H, m, CH2). 13C NMR (CDCl3, 63 MHz): 148.7 (C), 147.8 (C), 147.3 (C), 143.4 (C), 132.7 (CH), 131.4 (CH), 119.9 (CH), 119.4 (CH), 63.6 (C), 47.9 [broad, N(CH2)2], 25.1 (CH2), 24.2 (CH2). MS (EI, m/z, %) 801.5 (M+, 42), 577.2 ([MH-C7H4F3O3S]+, 46), 466.1 (100). HRMS (EI) calcd. for C41H46F3N9O3S (M+) 801.3396, found 801.3394. 1,1′,1″-((1E,1′E,1″E)-(((4-((trimethylsilyl)ethynyl)phenyl)methanetriyl)tris(benzene-4,1-diyl))tris(diazene-2,1-diyl))tripiperidine (14). A Schlenk tube equipped with a stirring bar was charged with 5a (179 mg, 0.22 mmol), CuI (4 mg, 0.022 mmol), PPh3 (12 mg, 0.045 mmol), and Pd(PPh3)4 (26 mg, 0.022 mmol). After vacuum-drying, activated 4 Å molecular sieves, piperidine (15 mL), and trimethylsilylacetylene (0.19 mL, 1.34 mmol) were added under argon. The Schlenk tube was sealed with a Teflon screw cap, and the reaction mixture was stirred at 100 °C during 12 h and then allowed to reach rt. The mixture was concentrated, and the residue was dissolved in EtOAc (10 mL) and washed with a saturated solution of NH4Cl (3 × 15 mL). The organic layer was dried and concentrated to give a residue, which was purified by flash chromatography (3:3:94 EtOAc/ CH2Cl2/hexanes) to give 14 as a yellowish solid [145 mg, 0.19 mmol, 87%, Rf = 0.60 (1:1:8 EtOAc/CH2Cl2/hexanes), mp >350 °C (dec 300−350 °C, color change to black)]. 1H NMR (CDCl3, 250 MHz): 7.31 (8H, m, Ar), 7.18 (8H, m, Ar), 3.76 (12H, broad s, CH2−N), 1.69 (18H, m, CH2), 0.24 (9H, s, TMS). 13C NMR (CDCl3, 63 MHz): 148.8 (C), 148.0 (C), 144.0 (C), 131.7 (CH), 131.6 (CH), 131.1 (CH), 120,5 (C), 119.6 (CH), 64.2 (C), 48.3 [broad, N(CH2)2], 25.4 (CH2), 24.5 (CH2), 0.12 (CH3). MS (EI, m/z, %) 749.1 (M+, 16), 638.4 ([MH-C5H10N3]+, 46), 414.2 ([MH-3x(C5H10N3)]+, 100). HRMS (EI) calcd. for C45H55N9Si+ (M+) 749.4350, found 749.4351. 1,1′,1″-((1E,1′E,1″E)-(((4-Ethynylphenyl)methanetriyl)tris(benzene-4,1-diyl))tris(diazene-2,1-diyl))tripiperidine (3). KOH (38 mg, 0.67 mmol) was added to a solution of 14 (50 mg, 0.067 mmol) in

CH2Cl2 (10 mL) and MeOH (1.5 mL) under argon, and the mixture was stirred at rt during 1 h. The reaction mixture was concentrated, and the residue was dissolved in CH2Cl2 (20 mL) and washed with brine (3 × 15 mL). The organic phase was dried and concentrated to give a residue that was dissolved in CH2Cl2. Precipitation with hexanes gave 3 as a white solid, which was immediately used in the next reaction without further purification [44 mg, 0.065 mmol, 98%, Rf = 0.55 (1:1:8 EtOAc/CH2Cl2/hexanes)]. 1H NMR (CDCl3, 250 MHz): 7.40−7.24 (8H, m, Ar), 7.24−7.10 (8H, m, Ar), 3.73 (12H, s, CH2− N), 3.01 (1H, s, CC−H), 1.66 (18H, s, CH2). 13C NMR (CDCl3, 63 MHz): 148.6 (C), 148.1 (C), 143.7 (C), 131.5 (CH), 131.0 (CH), 130.9 (CH), 119.3 (CH), 83.6 (C), 63.9 (C), 48.0 [broad, N(CH2)2], 25.1 (CH2), 24.2 (CH2). tetrakis(4-Iodophenyl)methane (2).14,26 A mixture of trityl chloride (4.0 g, 14.3 mmol) and aniline (10 mL, 110 mmol) was heated at 220 °C for 20 min. Then it was allowed to reach rt and was vigorously stirred to give a violet paste that was treated with a solution of HCl (10 mL, 2.0 M) and MeOH (10 mL) and refluxed for 10 min. The precipitate was filtered at rt and washed with water and dried to give 4-tritylaniline26,27 as a violet solid [4.71 g, 14.0 mmol, 98%, Rf = 0.25 (20% EtOAc/hexanes), mp 255−257 °C]. 1H NMR (CDCl3, 250 MHz): 7.22 (15H, m, Ar), 7.01 (2H, d, J = 8.5 Hz, ArNH2), 6.69 (2H, d, J = 8.5 Hz, ArNH2), 4.34 (2H, broad s, NH2). 13C NMR (CDCl3, 63 MHz): 147.1 (C), 141.6 (C), 138.8 (C), 132.3 (CH), 131.2 (CH), 127.5 (CH), 125.9 (CH), 115.5 (CH), 64.4 (C). A solution of 4tritylaniline (4.0 g, 11.9 mmol) in acetone (150 mL) was cooled at 0 °C, and HCl (40 mL, 10%) was added. After 10 min, a solution of NaNO2 (6.8 mL, 3.0 M) was added. The mixture was stirred during 30 min, a solution of H3PO2 (2 mL, 50%) was added, and the reaction mixture was refluxed during 1 h. A brown solid was formed, which was filtered, washed with water, and dried to give tetraphenylmethane26,14b as a brownish solid (2.88 g, 9.0 mmol, 75%, mp 276−282 °C). 1H NMR (CDCl3, 250 MHz): 7.2 (20H, m, Ar). 13C NMR (CDCl3, 63 MHz): 146.7 (C), 131.1 (CH), 127.4 (CH), 125.8 (CH), 64.9 (C). BTI (8.04 g, 18.7 mmol) was added to a suspension of tetraphenylmethane (2.6 g, 8.13 mmol) and I2 (4.11 g, 16.3 mmol) in CCl4 (50 mL) under argon, and the mixture was heated at 60 °C during 4 h. The reaction mixture was allowed to reach rt and concentrated. The residue was washed with EtOH and the resulting white solid was suspended in CHCl3, refluxed for 10 h, filtered, suspended again in CHCl3, refluxed for 3 h, filtered, and dried to give 214 as a white fine power (3.0 g, 3.60 mmol, 45%, Rf = 0.65 (hexanes), mp >350 °C]. 1H NMR (THF-d8, 250 MHz): 7.61 (8H, m), 6.97 (8H, m). 13C NMR (THF-d8, 63 MHz): 146.2 (C), 138.0 (CH), 133.5 (CH), 93.0 (C), 64.5 (C). Dendrimer 15. A Schlenk tube equipped with a stirring bar was charged with freshly prepared 3 (250 mg, 0.37 mmol), 2 (68 mg, 0.082 mmol), CuI (14 mg, 0.074 mmol), PPh3 (39 mg, 0.15 mmol), and Pd(PPh3)4 (170 mg, 0.15 mmol). After vacuum-drying, activated 4 Å molecular sieves and piperidine (15 mL) were added under argon. The Schlenk tube was sealed with a Teflon screw cap and the reaction mixture was stirred at 100 °C during 12 h, and then it was allowed to reach rt. The mixture was concentrated and the residue was dissolved in CH2Cl2 (30 mL) and washed with a saturated solution of NH4Cl (3 × 20 mL). The organic layer was dried and concentrated to give a residue, which was purified by SEC (Bio-Beads SX-1, THF) and precipitation (THF/hexanes) to give 15 as a brown solid (178 mg, 0.059 mmol, 72%). 1H NMR (CDCl3, 250 MHz): 7.45−7.10 (80H, m, Ar), 3.73 (48H, s, CH2−N), 1.66 (72H, s, CH2). 13C NMR (CDCl3, 63 MHz): 148.5 (C), 147.6 (C), 145.7 (C), 143.9 (C), 131.5 (CH), 131.0 (CH), 130.7 (CH), 130.5 (CH), 121.2 (C), 120.3 (C), 119.3 (CH), 89.6 (C), 88.8 (C), 63.9 (C), 48.1 [broad, N(CH2)2], 25.1 (CH2), 24.3 (CH2). Dendrimer 4. A Schlenk tube equipped with a stirring bar was charged with 15 (100 mg, 0.033 mmol) and vacuum-dried. Then, MeI (10 mL) was slowly added under argon. The Schlenk tube was sealed with a Teflon screw cap and the reaction mixture was stirred at 120 °C during 12 h and allowed to reach rt. Then CH2Cl2 (50 mL) was added and the solution was washed with a saturated solution of Na2S2O3 (3 × 10 mL) and a saturated solution of NH4Cl (3 × 10 mL). The organic 13236


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The Journal of Organic Chemistry layer was dried and concentrated to give a residue which was triturated with refluxing hexanes followed by refluxing MeOH to give dendrimer 4 as a yellowish solid [72 mg, 0.023 mmol, 68%, mp 206−210 °C]. 1H NMR (CDCl3, 250 MHz): 7.56 (24H, d, J = 7.5 Hz, ArI), 7.37 (16H, d, J = 8.2 Hz, Ar), 7.09 (16H, d, J = 8.2 Hz, Ar), 6.88 (24H, d, J = 7.5 Hz ArI). 13C NMR (CDCl3, 63 MHz): 145.0 (C), 136.9 (CH), 132.5 (CH), 131.0 (CH), 130.5 (CH), 92.3 (C), 89.1 (C), 64.1 (C). Dendrimer 1. A Schlenk tube equipped with a stirring bar was charged with freshly prepared 1612 (110 mg, 0.25 mmol), 4 (30 mg, 0.0094 mmol), CuI (4 mg, 0.023 mmol), PPh3 (6 mg, 0.023 mmol), and Pd(PPh3)4 (26 mg, 0.023 mmol). After vacuum-drying, activated 4 Å molecular sieves and piperidine (15 mL) were added under argon. The Schlenk tube was sealed with a Teflon screw cap, and the reaction mixture was stirred at 100 °C during 12 h. The resulting mixture was allowed to reach rt and concentrated. The residue was dissolved in CH2Cl2 (30 mL) and washed with a saturated solution of NH4Cl (3 × 20 mL). The organic layer was dried and concentrated to give a residue, which was purified by SEC (Bio-Beads SX-1, THF) and flash chromatography (0−0.5% iPrOH/CH2Cl2) to give 112 as a white solid [50 mg, 0.073 mmol, Rf = 0.60 (1% THF/CH2Cl2), 77%]. 1H NMR (CDCl3, 250 MHz): 7.40 (64H, m, Ar), 7.17 (64H, m, Ar), 7.07 (72H, d, J = 8.9 Hz, ArOMe), 6.77 (72H, d, J = 8.9 Hz, ArOMe), 3.77 (108H, s, OMe). 13C NMR (CDCl3, 63 MHz): 158.1, 148.7, 146.5, 139.7, 132.6, 131.7, 131.6, 131.5, 131.3, 122.0, 121.1, 113.4, 90.4, 90.0, 89.5, 65.5, 63.5, 55.8. MS ([MALDI-TOF], m/z) 6988 (MAg+, calcd. 6988), 6773 ([M-C7H7O]+, calcd. 6773), 6369 ([M-C36H29O3]+, calcd. 6371). [The observed fragmentations correspond to the formation of trityl type cations: C7H7O = C6H4OCH3, C36H29O3 = (C6H4)C2(C6H4)C(C6H4OCH3)3].

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(c) Uribe-Romo, F. J.; Hunt, J. R.; Furukawa, H.; Klöck, C.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2009, 131, 4570. (d) El-Kaderi, H. M.; Hunt, J. R.; Mendoza-Cortes, J. L.; Côté, A. P.; Taylor, R. E.; O’Keeffe, M.; Yaghi, O. M. Science 2007, 316, 268. (e) Côté, A. P.; Benin, A. I.; Ockwig, N. W.; O’Keeffe, M.; Matzger, A. J.; Yaghi, O. M. Science 2005, 310, 1166. (4) Jin, Y.; Jin, A.; McCaffrey, R.; Long, H.; Zhang, W. J. Org. Chem. 2012, 77, 7392. (5) For selected examples see: (a) Verde-Sesto, E.; Pintado-Sierra, M.; Corma, A.; Maya, E. M.; de la Campa, J. G.; Iglesias, M.; Sánchez, F. Chem.Eur. J. 2014, 20, 5111 and references cited therein. (b) Wang, Z.; Zhang, B.; Yu, H.; Sun, L.; Jiao, C.; Liu, W. Chem. Commun. 2010, 46, 7730. (c) Holst, J. R.; Stöckel, E.; Adams, D. J.; Cooper, A. I. Macromolecules 2010, 43, 8531. (6) (a) Campagna, S.; Ceroni, P.; Puntoriero, F. Designing Dendrimers; Willey: Hoboken, NJ, 2012. (b) Vögtle, F.; Richardt, G.; Werner, N. Dendrimer Chemistry: Concepts, Synthesis, Properties, Applications; Wiley-VCH: Weinheim, Germany, 2009. (c) Newkome, G. R.; Moorefield, C. N.; Vögtle, F. Dendrimer Chemistry: Concepts, Synthesis, Properties, Applications; Wiley-VCH: Weinheim, Germany, 2001. (d) Fréchet, J. M.; Tomalia, D. A. Dendrimers and Other Dendritic Polymers; Wiley: Chichester, U.K., 2001. (e) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (7) (a) Astruc, D.; Boisselier, E.; Ornelas, C. Chem. Rev. 2010, 110, 1857. (b) Caminade, A.-M.; Turrin, C.-O.; Laurent, R.; Ouali, A.; Delavaux-Nicot, B. Dendrimers: Towards Catalytic, Material and Biomedical Uses; Wiley: Chichester, U.K., 2011. (8) (a) Pinto, L. F.; Correa, J.; Martin-Pastor, M.; Riguera, R.; Fernandez-Megia, E. J. Am. Chem. Soc. 2013, 135, 1972. (b) Ballauff, M.; Likos, C. N. Angew. Chem., Int. Ed. 2004, 43, 2998. (c) Lescanec, R. L.; Muthukumar, M. Macromolecules 1990, 23, 2280. (9) For selected examples see: (a) Hammer, B. A. G.; Moritz, R.; Stangenberg, R.; Baumgarten, M.; Müllen, K. Chem. Soc. Rev. 2015, 44, 4072. (b) Albrecht, K.; Yamamoto, K. J. Am. Chem. Soc. 2009, 131, 2244. (c) Mishra, A.; Ma, C.-Q.; Janssen, R. A. J.; Bauerle, P. Chem. Eur. J. 2009, 15, 13521. (d) Wang, J.-L.; Yan, J.; Tang, Z.-M.; Xiao, Q.; Ma, Y.; Pei, J. J. Am. Chem. Soc. 2008, 130, 9952. (e) Figueira-Duarte, T. M.; Simon, S. C.; Wagner, M.; Druzhinin, S. I.; Zachariasse, K. A.; Müllen, K. Angew. Chem., Int. Ed. 2008, 47, 10175. (f) Xia, C.; Fan, X.; Locklin, J.; Advincula, R. C.; Gies, A.; Nonidez, W. J. Am. Chem. Soc. 2004, 126, 8735. (g) Cao, Y.; Zhang, W.-B.; Wang, J.-L.; Zhou, X.-H.; Lu, H.; Pei, J. J. Am. Chem. Soc. 2003, 125, 12430. (h) Moore, J. S. Acc. Chem. Res. 1997, 30, 402. (10) A dense shell structure with internal voids was achieved for pollyphenylene dendrimers with tetraphenylmethane as core: Brutschy, M.; Stangenberg, R.; Beer, C.; Lubczyk, D.; Baumgarten, M.; Müllen, K.; Waldvogel, S. R. ChemPlusChem 2015, 80, 54. (11) Jiang, X.; O’Brien, Z. J.; Yang, S.; Lai, L. H.; Buenaflor, J.; Tan, C.; Khan, S.; Houk, K. N.; García-Garibay, M. A. J. Am. Chem. Soc. 2016, 138, 4650. (12) Urzúa, J. I.; Regueira, M. A.; Lazzari, M.; Torneiro, M. Polym. Chem. 2016, 7, 5641. (13) Chinchilla, R.; Najera, C. Chem. Rev. 2014, 114, 1783. (14) (a) Li, Q.; Rukavishnikov, A. V.; Petukhov, P. A.; Zaikova, T. O.; Keana, J. F. W. Org. Lett. 2002, 4, 3631. (b) Su, D.; Menger, F. M. Tetrahedron Lett. 1997, 38, 1485. (c) Simard, M.; Su, D.; Wuest, J. D. J. Am. Chem. Soc. 1991, 113, 4696. (15) Kimball, D. B.; Haley, M. M. Angew. Chem., Int. Ed. 2002, 41, 3338. (16) Gallina, M. E.; Baytekin, B.; Schalley, C.; Ceroni, P. Chem. Eur. J. 2012, 18, 1528. (17) Wu, X. F.; Darcel, C. Eur. J. Org. Chem. 2009, 28, 4753. (18) An intractable complex mixture was obtained when using ammonia with L-proline, CuI and K2CO3 in DMSO (80 °C, 24 h): Kim, J.; Chang, S. Chem. Commun. 2008, 26, 3052. (19) (a) Patrick, T. B.; Willaredt, R. P.; DeGonia, D. J. J. Org. Chem. 1985, 50, 2232. (b) Li, W.; Beller, M.; Wu, X.-F. Chem. Commun. 2014, 50, 9513.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b02302. Unsuccessful synthetic approaches to 5. Copies of NMR spectra. MS spectrum and GPC data of 1. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mercedes Torneiro: 0000-0003-0914-1769 Present Address †

Comisión Chilena de Energı ́a Nuclear, Departamento de Materiales Avanzados, Amunátegui 95, Santiago de Chile 8340701, Chile. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Spanish MINECO and FEDER (Grant MAT201342425-R) and Xunta de Galicia (Grant 08TMT002209PR) for financial support and Prof. M. Lazzari for help with the GPC analysis. J.I.U. acknowledges the Chile Government for a predoctoral fellowship (CONICYT Becas Chile).

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REFERENCES

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