A study of binding interactions between terpyridine derivatives and cucurbit[10]uril

Article abstract of DOI:10.1080/10610278.2018.1455977

The binding interactions of a series of 2,2′:6′,2″-terpyridine (TPY) derivatives and their metal complexes with cucurbit[10]uril (CB[10]) were investigated by ¹H NMR, UV/Vis, emission spectroscopy, and ESI mass spectrometry. ¹H NMR titrations revealed CB[10] could encapsulate methylated TPY (MTPY), and the binding ratio between guest MTPY and host was 1:1 and 2:1 via ESI-MS characterization. For the transition metal complexes composed of Fe(II) or Ru(II) or Rh(III) and TPY derivatives, the octahedral TPY?metal?TPY core can be included in the cavity of CB[10]. Three binding modes (1:1, 1:2 and 1:3) have been detected for the binding of the metal?MPTY complexes with CB[10] by ESI-MS.

Full text of DOI:10.1080/10610278.2018.1455977

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
A study of binding interactions between
terpyridine derivatives and cucurbit[10]uril
Yuanyuan Deng, Hang Yin, Zhiyong Zhao, Ruibing Wang & Simin Liu
To cite this article: Yuanyuan Deng, Hang Yin, Zhiyong Zhao, Ruibing Wang & Simin Liu (2018):
A study of binding interactions between terpyridine derivatives and cucurbit[10]uril, Supramolecular
Chemistry, DOI: 10.1080/10610278.2018.1455977
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Supramolecular chemiStry, 2018
https://doi.org/10.1080/10610278.2018.1455977
A study of binding interactions between terpyridine derivatives and cucurbit[10]
uril
Yuanyuan Denga, Hang Yinb , Zhiyong Zhaoa, Ruibing Wangb and Simin Liua
athe State Key laboratory of refractories and metallurgy, School of chemistry and chemical engineering, Wuhan university of Science and
technology, Wuhan, china; bState Key laboratory of Quality research in chinese medicine, institute of chinese medical Sciences, university of
macau, macau, china
ABSTRACT
ARTICLE HISTORY
received 29 January 2018
accepted 18 march 2018
Thebindinginteractionsofaseriesof2,2′:6′,2″-terpyridine(TPY)derivativesandtheirmetalcomplexes
with cucurbit[10]uril (CB[10]) were investigated by 1H NMR, UV/Vis, emission spectroscopy, and ESI
1
mass spectrometry. H NMR titrations revealed CB[10] could encapsulate methylated TPY (MTPY),
KEYWORDS
and the binding ratio between guest MTPY and host was 1:1 and 2:1 via ESI-MS characterization.
For the transition metal complexes composed of Fe(II) or Ru(II) or Rh(III) and TPY derivatives, the
octahedral TPY−metal−TPY core can be included in the cavity of CB[10]. Three binding modes (1:1,
1:2 and 1:3) have been detected for the binding of the metal−MPTY complexes with CB[10] by ESI-
MS.
host–guest interaction;
cucurbit[n]urils; metal–
terpyridine complex;
encapsulation
1. Introduction
widely used as photosensitizers, molecule probes, or for
the construction of supramolecular functional materials
(5–9).
Since the isolation of 2,2′:6′,2″-terpyridine (TPY) by Burstall
and Morgan in 1930s (1), increasing attention has been
paid to this heterocyclic compound derived from pyridine.
TPY and its derivatives can form planar or octahedral com-
plexes with numerous transition metal ions at 1:1 or 2:1
ratios due to its conjugate π bond and the high activity
of the three nitrogen atoms (2). Moreover, the metal−TPY
complexes have characteristic optical and electrochemical
properties as a consequence of intramolecular energy and
electron transfer (3,4). Due to its inherent fluorescence, TPY
derivatives and the related metal ion complexes have been
Cucurbit[n]urils (CB[n]s, n = 5–8, 10, 13–15) are macrocy-
clic hosts composed of variable methylene linked glycoluril
units (10–14). Owing to the hydrophobic cavity and elec-
tronegative portals and relatively rigid structure that they
possess, CB[n]s are intriguing hosts for binding hydropho-
bic molecules especially for organic cations. They are used
in a variety of applications in nanoreactors/catalyst, soft
materials, nanoconstructed materials, and drug delivery
(15–24). Although ‘larger’ twisted CB[n]s (n = 13–15) have
CONTACT Simin liu
liusimin@wust.edu.cn
Supplemental data for this article can be accessed https://doi.org/10.1080/10610278.2018.1455977
© 2018 informa uK limited, trading as taylor & Francis Group

2
Y. DENG ET AL.
Figure 1. Structures of: (a) cB[10]; (b) terpyridine (tpy) and its derivatives; and (c) metal−tpy complexes.
been discovered, CB[10] (Figure 1) is still considered to
have the lagest cavity among the traditional members
of the CB[n]s family, with a cavity volume of ~870 Å3 and
portal diameter of ~10 Å (12). Thus, CB[10] can bind larger
guests such as calix[4]arene, metalloporphyrin, as well as
blue-box and anti-cancer drugs. In addition, CB[10] is use-
ful in the construction of CB[10]-anions network and in the
selective isolation of metal cations (25).
Thus far, supramolecular assembled polymeric mate-
rial involving the connection of transition metal-ligand
complexes (bipyridine and terpyridine metal complex)
with CB[8] have been investigated by Zhang, Masson and
Li (26–28). Sun and coworkers observed a photoinduced
long-lived charge-separated state in a bipyridine-Ru com-
plex bearing a methylviologen group upon the encapsula-
tion of the methylviologen group in CB[8] (29). In all cases,
the core metal-ligand moiety was not included by CB[8].
Interestingly, upon the encapsulation in the large cavity of
CB[10], Ir(III) cyclometalated complexes showed markedly
enhanced luminescent emission (30,31). Herein, we report
our investigation on the binding interactions of TPY deriv-
atives and their transition metal complexes (Figure 1) with
CB[10] by 1H NMR, UV/Vis, emission spectroscopy, and ESI
mass spectrometry.
2. Results and discussion
2.1. Synthesis of terpyridine and its derivatives
Scheme 1 shows the synthetic route used to prepare
the guest compounds. The 2,6-dipyridin-2-yl-4-pyridin-
4-ylpyridine (PyTpy) was synthesized using 2-acetylpyri-
dine and 4-pyridinecarboxaldehyde as starting materials
(32). Compound 3 (MTPY) was prepared via methylation
of PyTpy. Compounds 4, 6, 7, 9, 10, and 11 were prepared
by coordinating the corresponding metal ions with com-
pound 1 or 3 (27). Nitryl-substituted terpyridine was syn-
thesized according to the Stille coupling reaction, and it
then reacted with FeCl2 followed by reduction to give 5
(33). Compound 2 was obtained via the destruction of a
metal-ligand interaction; compound 8 was prepared by
coordinating Ru ions with compound 2 (33).
2.2. Binding studies of TPY derivatives with CB[10]
Unlike other CB[n]s that are more or less soluble in water,
CB[10] is extremely insoluble in pure water even weakly
acidic aqueous solutions. After complexation with a
water-soluble guest, it sometimes can bring CB[10] into
water phase. Binding studies of TPY and its substituted

SUPRAMOLECULAR CHEMISTRY
3
Scheme 1. Synthesis of the guest compounds.
derivatives with CB[10] started with 2,2′:6′,2″-terpyridine
(1). However, 1 and CB[10] are mostly insoluble in water,
and this is a huge obstacle for exploring their interactions
in aqueous solutions. While 2 could be dissolved in an
acidic aqueous solution, no interactions between 2 and
CB[10] were detected. To determine whether or not the
TPY moiety can enter the cavity of CB[10], compound 3
was further synthesized as a water-soluble TPY derivative
bearing a cationic group (N-methyl pyridyl).
This unusual movement of proton signals often suggests
switching between multi-binding modes in the host−
guest complex (34). Due to the poor solubility of CB[10]
and weak binding between the host and guest, the real
ratio between 3 and CB[10] was ~1:0.6 making it impossi-
ble to determine the binding modes by 1H NMR or UV/Vis
spectroscopic titrations.
The complexation was also confirmed by UV/Vis and flu-
orescence spectroscopy because CB[n]s can alter the UV/
Vis absorption and fluorescence of chromophoric guests
(35). In the case of 3, there was a significant decrease in
the intensity of absorbance and a slight red-shift of the
Dissolution of CB[10] in an aqueous solution of 3
allowed us to further investigate their binding interac-
1
tions via H NMR spectroscopy. Figure 2 shows that the
binding of 3 with CB[10] is fast exchange kinetics on the
1H NMR time scale. Upon additions of ~0.3 eq of CB[10],
the signals for H1–6 protons of 3 significantly shifted upfield
and the signal for H8 proton remained constant. This indi-
cates that the TPY moiety was truly encapsulated in the
cavity of CB[10] (Figure 2(a)–(d)). As the equivalence of
the host increases (above ~0.4 eq of CB[10]), the signals
for H1–6 protons continued shifting upfield, but both H7
and H8 proton signals shifted downfield (Figure 2(e)–(g)).
λmax upon addition of CB[10] (Figure S1). Moreover, there
was an interesting movement of the emission spectra: the
emission λmax of 3 shifted from 430 to 410 nm upon addi-
tion of less CB [10] (~0.3 eq). As the equivalence of CB[10]
increases, the emission λmax of 3 experienced a red-shift to
445 nm (Figure S2). These shifts in the emission λmax also
imply the existence of multi-binding modes. The binding
behavior between 3 and CB[10] could be explained via
the ESI-MS data (Figure S3). Both 1:1 (m/z = 933.3) and 1:2

4
Y. DENG ET AL.
(m/z = 1155.4) host–guest complexes were detected in a
solution of 3 with excess CB[10].
2.3. Binding studies of metal–TPY complexes with
CB[10]
AlthoughTPY itself could not be dissolved in water, its tran-
sition metal complex is quite soluble. Based on the above
results showing the encapsulation of two TPY moieties in
CB[10], we wondered whether an octahedral TPY–metal–
TPY moiety could be included within CB[10]. The prelim-
inary result data showed that CB[10] can be dissolved in
the aqueous solution of 4, 7 and 10, and 1H NMR titration
was utilized to characterize their binding behaviors. Upon
complexation with CB[10], all signals for the protons on the
guests 4, 7 and 10 shifted upfield, reflecting the encapsu-
lation of octahedral TPY–metal–TPY moiety of all guests in
the cavity of CB[10].
Although complexation between 4, 7 and 10 and
CB[10] exhibits fast exchange kinetics on the 1H NMR time
scale, the ratio between the guest and host in the solu-
tion was ~1:1 based on the integrals of the corresponding
proton resonances. This suggests that the binding mode
between the guest and host could be 1:1. Interestingly,
the change in the chemical shift of H5 proton was larger
than that of the other protons in all cases. This indicates
that H5 is closer to the equatorial plane of CB[10] cavity
than H1 upon the encapsulation of guest. Since there is
no cationic binding site on these compounds, the hydro-
phobic effect surely dominates the binding as well as the
size-matching effect.
1
Figure 2. h Nmr spectra (298 K, D2o) of 3 (1.0 mm) with: (a) no
cB[10]; (b) 0.08 eq cB[10]; (c) 0.15 eq cB[10]; (d) 0.30 eq cB[10];
(e) 0.40 eq cB[10]; (f) 0.48 eq cB[10]; and (g) 0.60 eq cB[10].
As was observed with compounds 4, 7 and 10, com-
pounds 5 and 8 can also be included within CB[10] accord-
1
ing to H NMR titrations (Figure S4). Unfortunately, even
with the ammonium group as the binding site, complex-
ation between compounds 5 or 8 and CB[10] still experi-
1
ence fast exchange kinetics on the H NMR time scale in
weakly acidic aqueous solutions.
Metal–MTPY complexes 6, 9 and 11 have a longer cati-
onic group, and their binding behaviors with CB[10] were
proven to be more complicated. For compound 6, the
maximum amount of dissolved CB[10] was about 0.6 eq.
relative to the guest amount (1.0 mM). The signals for H1–5
protons on the TPY–metal–TPY core shifted upfield while
H6 proton shifted downfield; the signals for H7–8 protons
remained constant (Figure 4(a), (b)). These observations
suggest the existence of the 1:1 binding mode. Rather than
binding to the cationic sites (N-methyl pyridyl), CB[10] pre-
ferred to encapsulate the TPY–metal–TPY core of 6 in this
mode and leave two cationic groups outside of the portals
of CB[10].
Figure 3. 1h Nmr spectra (298 K, D2o) of: (a) free 4 (1.0 mm); (b) 4
(1.0 mm) with saturated cB[10]; (c) free 7 (1.0 mm); (d) 7 (1.0 mm)
with saturated cB[10]; (e) free 10 (1.0 mm); and (f) 10 (1.0 mm)
with saturated cB[10].
For compound 9, the maximum amount of dissolved
CB[10] was about 1.1 eq. The binding NMR spectra of 9 with

SUPRAMOLECULAR CHEMISTRY
5
1
Figure 4. h Nmr spectra (298 K, D2o) of: (a) free 6 (1.0 mm); (b)
Figure 5. eSi-mS spectra of: (a) guest 6 with saturated cB[10]; (b)
guest 9 with saturated cB[10]; and (c) guest 11 with saturated
cB[10].
6 (1.0 mm) with saturated cB[10] (ratio between 6 and cB [10]
was ~1:0.6 by integration); (c) free 9 (1.0 mm); (d) 9 (1.0 mm)
with saturated cB[10] (ratio between 9 and cB [10] was ~1:1.1 by
integration); (e) free 11 (1.0 mm); f() 11 (1.0 mm) with saturated
cB[10] (ratio between 11 and cB [10] was ~1:2.2 by integration).
pyridyl tails that are included within CB[7] (Figure S5–S7).
The downfield shifts of H1 and H4 resonances were due
to the close CH···O contact between the guest and host
as stated by Masson (27). The 1:2 binding mode of com-
pounds 6, 9, or 11 with CB[7] could explain the upfield shift
of H8 resonance and the downfield shift (could be H4 pro-
ton) in Figure 4(d) when compound 9 was encapsulated
in CB[10]. Complicated binding NMR was also reported
by Wallace when some of Ir(III) cyclometalated complexes
were encapsulated in CB[10] (31).
Although the NMR studies could give possible bind-
ing modes for complexation of compound 6, 9 and 11
with CB[10], we used ESI-MS to confirm that. Besides the
ion peak at m/z: 591.9 that corresponds to the 1:1 com-
plex CB[10]·6 ([CB[10] + 6]4+ = 591.9), ion peaks at m/z
1006.8 and 1421.9 were observed that correspond to the
2:1 complex CB[10]2·6 and 3:1 complex CB[10]3·6 ([2CB[10]
+ 6]4+ = 1006.8; [3CB[10] + 6]4+ = 1421.9) (Figure 5(a)).
For the mixture of 9 and CB[10], ion peaks at m/z: 603.2
and 815.9 corresponding to the 1:1 complex CB[10]·9
([CB[10] + 9]4+ = 603.2; [CB[10] + 9 + Cl−]3+ = 815.9), ion
peaks at m/z: 1018.5 and 1369.7 corresponding to the 2:1
complex CB[10]2·9 ([2CB[10] + 9]4+ = 1018.5; [2CB[10] +
9 + Cl−]3+ = 1369.7) and ion peak at m/z: 1433.4 correspond-
ing to the 3:1 complex CB[10]3·9 ([CB[10]3 + 9]4+ = 1433.4)
were observed (Figure 5(b)). As for compound 11, besides
ion peaks representing 1:1 and 2:1 complexes, a strong
CB[10] is quite complicated (Figure 4(c), (d)). The observa-
tion of multiple sets of proton signals of CB[10] at 5.5–
6.0 ppm (no proton signals of free CB[10]) and compound
9 show two binding modes. Exchange between these
1
modes is slow on the H NMR time scale. Unfortunately,
not all signals could be clearly assigned even by COSY NMR
due to the broadness and peak overlap. We assumed a
1:2 (guest:host) binding mode with two N-methyl pyridyl
encapsulated in CB[10] based on the signal of bound H8
proton at ~4.0 ppm and bound guest proton at ~9.8 ppm
(could be bound H4 proton) in Figure 4(d).
For compound 11, the maximum amount of dissolved
CB[10] was about 2.2 eq. In this case, all signals of the
guest protons shifted upfield (Figure 4(e), (f)). Considering
the maximum dissolution of CB[10] and the structure of
compound 11, the existence of a 1:3 (guest:host) binding
mode could be hypothesized. Again, the fast exchange
of binding on the 1H NMR time scale did not yield further
information. To further understand the binding behavior
of these guests with CB[10] including compound 9, CB[7]
was employed to check their binding. Upon complexa-
tion with CB[7], the signals corresponding to H2, H3 and
H5 protons on the TPY moiety of compound 6, 9, or 11
nearly remained constant. The signals for H6–8 protons on
the N-methyl pyridyl shifted upfield reflecting the two

6
Y. DENG ET AL.
ion peak at m/z: 1147.1 corresponding to the 3:1 complex
CB[10]3·11 ([3CB[10] + 11]5+ = 1147.1) was also detected
(Figure 5(c)). The ESI-MS is a powerful tool to character-
ize host–guest systems (36), but also highly related to
the determination conditions and the ratio between the
host and guest. Thus, the distribution of different binding
modes could not only use the intensity of the ion peak.
Considering the three binding sites in the metal–MTPY
complexes, we believe that in the dynamic binding pro-
cess, CB[10] prefers to bind to the middle of metal–MTPY
complexes and then bind to the two ends. It then finally
binds to three binding sites to form a 1:3 inclusion complex
when sufficient CB[10] is present. However, we also believe
that these three modes should exist concurrently depend-
ing on the amount of CB[10] in the solution. Attempts to
collect a single crystal of complex CB[10]n∙9 for structural
determination failed. This might be due to the existence
of multiple binding modes in the solution. Calculations by
MMFF tell that CB[10] could bind with the TPY–metal–TPY
core to form a 1:1 complex or with the methyl pyridine
moieties to form a 2:1 complex (Figure S8).
including catalysis and design of solar cells, we believe our
primary results would benefit these research areas.
4. Experimental section
CB[7] and CB[10] was synthesized and purified according
to the literature (11,14). Other compounds were purchased
from commercial suppliers and used without further puri-
fication. The 1H NMR spectra were recorded on an Agilent
600 MHz DD2 spectrometer, and chemical shifts were
recorded in parts per million. UV/Vis measurements were
performed on a SHIMADZU UV-3600 instrument with 1 cm
path length cells at 298 K. The ESI-MS spectrometry analy-
sis was conducted with a Thermo LTQ OrbiTrap XL instru-
ment equipped with an ESI/APCI multiprobe.
Funding
This work was financially supported by the National Natu-
ral Science Foundation of China [grant numbers 21472143,
21604066]; and the Thousand Youth Talents Program of China.
UV/Vis spectra were recorded for these metal–TPY
derivatives (complexes) as well as their inclusion com-
plexes with CB[10] in aqueous solution (Figure S9).
Compared to 3, all metal complexes showed an absorb-
ance near 300 to 600 nm related to the MLCT transition.
The different positions for λmax are due to the π*-π and
π-n transition at various energies (37). Versus the free
metal complexes, all inclusion complexes of these metal
complexes with CB[10] exhibited a slight red shift of λmax
and a decrease in the intensity. This is quite similar to the
absorption change of Ir(III) cyclometalated complexes or
metalloporphyrins upon encapsulation in CB[10] (31,38).
Although the middle metals are different in metal−TPY
complexes (4, 7 and 10) or metal−MTPY complexes
(6, 9 and 11), their binding behaviors with CB[10] are
quite similar. Differences in binding NMR (Figure 4) for
metal−MTPY complexes (6, 9 and 11) might be caused
by the charges and electropositivity of transition met-
als because the size and shape of these compounds are
nearly identical.
ORCID
Hang Yin
Simin Liu
http://orcid.org/0000-0002-2898-844X
http://orcid.org/0000-0002-8696-4833
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