University of Groningen

Molecular motors: new designs and applicationsRoke, 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.

86

Chapter 5

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

Photoresponsive supramolecular coordination cage based on overcrowded alkenes

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

Chapter 5

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

Photoresponsive supramolecular coordination cage based on overcrowded alkenes

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

Z-1b 67.7 ± 2.0 -106 ± 5.6 101 ± 0.1

90

Chapter 5

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 m

2 s

-1 for cage 4 in CD3CN at 23 °C) can be

91

Photoresponsive supramolecular coordination cage based on overcrowded alkenes

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)2

2+, 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.

Figure 5.4. DOSY NMR spectra of cage 3 (top) and cage 4 (bottom) in CD3CN.

92

Chapter 5

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

Photoresponsive supramolecular coordination cage based on overcrowded alkenes

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

favored as well, by at least 19 kJ mol-1

(Table 5.3).

94

Chapter 5

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

Photoresponsive supramolecular coordination cage based on overcrowded alkenes

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

Chapter 5

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

[HG

] (m

M)

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

[HG

] (m

M)

x

97

Photoresponsive supramolecular coordination cage based on overcrowded alkenes

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 1H-

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.

Scheme 5.3. Photochemical isomerization behavior of cages 3-5.

98

Chapter 5

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

Photoresponsive supramolecular coordination cage based on overcrowded alkenes

(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. 1

H 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. 1

H 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). 13

C 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

Chapter 5

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

Photoresponsive supramolecular coordination cage based on overcrowded alkenes

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 ·𝛿obs−𝛿0

𝛿complex−𝛿0 (1)

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