Embedding a guest gold cluster in an organic host. Evaluation of the encapsulation nature in a Au₁₈–superphane host–guest aggregate

Abstract

Formation of supramolecular aggregates incorporating Au₁₈ into a suitable bioinspired polyfunctional superphane cavity provides novel functionality to the overall structure. We evaluated the favorable incorporation of the Au₁₈ cluster into the superphane cavity. This amounted to −145.3 kcal mol⁻¹, provided mainly by electrostatic-type interactions (54.9%). Charge transfer characteristics involving host ← guest and host → guest backdonation through S ← Au and S → Au contacts led to overall Au₁₈ → 1 superphane charge transfer. Charge transfer consisted of a charge hopping rate (k_CT) in the range of ultrafast electron transfer, calculated to be 2.2 × 10¹³ s⁻¹ at 300 K. Thus, Au₁₈ → 1 charge transfer was driven by coordinating and short contacts towards the superphane available cavity, resulting in a supramolecular structure of the donor–acceptor (D–A) system. We expect that the current approach can be useful for further rationalizing the relevant stabilizing factors to ensure the stable aggregation of metallic clusters in organic host cavities during the making of novel functional cluster-based host–guest aggregates.

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Introduction

A supramolecular assembly represents a useful strategy to design and achieve multifunctional aggregate materials with applications in a wide range of fields,^1–6 highlighting the use of non-covalent interactions in creating ordered architectures. Host–guest chemistry takes advantage of such interactions by including guest units into a suitable host cavity based on the mutual recognition between molecular constituents,^7–17 largely exemplified by involving organic pairs.

Recently, Zhang and coworkers reported the ferritin-inspired design,¹⁸ characterizations and performance of a host–guest structure of a bare gold nanocluster (AuNC) embedded into a highly polyfunctional superphane cavity.¹⁹ It involved several coordination sites given by nitrogen, oxygen and sulfur atoms from imine, BINOL (BINOL = 1,1-binaphthyl-2,2 diol) dimethyl ether,²⁰ and thiophene groups. The resulting superphane appeared as a prototypical organic cavity with multiple coordinating sites serving as guidance for further development of host structures prone to incorporate bare clusters, favoring a controlled synthesis, size-selectivity purification, solubility in non-polar solvents, incorporation into organic electronic devices, among other issues relevant for emergent applications of atomically precise metal nanoclusters.^21–33

The AuNC–superphane host–guest pair by Zhang and colleagues was obtained by reacting the hollow superphane with AuCl₃ in CH₂Cl₂, followed by the addition of NaBH₃CN as a mild reducing agent,¹⁹ and confirmation of the inclusion of the atomically-precise Au₁₈ bare cluster via electrospray ionization-mass spectrometry (ESI-MS). The resulting organic–inorganic host–guest structure from AuNC–superphane gives rise to a supramolecular hybrid system that can modify the inherent physical–chemical characteristics of each constituent significantly,¹⁹ as observed for different hybrid assemblies.^34–41 In particular, this aggregate enhances the stability of AuNCs, providing unexpected functionality as given by a broad absorption, improving the sunlight absorption capabilities, with a promising photothermal conversion efficiency of 92.8% desired for solar-to-vapor generation.¹⁹

Interestingly, the Au₁₈ cluster features 18 cluster electrons (18-ce) fulfilling an electronic shell order in analogy to isolated atoms⁴² ascribed by 1S²1P⁶1D¹⁰, in line with the superatom approach of molecular clusters providing chemical stability.^43–45 Theoretical calculations on the structure of the Au₁₈–superphane species¹⁹ have denoted a favorable match between the available host cavity and guest size, showing coordination mainly ascribed to the thiophene and BINOL sites.

Herein, we rationalized the nature of the superphane–Au₁₈ interaction to further clarify the stabilizing factors that provide efficient cluster encapsulation into the available organic cavity. The intermolecular interaction between Au₁₈ and the superphane host cavity was evaluated by energy decomposition analysis (EDA),^46,47 electrostatic potential maps,^48–50 electron density difference maps, and non-covalent index (NCI) analysis^51,52 to reveal the contributing role of the different constituent sections of the organic cavity. Moreover, the charge transfer of electrons was evaluated within the Marcus theory framework to account for the reorganization energy (λ) and electronic coupling (V) involved in the processes determining the charge hopping rate in the resulting Au₁₈–superphane aggregate. This was done to further explore the charge transfer parameters in metallic clusters embedded in suitable organic cavities.

Computational details

Calculations were done using the ADF2024 code.⁵³ We used the triple-ξ and two polarization functions (STO-TZ2P) basis set within the generalized gradient approximation (GGA) according to the BP86 exchange–correlation functional and the empirical dispersion correction to DFT (DFT-D) given by the pairwise Grimme correction (D3)^54–57 and Becke–Johnson damping functions.^58,59 Dispersion-corrected DFT methods offer reliable results and improved performance in the description of supramolecular interactions at a moderate computational cost for larger systems.^55,60,61 Relaxed structures were obtained through the analytical energy gradient method implemented by Versluis and Ziegler⁶² at the TZ2P/BP86-D3 level without any symmetry restrain. The energy convergence criterion was set to 10⁻⁵ Hartree, gradient convergence criteria to 10⁻⁴ Hartree per Å, and radial convergence criteria to 10⁻³ Å to achieve final optimized structures. The counterpoise correction scheme was employed to overcome basis set superposition error (BSSE) in the interaction energy analysis owing to the systematic error introduced by the use of finite basis sets, overbinding van der Waals aggregates.^63–65 Solvent effects were taken into account via the conductor-like screening model (COSMO) for explicit solvation using dichloromethane (CH₂Cl₂) as the solvent, as implemented in the ADF code.^66,67 The charge hopping rate (k_CT) in the Au₁₈–superphane aggregate was modeled using the high-temperature limit of the Marcus theory,^68–72 as implemented in the ADFcode.

Molecular dynamics trajectories were obtained via eXtended tight binding (xTB) methods at the GFN0-xTB level as implemented in the standalone xtb code version.⁷³ The temperature was set to 298.15 K for thermostatistical evaluation, whereby hydrogens were treated as deuterium atoms with an accuracy set to 2.0.

Results and discussion

The structure of Au₁₈–superphane has been provided computationally by Zhang and coworkers.¹⁹ It denotes the suitable incorporation of the Au₁₈ cluster into the organic host cavity. The thiophene–BINOL-based superphane host (1)¹⁹ has been inspired by the unique binding pockets from ferritin iron storage proteins,¹⁸ leading to the encapsulation and stabilization of metal ions or clusters, thereby ensuring long-term stability. The resulting host–guest structure from Au₁₈ and 1 involves several coordinating sites retaining the gold nanoparticle (Scheme 1). This offers an interesting case of an amenable host cavity to evaluate the role of different stabilizing contributions resulting in efficient aggregation. This approach led to the characterized Au₁₈@1 as probed by ESI-MS, UV-vis spectroscopy, CD spectroscopy, powder X-ray diffraction (PXRD), and X-ray photoelectron spectroscopy (XPS), among other techniques.¹⁹

Scheme 1 A schematic representation of the thiophene–BINOL-based superphane host (1) and its host–guest complex with Au₁₈ (Au₁₈@1).

Our calculations revealed a similar structure to that reported previously.¹⁹ We documented shorter Au₁₈ superphane bond lengths in the range 2.406–2.517 Å involving Au–S coordinating bonds from thiophene groups, and Au–O bonds in the range 2.466–3.393 Å from methyl ether groups from methylated BINOL, ascribed mainly to the upper and central sections of the organic cavity, respectively (Fig. 1c), locating the Au₁₈ at one side of the available cavity. Such sections featured the main coordinating sites from the host cage interacting towards the bare Au₁₈ cluster, with complementary Au⋯H₃C- and Au⋯H–thiophene contacts (Fig. 1d). For comparison, the central disposition of the Au₁₈ cluster into 1 was evaluated, which was disfavored by 18.6 kcal mol⁻¹.

Fig. 1 Side (a) and upper (b) views of Au₁₈@1, denoting the location of methyl ether groups from methylated BINOL (c) and Au⋯H₃C- and Au⋯H–thiophene contacts (d). The available internal cavity size is given in green from MoloVol suite⁷⁴ calculations (e).

The available cavity size in 1 was evaluated by MoloVol suite⁷⁴ employing two spherical probes to define the available cavities and the related surfaces and volumes. This evaluation led to an inner cavity in 1 of 731.84 ų (Fig. 1d and e), which was very suitable for incorporating the Au₁₈ structure with a volume of 552.98 ų.

The embedded Au₁₈ cluster features 18 cluster electrons as provided by the respective set of 6s¹ atomic shells,^44,75 building up an electronic shell resembling atomic orbitals fulfilling an 1S²1P⁶1D¹⁰ order, ascribed as a superatom, denoting particular stability.^76–79 The 1D¹⁰ shell contributed to the formation of frontier orbitals in the overall Au₁₈@1 aggregate (Fig. S1, SI). This superatom cluster showed a distorted structure, which was located 34.6 kcal mol⁻¹ above the preferred isomer (Fig. S2, SI) observed from infrared multiple photon dissociation (IR-MPD) experiments.⁸⁰ Hence, the Au₁₈ cluster could modify its structure to maximize the interaction towards the organic cavity, retaining a similar electronic structure. We wished to evaluate the interaction energy leading to the Au₁₈–superphane host–guest aggregate. Hence, the interaction between the Au₁₈ unit and superphane structure 1 was calculated, resulting in a sizable stabilization of −145.3 kcal mol⁻¹ (Table 1).^81–83 To bring host and guest units from their isolated relaxed structures to their structure in the resulting host–guest system, the involved geometric and electronic destabilizing deformation is accounted for by the preparation energy ΔE_prep.^81–84 The estimated ΔE_prep amounted to 34.6 kcal mol⁻¹ for Au₁₈ and 41.8 kcal mol⁻¹ for the superphane cage, leading to an overall ΔE_prep of 76.4 kcal mol⁻¹. Hence, the structure could overcome the required structural modification to give rise to the host–guest aggregate.

Table 1 Energy decomposition analysis for the Au₁₈–1 interaction and for related models of 1. Values are in kcal mol⁻¹. Percentage contributions for stabilizing terms are provided

	Au18–1		Au18–(C4H4S)4		Au18–(OMe2)12
ΔEprep	76.4
ΔEPauli	783.8		387.2		301.46
ΔEelstat	−510.4	54.9%	−263.3	59.7%	−188.8	52.3%
ΔEorb	−277.7	29.9%	−153.8	34.9%	−112.9	31.3%
ΔEdisp	−140.8	15.2%	−24.1	5.5%	−59.5	16.5%
ΔEint	−145.3		−53.9		−59.7

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The characteristics of the stabilizing host–guest interaction waws further unraveled by the role of different contributing terms to the resulting interaction energy (ΔE_int) evaluated via the energy decomposition analysis (EDA) given by Ziegler and Rauk,^46,85,86 according to eqn (1):

ΔE_int = ΔE_Pauli + ΔE_elstat + ΔE_orb + ΔE_disp (1)

In this framework, the Pauli repulsion (ΔE_Pauli) results from the four electrons/two orbitals between occupied orbitals from Au₁₈ and the superphane cavity, reflecting the steric effect associated with the interaction,⁸⁷ which amounted to 783.8 kcal mol⁻¹ (Table 1). Moreover, the stabilizing electronic part of the interaction involving electrostatic (ΔE_elstat) and orbital (ΔE_orb) terms accounts for the Coulomb interaction between the charge densities (ΔE_elstat) and polarization and charge transfer contribution after relaxing the orbitals (ΔE_orb) to the in the host–guest system.⁸⁸ The dispersion interaction (ΔE_disp) was evaluated via the pairwise correction of Grimme (DFT-D3).⁵⁷ The ΔE_elstat and ΔE_orb terms amounted to −510.4 and −277.7 kcal mol⁻¹, respectively, complemented with the ΔE_disp term of −140.8 kcal mol⁻¹, which could overcome the Pauli repulsion. The percentage relationship between the stabilizing terms (ΔE_orb, ΔE_elstat, and ΔE_disp) characterize the overall nature of the host–guest interaction, which was of mainly electrostatic character (54.9%), followed by an orbital contribution of 29.9% and, lastly, 15.2% from the dispersion interaction.

To reveal the spatial distribution of the main electrostatic interaction accounting for the ΔE_elstat term, the charge reorganization at the van der Waals surface for Au₁₈ was obtained by representing the electrostatic potential⁴⁸ over an electron density surface of 0.001 electrons per Bohr³.^89,90 The electrostatic potential for the isolated Au₁₈ guest unit showed the formation of charge depletion regions as Lewis acidic sites at the low connected Au atoms (Fig. 2a) as the maxima in the surface electrostatic potential (V_S,max), similar to that obtained for the Au₁₃ cluster,⁵⁰ denoted as σ-hole regions accounting for reactive sites in metallic clusters.^50,91–93 Interestingly, the electrostatic potential for the overall Au₁₈–superphane structure showed charge reorganization over the van der Waals Au₁₈ surface at the Au–S and Au–O coordinating sites and also for Au⋯H₃C- and Au⋯H–thiophene contacts (Fig. 2b). These data showed that the stabilizing ΔE_elstat term was given by the contribution from different complementary sites within the superphane cavity contributing to encapsulation of the Au₁₈ bare superatom.

Fig. 2 The electrostatic potential surface for the isolated Au₁₈ cluster (a) and overall Au₁₈@1 (b) aggregate drawn at a 0.001 electrons per Bohr³ electron density for the Au₁₈ cluster (including the 1 host structure for graphical guidance). Blue denotes electro-positive sites prone to interact with Lewis bases.

Moreover, ΔE_orb can be evaluated in terms of individual deformation density channels accounting for individual bonding contributions^94,95 via the natural orbitals for chemical valence^96–98 extension of EDA (EDA-NOCV).⁹⁸ We documented sixteen main individual deformation density channels (Δρ1–Δρ16) (Fig. S3, SI) contributing between −20.0 to −5.2 kcal mol⁻¹ (Table S1, SI). These data suggested host ← guest charge transfer through S ← Au contacts (Δρ1–Δρ4) (Fig. 3a), and host → guest charge transfer via S → Au contacts. These results suggested a donation and backdonation of charge leading to an overall Au₁₈ → 1 superphane charge transfer of +0.79e as obtained from Hirshfeld charge analyses. The spatial distribution of the resulting charge transfer was denoted by the difference in electron density between the host–guest aggregate and respective fragments (Δρ(r) = ρ(r)^total − (ρ(r)^Au18 + ρ(r)^superphane)) (Fig. 3b). These data suggested that charge accumulation remained at the host cavity near the Au₁₈ cluster, with charge depletion at the S–Au coordinating contacts. Thus, the Au₁₈ → 1 charge transfer was driven by coordinating and short contacts towards the superphane available cavity, resulting in a supramolecular donor–acceptor (D–A) system structure. In addition, TZ2P/PBE-D3 and TZ2P/B3LYP-D3 levels of theory were evaluated, which delivered similar features than for those calculated at the TZ2P/BP86-D3 level, supporting the calculated data at different computational levels (Table S2, SI). The obtained Au₁₈–1 interaction energy amounted to −133.6 and −155.5 kcal mol⁻¹ for TZ2P/PBE-D3 and TZ2P/B3LYP-D3 levels, respectively, in line with that calculated at TZ2P/BP86-D3 (−145.2 kcal mol⁻¹). In addition, different levels of theory can be employed to gain insights into the characteristics of the Au₁₈ inclusion into 1.⁹⁹

Fig. 3 Side (a) and upper (b) views of Au₁₈@1, denoting the location of methyl ether groups from methylated BINOL (c) and Au⋯H₃C– and Au⋯H–thiophene contacts.

Furthermore, the contribution of London interactions as complementary weak intermolecular interactions for the overall host–guest aggregation were evaluated in the interacting structure via the independent gradient model (IGM).^100–102 This approach enabled isolation of the intermolecular interaction between the host and guest via the proposed electron-density based δg_inter descriptor by Hénon¹⁰² and coworkers. The stabilizing and repulsive characteristics of the intermolecular interaction can be evaluated by the second eigenvalue of the electron density Hessian (λ₂). This accounts for an accumulation (attractive) or depletion (repulsive) of electron density, with the sign (λ₂)ρ term as a useful descriptor for stabilizing (λ₂ < 0), van der Waals (λ₂ ≈ 0) or repulsive-type interactions (λ₂ > 0).^51,52,103 IGM analysis was carried out as implemented in the Multiwfn suite.¹⁰⁴

The resulting intermolecular interactions from the IGM analysis (Fig. 4) were mainly ascribed at S–Au contact sites, regions below the methyl ether units from BINOL motifs given as Au⋯OMe and Au⋯H₃C–, and Au⋯H–thiophene contacts. These results suggested that non-covalent contributions to the Au₁₈ encapsulation interactions were due to the multiple interacting sites from the organic cavity.

Fig. 4 The independent gradient model (IGM) isosurface (0.005 a.u.), colored in the −0.10 a.u. < sign (λ₂)ρ < +0.10 a.u. range for the Au₁₈–1 interaction in Au₁₈@1. Blue/red accounts for stabilizing/destabilizing non-covalent interactions, whereas green isosurfaces denote London-type interactions.

Moreover, a simplified model involving the four closest thiophene groups retaining their structure in the overall host–guest aggregate (Au₁₈@(SC₄H₄)₄) was used to evaluate the contribution of units from Au–S and Au⋯H–thiophene contacts (Fig. S4, SI). This model revealed an interaction energy of −53.9 kcal mol⁻¹, which accounted for 37.1% from the overall Au₁₈–1 interaction energy in the Au₁₈@1 aggregate. Similarly, for the simplified model accounting for Au⋯OMe and Au⋯H₃C– interactions (Au₁₈@(OMe₂)₁₂), the interaction energy towards the Au₁₈ cluster amounted to −59.7 kcal mol⁻¹, representing 41.1% from the overall interaction energy related to the encapsulation of Au₁₈ into 1 superphane. The remaining contribution to the overall interaction energy related to formation of the Au₁₈@1 aggregate was given by weaker contributions from several Au₁₈⋯HC=N– contacts at one side of the cavity. Hence, efficient encapsulation of the Au₁₈ cluster was provided by the contribution of several weak interactions driven by the contacts to the superphane cavity.

Furthermore, the resulting incorporation of Au₁₈ can involve complex and diverse conformational landscapes.^105,106 Conformational exploration was evaluated through molecular dynamics via eXtended tight binding (xTB) methods at the GFN0-xTB level within the xtb code.⁷³ The resulting trajectory involved structures with a more disperse Au₁₈ cluster within the 1 cage (red arrow in Fig. 5), a partially aggregated Au₁₈ structure (blue arrow), and a completely aggregated Au₁₈ as the most favorable structure (green arrow). Along with the trajectory steps, different Au–cage interactions evolve whereby Au₁₈ tends to aggregate as characterized previously.¹⁹ Recently, a benchmark report on the capabilities of xTB Hamiltonians (GFN0-xTB, GFN1-xTB, and GFN2-xTB) in achieving host–guest binding features with implicit solvent models¹⁰⁷ denoted their performance in relation to MM-based techniques, appearing to be worthy alternatives if MM-based techniques are not applicable. In our case, due to the hybrid metallic–organic nature of the Au₁₈–superphane aggregate, only GFN0-xTB could be applied successfully, and the resulting trajectory should be regarded as a qualitative conformational sampling, whereby quantitative interaction energy analyses were reliant upon DFT calculations.

Fig. 5 Molecular dynamic trajectories for the Au₁₈@1 structure featuring a disperse Au₁₈ cluster isomer (red arrow), a partially aggregated Au₁₈ structure (blue arrow), and a complete aggregated Au₁₈ structure (green arrow). The x-axis corresponds to 200 steps, covering a total simulation time of 10 ps.

The initial and middle stages of the calculated molecular dynamics trajectory in Fig. 5 are shown by red and blue arrows, respectively. The calculated Au₁₈–superphane interaction energy at these stages (Table S3) showed a repulsive interaction of +82.6 kcal mol⁻¹ at the initial step, which stabilized to −35.2 kcal mol⁻¹ at the middle position of the trajectory, and settled at the interaction value of −145.2 kcal mol⁻¹ for the Au₁₈@1 structure discussed above. The main destabilizing factor was provided by an increase in the Pauli repulsion contribution at initial and middle steps. Hence, the more compact Au₁₈ structure (final), as discussed above, provided more favorable encapsulation reducing the steric repulsion towards the overall cavity, in contrast to when more gold atoms are spread around the cavity. Thus, the compact metal cluster distribution favored a reduction in the steric repulsion upon encapsulation, thereby leading to more stable cluster incorporation.

We wished to evaluate the intermolecular charge transfer characteristics in the formation of Au₁₈@1. Hence, the electronic coupling between Au₁₈ and superphane host was calculated to quantify charge transport capabilities. Calculations revealed strong electronic coupling of 52.1 meV for electron transport (V_e) and 45.8 meV for hole transport (V_h) integrals. These values are in the range of one of the most efficient p-type semiconductors: benzothienobenzothiophene (BTBT) (33 meV for hole transport).¹⁰⁸ The charge hopping rate (k_CT) in Au₁₈@1 can be modeled using the high-temperature limit of the Marcus theory.^68–72 The latter is governed by two key parameters, the reorganization energy (λ) and intermolecular effective electronic coupling for electron transport (V),^68–72 where T is the temperature and k_b is the Boltzmann constant, as given by eqn (2):

The adiabatic potential energy surfaces method was used to calculate the reorganization energy (λ).¹⁰⁹ This involved the neutral and anionic optimized geometries of the isolated guest and host at neutral and anionic states, given by ^GE₀ and ^GE₋, and ^HE₀ and ^HE₋, respectively, taking into account solvation effects via the COSMO approach amounting to 0.17 eV, which is given by eqn (3):

λ = λ₀ + λ₋ = (^HE^anion₀ − ^HE^neutral₀ + ^GE^anion₀ − ^GE^neutral₀) + (^HE^neutral₋ − ^HE^anion₋ + ^GE^neutral₋ − ^GE^anion₋) (3)

As a result, the estimated intermolecular charge hopping rate (k_CT) was 2.2 × 10¹³ s⁻¹ at 300 K, in the range of ultrafast electron transfer in supramolecular aggregates (4.0 × 10¹¹ s⁻¹),¹¹⁰ and from other intermolecular interactions (5.0 × 10¹² s⁻¹).¹¹¹ Thus, the Au₁₈ → 1 charge transfer was driven by coordinating and short contacts towards the superphane available cavity, resulting in a supramolecular structure of donor(Au₁₈)–acceptor(superphane) D–A system.

Conclusions

The favorable incorporation of the Au₁₈ cluster into the polyfunctional superphane cavity 1 amounted to −145.3 kcal mol⁻¹, mainly provided by electrostatic-type interactions (54.9%), leading to a stable Au₁₈@1 aggregate. The electrostatic contribution was given by charge reorganization over Au₁₈, which enhanced the interaction towards the 1 cavity, and accounted for the observed stability of the overall host–guest pair. Charge transfer involved host ← guest and host → guest backdonation through S ← Au and S → Au contacts, leading to an overall Au₁₈ → 1 superphane charge transfer of +0.79e. Hypothetical models involving isolated cavity units suggested that the Au18–thiophane interaction amounted to −53.9 kcal mol⁻¹, accounting for 37.1% of the overall stabilization, whereas the isolated methyl ether units from BINOL motifs amounted to −59.7 kcal mol⁻¹, accounting for 41.1% of the Au₁₈@1 aggregate stabilization. Moreover, non-covalent interactions contributed to a lesser extent to Au₁₈@1 formation.

The resulting aggregate showed favorable charge transfer. A charge hopping rate (k_CT) in the range of ultrafast electron transfer, calculated to be 2.2 × 10¹³ s⁻¹ at 300 K, was documented. Thus, the Au₁₈ → 1 charge transfer was driven by coordinating and short contacts towards the superphane available cavity, resulting in the supramolecular structure of the D–A system.

We expect that the current approach could help to characterize further the stabilizing factors leading to the aggregation of metallic clusters into organic host cavities. This strategy may aid rational design and explorative synthetic efforts, providing novel functionality for host–guest aggregates.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5cp01989c.

Acknowledgements

M. P.-C. acknowledges a PhD scholarship from Vicerrectoría de Investigación y Doctorados, Universidad San Sebastián. This work was supported by ANID FONDECYT Regular 1221676. P. L. R.-K. would like to thank the CIMAT Supercomputing Laboratories of Guanajuato and Puerto Interior for their support.

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