Article pubs.acs.org/IC
Functionalized Triptycene-Derived Tripodal Ligands: Privileged Formation of Tetranuclear Cage Assemblies with Larger Ln(III) Alexandra Vuillamy,† Soumaila Zebret,‡ Céline Besnard,§ Virginie Placide,† Stéphane Petoud,† and Josef Hamacek*,† †
CBM−CNRS Orléans, Rue Charles Sadron, 45071 Orléans Cedex 2, France Department of Inorganic and Analytical Chemistry, University of Geneva, 30 quai Ernest-Ansermet, 1211 Geneva 4, Switzerland § Laboratory of Crystallography, University of Geneva, 24 quai Ernest-Ansermet, 1211 Geneva 4, Switzerland ‡
S Supporting Information *
ABSTRACT: In this Article, we report the self-assembly of lanthanide complexes formed with two new tripodal ligands, L2 and L3, where binding strands are connected to a rigid triptycene anchor. The pyridine moieties are functionalized with methoxy and PEG groups to enhance ligand solubility and to evaluate the effect of these substituents on lanthanide coordination. These ligands were successfully synthesized and characterized, and their coordination properties were examined along the lanthanide series through speciation studies with NMR and ESI-MS. Well-defined tetranuclear complexes are formed with both ligands, but their stabilities with heavier lanthanides are considerably reduced, especially for complexes with L3. This is attributed to a destabilizing effect of pending PEG arms in combination with increased steric hindrance between binding strands upon complexation with smaller cations. The sensitization of lanthanide luminescence in tetranuclear complexes occurs despite one water molecule being coordinated to a metal ion.
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INTRODUCTION The introduction of supramolecular concepts in the 90s had important consequences on the development of new nanomaterials for various applications because relatively complex systems can be obtained from simple building blocks by spontaneous self-organization.1 Behind this new semantics, we often deal with coordination chemistry governed by classical thermodynamic principles,2 whose control and programming on the molecular level allow self-organized structures with specific microscopic and macroscopic properties to be conceived and prepared.3 Indeed, metal ions incorporated in such assemblies very often act as building elements responsible for targeted properties. Starting from relatively simple 1D and 2D arrangements of metal ions, the interest of chemists has progressively moved toward more challenging 3D assemblies as precursors for molecular functional devices and nanomachines.4,5 In this context, transition metal ions such as ruthenium, platinum, palladium, gallium, etc. have been extensively used because of the relative kinetic inertness of the complexes that they form, allowing for the creation of stable self-assembled structures.6 The same evolution is encountered for lanthanide complexes that are, however, kinetically more labile. After initial investigations of Ln(III) complexes with linear or 2D topologies,7 the self-assembly of 3D structures has been extensively studied over the past several years. Our research mainly deals with helicoidal assemblies, where the building blocks are tripodal organic receptors. Their design plays a © XXXX American Chemical Society
crucial role in programming self-assemblies with Ln(III) in order to match their coordination preferences. Chemical structures of such ligands consist of three metal binding moieties attached to a suitable triamine anchor, which is sufficiently rigid to avoid the complexation of all three strands to the same metal ion. A series of polynuclear complexes has been built with ligands where pyridinedicarboxamide binding moieties were attached to 1,1,1-trisaminomethylethane (TAME), a short aliphatic anchor. The resulting lanthanide complexes are tetra-,8,9 penta-,10 and octanuclear,11 but the central structurally identical tetrametallic cluster represents a common feature of these nanoobjects. Interestingly, the complexes exhibit the same chirality for each metal ion, but they exist as racemic mixtures. In addition, cage tetranuclear complexes were obtained using 1,1,1-trishydroxymethylethane (THME) and undergo host−guest interactions with anions.12 Highly symmetrical arrangements may also have synergic effects on optical properties, e.g., on emission intensity or energy transfer.13 In search of a suitable rigid aromatic spacer, we have resorted to triamino-triptycene. Its coupling with dicarbonylpyridine moieties results in ligand L1, and the investigation of its Eu(III) tetranuclear complexes was described in a previous communication.14 Interestingly, in comparison with ligands derived from aliphatic spacers, the excitation wavelengths is shifted toward the visible and an Received: December 2, 2016
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DOI: 10.1021/acs.inorgchem.6b02900 Inorg. Chem. XXXX, XXX, XXX−XXX
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Structure of Tetranuclear Complexes Formed with L2. The formation of polynuclear complexes was first investigated with Eu(III), which is considered to be the representative cation from the middle of the series. Indeed, the tetranuclear complex [Eu4L24]12+ was prepared by mixing equimolar amounts of Eu(ClO4)3 and L2 in CH2Cl2/CH3CN. Upon diffusion of t-BME, solid crystals suitable for X-ray crystallography were obtained. The Eu(III) complex crystallizes as a mixture of helical enantiomers in the cubic unit cell containing eight cations [Eu4L24]12+. The structure of the complex entity (Figure 1a) is globally similar to the previously published structure with L1. The four Eu(III) cations are located at the vertices of a tetrahedron. The inner cavity contains an encapsulated perchlorate anion, whose oxygens are oriented toward Eu(III) cations. The ligand strands are wrapped about the metallic cations in a helical fashion. The complex possesses a crystallographic C3 axis that passes through one nonacoordinated europium cation. The remaining three cations are deca-coordinated by three tridentate ligand strands and one water molecule. This implies that the asymmetric unit constitutes one-third of the complex. Therefore, two types of intermetallic distances are found in the complex: 11.755 Å between two deca-coordinated cations (bottom of the tetrahedron) and 11.593 Å between those cations and the one nona-coordinated cation localized on the C3 axis. Each phenyl cycle is involved in intramolecular π−π stacking interactions with the pyridine moiety of the neighboring ligand. The crystal structure reveals additional interesting features. In the proximity of the nona-coordinated Eu(III) cation, one acetonitrile molecule is found at a distance of 4.76 Å (Figure 1b). Coordinated water molecules are systematically involved in hydrogen bonding with a perchlorate anion, as illustrated in Figure S2a. Moreover, perchlorate anions undergoing interactions with one or more amide protons are found in the proximity of Eu(III) cations and fill the space between complexes (Figure S2b). In contrast, no intermolecular πstacking interactions are observed in the crystal structure (Figure S2c). The structure of the isolated complex [Eu4L24]12+ in solution was examined with NMR. The 1H NMR spectrum of [Eu4L24]12+ (Figure 2) shows 10 signals related to the protons of L2, which is compatible with the average C3 symmetrical environment in the tetrahedral complex evidenced by the crystal structure. Protons H1 and H5, situated inside the complex cavity, are exposed to the strongest influence of paramagnetic Eu(III) cations and undergo a significant chemical shift. All proton signals were completely assigned using two-dimensional techniques (COSY and HSQC). The stoichiometry of the complex in acetonitrile was confirmed with ESI-MS, which shows a series of peaks typical for multicharged complex cations in the form of adducts with perchlorates (Figure 3). A superposition of the crystal structures of [Eu4L14]12+ and [Eu4L24]12+ reveals that the structures of the central core are almost identical, but the tridentate complexing moieties in [Eu4L24]12+ are slightly shifted, allowing the coordination of one water molecule to the metal cation. This is further facilitated by replacing the terminal diethylcarboxamide moieties in L1 by esters in L2. In addition to the crystal structure of [Eu4L24]12+, an analogous complex with La(III) was prepared and analyzed with X-ray crystallography. Despite the low quality of diffraction data, the structure of the [La4L24]12+ complex was
efficient energy transfer to the Eu(III) cation occurs. In addition to polynuclear edifices of interest, different complex species can be generated as a function of the [Ln]/[L] ratio and their thermodynamic stabilities. A global understanding of the solution chemistry of multicomponent systems is thus required for the quantitative production of targeted assemblies. In this contribution, we focus on the investigation of lanthanide complexation with ligands L2 and L3, where the terminal bulky diethylamide groups (R1) were replaced with ester groups (Scheme 1). The latter are more suitable for Scheme 1. Structure of Ligands L1−L3
subsequent hydrolysis under soft conditions and can provide related carboxylate complexes with higher stabilities in aqueous media. In addition, the para position of the pyridine was substituted by a methoxy group in L2. Similarly, PEG groups were introduced instead of methoxy groups in L3. This functionalization should increase the solubility of tetranuclear clusters in polar solvents and allow further vectorization for the future applications of such compounds. In this context, it is important to determine how these structural changes affect selfassembly processes in solution in order to make reliable predictions for the ligand structure as well as for the choice of Ln(III) cation. Speciation and characterization studies of lanthanide complexes were performed along the series using NMR techniques in combination with mass spectrometry and spectroscopy. The differences in relative thermodynamic stabilities are discussed in relation to the ligand structure and lanthanide cation radii.
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RESULTS AND DISCUSSION Synthesis of Ligands L2 and L3. The synthesis of 9,10dihydro-9,10-[1,2]benzenoanthracene-2,7,14-triamine (6) was carried out according to a procedure published previously,15 except for the final reduction. To prepare tripodal ligands L2 and L3, 4-methoxy-2,6-pyridine-dicarboxylic acid monomethyl ester (5a) or the methyltriglycol (PEG) analogue (5b) (Scheme 2a) was coupled with 2,7,14-triamino-triptycene (6), as shown in Scheme 2b,c. It is worth noting that the use of ethyl ester protective groups in 4b allows for better control of its monohydrolysis. The obtained ligands were then purified by column chromatography and characterized. The 1H NMR spectrum of L2 shows signals corresponding to 10 protons, which is compatible with its dynamically average C3v symmetrical structure (Figure S1a). The NMR spectrum of L3 is similar except for the presence of additional peaks related to the PEG moiety (Figure S1b). The proton peaks were fully assigned with the help of two-dimensional NMR techniques (NOESY, COSY, HSQC). B
DOI: 10.1021/acs.inorgchem.6b02900 Inorg. Chem. XXXX, XXX, XXX−XXX
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Scheme 2. (a) Synthesis of Precursors 5a and 5b and Coupling of 5a and 5b with 2,7,14-Triamino-triptycene To Provide L2 (b) or L3 (c), Respectively
extracted (Figure S3). These data confirm the tetrahedral arrangement of La(III) cations with shorter intermetallic distance (11.34 and 11.62 Å), which points to a slightly more compact structure in comparison with that of [Eu4L24]12+, despite the larger ionic radius of La(III). Moreover, all four La(III) cations are deca-coordinated by three tridentate strands and one water molecule, which reflects their preferences for higher coordination numbers. The analyses of [La4L24]12+ in solution are compatible with the X-ray structure and are discussed in detail below. Speciation Studies of Lanthanide Complexes with L2 along the Series. As shown for Eu(III) and La(III), the formation of tetrahedral complexes [Ln4L24]12+ may occur under stoichiometric conditions, a priori for all lanthanides along the series. However, other coordination complexes can be formed not only for various [Ln]/[L] ratios but also for different Ln(III) in relation with their ionic radii. Therefore, deeper speciation studies were undertaken for selected lanthanides Ln = La, Nd, Sm, Eu, Yb, and Lu. 1H NMR
Figure 1. (a) X-ray crystal structure of [Eu4L24]12+. The ligands are colored to better distinguish structural features. Protons are omitted for the sake of clarity. (b) View of coordinated water molecules, acetonitrile, and central perchlorate anion (balls) within the crystal structure.
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DOI: 10.1021/acs.inorgchem.6b02900 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 2. 1H NMR spectrum of [Eu4L24]12+ in CD3CN (298 K, 600 MHz).
Figure 3. ESI-MS spectrum of [Eu4L24]12+ (positive mode, CH3CN).
be determined. This behavior is observed for lanthanides with smaller ionic radii (i.e., Eu to Lu). Interestingly, a simplified spectrum appears for the titration with Lu(III) at [Lu]/[L2] ∼ 0.7 (Figure S4). The integrals of related peaks in the spectrum (e.g., H6 protons at 9.7 and 10.3 ppm) are close to a 1:2 ratio. Indeed, it may be compatible with triple-stranded helicates Lu2L23, where two strands of each L2 are coordinated to two different Lu(III) cations (Scheme 3a). Smaller peaks are thus
titrations were adapted for these investigations and performed in the mixed solvent CDCl3/CD3CN (4/3). The composition and stoichiometry of Ln(III) complexes with L2 were ascertained with mass spectra recorded under soft ionization conditions. After the addition of each metal perchlorate, the mixture was equilibrated until no change in the NMR spectrum occurred for at least 30 min. It is worth noting that restoring thermodynamic equilibria may lead to slow kinetics. Consequently, the spectra of relatively stable tetranuclear complexes in metal excess may evolve on the scale of several weeks, but these slow processes have not been analyzed here. Ln(III) Complexes in Excess of L2. The titrations of L2 ([L2] ∼ 3 mM) with successive additions of Ln(III) perchlorates into L2 solutions generally show a decrease in the 1H signals of free L2 until close to [Ln]/[L2] ∼ 1. We observed the appearance of several peaks attributed to some low symmetrical transition species, whose symmetry could not
Scheme 3. Suggested Structures of Ln(III) Complexes (a) Ln2L23, (b) Ln3L22, and (c) Ln4L22
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DOI: 10.1021/acs.inorgchem.6b02900 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 4. Comparison of the 1H NMR spectra of La(III) complexes: (a) [La4L24]12+ (red) and (b) [La4L34]12+ (black). PEG protons are not shown for the sake of clarity (298 K, 600 MHz).
formations are evidenced by the NMR titration of L2 with Eu(III) in Figure S9, which shows a complicated spectral evolution. In order to characterize these species in metal excess, we attempted to directly prepare trinuclear complex [Eu3L22]9+ by mixing Eu(III) perchlorate in CD3CN with L2 in CDCl3 in a [Eu]/[L2] ratio of 1.5. The 1H NMR spectrum of the mixture (Figure S10) shows 10 different signals belonging to one ligand strand. These signals are also found in the NMR titration. The ESI-MS spectrum of the above solution confirms the predominant presence of the trinuclear complex [Eu3L22]9+ in the form of typical adducts (Figure S11). These observations may be explained by a global D3 symmetry of the [Eu3L22]9+ complex, where two coordinating ligands are equivalent at rt. In order to better visualize the structure, the trinuclear species can be described as a sandwich-type complex, where each of three cations is coordinated with two strands of different tripodal ligands (Scheme 3b) and which was observed during previous studies.8,14 The complete assignment of peaks was achieved with two-dimensional NMR techniques. One may notice the remarkable chemical shifts for H5, H6, and H7, which are in contrast with those found in the tetranuclear complexes with Eu(III). It is worth noting that new minor 1H NMR signals appear for [Ln]/[L2] > 4 for Ln = Eu and Sm, but the ESI-MS analyses still show species corresponding to Ln3L22. They may be tentatively attributed to some further reorganization of the trinuclear species. While the tetranuclear complex formed with Yb(III) is readily converted into [Yb3L22]9+, the NMR spectrum for [Lu]/[L2] ∼ 2 still shows the tetranuclear [Lu4L24]12+ complex as the major species. The quantity of the trinuclear species increases with time, but the conversion under these conditions progresses relatively slowly (days to weeks). The ESI-MS spectrum of the solution in excess Lu(III) finally confirms the presence of trinuclear complexes [Lu3L22]9+. An occasional presence of Ln4L22 species is observed in ESIMS spectra in parallel with Ln3L22 species. Since the NMR
related to the third uncomplexed ligand strand, as indicated by chemical shifts close to those of the free ligand. The tentative attribution of peaks in the 1H NMR spectrum is given in Figure S5 by considering an averaged D3 symmetrical structure of presumed dinuclear species Lu2L23. Several attempts to isolate this complex have been performed without success. Tetranuclear Complexes [Ln4L24]12+. When the [Ln]/ [L2] ratio approaches 1, newly arising 1H NMR signals are attributed to highly symmetrical tetranuclear complexes [Ln4L24]12+. For La(III) and Nd(III), intermediate complexes in ligand excess are not observed, and their tetranuclear complexes coexist with L2. Tetranuclear helicates with L2 are globally formed with all selected Ln(III) along the series, and their corresponding NMR spectra are compatible with the tetrahedral structure given in Figure 1. Chemical shifts for [Ln4L24]12+ are given in the Supporting Information, and an example 1H NMR spectrum is shown for [Sm4L24]12+ in Figure S6. The 4:4 stoichiometries are clearly confirmed with ESI-MS spectra, whereby the detected species are exclusively the tetranuclear complexes possessing different charges due to the formation of adducts with perchlorate anions. Additional peaks with a lower m/z correspond to a loss of the ion pair H+·ClO4− in the gas phase. Figure S7 illustrates these effects for [La4L24]12+ and [Yb4L24]12+. High-resolution spectra of all complexes show isotopic profiles in agreement with calculated spectra (e.g., Sm(III) complex in Figure S8). Ln(III) Complexes with L2 in Metal Excess. The tetranuclear complexes are progressively transformed into species with another stoichiometry in metal excess. However, this tendency is not the same along the series. For La(III) and Nd(III), new 1H NMR signals start to appear only at [Ln]/ [L2] and ESI-MS analysis reveals the presence of additional species with Ln3L22 and Ln4L22 stoichiometries. For smaller cations, new species already appear at [Ln]/[L2] ∼ 1−2. Under these experimental conditions, tetranuclear complexes Ln4L24 and Ln4L22 and trinuclear species Ln3L22 are simultaneously detected with ESI-MS. These transE
DOI: 10.1021/acs.inorgchem.6b02900 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry spectra at [Ln]/[L2] ∼ 3 do not contain additional signals in relation to a plausible structure shown in Scheme 3c, we tentatively suggest that this species is formed under these conditions only in the gas phase as an adduct of trinuclear complexes and Ln(III) cations compatible with the formula [Ln3L22(ClO4)n]9−n·Ln(ClO4)3. In spite of this, the formation of the true species Ln4L22 cannot be completely excluded, especially at higher metal excess or after a long re-equilibration time. Ln(III) Complexes with L3. The introduction of methyltriglycol moieties (PEG) in L3 should allow for an elegant functionalization of Ln(III) tetrahedral complexes. A simple model of the 3D structure of [La4L34]12+, where the PEG moieties encompass the tetranuclear core, is shown in Figure S12. Since self-assembly with L3 can be different in comparison to that with L2, speciation studies of Ln(III) complexes were carried out with the same cations by means of NMR and ESI-MS with a special focus on the formation of tetranuclear edifices. 1 H NMR titrations with larger metal ions Ln = La, Nd (see, e.g., the titration with Nd(III) in Figure S13) show that the tetranuclear [Ln4L34]12+ complexes are quantitatively formed under stoichiometric conditions and remain stable even in a moderate excess of the metal. Upon successive metal additions to L3 in acetonitrile (ligand excess), a free ligand coexists with the tetranuclear complex and no intermediate species are observed. No significant changes in the NMR spectra are detected at [Ln]/[L3] = 1−2, which means that tetranuclear complexes are relatively stable under these conditions. The NMR spectra for [Ln4L34]12+ (Ln = La, Nd) are compatible with the expected tetrahedral structure of the core cluster obtained with L2 (Figure 1). Since the complexation mode is identical, the chemical shifts of related protons in both [Ln4LX4]12+ (X = 2, 3) complexes are very close, as illustrated in Figure 4 for the assemblies with La(III). Concerning the signals of the PEG moiety between 3 and 4.5 ppm, only the protons near the pyridine moiety are significantly influenced by the complexation. The NMR spectra were completely assigned using 2D NMR techniques. The tetranuclear complexes formed with La(III) and Nd(III) were isolated by the slow diffusion of t-BME as off-white powders. Their ESI-MS analysis exclusively shows typical multicharged perchlorate adducts and confirms their composition (Figure S14). In addition, DOSY NMR experiments were performed with the latter tetranuclear complexes in CD3CN. The diffusion coefficients of [La4L34]12+ and [Nd4L34]12+ amount to 4.1(2) × 10−10 and 4.1(3) × 10−10 m2 s−1, respectively. These values significantly differ from the diffusion coefficient determined under the same conditions for [Eu4L24]12+ (D = 5.0(2) × 10−10 m2 s−1), which reflects hindered diffusion for the larger PEGylated assemblies. It is worth noting that all of these values are in close correlation with the diffusion coefficients for analogous tetranuclear8 or octanuclear complexes11 determined previously in CD3CN. In large metal excess ([Ln]/[L3] > 2), new minor signals are apparent in the 1H NMR spectra. ESI-MS spectra suggest the formation of [Ln3L32]9+ species, in analogy with the complexes of L2. For lanthanides located in the middle of the series (Ln = Sm, Eu), 1H NMR spectra evolve more dramatically. The titrations of L3 evidence intermediate species in ligand excess (Figure S15), corresponding to partially complexed ligands, but more precise structural information could not be obtained from the
spectra. The tetranuclear complexes are progressively formed until [Ln]/[L3] = 1. [Sm4L34]12+ and [Eu4L34]12+ were isolated and characterized; however, these complexes must be prepared under strict stoichiometric conditions. Another complex starts to appear at a small excess of the metal, mainly in the case of Eu(III). As expected, the present species is identified by ESI-MS as a trinuclear complex [Ln3L32]9+ and becomes a major compound in metal excess. In addition, the species with stoichiometries Ln5L34 and Ln4L32 were also detected in the mass spectra (Figure S16). Since the NMR spectrum does not significantly change, these “exotic” complexes observed under the conditions of electrospray correspond most likely to adducts of the tetranuclear and trinuclear complexes with metal ions in excess, where Ln(III) cations could be retained by the PEG chains. Speciation studies with smaller lanthanides (Ln = Yb, Lu) show the formation of intermediate species in ligand excess, but the spectrum at [Ln]/[L3] ∼ 1 appears very broad (Figure S17), indicating the absence of well-organized structures. The expected tetranuclear compounds are thus not formed with these Ln(III). Nevertheless, trinuclear D3-symmetrical species are observed in metal excess for Yb(III), as simultaneously confirmed by ESI-MS. Luminescence of Tetranuclear Complexes in Acetonitrile. Photophysical studies of tetrahedral complexes [Ln4LX4]12+ (X = 2, 3) with emitting lanthanides (Ln = Nd, Sm, Eu, Yb) were studied in acetonitrile at rt. According to the excitation spectra, the optimal excitation wavelength for lanthanide sensitization was fixed at 350 nm, but this wavelength can possibly be shifted to 400 nm. The emission wavelength (in the visible or near-infrared) can be tuned by complexing appropriate Ln(III) cations. The emission spectrum of [Eu4LX4]12+ (X = 2, 3) shows typical bands related to 5D0 → 7Fj transitions with the maximum intensity at 615 nm. The luminescence lifetimes of these two complexes amount to 0.63(4) and 0.46(2) ms, respectively, but these are significantly smaller than the lifetime measured for nona-coordinated Eu(III) cations in [Eu4L14]12+ (1.88(1) ms).14 This luminescence quenching suggests that the coordination sphere of the cations is not completely saturated by tripodal ligands but also contains at least one water molecule, as evidenced by the crystal structure of [Eu4L24]12+. In addition, typical emission spectra have been recorded for other complexes, [Ln4L24]12+ (Ln = Nd, Sm, Yb) and [Ln4L34]12+ (Ln = Nd, Sm). Since the tetranuclear complexes with L3 are quantitatively formed only with larger lanthanides, Figure 5 shows the emission spectra of the isolated complexes with Nd(III) and Sm(III). Interestingly, the latter complex emits in both the visible and near-infrared regions, as has been described previously for another system.16 Lanthanide luminescence is thus sensitized with both ligands despite the unsaturated coordination sphere of Ln(III).
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CONCLUSIONS This work deals with the speciation of lanthanide complexes with new tripodal ligands based on a triptycene anchor. The main interest lies in the controlled assembly of tetranuclear helicates in view of further applications of these supramolecular systems. Herein, the tripodal scaffold was functionalized at each strand by introducing methoxy (L2) and PEG (L3) groups, which indeed increase the solubility of L3 and its complexes. The effect of these structural modifications on lanthanide complexation was followed in detail by NMR and mass F
DOI: 10.1021/acs.inorgchem.6b02900 Inorg. Chem. XXXX, XXX, XXX−XXX
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were distilled over CaH2. The lanthanide perchlorate salts (Ln = La, Nd, Sm, Eu, Yb, Lu) were prepared from the corresponding oxides (Rhodia and Aldrich, 99.99%) and dried according to published procedures. Caution! Perchlorate salts are potentially explosive and should be handled caref ully in small quantities. The Ln(III) content in solid salts was determined by complexometric titrations with Titriplex III (Merck) in the presence of urotropine and xylene orange. Compounds 2a and 4a were synthesized according to literature procedures.17 The syntheses of 5a, 2b, 4b, and 5b are given in the Supporting Information. Synthesis of L2. For L2, 42 mg (0.2 mmol) of 4-methoxy-2,6pyridine-dicarboxylic acid monomethyl ester (5a)18 was dissolved in 10 mL of CH2Cl2. After adding 5 μL of DMF and 150 μL of SOCl2 (10 equiv), the mixture was refluxed for 2.5 h. The excess of SOCl2 was evaporated, and the residue was dried under vacuum. After dissolving in CH2Cl2, a mixture of triamine 615 (15 mg, 0.05 mmol) and triethylamine (50 μL) was added dropwise to this solution. The reaction was stopped after 16 h of mixing at rt. The organic solvent was evaporated. The solid residue was partitioned between CH2Cl2 and saturated aqueous NaHCO3. The latter was then extracted twice by CH2Cl2. The combined organic phases were evaporated, and the residue was purified on a chromatographic column (SiO2, MeOH/ CH2Cl2, 0−1% (v/v)). Finally, the isolated L2 was dried under vacuum (31 mg, 70% yield). L2: 1H NMR (600 MHz, CDCl3) δ ppm = 4.00 (s, 18H, CH3), 5.61 (s, 1H, CH), 5.68 (s, 1H, CH), 7.41−7.47 (m, 6H, CH), 7.74 (s, 3H, CH), 7.88 (s, 3H, CH), 8.03 (s, 3H, CH), 9.99 (s, 3H, CH). 13C NMR (150 MHz, CDCl3) δ ppm = 52.7, 53.4, 54.45, 57.01, 111.19, 114.83, 117.18, 117.57, 124.54, 135.85, 142.45, 146.7, 149.34, 152.76, 161.94, 165.51, 169.22. ESI-MS m/z: calc. for [L2 + H+]+, 879.3; obs., 879.7; calc. for [L2 + 2H]2+, 1758.5; obs., 1758.2. Synthesis of L3. 5b (147 mg, 0.41 mmol) was solubilized in dry chloroform under argon. Thionyl chloride (0.3 mL; 4.1 mmol) was added slowly; then, 20 μL of distillated DMF was added. The reaction was stirred at 45 °C. After 3 h, the mixture was cooled to room temperature, and thionyl chloride was evaporated. The obtained oil was solubilized in 15 mL of dry chloroform, under argon. Thirty milligrams of 6 and 200 μL (1.46 mmol) of Et3N were added. The reaction mixture was stirred at rt overnight. The solvent was evaporated under vacuum. The oil was solubilized in dichloromethane and washed with a saturated aqueous NaHCO3 solution. The organic layers were combined, dried with anhydrous Na2SO4, filtered, and evaporated under vacuum to give ligand L3 (100 mg, 95%). 1H NMR (600 MHz, CD3CN) δ ppm = 1.42 (t, 9H, CH3), 3.26 (s, 9H, CH3), 3.43 (m, 6H, CH2), 3.53 (m, 6H, CH2), 3.56 (m, 6H, CH2), 3.62 (m, 6H, CH2), 3.82 (t, 6H, CH2), 4.33 (t, 6H, CH2), 4.42 (q, 6H, CH2), 5.6 (s, 1H, CH), 5.7 (s, 1H, CH), 7.41 (dd, 3H, CH), 7.45 (d, 3H, CH), 7.74 (d, 3H, CH), 7.88 (d, 3H, CH), 7.99 (d, 3H, CH), 9.98 (s, 3H, NH). 13C NMR (600 MHz, CD3CN) δ ppm = 14.52, 52.78, 54.54, 58.87, 62.91, 69.55, 69.68, 70.99, 71.06, 71.4, 72.56, 111.62, 115.25, 117.32, 117.65, 124.71, 136.02, 142.66, 146.91, 149.54, 152.93, 162.13, 165.08, 168.42. ESI-MS m/z: calc. for [L3 + H+]+, 1317.5; obs., 1317.6; calc. for [L3 + 2H]2+, 659.5; obs., 659.3. Characterization of the Ln(III) Complexes with L2 and L3. The Ln(III) complexes were prepared by adding a solution of Ln(III) perchlorate salts in CD3CN to an equivalent amount of L2 dissolved in CDCl3/CD3CN (50/50 v/v). The mixture was allowed to equilibrate before measuring NMR spectra. The chemical shifts for lanthanide complexes with L2 and L3 are given in the Supporting Information. The solid compounds were isolated after diffusing t-BME into the above solutions by filtration of precipitates followed by drying under vacuum. The complexes were redissolved in CH3CN or CD3CN for further analyses by mass spectrometry and NMR. Spectroscopic and Physical Measurements. Electrospray ionization mass spectra (ESI-MS) of Eu(III) complexes were recorded on a Finnigan SSQ7000 instrument with the optimized ionization temperature (120−180 °C) from acetonitrile solutions. Highresolution spectra were recorded on a 4 GHz MaXis ultra-highresolution QTOF mass spectrometer from Bruker Daltonics (Germany). 1H and 13C spectra including 2D experiments (COSY,
Figure 5. Excitation (blue) and emission (λex = 350 nm) spectra of tetranuclear [Sm4L34]12+ (green) and [Nd4L34]12+ (red) complexes in acetonitrile.
spectrometry, and the complex speciation with both ligands is summarized in Table S2. Tetranuclear species [Ln4LX4]12+ are formed with L2 under stoichiometric conditions along the lanthanide series, but those formed with larger cations are relatively stable with respect to the transformation into trinuclear species, which occurs only at higher metal excess. This observation can be explained by a better fit of the coordinating strands with larger Ln(III) cations, where the coordination sphere is completed with one water molecule per metal ion (CN = 10). This coordination environment is evidenced with the crystal structures of [Eu4L24]12+ and [La4L24]12+, which also show one perchlorate anion captured inside the tetranuclear assembly. [Ln4L24]12+ is still formed with smaller lanthanides, but it is readily transformed in trinuclear complexes. In case of L3, the tetranuclear species are observed only for larger cations La− Eu, but the tetranuclear complexes with La(III) and Nd(III) are particularly stable in excess of both ligand and metal. Since the coordination moieties are identical for both ligands, this difference is tentatively attributed to the destabilizing effect of pending PEG groups attached to coordinating pyridine moieties. In addition, the smaller size of the lanthanides destabilizes the tetranuclear species, and only more compact trinuclear complexes are observed. It can be concluded that thermodynamic stabilities of different species evolve across the lanthanide series and are mediated by the substituents on coordinating pyridyldicarbonyl moieties. The spectroscopic characterization of tetranuclear complexes in acetonitrile shows that both triptycene-based ligands can be used for the sensitization of lanthanides emitting not only in the visible region but also in the near-infrared. Interestingly, complexes with Sm(III) exhibit emission in both regions. Finally, the tetranuclear complexes are generally transformed into well-defined trinuclear complexes [Ln3LX2]9+ in metal excess. Their NMR spectra are compatible with a D3symmetrical sandwich-like structure.
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EXPERIMENTAL SECTION
Solvents and Starting Materials. Chemicals were purchased from Acros Organics, Fluka AG, and Aldrich and used without further purification unless otherwise stated. Acetonitrile and dichloromethane G
DOI: 10.1021/acs.inorgchem.6b02900 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
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NOESY, HSQC) were measured on a 600 MHz NMR spectrometer Bruker Avance III at 298 K. DOSY (diffusion ordered spectroscopy) NMR experiments with Ln(III) complexes in CD3CN were performed on a 700 MHz Bruker Avance III-HD equipped with a cryo-probe at 298 K applying the ledbpgp2s pulse sequence. The recorded data were analyzed with the TopSpin T1/T2 module. Excitation and emission spectra and lifetimes of Ln(III) complexes in acetonitrile were measured at rt on a Fluorolog-3 spectrofluorimeter (Horiba) equipped with either a visible photomultiplier tube (PMT) (220−850 nm, R928P; Hamamatsu), a NIR solid-state InGaAs detector cooled to 77 K (800−1600 nm, DSS-IGA020L; Horiba Scientific), or a NIR PMT (950−1650 nm, H10330-75; Hamamatsu). Excitation and emission spectra were corrected for the instrumental functions. The crystallographic data were collected at 180 K on an Agilent Supernova diffractometer using Cu Kα radiation. CCDC 1514581 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02900. Detailed synthetic procedures for 5a, 2b, 4b, 5b, and Ln(III) complexes, crystallographic data, NMR data and spectra, MS spectra, and calculated molecular model (PDF) Crystallographic data (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Josef Hamacek: 0000-0003-4641-7357 Funding
Financial support from La Région Centre (project Deep-PDT), Agence Nationale de la Recherche (project Lumiphage ANR13-BSV5-009 and project Lumzif ANR-12-BS07-0012), La Ligue Contre le Cancer, CNRS, and the Swiss National Science Foundation is gratefully acknowledged. S.P. acknowledges support from Institut National de la Santé et de la Recherche Médicale (INSERM). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank G. Gabant (mass spectrometry platform at the Centre for Molecular Biophysics (CBM) Orléans) for measuring high-resolution mass spectra and H. Meudal (NMR platform at CBM) and K. Loth for their help with DOSY measurements. We are grateful to L. Peterhans and A. Homberg for initiating experiments with L1.
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DEDICATION This manuscript is dedicated to the memory of Dr. Josef Dolezal, a young talented chemist who passed away 20 years ago.
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REFERENCES
(1) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry; Wiley: New York, 2009. H
DOI: 10.1021/acs.inorgchem.6b02900 Inorg. Chem. XXXX, XXX, XXX−XXX
