DOI: 10.1002/chem.201500006
Communication
& Supramolecular Tetrapods
Designing Artificial 3D Helicates: Unprecedented Self-Assembly of Homo-octanuclear Tetrapods with Europium Soumaila Zebret,[a] Eliane Vçgele,[a] Tomas Klumpler,[b] and Josef Hamacek*[a, c] Abstract: Herein, we report on the rational design, preparation and characterization of a novel homo-octanuclear helicate, which results from a spatial extension of the central tetranuclear platform. The 3D supramolecular assembly is obtained by complexing europium(III) with a new hexatopic tripodal ligand. The isolated octanuclear helicate is fully characterized by different methods clearly evidencing the structure predicted with molecular modelling. The ligand preorganization plays a crucial role in a successful self-assembly process and induces the formation of a well-defined triple-stranded helical structure. This prototypal octanuclear edifice accommodating functional lanthanides within a 3D scaffold offers attractive perspectives for further applications.
After intense development of supramolecular chemistry during the last two decades,[1] the conception of new multicomponent assemblies is nowadays focused on designing more sophisticated systems with exciting functionalities. In many cases, these functions are mediated by metal ions inserted in the structure of polynuclear homo- and/or heterometallic compounds. In this context, lanthanide-containing systems[2] and nanomaterials[3] are of persistent interest because of their peculiar optical and magnetic properties.[4] The incorporation of a high number of lanthanide cations in a supramolecular scaffold may exhibit synergic effects on desired behaviour and properties (i.e., relaxivity,[5] light emission,[6] etc.). In addition, accommodating different metal ions within the same molecule can be exploited, for example, in magnetic[7] and/or optical devices[8] and in multimodal agents for biomedical imaging,[9, 10] but their controlled preparation remains challenging, especially in the case of 4f heteronuclear systems.[11, 12]
[a] Dr. S. Zebret, E. Vçgele, Prof. J. Hamacek Department of Inorganic and Analytical Chemistry, University of Geneva 30 quai Ernest-Ansermet, 1211 Geneva 4 (Switzerland) E-mail: [email protected] [b] Dr. T. Klumpler Core Facility X-ray Diffraction and Bio-SAXS Central European Technology Institute Kamenice 753/5, 625 00 Brno (Czech Republic) [c] Prof. J. Hamacek Current address: CBM CNRS Orl¦ans/University of Orl¦ans Rue Charles Sadron, 45071 Orleans Cedex 2 (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500006. Chem. Eur. J. 2015, 21, 6695 – 6699
Recently, several tetranuclear tetrahedral complexes built with lanthanides have been investigated.[13–15] These polynuclear edifices may exhibit several functionalities such as selective host–guest interactions,[16, 17] light emission in the NIR–visible region, energy transfer and conversion. In this context, a spatial extension of the central tetrahedron in the direction of C3 axes is a challenging task allowing the introduction of additional binding sites for metallic cations in order to obtain helicoidal homo- or hetero-octanuclear complexes with tetrahedral topology. There are only few reports on octanuclear lanthanide-containing assemblies,[18] but any system exploits similar concept. Herein we report on an unprecedented strict selfassembly of an octanuclear helicate resulting from the reaction of a new polydentate ligand L1 with europium(III). This original supramolecular platform accommodates eight metal ions in a single 3D helical scaffold and its structure is unambiguously characterised with physicochemical methods assisted by computational modelling. A tripodal receptor leading potentially to the desired octanuclear complex should be carefully designed, since small structural deviations may significantly influence the assembly process with lanthanide cations. We have considered a new hexatopic tripodal ligand L1 as the extension of the tripodal ligands L2[13] and L3.[19] In L1, three strands are linked to the same central anchor and bear two pyridyldicarbonyl coordination moieties connected through a spacer, X, which may vary in length or rigidity (Scheme 1). Our choice was finally turned to a diphenylmethane spacer (dpm = X). The targeted tripodal ligand L1 is synthesized by coupling the activated tris-carboxylic acid 2 with the monoamine compound 3.[19] The targeted octanuclear complex [Eu8L14]24 + is supposed to be formed by the treatment of L1 with EuIII, which is chosen as a representative cation for the lanthanide series. Moreover, its complexation can be conveniently followed by NMR spectroscopy or spectrophotometric methods. To obtain better structural insight in this assembly, a molecular model (MM) was built by combining crystal structures of the dinuclear NdIII helicate with L4[21] and the TbIII tetrahedral complex.[13] The geometry of the merged system with EuIII was further optimized with the parameters Sparkle/AM1[20] using the MOPAC 2009 program and this routine converges to the supramolecular structure shown in Figure 1 and Figure S1 in the Supporting Information. The structure of [Eu8L14]24 + possesses a global tetrahedral symmetry. Each ligand coordinates six different EuIII cations and each of the eight cations is nine coordinated by three dicarbonylpyridine units from three different ligands. In term of shape, the assembly is compared with a giant tetrahedron
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Scheme 1. Structure of ligands L1–L4.
with the central tetranuclear cluster and four vertices corresponding to outer cations coordinated by ligand strands. If we approximate the shape of the octanuclear complex as spherical, the radius of this sphere RMM can be estimated from the volume delimited by the Connolly surface[21] constructed around the MM and accessible to solvent molecules (Figure S1c).[22] Accordingly, this volume amounts to 7625 æ3 and the related radius is equal to 12.2 æ.
Figure 1. View of the calculated molecular structure of [Eu8L14]24 + (Sparkle/ AM1). Hydrogen atoms are omitted for clarity. Ligands are represented in greyscale.
To probe the formation of octanuclear complexes, 1H NMR titration of L1 dissolved in a mixed solvent (CDCl3/CD3CN) was performed by stepwise additions of Eu(ClO4)3 in CD3CN. However, the peaks of solvents dominate the spectrum during the titration and small peaks close to the baseline indicate the presence of some low-symmetrical species. At the same time, a colloidal precipitate points to the formation of low-soluble intermediates, which occur prior to a thermodynamic reorganization of the system. In order to circumvent these problems, a reversed titration in metal excess was carried out by adding L1 to a solution of Eu(ClO4)3. The ratio [Eu]/[L1] varied from six to two and at least 18 h was permitted between the metal addition and spectrum recording. No precipitation occurred in these conditions. When the stoichiometric ratio [Eu]/[L1] = 2 was reached, the evolution of the 1H NMR spectrum was folChem. Eur. J. 2015, 21, 6695 – 6699
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lowed during one month to allow equilibration. The main changes are detected during the first week and relatively well-resolved spectra are afforded after nine days. Only minor changes are noticeable thereafter (Figure S2 in the Supporting Information). In comparison with the 1 H NMR spectrum of L1, the differences in chemical shifts clearly indicate a complexation with EuIII. The number of major peaks is reminiscent with that of the free ligand, which suggests a high symmetry of the complex. ESI mass spectrometry was employed for confirming the presence and the stoichiometry of the putative octanuclear complex. Indeed, the spectrum in Figure S3a in the Supporting Information measured for the solution with [Eu]/[L1] = 2, clearly shows intense signals for the series of the perchlorate adducts [Eu8L14(ClO4)n](24¢n) + . Additional peaks are attributed to the species [Eu8L14(ClO4)n(¢m H + )](24¢n¢m) + appearing predominantly for highly charged adducts and at higher ionization voltage. The comparison of experimental (high resolution ESI-MS) and calculated isotopic profiles shows very good agreement and exactly reflects the composition and the charge of octanuclear species (Figure S4 in the Supporting Information). The latter NMR titration also shows that the octanuclear self-assembly readily occurs in metal excess, which favours a thermodynamic re-equilibration of the mixture into stable octanuclear complexes. EuIII cations may thus “catalyse” self-repairing steps by facilitating the ligand exchange, in line with the formation mechanism reported for dinuclear helicates.[23] In order to verify and support this observation, the reaction mixtures with the ratio [Eu]/[L1] equal to 4, 6 and 8 were prepared. Their NMR spectra changed considerably within the first two days and then evolved slowly up to one week. Indeed, the final spectra for these solutions are comparable with the spectrum for [Eu]/[L1] = 2 (Figure S5 in the Supporting Information), which points to the presence of the same complex [Eu8L14]24 + . This is also shown with the ESI-MS analyses, confirming octanuclear complexes as major species in these solutions (Figure S3 in the Supporting Information). Finally, the targeted octanuclear complex was prepared by treating L1 with 4 equiv of EuIII. The ESMS spectrum of the isolated octanuclear complex is given in Figure 2 and shows the series of its different perchlorate adducts as discussed above. The NMR spectrum in Figure 3 is relatively simple with well resolved peaks despite some broadening due to a higher correlation time and due to the paramagnetic effect of EuIII. The spectrum with 23 signals is fully consistent with the molecular model in Figure 1. All four ligands L1 in the complex are equivalent, and all three strands of each are related in a C3 symmetry. Full assignment of proton signals is achieved with the help of 2D NMR spectroscopy techniques like COSY (Figure S6 in the Supporting Information), HSQC and NOESY. A strong para-
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Communication magnetic shift of H1 (Dd > 10 ppm) indicates its location in the centre of the tetrahedron (endo-CH3 conformation). This chemical shift at 298 K is smaller than in [Eu4L24]12 + , which may point to some subtle structural deviations within the octanuclear complex due to the spatial extension. Six signals are attributed to diastereotopic protons of methylene groups (H2H2’, H16-H16’ and H18-H18’), which indicates that the complexation of L1 strands occurs in a helical fashion. Moreover, since H20 is enantiotopic, this helical twist about EuIII cations is propagated from the central tetrahedron toward external sites to provide homochiral structures (P8- and M8-forms). In addition, the DOSY analysis provides the diffusion coefficient in acetonitrile (4.0(1) Õ 10¢10 m2 s¢1). Considering a spherical shape of [Eu8L14]24 + , the hydrodynamic diameter of the complex corresponds to 14.8 æ, which reasonably agrees with the RMM. The NMR spectrum of the octanuclear complex [Eu8L14]24 + can be compared with the parent compounds: the tetranuclear complex [Eu4L24]12 + and the dinuclear helicate [Eu2L43]6 + . Their
Figure 2. ESI-MS spectrum of the isolated octanuclear complex redissolved in acetonitrile. Only main perchlorate adducts are annotated. Minor peaks (* [Eu6L14(ClO4)n]18¢n ; # [EuL1(ClO4)]2 + ) result from a partial fragmentation in the electrospray.
Figure 3. 1H NMR spectrum of the octanuclear complex [Eu8L14]24 + with peak assignment (CD3CN, 298 K). Chem. Eur. J. 2015, 21, 6695 – 6699
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spectra are traced together in Figure S7 in the Supporting Information and a good correspondence between related peaks is found. Since only poor X-ray diffraction data were obtained despite many crystallization attempts, we resorted to the small angle X-ray scattering (SAXS) methodology[24] in order to correlate the calculated molecular model with the structure of [Eu8L14]24 + in acetonitrile. The tools common in the low-resolution structural studies of proteins and macromolecular complexes such as theoretical scattering evaluation and fitting and dummy atom modelling were applied. The SAXS data collection of the acetonitrile solution of [Eu8L14]24 + is summarized in Table S1 in Supporting Information. Experimentally determined SAXS-derived parameters of the [Eu8L14]24 + complex are the radius of gyration (Rg) of 17.4 æ, the maximum particle diameter of 50 æ and the particle volume estimate of 11 335 æ3. In order to establish the relation between experimental parameters and the molecular structure of [Eu8L14]24 + calculated by Sparkle/AM1, we resorted to the evaluation of the theoretical solution scattering of the EuIII octanuclear complexes with CRYSOL.[25] Indeed, the theoretical structural parameters of the [Eu8L14]24 + molecular model as the envelope Rg of 18.7 æ, the envelope volume 13 740 æ3 and the envelope diameter 54 æ are well comparable with SAXS parameters derived directly from experimental data. The overall CRYSOL fit of the theoretical [Eu8L14]24 + molecular model scattering against experimental SAXS data is very good (Figure S8 in the Supporting Information) with the final c value of 1.024. The ab initio shape reconstruction of the octanuclear complex in solution was performed using dummy atom modelling with P3 symmetry constrain as it is implemented in DAMMIF.[26] The resulting ab initio model has the tetrapodal shape compatible with the [Eu8L14]24 + molecular model, as shown in Figure 4. The results of the SAXS analysis are very satisfactory and bring experimental insight into the solution shape of octanuclear complexes. The speciation of the Eu–L1 system was elucidated by performing a batch spectrophotometric titration of L1 with Eu(ClO4)3 (Figure S9a in the Supporting Information). The variation of absorption as a function of the [Eu]/[L1] ratio is shown for selected wavelengths in Figure S9b. The whole set of spectrophotometric data was analysed with the SPECFIT program
Figure 4. Superimposition of the ab initio SAXS envelope contoured at 8 æ resolution (grey mesh) and the [Eu8L14]24 + molecular model generated by Sparkle/AM1. a) Top view, b) side view.
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Communication and the best fit indicates a successive formation of complex species with stoichiometries EuL12, Eu4L14, Eu6L14 and Eu8L14. The calculated electronic spectra of these species are given in Figure S9c. The stability constant of [Eu8L14]24 + fitted with SPECFIT amounts to log b = 62.1(8). The species distribution along the titration is shown in Figure S9d and illustrates the zone of predominance of [Eu8L14]24 + . Interestingly, the stability constant of the octanuclear complex can be compared with the one predicted with the thermodynamic affinity free energy model.[27] This modern tool considers different microscopic contributions to characterize supramolecular assemblies (see the Supporting Information) and understand driving forces. Using judicious parameterisation of the octanuclear system and the values fitted previously for [Eu4L24]12 + , the calculated semi-empirical stability constant of [Eu8L14]24 + amounts to log b = 69(7), which closely agrees, within estimated errors, with spectrophotometric data. In this context let us comment on stabilizing contributions appearing in the model. The statistical factor, w, accounting for the change in rotational degeneracy during the octanuclear assembly (log w = 7) reflects relatively important entropy gain (~ 40 kJ mol¢1 at 298 K) due to a high molecular symmetry of [Eu8L14]24 + . The microscopic affinity, k, associated with each connection between one tridentate moiety of L1 and EuIII cation represents a favourable free energy change. In the complex, 24 virtual connections exist, which globally generate a sufficient energy gain to overcome repulsive interactions between cations. The overall thermodynamic balance is thus favourable for the assembly process and the octanuclear edifice is stabilized even in metal excess, while [Eu4L24]12 + with only 12 connections is destroyed.[14] In summary, the rational design of the octanuclear edifice with europium, efficiently supported by molecular modelling, exploits axial extensions of the tetranuclear platform investigated previously. Indeed, the treatment of L1 with europium provides octanuclear helicates [Eu8L14]24 + , the stoichiometry and structure of which were undoubtedly confirmed by mass spectrometry and NMR analyses. To the best of our knowledge, it is the first discrete octanuclear assembly with a triple-stranded helical structure in 3D. All ligand strands are wrapped about europium cations with the same helicity and the helical twist is cooperatively propagated from the central tetrahedron toward four more distant binding sites. The complex exists in solution as a racemic mixture of homochiral enantiomers. Direct evidence for the molecular shape of [Eu8L14]24 + is obtained from SAXS measurements. The ab initio SAXS envelope is reminiscent of a tetrapod and corresponds excellently with the molecular model. Individual well-defined nanoparticles have the hydrodynamic diameter of 3 nm and the mass of approximately 8 kDa (without counterions), which compares with a small protein of about 74 amino acids. The europium octanuclear complex is obtained as a thermodynamic product at stoichiometry and in metal excess. The spectrophotometric titration allowed the determination of its stability constant, which in a good agreement with the predicted value. In addition to appealing fundamental structural aspects, the self-assembly of the octanuclear 3D platform paves the way for further develChem. Eur. J. 2015, 21, 6695 – 6699
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opment of different applications by a judicious design of polytopic tripodal receptors, for example, light-converting devices and nanomaterials.
Experimental Section Starting materials and synthesis Chemicals were purchased from Acros Organics and Sigma-Aldrich and were used without further purification. The synthesis and characterization of L1 is described in the Supporting Information.
Physical measurements 1
H, 13C, COSY, HSQC, NOESY and DOSY NMR spectra were recorded on a high-field NMR spectrometer (400 MHz or 600 MHz, Bruker). Standard electrospray mass spectra (ESI-MS) were recorded with an API 150EX LC/MS system (Applied Biosystems/MDS SCIEX). ESI-MS spectra of EuIII complexes were recorded from 10¢4–10¢5 m acetonitrile solutions on a Finnigan SSQ7000 instrument with the optimized ionization temperature (180 8C). High-resolution spectra were recorded on a 4 GHz MaXis ultra-high-resolution QTOF mass spectrometer from Bruker Daltonics (Germany). Absorption spectra were recorded with a Perkin–Elmer Lambda 900 spectrometer using quartz cells of 1 mm pathlength. Excitation and emission spectra were recorded on a Perkin–Elmer LS-50B spectrometer.
Synthesis and characterisation of [Eu8L14](ClO4)24 + In a typical procedure, 5 mg (0.05 mmoles) of L1 was dissolved in 0.6 mL of CH3CN/CHCl3 (1:1 v/v) and added slowly under stirring to four equivalents of Eu(ClO4)3·x H2O dissolved in 1 mL of CH3CN. The mixture was allowed to equilibrate for 3 days at 50 8C. This solution was concentrated by evaporating solvents and a slow diffusion of tert-butylmethylether gave a white-off material, which was isolated by filtration. The solid was further washed with tert-butylmethylether and dried under vacuum to collect 7–8 mg of the complex. Related analytical data and the spectrophotometric titration are described in the Supporting Information.
SAXS analysis and shape reconstitution The SAXS data were collected on the BioSAXS-1000, Rigaku at CEITEC (Brno). Data were collected at 293.15 K with X-ray beam wavelength 1.54 æ. Sample to detector (PILATUS 100 K, Dectris Ltd.) distance was 0.4 m covering a scattering vector range from 0.033 to 0.65 æ¢1. For solvent and sample one two-dimensional image was collected with an exposure time of 20 min per image. Radial averaging of two-dimensional scattering images was performed using SAXSLab3.0.0r1, Rigaku. The solvent subtraction was performed using PRIMUS.[28] The evaluation of the theoretical solution scattering of the [Eu8L14]24 + atomic model generated by Sparkle/AM1 and fitting to experimental data was performed with CRYSOL,[25] where the maximum scattering vector was set to 0.2 æ¢1, explicit hydrogen atoms were taken in account, automatic constant subtraction was allowed, while other parameters were kept at default. The ab initio shape reconstruction was performed with DAMMIF,[26] where the symmetry was set to P3, while other parameters were kept at default. The superimposition of the ab initio SAXS envelope and the atomic model of [Eu8L14]24 + generated by Sparkle/AM1 was performed with SUPCOMB.[29] For graphical representation of ab initio SAXS envelopes, the density map contoured at 8 æ resolution of the DAMMIF dummy atom model was generated with UCSF Chimera.[30]
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Acknowledgements Financial supports from the University of Geneva and Swiss National Science Foundation are gratefully acknowledged. We acknowledge E. Sandmeier for measuring ESMS spectra, K. Buchwalder for performing the elemental analyses, and P.-Y. Morgantini for helping with molecular modelling (all from the University of Geneva). We thank G. Gabant from the platform of mass spectrometry and proteomics at CBM in Orl¦ans (France) for measuring high-resolution mass spectra. The X-ray part of the work was realized in the Central European Institute of Technology with research infrastructure supported by the project CZ.1.05/1.1.00/02.0068 financed from European Regional Development Fund. This research was supported by project Employment of Best Young Scientists for International Cooperation Empowerment, reg. number CZ.1.07/2.3.00/30.0037 co-financed by the European Social Fund and the state budget of the Czech Republic. Keywords: europium · helicates · octanuclear · self-assembly · supramolecular chemistry [1] J. W. Steed, J. L. Atwood, in Supramolecular Chemistry, Wiley, New York, 2009. [2] C. Piguet, J.-C. G. Bìnzli in Handbook on the Physics and Chemistry of Rare Earths, Vol. 40 (Eds.: K. A. Gschneidner, J.-C. G. Bìnzli, V. K. Pecharsky), Elsevier, Amsterdam, 2010, pp. 301 – 553. [3] a) K. Binnemans, Chem. Rev. 2009, 109, 4283 – 4374; b) J. Feng, H. Zhang, Chem. Soc. Rev. 2013, 42, 387 – 410. [4] J.-C. G. Bìnzli in Lanthanide Probes in Life, Chemical and Earth Sciences (Eds.: J.-C. G. Bìnzli, G. R. Choppin,), Elsevier, Amsterdam, 1989. [5] G. M. Nicolle, E. Toth, H. Schmitt-Willich, B. Radìchel, A. Merbach, Chem. Eur. J. 2002, 8, 1040 – 1048. [6] A. Foucault-Collet, C. M. Shade, I. Nazarenko, S. Petoud, S. V. Eliseeva, Angew. Chem. Int. Ed. 2014, 53, 2927 – 2930; Angew. Chem. 2014, 126, 2971 – 2974. [7] a) J. Jankolovits, J. W. Kampf, V. L. Pecoraro, Polyhedron 2013, 52, 491 – 499; b) V. Chandrasekhar, P. Bag, W. Kroener, K. Gieb, P. Mìller, Inorg. Chem. 2013, 52, 13078 – 13086.
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[8] L. Aboshyan-Sorgho, C. Besnard, P. Pattison, K. R. Kittilstved, A. Aebischer, J.-C. G. Bìnzli, A. Hauser, C. Piguet, Angew. Chem. Int. Ed. 2011, 50, 4108 – 4112; Angew. Chem. 2011, 123, 4194 – 4198. [9] C. S. Bonnet, F. Buron, F. Caill¦, C. M. Shade, B. Drahosˇ, L. Pellegatti, J. Zhang, S. Villette, L. Helm, C. Pichon, F. Suzenet, S. Petoud, Ê. Tûth, Chem. Eur. J. 2012, 18, 1419 – 1431. [10] J. Cheon, J.-H. Lee, Acc. Chem. Res. 2008, 41, 1630 – 1640. [11] M. P. Placidi, A. J. L. Villaraza, L. S. Natrajan, D. Sykes, A. M. Kenwright, S. Faulkner, J. Am. Chem. Soc. 2009, 131, 9916 – 9917. [12] a) A. M. Johnson, M. C. Young, X. Zhang, R. R. Julian, R. J. Hooley, J. Am. Chem. Soc. 2013, 135, 17723 – 17726; b) N. Andr¦, T. B. Jensen, R. Scopelliti, D. Imbert, M. Elhabiri, G. Hopfgartner, C. Piguet, J.-C. G. Bìnzli, Inorg. Chem. 2004, 43, 515 – 529. [13] J. Hamacek, G. Bernardinelli, Y. Filinchuk, Eur. J. Inorg. Chem. 2008, 3419 – 3422. [14] J. Hamacek, C. Besnard, T. Penhouet, P.-Y. Morgantini, Chem. Eur. J. 2011, 17, 6753 – 6764. [15] S. Zebret, C. Besnard, G. Bernardinelli, J. Hamacek, Eur. J. Inorg. Chem. 2012, 2409 – 2417. [16] B. El Aroussi, L. Gu¦n¦e, P. Pal, J. Hamacek, Inorg. Chem. 2011, 50, 8588 – 8597. [17] J. Hamacek, D. Poggiali, S. Zebret, B. E. Aroussi, M. W. Schneider, M. Mastalerz, Chem. Commun. 2012, 48, 1281 – 1283. [18] a) J. Xu, K. N. Raymond, Angew. Chem. Int. Ed. 2000, 39, 2745 – 2747; Angew. Chem. 2000, 112, 2857 – 2859; b) H. Hou, Y. Wei, Y. Song, Y. Fan, Y. Zhu, Inorg. Chem. 2004, 43, 1323 – 1327; c) V. Chandrasekhar, P. Bag, E. Colacio, Inorg. Chem. 2013, 52, 4562 – 4570; d) A. J. Metherell, M. D. Ward, RSC Adv. 2013, 3, 14281 – 14285. [19] B. El Aroussi, S. Zebret, C. Besnard, P. Perrottet, J. Hamacek, J. Am. Chem. Soc. 2011, 133, 10764 – 10767. [20] R. O. Freire, G. B. Rocha, A. M. Simas, Inorg. Chem. 2005, 44, 3299 – 3310. [21] M. Connolly, J. Appl. Crystallogr. 1983, 16, 548 – 558. [22] N. Dalla Favera, L. Gu¦n¦e, G. Bernardinelli, C. Piguet, Dalton Trans. 2009, 7625 – 7638. [23] M. Elhabiri, J. Hamacek, J.-C. G. Bìnzli, A. M. Albrecht-Gary, Eur. J. Inorg. Chem. 2004, 51 – 62. [24] S. Yang, Adv. Mater. 2014, 26, 7902 – 7910. [25] D. Svergun, C. Barberato, M. H. J. Koch, J. Appl. Crystallogr. 1995, 28, 768 – 773. [26] D. Franke, D. I. Svergun, J. Appl. Crystallogr. 2009, 42, 342 – 346. [27] a) C. Piguet, Chem. Commun. 2010, 46, 6209 – 6231; b) J. Hamacek, in Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics (Eds.: M. Albrecht, G. Mayaan), Wiley, New York, 2013, pp. 91 – 120. [28] P. V. Konarev, V. V. Volkov, A. V. Sokolova, M. H. J. Koch, D. I. Svergun, J. Appl. Crystallogr. 2003, 36, 1277 – 1282. [29] M. B. Kozin, D. I. Svergun, J. Appl. Crystallogr. 2001, 34, 33 – 41. [30] E. F. Pettersen, T. D. Goddard, C. C. Huang, G. S. Couch, D. M. Greenblatt, E. C. Meng, T. E. Ferrin, J. Comput. Chem. 2004, 25, 1605 – 1612. Received: January 1, 2015 Published online on March 12, 2015
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& Supramolecular Tetrapods
Designing Artificial 3D Helicates: Unprecedented Self-Assembly of Homo-octanuclear Tetrapods with Europium Soumaila Zebret,[a] Eliane Vçgele,[a] Tomas Klumpler,[b] and Josef Hamacek*[a, c] Abstract: Herein, we report on the rational design, preparation and characterization of a novel homo-octanuclear helicate, which results from a spatial extension of the central tetranuclear platform. The 3D supramolecular assembly is obtained by complexing europium(III) with a new hexatopic tripodal ligand. The isolated octanuclear helicate is fully characterized by different methods clearly evidencing the structure predicted with molecular modelling. The ligand preorganization plays a crucial role in a successful self-assembly process and induces the formation of a well-defined triple-stranded helical structure. This prototypal octanuclear edifice accommodating functional lanthanides within a 3D scaffold offers attractive perspectives for further applications.
After intense development of supramolecular chemistry during the last two decades,[1] the conception of new multicomponent assemblies is nowadays focused on designing more sophisticated systems with exciting functionalities. In many cases, these functions are mediated by metal ions inserted in the structure of polynuclear homo- and/or heterometallic compounds. In this context, lanthanide-containing systems[2] and nanomaterials[3] are of persistent interest because of their peculiar optical and magnetic properties.[4] The incorporation of a high number of lanthanide cations in a supramolecular scaffold may exhibit synergic effects on desired behaviour and properties (i.e., relaxivity,[5] light emission,[6] etc.). In addition, accommodating different metal ions within the same molecule can be exploited, for example, in magnetic[7] and/or optical devices[8] and in multimodal agents for biomedical imaging,[9, 10] but their controlled preparation remains challenging, especially in the case of 4f heteronuclear systems.[11, 12]
[a] Dr. S. Zebret, E. Vçgele, Prof. J. Hamacek Department of Inorganic and Analytical Chemistry, University of Geneva 30 quai Ernest-Ansermet, 1211 Geneva 4 (Switzerland) E-mail: [email protected] [b] Dr. T. Klumpler Core Facility X-ray Diffraction and Bio-SAXS Central European Technology Institute Kamenice 753/5, 625 00 Brno (Czech Republic) [c] Prof. J. Hamacek Current address: CBM CNRS Orl¦ans/University of Orl¦ans Rue Charles Sadron, 45071 Orleans Cedex 2 (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500006. Chem. Eur. J. 2015, 21, 6695 – 6699
Recently, several tetranuclear tetrahedral complexes built with lanthanides have been investigated.[13–15] These polynuclear edifices may exhibit several functionalities such as selective host–guest interactions,[16, 17] light emission in the NIR–visible region, energy transfer and conversion. In this context, a spatial extension of the central tetrahedron in the direction of C3 axes is a challenging task allowing the introduction of additional binding sites for metallic cations in order to obtain helicoidal homo- or hetero-octanuclear complexes with tetrahedral topology. There are only few reports on octanuclear lanthanide-containing assemblies,[18] but any system exploits similar concept. Herein we report on an unprecedented strict selfassembly of an octanuclear helicate resulting from the reaction of a new polydentate ligand L1 with europium(III). This original supramolecular platform accommodates eight metal ions in a single 3D helical scaffold and its structure is unambiguously characterised with physicochemical methods assisted by computational modelling. A tripodal receptor leading potentially to the desired octanuclear complex should be carefully designed, since small structural deviations may significantly influence the assembly process with lanthanide cations. We have considered a new hexatopic tripodal ligand L1 as the extension of the tripodal ligands L2[13] and L3.[19] In L1, three strands are linked to the same central anchor and bear two pyridyldicarbonyl coordination moieties connected through a spacer, X, which may vary in length or rigidity (Scheme 1). Our choice was finally turned to a diphenylmethane spacer (dpm = X). The targeted tripodal ligand L1 is synthesized by coupling the activated tris-carboxylic acid 2 with the monoamine compound 3.[19] The targeted octanuclear complex [Eu8L14]24 + is supposed to be formed by the treatment of L1 with EuIII, which is chosen as a representative cation for the lanthanide series. Moreover, its complexation can be conveniently followed by NMR spectroscopy or spectrophotometric methods. To obtain better structural insight in this assembly, a molecular model (MM) was built by combining crystal structures of the dinuclear NdIII helicate with L4[21] and the TbIII tetrahedral complex.[13] The geometry of the merged system with EuIII was further optimized with the parameters Sparkle/AM1[20] using the MOPAC 2009 program and this routine converges to the supramolecular structure shown in Figure 1 and Figure S1 in the Supporting Information. The structure of [Eu8L14]24 + possesses a global tetrahedral symmetry. Each ligand coordinates six different EuIII cations and each of the eight cations is nine coordinated by three dicarbonylpyridine units from three different ligands. In term of shape, the assembly is compared with a giant tetrahedron
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Scheme 1. Structure of ligands L1–L4.
with the central tetranuclear cluster and four vertices corresponding to outer cations coordinated by ligand strands. If we approximate the shape of the octanuclear complex as spherical, the radius of this sphere RMM can be estimated from the volume delimited by the Connolly surface[21] constructed around the MM and accessible to solvent molecules (Figure S1c).[22] Accordingly, this volume amounts to 7625 æ3 and the related radius is equal to 12.2 æ.
Figure 1. View of the calculated molecular structure of [Eu8L14]24 + (Sparkle/ AM1). Hydrogen atoms are omitted for clarity. Ligands are represented in greyscale.
To probe the formation of octanuclear complexes, 1H NMR titration of L1 dissolved in a mixed solvent (CDCl3/CD3CN) was performed by stepwise additions of Eu(ClO4)3 in CD3CN. However, the peaks of solvents dominate the spectrum during the titration and small peaks close to the baseline indicate the presence of some low-symmetrical species. At the same time, a colloidal precipitate points to the formation of low-soluble intermediates, which occur prior to a thermodynamic reorganization of the system. In order to circumvent these problems, a reversed titration in metal excess was carried out by adding L1 to a solution of Eu(ClO4)3. The ratio [Eu]/[L1] varied from six to two and at least 18 h was permitted between the metal addition and spectrum recording. No precipitation occurred in these conditions. When the stoichiometric ratio [Eu]/[L1] = 2 was reached, the evolution of the 1H NMR spectrum was folChem. Eur. J. 2015, 21, 6695 – 6699
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lowed during one month to allow equilibration. The main changes are detected during the first week and relatively well-resolved spectra are afforded after nine days. Only minor changes are noticeable thereafter (Figure S2 in the Supporting Information). In comparison with the 1 H NMR spectrum of L1, the differences in chemical shifts clearly indicate a complexation with EuIII. The number of major peaks is reminiscent with that of the free ligand, which suggests a high symmetry of the complex. ESI mass spectrometry was employed for confirming the presence and the stoichiometry of the putative octanuclear complex. Indeed, the spectrum in Figure S3a in the Supporting Information measured for the solution with [Eu]/[L1] = 2, clearly shows intense signals for the series of the perchlorate adducts [Eu8L14(ClO4)n](24¢n) + . Additional peaks are attributed to the species [Eu8L14(ClO4)n(¢m H + )](24¢n¢m) + appearing predominantly for highly charged adducts and at higher ionization voltage. The comparison of experimental (high resolution ESI-MS) and calculated isotopic profiles shows very good agreement and exactly reflects the composition and the charge of octanuclear species (Figure S4 in the Supporting Information). The latter NMR titration also shows that the octanuclear self-assembly readily occurs in metal excess, which favours a thermodynamic re-equilibration of the mixture into stable octanuclear complexes. EuIII cations may thus “catalyse” self-repairing steps by facilitating the ligand exchange, in line with the formation mechanism reported for dinuclear helicates.[23] In order to verify and support this observation, the reaction mixtures with the ratio [Eu]/[L1] equal to 4, 6 and 8 were prepared. Their NMR spectra changed considerably within the first two days and then evolved slowly up to one week. Indeed, the final spectra for these solutions are comparable with the spectrum for [Eu]/[L1] = 2 (Figure S5 in the Supporting Information), which points to the presence of the same complex [Eu8L14]24 + . This is also shown with the ESI-MS analyses, confirming octanuclear complexes as major species in these solutions (Figure S3 in the Supporting Information). Finally, the targeted octanuclear complex was prepared by treating L1 with 4 equiv of EuIII. The ESMS spectrum of the isolated octanuclear complex is given in Figure 2 and shows the series of its different perchlorate adducts as discussed above. The NMR spectrum in Figure 3 is relatively simple with well resolved peaks despite some broadening due to a higher correlation time and due to the paramagnetic effect of EuIII. The spectrum with 23 signals is fully consistent with the molecular model in Figure 1. All four ligands L1 in the complex are equivalent, and all three strands of each are related in a C3 symmetry. Full assignment of proton signals is achieved with the help of 2D NMR spectroscopy techniques like COSY (Figure S6 in the Supporting Information), HSQC and NOESY. A strong para-
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Communication magnetic shift of H1 (Dd > 10 ppm) indicates its location in the centre of the tetrahedron (endo-CH3 conformation). This chemical shift at 298 K is smaller than in [Eu4L24]12 + , which may point to some subtle structural deviations within the octanuclear complex due to the spatial extension. Six signals are attributed to diastereotopic protons of methylene groups (H2H2’, H16-H16’ and H18-H18’), which indicates that the complexation of L1 strands occurs in a helical fashion. Moreover, since H20 is enantiotopic, this helical twist about EuIII cations is propagated from the central tetrahedron toward external sites to provide homochiral structures (P8- and M8-forms). In addition, the DOSY analysis provides the diffusion coefficient in acetonitrile (4.0(1) Õ 10¢10 m2 s¢1). Considering a spherical shape of [Eu8L14]24 + , the hydrodynamic diameter of the complex corresponds to 14.8 æ, which reasonably agrees with the RMM. The NMR spectrum of the octanuclear complex [Eu8L14]24 + can be compared with the parent compounds: the tetranuclear complex [Eu4L24]12 + and the dinuclear helicate [Eu2L43]6 + . Their
Figure 2. ESI-MS spectrum of the isolated octanuclear complex redissolved in acetonitrile. Only main perchlorate adducts are annotated. Minor peaks (* [Eu6L14(ClO4)n]18¢n ; # [EuL1(ClO4)]2 + ) result from a partial fragmentation in the electrospray.
Figure 3. 1H NMR spectrum of the octanuclear complex [Eu8L14]24 + with peak assignment (CD3CN, 298 K). Chem. Eur. J. 2015, 21, 6695 – 6699
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spectra are traced together in Figure S7 in the Supporting Information and a good correspondence between related peaks is found. Since only poor X-ray diffraction data were obtained despite many crystallization attempts, we resorted to the small angle X-ray scattering (SAXS) methodology[24] in order to correlate the calculated molecular model with the structure of [Eu8L14]24 + in acetonitrile. The tools common in the low-resolution structural studies of proteins and macromolecular complexes such as theoretical scattering evaluation and fitting and dummy atom modelling were applied. The SAXS data collection of the acetonitrile solution of [Eu8L14]24 + is summarized in Table S1 in Supporting Information. Experimentally determined SAXS-derived parameters of the [Eu8L14]24 + complex are the radius of gyration (Rg) of 17.4 æ, the maximum particle diameter of 50 æ and the particle volume estimate of 11 335 æ3. In order to establish the relation between experimental parameters and the molecular structure of [Eu8L14]24 + calculated by Sparkle/AM1, we resorted to the evaluation of the theoretical solution scattering of the EuIII octanuclear complexes with CRYSOL.[25] Indeed, the theoretical structural parameters of the [Eu8L14]24 + molecular model as the envelope Rg of 18.7 æ, the envelope volume 13 740 æ3 and the envelope diameter 54 æ are well comparable with SAXS parameters derived directly from experimental data. The overall CRYSOL fit of the theoretical [Eu8L14]24 + molecular model scattering against experimental SAXS data is very good (Figure S8 in the Supporting Information) with the final c value of 1.024. The ab initio shape reconstruction of the octanuclear complex in solution was performed using dummy atom modelling with P3 symmetry constrain as it is implemented in DAMMIF.[26] The resulting ab initio model has the tetrapodal shape compatible with the [Eu8L14]24 + molecular model, as shown in Figure 4. The results of the SAXS analysis are very satisfactory and bring experimental insight into the solution shape of octanuclear complexes. The speciation of the Eu–L1 system was elucidated by performing a batch spectrophotometric titration of L1 with Eu(ClO4)3 (Figure S9a in the Supporting Information). The variation of absorption as a function of the [Eu]/[L1] ratio is shown for selected wavelengths in Figure S9b. The whole set of spectrophotometric data was analysed with the SPECFIT program
Figure 4. Superimposition of the ab initio SAXS envelope contoured at 8 æ resolution (grey mesh) and the [Eu8L14]24 + molecular model generated by Sparkle/AM1. a) Top view, b) side view.
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Communication and the best fit indicates a successive formation of complex species with stoichiometries EuL12, Eu4L14, Eu6L14 and Eu8L14. The calculated electronic spectra of these species are given in Figure S9c. The stability constant of [Eu8L14]24 + fitted with SPECFIT amounts to log b = 62.1(8). The species distribution along the titration is shown in Figure S9d and illustrates the zone of predominance of [Eu8L14]24 + . Interestingly, the stability constant of the octanuclear complex can be compared with the one predicted with the thermodynamic affinity free energy model.[27] This modern tool considers different microscopic contributions to characterize supramolecular assemblies (see the Supporting Information) and understand driving forces. Using judicious parameterisation of the octanuclear system and the values fitted previously for [Eu4L24]12 + , the calculated semi-empirical stability constant of [Eu8L14]24 + amounts to log b = 69(7), which closely agrees, within estimated errors, with spectrophotometric data. In this context let us comment on stabilizing contributions appearing in the model. The statistical factor, w, accounting for the change in rotational degeneracy during the octanuclear assembly (log w = 7) reflects relatively important entropy gain (~ 40 kJ mol¢1 at 298 K) due to a high molecular symmetry of [Eu8L14]24 + . The microscopic affinity, k, associated with each connection between one tridentate moiety of L1 and EuIII cation represents a favourable free energy change. In the complex, 24 virtual connections exist, which globally generate a sufficient energy gain to overcome repulsive interactions between cations. The overall thermodynamic balance is thus favourable for the assembly process and the octanuclear edifice is stabilized even in metal excess, while [Eu4L24]12 + with only 12 connections is destroyed.[14] In summary, the rational design of the octanuclear edifice with europium, efficiently supported by molecular modelling, exploits axial extensions of the tetranuclear platform investigated previously. Indeed, the treatment of L1 with europium provides octanuclear helicates [Eu8L14]24 + , the stoichiometry and structure of which were undoubtedly confirmed by mass spectrometry and NMR analyses. To the best of our knowledge, it is the first discrete octanuclear assembly with a triple-stranded helical structure in 3D. All ligand strands are wrapped about europium cations with the same helicity and the helical twist is cooperatively propagated from the central tetrahedron toward four more distant binding sites. The complex exists in solution as a racemic mixture of homochiral enantiomers. Direct evidence for the molecular shape of [Eu8L14]24 + is obtained from SAXS measurements. The ab initio SAXS envelope is reminiscent of a tetrapod and corresponds excellently with the molecular model. Individual well-defined nanoparticles have the hydrodynamic diameter of 3 nm and the mass of approximately 8 kDa (without counterions), which compares with a small protein of about 74 amino acids. The europium octanuclear complex is obtained as a thermodynamic product at stoichiometry and in metal excess. The spectrophotometric titration allowed the determination of its stability constant, which in a good agreement with the predicted value. In addition to appealing fundamental structural aspects, the self-assembly of the octanuclear 3D platform paves the way for further develChem. Eur. J. 2015, 21, 6695 – 6699
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opment of different applications by a judicious design of polytopic tripodal receptors, for example, light-converting devices and nanomaterials.
Experimental Section Starting materials and synthesis Chemicals were purchased from Acros Organics and Sigma-Aldrich and were used without further purification. The synthesis and characterization of L1 is described in the Supporting Information.
Physical measurements 1
H, 13C, COSY, HSQC, NOESY and DOSY NMR spectra were recorded on a high-field NMR spectrometer (400 MHz or 600 MHz, Bruker). Standard electrospray mass spectra (ESI-MS) were recorded with an API 150EX LC/MS system (Applied Biosystems/MDS SCIEX). ESI-MS spectra of EuIII complexes were recorded from 10¢4–10¢5 m acetonitrile solutions on a Finnigan SSQ7000 instrument with the optimized ionization temperature (180 8C). High-resolution spectra were recorded on a 4 GHz MaXis ultra-high-resolution QTOF mass spectrometer from Bruker Daltonics (Germany). Absorption spectra were recorded with a Perkin–Elmer Lambda 900 spectrometer using quartz cells of 1 mm pathlength. Excitation and emission spectra were recorded on a Perkin–Elmer LS-50B spectrometer.
Synthesis and characterisation of [Eu8L14](ClO4)24 + In a typical procedure, 5 mg (0.05 mmoles) of L1 was dissolved in 0.6 mL of CH3CN/CHCl3 (1:1 v/v) and added slowly under stirring to four equivalents of Eu(ClO4)3·x H2O dissolved in 1 mL of CH3CN. The mixture was allowed to equilibrate for 3 days at 50 8C. This solution was concentrated by evaporating solvents and a slow diffusion of tert-butylmethylether gave a white-off material, which was isolated by filtration. The solid was further washed with tert-butylmethylether and dried under vacuum to collect 7–8 mg of the complex. Related analytical data and the spectrophotometric titration are described in the Supporting Information.
SAXS analysis and shape reconstitution The SAXS data were collected on the BioSAXS-1000, Rigaku at CEITEC (Brno). Data were collected at 293.15 K with X-ray beam wavelength 1.54 æ. Sample to detector (PILATUS 100 K, Dectris Ltd.) distance was 0.4 m covering a scattering vector range from 0.033 to 0.65 æ¢1. For solvent and sample one two-dimensional image was collected with an exposure time of 20 min per image. Radial averaging of two-dimensional scattering images was performed using SAXSLab3.0.0r1, Rigaku. The solvent subtraction was performed using PRIMUS.[28] The evaluation of the theoretical solution scattering of the [Eu8L14]24 + atomic model generated by Sparkle/AM1 and fitting to experimental data was performed with CRYSOL,[25] where the maximum scattering vector was set to 0.2 æ¢1, explicit hydrogen atoms were taken in account, automatic constant subtraction was allowed, while other parameters were kept at default. The ab initio shape reconstruction was performed with DAMMIF,[26] where the symmetry was set to P3, while other parameters were kept at default. The superimposition of the ab initio SAXS envelope and the atomic model of [Eu8L14]24 + generated by Sparkle/AM1 was performed with SUPCOMB.[29] For graphical representation of ab initio SAXS envelopes, the density map contoured at 8 æ resolution of the DAMMIF dummy atom model was generated with UCSF Chimera.[30]
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Acknowledgements Financial supports from the University of Geneva and Swiss National Science Foundation are gratefully acknowledged. We acknowledge E. Sandmeier for measuring ESMS spectra, K. Buchwalder for performing the elemental analyses, and P.-Y. Morgantini for helping with molecular modelling (all from the University of Geneva). We thank G. Gabant from the platform of mass spectrometry and proteomics at CBM in Orl¦ans (France) for measuring high-resolution mass spectra. The X-ray part of the work was realized in the Central European Institute of Technology with research infrastructure supported by the project CZ.1.05/1.1.00/02.0068 financed from European Regional Development Fund. This research was supported by project Employment of Best Young Scientists for International Cooperation Empowerment, reg. number CZ.1.07/2.3.00/30.0037 co-financed by the European Social Fund and the state budget of the Czech Republic. Keywords: europium · helicates · octanuclear · self-assembly · supramolecular chemistry [1] J. W. Steed, J. L. Atwood, in Supramolecular Chemistry, Wiley, New York, 2009. [2] C. Piguet, J.-C. G. Bìnzli in Handbook on the Physics and Chemistry of Rare Earths, Vol. 40 (Eds.: K. A. Gschneidner, J.-C. G. Bìnzli, V. K. Pecharsky), Elsevier, Amsterdam, 2010, pp. 301 – 553. [3] a) K. Binnemans, Chem. Rev. 2009, 109, 4283 – 4374; b) J. Feng, H. Zhang, Chem. Soc. Rev. 2013, 42, 387 – 410. [4] J.-C. G. Bìnzli in Lanthanide Probes in Life, Chemical and Earth Sciences (Eds.: J.-C. G. Bìnzli, G. R. Choppin,), Elsevier, Amsterdam, 1989. [5] G. M. Nicolle, E. Toth, H. Schmitt-Willich, B. Radìchel, A. Merbach, Chem. Eur. J. 2002, 8, 1040 – 1048. [6] A. Foucault-Collet, C. M. Shade, I. Nazarenko, S. Petoud, S. V. Eliseeva, Angew. Chem. Int. Ed. 2014, 53, 2927 – 2930; Angew. Chem. 2014, 126, 2971 – 2974. [7] a) J. Jankolovits, J. W. Kampf, V. L. Pecoraro, Polyhedron 2013, 52, 491 – 499; b) V. Chandrasekhar, P. Bag, W. Kroener, K. Gieb, P. Mìller, Inorg. Chem. 2013, 52, 13078 – 13086.
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