Spiroborate-Based Double-Stranded Helicates: Meso-to-Racemo

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Spiroborate-Based Double-Stranded Helicates: Meso-to-Racemo Isomerization and Ion-Triggered Spring-Like Motion of the Racemo-Helicate Naoki Ousaka, Kaori Shimizu, Yoshimasa Suzuki, Takuya Iwata, Manabu Itakura, Daisuke Taura, Hiroki Iida, Yoshio Furusho, Tadashi Mori, and Eiji Yashima J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08268 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 19, 2018

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Journal of the American Chemical Society

Spiroborate-Based Double-Stranded Helicates: Meso-to-Racemo Isomerization and Ion-Triggered Spring-Like Motion of the RacemoHelicate Naoki Ousaka,†,‡ Kaori Shimizu,‡ Yoshimasa Suzuki,‡ Takuya Iwata,‡ Manabu Itakura,† Daisuke Taura,†,‡ Hiroki Iida,‡,§ Yoshio Furusho,‡,∥ Tadashi Mori,# and Eiji Yashima*†,‡ †

Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan ‡ Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan # Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 5650871, Japan ABSTRACT: A one-handed double-stranded spiroborate helicate exhibits a unique reversible extension–contraction motion cou+ pled with a twisting motion in one direction triggered by binding and release of a Na ion while retaining its handedness. Here we report that an extended meso-helicate was also produced together with the racemo-helicate, and the meso-helicate was readily con+ verted to the racemo-helicate assisted by a Na ion as a template in the presence of water. The thermodynamic analyses of the iontriggered spring-like motion of the racemo-helicate using a series of monovalent cations with different sizes revealed that the asso+ + + + + + + ciation constants of the extended racemo-helicate decreased in the following order: Li > Na > NH4 > Ag ≥ K > Cs > Rb , + which roughly agrees with the reverse order of their ionic radii except for the NH4 ion due to the more elongated contracted helicates when complexed with larger cations as supported by the crystal and DFT calculated structures. The one-handed contracted helicates showed characteristic CD spectra depending on the cation species due to the differences in their contracted helical structures and its absolute handedness of the spiroborate helicate was determined by X-ray crystallography. The kinetic studies of the spring-like motions of the racemo-helicate showed that the exchange rate between the extended and contracted helicates tend to + + + + + + + increase in the following order: Li < Na < K ≤ NH4 < Rb < Cs < Ag as anticipated from the association constants, being in + good agreement with the order of the cation sizes except for Ag .

Introduction Molecules and polymers that enable reversible extension and contraction motions in response to external stimuli have been attracting considerable interest because of their potentials for constructing intelligent molecular machines with nanomechanical functions.1 To this end, a number of synthetic molecular and polymer systems that undergo muscle-like elastic motions, thereby expressing their motions as macroscopic work,2 has been developed.1 However, molecular springs that twist in one particular direction (anisotropic twisting) have rarely been achieved, even though it is common in sophisticated biological systems.3 Helical molecules and polymers have a great advantage over other structures such that an intriguing unidirectional twisting motion during the extension-contraction process may be expected because of their inherently chiral topological structures.2a,c,4-6 Nevertheless, the control of unidirectional twisting motions with synthetic molecular and helical systems remains a challenge. We previously reported a unique double-stranded racemo + (rac)-helicate (rac-DH1BNaB ·Na ) consisting of two ortholinked tetraphenol strands with a biphenylene unit in the middle doubly bridged by two spiroborate groups in which a Na +

ion is encapsulated within the helicate stabilized by negatively-charged boronates (Figure 1a), resulting in the contracted form.7 The central Na+ ion embedded in the contracted helicate can be completely removed by addition of a cryptand [2.2.1] 2to produce the dianionic extended helicate (rac-DH1BB · + + (Na )2), thereby leading to Na ion-triggered, reversible extension–contraction motions coupled with a twisting motion in one direction upon the release and binding of the Na+ ion (Figure 1b) as unambiguously revealed by the crystal structures of the contracted and extended helicates.7a,8 During this unidirectional spring-like motion, the helicate maintained its helical handedness as revealed by complete reversible circular dichroism (CD) spectral changes when an optically-active DH1 was used.7a The observed ion-triggered molecular motions are of key importance in a number of biological processes and reminiscent of fascinating biological machines operating in muscle cells.9 On the other hand, an analogous spiroborate helicate composed of two ortho-linked hexaphenol strands + (rac-DH2BNaB ·Na ) showed no spring-like motions due to the + strong coordination of the Na ion to the central biphenol oxy10 gen atoms (Figure 1a).

1 ACS Paragon Plus Environment

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+

-

During the course of our studies on the spiroborate racemohelicate formation, we have, incidentally, found that its meso+ 2helicate (meso-DH1BB ·(Na )2) was also formed, then converted into the contracted racemo-helicate catalyzed and as+ sisted by water and a Na ion as a template (Figure 1c).13 Based on this finding, we have investigated the mechanism of the meso-to-racemo isomerization of the spiroborate helicate and the effects of a series of monovalent cations, such as alkali + + + + + + + metals (Li , Na , K , Rb , and Cs ), Ag , and NH4 ions of different sizes (Figure 1d) on the thermodynamics and kinetics of the ion-triggered spring-like motions of the racemo-helicate (rac-DH1) upon the binding and release of cations

The structures of the rac-DH1BNaB ·Na and rac-DH2BNaB + ·Na suggested that the biphenylene and biphenol linker units in the middle of each strand could be replaced with other functional residues while maintaining the double-stranded spiroborate helicate structure. In fact, one-handed double-stranded spiroborate helicates bearing 4,4’-linked 2,2’-bipyridine (bpy)11 and m-diphenylporphyrin units12 in the middle underwent a unique reversible unidirectional spring-like motion in a highly homotropic allosteric manner regulated by the coopera+ tive binding and release of protons or Cu ions to the two covalently-linked bpy units and guest encapsulation-triggered unidirectional dual rotary and twisting motions resulting from expansion of the bisporphyrin cavity, respectively.

(a)

tBu

tBu

tBu

OO B OO

tBu

tBu

tBu

OO B OO

Na+

tBu

Page 2 of 15

tBu tBu

tBu

OO B OO

Na+

tBu

tBu

tBu tBu

rac-DH1BNaB–·Na+

OO B OO

HO OH Na+ HO OH

tBu tBu

tBu tBu

Na+

B

N

B

B O

OH

tBu

Cl(CH2)2Cl/EtOH = 6/1 50 °C, 28 h

2

tBu

tBu

tBu

O O B O O

NaBH4 tBu

cryptand [2.2.1]

rac-DH1BB2–·(Na+)2 Extended form tBu

OH

O N

Unidirectional twisting motion

(c)

O O

NaPF6

Contracted form

tBu

O

B

rac-DH1BNaB–·Na+

Na+

rac-DH2BNaB–·Na+

cryptand [2.2.1]

(b)

tBu

O O B O O

2X + rac-DH1BNaB–·Na+

L1 tBu

tBu

tBu

2–·(Na+)

tBu

Na+

meso-DH1BB 2 :X= meso-DH1BB2–·(TBA+)2 : X = TBA+

(d) B

B

B

B

B

Li+

Na+

K+

Rb+

Cs+

B

B

B

B

B

M+

B

B rac-DH1BB2–·(TBA+)2 Extended form

(M+ = Li+, Na+, K+, Rb+, Cs+, Ag+, NH4+)

Ag+

NH4+

Figure 1. (a) Chemical structures of double-stranded spiroborate helicates rac-DH1BNaB–·Na+ and rac-DH2BNaB–·Na+. (b) Schematic representation of the unidirectional spring-like motion upon Na+-ion release and binding. (c) Synthesis of double-stranded spiroborate helicates meso-DH1BB2–·(X)2 (X = Na+ or TBA+) and rac-DH1BNaB–·Na+. (d) Schematic representation of the inclusion complex formation of rac+ + DH1BB2–·(TBA+)2 with alkali metals, Ag , and NH4 ions. TBA: tetrabutylammonium.


Page 3 of 15 tBu

tBu

tBu

g h

tBu

f

a

i

O O B O O

O O B O O

No extension

2Na+

e

b tBu

c d

tBu

j

tBu

N

tBu

O

k

meso-DH1BB tBu

tBu

2–·(Na+)

O

2

cryptand [2.2.1]

tBu

f

a

i

O O B O O

O O B O O

Na+

b

meso-DH1BB2–·[Na+⊂[2.2.1]]2

O N

tBu

g h

O O

tBu

Na+ a

j

c

d

tBu

tBu

k

cd g e b h f

i

(b) meso-DH1BB2–·(Na+)2 d c f

(c) (b) + [2.2.1] (3.0 equiv) d c f

(d) rac-DH1BNaB–·Na+

gf h

(e) (d) + [2.2.1] (3.0 equiv) d c f

a

cd

tBu

j

k

g

e b h

i

e b h

i

e

d

e i b h

h

7.0

tBu

tBu

tBu

k

extended rac-DH1BB2–·[Na+⊂[2.2.1]]2

i

L1

kj 2

k j

k j

g

i

c

g

OO B OO

g

a

a

tBu

i

e f g h

tBu

OH

7.5

OH

a b

8.0

c d

j OH

tBu

ef

tBu

tBu

contracted rac-DH1BNaB–·Na+ (a) Ligand L1

OO B OO

b

e

tBu

tBu

j

k

a

6.5

6.0

5.5

j

k

a

b

5.0

≈

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.6

1.4

1.2

1.0

0.8

δ / ppm

Figure 2. 1H NMR spectra (500 MHz, CD3CN, 0.50 mM, 25 °C) of L1 (a), meso-DH1BB2–·(Na+)2 (b), (b) + [2.2.1] (3.0 equiv) (c), racDH1BNaB–·Na+ (d), and (d) + [2.2.1] (3.0 equiv) (e).

by measuring the absorption, CD, and 1H NMR spectroscopies along with determining the structures of the contracted racemo-helicates complexed with a series of cations by crystallographic analysis and/or density functional theory (DFT) calculations. The results will not only contribute to a fundamental understanding of the biological machinery systems,3,9 but also provide the rationale for the molecular design of artificial molecular springs with specific functions.1f,14

RESULTS AND DISCUSSION Meso- and Racemo-Helicates Formations. We previously reported that the spiroborate helicate with a double-stranded + helical structure (rac-DH1BNaB ·Na ) was produced as the major product in 1,2-dichloroethane/EtOH (6/1, v/v) accompanied by a white precipitate during the reaction of the tetraphenol L1 with NaBH4 (1 equiv) at 80 °C for 20 h.7a By careful investigations of the reaction conditions and the products, we found that a relatively large amount of an extended diva+ 2lent meso-helicate (meso-DH1BB ·(Na )2) was produced as a precipitate along with the racemo-helicate that was soluble in

the reaction medium (1,2-dichloroethane/EtOH (6/1, v/v)) when L1 was allowed to react with NaBH4 (1 equiv) at 50 °C for 28 h (Figure 1c). The meso-structure with a plane of symmetry was confirmed by the X-ray crystallographic analysis of the single crystals obtained after slow evaporation from a solution of the precipitate with tetraphenylphosphonium bromide (TPP+·Br–) (10 equiv) in CH3CN at room temperature (Figures 3d and S5) and further supported by high-resolution electronspray ionization (ESI) mass spectrometry (Figure S35a). The two tetraphenol strands are bridged by two spiroborates in a parallel way to each other with the B-B distance (dB-B) of 11.8 Å, which was similar to that of the extended divalent racemohelicate complexed with benzyltrimethylammonium (BTMA+) + 2(rac-DH1BB ·(BTMA )2) (13.0 Å) determined by X-ray crystallography (Figure 3e).7a 2-

As expected from the crystal structure of meso-DH1BB 2+ 1 + ·(TPP )2, the H NMR spectrum of meso-DH1BB ·(Na )2 was similar in pattern to that of the extended racemo-helicate rac2DH1BB ·(Na+Ì[2.2.1])2 (Figure 2b,e); some aromatic protons were slightly upfield in CD3CN as compared to those of the


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tBu

Page 4 of 15 tBu

tBu

tBu

(c) NaPF6 (5 equiv) CD3CN, 70 °C, 1 h

O O B O O

tBu

tBu

O O B O O

Na+

tBu –·X

X

tBu

rac-DH1BNaB X = Na+ or TBA+ tBu

tBu

tBu

tBu

tBu

(a) CD3CN/H2O,

tBu

tBu

tBu

70 ºC, 6.3 h O O B O O

O O B O O

OO B OO

2TBA+

(b) dry CH3CN, tBu

tBu

tBu

tBu

tBu

70 ºC, 2 h

meso-DH1BB2–·(TBA+)2 (d)

meso-DH1BB2–·(TPP+)2

OO B OO

tBu

tBu

2TBA+

tBu

rac-DH1BB2–·(TBA+)2 +17.5°

(e)

extended rac-DH1BB2–·(BTMA+)2 164° Twist angle

B

B

11.8 Å

B

B

–17.5°

B

B

13.0 Å

Figure 3. Interconversion between meso-DH1BB2–·(TBA+)2 and rac-DH1BB2–·(TBA+)2 in CD3CN containing ca. 1000 equiv H2O (ca. 13 µM) at 70 °C (a), which does not take place in the absence of H2O (b). (c) Na+-ion-templated quantitative conversion of meso-DH1BB2– ·(TBA+)2 into rac-DH1BNaB–·X (X = Na+ or TBA+) in the presence of ca. 340 equiv H2O (ca. 18 µM) and NaPF6 (5 equiv). (d,e) Cappedstick representations of the X-ray crystal structures of meso-DH1BB2–·(TPP+)2 (d) and rac-DH1BB2–·(BTMA+)27a (e) (side (left) and top (right) views). All the hydrogen atoms, solvent molecules, and countercations are omitted for clarity. For (e), a right-handed (P)-doublehelical structure is only depicted.

corresponding ligand L1, but were not significant when compared to that of the contracted racemo-helicate rac-DH1BNaB + ·Na (Figure 2d). In addition, characteristic cross-peaks between the strands (k and g/h protons in Figure 2) as anticipated from its crystal structure were observed in the twodimensional (2D) ROESY spectrum (Figure S29), indicating + 2that the meso-DH1BB ·(Na )2 retained its double-stranded meso-structure in solution (for complete signal assignments, see 2D COSY and ROESY spectra (Figures S25–S29). In con2trast, the extended rac-DH1BB ·(Na+Ì[2.2.1])2 showed no 7a ROE cross peak in CD3CN. Because of the extended rigid + 2meso-structure, meso-DH1BB ·(Na )2 cannot accommodate a + Na ion in the center. As a result, the spring-like extension– + contraction motions observed in the rac-DH1BNaB ·Na did not + proceed upon the Na ion release by the [2.2.1]cryptand and + further binding of a Na ion as supported by negligible chang+ 21 es in the H NMR spectrum of meso-DH1BB ·(Na )2 after the

addition of 3 equiv of the [2.2.1]cryptand in CD3CN (Figure 2b,c). Meso-to-Racemo Isomerization of the Spiroborate Helicate. To gain insight into the mechanism for the meso- and racemohelicates formations, the meso-to-racemo isomerization of the isolated meso-helicate after countercation exchange with n+ – 2tetrabutylammonium bromide (TBA ·Br ) (meso-DH1BB + ·(TBA )2) (see the Supporting Information (SI), Section 2) was investigated by 1H NMR in CD3CN in the absence and + presence of water and Na ions (Figure 3a-c and Section 3 of SI). The meso-to-racemo isomerization of the spiroborate helicate requires at least cleavages of one of the four covalent B– O bonds at the spiroborate groups followed by reformation of the spiroborate residues, which could be catalyzed by water. In 1 + 2fact, the H NMR spectrum of meso-DH1BB ·(TBA )2 almost remained unchanged in dehydrated CD3CN


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tBu

g

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h

tBu

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f

a b tBu

O O B O O

2TBA+

CD3CN 70 °C, 1 h

j

tBu

tBu

i

O O B O O

NaPF6 (5 equiv)

e d

tBu

h

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f i

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a b

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TBA+

e c

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d

j

k

O O B O O

Na+

tBu

tBu

tBu

k

contracted rac-DH1BNaB–·TBA+

meso-DH1BB2–·(TBA+)2 (a) meso-DH1BB2–·(TBA+)2

k

j

a dc f

e

i

b h

#

g

#

#

k

(b) after heating at 70 ºC for 1 h in the presence of NaPF6 e

ghf

d

c

i

a b

(c) after heating at 70 ºC for 6.3 h in the absence of NaPF6

k

a dc f

* **

e b h

i

* **

g

*

j

*

*

j

*

≈

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8.0

7.5

7.0

6.5

6.0

5.5

5.0

1.6

1.4

1.2

1.0

0.8

δ / ppm Figure 4. 1H NMR spectra (500 MHz, CD3CN, rt) of meso-DH1BB2–·(TBA+)2 (18 µM, CD3CN containing ca. 340 equiv of H2O) in the presence of 5 equiv of NaPF6 before (a) and after (b) heating at 70 °C for 1 h, affording rac-DH1BNaB–·TBA+ quantitatively. (c) 1H NMR spectrum (ca. 13 µM, CD3CN containing ca. 1000 equiv of H2O) of meso-DH1BB2–·(TBA+)2 in the absence of NaPF6 after heating at 70 °C for 6.3 h, reaching an equilibrium (meso : racemo =71 : 29) (see Figure S3). # and * denote the protons from TBA+ and rac-DH1BB2– ·(TBA+)2, respectively.

after heating at 70 °C for 2 h (Figures 3b and S1). However, the meso-helicate was completely converted to the contracted + rac-DH1BNaB ·(TBA ) within 1 h upon heating at 70 °C in 2aqueous CD3CN (ca. 340 equiv of H2O to meso-DH1BB + ·(TBA )2) in the presence of 5 equiv of NaPF6 (Figures 3c and 4a,b).15 The formation of the racemic double-stranded helical + + structure of rac-DH1BNaB ·(TBA ), in which a Na ion was embraced within the helicate at the middle, was confirmed by 1 the characteristic proton resonances of its H NMR spectrum due to the contracted spiroborate helicate as previously report+ ed.7a The contracted rac-DH1BNaB ·(TBA ) was also stable in dehydrated CD3CN and maintained its helical structure after heating at 80 °C for 24 h.7a These results clearly indicated that the meso- and racemo-helicates are kinetically inert toward isomerization in the absence of water, but the meso-helicate readily converts to the contracted racemo-helicate (rac+ + DH1BNaB ·(TBA )) in the presence of Na ions in aqueous solvents through a water-mediated B-O bond cleav+ age/reformation of the spiroborate groups, in which the Na 16 ions serve as a template (Figure 3c). The pseudo-first-order rate constants (krac, s-1) for the meso2to-racemo isomerization of the meso-helicate (DH1BB + ·(TBA )2) catalyzed by water in the presence of 5 equiv of 1 NaPF6 were then estimated by following the H NMR spectral

changes of the meso-helicate at different temperatures in CD3CN, providing the activation parameters by the Eyring and Arrhenius plots of the kinetic data (Figure S2): Ea = 99.8 kJ mol–1, DG‡298 = 103 kJ mol–1, DH‡ = 98 kJ mol–1, and DS‡ = – + 15 J mol–1 (T = 298 K). The important role of the Na ion template in the contracted racemo-helicate formation was supported by the fact that the meso-helicate very slowly converted + to the racemo-helicate in the absence of Na ions in aqueous CD3CN at 70 °C and finally reached an equilibrium after ca. 6 h, yielding a 71:29 mixture of the meso- and racemo-helicates (Figures 3a, 4c, and S3). Binding Affinity of Extended Racemo-Helicate towards Various Monovalent Cations to Form Contracted Racemo2Helicate. The extended divalent racemo-helicate rac-DH1BB + + ·(TBA )2 binds a Na ion in the center of the helicate to form + the contracted helicate rac-DH1BNaB ·(TBA ), which is accompanied by a unidirectional twisting motion, and the association constant (Ka) was estimated to be 2.68 × 106 M–1 in CH3CN at 25 °C by the competitive binding titration experi+ 2ment between rac-DH1BB ·(TBA )2 and dicylohexano-181 crown-6 ether (DC18C6) using H NMR spectroscopy.7a We then investigated the binding affinity of the extended rac+ 2DH1BB ·(TBA )2 toward a series of monovalent cations



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Table 1. Association Constants (Ka), Exchange Rates (kex), Free Energies of Activation (DG‡), and Coalescence Temperatures (Tc) of Extended rac-DH1BB2– with Li+, Na+, K+, Rb+, Cs+, Ag+, and NH4+ helicate

cation

Ka (M-1) a

kex (s-1) b

DG‡ (kJ mol-1) c

Tc (K) c

extended helicate rac-DH1BB2–

Li+

3.65 ´ 106 d

0.278 (298 K)

76.2

408

Na+

2.68 ´ 106 e

1.66 (298 K)

71.7

384

K+

1.79 ´ 105

36.1 (268 K)

57.4

306

51.0

271

49.3

262

48.6

264

57.4

304

[537 (298 K)] Rb+

1.10 ´ 104

c

32.5 (238 K) 3

[7.26 ´ 10 (298 K)] Cs+

3.82 ´ 104

76.4 (238 K) 4

[1.44 ´ 10 (298 K)] Ag+

1.83 ´ 105

c

109 (238 K) 4

[1.91 ´ 10 (298 K)] 1.35 ´ 106

NH4+

c

12.9 (258 K) [541 (298 K)]

c

c

a

Estimated by UV titration. bEstimated by 2D EXSY. cEstimated from the following equations (1) kex = t298-1 = (kBT/h)exp(–DG‡/RT) and (2) DG‡ = RTcln[21/2kBTc/ph(Du)], where Du is difference in the chemical shifts (Hz). For more details, see Section 11 of SI. dEstimated by 1 H NMR of rac-DH1BNaB–·BTMA+ in the presence of 0.5 equiv of LiPF6 (Figure S11). eTaken from reference 7a. +

+

+

+

+

+

including alkali metals (Li , K , Rb , and Cs ), Ag , and NH4 ions by estimating their Ka values at 25 °C in CH3CN based on + UV titrations (Figure S10) except for Li whose Ka value was obtained by a competitive binding experiment in CD3CN using 1 + 2H NMR based on the Ka value of rac-DH1BB ·(TBA )2 to+ ward a Na ion (Figure S11), and the results are summarized in Table 1.17 The results revealed that the association constants + + + + decreased in the following order: Li > Na > NH4 > Ag ≥ + + + K > Cs > Rb , which roughly agrees with the reverse order of their ionic radii except for NH4+ (Figure 5a) probably be+ cause an NH4 cation encapsulated within the helicate at the middle was stabilized by the negatively charged boronates as well as by hydrogen bonding interactions with the boronates as reported for DC18C6.18,19 A comparison of the observed Ka values with those for the macrocyclic polyethers revealed that + 2the binding affinities of the extended rac-DH1BB ·(TBA )2 toward cations are higher than those with 15-crown-5 ether (15C5) and comparable to those with 18-crown-6 ether (18C6) and DC18C6.20b,21 Ion-Triggered Extension and Contraction Motions of Rac+ emo-Helicate. The Na ion-triggered extension–contraction motions observed for the racemo-helicate rac-DH1 are a quite unique since such a spring-like motion is coupled with the twisting motion of the helicate in one direction that reversibly + takes place upon the binding and release of a Na ion.7a We anticipated that the ion-triggered spring-like motion of racDH1 would depend on the ion species encapsulated within the helicate due to a structural change in the contracted helicate complexed with cations of different sizes, which can be readi1 ly followed and quantified by H NMR, absorption, and CD

spectroscopies because the racemic helicate can be easily separated into enantiomers by diastereomeric salt formation using an optically-active ammonium salt, followed by cation ex+ change with an achiral TBA salt (see Section 4 of SI and be7a low). +

Upon the addition of 0.5 equiv of alkali metals, Ag , and + + + + + + + NH4 salts (MPF6 (M = Li , Na , K , Rb , Ag , and NH4 ) and + 222 CsBPh4) to a solution of the extended rac-DH1BB ·(TBA )2 1 helicate in CD3CN, its H NMR spectrum significantly changed depending on the ion species; a new set of signals 1 attributed to the contracted helicate appeared in the H NMR + + spectra with Li and Na ions (Figure S25a-c), while the signals became considerably broadened in the presence of 0.5 equiv of the other ions (Figures S24 and S25d-h), indicating + + + + + + + slow (Li and Na ) and fast (K , Rb , Cs , Ag , and NH4 ) exchange between the extended and contracted forms on the present NMR time scale. The further addition of a large amount of ions (6.0 or 20 equiv) quantitatively produced the contracted helicates (Figure 6), in which each cation was embraced within the helicate as confirmed by ESI mass spectrometry (Figure S35b-h) and further revealed by the singlecrystal X-ray (Figure 7) and DFT calculated structures (Figure S9) of the contracted helicates complexed with those series of cations (see below). 2-

Upon the binding of the cations, the extended rac-DH1BB + ·(TBA )2 helicate contracted accompanied by significant upfield shifts of the terminal tBu signals (j) and the aromatic protons (a–c) on the terminal benzene rings due to the ring current effect of the aromatic rings of the other strand (Figure 6). The magnitude of the observed upfield shifts of


Page 7 of 15

Journal of the American Chemical Society +

Li+

5

J

NH4+

Na+

J J

J

Ag+

K+

4

Cs+ J J

Rb+

3 2

40

Cs+ Rb+ J J J NH4+ G

30 20

Li+ J

1.2

Na+

J: Peak a J: Peak j

+ J Ag J

⎮Average dihedral angle⎮ (°)

1.6

(e) K+

0 55

Rb+

0.8

J G

J

0.4

J

Cs+

J J J JJ

0 0.6

1.0 1.2 Ionic radius (Å)

1.8

0 X-ray

•

DFT J J

J

45

J

J

J

E

J E E

40 8

◦

X-ray

• DFT

J J

E E

J E J

EJJJ E J J

J E

◦

4 0.6

300 J

J

6 5

J

◦

0.3

Na+

50

7

JJ

NH4+

0.6

J G

10

J

dBB (Å)

(b)

(d)

0.9

Ag+ G J

1.8

1.2

J G

Li+


1.5

K+

1 0 0.6

G

G

1.8

ε325 × 10–4 (M–1 cm–1)

Log Ka (M–1)

6

(c) J

X-ray

J J

• DFT


290 280 270 260

Twist angle (°)

7

Δε325 (M–1 cm–1)

(a)

Δδ (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

250 1.8

Figure 5. (a) Association constants (Ka) of extended rac-DH1BB2– with M+ (alkali metals, Ag+, and NH4+ ions) plotted versus the ionic radius of M+. The ionic radii of Li+, Na+, K+, Rb+, Cs+, Ag+, and NH4+ are 0.76, 1.02, 1.38, 1.52, 1.67, 1.15,20a,b and 1.48 Å,20c respectively. (b) Plots of the differences in the chemical shifts (Dd) of a and j (see Figure 6) between extended rac-DH1BB2– ·(TBA+)2 and contracted rac-DH1BMB– formed after binding with M+ versus the ionic radius of M+. (c) The relationships between the CD (blue circles) and absorption (red open squares) intensities at 325 nm of (P)-(+)-DH1BB2–·(TBA+)2 in the presence of M+ (Figure 9a,b) plotted versus the ionic radius of M+. (d,e) Plots of the average dihedral angles between the benzene rings (absolute values) (red circles) (d), the distance between the two boron atoms (dBB, orange circles) (e), and the twist angles of the two terminal benzene rings of each strand (magenta circles) (e) of the energyminimized structures of contracted (M)-DH1BMB– complexed with M+ (alkali metals, Ag+, and NH4+ ions) obtained by the DFT calculations (Figure S9) and those (open circles) obtained from the crystal structures of DH1BMB– (M = Li+, Na+, and K+) (Figure 7) versus the ionic radius of M+. The dotted and solid lines are drawn to guide the eyes.

these protons, namely the differences in the chemical shifts (Dd) of the tBu (j) and aromatic (a) protons between the extended and contracted forms tends to decrease with an increase in the ionic radius of the cations entrapped within the contracted helicate (Figures 5b and 6), which most likely reflects a crucial difference in the structures of the contracted helicates. That is, the contracted helicates embracing the larger cations adopted more elongated structures by an induced-fit mechanism, which is in good agreement with their crystal and DFT calculated structures of the contracted helicates complexed with a series of cations (Figures 7 and S9); the distance between the two boron atoms of the contracted helicates (dBB) increased with the increasing size of the cations from 5.2 Å

+

(Li ) to 7.1 Å (Cs ) (Figure 5e and Table 2). In accordance with these changes in the B–B distance, a twisting angle between the terminal benzene rings of each strand also changed + + + + + + from 284–286° (Li , Na , Ag , NH4 , K ) to 267-268° (Rb + 23 and Cs ) (Figure 5e and Table 2), suggesting that the extended spiroborate helicate was not fully contracted upon binding of the larger cations. In other words, the extended helicate more tightly twists to contract when complexed with the + + + smaller cations, such as Li , Na , and Ag ions. The weakly + + + + bound central ions, such as Li , Na , K , and Ag ions, could be successfully removed from the corresponding contracted + + + helicates by cryptand [2.2.1] (Li , Na ,7a and Ag ) or [2.2.2] + (K ), resulting in the quantitative regeneration of the extended 1 2rac-DH1BB helicate, as evidenced by their H NMR spectral changes (Figures S12–S14), leading to reversible spring-like motions. Ion-Triggered Unidirectional Spring-Like Motions of One+ Handed Helicate. The racemic helicate rac-DH1BNaB ·(TBA ) was readily resolved into the enantiomers by diastereomeric salt formation using optically pure (–)-N-dodecyl-N+ methylephedrinium bromide ((–)-DMEph ·Br–), followed by + removal of the central Na ion using cryptand [2.2.1] and cati+ – on exchange with the achiral TBA ·Br ,7a yielding the optical2ly pure double-stranded extended helicate ((+)-DH1BB + ·(TBA )2) with the enantiomeric excess (ee) of >99% as de1 termined by H NMR in the presence of Nbenzylcinchonidinum bromide (3+·Br–) as a chiral shift reagent (Figure S4 and the prefix (+) denotes the sign of the Cotton effect at 244 nm). Fortunately, we obtained the single crystals of (–)-DH1BNaB complexed with the optically pure (5aS,10bR)-2-(2,3,4,5,6-pentafluorophenyl)-6,10b-dihydro4H,5aH-indeno[2,1-b][1,2,4]triazolo[4,3-d][1,4]oxazin-2-ium + ((5aS,10bR)-TO ) suitable for X-ray analysis, which allowed us to unambiguously determine the helix-sense of the contracted (–)-DH1BNaB helicate based on the absolute configura+ tion of the countercation ((5aS,10bR)-TO ) (Figures 8b and + S6). (–)-DH1BNaB ·(TO ) adopts a left-handed (M)-doublehelical structure with a B–B distance of 5.9 Å and the twist angle between the two terminal benzene rings of each strand of 284° (Figure 8b), the structure of which is nearly identical + to that of the contracted DH1BNaB ·BTMA (Figure 7b). The + obtained single crystal of (M)-(–)-DH1BNaB ·(TO ) dissolved in CH3CN exhibited the Cotton effects that are almost mirror + images of those of the contracted (+)-DH1BNaB ·TBA (Figure 8a). Therefore, the absolute handedness of the spiroborate + helicate (+)-DH1BNaB ·TBA derived from the extended (+)+ 2DH1BB ·(TBA )2 can be determined to be a right-handed (P)double-helical structure. -

The absolute handedness of the extended (P)-DH1BB2 helicate was further confirmed by the direct comparison of the experimental CD spectrum with the theoretically calculated one using the time-dependent density functional theory (TDDFT) method24 with the M06-2X functional25 (Figure S16a).26 Figure 9a,b displays the absorption and CD spectra of the + right-handed (P)-contracted helicates ((P)-DH1BMB ·TBA ) + complexed with a series of cations (M ) with different sizes in CH3CN, which are quite different from that of the (P)+ 2extended helicate DH1BB ·(TBA )2, and their absorption and CD spectral patterns were highly dependent on


Journal of the American Chemical Society tBu

tBu

tBu

tBu

tBu

tBu

tBu

g h

tBu

f

a

OO B OO

b

h ef

tBu

j

c d

OO B OO

i

M+

2TBA+

CD3CN

g

tBu

tBu

M+ M+

tBu

a

= Li, Na, K, Rb, Cs, = Ag, NH4

i

O O B O O e

tBu

c

d

tBu

j

k

f

dc

d

f, g, h d or e e

f

e

f

e

f

* *

7.0

j

k

Li+

a

i b

i

c

#

#

a

b

k

j

k

j

Na+

a

b

k

a

b a

j

k

j

k

j

b

d c

e g i *h d

7.5

c

id c h, g

h, gi

(g) (a) + RbPF6 (20 eq.)

8.0

d

g f h

(f) (a) + NH4PF6 (6.0 eq.)

i

d or e c i

(d) (a) + AgPF6 (6.0 eq.)

(h) (a) + CsBPh4 (6.0 eq.)

g

e

e

h (c) (a) + NaPF6 (6.0 eq.) g f

(e) (a) + KPF6 (6.0 eq.)

j

k a #

gh f

(b) (a) + LiPF6 (6.0 eq.)

tBu

contracted rac-DH1BMB–

i eb h

dc

tBu

k

extended rac-DH1BB2–·(TBA+)2 extended (a) rac-DH1BB2–·(TBA+)2

O O B O O

M+

b

a

b

6.5

6.0

Ag+

K+

Rb+

k

a

b

NH4+

j Cs+

5.5

5.0

≈

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 15

1.6

1.4

1.2

1.0

0.8

δ / ppm Figure 6. Partial 1H NMR spectra (500 MHz, CD3CN, 0.50 mM, rt) of (a) rac-DH1BB2–·(TBA+)2, (b) (a) + LiPF6 (6.0 equiv), (c) (a) + NaPF6 (6.0 equiv), (d) (a) + AgPF6 (6.0 equiv), (e) (a) + KPF6 (6.0 equiv), (f) (a) + NH4PF6 (6.0 equiv), (g) (a) + RbPF6 (20 equiv), and (h) (a) + CsBPh4 (6.0 equiv). # and * denote the protons from TBA+ and BPh4–, respectively.

Figure 7. Capped-stick representations of the X-ray crystal structures of contracted rac-DH1BLiB–·Na+Ì[2.2.1] (a), rac-DH1BNaB–·BTMA+ (b),7a and rac-DH1BKB–·BTMA+ (c) (side (top) and top (bottom) views). All the hydrogen atoms, solvent molecules, and countercations are omitted for clarity and (M)-double-helical structures are only shown. The two terminal benzene rings of each strand are twisted by 285°. Li+, Na+, and K+ ions are highlighted as colored balls.


Page 9 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Bond Distances (dBB) between Two Boron Atoms, Average Dihedral Angles, and Twist Angels of the Crystal Structures of Contracted rac-DH1BMB– (M = Li+, Na+, and K+), (M)-(–)-DH1BNaB–·(5aS,10bR)-TO+, Extended rac-DH1BB2–, and meso-DH1BB2– and Those of the DFT-calculated Structures of Contracted (M)-DH1BMB– (M = Li+, Na+, K+, Rb+, Cs+, Ag+, and NH4+) and CH3CN-coordinated (M)-DH1BMB– (M = Na+ and K+) helicate

method

dBB (Å)

½dihedral angle½a (°)

twist angle (°) b

contracted rac-DH1BLiB–·Na+Ì[2.2.1]

X-ray

5.2

42.3

285

contracted rac-DH1BNaB–·BTMA+·(CH3CN) c,d

X-ray

6.0 c

42.8

285

contracted rac-DH1BKB–·BTMA+·(CH3CN) d

X-ray

6.4

44.3

285

X-ray

5.9

42.2

284

contracted (M)-DH1BLiB– e

[DFT]

[5.2] f

[43.1] f

[287] f

contracted (M)-DH1BNaB– e

[DFT]

[5.8] f

[44.3] f

[286] f

contracted (M)-DH1BNaB–·(CH3CN) d,e

[DFT]

[6.0] f

[44.5] f

[282] f

contracted (M)-DH1BKB–·(CH3CN) d,e

[DFT]

[6.5] f

[46.6] f

[281] f

contracted (M)-DH1BKB– e

[DFT]

[6.3] f

[45.9] f

[269] f

contracted (M)-DH1BRbB– e

[DFT]

[6.7] f

[50.1] f

[267] f

contracted (M)-DH1BCsB– e

[DFT]

[7.1] f

[51.8] f

[268] f

contracted (M)-DH1BAgB– e

[DFT]

[6.1] f

[45.3] f

[286] f

contracted (M)-DH1BNH4B– e

[DFT]

[6.6] f

[46.7] f

[285] f

extended rac-DH1BB2–·(BTMA+)2 c

X-ray

13.0 c

103.0

164

meso-DH1BB2–·(TPP+)2

X-ray

11.8

79.9

±17.5

+

–

contracted (M)-(–)-DH1BNaB ·(5aS,10bR)-TO ·(CH3CN) (Molecule A)

d

a

Average absolute values. bTwist angles between the two terminal benzene rings (R1 and R6, Ra and Rf) of each strand (see Table S5). Taken from reference 7a. dAn CH3CN molecule coordinates to an entrapped alkali metal ion. eDFT calculations were carried out without a non-entrapped counter cation. fThe values in parentheses were obtained from the DFT-calculated structures. For more detailed structural data, see Tables S5 and S6. c

the cation species, i.e., their sizes (ionic radii), demonstrating the difference in their contracted helical structures as previous+ ly mentioned. The helicate complexed with the smallest Li + cation (P)-DH1BLiB ·TBA showed the most intense split-type CD in the wavelength region of ca. 235 – 295 nm accompanied by an apparent absorption maximum around 300 nm together with the positive first Cotton effect in the longer wavelength region (Figure 9a). This is most likely due to its tightly + twisted helical structure complexed with a Li ion as supported by the shortest B-B distance (5.2 Å) (Table 2). Upon complex+ + ation with the larger cations, such as the Na and Ag ions, their CD spectral patterns and intensities changed (Figure 9a), while further dramatic changes in their CD spectral patterns took place upon binding of even the larger cations, such as the + + + + K , Rb , Cs , and NH4 ions, accompanied by the appearance of a new positive CD band around 254 nm which was similar to that observed for the extended helicate (Figures 9b and S15), indicating more elongated helical structures when complexed with larger cations being in good agreement with the NMR results (Figure 6) as supported by the structures of the corresponding contracted helicates (Figures 7 and S9). The changes in the CD and absorption intensities at 325 nm of the contracted helicates complexed with a series of cations showed a clear tendency that both the CD and absorption in-

tensities increased with an increase in the ionic radius of the cations contained in the helicate cavity (Figure 5c), which also supported the elongated helical structures of the contracted helicates complexed with larger cations, resulting in a significant absorbance in the longer wavelength regions (Figures 5c and S15). In relation to the increase in the CD and absorption intensities at 325 nm, the average dihedral angles between the benzene rings (absolute values) of the contracted helicates as well as the B-B distances also tended to increase with the increasing ionic radius of the cations entrapped within the contracted helicate, whereas the twist angle of the helicates tended to decrease in reverse proportion to the B-B distances (Figure 5d,e and Table 2). Such a helical-pitch dependence of the CD spectral changes has been observed and theoretically revealed for a series of fully organic carbo[n]helicenes with a different number of fused benzene rings.28 We then calculated the CD spectra of the contracted (P)-DH1BMB helicates complexed with a series + + + + + of cations with different sizes (Li , Na , K , Rb , Ag , and + NH4 ) based on the TD-DFT method with the M06-2X/def2SVP functional (Figure S16d,e). The calculated CD spectral patterns and intensities of the contracted (P)-DH1BMB helicates were properly reproduced, especially for those com+ + plexed with the smaller cations, such as the K and Na ions



tBu

tBu

tBu

F O O B O O

F

O O B O O

Na+

F F

N N

N

F

O tBu

tBu

tBu

80 60 40 20 0 –20 –40 –60 –80

(P)-(+)-DH1BNaB–·TBA+

(M)-(–)-DH1BNaB–·(5aS,10bR)-TO+

tBu

(M)-(–)-DH1BNaB–·(5aS,10bR)-TO+

200

250 300 350 Wavelength (nm)

2.0 1.5 1.0 0.5 0 400

Norm. Abs.

tBu

Norm. CD

(a)

(b) B

284°

Na

5.9 Å

B Figure 8. (a) Normalized CD and absorption spectra (normalized at 310 nm) of the single crystal of the left-handed helical (M)-(–)DH1BNaB–·(5aS,10bR)-TO+ (CH3CN, 1.0-cm cell, rt) (blue lines) used for the X-ray diffraction measurements and (P)-(+)-DH1BNaB–·TBA+ (CH3CN, 1.0 mM, 0.01-cm cell, 25 ºC) (red lines) obtained from (P)-(+)-DH1BB2–·(TBA+)2 in the presence of NaPF6 (1.25 equiv). (b) Capped-stick representations of the X-ray crystal structure of (M)-(–)-DH1BNaB–·(5aS,10bR)-TO+ (side (top) and top (bottom) views). One of the crystallographically independent molecules (molecule A) (see Figure S6) is shown. All the hydrogen atoms, solvent molecules, and countercations except for the side view of (M)-(–)-DH1BNaB–·(5aS,10bR)-TO+ are omitted for clarity.

2.0 1.0 0

Li+ Ag+ Na+ M+

–1.0 –2.0

2.0

None

1.5 None Ag+ Na+ Li+

200


1.0 0.5 0 400

3.0 2.0 1.0 0

None Rb+ K+

Cs+ NH4+ M+

–1.0 –2.0

None None Cs+ NH4+ Rb+ K+

200


2.0 1.5 1.0 0.5 0 400

ε × 10–5 (M–1 cm–1)

(b) None

Δε × 10–2 (M–1 cm–1)

3.0

ε × 10–5 (M–1 cm–1)

(a) Δε × 10–2 (M–1 cm–1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 15

Figure 9. (a and b) CD and absorption spectra (CH3CN, 1.0 mM, 0.01-cm cell, 25 ºC) of extended (P)-(+)-DH1BB2–·(TBA+)2 in the absence and presence of LiPF6 (1.25 equiv), NaPF6 (1.25 equiv), and AgPF6 (1.90 equiv) (a) and those in the absence and presence of KPF6 (1.80 equiv), RbPF6 (12.5 equiv), CsBPh4 (4.50 equiv), and NH4PF6 (1.30 equiv) (b). For combined CD and absorption spectra, see Figure, S15. Under the conditions, the extended helicate was completely converted to the corresponding contracted helicates (> 99 mol%) based on their association constants (Table 1). (Figure S16d), while the theoretical prediction turned out to be not suitable for the larger cation complexes at the present level of calculations (Section 10 of SI), suggesting that the observed CD spectral changes of the contracted helicates upon complexation with various cations are most likely derived from the

structural changes (helix elongation) of the helicate as well as the electronic effect (interactions with the cations). Upon the further addition of a small excess of cryptand [2.1.1], [2.2.1], or [2.2.2] in CH3CN, the contracted helicates + ((P)-DH1BMB ·TBA ) complexed with metal ions


MPF6 B

B

B

M+

B

cryptand

N

3.0 Li+ 2.0 1.0 0 –1.0 –2.0

200

O

(i), (iii)

–1.0 –2.0

(i) (P)-(+)-DH1BB2–·(TBA+)2 (ii) (i) + NaPF6 (1.25 equiv) (iii) (ii) + [2.2.1] (1.5 equiv)

2.0 1.5 1.0

200


0.5 0 400

ε × 10–5 (M–1 cm–1)

N [2.2.1]

(ii)

N

(i), (iii) (i) (P)-(+)-DH1BB2–·(TBA+)2 (ii) (i) + LiPF6 (1.25 equiv) (iii) (ii) + [2.1.1] (1.5 equiv)

+ 2.0 K 1.0 0

–1.0 –2.0

O

O O

O O

N [2.2.2]

(ii) (i), (iii)

(i) (P)-(+)-DH1BB2–·(TBA+)2 (ii) (i) + KPF6 (1.8 equiv) (iii) (ii) + [2.2.2] (2.0 equiv)

2.0 1.5 1.0

200



(d)

N O

3.0

0.5 0 400

ε × 10–5 (M–1 cm–1)

O

Δε × 10–2 (M–1 cm–1)

O O

Δε × 10–2 (M–1 cm–1)

(c)

N O

3.0 Na+ 2.0 1.0 0

O O

[2.1.1]

contracted (P)-(+)-DH1BMB–·M+

extended (P)-(+)-DH1BB2–·(TBA+)2

(b)

(ii)

O O

2.0 1.5 1.0

0.5 0 400

ε × 10–5 (M–1 cm–1)

Δε × 10–2 (M–1 cm–1)

(a)

Δε × 10–2 (M–1 cm–1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N O

3.0

+ 2.0 Ag 1.0 0

–1.0 –2.0

O O

(ii)

O

O N [2.2.1]

(i), (iii) (i) (P)-(+)-DH1BB2–·(TBA+)2 (ii) (i) + AgPF6 (1.9 equiv) (iii) (ii) + [2.2.1] (2.1 equiv)

2.0 1.5 1.0

200


0.5 0 400

ε × 10–5 (M–1 cm–1)

Page 11 of 15

Figure 10. Unidirectional twisting of the helicate without racemization. (a-d) CD and absorption spectra (CH3CN, 1.0 mM, 0.01-cm cell, 25 ºC) of extended (P)-(+)-DH1BB2–·(TBA+)2 (i), (a) (i) + LiPF6 (1.25 equiv) (ii) and (ii) + [2.1.1] (1.5 equiv) (iii); (b) (i) + NaPF6 (1.25 equiv) (ii) and (ii) + [2.2.1] (1.5 equiv) (iii); (c) (i) + KPF6 (1.8 equiv) (ii) and (ii) + [2.2.2] (2.0 equiv) (iii); (d) (i) + AgPF6 (1.9 equiv) (ii) and (ii) + [2.2.1] (2.1 equiv) (iii). Chemical structures of cryptands [2.1.1], [2.2.1], and [2.2.2] are also shown. +

+

+

+

+

(M = Li , Na , K , or Ag ) readily and quantitatively reverted 2back to the original extended helicate (P)-DH1BB , while maintaining its CD and absorption spectra (Figure 10), thus indicating that neither the racemization nor breaking of the spiroborate groups took place during the ion-triggered extension–contraction motions of the helicate independent of the tested ion species, which proceeds reversibly coupled with the twisting in one direction.29 Kinetic Studies of Ion-Triggered Spring-Like Motions of Racemo-Helicate. Of particular interest is the kinetics of the extension and contraction motions of the present molecular spring. Previously, we attempted to estimate the rate constants of the extension (kext) and contraction (kcont) events of an optically-active contracted DH1BNaB helicate induced by cryptand 2[2.2.1] and the corresponding extended DH1BB helicate trig+ gered by Na ions, respectively, using a stopped-flow CD method in CH3CN at 22 °C.7a The extension event of DH1BNaB induced by cryptand [2.2.1] slowly took place within 5 sec to 3 completion, yielding its rate constant kext of (5.38 ± 0.05) × 10 -1 -1 M s . However, the contraction event was too fast to follow and its rate constant kcont could not be accurately estimated.7a We recently reported that analogous spiroborate-based helicates composed of 4,4′-linked 2,2′-bpy and its N,N′-dioxide units in the middle also showed a similar unidirectional spring-like motion upon the binding and release of protons.11 Although the binding affinities of these extended helicates + toward Na ions were very weak compared to that of the ex-

2-

tended helicate DH1BB because of an unfavorable synconformation of the bpy and its N,N′-dioxide residues for con+ tracted helicate formations that prevents a Na ion embraced in 11 the center of the contracted helicates. However, the kinetics (exchange rate constants (kex) between the extended and contracted helicates) of the proton-assisted spring-like motion of the bpy- and its N,N′-dioxide-bound helicates were able to be estimated by the 2D EXSY measurements30 of a 1:1 mixture of the extended and contracted helicates generated with 1 equiv of trifluoroacetic acid.11 We then applied the 2D EXSY technique to estimate the 2exchange rate constants (kex) between the extended (DH1BB ) and contracted (DH1BMB ) helicates triggered by a series of + cations (M ) in CD3CN at 25 °C (Section 11 of SI). However, as previously mentioned, upon the addition of 0.5 equiv of + + alkali metals, Ag , and NH4 ions to a solution of the extended + 2helicate rac-DH1BB ·(TBA )2 in CD3CN at 25 °C, only the + + helicates with 0.5 equiv of Li and Na ions appeared as two sets of clear signals due to the extended and contracted helicates because of the slow exchange between them (Figure S25b,c). We then measured the 2D EXSY spectra of a mixture 2of the extended and contracted helicates (DH1BB and + DH1BNaB ) generated with 0.5 equiv of Na ions acquired at different mixing times in CD3CN at 25 °C (Figure S18). Among a number of chemical exchange cross-peaks between the extended and contracted helicates, an isolated cross-peak between the a and A protons in Figure S18 was selected, and the peak volumes of the cross and diagonal peaks at different



5 J

+

425

Ag+

Cs+

Rb+ J

J

4

J

400

J

375

K+

3

J J NH4

+

350

2 325 1

J J

Na+ 0

300

J

Li+ 0.8

275

J J

J

–1 0.6

Tc (K)

Log kex (s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.0

1.2

Page 12 of 15

size, except for Ag (Figure 11). In other words, the extension and contraction rates of the double-stranded helicate can be controlled by cation species. The coalescence temperatures (Tc), which are associated with the spring-like motion rates of the helicate upon binding and release of cations, tended to + + + + + increase in the following order: Cs ≤ Ag < Rb < NH4 ≤ K + + < Na < Li , as anticipated from the kex and association constant (Ka) values, which is in agreement with the reverse order + of the cation size except for Ag (Figure 11). The reason for + the observed fastest binding and release of the Ag ion in the helicate while maintaining its high association constant (Ka) is + not clear at present, but specific solvation of the Ag ions by 21a,31 CH3CN may be plausible.

J

1.4

1.6

250 1.8

Ionic radius (Å) Figure 11. Plots of the logarithm of the exchange rates (kex, black circles) at 298 K and coalescence temperatures (Tc, red circles) versus the ionic radius of M+. The dotted and solid lines are drawn to guide the eyes.

mixing times were measured to estimate the apparent exchange rate constant (kex) between the extended and contracted -1 2helicates (DH1BB and DH1BNaB ) to be 1.66 s (Table 1). The corresponding kex value for the spring-like motion triggered by + -1 Li ions at 25 °C was also estimated to be 0.278 s (Figure + S17), which was smaller than that by Na ions as anticipated + from its Ka value higher than that with Na ions. 1 We then measured the H NMR spectra of the extended + 2helicate rac-DH1BB ·(TBA )2 in the presence of 0.5 equiv of + + + + + K , Rb , Cs , Ag , and NH4 ions in CD3CN at low temperatures to investigate the temperature-dependent extension– contraction motion of the helicate (Figure S24). Upon cooling, the broad signals due to fast exchange between the extended and contracted helicates became further broadened or gradually sharpened as the temperature was lowered and then finally split into two sets of apparent signals ascribed to the extended and contracted forms through a coalescence temperature (Tc). The 2D EXSY measurements were then performed to estimate the kex values for the spring-like motions of the helicate trig+ + + + + gered by K , Rb , Cs , Ag , and NH4 ions at specific tempera+ + + tures (Table 1); –5 °C for the K ion, –35 °C for the Rb , Cs , + + and Ag ions, and –15 °C for the NH4 ion, since at these low temperatures, the chemical exchange peaks between the extended and contracted helicates (a and A protons) appeared as isolated cross-peaks (Figures S19–S23). Based on the kex values obtained at specific temperatures, the free energies of activation (ΔG⧧) for the extended and contracted events of the helicate were estimated, then the kex values at 298 K along with the coalescence temperatures (Tc) could be also roughly estimated using the chemical shift differences between the signals (Δν) at low temperatures (Figure S24) (see Section 12 of SI) and the results are summarized in Table 1. These results suggest that the ion-assisted extension–contraction motions of the helicate were significantly dependent on the ion species, namely, the size of the cations and the exchange rate (kex) between the extended and contracted helicates tended to increase + + + + + + in the following order: Li < Na < K ≤ NH4 < Rb < Cs < + Ag , which is in good agreement with the order of the cation

CONCLUSION We disclosed in this study that the ion-assisted extension– contraction motions of the spiroborate-helicate have a remarkable dependence on the ion species, namely the size of the cations. X-ray crystallography together with DFT calculations and 1D and 2D NMR and CD spectroscopies unraveled the structural and mechanistic details, thermodynamics, and kinetics of the spring-like motions of the helicate; the binding affinity of the extended helicate toward a series of monovalent + + + cations decreased in the following order: Li > Na > NH4 > + + + + Ag ≥ K > Cs > Rb , while the exchange rate (kex) between the extended and contracted helicates increased in the follow+ + + + + + + ing order: Li < Na < K ≤ NH4 < Rb < Cs < Ag . The observed tendency can be mostly rationalized in terms of the cation sizes. Namely, the extended helicate more tightly binds + + + smaller cations, such as Li , Na , and Ag ions, to form a compact contracted helicate, whereas the helicate adopts a more elongated contracted structure upon binding with the larger cations, resulting in a lower binding affinity thereby showing a faster extension–contraction motion. Importantly, the present ion-triggered extension–contraction motions of the spiroborate-helicate are fully reversible and controllable coupled with a unidirectional twisting motion of the double helix without racemization. In addition, the speed of the unidirectional elastic motion and its twist-sense can be regulated by cation species and controlled by the helical handedness of the helicate, respectively. The present systematic studies provide a clue for developing novel chiral molecular springs with specific functions, such as supramolecular asymmetric catalysis14,32 and chiral sensing33 associated with the ion binding and release events by introducing functional units in the middle and/or at both ends, whose catalytic activity and enantioselectivity will be modulated and switched by the unidirectional twisting motion in one direction. Work along this line is currently ongoing in our laboratory.

ASSOCIATED CONTENT Supporting Information Full experimental details and additional spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Present Addresses


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§ Department of Chemistry, Graduate School of Natural Science and Technology, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan ∥Department of Chemistry, Shiga University of Medical Science, Seta, Otsu 520-2192, Japan

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Riho Ikeda (Nagoya University) for her help in the experimental assistance. This work was supported in part by JSPS KAKENHI (Grant-in-Aid for Scientific Research (S), no. 25220804 (E.Y.) and Grant-in-Aid for Young Scientists (B), no. 17K14470 (N.O.).

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(21) (a) Buschmann, H.-J. J. Solution Chem. 1988, 17, 277-286. (b) Gokel, G. W.; Leevy, W. M.; Weber, M. E. Chem. Rev. 2004, 104, 2723-2750. (22) We used CsBPh4 instead of CsPF6 because CsPF6 is hardly soluble in CH3CN. (23) The twisting angle of the crystal structure of the contracted racDH1BKB- with an CH3CN molecule coordinating to the entrapped K+ ion (285°) was different from that of the DFT calculated structure without such a coordinated CH3CN molecule (269°), but well matched the DFT calculated structure with one CH3CN molecule (281°) (Figures 7c and S9c,i and Table 2), although only a small difference in the twisting angles of the crystal and DFT calculated structures of rac-DH1BNaB- was observed irrespective of the CH3CN molecule (Figures 7b and S9b,h and Table 2). (24) TD-DFT has been successfully applied as the cost-efficient chiroptical property prediction method for medium to large-sized molecules. See: (a) Warnke, I.; Furche, F. WIREs Comput. Mol. Sci. 2012, 2, 150–166. (b) Autschbach, J. Chirality 2009, 21, E116–E152. (c) Laurent, A. D.; Jacquemin, D. Int. J. Quantum Chem. 2013, 113, 2019–2039. (d) Adamo, C.; Jacquemin, D. Chem. Soc. Rev. 2013, 42, 845–856. (25) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215–241. (26) For further discussion and details about the computation, such as the functional dependence and basis-set convergence, see Section 10 of SI. We note that the more sophisticated ansatz, such as the coupled-cluster calculation,27 was not applied in this study, because the method may not be practically feasible for the contracted helicates complexed with cations including alkali metals at this moment (see below).

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(27) (a) Christiansen, O.; Koch, H., Jørgensen, P. Chem. Phys. Lett. 1995, 243, 409–418. (b) Hättig, C.; Köhn, A. J. Chem. Phys. 2002, 117, 6939–6951. (28) Nakai, Y.; Mori, T.; Inoue, Y. J. Phys. Chem. A 2012, 116, 7372–7385. (29) The ion-triggered extension–contraction motions of the helicate by Rb+ and Cs+ ions could not be attempted because suitable hosts which can strongly bind them are not available. (30) (a) Perrin, C. L.; Dwyer, T. J. Chem. Rev. 1990, 90, 935-967. (b) Miyawaki, A.; Kuad, P.; Takashima, Y.; Yamaguchi, H.; Harada, A. J. Am. Chem. Soc. 2008, 130, 17062-17069. (31) Burchard, T.; Cox, B. G.; Firman, P.; Schneider, H. Ber. Bunsenges. Phys. Chem. 1994, 98, 1526-1533. (32) (a) Wiester, M. J.; Ulmann, P. A.; Mirkin, C. A. Angew. Chem., Int. Ed. 2011, 50, 114-137. (b) Raynal, M.; Ballester, P.; Vidal-Ferran, A.; van Leeuwen, P. W. N. M. Chem. Soc. Rev. 2014, 43, 1660-1733. (c) Raynal, M.; Ballester, P.; Vidal-Ferran, A.; van Leeuwen, P. W. N. M. Chem. Soc. Rev. 2014, 43, 1734-1787. (d) Blanco, V.; Leigh, D. A.; Marcos, V. Chem. Soc. Rev. 2015, 44, 5341-5370. (33) (a) Hembury, G. A., Borovkov, V. V., and Inoue, Y. Chem. Rev. 2008, 108, 1-73. (b) Berova, N.; Polavarapu, P. L.; Nakanishi, K.; Woody, R. W. Comprehensive Chiroptical Spectroscopy.; John Wiley & Sons: New York, 2012; Vols. 1 and 2. (c) You, L.; Zha, D.; Anslyn, E. V. Chem. Rev. 2015, 115, 7840-7892.


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Journal of the American Chemical Society Table of Contents Artwork


Spiroborate-Based Double-Stranded Helicates: Meso-to-Racemo

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