The Effect of Solvent and Coordination Environment of Metal Source

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The Effect of Solvent and Coordination Environment of Metal Source on the Self-Assembly Pathway of a Pd(II)-Mediated Coordination Capsule Shumpei Kai,† Yui Sakuma,‡ Takako Mashiko,‡ Tatsuo Kojima,† Masanori Tachikawa,‡ and Shuichi Hiraoka*,† †

Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan ‡ Quantum Chemistry Division, Graduate School of Science, Yokohama City University 22-2 Seto, Kanazawa-ku, Yokohama, Kanagawa 236-0027, Japan S Supporting Information *

ABSTRACT: The effect of reaction environment on the selfassembly process of an octahedron-shaped Pd6L8 capsule was investigated. Quantitative analysis of self-assembly process with 1H NMR spectroscopy revealed that the self-assembly pathway of the capsule was altered by solvent and a leaving ligand coordinating to the metal source, which are not the components of the final self-assembly. Solvents definitively determine the pathway of the self-assembly at a very early stage of the self-assembly. Contrary to the expectation that the weaker the coordination ability of the leaving ligand is, the faster the formation of the final assembly becomes, a leaving ligand with weak coordination ability tends to generate a kinetically trapped species to prevent the capsule formation under mild conditions.

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INTRODUCTION In chemical reactions in solution, there are many examples that the pathway of reaction (chemo- and stereoselectivities) as well as the rate of reaction is dramatically altered by solvent due to differences in the stabilization of substrates, transition states, and products.1 Similarly, in molecular self-assembly, in which well-defined structures are constructed through multiple reversible reactions or molecular interactions, it is natural that not only the formation of self-assembled structures but also the corresponding self-assembly pathway should strongly depend on solvent. This is quite reasonable because the chemical bonds and molecular interactions employed in the molecular selfassembly such as electrostatic interaction, hydrogen bond, coordination bond, van der Waals interaction, and hydrophobic effect are intrinsically affected by solvent. As to the formation of coordination self-assemblies consisting of metal ions (M) and multitopic organic ligands (L) in solution,2−19 square-planar Pd(II) and Pt(II) ions have widely been used. 20−34 Coordination self-assembly with Pd(II) and Pt(II) ions takes place through ligand exchange of a leaving ligand (X) coordinating to metal source (MX4, M = Pd or Pt) with a multitopic ligand (L), transiently producing five-coordinate intermediates and a transition state (Figure 1).35−39 As the coordination bonds are reversible through ligand exchange, even if inappropriate intermediates that cannot convert into the final assemblies without bond recombination are kinetically produced during the coordination self-assembly, the ligand © 2017 American Chemical Society

exchange causes such kinetically trapped species to convert into the final assemblies under thermodynamic control. Solvents with coordination ability such as CH3CN and DMSO are often effective to efficiently afford the thermodynamically most stable species,40−50 as such solvents accelerate the ligand exchange, playing a role of incoming and leaving ligands (Figure 1). Thus, it is natural to expect that solvent should affect not only the rate of the product formation but also the self-assembly process itself. However, such a kinetic aspect of solvent on the selfassembly has yet to be explored. As to the leaving ligand (X) on the metal source (MXn), it is predictable how the leaving ligand affects the self-assembly in equilibrium. When metal source with coordinatively weaker X is used, the formation of the self-assembly is more favorable. However, kinetic effect of the leaving ligand on the selfassembly is not easy to presume. Good leaving ligands, X, accelerate ligand exchange, so if the self-assembly takes place through the same pathway regardless of the leaving ligand, a better leaving ligand would lead the system to the final selfassembly faster. But if the leaving ligand affects or alters the self-assembly pathway, just because good leaving ligands accelerate the ligand exchange, it does not follow that the self-assembly with good leaving ligands, X, takes place efficiently. Received: August 24, 2017 Published: September 25, 2017 12652

DOI: 10.1021/acs.inorgchem.7b02152 Inorg. Chem. 2017, 56, 12652−12663

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Inorganic Chemistry

Figure 1. Ligand exchange mechanism on a square-planar Pd(II) complex, [PdPy4]2+, with Py, in which five-coordinate square-pyramidal intermediates and a trigonal-bipyramidal transition state are produced.

assembly process is contrary to the intuition that the selfassembly proceeds fast starting from the metal source with good leaving ligands even though the ligand exchange is truly accelerated with good leaving ligands.

In biological self-assembly, there are some examples where molecules not finally incorporated into the assemblies such as molecular chaperone51,52 and the environment play a leading role to facilitate the self-assembly, which turned our attention to investigating the effect of molecules that are not the component of assembled structures, such as solvent and leaving ligand, on the coordination self-assembly process. Previously we reported the self-assembly process of an octahedron-shaped Pd618 capsule (Figure 2 and eq 1) by our recently developed

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Reliable investigation of the self-assembly process needs to follow major intermediates produced in the self-assembly, not the only intermediates rarely observed by spectroscopy. However, it is practically impossible to detect and quantify most of the intermediates that appear in the self-assembly process because of the formation of many kinds of intermediates with low symmetry through many pathways and of very short lifetime of most of the intermediates, which are the main reason why kinetic aspects of molecular selfassembly have scarcely been investigated. The idea of QASAP is that the information about all of the not observed intermediates is obtained from quantification of all the observable and characterized species. In other words, the average composition of the intermediates not observed by spectroscopy can be determined by quantifying all the observable species. In most cases, such observable species are the substrates (metal source and multitopic organic ligand) and the products (selfassembled structure and leaving ligand, X) in eq 1. In order to quantify metal source and leaving ligand in the reaction mixture, a metal complex possessing NMR-detectable monodentate ligand (X), [PdX4]2+, is used as metal source. As multitopic ligand, L, and the self-assembled product, Pd618, can be quantified by 1H NMR spectroscopy, time variation of all the species other than the intermediates, [PdX4]2+, L, Pd618, and X in eq 1, can thus be monitored. As a result, the average composition of all the intermediates, M⟨a⟩L⟨b⟩X⟨c⟩, can be obtained even if none of the intermediates are observed by 1H NMR spectroscopy (details are shown in the SI). In order to clarify the coordination environment and the composition in the intermediates, the following two parameters, ⟨n⟩ and ⟨k⟩, are defined:

Figure 2. Chemical structure of tritopic ligand, 1, and a schematic representation of the octahedron-shaped [Pd618]12+ capsule.

NMR-based quantitative method: quantitative analysis of selfassembly process (QASAP), which enables us to discuss the self-assembly process even though none of the intermediates are observed (the (n, k) analysis, vide inf ra).53−55 Very recently, theoretical analysis of the self-assembly process of the Pd618 capsule by a master equation approach, which is totally different from either molecular dynamic simulation or traditional rate equation, was reported for the speciation of the intermediates that cannot be determined by experiments.56 6·PdX4·(OTf)2 + 8·1 ⇄ Pd618 ·(OTf)12 + 24· X

QUANTITATIVE ANALYSIS OF SELF-ASSEMBLY PROCESS (QASAP)

(1)

Here we report the effect of solvent and leaving ligands on the self-assembly process of the Pd618 capsule. We found that solvents with coordination ability not only convert kinetically trapped species into thermodynamically most stable species but also affect a very early stage of the self-assembly to prevent primitive intermediates from leading to kinetically trapped species and that the effect of leaving ligands, X, on the self-

⟨n⟩ = 12653

N ⟨a⟩ − ⟨c⟩ ⟨b⟩

(2) DOI: 10.1021/acs.inorgchem.7b02152 Inorg. Chem. 2017, 56, 12652−12663

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Inorganic Chemistry ⟨k⟩ =

⟨a⟩ ⟨b⟩

concentrated in vacuo. The residue was washed with a small amount of CHCl3 to remove residual Py* and with water to remove residual AgOTf and dried in vacuo to afford PdPy*4(OTf)2 as a slightly yellow solid (143 mg, 58%). 1H NMR (500 MHz, CD3NO2, 298 K) δ 9.04 (d, J = 2.1 Hz, 4H), 8.95 (d, J = 5.6 Hz, 4H), 8.05 (ddd, J = 8.4, 2.3, 1.3 Hz, 4H), 7.59 (dd, J = 8.2, 5.5 Hz, 4H). PdPy4Me4(OTf)2. A solution of PdCl2 (100 mg, 0.56 mmol) and AgOTf (288 mg, 1.12 mmol) in CH3CN (5.6 mL) was stirred at 60 °C for 30 min. Into the solution was added 4-picoline (0.24 mL, 2.5 mmol). Then the precipitated AgCl was removed by centrifugation, and the precipitation was washed with CH3CN. The obtained solution was concentrated in vacuo to afford Pd(Py4Me)4(OTf)2 as a slightly yellow solid (402 mg, 92%). 1H NMR (500 MHz, CD3NO2, 298 K) δ 8.46 (d, J = 6.0 Hz, 8H), 7.25 (d, J = 5.7 Hz, 8H), 4.17 (s, 12H). Monitoring the Self-Assembly of [Pd618]12+ in CD3NO2/ CD2Cl2 (v/v = 4/1) by 1H NMR. From PdX4(OTf)2 (X: Py, Py*, or CH3CN). A 2.4 mM solution of [2.2]paracyclophane in CHCl3 (125 μL), which was used as an internal standard, was added to two NMR tubes (tubes A and B), and the solvent was removed in vacuo. A solution of PdX4(OTf)2 (X: Py, Py*, or CH3CN) (18 mM) in CD3NO2 was prepared (solution A). Solution A (50 μL) and CD3NO2 (450 μL) were added to tube A. The exact concentration of solution A was determined based on [2.2]paracyclophane by 1H NMR. A solution of tritopic ligand 1 (12 mM) in CHCl3 (100 μL) was added to tube B, and the solvent was removed in vacuo. Then CD2Cl2 (110 μL) and CD3NO2 (390 μL) were added to tube B, and the exact amount of 1 in tube B was determined based on [2.2]paracyclophane by 1H NMR. Into tube B was added 0.75 equiv (against the amount of ligand 1 in tube B) of PdX4(OTf)2 in solution A at 273 K, and then the selfassembly of the capsule was monitored at 298 K by 1H NMR. The quantities of 1, [PdX4]2+, [Pd618]12+, and X were quantified by the integral of each 1H signal against the signal of the internal standard ([2.2]paracyclophane). In the case of the self-assembly of the capsule from [Pd(CH 3CN)4 ]2+ and 1, due to the difficulty of the quantification of [Pd(CH3CN)4]2+ and CH3CN, only the consumption of 1 and the formation of the capsule were monitored by 1 H NMR. In order to confirm the reproducibility, the same experiments were carried out three times (runs 1−3). The existence ratios of the substrates (1 and [PdX4]2+) and the products ([Pd618]12+ and X) are plotted in Figures S1, S3, and S5 and the (⟨n⟩, ⟨k⟩) plots are shown in Figures S2 and S4. These data are listed in Tables S1−S9. From Pd(Py4Me)4(OTf)2. A 2.4 mM solution of [2.2]paracyclophane in CHCl3 (125 μL), which was used as an internal standard, was added to two NMR tubes (tubes A and B), and the solvent was removed in vacuo. A solution of Pd(Py4Me)4(OTf)2 (18 mM) and 1,3,5trimethoxybenzene (15 mM), which was used as a secondary internal standard, in CD3NO2 was prepared (solution A). Solution A (50 μL) and CD3NO2 (450 μL) were added to tube A. The exact concentration of Pd(Py4Me)4(OTf)2 and 1,3,5-trimethoxybenzene in solution A was determined based on [2.2]paracyclophane by 1H NMR. A solution of ditopic ligand 1 (12 mM) in CHCl3 (100 μL) was added to tube B, and the solvent was removed in vacuo. Then CD2Cl2 (110 μL) and CD3NO2 (390 μL) were added to tube B, and the exact amount of 1 in tube B was determined based on [2.2]paracyclophane by 1H NMR. Into tube B was added 0.75 equiv (against the amount of ligand 1 in tube B) of Pd(Py4Me)4(OTf)2 in solution A at 263 K, and then the self-assembly of the capsule [Pd618]12+ was monitored at 298 K by 1H NMR measurement. The exact ratio of 1 and Pd(Py4Me)4(OTf)2 was unambiguously determined by the comparison of the integral value of each 1H signal of [2.2]paracyclophane and 1,3,5-trimethoxybenzene. The quantities of 1, [Pd(Py4Me)4]2+, [Pd618]12+, and Py4Me were quantified by the integral value of each 1 H signal against the signal of the internal standard ([2.2]paracyclophane). In order to confirm the reproducibility, the same experiments were carried out three times (runs 1−3). The existence ratios of the substrates (1 and [Pd(Py4Me)4]2+) and the products ([Pd618]12+ and Py4Me) are plotted in Figure S6 and the (⟨n⟩, ⟨k⟩) plots are shown in Figure S7. These data are listed in Tables S10−S12. ESI-TOF Mass Study. The Self-Assembly from 1 and PdX4(OTf)2 (X: Py, Py*, or Py4Me). Time-dependent ESI-TOF mass spectra were

(3)

where N is a coordination number of a metal ion, Mm+. Parameter n is the average number of metal ions binding to a ligand, L, and thus the maximum value of n is the number of binding sites of L. Parameter k is the ratio of a metal to ligand of an intermediate. Previously we investigated the self-assembly process of the octahedron-shaped Pd618 capsule from PdPy4· (OTf)2 (Py: pyridine) and tritopic ligand, 1, in CD3CN/ CD2Cl2 (v/v = 4/1) by the (n, k) analysis (Figure 5).53 Upon mixing the two starting materials, both were quickly consumed to produce a lot of intermediates. Interestingly, the (⟨n⟩, ⟨k⟩) value stayed around (2.88, 0.75) during the formation of the capsule (Figure 5, blue solid circles), which indicates that the species whose (n, k) value is (2.88, 0.75), [Pd618Py1]12+, exists as a dominant intermediate and that the step from [Pd618Py1]12+ to the Pd618 capsule, which is the final step of the capsule formation, is the rate-determining step of the selfassembly. It is worth noting that the (n, k) analysis does not necessarily indicate that the intermediate whose (n, k) value is equal to experimentally obtained (⟨n⟩, ⟨k⟩) value exists in the reaction mixture except some special cases, one of which is the above case where the self-assembled structure is produced from the intermediates without changing the (⟨n⟩, ⟨k⟩) value. Thus, we have to carefully discuss the (⟨n⟩, ⟨k⟩) value, but the time variation of the (⟨n⟩, ⟨k⟩) value also provides us valuable information about the self-assembly process. For example, the decrease in the ⟨k⟩ value with time suggests that L is more incorporated into the intermediates than the metal source. When all the substrates are consumed, the case where the ⟨n⟩ value increases with time accompanied by a constant value of ⟨k⟩ suggests the growth of intermediates by intermolecular connection between the intermediates (eq 4) and/or the closing process by intramolecular linking between neighboring metal ions by a multitopic ligand in the intermediates (eq 5). [PdaLbXc]2a + + [PdxLyX z]2x + ·

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→ [Pda + xLb + yXc + z − w]2(a + x) + + w·X

(4)

[PdaLbXc]2a + → [PdaLbXc − v]2a + + v ·X

(5)

EXPERIMENTAL SECTION

General Information. 1H NMR spectra were recorded using a Bruker AV-500 (500 MHz) spectrometer. All the 1H NMR spectra were referenced using residual solvent peaks, CD3CN (δ 1.94) or CD3NO2 (δ 4.33). Electrospray ionization time-of-flight (ESI-TOF) mass spectra were obtained using a Waters Xevo G2-S Tof mass spectrometer. Dynamic light scattering (DLS) measurements were performed using a Malvern Zetasizer Nano ZS instrument equipped with a He−Ne laser operating at 2 mW power and 633 nm wavelength and a computer-controlled correlator at 173° accumulation angle. Materials. Unless otherwise noted, all solvents and reagents were obtained from commercial suppliers (TCI Co., Ltd., WAKO Pure Chemical Industries Ltd., KANTO Chemical Co., Inc., and SigmaAldrich Co.) and were used as received. Tritopic ligand 1 and PdPy4(OTf)2 were prepared according to refs 57 and 53, respectively. Synthesis. PdPy*4(OTf)2. A solution of PdCl2 (50.9 mg, 0.287 μmol) and AgOTf (155 mg, 0.603 μmol) in CH3CN (4 mL) was stirred at 80 °C for 2 h. Then the precipitated AgCl was removed by centrifugation, and the precipitation was washed with CH3CN. The obtained solution was concentrated in vacuo. Then to the residue were added CH3NO2 (4 mL) and 3-chloropyridine (Py*) (261 mg, 2.30 μmol). The solution was stirred at 80 °C for 1 h and then was 12654

DOI: 10.1021/acs.inorgchem.7b02152 Inorg. Chem. 2017, 56, 12652−12663

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Inorganic Chemistry measured to investigate the intermediates. PdX4(OTf)2 (1.8 μmol) and 1 (1.94 mg, 2.4 μmol) were quantified and mixed in CD3NO2/ CD2Cl2 (v/v = 4/1) (ca. 1100 μL) in the same manner as the 1H NMR monitor of the self-assembly described in the previous section. At each time point, 50 μL of the resulting solution was taken, diluted with CH3NO2 (500 μL), filtered through a membrane filter (pore size: 0.20 μm), and injected to the mass spectrometer with 3 μL/min flow rate to obtain an ESI-TOF mass spectrum. The ESI-TOF mass spectra and the time variation of the observed species are shown in Figures S10, S11, and S13 and Tables S14, S15, and S17, respectively. The Self-Assembly from 1 and Pd(CH3CN)4(OTf)2. Time-dependent ESI-TOF mass spectra were measured to investigate the intermediates. Pd(CH3CN)4(OTf)2 (0.51 mg, 0.90 μmol) and 1 (0.97 mg, 1.2 μmol) were quantified and mixed in CD3NO2/CD2Cl2 (v/v = 4/1) (ca. 550 μL) in the same manner as the 1H NMR monitor of the self-assembly described in the previous section. The solution was left for 30 min, and then to the solution was added the solution (0.60 mM) of NBu4OTf in CH3NO2 (5 μL) as an internal standard in order to quantify the ion intensity in the mass measurements. At each time point, 50 μL of the resulting solution was taken, diluted with CH3NO2 (500 μL), filtered through a membrane filter (pore size: 0.20 μm), and injected to the mass spectrometer with 3 μL/min flow rate to obtain an ESI-TOF mass spectrum. The ESI-TOF mass spectra and the time variation of the observed species are shown in Figure S12 and Table S16, respectively. The Effect of Solvent on the Self-Assembly of [Pd618]12+. A general procedure for solvent replacement after the convergence of the self-assembly: PdX4(OTf)2 (X: Py, Py*, or CH3CN) (0.90 μmol) and 1 (0.97 mg, 1.2 μmol) were quantified and mixed in CD3NO2/CD2Cl2 (v/v = 4/1) (ca. 550 μL) in the same manner as the 1H NMR monitor of the self-assembly described in the previous section. The selfassembly of the capsule was monitored at 298 K by 1H NMR. The quantity of [Pd618]12+ was quantified by the integral of the 1H signal against the signal of the internal standard ([2.2]paracyclophane). After the convergence of the self-assembly, the solution was heated at 353 K, and the self-assembly of the capsule at 353 K was monitored by 1H NMR. After the convergence of the self-assembly, the solution was completely concentrated in vacuo. Then to the residue was added CD3CN (500 μL), and the self-assembly of the capsule at 353 K was monitored by 1H NMR. After the convergence of the self-assembly, the solution was completely concentrated in vacuo. Then to the residue was added DMSO-d6 (500 μL), and the self-assembly of the capsule at 353 K was monitored by 1H NMR. The results are shown in Figure 6. A general procedure for solvent replacement at a very early stage of the self-assembly: PdPy4(OTf)2 (0.77 mg, 0.90 μmol) and 1 (0.97 mg, 1.2 μmol) were quantified and mixed in CD3NO2/CD2Cl2 (v/v = 4/1) or CD3CN/CD2Cl2 (v/v = 4/1) (ca. 550 μL) in the same manner as the 1H NMR monitor of the self-assembly described in the previous section. The solution was left for 5 min, and then the solution was carefully concentrated up to 100 μL in vacuo at 298 K in order to remove only the solvent. Then 400 μL of another solvent (CD3CN or CD3NO2) was added to the solution, and the self-assembly of the capsule at 298 K was monitored by 1H NMR. The quantity of Py after the replacement of the solvent was checked based on the internal standard ([2.2]paracyclophane). The quantity of [Pd618]12+ was determined by the integral of the 1H signal of the capsule against the signal of the internal standard. The data are shown in Figure 7. DFT Calculations for the Ligand Exchange on a SquarePlanar Pd(II) Center. In the ligand exchange between X (X: Py, Py*, or CH3CN) of [PdX4]2+ and free Py, the initial state of [PdX4]2+ and Py, reactant, transition state (TS), product, and final state of [PdX3Py]2+ and X were calculated by long-range corrected density functional theory (LC-DFT) with LC-OLYP functional. LanL2DZ and 6-31G(d) basis sets were employed for Pd and other atoms, respectively. First, geometry optimization for each state structure was performed in gas phase. Vibrational harmonic frequencies were also carried out at the same level of theory. We have confirmed that the reactant and product reported are equilibrium structure with no imaginary frequency and TS structure with one imaginary frequency.

For these optimized structures, the single point calculation was performed in CH3NO2 solvent (ε = 36.562) with a polarizable continuum model (PCM). The relative energies are shown in Table 1.

Table 1. Computationally Determined Activation Energies of the Ligand Exchange of X in [PdX4]2+ (X = Py, Py*, and CH3CN) with Py in CH3NO2 X

activation energy (kcal mol−1)

Py Py* CH3CN

15.1 14.2 6.4

The optimized structures before and after the ligand exchange (reactant and product) and the TS are all five-coordinated species. All calculations were performed with Gaussian 09 program package.58

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RESULTS AND DISCUSSION The Effect of Solvent on the Self-Assembly Process. First, we investigated the effect of coordination ability of solvent on the rate of the ligand exchange between Py of PdPy4· (OTf)2 and free Py (Figure S8). The rate of the ligand exchange at 298 K in CD3CN was determined to be 2.0 × 10−2 s−1 by 1H NMR spectroscopy, which is about six times faster than that in CD3NO2, 0.34 × 10−2 s−1. This result indicates that CD3NO2, which has little coordination ability, decelerates the ligand exchange because of the lack of the solvent-assisted exchange process (Figure 1). Due to the low solubility of 1 in CD3CN or CD3NO2, the self-assembly was performed in CD3CN/CD2Cl2 (v/v = 4/1) or CD3NO2/CD2Cl2 (v/v = 4/ 1), but CD2Cl2 did not much affect the rate of the ligand exchange. Indeed the rate of the ligand exchange between Py of [PdPy4]2+ and free Py at 298 K in CD3CN/CD2Cl2 (v/v = 4/1) was found to be 1.9 × 10−2 s−1, which is about four times faster than that in CD3NO2/CD2Cl2 (v/v = 4/1), 0.46 × 10−2 s−1. Next, the effect of solvent on the self-assembly process of the Pd618 capsule from 1 and PdPy4·(OTf)2 was investigated in the solvents with different coordination ability. Some 1H NMR spectra measured during the self-assembly are shown in Figure 3, in which the signals for the capsule were found, whereas none of the signals for the intermediates were observed. The time variation of the existence ratio of the substrates and the products in eq 1 is shown in Figure 4. As expected, the consumption of 1 and [PdPy4]2+ and the release of Py in CD3NO2/CD2Cl2 (v/v = 4/1) are slower than those in CD3CN/CD2Cl2 (v/v = 4/1) (Figure 4a−c). On the other hand, the self-assembly of the capsule in CD3NO2/CD2Cl2 (v/v = 4/1) stopped at 22% yield even though a large part of 1 and [PdPy4]2+ was consumed (Figure 4d). This result suggests that most of the starting materials were converted into a mixture of kinetically trapped species not observed by 1H NMR spectroscopy probably because of many chemically inequivalent signals arising from low symmetrical structures and because of intramolecular ligand exchanges that are comparable to NMR time-scale (a possibility of large species that cannot be detected by NMR is ruled out by DLS measurement (vide infra)). Considering the fact that in CD3CN/CD2Cl2 (v/v = 4/1) the capsule was efficiently produced at 298 K in over 90% yield after 6 h, the solvent with little coordination ability not only retarded the ligand exchange but also altered the self-assembly pathway. The change of the self-assembly process depending on the solvent was also confirmed by the time variation of the (⟨n⟩, 12655

DOI: 10.1021/acs.inorgchem.7b02152 Inorg. Chem. 2017, 56, 12652−12663

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to 60 min, the ⟨n⟩ value increased with constant value of the ⟨k⟩ value, indicating that reactions in eqs 4 and 5 mainly took place, although small amount of the substrates were consumed. The cage formation started at 25 min, and 10% of the cage was produced at 60 min with decrease in the ⟨k⟩ value, suggesting that the incorporation of 1 into the intermediates after 60 min brought about the capsule formation with the release of Py. The formation of the capsule slowed down after 120 min and stopped (22%) at 360 min, after which the (⟨n⟩, ⟨k⟩) value stayed around (2.80, 0.75). The self-assembly of the capsule from PdPy4·(OTf)2 and 1 in CD3NO2/CD2Cl2 (v/v = 4/1) was monitored by ESI-TOF mass spectrometry (Table S14 and Figure S10), which indicates the growth of intermediates smaller than the capsule with time. None of the species that contain more Pd(II) ions and/or 1 than the capsule were observed. The species whose Pd(II) center(s) is/are coordinatively unsaturated (for example, [Pd1]2+ indicated as (1,1,0) in Table S14) were observed. Considering the result that none of the Py of [PdPy4]2+ was detached in CD3NO2/CD2Cl2 (v/v = 4/1), such species should come from the release of Py during ionization. Thus, truly existing species in the reaction mixture may contain more Py than were detected by mass spectrometry. After the convergence of the self-assembly of the capsule from PdPy4·(OTf)2 and 1 in CD3NO2/CD2Cl2 (v/v = 4/1) at 298 K, DLS measurement of the reaction mixture containing 24% of the capsule was carried out, and 2−3 nm-sized species (black line) were found (Figure S14a), which is slightly smaller than the capsule (red line in Figure S14), suggesting that the kinetically trapped species are a little smaller than the capsule. Next, the stability of the kinetically trapped species was investigated (Figure 6a). By heating the reaction mixture, which was prepared from 1 and PdPy4·(OTf)2 in CD3NO2/CD2Cl2 (v/v = 4/1), at 353 K, the yield of the capsule increased up to 43%, but the prolonged heating did not improve the yield. Then the solvent was replaced with CD3CN, and the solution

Figure 3. Partial 1H NMR spectra (500 MHz, CD3NO2/CD2Cl2 (v/v = 4/1), 298 K) for the self-assembly of the Pd618 capsule from PdPy4· (OTf)2 and 1 in CD3NO2/CD2Cl2 (v/v = 4/1) at 298 K. (a) 1, (b) PdPy4·(OTf)2, and (c) Py. (d−f) The reaction mixture measured at 60, 120, and 720 min, respectively. The signals colored in red, green, blue, and orange indicate the capsule, [PdPy4]2+, 1, and Py, respectively. The pound sign (#) indicates one of the proton signals for [2.2]paracyclophane, which was added as the internal standard.

⟨k⟩) value for the self-assembly of the capsule (Figure 5). When CD3NO2/CD2Cl2 (v/v = 4/1) was used as the solvent (red solid circles), the ⟨k⟩ value first decreased with increase of the ⟨n⟩ value until 30 min. As the consumption of 1 and [PdPy4]2+ continued during the self-assembly (Figure 4a,b, Table S1), the decrease in the ⟨k⟩ value indicates that 1 was more incorporated into the intermediates than Pd(II) ion. From 30

Figure 4. Time variations of the existence ratio of (a) 1, (b) [PdPy4]2+, (c) Py, and (d) [Pd618]12+ for the self-assembly of the Pd618 capsule in different solvents at 298 K. Blue and red solid circles indicate data obtained in CD3CN/CD2Cl2 (v/v = 4/1) and in CD3NO2/CD2Cl2 (v/v = 4/1), respectively. 12656

DOI: 10.1021/acs.inorgchem.7b02152 Inorg. Chem. 2017, 56, 12652−12663

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Inorganic Chemistry

Figure 5. (a) The (⟨n⟩, ⟨k⟩) plots for the self-assembly of the Pd618 capsule from 1 and [PdPy4]2+ in different solvents at 298 K. Blue and red solid circles indicate the (⟨n⟩, ⟨k⟩) plots obtained in CD3CN/CD2Cl2 (v/v = 4/1) and in CD3NO2/CD2Cl2 (v/v = 4/1), respectively. (b) A magnified view.

CD3CN, and the solution was heated at 353 K to increase the yield up to 88%. These results indicate that the kinetically trapped species formed from 1 and PdPy4·(OTf)2 in CD3NO2/ CD2Cl2 (v/v = 4/1) at 298 K were stable in CD3CN even with heat but were converted into the capsule by heating in DMSOd6. Compared with the result that the capsule was smoothly formed within 6 h in CD3CN/CD2Cl2 (v/v = 4/1) at 298 K (Figure 4d, blue solid circles), CD3CN solvent plays a key role to prevent primitive intermediates produced in a very early stage of the self-assembly from going through the pathways leading to the kinetically trapped species. But the coordination ability of CD3CN is not high enough to transform the kinetically trapped species produced in less coordinative solvent (CD3NO2/CD2Cl2 (v/v = 4/1)) into the capsule. Finally, to investigate whether or not the solvent destines the self-assembly process at a very early stage, the following experiments were performed. The self-assembly of the capsule from 1 and PdPy4·(OTf)2 started in less coordinative solvent, CD3NO2/CD2Cl2 (v/v = 4/1), and after 5 min, the solvent was replaced with coordinative solvent, CD3CN/CD2Cl2 (v/v = 4/ 1), and the self-assembly was monitored by 1H NMR spectroscopy. The yield of the capsule reached 28% at 3 h (Figure 7, red solid circles), which is similar to that carried out in CD3NO2/CD2Cl2 (v/v = 4/1), and further increase of the yield was not observed, although less coordinative solvent was

Figure 6. Time variation of the yield of the Pd618 capsule using various PdX4·(OTf)2. The formation of the Pd618 capsule (a) from 1 and [PdPy4]2+, (b) from 1 and [PdPy*4]2+, (c) from 1 and [Pd(CH3CN)4]2+, and (d) from 1 and [Pd(Py4Me)4]2+.

Figure 7. Time variation of the yield of the Pd618 capsule assembled from PdPy4·(OTf)2 and 1 after the replacement of the solvent at 5 min at 298 K. Blue solid circles indicate the capsule formation ratio with time when the solvent was replaced from CD3CN/CD2Cl2 (v/v = 4/ 1) to CD3NO2/CD2Cl2 (v/v = 4/1) at 5 min. Red solid circles indicate the capsule formation ratio with time when the solvent was replaced from CD3NO2/CD2Cl2 (v/v = 4/1) to CD3CN/CD2Cl2 (v/v = 4/1) at 5 min.

was heated at 353 K, but no further progress of the selfassembly was found. Finally, the solvent was replaced with DMSO-d6, which possesses stronger coordination ability than 12657

DOI: 10.1021/acs.inorgchem.7b02152 Inorg. Chem. 2017, 56, 12652−12663

Article

Inorganic Chemistry

lower than that from with less labile leaving ligands. In addition, the stronger the coordination ability of the leaving ligand in [PdX4]2+ is, the faster the rate of the capsule formation became. These results indicate that the self-assembly of the capsule at 298 K took place not under thermodynamic control and that the use of metal source with relatively good leaving ligand tends to produce kinetically trapped species, which prevent the capsule formation, whereas a leaving ligand X with appropriate coordination ability causes the self-assembly of the capsule efficiently even under mild condition. The efficiency of the selfassembly did not simply correlate with the rate of the ligand exchange. Self-Assembly Process of the Capsule from 1 and [PdPy*4]2+. The intermediates formed from 1 and PdPy*4· (OTf)2 were investigated by the (n, k) analysis (Figure 13, green solid circles). In this case, as all of the tritopic ligand, 1, and [PdPy*4]2+ were consumed within 5 min, the ⟨k⟩ value always indicates 0.5, and the change in the ⟨n⟩ value is dependent on the release of Py* and the formation of the capsule from the intermediates. None of the capsules were produced until 25 min, and the intermolecular reaction of the intermediates to lead to large intermediates (eq 4) and/or the intramolecular linking to lead to closed structures (eq 5) took place until 25 min. The rate of the formation of the capsule was very slow, and the yield of the capsule after convergence was only 12%, indicating that most of the substrates were converted into a mixture of kinetically trapped species. The high ⟨n⟩ value after 720 min suggests that the kinetically trapped species are closed structures with quite a few Py*, which is consistent with the fact that the release ratio of Py* is over 90% (Figure 12c, green solid circles). The growth of the intermediates smaller than the capsule during the self-assembly of the capsule from PdPy*4·(OTf)2 and 1 in CD3NO2/CD2Cl2 (v/v = 4/1) at 298 K was monitored by time-dependent ESI-TOF mass spectrometry (Table S15 and Figure S11). As was observed in the selfassembly of the capsule from [PdPy4]2+ and 1, the species whose Pd(II) center(s) is/are coordinatively unsaturated were detected, which is reasonable because the coordination ability of Py* is weaker than that of Py. The signal for the capsule (6,8,0) was observed after 5 min, while the 1H NMR signals of the capsule did not appear until 30 min. An earlier observation of the capsule by mass spectrometry would arise from higher sensitivity for the capsule with higher stability than the other species. The species larger than the capsule was not observed (Table S15), which is consistent with the observation of the peak around 2 nm for kinetically trapped species a little smaller than the capsule by DLS measurement (Figure S14b). Self-Assembly Process of the Capsule from 1 and [Pd(CH3CN)4]2+. For the self-assembly of the capsule from Pd(CH3CN)4·(OTf)2 and 1, the capsule was produced in no more than 1% (Figure 12d, yellow solid circles) with the generation of many kinetically trapped species. As only very broad signals were observed by 1H NMR spectroscopy (Figure 10), the self-assembly was monitored by ESI-TOF mass spectrometry (Table S16 and Figure S12). Four species mainly observed were the ones with no or only one leaving ligand, which is due to the detachment of the very labile leaving ligand, CH3CN, from Pd(II) centers in the intermediates during ionization. It is obvious that more species were detected in the self-assembly of the capsule from Pd(II) ion source with more inert leaving ligands, X, which reflects the relative stability of the intermediates. This result implies that the species

replaced after 5 min with a better solvent for the capsule formation. This result indicates that most of the intermediates formed in CD3NO2/CD2Cl2 (v/v = 4/1) in 5 min are already destined to lead to the kinetically trapped species regardless of the solvent used after 5 min. On the other hand, when the selfassembly started in CD3CN/CD2Cl2 (v/v = 4/1) and then the solvent was replaced with CD3NO2/CD2Cl2 (v/v = 4/1) at 5 min, the capsule was produced in over 60% yield (Figure 7, blue solid circles). These results indicate that the intermediates formed at a very early stage of the self-assembly in CD3NO2/ CD2Cl2 (v/v = 4/1) and in CD3CN/CD2Cl2 (v/v = 4/1) are completely different, which is also suggested by the (n, k) analysis (Figure 5) and that the self-assembly process is definitively determined by the solvent employed at the beginning of the self-assembly. The Effect of Leaving Ligands on the Self-Assembly Process. The effect of leaving ligands on the self-assembly process of the capsule was investigated by using four kinds of leaving ligands with different coordination ability, CH3CN, pyridine (Py), 3-chloropyridine (Py*), and 4-picoline (Py4Me). Because of the partial detachment of Py* from PdPy*4·(OTf)2 in CD3CN/CD2Cl2 (v/v = 4/1), the self-assembly was carried out in CD3NO2/CD2Cl2 (v/v = 4/1). Prior to the experiment, the ligand exchange of X in [PdX4]2+ (X: Py, Py*, and CH3CN) with Py was studied by density functional theory (DFT) calculations. First, a square-pyramidal structure of [PdX4Py]2+ was optimized and then was transformed to a trigonalbipyramidal transition structure [PdX 4Py]2+ via Berry’s pseudorotation as shown in Figure 8. The activation energies

Figure 8. A potential energy profile of the ligand exchange between Py* in [PdPy*4]2+ and Py. Color labels: gray, C; white, H; blue, N; light green, Cl; light blue, Pd.

for various X (Py, Py*, and CH3CN) are listed in Table 1. The activation energy of the ligand exchange with better leaving ligand, X, is lower, which suggests that if the self-assembly pathway does not depend on X, the self-assembly of the capsule with better leaving ligand should take place faster. 1 H NMR spectra for the self-assembly from 1 and PdX4· (OTf)2 (X = Py*, CH3CN, and Py4Me) in CD3NO2/CD2Cl2 (v/v = 4/1) at 298 K are shown in Figures 9−11. In every case, no signals besides the substrates (1 and [PdX4]2+) and the products (Pd618 and X) were observed, so the information about the intermediates could not directly be obtained by NMR spectroscopy. Based on thermodynamic consideration, the weaker the coordination ability of X is, the higher the yield of the capsule is. However, the opposite result was found in the self-assembly of the Pd618 capsule (Figure 12d); the yield of the capsule starting from [PdX4]2+ with good leaving ligands, X, is 12658

DOI: 10.1021/acs.inorgchem.7b02152 Inorg. Chem. 2017, 56, 12652−12663

Article

Inorganic Chemistry

Figure 9. Partial 1H NMR spectra (500 MHz, CD3NO2/CD2Cl2 (v/v = 4/1), 298 K) for the self-assembly of the Pd618 capsule from PdPy*4·(OTf)2 and 1 in CD3NO2/CD2Cl2 (v/v = 4/1) at 298 K. (a) 1, (b) PdPy*4·(OTf)2, and (c) Py*. (d−f) The reaction mixture measured at 60, 120, and 720 min, respectively. The signals colored in red, green, blue, and orange indicate the capsule, PdPy*4·(OTf)2, 1, and Py*, respectively. The pound sign (#) indicates one of the proton signals for [2.2]paracyclophane, which was added as the internal standard.

Figure 10. Partial 1H NMR spectra (500 MHz, CD3NO2/CD2Cl2 (v/v = 4/1), 298 K) for the self-assembly of the Pd618 capsule from Pd(CH3CN)4·(OTf)2 and 1 in CD3NO2/CD2Cl2 (v/v = 4/1). (a) 1. (b−d) The reaction mixture measured at 60, 120, and 720 min, respectively. The pound sign (#) indicates one of the proton signals for [2.2]paracyclophane, which was added as the internal standard. The lettering refers to those shown in Figure 2.

d6 at 353 K, indicating that these species were kinetically trapped in less polar solvent. Self-Assembly Process of the Capsule from 1 and [Pd(Py4Me)4]2+. When 4-picoline (Py4Me), whose coordination ability is stronger than that of the other leaving ligands used in this study, was used as a leaving ligand, X, the capsule formation started at 5 min, which is much faster than when the other leaving ligands were used (Figure 12d). The rates of the consumption of the substrates and the release of X (= Py4Me) were faster than when Py was used as X. These results are contrary to the idea that the self-assembly of the capsule becomes faster when the metal source with good leaving ligand is used under the assumption that the self-assembly takes place through the same pathway regardless of the leaving ligand, suggesting that the self-assembly process of the capsule was altered by the leaving ligand and that inert leaving ligand tends

possessing labile X tend to be decomposed during the ionization and that more intermediates than were observed by ESI-TOF mass spectrometry would actually be formed in the self-assembly of 1 and [PdX4]2+ with labile X. DLS measurement for the reaction mixture, which contained