Acid-induced formation of hydrogen-bonded double helix based on chiral polyphenyl-bridged bis(2,2′-bipyridine) ligands

Article abstract of DOI:10.1039/c4ra00849a

A series of chiral polyphenyl-bridged bis(2,2′-bipyridine) ligands comprising one to four phenyl units were synthesized. The ligands give a weak signal in the CD spectra, but upon addition of tetrachloroferric acid or perchloric acid, a more intense CD si

Full text of DOI:10.1039/c4ra00849a

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Acid-induced formation of hydrogen-bonded
double helix based on chiral polyphenyl-bridged
Cite this: RSC Adv., 2014, 4, 14513
bis(2,20-bipyridine) ligands†
Kiu-Chor Sham, Chi-Chung Yee, Yi Pan, Kai-Chung Lau, Shek-Man Yiu
*
and Hoi-Lun Kwong
A series of chiral polyphenyl-bridged bis(2,20-bipyridine) ligands comprising one to four phenyl units were
synthesized. The ligands give a weak signal in the CD spectra, but upon addition of tetrachloroferric acid or
perchloric acid, a more intense CD signal is observed for ligands having two or more phenyl units. Titration
experiments show that the CD signal comes from a monoprotonated species which gives broadened and
upfield shifted 1H NMR signals. Variable temperature NMR experiments split the broadened signals into two
sets of signals when the temperature is decreased. One of the sets is remarkably upfield while the other has
chemical shifts similar to those of the free ligand. The X-ray crystal structures of a free ligand (mono phenyl),
a monoprotonated ligand (biphenyl) and a biprotonated ligand (tetraphenyl) were obtained and the
structure of the monoprotonated ligand shows that it is a double-stranded helix, which is stabilized by
interior hydrogen bonding between the pyridinium N–H and the pyridine N of another ligand strand, and
Received 29th January 2014
Accepted 10th March 2014
ꢀ
exterior CH/Cl hydrogen bonding between FeCl4 and the two ligand strands. Theoretical DFT
DOI: 10.1039/c4ra00849a
calculations show that there is such stabilization in solution as well. With the perchlorate anion, the helix
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formation process is reversible with Et3N which accompanies an on/off CD signal change.
bonds, respectively, to both ligand strands to stabilize the
double helices.
Introduction
Proton, the smallest cation, can interact with the lone pair
electron of a ligand strand and lead to hydrogen bonds. Huc
et al.33 and Aida et al.34 have both reported the use of proton
to induced formation of single-stranded helices, however, to
the best of our knowledge, double helix formation induced by
proton is not known. We have previously reported the
synthesis of Mn double-stranded helicates with mono- to tri-
phenyl-bridged bis(2,20-bipyridine) L1–3 ligands.35 Herein,
together with a tetraphenyl-bridged bis(2,20-bipyridine) L4,
we report the synthesis of this series of ligands. Upon
protonation, ligands L2–4 give intense CD signal in the
presence of FeCl4 or ClO4 anion. CD and NMR titration
experiments suggest that a monoprontonated ligand species
is responsible for the intense CD signals and X-ray crystal
structures of monoprotonated L2 shows that a double-
stranded helix is responsible for the signal.
The double helix is ubiquitous in biological molecules and its
sophisticated structure is oen closely related to its biological
function.1 Inspired by nature, chemists have great interest to
prepare articial molecules with double helical structure.2–8
Self-assembly is the most commonly used approach in which
non-covalent interactions, such as electrostatic,9,10 metal–
ligand,7,8,11–16 p–p stacking17–19 and hydrogen bonding,20–24 drive
the two strands to intertwine in the formation process. These
interactions of the two strands can come from motifs that are
incorporated into the strands during synthesis20 or from a third
component, like a metal ion, which induces the interactions.7,8
Important to both biological and chemical process,25,26
anions can form hydrogen-bond and this hydrogen-bonding
interaction has been shown to play curial role in the forma-
tion of many supramolecular system27,28 which include
double helices.29–32 In the double helix examples, chloride,
uoride or sulfate, are locate at the helical axis, and form
multiple strong N–H/Cl, N–H/F and N–H/O hydrogen
ꢀ
ꢀ
Experimental
Chemicals and starting materials
Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue,
Kowloon, Hong Kong SAR, China. E-mail: bhhoik@cityu.edu.hk; Fax: +852 3442 0522;
Tel: +852 3442 7304
Solvents used for synthesis were of analytical grade. All
starting chemicals were of reagent-grade quality and were
obtained commercially and used as received without further
purication. Synthesis of chiral bromobipyridine 1 was
reported previously.36
† Electronic supplementary information (ESI) available. CCDC 983706–983708.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ra00849a
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Physical measurements and instrumentation
Procedure for synthesis of 2 and 3
1H, COSY and NOESY NMR spectra were recorded on Bruker 400 A solution of 1 (1.45 mmol) and tetrakis(triphenylphospine)
MHz instrument. The 1H and chemical shi was referred to palladium(0) (0.06 mmol) in degased toluene (6 ml) was treated
TMS as reference. Electrospray (ESI) mass spectra were with a solution of sodium carbonate (2.0 mmol) in H2O (3 ml). A
measured by a PE SCIEX API 150 EX system. CD spectra were methanolic solution (3 ml) of 3-chlorophenylboronic acid
recorded on a Biokin MOS-450 instrument with a 1 mm cell.
(1.45 mmol) or 3-bromophenylboronic acid (1.19 mmol) was
added. The mixture was stirred at 80 ꢁC for 3 h under nitrogen.
Aer cooling to room temperature, aqueous NH3 (30 ml) was
added and the mixture was extracted by CH2Cl2. The combined
organic layers were dried by MgSO4. Solvent was removed under
reduced pressure. The crude product was puried by column
chromatography (petroleum ether : ethyl acetate ¼ 10 : 1).
Products were isolated as white solid in 95% and 83% yield for 2
and 3, respectively. 1H NMR of 2 (300 MHz, CDCl3) d 8.94 (d, J ¼
7.8 Hz, 1H), 8.60 (s, 1H), 8.49 (s, 1H), 8.05 (m, 2H), 7.95 (m, 1H),
7.85 (d, J ¼ 8 Hz, 1H), 7.42 (m, 2H), 3.29 (d, J ¼ 2.4 Hz, 2H), 3.02
(t, J ¼ 5.4 Hz, 1H), 2.8 (m, 1H), 2.41 (s, 1H), 1.43 (s, 3H), 1.23 (d, J
¼ 10.3 Hz, 1H), 0.64 (s, 3H). 1H NMR of 3 (300 MHz, CD2Cl2): d
8.37 (s, 1H), 8.35 (d, 1H, J ¼ 8.1 Hz), 8.31 (t, 1H, J ¼ 1.8 Hz), 8.22
(s, 1H), 8.02 (d, 1H, J ¼ 7.8 Hz), 7.85 (t, 1H, J ¼ 7.8 Hz), 7.67 (d,
1H, 6.9 Hz), 7.55 (d, 1H, J ¼ 7.8 Hz), 7.36 (t, 1H, J ¼ 7.8 Hz), 3.10
(d, 1H, J ¼ 2.7 Hz), 2.88 (t, 1H, J ¼ 5.4 Hz), 2.70 (m, 1H), 2.33 (m,
1H), 1.42 (s, 3H), 1.21 (d, 1H, J ¼ 6.9 Hz), 0.67 (s, 3H).
Crystal structure determination
For crystal structure of L1, data was collected at 293 K with an
Oxford Diffraction Gemini S Ultra X-ray single crystal diffrac-
tometer using graphite monochromatized Mo-Ka radiation
˚
(l ¼ 0.71073 A). For structure of [(L2)2H2](ClO4)2 and [(L4)
H2(Cl)](FeCl4), data were collected at 133 K with an Oxford
Diffraction Gemini S Ultra X-ray single crystal diffractometer
using graphite monochromatized Cu-Ka radiation (l ¼ 1.54178
˚
A). All collected frames were processed with the soware SAINT,
and absorption correction was applied (SAD-ABS) to the
collected reections. The structure of the complex was solved by
direct methods (SHELXTL) in conjunction with standard
difference Fourier syntheses. All non-hydrogen atoms were
assigned with anisotropic displacement parameters. The
hydrogen atoms were generated in their idealized positions and
allowed to ride on the respective carbon atoms. Crystal data of
L1. C80H76N8O, M ¼ 1165.49, orthorhombic, a ¼ 10.0224(4),
3
Procedure for synthesis of 4
˚
˚
b ¼ 13.5194(6), c ¼ 24.219(1) A, U ¼ 3281.6(2) A , space group
P212121, Z ¼ 4, 10071 reections measured, 5493 unique A solution of 3 (11.6 mmol) and tetrakis(triphenylphospine)
(Rint ¼ 0.0228) which were used in all calculation. The nal palladium(0) (5 mol%) in degased toluene (36 ml) was treated
wR(F2) was 0.0804 (all data). Crystal data of [(L2)2H2](FeCl4)2. with a solution of sodium carbonate (20 mmol) in H2O
C
96H94Cl8Fe2N8O1.75, M ¼ 1783.09, monoclinic, a ¼ 30.7135(9), (16.5 ml). A methanolic solution (16.5 ml) of 3-chlor-
3
˚
˚
b ¼ 21.8661(6), c ¼ 15.6061(5) A, U ¼ 10290.2(5) A , space group ophenylboronic acid (11.6 mmol) was added. The mixture
ꢁ
C2,
Z
¼
4, 24178 reections measured, 14245 unique was stirred at 95 C for 24 h under nitrogen. Aer cooling to
(Rint ¼ 0.0346) which were used in all calculation. The nal room temperature, aqueous NH3 (50 ml) was added and the
wR(F2) was 0.1416 (all data). Crystal data of [(L2)2H2](FeCl4)2. mixture was extracted by CH2Cl2. The combined organic
C
96H94Cl8Fe2N8O1.75, M ¼ 1783.09, monoclinic, a ¼ 30.7135(9), layers were dried by MgSO4. Solvent was removed under
3
˚
˚
b ¼ 21.8661(6), c ¼ 15.6061(5) A, U ¼ 10290.2(5) A , space group reduced pressure. The crude product was puried by column
C2, Z ¼ 4, 24178 reections measured, 14245 unique (Rint
¼
chromatography (petroleum ether : ethyl acetate ¼ 10 : 1).
0.0346) which were used in all calculation. The nal wR(F2) was Products were isolated as white solid with 93% yield. 1H NMR
0.1416 (all data). Crystal data of [(L4)H2](Cl)(FeCl4). (400 MHz, CDCl3) d 8.56 (d, J ¼ 7.8 Hz, 1H), 8.49 (s, 1H), 8.33
C
59H56Cl5FeN4O, M ¼ 1070.18, monoclinic, a ¼ 11.4756(3), (s, 1H), 8.29 (t, J ¼ 1.5 Hz, 1H), 8.16 (d, J ¼ 7.3 Hz, 1H), 7.98 (t,
3
˚
˚
b ¼ 17.6487(5), c ¼ 26.0159(7) A, U ¼ 5182.0(2) A , space group J ¼ 7.8 Hz, 1H), 7.85 (d, J ¼ 7.2 Hz, 1H), 7.70 (t, J ¼ 1.7 Hz, 1H),
C1211, Z ¼ 4, 12966 reections measured, 9565 unique 7.66 (d, J ¼ 7.7 Hz, 1H), 7.63 (d, J ¼ 7.6 Hz, 1H), 7.59 (d, J ¼ 7.6
(Rint ¼ 0.0259) which were used in all calculation. The nal Hz, 1H), 7.43 (t, J ¼ 7.7 Hz, 1H), 7.38 (d, J ¼ 8.0 Hz, 1H), 3.18
wR(F2) was 0.099 (all data).
(d, J ¼ 2.5 Hz, 2H), 2.96 (t, J ¼ 5.4 Hz, 1H), 2.8 (m, 1H), 2.4 (m,
1H), 1.46 (s, 3H), 1.27 (t, J ¼ 8.2 Hz, 1H), 0.69 (s, 3H).
DFT calculations
Procedure for synthesis of L1 and L3
All calculations were done at M06-2X (hybrid meta exchange-
correlation functional with double the amount of nonlocal Degas toluene (16 ml), MeOH (6 ml) and water (6 ml) were added
exchange) functional37 using LanL2DZ basis set for Fe,38–40 to a ask containing Pd(PPh3)4 (0.17 g, 3 mol%), 1 or 3 (4.4
6-31G(d) basis sets for H, C, N, and 6-31 + G(d) basis sets for O, mmol), 1,3-phenyldiboronic acid (0.34 g, 2 mmol) and sodium
Cl atoms. The solvent effect is taken account by the Polarizable carbonate (0.92 g). The mixture was heated at 80 ꢁC for 24 h. It
Continuum Model.41,42 Atom-in-molecule (AIM) analysis is per- was cooled to room temperature. Ammonia solution in saturated
formed with AIM2000 program.43 The wavefunction was taken Na2CO3 solution was added and the solution was stirred for 5
from the optimized structure at the M06-2X level with using min. It was extracted by CH2Cl2. Solvent was dried under vacuo,
LanL2DZ basis set for Fe and 6-31G(d) basis set for all non- and the compound was puried by column chromatograph with
metal elements.
solvent (dichloromethane : diethyl ether ¼ 10 : 1). The crude
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yellow solution can be further puried by washing with MeCN. 1H, J ¼ 7.8 Hz), 7.64 (t, 1H, J ¼ 7.5 Hz), 3.05 (d, 2H, J ¼ 2.7 Hz),
Products were isolated as pale yellow solid with 57% and 30% 2.88 (t, 1H, J ¼ 5.4 Hz), 2.70 (m, 1H), 2.31 (m, 1H), 1.42 (s, 3H),
yield for L1 and L2, respectively. 1H NMR of L1 (400 MHz, 1.21 (d, 1H, J ¼ 6.9 Hz), 0.66 (s, 3H). 13C NMR (CDCl3): d 156.43,
CDCl3): d 8.81 (t, 1H, J ¼ 1.6 Hz), 8.85 (s, 2H), 8.38 (d, 2H, J ¼ 6.7 156.38, 154.63, 145.46, 145.39, 143.11, 140.13, 137.68, 129.18,
Hz), 8.27 (d, 2H, J ¼ 7.8 Hz), 8.25 (s, 2H), 7.93 (t, 2H, J ¼ 7.7 Hz), 127.73, 125.57, 120.70, 120.23, 119.28, 44.56, 40.12, 39.30,
7.87 (d, 2H, J ¼ 6.8 Hz), 7.69 (t, 1H, J ¼ 7.7 Hz) 3.11 (d, 4H, J ¼ 2.6 33.07, 31.85, 26.05, 21.44.
Hz), 2.90 (t, 2H, J ¼ 5.4 Hz), 2.73 (m, 2H), 2.34 (m, 2H), 1.44
(s, 6H), 1.27 (d, 2H, J ¼ 9.6 Hz), 0.69 (s, 6H). 13C NMR (CDCl3): d
Procedure for synthesis of L4
156.65, 156.61, 154.86, 145.68, 145.57, 143.30, 140.30, 140.36,
To a solution of NiCl2$6H2O (3.8 mmol) in degassed DMF
137.86, 129.38, 127.93, 125.80, 120.90, 120.42, 119.50, 44.82,
(17 ml) at 70 ꢁC under nitrogen, triphenylphosphine (16 mmol)
was added to give a blue solution. Zinc powder (6.4 mmol) was
then added and the resulting mixture was stirred for an hour,
in which dark-brown mixture was formed. Compound 4
(3.2 mmol) in degassed DMF (9 ml) was added slowly and the
mixture stirred at 70 ꢁC for 72 hours. The mixture was then
allowed to cool to room temperature and 25% aqueous NH3
(100 ml) was added. The layers were separated, and the organic
layers were washed with CH2Cl2. The solution was dried with
MgSO4 and solvent was removed under reduced pressure.
Crude product was puried by column chromatography
40.38, 39.54, 33.30, 32.08, 26.28, 21.66. 1H NMR of L3 (300 MHz,
CDCl3): d 8.43 (m, 4H), 8.36 (d, 2H, J ¼ 6 Hz), 8.22 (s, 2H), 8.16
(d, 2H, J ¼ 6 Hz), 8.05 (t, 1H, J ¼ 1.8 Hz), 7.90 (t, 2H, J ¼ 7.8 Hz),
7.81 (d, 2H, J ¼ 7.8 Hz), 7.75 (t, 4H, J ¼ 7.5 Hz), 7.63 (t, 3H, J ¼ 7.5
Hz), 3.05 (d, 4H, J ¼ 2.7 Hz), 2.88 (t, 2H, J ¼ 5.4 Hz), 2.70 (m, 2H),
2.30 (m, 2H), 1.41 (s, 6H), 1.21 (d, 2H, J ¼ 6.9 Hz), 0.65 (s, 6H). 13
C
NMR (CDCl3): d 156.62, 156.54, 154.75, 145.65, 145.58, 143.31,
142.15, 141.91, 140.53, 137.87, 129.52, 129.43, 128.09, 126.67,
126.44, 126.26, 120.88, 120.35, 119.48, 44.80, 40.37, 39.49, 33.23,
32.06, 26.25, 21.63.
(dichloromethane : n-hexane : diethyl ether
¼
30 : 6 : 1).
Procedure for synthesis of L2
Product was isolated as a pale brown solid in 26% yield. 1H
NMR (400 MHz, CDCl3) d 8.42 (d, J ¼ 2.8 Hz, 1H), 8.37 (d, J ¼ 7.6
Hz, 1H), 8.22 (s, 1H), 8.16 (d, J ¼ 7.3 Hz, 1H), 8.02 (s, 1H), 7.90
(t, J ¼ 7.8 Hz, 1H), 7.80 (d, J ¼ 7.7 Hz, 1H), 7.74 (t, J ¼ 7.2 Hz,
2H), 7.62 (d, J ¼ 13.8, 7.6 Hz), 3.06 (d, J ¼ 2.3 Hz, 1H), 2.89 (t, J ¼
5.4 Hz, 1H), 2.7 (m, 1H), 2.32 (s, 1H), 1.43 (s, 2H), 1.25 (d, J ¼ 9.8
Hz, 1H), 0.67 (s, 2H). 13C NMR (CDCl3): d 156.40, 156.33, 254.57,
145.46, 125.40, 143.12, 141.98, 141.84, 141.65, 140.31, 137.71,
129.28, 127.90, 126.54, 126.43, 126.30, 126.05, 120.72, 120.19,
119.33, 44.57, 40.14, 39.28, 33.02, 31.87, 23.06, 21.45.
To a solution of NiCl2$6H2O (1.2 mmol) in degassed DMF
(15 ml) at 70 ꢁC under nitrogen, triphenylphosphine (4.8 mmol)
was added to give a blue solution. Zinc powder (2.6 mmol) was
then added and the resulting mixture was stirred for an hour, in
which dark-brown mixture was formed. 2 (1 mmol) in degassed
DMF (5 ml) was added slowly and the mixture stirred at 70 ꢁC for
72 hours. The mixture was then allowed to cool to room
temperature and 25% aqueous NH3 (25 ml) was added. The
layers were separated, and the aqueous layers were extracted
with CH2Cl2. The solution was dried with MgSO4 and solvent
was removed under reduced pressure. Crude product was
puried by column chromatography. Product was isolated as a
Procedure for reversibility experiment
pale yellow solid. Yield: 37%. 1H NMR of L2 (300 MHz, CDCl3): d To a solution of L (L2–4, 3 ꢂ 10ꢀ4 M in CH2Cl2) in a 3 mm cell, 1
8.43 (m, 2H), 8.37 (d, 2H, J ¼ 7.5 Hz), 8.22 (s, 1H), 8.18 (d, 1H, J ¼ equiv. of HClO4 was added, and the CD spectrum of the solution
8.1 Hz), 7.90 (t, 1H, J ¼ 7.8 Hz), 7.83 (d, 1H, J ¼ 7.2 Hz), 7.78 (d, was obtained. Aer that, 1.2 equiv. of NEt3 was added to the
Scheme 1 Preparation of polyphenyl-bridged bis(2,20-bipyridine) L1–4.
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above solution to fully restore the original spectrum. The
experiments were repeated by following the sequence, and the
absorption at 334 nm which is the maximum of the induced CD
signals was plotted.
Results and discussion
Synthesis of polyphenyl-bridged bis(2,20-bipyridine) ligands
Chiral polyphenyl-bridged bis(2,20-bipyridine) ligands L1–4
with pinene-based chiral substituents at the 4,5-position of
terminal pyridine rings were prepared from bipyridine inter-
¨
mediate 1, which was obtained from reported Krohnke
condensation between of pyridinium iodide and a,b-unsatu-
rated ketone.36 The Pd-catalyzed Suzuki coupling between 1 and
m-phenylenediboronic acid with Na2CO3 in a mixture of H2O,
MeOH and toluene yielded the monophenyl-bridged L1 in 57%
yield. For the synthesis of biphenyl-bridged L2, Pd-catalyzed
Suzuki coupling between 1 and 3-chlorophenylboronic acid
resulted in the chlorophenylbipyridine intermediate 2. Ni(0)-
mediated homocoupling of 2 in DMF gave L2 in 37% yield. For
the synthesis of triphenyl-bridged L3, the reaction between 2
and m-phenylenediboronic acid with Pd(PPh3)4 as catalyst was
rst tried, but no reaction was observed. Then, bromophe-
nylbipyridine intermediate 3, prepared by Pd-catalyzed Suzuki
coupling between 1 and 3-bromophenylboronic acid, was
employed. A sub-stoichiometric amount of 3-bromophenylbor-
onic acid was used to reduce the formation of the bromobi-
phenylbipyridine and Suzuki coupling between 3 and m-
phenylenediboronic acid yielded L3 in 30% yield. For the
synthesis of tetraphenyl-bridged L4, intermediate 4, synthe-
sized by the reaction between 3 and 3-chlorophenylboronic acid
with Pd(PPh3)4 as catalyst, was used. Ni(0)-mediated homo-
coupling of 4 yielded L4 in 26% yield (Scheme 1).
Effect of acid on the CD spectra of polyphenyl-bridged bis-
(2,20-bipyridine) ligands
Although L1–4 contains chiral substitution at the 4,5-position of
the terminal pyridine rings, they gave only a weak CD absorp-
tion. Fig. 1a–d show the effect of different acids on the CD
spectrum of the ligands. The phenyl-bridged L1 does not show
much effect with addition of acid. Only very small change in the
CD spectrum of L1 is observed (Fig. 1a). However, L2–4 show
much greater change with addition of some acid. Fig. 1b shows
the results obtained with the biphenyl-bridged L2. Addition of
HFeCl4 gives an intense induced CD signal absorption at 334
nm with D3 ¼ 6.0 Mꢀ1 cmꢀ1
.
HClO4 and HOTf give similar
44
change but with weaker intensity, D3 ¼ 4.0 and 2.0 Mꢀ1 cmꢀ1
,
respectively. Fig. 1c shows the results obtained with the tri-
phenyl-bridged L3. Similar to the results of L2, addition of
HFeCl4, HClO4 and HOTf to L3 leads to induced CD signals at
333 nm with D3 ¼ 10.2, 4.3, and 3.0 Mꢀ1 cmꢀ1, respectively.
These signals have larger intensity when compared to L2.
Fig. 1 CD spectra of L1–4 (3 ꢂ 10ꢀ4 M) with addition of different acids
(1 equiv.).
Fig. 1d shows the results obtained with the tetraphenyl-bridged of HFeCl4 giving the strongest absorption, followed by HClO4
L4. Intense induced CD signals with D3 ¼ 9.0, 5.0, and 1.7 Mꢀ1 and then HOTf, is very similar to L2 and L3. With the additions
cmꢀ1 are observed at 330 nm with addition of HFeCl4, HClO4 of acids other than HFeCl4, HClO4 and HOTf, the CD spectra of
and HOTf, respectively, which are comparable to L3. The trend L2–4 give very minimal change.
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Fig. 3 CD spectrum of L2 (3 ꢂ 10ꢀ4 M) in CH2Cl2 with addition of HFeCl4
(0 to 4 equiv.). The inset shows the change in absorption at 334 nm.
Fig. 4 CD spectrum of L3 in CH2Cl2 (3 ꢂ 10ꢀ4 M) with addition of HFeCl4
(0 to 4 equiv.). The inset shows the change in absorption at 333 nm.
Fig. 5 CD spectrum of L4 in CH2Cl2 (3 ꢂ 10ꢀ4 M) with addition of HFeCl4
(0 to 4 equiv.). The inset shows the change in absorption at 332 nm.
upon protonation. For L2–4, the CD signal change does not
seem to follow the trend of the pKa value; for example, HClO4
(pKa ¼ ꢀ10.0) give a more intense signal than HOTf (pKa
¼
ꢀ14.0). Other acids like HOAc (pKa ¼ ꢀ4.8), HCl (pKa ¼ ꢀ8.0),
HNO3 (pKa ¼ ꢀ1.3), H2SO4 (pKa ¼ ꢀ3.0) give only very small or
no CD change. These results suggest that the CD signal change
is not triggered by protonation alone. Anion seems to have a role
in the change of the CD signal as well.
Fig. 2 ESI-MS spectrum of a CH2Cl2 solution of L1–4 with addition of
HFeCl4.
ESI-MS study
With HFeCl4 giving the largest CD signals, the ESI-MS spectra
When comparing the polyphenyl-bridged ligands, much of the ligands with 1 equiv. of HFeCl4 were obtained for L1–4
stronger acid-induced CD spectral change are observed with L2– (3 ꢂ 10ꢀ4 M). The spectrum with L1 is shown in Fig. 2a. The
4 than L1 and it seems to suggest that L2–4 give similar species spectrum show peaks at m/z 575.8 and 1151.6, which can be
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Fig. 6 1H NMR spectra (400 MHz) of (a) L2 (3 ꢂ 10ꢀ4 M) in CD2Cl2 with
Fig. 9 1H NMR spectra for (a) L4 (3 ꢂ 10ꢀ4 M) in CD2Cl2 with addition
addition of (b) 1 equiv. (c) 2 equiv. (d) 3 equiv. (e) 4 equiv. of HClO4.
of (b) 0.5 equiv. (c) 1 equiv. (d) 2 equiv. (e) 3 equiv. of HClO4.
Acid titration monitored by CD
In order to have a better understanding of the change in
CD signals, titration experiments were carried out. As shown
in Fig. 3, addition of various concentration of HFeCl4 into a
3 ꢂ 10ꢀ4 M solution of L2 rst give positive absorption, then
decrease in absorption. The inset shows the change in
intensity of the absorption at 334 nm. The intensity of the
signal reaches maximum when there is 1 equiv. of HFeCl4.
Further addition decreases the signal intensity, and
nally the positive CD absorption completely disappeared
when 3 equiv. of HFeCl4 is added. A similar trend in the
Fig. 7 1H NMR spectra for (a) L2 (2 ꢂ 10ꢀ2 M) in CD2Cl2 with addition
of (b) 0.5 equiv. (c) 1.0 equiv. (d) 1.5 equiv. (e) 2.0 equiv. of HClO4.
Fig. 8 1H NMR spectra for (a) L3 (3 ꢂ 10ꢀ4 M) in CD2Cl2 with addition
of (b) 0.5 equiv. (c) 1 equiv. (d) 2 equiv. (e) 3 equiv. of HClO4.
assigned to [(L1)H]+ and [(L1)2H]+, respectively, indicating
protonation of L1 by HFeCl4. Fig. 2b shows the spectrum with
L2. Similar to L1, peaks corresponding to [(L2)H]+ and
[(L2)2H]+ are observed, however, in addition to these peaks, a
peak at 1500.8 which can be assigned to [(L2)2H2](FeCl4)+ is
also observed. This formula indicates that a dimeric form of
monoprotonated L2 may have been formed. The spectrum
with L3, which is shown in Fig. 2c, has peaks at m/z 729.1,
1452.4 which corresponds to [(L3)H]+ and [(L3)2H]+, respec-
tively. Again, the spectrum of L3 shows peak corresponded to
a dimeric species, [(L3)2H2](FeCl4)+, at 1654.3. For L4, the
spectrum show peaks corresponded to [(L4)H]+, [(L4)2H]+,
and [(L4)2H2](FeCl4)+ at m/z 805.7, 1606.4, and 1806.5
respectively (Fig. 2d). The signals of [(L)2H2](FeCl4)+ (L ¼ L2–
4) observed in the spectra suggest the presences of dimeric
species [(L)2H2](FeCl4)2, but this species is not observed
with L1.
Fig. 10 NOESY spectrum (400 MHz) of L2 in CD2Cl2.
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Acid titration monitored by NMR
With the paramagnetic nature of HFeCl4, NMR informations
were obtained by using HClO4. With L2–4, titration experiments
were carried out under the same concentration as the CD
experiments. Fig. 6 shows the results with L2. With addition of
HClO4, there is an immediate upeld shi and broadening of
1
the H NMR signals. The most upeld and broadened signals
appear when 2 equiv. of HClO4 is added. Further addition of
HClO4 leads to a downeld shi and sharpening of the signals.
Sharpened signals with chemical shi similar to the unproto-
nated L2 is obtained when more acid is added. However, unlike
CD titration where the concentration is limited, the NMR
titration can be carried out at a much higher concentration.
Fig. 7 shows the spectra when the experiment is carried out at 2
ꢂ 10ꢀ2 M of L2, only 1 equiv. of HClO4 is needed to lead to the
upeld signals. Fig. 8 and 9 are the spectra of titrations with L3
and L4 which also show similar change, but in both of this case
only one equiv. of HClO4 is needed for reaching the most
upeld signal at the CD experiments concentration. These
results suggest that the upeld signals may come from the
monoprotonation of the polyphenyl-bridged bis(2,20-bipyridine)
ligands. By considering that the induced CD signals and the
1
upeld H signals occurred at the same time, we believe that
they come from the same protonated species.
Fig. 11 NOESY spectrum (400 MHz) of L2 with addition of 2 equiv. of
HClO4 in CD2Cl2.
1H NMR assignment
To have a better understanding of the species, 2D NMR was
carried out. The NOESY spectrum of L2 is shown in Fig. 10. The
correlation signals between the pyridine proton H5 and both
protons at the 2- and 6-position of the bridging phenyl ring, H6
and H9, suggests that L2 interconverts between two conforma-
tions with H5 syn either to H6 or H9. Although broadened signals
observed with addition of 1 equiv. of HClO4 cannot be assigned,
the sharpened signals observed at 2 equiv. of HClO4 were fully
assigned. The syn-conformation of the mono-protonated 2,20-
bipyridine is well established by both X-ray crystal structure and
change of CD signals was also observed in the titration
experiments of L3 and L4. Fig. 4 and 5 show the change
of the intensity of the CD signals with concentration of
HFeCl4. The signal reach maximum when there is 1 equiv. of
HFeCl4, and the intensity of signal decreases with further
increase of HFeCl4. These titration experiments suggest that
the CD signals are originated from a monoprotonated
state, and the acid-induced signal is lost upon further
protonation.
Fig. 12 1H NMR spectra for L2 (3 ꢂ 10ꢀ4 M) in CD2Cl2 with HClO4 (2 equiv.) at variable temperature. The species giving upfield and downfield
signals are labelled , and B, respectively.
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Fig. 13 1H NMR spectra for L3 (3 ꢂ 10ꢀ4 M) in CD2Cl2 with HClO4 (1 equiv.) at variable temperature. The species giving upfield and downfield
signals are labelled , and B, respectively.
Fig. 14 1H NMR spectra for L4 (3 ꢂ 10ꢀ4 M) in CD2Cl2 with HClO4 (1 equiv.) at variable temperature. The species giving upfield and downfield
signals are labelled , and B, respectively.
theoretical calculation.45,46 The correlation signal between H2 Variable temperature NMR experiments were then carried out
and H3 observed in the NOESY spectrum (Fig. 11) suggests that to study the broadened signals. Fig. 12–14 shows the effect of
the pyridyl rings of both bipyridine units are in syn-conforma- the temperature on the 1H NMR signals. Fig. 12 shows that the
tion which is consistence with the mono-protonation of the broadened signals of L2 with 2 equiv. of HClO4 become even
bipyridine units. By considering that proton at the 2-position of broader when the sample is cooled to 0 ꢁC. Further decrease in
the phenyl ring, H9, gives correlation signals to both protons at temperature leads to sharpening and splitting of the signals
the 4-position of the phenyl ring, H8, and the 50-position of the into two sets with one set signicantly upeld shied while the
bipyridine, H5, a biprotonated species with a linear conforma- other set shied downeld to chemical shi similar to that of
tion is proposed. This biprotonated species comes from the the free ligand. Fig. 13 shows the spectra obtained with L3 and 1
further protonation of the species giving the upeld signals, equiv. of HClO4. Similar signals splitting is observed, but it
which is consistent with the suggestion that the upeld signal is starts at higher temperature of 0 ꢁC. The most sharpened
most likely a monoprotonated species of L2.
signals are observed at ꢀ40 ꢁC. Fig. 14 shows that signals of L4
with 1 equiv. HClO4 also split into in two sets at 0 ꢁC. The line-
shape of the signals does not change much when the temper-
ature is below ꢀ30 ꢁC. These results suggest that the broad-
1
Low temperature H NMR study
Apart from the upeld shi, broadening of signals of were also ening of signals observed with L2–4 and HClO4 come from the
observed in NMR spectra of L2–4 when HClO4 was added. exchange between an upelded and a downelded species. The
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Fig. 17 (a) Capped stick drawing of X-ray crystal structure of
[(L2)2H2](FeCl4)2. Dotted lines show hydrogen bonding within the
double helix. (b) Spacefilling model of X-ray crystal structure of
[(L2)2H2](FeCl4)2.
Fig. 15 The ORTEP plot of crystal structure of L1. Hydrogen atoms are
removed for clarity.
believe that the broadening of signals may be due to the
exchange between the dimer and its monomeric form, and
the stacking between the strands of the dimeric species is the
differences in temperature in giving the splitting and change in reason that leads to anisotropic effects of the aromatic rings
line-shape of the signals suggest that the exchange rates are and upeld signals.
different. Analysis of the linewidth of the signals at temperature
below coalescence give the free energy of activation for the
X-ray characterization
exchange process47,48 of 13.0, 13.4, and 14.0 kcal molꢀ1 for L2,
L3 and L4, respectively. By considering that the signals broad- Single crystals suitable for X-ray analysis were obtained for the
ening and the upeld shi of signals occur at the same time, nonprotonated state of L1, the biprotonated state of L4, and the
we believe that the exchanges involve monoprotonated species dimeric form of the monoprotonated L2. Crystals of L1 were
of L2–4.
obtained by slowly evaporation of a diethyl ether solution. It is
Aer the variable temperature experiments, the NMR solu- crystalized in a P212121 space group. Fig. 15 shows the ORTEP
tions were then characterized by ESI-MS at room temperature diagram. The pyridine rings adopt a transoid geometry and L1 is
(Fig. S1†). The spectra show similar results to that of HFeCl4 as not coplanar. Twisting is observed between the aromatic rings
both [(L)2H2](ClO4)+ and [(L)H]+ were observed. These monop- with the torsional angles between the pyridyl rings being 2.8ꢁ
rotonated species can be considered monomer and dimer. We and 19.1ꢁ, and between the phenyl and pyridyl rings being 22.7ꢁ
and 24.8ꢁ. The ligand is not long enough to have a complete
helical turn, and there is no stacking interaction.
Single crystals of the biprotonated L4 were obtained by ether
diffusion into a methanol and chloroform solution of L4 and
HFeCl4. The compound crystallized in a P1211 space group.
Although the mixture was prepared using a one to one molar
ratio of L4 and HFeCl4, the biprotonated species of L4 is a
structure with a chloride and a tetrachloroferrate anions. The
biprotonated L4 coils up around the chloride ion to give two
helical structures, the P- and M-helices, which are coexist in the
crystal lattice. The structures of P- and M-helices are shown in
Fig. 16a–d. Beside the methyl groups of the P-form is pointing
towards the chloride and the methyl groups of the M-form is
pointing away from the chloride, the structures of the two
helices are very similar. In both structures, the pyridyl rings
adopt a cisoid geometry and point toward the chloride ion. The
coiling of L4 comes from the twisting between the aromatic
rings with most of it is contributed by the bridging phenyl rings.
The dihedral angles between the phenyl rings are in the range
33.53–48.49ꢁ. The chloride anion has a close proximity with the
Fig. 16 Capped stick drawing of X-ray crystal structure of (a) P- and (b)
terminal pyridine rings with N–H/Cl distances and angles in
M- [(L4)H2](Cl)(FeCl4). Dotted lines show hydrogen bonding with the
ꢁ
˚
chloride. The FeCl4ꢀ ion is not shown for clearance. (c) and (d) are the
range 2.28–2.41 A and 139.37–147.24 , respectively, which
spacefilling model of P- and M-[(L4)H2](Cl)(FeCl4) respectively.
suggests the presence of hydrogen bonding. Although these
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Fig. 19 Crystal structure of [(L2)2H2](FeCl4)2 showing crystal packing
Fig. 18 Crystal structꢀure of [(L2)2H2](FeCl4)2 showing crystal packing
and location of FeCl4 anions surrounding the [(L2)2H2]2+ core.
and location of FeCl4 anions surrounding [(L2)2H2]2+ core. Dotted
ꢀ
ꢀ
lines shows some of the hydrogen bonding between FeCl4 and
the helix.
Table 1 Hydrogen bonding parameters of [(L2)2H2]2+ and surrounding
FeCl4ꢀ observed in X-ray crystal structure of [(L2)2H2](FeCl4)2
D–H/A/ꢁ
˚
D–H/A
H/A/A
N1–H1/N5
N8–H8/N4
C–H77/Cl1
C–H11/Cl2
C–H14/Cl3
C–H78/Cl5
C–H79/Cl6
C–H16/Cl7
C–H71/Cl6
C–H72/Cl6
C–H67/Cl8
C–H32/Cl1
C–H62/Cl3
C–H65/Cl4
C–H27/Cl8
C–H20/Cl4
C–H21/Cl4
1.987
1.989
3.036
2.713
3.119
2.906
3.097
3.057
3.042
3.090
3.031
2.880
2.996
2.747
2.994
3.125
2.768
156.14
154.98
165.66
162.04
124.83
139.35
136.80
132.81
128.50
126.66
137.48
162.13
136.51
128.67
171.65
152.81
166.47
Fig. 20 Energy profile of the formation of [(L2)2H2](X)2 (X ¼ ClO4ꢀ or
FeCl4ꢀ) obtained by calculation at M06-2X/6-31G(d)/LANL2DZ with
solvent effect. The red line indicates the formation of [(L2)2H2](ClO4)2
and black line indicates the formation of [(L2)2H2](FeCl4)2. The relative
energy is given in kcal molꢀ1
.
parameter ¼ 0.026(7)). The two molecules of L2 are held tightly
together by intermolecular hydrogen bonding between the
pyridinium proton of one strand and pyridine ring of the other
˚
strand. The N–H/N distances are 1.987 and 1.989 A, and the
angles N–H/N are 154.14 and 156.98ꢁ. The helix is also stabi-
species have helical structures, by considering that the anions lized by p–p stacking interactions. There are extensive stacking
are different, and the coexistence of the P- and M-form, these interactions between the aromatic rings of the two molecules
species are not related to the CD signals observed in the CD which start from the stacking of the pyridinium of one strand
experiments. In addition, these biprotonated species shows that with the forth aromatic ring of the other, and extend along the
the tetraphenyl-bridged L4 is long enough to give a complete whole ligand strand. Fig. 18 shows the crystal lattice of
helical turn, and there are stacking interactions between the [(L2)2H2](FeCl4)2. There are FeCl4ꢀ anions surrounding the helix
terminal pyridine rings, however, no upeld signals shi is core. Some of the chloride atoms of the FeCl4ꢀ are close to the
observed while the biprotonated species is formed in the 1H aromatic hydrogens of the helix, and their distances and angles
NMR studies with HClO4. It is not likely that this helical form of are summarized in Table 1. The C–H/Cl distances and angles
ꢀ
ꢁ
˚
biprotonated L4 is presented when only ClO4 anion is are in the range 2.713–3.125 A and 124.83–171.65 respectively.
presented.
These results suggest the presence of the C–H/Cl hydrogen
ꢀ
Single crystals of the monoprotonated L2 was obtained by bonding interactions between the helix and FeCl4 anions.49–51
ether diffusion into a solution of L2 in the presence of 1 equiv. Fig. 19 shows some of these interactions.
of HFeCl4. It crystallizes in a monoclinic C2 space group and it
is a dimeric form of formula, [(L2)2H2](FeCl4)2. The structure,
shown in Fig. 17, has syn pyridine rings and the two protonated
Solution state structure by DFT calculation
strands twist around each other resulting in the double- From the crystal structure of [(L2)2H2](FeCl4)2, we believe that a
stranded helical structure with P-helical chirality (Flack double-stranded helix is formed in solution when L2 is
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ꢀ
Fig. 21 Energy profile of the formation of [(L3)2H2](X)2 (X ¼ ClO4 or
FeCl4ꢀ) obtained by calculation at M06-2X/6-31G(d)/LANL2DZ with
solvent effect. The red line indicates the formation of [(L3)2H2](ClO4)2
and black line indicates the formation of [(L3)2H2](FeCl4)2. The relative
Fig. 23 Minimized energy model of (a) [(L2)2H2](FeCl4)2, and (b)
[(L2)2H2](ClO4)2 obtained from theoretical calculation at M06-2X/6-
31G(d)/LANL2DZ. Hydrogen bonding interactions are shown with
dotted lines on only one of the anions for clearance.
energy is given in kcal molꢀ1
.
Fig. 22 Energy profile of the formation of [(L4)2H2](X)2 (X ¼ ClO4ꢀ or
FeCl4ꢀ) obtained by calculation at M06-2X/6-31G(d)/LANL2DZ with
solvent effect. The red line indicates the formation of [(L4)2H2](ClO4)2
and black line indicates the formation of [(L4)2H2](FeCl4)2. The relative
Fig. 24 Minimized energy model of (a) [(L3)2H2](FeCl4)2, and (b)
[(L3)2H2](ClO4)2 obtained from theoretical calculation at M06-2X/6-
31G(d)/LANL2DZ. Hydrogen bonding interactions are shown with
dotted lines on only one of the anions for clearance.
energy is given in kcal molꢀ1
.
monoprotonated in the presence of FeCl4 or ClO4ꢀ, and the
formation of the double helix is the origin the spectro-change
observed in CD and NMR. The P-helical chirality of the
[(L2)2H2](FeCl4)2 is consistent with the rst positive CD signals
observed in the CD spectra52 which is similar to the spectra of
the rigid Mn double-stranded helicates of L2 and L3.35 The
extensive p–p stacking of the aromatic rings of the strands of
the double-stranded helical structure which lead to anisotropic
effect also agree well with the upeld signals observed in the
NMR spectra. Considering the observation and results obtained
with L2, L3 and L4 in the experiments, we believe that the
monoprotonated form of triphenyl-bridged L3 and tetraphenyl-
bridged L4 also form double-stranded helical structure. To
show that this is the case and to have a better understanding to
the helices in solution, DFT calculations were carried out on thꢀe
monoproꢀtonated species of L2, L3 and L4 using both the FeCl4
and ClO4 anions.
ꢀ
Fig. 25 Minimized energy model of (a) [(L4)2H2](FeCl4)2, and (b)
[(L4)2H2](ClO4)2 obtained from theoretical calculation at M06-2X/6-
31G(d)/LANL2DZ. Hydrogen bonding interactions are shown with
dotted lines on only one of the anions for clearance.
Models of [(L)2H2](FeCl4)2 and [(L)2H2](ClO4)2 (L ¼ L2–4)
were rst constructed, then their formation were investigated HClO4. The energy different is the sum of dissociation of
using theoretical DFT calculation at M06-2X level, with the HFeCl4, protonation of L2, the stacking between the
solvent effect taken account by the Polarizable Continuum ligands and the pyridinium pyridine hydrogꢀen bondings.
ꢀ
Model. Fig. 20 shows the calculation with the biphenyl- Interactions between [(L2)2H2]2+ with FeCl4 and ClO4
bridged L2. The calculation starts from protonation of L2 and anion further stabilize the system by 30.1 and 26.1 kcal
formation of the double-stranded helical core [(L2)2H2]2+
.
molꢀ1
,
respectively, which give [(L2)2H2](FeCl4)2 and
The formation of the helical core stabilizes the system by [(L2)2H2](ClO4)2 with an overall stabilization 194.1 and
164.4 and 107.8 kcal molꢀ1, respectively, with HFeCl4 and 133.9 kcal molꢀ1, respectively.
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Table 2 A summary of the selected bond lengths and angles of X-ray and calculated data of [(L2)2H2](FeCl4)2
X-ray structure of [(L2)2H2](FeCl4)2
Model of [(L2)2H2](FeCl4)2
Atoms
Bond lengths or angles
Atoms
Bond lengths or angles
˚
˚
N1–H1/N5
N1–H1/N5
N8–H8/N4
1.987 A
N89–H195/N194
N89–H195/N194
N181–H196/N92
1.890 A
156.14ꢁ
157.00ꢁ
˚
˚
1.989 A
1.894 A
N8–H8/N4
154.98ꢁ
17.58ꢁ
20.49ꢁ
32.74ꢁ
24.58ꢁ
21.82ꢁ
24.04ꢁ
22.81ꢁ
34.24ꢁ
13.29ꢁ
20.35ꢁ
N181–H196/N92
156.48ꢁ
23.83ꢁ
12.87ꢁ
29.67ꢁ
20.82ꢁ
21.30ꢁ
23.85ꢁ
11.47ꢁ
30.08ꢁ
21.48ꢁ
20.64ꢁ
N1–C12–C13–N2
N2–C17–C18–C23
C23–C22–C24–C29
C29–C28–C30–N3
N3–C34–C35–N4
N5–C58–C59–N6
N6–C63–C64–C69
C69–C68–C70–C75
C75–C74–C76–N7
N7–C80–C81–N8
N89–C26–C27–N90
N90–C34–C35–C43
C43–C42–C45–C53
C53–C52–C55–N91
N91–C62–C63–N92
N181–C116–C119–N182
N182–C126–C127–C135
C135–C134–C137–C145
C145–C144–C147–N183
N183–C154–C155–N184
ꢀ
Scheme 2 Formation of hydrogen-bonded double-stranded helix [(L2)2H2](ClO4)2 with ClO4 anions represented by
.
Similar calculations were carried out using L3 and L4 and the pyridine hydrogen bonding, there are C–H/Cl type hydrogen
results are shown in Fig. 21 and 22. The results suggest that the bonding interactions between the [(L2)2H2]2+ core and the two
ꢀ
formation of the double-stranded helices with L3 and L4 are FeCl4 anions very much similar to that observed in the X-ray
also feasible, and also give additional information to the structure. The bond distances and angles are summarized in
difference between these systems. In the presence of HFeCl4, Table S1.† The presence of C–H/Cl hydrogen bondings are also
the formation of the double-stranded helical core [(L)2H2]2+ conrmed by Atom in Molecules (AIM) analysis, and the results
with L4 being the most stable (ꢀ173.4 kcal molꢀ1), which is are shown in Table S2.† The FeCl4ꢀ anions use three out of the
followed by L3 (ꢀ172.1 kcal molꢀ1), and then followed by L2 four chloride atoms to hold the double-stranded helical struc-
(ꢀ164.4 kcal molꢀ1). The same trend also occurs with HClO4. ture by forming multi-centered hydrogen bonding to the protons
These trends seem most likely due to the increase in the on the rst and the second aromatic rings of one strands, and
number of the bridging phenyl-rings of the ligands which the protons on the forth and rth aromatic rings of the other.
increase the stacking interactions. Interactions between The model of [(L2)2H2](ClO4)2 is shown in Fig. 21b. There arꢀe
ꢀ
[(L)2H2]2+ and FeCl4 are in range ꢀ30.1 to ꢀ31.7 kcal molꢀ1
.
C–H/O type hydrogen bonding interactions. The smaller ClO4
With ClO4ꢀ, the interactions are in the range ꢀ25.3 to ꢀ28.3 anions use three out of the four its oxygen atoms to form
kcal molꢀ1. The FeCl4ꢀ anion seems to have better stabilization
than ClO4ꢀ. With the same anion, the differences in energy
among L2–4 suggest that the helices interact differently with
different anions. The energy of ꢀ133.9 kcal molꢀ1 for
[(L2)2H2](ClO4)2 is less stable than other helices which agrees
quite well with the observations in the CD experiments.
The models of [(L)2H2](FeCl4)2 and [(L)2H2](ClO4)2 (L ¼ L2–4)
are shown in Fig. 23–25. Fig. 23a shows the model of
[(L2)2H2](FeCl4)2. The optimized geometry is in satisfactory
agreement with its crystal structure, and the X-ray and calculated
data are summarized in Table 2. Beside the pyridinium N+H and alternate addition of HClO4 and NEt3.
Fig. 26 CD absorption at 334 nm of (a) L2, (b) L3, and (c) L4, with
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hydrogen bondings to the same aromatic protons on the back- (Fig. 26b and c). The results suggest that these systems have
ꢀ
bone of both strands as FeCl4 but with slightly different potential to be developed as chiroptical switches.
orientation. The data are summarized in Tables S3 and S4.†
Models with L3 are shown in Fig. 22a and b. The stacking
Conclusion
between the aromatic rings observed in [(L3)2H2]2+ are different
from [(L2)2H2]2+. In the case of [(L2)2H2]2+, the pyridinium of We have presented the synthesis of a series of chiral polyphenyl-
one strand stacks with the forth aromatic ring of the other, but bridged bis(2,20-bipyridine) ligands. Protonation of the ligands
the pyridinium stacks with the third aromatic ring in case of give intense CD signals. The protonation processes were studied
[(L3)2H2]2+. Exterior hydrogen bindings between the helices and in detail by both CD and 1H NMR. The results suggest that the
the anions are also observed. Data are summarized in Table S5– CD signals come from monoprotonated species. X-ray crystal
8.† Some differences are observed when comparing to L2. For structures of free ligand and protonated ligands were obtained.
L3, the FeCl4ꢀ forms hydrogen bonds to the protons on the rst The structure of the monoprotonated ligand shows that it is a
and the second aromatic rings of one strands, but to the protons double-stranded helix that is stabilized by a delicate combina-
on the third and the forth aromatic rings of the other. In the tion of interior hydrogen bonds between the ligand strands and
presence of ClO4ꢀ, it ꢀforms hydrogen bonds to the same exterior hydrogen bonds between the helical core and anions.
aromatic rings as FeCl4 but with slightly different protons.
Theoretical DFT calculations on the formation of hydrogen-
Models with L4 are shown in Fig. 23a and b. The stacking bonded double helices with the series of ligands were carried
observed in the helix of L4 is different from L2 and L3. The out. We believe that hydrogen-bonded double helices are
pyridinium of one strand stacks with the second aromatic ring of formed upon monoprotonation of the ligands and they are
the other. The close proximity of the anions with the aromatic stabile in solution as well. With perchlorate anion, the system
protons again suggest the presents of hydrogen bondings. The can be interconverted between non-helical and helical states by
data are summarized in Table S9–11.† However, due to the size of addition and removal of proton, making it an on/off chiroptical
the model of [(L4)2H2](FeCl4)2 excess the limit of the soware, switch. Further investigation of these properties for other
ꢀ
AIM analysis was only carried out on [(L4)2H2](ClO4)2. The FeCl4
forms hydrogen bonds to the protons on the rst and the second
aromatic rings of one strand, but to the protons on the third
applications line is now in progress.
Acknowledgements
aromatic rings of the other, which is different from helix of L2
ꢀ
and L3. The ClO4 also form hydrogen bonds to the same The work described in this paper was supported by grants from
aromatic protons. Although the C–H/O and C–H/Cl hydrogen the University Grants Committee of Hong Kong Special
bonding are generally considered as weak hydrogen bonding,49 Adminstrative Region, China (Project no. [AoE/P-03/08] and
the calculation results show that the sum of the interactions from CityU 7004018).
the two anions are not weak (in range ꢀ30.1 to ꢀ31.7 kcal molꢀ1
for FeCl4ꢀ, and ꢀ25.3 to ꢀ28.3 kcal molꢀ1 for ClO4ꢀ), and is
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