Self-Assembly and Catalytic Reactivity of BINOL-Bridged Bis

Nov 15, 2017 - underwent chiral self-sorting to afford two homochiral metal- locages. The narcissistic self-sorting process of the metallocages was ob...
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Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

Self-Assembly and Catalytic Reactivity of BINOL-Bridged Bis(phenanthroline) Metallocages Kai-Yu Cheng, Shi-Cheng Wang, Yu-Sheng Chen, and Yi-Tsu Chan* Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan S Supporting Information *

ABSTRACT: Upon treatment with ZnII ions, a series of BINOLbridged bis(phenanthroline) ligands was self-assembled into [M2L3] metallocages, which were carefully characterized by NMR spectroscopy and ESI-MS spectrometry. Among them, a racemic mixture of the BINOL-bridged bis(phenanthrolines) underwent chiral self-sorting to afford two homochiral metallocages. The narcissistic self-sorting process of the metallocages was observed in the complexation reaction of the constitutionally isomeric bis(phenanthrolines) with varying connection positions. Moreover, the endo hydroxyl-functionalized metallocage [Zn2{(S)-L2OH}3] exhibited catalytic activity and substrate selectivity for the Knoevenagel condensation reactions of aromatic tricarbaldehydes with malononitrile.

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lenes35,36 have been assembled into a single diastereomer upon coordination to suitable transition-metal ions. BINOL is one of the chiral motifs ubiquitously utilized to create an optically active environment, and its inward hydroxyl groups could be easily converted to various derivatives used in diverse catalytic modes, such as metal complexes, hydrogen bonding, Brønsted acids, and ion pairing.37 Incorporation of catalytically active sites into supramolecular frameworks is a straightforward method to confer catalytic functions. For instance, the enantioselective addition of Et2Zn to aldehydes could be catalyzed by a combination of the Pt-alkynyl BINOL-based metallotriangle and Ti(OiPr)4, demonstrating that the confined active sites led to enhanced product selectivity.38 Herein, we bridge two 1,10-phenanthroline (phen) ligands with BINOL at the 3- and 5-positions, respectively, for selfassembly of metallocages, because the BINOL unit provides not only axial chirality but also a potential substrate binding site for catalysis. The self-assembly and self-sorting behavior of the BINOL-bridged bis(phenanthrolines) are investigated. Inspired by the catalytic Knoevenagel condensation reactions using metal−organic frameworks (MOFs),39 organic cages,40 and metallocages41−43 under mild conditions, the newly prepared metallocage [Zn2{(S)-L2OH}3] was subjected to condensation tests and showed distinctive catalytic activity and selectivity for tricarbaldehyde substrates.

INTRODUCTION In the field of supramolecular chemistry, chemists are often inspired by precise self-assembly of biological subunits into well-defined and functional structures.1−5 The metal-coordination-driven approach6−11 providing predictable directionality and proper bond strength allows chemists to efficiently prepare highly ordered architectures, such as grids,12 helicates,13,14 knots,15,16 and polygons and polyhedra.8,10,11,17 In recent years, the focus of supramolecular coordination chemistry has gradually shifted from the creation of aesthetic structures toward exploiting potential functions in catalysis,18−20 drug delivery,21 molecular recognition,22 and others.23 In order to increase functionalities and complexities, the precise control of the arrangement of different ligands into a single discrete assembly has drawn tremendous research interest. Self-sorting is a common strategy to do so and can be considered as a spontaneous process by which building blocks in a complicated mixture undergo self- and/or nonselfrecognition to produce well-defined assemblies.24−26 To realize high-fidelity self-sorting within a multicomponent system, a delicate balance between intra-/intermolecular interactions is needed. Molecular chirality is one of the most prominent driving forces for successful self-sorting, especially for trischelate metal complexes with octahedral coordination geometry, whose configurations can be remotely controlled by stereocenters elsewhere within the scaffold.27−29 Over the past decades, many stereoselectively self-assembled helicates and metallocages have been achieved by using enantiopure organic ligands capable of propagating their chiral information to the metal centers.29 Along this trend, enantiomerically pure ligands based on 1,1′-bi-2-naphthol (BINOL),30−32 Tröger’s base derivatives,33 9,9′-spirobifluorene,34 and dissymmetric al© XXXX American Chemical Society

Special Issue: Self-Assembled Cages and Macrocycles Received: October 3, 2017

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DOI: 10.1021/acs.inorgchem.7b02540 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Ligand Synthesisa

a Reagents and conditions: (i) ethynylphenanthroline, CuI, Pd(PPh3)4, DIPA/DMF, 70 °C, 1−4 days; (ii) concentrated HCl(aq), dioxane, reflux, 1 h.

Scheme 2. Cartoon Representations for Enantiopure Bis(phenanthroline) Ligands

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RESULTS AND DISCUSSION Synthesis of Ligands. Lützen and co-workers have developed an efficient synthetic route to prepare a number of bis(bipyridyl) BINOL ligands through the Sonogashira crosscoupling reactions of 5-ethynyl-2,2′-bipyridine with 3,3′-diiodoBINOL derivatives.30,31 According to this synthetic strategy, similarly a variety of BINOL-based bis(phenanthroline) ligands can be generated, and the ligand geometry is tunable by varying the ethynyl position on the phenanthroline unit (Scheme 1). In this work, L1 was prepared in good yield by the Pd-catalyzed Sonogashira coupling reaction of 5-ethynylphenanthroline with 1. The further deprotection of L1 under acidic conditions afforded a ligand with two hydroxyl groups, but it showed poor solubility after complexation. In order to improve the ligand solubility, two n-octyl groups were incorporated at the 6,6′positions of (S)-BINOL by a Kumada coupling reaction. Ligands (S)-L2 and (S)-L3 (Scheme 2) were synthesized from diiodo precursor (S)-2 and then deprotected to generate (S)L2OH and (S)-L3OH, respectively. Complexation Reactions. The BINOL-bridged ligand (S)L1 was treated with Zn(OTf)2 (3/2 ratio) in a CHCl3/MeOH mixed solvent (1/1, v/v) to afford the complex. The highresolution electrospray ionization (ESI) mass spectrum of the resultant complex revealed three intense signals corresponding to the ions of [Zn2{(S)-L1}3] with charge states ranging from 2+ to 4+ (Figure S15 in the Supporting Information). The 1H NMR spectrum of [Zn2{(S)-L1}3] (Figure 1c) exhibited only one set of peaks for the phenanthroline units, and the DOSY spectrum (Figure S14 in the Supporting Information) suggested the formation of a single species in a CDCl3/ CD3OD mixture (1/1, v/v). The complete 1H NMR assignments were further confirmed by COSY, NOESY, and

HMQC spectra (Figures S12 and S13 in the Supporting Information). In comparison with uncoordinated (S)-L1, the characteristic upfield shifts for phen-H9 (δ 8.89 ppm, Δδ = −0.29 ppm) and phen-H2 (δ 7.69 ppm, Δδ = −1.54 ppm) were observed after the complexation reaction. The simple 1H NMR pattern indicated that the metal centers in [Zn2{(S)-L1}3] have the same configuration (Δ,Δ or Λ,Λ) instead of forming a mesocate with Δ,Λ configuration, which was also supported by the opposite Cotton effects of the two enantiomeric complexes in the circular dichroism (CD) spectra (Figure 1e). Attempts to grow single crystals suitable for X-ray diffraction analysis were unsuccessful. Thus, the absolute configurations were assigned by computational methods. Among the three possible energyminimized structures, (Δ,Δ)-[Zn2{(S)-L1}3] showed the lowest energy (Table S1 in the Supporting Information), presumably because the suitable configurations of the octahedral complexes could release the torsional strain derived from the mismatched configurations. A similar stereoselective self-assembly has been reported for bipyridine-based triplestranded helicates.30 In addition to (S)-L1 and (R)-L1, a racemic mixture, rac-L1, was also employed to examine the chiral self-sorting behavior. In principle, rac-L1 may produce four different isomers upon coordination to ZnII ions, including [Zn2{(R,R,R)-L13}], [Zn2{(S,S,S)-L13}], [Zn2{(R,R,S)-L13}], and [Zn2{(S,S,R)L13}]. When rac-L1 was mixed with ZnII ions in a ratio of 3/ 2, the exact same 1H NMR signals as those for [Zn2{(S)-L1}3] were observed (Figure 1d), strongly supporting that the reaction underwent completely chiral narcissistic self-sorting to generate a racemic mixture of metallocages, [Zn2{(R)-L1}3] and [Zn2{(S)-L1}3], which was also evident from its silent CD spectrum (Figure 1e). B

DOI: 10.1021/acs.inorgchem.7b02540 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Self-assembly of [Zn2{(S)-L1}3] and the chiral self-sorting process. 1H NMR spectra of (b) (S)-L1 in CDCl3 and (c) [Zn2{(S)-L1}3] and (d) [Zn2{(rac)-L1}3] in CDCl3/CD3OD (1/1, v/v). (e) CD spectra of [Zn2{(S)-L1}3], [Zn2{(R)-L1}3], and [Zn2{(rac)-L1}3] in CHCl3/ MeOH (1/1, v/v).

Upon complexation of (S)-L2 with Zn II ions, the spontaneous formation of the desired [Zn2{(S)-L2}3] was found, as evidenced by the NMR experiments (Figure 2b and Figures S16−S19 in the Supporting Information) and the characteristic ESI-MS peaks as well as the explicit isotope patterns (Figure S20 in the Supporting Information). On the other hand, (S)-L3 is a constitutional isomer of (S)-L2, which has a connection at the phenanthrolinyl 3-position. Similarly, (S)-L3 was treated with ZnII ions in a 3/2 ratio to afford [Zn2{(S)-L3}3] in quantitative yield. COSY and NOESY experiments (Figures S22 and S23 in the Supporting Information) were conducted to ensure the proper 1H NMR assignments (Figure 2c). The computational energy-minimized structures (Figure 3) suggested that the constitutional isomers (S)-L2 and (S)-L3 led to self-assembled metallocages with distinct shapes. Notably, the molecular geometry of [Zn2{(S)L3}3] is analogous to those of the reported triple-stranded helicates.30,35 In order to gain more structural insights into

these two metallocages, ESI interfaced with traveling wave ionmobility mass spectrometry (TWIM-MS) was utilized to obtain the corresponding collision cross sections (CCSs).44−48 Although [Zn2{(S)-L2}3] and [Zn2{(S)-L3}3] have the same molecular weight, the TWIM-MS analyses (Figure S36 in the Supporting Information) showed that they exhibited distinguishable drift time distributions for the triply and quadruply charged ions. In addition, the average experimental CCS of [Zn2{(S)-L2}3] (616.3 ± 12.8 Å2) derived from the calibration curve is larger than that of [Zn2{(S)-L3}3] (492.8 ± 10.2 Å2), indicating that [Zn2{(S)-L3}3] has a more compact geometry. These results are in good agreement with the theoretical values. To explore the self-sorting behavior of the isomeric ligands, equimolar amounts of (S)-L2 and (S)-L3 in CHCl3 were treated with a MeOH solution of Zn(OTf)2, and the mixture was stirred at 25 °C for 30 min. After the resultant complexes were counterion-exchanged with PF6−, the 1H NMR spectrum taken in CDCl3/CD3OD (5/1, v/v) displayed two distinct sets C

DOI: 10.1021/acs.inorgchem.7b02540 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) Illustration of the narcissistic self-sorting process for complexation of (S)-L2 and (S)-L3. 1H NMR spectra of (b) [Zn2{(S)-L2}3], (c) [Zn2{(S)-L3}3], and (d) the self-sorted complexes. (e) ESI-TWIM-MS plots of the self-sorted complexes.

Figure 3. Energy-minimized structures of (a) [Zn2{(S)-L2}3] and (b) [Zn2{(S)-L3}3]. The n-octyl groups are omitted for clarity.

of signals assigned to [Zn2{(S)-L2}3] and [Zn2{(S)-L3}3], respectively (Figure 2d), demonstrating a high-fidelity narcis-

sistic self-sorting process. The mixture generated from the selfsorting experiment was analyzed with ESI-TWIM-MS to D

DOI: 10.1021/acs.inorgchem.7b02540 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. 1H NMR spectra of (a) (S)-L2OH, (b) [Zn2{(S)-L2OH}3], (c) (S)-L3OH, and (d) [Zn2{(S)-L3OH}3]. ESI-MS spectra of (e) [Zn2{(S)L2OH}3] and (f) [Zn2{(S)-L3OH}3].

(Figure 4d), and similarly the trimeric composition was supported by ESI-MS (Figure 4f). Catalytic Reactivity. The Knoevenagel condensation,49 a modified aldol condensation reaction, is a nucleophilic addition between a carbonyl group and an active hydrogen compound followed by dehydration. Hence, it is challenging to perform the Knoevenagel condensation reactions under neutral and hydrous conditions. Since [Zn2{(S)-L2OH}3] is a triple-stranded cage possessing three pairs of inward hydroxyl groups, it may potentially interact with a trifunctional Knoevenagel condensation substrate, such as 1,3,5-benzenetricarbaldehyde (3), to enhance the carbonyl electrophilicity. When 3 was treated with 6 equiv of malononitrile and 10 mol % of [Zn2{(S)L2OH}3] in CDCl3/CD3CN (5/2, v/v) at room temperature, the Knoevenagel condensation product was obtained in 93% yield after 2 days (Table 1, entry 3). In general, the Knoevenagel condensation reactions are conducted under basic conditions in anhydrous media to offer good yields.50 In contrast, in this test, the reaction took place under nonbasic and hydrous conditions. Therefore, a series of control experiments was conducted to confirm the catalytic activity of [Zn2{(S)-L2OH}3] (Table 1). In the absence of [Zn2{(S)L2OH}3], entry 1 gave a yield of 4% under the same reaction

differentiate the isomeric metallocages. It is worth noting that the TWIM-MS plot (Figure 2e) showed two separate drift time distributions for the triply charged species [[M2L3]·(PF6)]3+. Two drift times at 9.70 and 13.45 ms corresponding to [Zn2{(S)-L3}3]3+ and [Zn2{(S)-L2}3]3+, respectively, were consistent with the previous observation for the individual metallocage (Figure S36 in the Supporting Information). Deprotected ligands (S)-L2OH and (S)-L3OH were reacted with Zn(OTf)2 to give the hydroxyl-functionalized metallocages [Zn2{(S)-L2OH}3] and [Zn2{(S)-L3OH}3] respectively, which were fully characterized by NMR and ESI-MS. The 1H NMR spectrum of [Zn2{(S)-L2OH}3] (Figure 4b) exhibited a significant upfield shift (δ 7.55 ppm, Δδ = −1.52 ppm) for phen-H2 in comparison to uncomplexed (S)-L2OH, confirming the formation of the octahedral complex [Zn(phen)3]. The [M2L3] composition was further verified by three major ESIMS peaks at m/z 1587.1195, 1008.4341, and 719.1054 derived from the 2+ to 4+ ions, respectively (Figure 4e). Even though the solubility of (S)-L3OH in CHCl3 is poor, [Zn2{(S)-L3OH}3] has moderate solubility in a mixed solvent of CHCl3 and MeCN (5/1, v/v). The 1H NMR spectrum of [Zn2{(S)L3OH}3] also showed a characteristic upfield shift (δ 7.92 ppm, Δδ = −1.28 ppm) for phen-H2 after coordination to ZnII ions E

DOI: 10.1021/acs.inorgchem.7b02540 Inorg. Chem. XXXX, XXX, XXX−XXX

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source of catalytic activity. Hence, it was envisioned that the hydroxyl groups densely localized in the interior of the cage allowed the substrate first to be included in the cavity through hydrogen bonding to enhance the carbonyl electrophilicity and then to be attacked by a nucleophile. Additionally, the catalyst recycling experiments of [Zn2{(S)L2OH}3] were carried out for the condensation reaction of 3 and malononitrile (Figure S39 in the Supporting Information). After the reaction, the product was precipitated from the solution, which could be easily removed by filtration. The filtrate containing the catalyst was employed in the next cycle. The results demonstrated that metallocage [Zn2{(S)-L2OH}3] could be reused at least twice without dramatically losing its catalytic reactivity. The interaction of the metallocage with 3 was verified by means of UV/vis spectroscopy in CHCl3/MeCN (5/2, v/v). Upon gradual addition of 3 into a solution of [Zn2{(S)L2OH}3], the intensity of the absorption band at 354 nm slightly decreased as the molar ratio of [Zn2{(S)-L2OH}3]/3 was changed from 1/0 to 1/22. On the basis of the UV/vis titration experiment (Figure S40 in the Supporting Information), the association constant (Ka) was determined to be (1.1 ± 0.2) × 105 M−1 by using a 1/1 isotherm model and nonlinear leastsquares curve fitting (Figure S41 in the Supporting Information).51 The titration experiment was also monitored by 1H NMR. The signal for the hydroxyl groups gradually disappeared with increasing concentration of 3, suggesting that the proton was in a fast exchange with the substrate (Figure S42 in the Supporting Information). To investigate the substrate selectivity, a number of aromatic tricarbaldehydes were chosen for the Knoevenagel condensation (Table 2). Intriguingly, the yield for methyl-substituted substrate 4 was only 6% even after 5 days, indicating that the methyl substituents might reduce the electrophilicity of the formyl groups (Figure S48 in the Supporting Information) and/or interrupt the interactions between the formyl groups and the hydroxyl groups within the confined space (Figure S43 in the Supporting Information). Building on this observation, an equimolar mixture of 3 and 4 was treated with malononitrile in the presence of [Zn2{(S)-L2OH}3] to realize selective catalysis. Indeed, only substrate 3 was reacted to afford a 95% yield of the condensation product. The control experiment done by using phen or (S)-L2OH as a base catalyst showed no selectivity for 3 and 4, in which both substrates were converted

Table 1. Catalyst Tests for Knoevenagel Condensation Reactionsa

entry

catalyst

amt (mol %)

yield (%)

1 2 3 4 5 6 7 8

none [Zn2{(S)-L2}3](OTf)4 [Zn2{(S)-L2OH}3](OTf)4 [Zn2{(S)-L3OH}3](OTf)4 BINOL (S)-L2OH Zn(OTf)2 [Zn(phen)3](OTf)2

10 10 10 30 30 20 20

4 6 93