Molecular motors: new designs and applications
Roke, Gerrit Dirk
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Chapter 5
Photoresponsive supramolecular
coordination cage based on
overcrowded alkenes
Supramolecular coordination cages with a Pd2L4 composition are formed using molecular
motors based on overcrowded alkene as a ligand. Characterization of these cages with NMR, HRMS, CD and X-Ray shows that the cages self-sort into homochiral assemblies, which are energetically favored over the diastereomeric complexes as shown by DFT calculations. The photochromic ligands can be switched between three states, each of them having the potential of forming discrete cage complexes, allowing cage-to-cage transformation. Moreover, the complexes were shown to bind a tosylate anion in their cavity.
This chapter will be published as: C. Stuckhardt, D. Roke, W. Danowski, S. J. Wezenberg, B. L. Feringa, manuscript in preparation
The experimental work described in this chapter was performed by C. Stuckhardt as a part of his Master’s thesis under the guidance of D. Roke.
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5.1
Introduction
Supramolecular coordination complexes (SCC’s) represent an exciting class of compounds which have been used in sophisticated molecular systems.[1–6] Making use of the vacant cavity inside these complexes, SCC’s have found applications for drug delivery,[6–8] supramolecular catalysis,[9–12] X-ray structure determination[13,14] and stabilization of reactive guests.[15–17] The use of reversible and hence dynamic bonds in supramolecular chemistry gives rise to systems that allow for error correction, a necessity for self-sorting,[18–20] and for adaption to external stimuli such as pH, anions, electric potential, concentration and light.[4,8,21–24] Using light to control the shape and function of SCCs is a very promising strategy as light is a non-invasive stimulus that can be easily controlled in a spatial and temporal manner as well as in terms of intensity and wavelength, without producing any waste. The field of photoswitchable SCC’s is, however, underdeveloped. Systems have been reported where photoisomerization of azobenzene-derived anions encapsulated in supramolecular palladium complexes caused immediate crystallization.[25] Moreover, azobenzenes have been used to functionalize both the interior[26] and exterior[27] of SCC’s to photochemically control guest binding and release. Incorporation of photoswitches into the backbone of the ligands has only been shown with dithienylethenes, which can be switched between an open and a closed state.[28–30] These ligands were used to control host-guest interactions,[31] structural composition of coordination cages[32] and sol-gel transitions.[33] However, up to now, these are the only examples of SCCs bearing photoswitchable ligands in the backbone and they are limited to the use of dithienylethene switches. Introducing photoswitches that have a larger geometric change upon switching has the potential to induce larger changes in properties. Employing molecular motors as ligands in SCCs is therefore an interesting strategy, as they feature a large geometric change upon switching and have the potential to induce chirality in the complex.[34]
Herein, we report a new photochromic coordination cage with ligands based on molecular motors (Scheme 5.1a). Cages with a Pd2L4 composition are formed from bent bidentate
bispyridyl ligands and Pd(II) ions with a square planar geometry, which have been widely studied.[6,35–37] The photochromic ligands can be switched between three states, each of them having the potential of forming separate discrete cage complexes, allowing cage-to-cage transformations (Scheme 5.1b). Moreover, the assemblies were found to be self-sorting, as only homochiral cages are formed. In addition, two of the cage isomers can bind a tosylate anion in solution by formation of a host-guest complex.
87
Scheme 5.1 a) Schematic representation of a photoresponsive cage with ligands based on overcrowded alkenes. b) Cage formation of overcrowded alkene switches 1 and 2 and their isomerization behavior.
5.2
Ligand synthesis and characterization
Ligands Z-1a and E-2a were synthesized by a Suzuki cross-coupling reaction of 3-pyridinylboronic acid with an E/Z mixture of reported overcrowded alkene precursors (Scheme 5.2).[38] The E and Z isomers were readily separated by column chromatography and identified using 2D NOESY NMR spectroscopy. Enantiopure ligands were synthesized in the same manner, starting from enantiopure motor 7, which was prepared according to literature procedures.[38]
88
Scheme 5.2. Synthesis of overcrowded alkene-based ligands Z-1a and E-2a.
The photochemical and thermal isomerization steps of ligands Z-1a and E-2a were characterized by detailed 1H-NMR studies (Figure 5.1), revealing the same behavior as structurally related molecular motors.[39] When a sample of stable Z-1a was irradiated at 312 nm at -55 °C, a new set of signals appeared, belonging to unstable E-2b (Figure 5.1ii). This can be seen most clearly for the signals of the protons on the central five membered ring (Ha-c). The sample was irradiated until no further changes were observed, and at this
photostationary state (PSS) the ratio of E-2b to Z-1a was 91:9. When allowing this sample to warm to room temperature, this photogenerated isomer undergoes a thermal helix inversion (THI), quantitatively forming stable E-2a. An Eyring analysis was performed to obtain the activation parameters for this process. The THI was followed at five different temperatures ranging from -46 to -26 °C using NMR spectroscopy. A Gibbs free energy barrier of 72.9 kJ mol-1 was obtained (Table 5.1), slightly lower than the barrier reported for the unsubstituted parent motor (‡G = 80 kJ mol-1).[40]
89
Figure 5.1. 1H-NMR spectrum of switching cycle of ligand 1 in CD2Cl2 at -55 °C. i) Stable Z-1a. ii) PSS 312, unstable E-2b. iii) THI, stable E-2a.
When a 1H-NMR sample of E-2a was irradiated at 312 nm in CD2Cl2, the formation Z-1b
could be observed (Figure 5.2). A slightly lower PSS ratio is obtained at this step: 77:23. Leaving this sample for 5 days at room temperature leads to the formation of Z-1a through a THI process. The activation parameters for this step were determined using an Eyring analysis as well. The THI of Z-1b to Z-1a was followed with UV/vis spectroscopy, by monitoring the increase in absorption at = 320 nm of a sample in heptane at five different temperatures ranging from 60 to 90 °C. A Gibbs free energy barrier of 101 kJ mol
-1 was obtained (Table 5.1), slightly higher than the barrier reported for the unsubstituted
parent motor (93 kJ mol-1).[40]
Figure 5.2. 1H-NMR spectra of switching cycle of ligand 2 in CD2Cl2. i) stable E-2a ii) PSS 312
nm, unstable Z-1b. iii) THI, stable Z-1a.
‡H (kJ mol-1) ‡S (J mol-1 K-1) ‡G (kJ mol-1)
E-2b 70.5 ± 2.4 -6.7 ± 10 72.9 ± 0.5
90
Table 5.1. Activation parameters for the THI of isomers E-2b and Z-1b. Values are given at 20 °C.
5.3
Cage formation and characterization
Heating a 2:1 mixture of racemic ligands Z-1a or E-2a with Pd(NO3)2 in acetonitrile at reflux
lead to the quantitative formation of cage 3 or 4, respectively, as evidenced by 1H NMR, DOSY and HRMS. The 1H-NMR signals of the pyridine moieties of the ligands (Ha-d) in the
assembled cages are shifted downfield compared to those of the free ligands, as expected due to metal coordination (Figure 5.3).[31] As the ligand exchange in Pd2L4 complexes is
slow on the NMR timescale, the discrete signals do not represent an average of quickly interconverting isomers.[41,42] Using a racemic mixture of ligands, four different diastereomeric cages can be formed ((S,S)4, (S,S)3(R,R), (S,S)2(R,R)2 and (S,S)(R,R)(S,S)(R,R)
and their enantiomeric pairs). However, in both cases, only one set of signals is observed, which is a strong indication that only one species with high symmetry is formed by chiral self-sorting without any sign of the formation of diastereomeric mixtures. The formation of cage complexes using enantiopure ligands (S,S)-Z-1a or (S,S)-E-2a, resulted in the exact same 1H-NMR spectrum as was obtained with the racemic ligands, indicating that the racemic ligands also form homochiral cages.
Figure 5.3. Aromatic region of stacked 1H-NMR spectra (in CD3CN) of Z-1a and cage 3 (top)
and E-2a and cage 4 (bottom).
Additionally, DOSY NMR spectroscopy revealed that the signals correspond to a single type of assembly in each case (Figure 5.4). The measured diffusion coefficients (D = 8.7 · 10-10 m2 s-1 for cage 3 and D = 7.9 · 10-10 m2 s-1 for cage 4 in CD3CN at 23 °C) can be
91 translated into hydrodynamic radii of rH = 7.2 Å for cage 3 and rH = 8.0 Å for cage 4 by
using the Stokes-Einstein equation.[43] By means of ESI high resolution mass spectrometry, we were able to identify the Pd2L4 constitution of both cages. The spectrum of cage 3
shows the signals for the cations Pd2-Z-1a4(NO3)3+, Pd2-Z-1a4(NO3)22+, Pd2-Z-1a4(NO3)3+,
Pd2-Z-1a44+ (Figure 5.5). For cage 4, the peaks corresponding to the cations Pd2
-E-2a4(NO3)22+ and Pd2-E-2a4(NO3)3+ were observed (Figure 5.5). For both isomers, the
experimental isotopic patterns and exact m/z values match the simulated patterns.
92
Figure 5.5. HRMS spectra of cage 3 (top) and cage 4 (bottom); Insets: comparison of simulated and measured isotopic patterns of Pd2L4(NO3)3+ ions
Formation of single crystals suitable for X-ray structure determination of the cages proved to be challenging and many attempts at obtaining suitable crystals were unsuccessful. Finally, one single crystal of cage 4 formed from a racemic mixture of ligand E-2a suitable for X-ray structure determination was grown by vapor diffusion of a 1:1 mixture of benzene and diethyl ether into a solution of the cage in a 1:1 mixture of acetonitrile and chloroform. The crystal structure shows cages with a Pd2L4 stoichiometry and one NO3
-counter ion and one molecule of acetonitrile are located inside each cage (Figure 5.6). In addition, a chloride ion is located close to the metal centers outside of the cage. This counter ion most likely originates from the solvent, as chloroform can contain considerable amounts of HCl. The crystal structure belongs to the P 4/n space group and the unit cell is occupied by a pair of enantiomeric cages in which the Pd-Pd axis of each cage is located at the 4-fold rotation axis. This means that the crystal structure represents a racemic mixture of cages which either only contain the (R,R) enantiomer or only the (S,S) enantiomer of the ligand.
93
Figure 5.6. Crystal structure of cage 4 (top left) and DFT optimized structures of cages 3-5. Color coding: C – Black, H – White, N – Blue, Pd – Cyan, Cl – Green, O – Red.
To further support that homochiral cages 3 and 4 are formed by chiral self-sorting, giving rise to only one diastereomer (and its enantiomer), DFT calculations were performed. The structures of all possible cage diastereomers were optimized using B3LYP/6-31G(d) for C,H,N and LANL2DZ with ECP for Pd in the gas phase without counter ions. The optimized structure of (E-2a)4Pd24+ is in good agreement with the solved X-ray structure (Figure 5.6).
Moreover, the calculations revealed that the homochiral cage [(S,S)-E-2a]4Pd24+ (and its
enantiomer) are energetically favored by at least 61 kJ mol-1 compared to the other possible diastereomers (Table 5.2). Similar calculations on the diastereomers of cage 3 revealed that the homochiral cage diastereomers [(S,S)-Z-1a]4Pd24+ are energetically
94
Cage 4 diastereomer Relative Gibbs free energy (kJ mol-1)
Pd2[(S,S)-E-2a]4 0
Pd2[(S,S)-E-2a]3[(R,R)-E-2a] +60.9
Pd2[(S,S)-E-2a]2[(R,R)-E-2a]2 +75.8
Pd2[(S,S)-E-2a][(R,R)-E-2a][(S,S)-E-2a][(R,R)-E-2a] +113.8
Table 5.2. DFT calculated relative Gibbs free energy of possible diastereomers of cage 4. Cage 3 diastereomer Relative Gibbs free energy (kJ mol-1)
Pd2[(S,S)-Z-1a]4 0
Pd2[(S,S)-Z-1a]3[(R,R)-Z-1a] +18.5
Pd2[(S,S)-Z-1a]2[(R,R)-Z-1a]2 +27.4
Pd2[(S,S)-Z-1a][(R,R)-Z-1a][(S,S)-Z-1a][(R,R)-Z-1a] +32.4
Table 5.3. DFT calculated relative Gibbs free energy of possible diasteomers of cage 3.
These calculations support that the cage is formed by chiral narcissistic self-sorting which was then probed experimentally by CD spectroscopy. Since the two homochiral enantiomers of cage 3 are expected to be the only optically active species in solution, we argue that the difference in the extinction coefficient (Δε) should have a linear dependency on the ee of the cage solution. On the other hand, if several different diastereomers were present, they should have different individual CD spectra which would all contribute to the overall obtained CD spectrum. As the ratio of these diastereomers would depend on the ee of the cage, this situation would cause a deviation from the linear dependency of Δε on the ee. To test this hypothesis, stock solutions of racemic and enantiopure cage 3 were mixed in different ratios to obtain a range of ee’s. In accordance to our expectations, we found that the amplitude in CD spectra of solutions containing cage 3 show a linear dependence on the ee of ligand Z-1a (Figure 5.7). Plotting the values for Δε found around the extrema at λ = 258, 320 and 360 nm versus the ee of Z-1a used to form the cage gave linear curves for each wavelength. Our predictions based on the DFT calculations were hence further supported experimentally.
95 260 280 300 320 340 360 380 400 -60 -40 -20 0 20 40 60 80 [mo l -1cm -1] [nm] 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Figure 5.7. CD spectra of cage 3 formed from ligand Z-1a (c = 1.1 x 10-5 M) with varying ee (left). Plot of versus ee, showing a linear dependency (right).
Next, we were interested in the guest binding abilities of cages 3 and 4. The tosylate anion has an appropriate size to fit inside the cages. 1H-NMR titrations with tetrabutylammonium tosylate was performed and the data was fitted against a 1:1 binding model using BindFit software (Figure 5.8).[44] The fitting revealed that both cages show equally strong binding towards OTs- (KB= 1604 ± 39 M-1 for 3; KB = 1758 ± 39 M-1 for 4
at 293 K). The binding constants were determined by titration of a stock solution of cage 3 (c = 3.0 x 10-4 M) or 4 (c = 2.7 x 10-4 M) with a stock solution of tetrabutylammonium tosylate (c = 4.0 x 10-3 M) that contained the guest in the same concentration to exclude dilution effects. A Job plot analysis was performed by plotting the host-guest concentration ([HG]) versus the molar fraction of the host (), revealing a 1:1 binding stoichiometry between both cage isomers and OTs- (Figure 5.9). This is in line with the idea that OTs- serves as a guest molecule which is encapsulated inside the cages.
0 20 40 60 80 100 -60 -40 -20 0 20 40 60 80 258 nm 320 nm 360 nm [mo l -1cm -1] ee [%]
96
Figure 5.8. Top: Fitting of 1H-NMR titration data of cage 3 (a) and 4 (b) with tetrabutylammonium tosylate using protons Ha-He as shown in figure 5.3. Bottom: residual
plots of fitting of 1H-NMR titration data of cage 3 (c) and 4 (d).
0.0 0.2 0.4 0.6 0.8 1.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 [H G ] (mM) x
Figure 5.9. Job plot of tosylate binding to cage 3 (left) and cage 4 (right).
0.0 0.2 0.4 0.6 0.8 1.0 0.00 0.02 0.04 0.06 0.08 [H G ] (mM) x
97
5.4
Photochemical isomerizations
The photochemical and thermal isomerizations of cages 3 and 4 were followed by 1H NMR studies (Scheme 5.3 and Figure 5.10). Irradiation of cage 3 in CD3CN:CD2Cl2 1:1 mixture at
312 nm at -70 °C was performed to isomerize ligand Z-1a to E-2b (vide infra), followed by allowing the sample to warm up to room temperature to form E-2a (Figure 5.10ii). The 1 H-NMR spectrum of this newly formed complex is identical to the spectrum of cage 4 prepared directly from E-2a (Figure 5.10iii), showing that cage 3 is effectively converted to cage 4. An intermediate complex containing ligands E-2b was never observed, even at low temperatures, most likely due to the low barrier for THI of this isomer. Conversion of cage 4 to cage 5 by photochemical E-Z isomerization of ligand E-2a to Z-1b was performed by irradiation of cage 4 at 312 nm at -20 °C (Figure 5.10iv). Signals of cage 4 disappeared and the formation of a new set of signals was observed. DOSY NMR confirmed the formation of an assembly with a similar hydrodynamic radius. Precipitation of the metal centers in this assembly using sodium glutamate liberates the organic ligands and they were identified as Z-1b. This confirms that the photogenerated complex is indeed cage 5, formed from ligand Z-1b. Subsequent irradiation of this sample containing cage 5 at -20 °C at 365 nm converts the Z-1b ligands back to E-2a, reforming cage 4 (Figure 5.10v). These experiments highlight the reversible formation of cage 5 through photochemical E-Z isomerization of the ligands.
On the other hand, allowing the THI of ligands Z-1b in cage 5 to take place by leaving the solution at room temperature for 5 d did not lead to the formation of cage 3, but to disassembly of the cage and formation of ill-defined complexes. Precipitation of the metal centers in these complexes identified the ligands as a mixture of both Z-1a and E-2a (originating from the PSS mixture), indicating that the THI does take place. A possible explanation could be that the mixture of Z-1a and E-2a does not form separate well-defined cage structures, but form mixed complexes.
98
Figure 5.10. Aromatic region of stacked 1H NMR spectra (CD3CN/CD2Cl2 1:1) of i) cage 3
formed from Z-1a; ii) cage 4 generated by irradiation of cage 3; iii) cage 4 prepared from E-2a; iv) cage 5 generated by irradiation of cage 4; v) cage 4 generated by irradiation of cage 5.
5.5
Conclusions
In summary, a new photoresponsive supramolecular coordination complex based on overcrowded alkenes is presented, allowing switching between three different cage structures. Interestingly, the cage structures with Pd2L4 constitution were shown to be
homochiral, forming single diastereomers as shown by NMR, CD and X-ray studies, supported by DFT calculations. Additionally, the cage structures were able to bind OTs -inside their cavity. Although photoswitching affords a large geometric change of the ligands, only minor changes were observed in binding constants of the different cage structures. These results show that by incorporation of overcrowded alkenes into SCCs the geometry of cage structures can be controlled by light. Different designs might be considered to translate these geometrical changes to changes in properties such as guest binding.
5.6
Experimental procedures
For general remarks regarding experimental procedures see Chapter 2.
Racemic and enantiopure ketone 6 and motor 7 were synthesized according to literature procedures.[38]
99
(Z)-3,3'-(2,2',4,4',7,7'-hexamethyl-2,2',3,3'-tetrahydro-[1,1'-biindenylidene]-5,5'-diyl)dipyridine (Z-1a) and (E)-3,3'-(2,2',4,4',7,7'-hexamethyl-2,2',3,3'-tetrahydro-[1,1'-biindenylidene]-5,5'-diyl)dipyridine (E-2a)
A 1.5:1 mixture of motors Z-7 and E-7 (700 mg, 1.48 mmol, 1.0 equiv.), 3-pyridinylboronic acid (454 mg, 3.69 mmol, 2.5 equiv.), PdCl2dppf complex with CH2Cl2 (60 mg, 73.8 μmol,
0.05 equiv.) and K2CO3 (1.40 g, 10.1 mmol, 6.8 equiv.) were dissolved in a mixture of water
(7.4 mL) and THF (21 mL). The mixture was degassed by purging with N2 for 30 min and
then stirred at 70 °C for 2 d. Then, the mixture was diluted with CH2Cl2 (50 mL) and
washed with brine (50 mL). The organic phases were combined and dried over Mg2SO4,
volatiles were removed in vacuo and the residue was purified using column chromatography (SiO2, CH2Cl2 + 2.5% MeOH) to give ligands Z-1a (343 mg, 0.73 mmol,
82%) and E-2a (233 mg, 0.49 mmol, 83%) as off-white solids.
Z-1a: Mp 269 °C. 1H NMR (400 MHz, CDCl3) δ (ppm) = 8.64 (s, 2H), 8.58 (s, 2H), 7.72 (d, J =
7.8 Hz, 2H), 7.37 (s, 2H), 6.86 (s, 2H), 3.43 (t, J = 6.6 Hz, 2H), 3.18 (dd, J = 14.9, 6.4 Hz, 2H), 2.53 (d, J = 15.4 Hz, 2H), 2.20 (s, 6H), 1.61 (s, 6H), 1.15 (d, J = 6.8 Hz, 6H). 13C NMR (101 MHz, CDCl3) δ (ppm) = 150.2, 147.7, 145.6, 141.0, 140.9, 138.0, 136.9, 136.8, 133.4, 130.0,
128.3, 123.1, 41.9, 39.6, 20.8, 20.7, 16.4. HRMS (ESI+): calcd for C34H35N2+ [M+H]+:
471.2795, found 471.2763.
E-2a: Mp 235-237 °C. 1H NMR (400 MHz, CDCl3) δ (ppm) = 8.66 (s, 2H), 8.59 (s, 2H), 7.73 (d,
J = 7.7 Hz, 2H), 7.38 (s, 2H), 7.00 (s, 2H), 3.22 – 2.89 (m, 2H), 2.79 (dd, J = 14.7, 5.7 Hz, 2H),
2.50 (s, 6H), 2.34 (d, J = 14.6 Hz, 2H), 2.13 (s, 6H), 1.17 (d, J = 6.5 Hz, 6H). 13C NMR (101 MHz, CDCl3) δ (ppm) = 150.3, 147.9, 144.0, 141.7, 141.0, 137.8, 136.9, 136.8, 131.3, 130.5,
129.1, 123.0, 42.2, 39.8, 22.0, 19.7, 16.3. HRMS (ESI+): calcd for C34H35N2+ [M+H]+:
471.2795, found 471.2764.
The enantiopure ligands (S,S)-Z-1a and (S,S)-E-2a were prepared according to the same procedure employing a mixture of enantiopure precursors (S,S)-Z-7 and (S,S)-E-7. The ee for (S,S)-Z-1a >99% as was determined by chiral HPLC analysis, Chiralpak AD-H (90% heptane/10% i-PrOH), 0.5 mL/min, retention times (min) 12.4 (major) and 15.5 (minor). The ee for (S,S)-E-2a >99%, Chiralpak AD-H (90% heptane/10% i-PrOH), 0.5 mL/min, retention times (min) 11.3 (major) and 12.6 (minor).
100
Cage formation
A solution (c ≤ 2.5 mM) of 1.0 equiv. of Pd(NO3)2 in CD3CN, alternatively in a mixture with
CD2Cl2, was added to 2.0 equiv. of either Z-1a or E-2a in a closed vial and the mixture was
heated at reflux until a clear solution was obtained to yield either cage 3 or cage 4 in solution.
Cage 3: 1H NMR (500 MHz, CD3CN) δ (ppm) = 8.81 (s, 8H), 8.67 (d, J = 5.8 Hz, 8H), 7.99 (d, J
= 8.4 Hz, 8H), 7.55 (dd, J = 8.1, 5.7 Hz, 8H), 7.00 (s, 8H), 3.52 – 3.31 (m, 8H), 3.05 (dd, J = 14.8, 6.2 Hz, 8H), 2.48 (dd, J = 14.9, 7.7 Hz, 8H), 1.97 (s, 24H), 1.61 (s, 24H), 1.02 (d, J = 6.7 Hz, 24H). HRMS (ESI+): calcd for C136H136N11O9Pd2+ ([Pd2Z-1a4](NO3)3+): 2280.8644, found
2280.8893; calcd for C136H136N10O6Pd22+ ([Pd2Z-1a4](NO3)22+): 1109.4380, found 1109.4513;
calcd for C136H136N9O3Pd23+ ([Pd2Z-1a4](NO3)3+): 718.9625, found 718.9712; calcd for
C136H136N8Pd24+ ([Pd2Z-1a4])4+): 523.7248, found 523.7303.
Cage 4: 1H NMR (500 MHz, CD3CN) δ (ppm) = 9.56 (br, 8H), 9.16 (d, J = 5.7 Hz, 8H), 8.13 –
7.85 (m, 8H), 7.67 (dd, J = 7.7, 5.8 Hz, 8H), 6.88 (s, 8H), 2.93 (t, J = 6.3 Hz, 8H), 2.55 – 2.47 (m, 8H), 2.43 (s, 24H), 2.12 (d, J = 12.7 Hz, 8H, *covered by solvent signal), 1.52 (s, 24H), 1.08 (d, J = 6.4 Hz, 24H). HRMS (ESI+): calcd for C136H136N10O6Pd22+ ([Pd2E-2a4](NO3)22+):
1109.4380, found 1109.4378; calcd for C136H136N9O3Pd23+ ([Pd2E-2a4](NO3)3+): 718.9625,
found 718.9613. Binding studies
The binding constants were determined by NMR titrations at 20 °C. Titration of a stock solution of cage 3 (c = 3.0 x 10-4 M) or 4 (c = 2.7 x 10-4 M) with a stock solution of tetrabutylammonium tosylate (c = 4.0 x 10-3 M) in a 1:1 mixture of CD2Cl2 and CD3CN that
contained the guest in the same concentration to exclude dilution effects was performed. The chemical shifts of Ha-d (cage 3) and Ha,c-e (cage 4) were plotted against the host to
guest ratio and fitted against a 1:1 binding model using BindFit software (Figure 5.8). Binding constants of 1604 ± 39 M-1 for 3 and 1758 ± 39 M-1 for 4 were obtained.
A Job plot analysis was performed by plotting the host-guest concentration ([HG]) versus the molar fraction of the host (). Different molar fractions were obtained by mixing stock
101 solutions (c = 3.4 · 10-4 M) of TBAOTs and cage 3 or 4 in a 1:1 mixture of CD3CN and CD2Cl2.
The host guest concentration ([HG]) was then determined by equation 1, in which [H]0 is
the total concentration of host,δobs is the measured chemical shift of proton Ha in the host
guest mixture, δ0 is the chemical shift of proton Ha for the pure host and δcomplex is the
chemical shift of proton Ha in the host guest complex which is assumed to be formed
completely for a host guest ratio of 1:9. Plotting [HG] versus the molar fraction of the host (x) yielded curves with maxima for x = 0.5, confirming a 1:1 binding stoichiometry (Figure 5.9).
[HG] = [H]0·𝛿𝛿obs−𝛿0
complex−𝛿0 (1)
5.7
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