Self-assembly and steric hindrance for further host–guest interactions of a tetrahedral cage FeII4L4

Article abstract of DOI:10.1007/s11696-021-01685-w

In this paper, a novel complex [Fe^II₄L₄]⁸⁺ was designed and synthesized from subcomponent self-assembly of C₃-symmetric 2,4,6-tris(4-aminophenoxy)triazine, 3-methylpyridine-2-carboxaldehyde and iron(II) trifluoromethanesulfonate in CH₃CN solution. The structure of target compound was confirmed by ¹H NMR and ESI-MS measurements as a tetrahedral cage, and the host–guest interactions between the complex and a series of guest molecules were studied. The results show that the intermolecular interactions are relatively weak by comparison with other similar ligand’s complexes, which may be due to the steric effect of methyl groups adjacent to the peripheral apertures of the cage.

Full text of DOI:10.1007/s11696-021-01685-w

Chemical Papers
https://doi.org/10.1007/s11696-021-01685-w
ORIGINAL PAPER
Self‑assembly and steric hindrance for further host–guest interactions
of a tetrahedral cage FeII L
4 4
Ning Bu1 · Wen‑Yuan Wu1 · Peng Jiang1 · Zi‑Yuan Zhan1 · Jin‑Long Wan1 · Zi‑Jing Wu1 · Rong Wan1
Received: 17 January 2021 / Accepted: 27 April 2021
© Institute of Chemistry, Slovak Academy of Sciences 2021
Abstract
In this paper, a novel complex [FeII L4]8+ was designed and synthesized from subcomponent self-assembly of C3-symmetric
4
2,4,6-tris(4-aminophenoxy)triazine, 3-methylpyridine-2-carboxaldehyde and iron(II) trifuoromethanesulfonate in CH3CN
solution. The structure of target compound was confrmed by 1H NMR and ESI-MS measurements as a tetrahedral cage,
and the host–guest interactions between the complex and a series of guest molecules were studied. The results show that the
intermolecular interactions are relatively weak by comparison with other similar ligand’s complexes, which may be due to
the steric efect of methyl groups adjacent to the peripheral apertures of the cage.
Keywords Cage complex · Self-assembly · Iron(II) · Host–guest interaction
Introduction
elaborate architectures could be amazingly prepared from
simple building blocks through the formation of dynamic-
covalent (C=N) and coordinative (N→M) bonds during the
subcomponent self-assembly process (Ronson et al. 2013;
Castilla et al. 2014; Zarra et al. 2015).
Supramolecular compounds have been a topic of great inter-
est in the past several decades due to their diverse character-
istics (Cram 1992; Liu et al. 2006; Du et al. 2008; Breniner
et al. 2011; Chakrabarty et al. 2011; Schenider et al. 2012).
One class of these molecules, hollow cage complexes, has
been widely studied for their appealing host–guest properties
(Furutani et al. 2009; Gupta et al. 2012). The construction
of these complexes is involved in specifc geometric shape,
three-dimensional confguration and excellent symmetry
(Hasenknopf et al. 1996; Caulder and Raymond 1999a;
Chichak et al. 2004; Fujita et al. 2005; Bonnet et al. 2006;
Sun et al. 2010). These well-defned cage complexes allow
for diverse applications, such as gas storage (Riddell et al.
2011), guests separation (Brenner et al. 2017), organic catal-
ysis (Murase et al. 2010; Neelakandan et al. 2015), molecu-
lar recognition (Nakabayashi et al. 2008; Clever et al. 2009;
Ariga et al. 2012), unstable species protection (Yoshizawa
et al. 2005; Horiuchi et al. 2011), fuorescent sensor (Li et al.
2012) and molecular magnet switching (Li et al. 2019). Such
Triazine-based ligands featuring C3-symmetry and other
attractive characteristics also have the advantages of fex-
ible synthetic routes, which provide convenience for design-
ing diverse metal–organic constructions, including discrete
complexes and framework structures (Yu et al. 2021). In our
previous paper (Fu et al. 2018), we reported the development
of a helicate complex [FeII L3]4+ through the self-assembly
2
of 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline, octahedral
iron(II) and 2-formyl pyridine (Scheme 1), and found that
it could bind electron-rich guest molecules by aromatic π-π
interactions.
Herein, we designed and synthesized a novel complex
by modifying the subcomponent triazine-trianiline via the
insertion of an ether group between triazine and aniline
structure (Scheme 2). This modifcation could improve the
fexibility of the ligand for the reason of the free rotation
ability of O–C single bond, which allows the ligand more
versatile to develop even more complicated metal–organic
structures. Then the complex formed by self-assembly was
confrmed by NMR and ESI-MS to be a much larger cage
structure other than the helicate (Scheme 3). The host–guest
relationships between the complex and selected guest mol-
ecules were investigated.
* Wen-Yuan Wu
wwy@njtech.edu.cn
* Rong Wan
rwan@njtech.edu.cn
1
College of Chemistry and Molecular Engineering, Nanjing
Tech University, Nanjing, People’s Republic of China
Vol.:(0123456789)
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Chemical Papers
Scheme 1 Self-assembly of the
helicate 1
Scheme 2 Synthesis of sub-
component triamine
Scheme 3 Self-assembly of
tetrahedral cage 1
Synthesis of the subcomponent triamine
Experimental
To obtain the subcomponent triamine 2,4,6-tri(4-aminophe-
noxy)triazine, two steps reactions are needed (Scheme 2).
First, to a stirred solution of cyanuric chloride (1.5 g,
8.2 mmol) in acetone (100 mL) was added slowly a solution
of p-nitrophenol (3.00 g, 25.2 mmol) and NaOH (1.00 g,
25.2 mmol) in H2O (100 mL) and acetone (20 mL) with gen-
tle cooling. The combined solution was heated and stirred
under refux for 2 h under 60 ℃. The crude product was
Materials and apparatus
All reagents and solvents were purchased from commercial
1
sources and used as supplied except otherwise stated. H
NMR spectra were recorded on a Bruker Avance DPX400
MHz spectrometer. High-resolution electrospray ioniza-
tion mass spectrometry (HRMS-ESI) was performed on a
Thermo Fisher Scientifc Q Exactive device.
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Chemical Papers
Self‑assembly of [FeII4L4]8+ complex 1
collected by fltration, washed with H2O (20 mL) and MeOH
(20 mL) for three times, and dried under vacuum to give
2,4,6-tris(4-nitrophenoxy)triazine as white crystalline solid.
Then, to a 250-mL flask, 1.78 g (3.7 mmol) of
2,4,6-tris(4-nitrophenoxy)triazine, 3 g (55 mmol) of ammo-
nium chloride, 60 mL of ethanol and 20 mL of water were
added. The mixture was stirred and heated to 80 ℃. Reduced
iron powder 3 g (55 mmol) was added and the reaction was
continued under refuxed for 2 h. Then the reaction mixture
was fltered and the flter cake was washed with hot ethanol
10 mL for twice. The combined fltrate was adjusted to pH
9 with sodium bicarbonate. The crude product was precipi-
tated, fltered and recrystallized from methanol to give pale
yellow 2,4,6-tri(4-aminophenoxy)triazine.
To a stirred solution of 2,4,6-tris(4-aminophenoxy)tria-
zine (25.0 mg, 0.062 mmol) in 10 mL CH3CN was added
3-methylpyridine 2-formaldehyde (21.9 mg, 0.186 mmol)
and iron(II) trifuoromethanesulfonate (Fe(OTf)2, 22.0 mg,
0.062 mmol)(Scheme 3). The mixture was stirred under
30 ℃ for 12 h to give a dark red solution. Then the solu-
tion was cooled to room temperature and 10 mL Et2O was
added. The compound of 1 was precipitated and collected
by fltration to give a purple red solid. The crude product
was purifed by recrystallization in acetonitrile.
[FeII4L4]8+ complex: purple red solid; yield 58.3%;
1
m.p. > 300 ℃; H NMR (400 MHz, CD3CN): δ 9.00
2,4,6-tris(4-nitrophenoxy)triazine: White crystalline
solid; yield 91%; m.p. 208–212 ℃; IR (KBr, ν/cm−1):
1614.40, 1567.46, 1487.24, 1374.32, 1346.28, 1527.10;
1H NMR (400 MHz, CDCl3): δ 8.33 (d, J = 8.76 Hz, 6H,
benzene-H), 7.37 (d, J=9.0 Hz, 6H, benzene-H); 13C NMR
(400 MHz, CDCl3):173.07, 162.56, 145.91, 125.55, 122.39.
2,4,6-tri(4-aminophenoxy)triazine: Pale yellow solid;
yield 68%; m.p. 225–228 ℃; IR (KBr, ν/cm−1): 3341.45,
1614.44, 1510.22, 1475.14, 1386.27; 1H NMR (400 MHz,
CDCl3): δ 6.94 (d, J = 8.76 Hz, 6H, benzene-H), 6.65 (d,
J=8.72 Hz, 6H, benzene-H), 3.70 (s, 6H, −NH2); 13C NMR
(400 MHz, (CD3)2SO): 174.09, 146.88, 142.24, 122.07,
114.64.
(s, 3He), 8.11 (d, J = 8.5 Hz, 3Hb), 7.59 (t, J = 12.9 Hz,
3Hc), 7.45 (d, J = 7.5 Hz, 3Hh), 7.18 (d, J = 7.1 Hz, 3Hi),
7.01 (d, J = 5.0 Hz, 3Hd), 5.79 (d, J = 9.5 Hz, 3Hg), 5.18
(d, J = 8.6 Hz, 3Hf), 2.79 (s, 3H, 9Ha); Anal Calcd for
C176H132Fe4N36O36S8F24: C, 49.59; H, 3.12; N, 11.83;
found: C, 49.08; H, 3.08; N, 12.01; ESI-MS of [1·OTf]7+
for C169H132Fe4N36O15SF3: m/z 459.83; ESI-MS of
[1·2OTf]6+ for C170H132Fe4N36O18S2F6: m/z 561.45; ESI-
MS of [1·3OTf]5+ for C171H132Fe4N36O21S3F9: m/z 703.54.
Fig. 1 1H NMR spectrum of
cage 1 in CD3CN at 293 K
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Chemical Papers
[FeIIL·OTf]+
Fig. 2 ESI-MS peaks relative to
cage 1 in CH3CN solution
Abundance
916.15
[FeII4L4·OTf]7+
459.83
[FeII4L4·2OTf]6+
561.45
[FeII4L4·3OTf]5+
703.54
m/z
500
1000
1500
achieved yet, the MM2 force feld model (CAChe Work
System Pro 2000–2006) based on the crystal structures of
a similar Fe4L4 cage series (Ferguson et al. 2014), where
diferent C3-symmetric triamines and formaldehyde have
been applied, was optimized and used to discuss the struc-
ture of our complex.
Through the simulation structure of cage 1 (Fig. 3), there
are four identical C3-symmetrical Schif base ligands cap-
ping the four faces of the tetrahedral cage, coordinated by
four octahedral Fe(II) ions at the four vertexes of the tetrahe-
dron. Compared with the structure of our formerly reported
Fe2L3 helicate (Fu et al. 2018), the ether groups added in
the ligand not only enlarge the Fe(II)–Fe(II) distance from
11.9 Å in helicate to 14.21–15.69 Å in cage 1, but also suc-
ceed in the preferred cage structure. The single σ bond on
both sides of ether group can rotate freely, greatly increas-
ing the spatial freedom of terminal coordination sites in the
ligand, which fully meets the coordination confguration of
the metal ions during the self-assembly process. Finally the
novel tetrahedral cage structure is achieved rather than the
much simpler helicate. The volume of the central cavity is
calculated to be about 185 Å3 by using VOIDOO program
(Kleywegt et al. 1994, Caulder and Raymond 1999b).
The inner core of the complex has the cavity, while along
the edges of the tetrahedral cage, there also exist peripheral
apertures. Inner cavity and peripheral apertures surely make
this tetrahedral cage become adventure land for guest mol-
ecules to enter and exit.
Fig. 3 MM2 model of tetrahedral cage 1
Results and discussion
Structural features of the tetrahedral cage
1
The tetrahedral cage 1 was confrmed by H NMR and
ESI-MS measurements (Figs. 1, 2). The only one set sin-
gles of ligand refects high-symmetrical arrangement of
cage 1 and ESI-MS spectrum clearly gives the peaks of
the [FeII L4·n(OTf)]8−n (n = 1 ~ 3) series. Also the peak
4
1H NMR studies of host–guest chemistry
of m/z = 916.15 could be assigned to the [FeIIL·OTf]+
fragment of cage structure during the ionization process.
The isotopic peaks analysis is presented in Fig.S6. As the
crystal of 1 suitable for X-ray determination has not been
Six various aromatic guests (Scheme 4) of high symmetry
were selected to explore the host–guest 1H NMR investiga-
tions. The host cage 1 (0.01 mmol, 1 eq) was dissolved in
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Chemical Papers
OH
planer neutral aromatic guests all failed in signifcant feld
shift in the host 1H signals (Table 1). It could be explained
why the π-π stacking could hardly occur in cage 1, for
the insertion of ether group breaks the conjugation system
between the triazine core and outstretching phenyl rings.
Thus the π-π stacking interactions between host and guest
in the formerly reported helicate vanished in the cage 1
system.
HO
OH
naphthalene-1,5-diol
pyren-1-ol
pyrene
-
OH
Na+
B
The only guest exhibiting moderate host–guest interac-
tions is sodium tetraphenylboron (NaBPh4). The tetrap-
henylborate is the anion and has extra electronic density
while the tetrahedral shape could ft the grooves of similar
C3-symmetric ligands in our formerly reported Zn3L2 tri-
angular double helicate 2 (Wu et al. 2018) and then ofers
the electronic perturbation to nearby hydrogens of the host
via C–H…π interactions.
HO
OH
benzene-1,3,5-triol
phenanthrene
sodium tetraphenylboron
Scheme 4 Guest molecules used in host–guest investigation
1 mL CD3CN and then one of the chosen guest molecules
(0.1 mmol, 10 eq) was added individually. After stirring
reaction (at least for 2 h) over, the whole system was ana-
lyzed by 1H NMR to obtain the data of host–guest interac-
tion (Figs. S7–S18).
1
As seen in Fig. 4a, the most up feld shifts of H sig-
nals (Ha and Hc) are from the outward pyridine ring, not
involved in the inner phenyl part of the ligand in 1 (Hf, Hg,
Hh and Hi). While in Fig. 4b, the most signifcant changes
of 1H signals (Hg′ and He′) occur to the groove between the
outstretched arms of the ligand in 2. Also the maximum
shifts (Δδ = − 0.18 ~ − 0.20 ppm) in 2 are also larger than
those in 1 (Δδ= − 0.08 ppm), which suggests the stronger
binding of guests around the groove part of host.
From the 1H NMR spectra, it’s observed that with difer-
ent guest molecules adding, the proton signals of the host
and guest molecules have more or less changed. The signals
attributed to the excessive guest molecules were kept in one
set and had not many change compared with the 1H NMR of
free guests (Δδ< 0.02 ppm), which declared the rapidly
exchange mode between free and possible bound states of
guests on the NMR time scale (Ronson et al. 2014).
Based on former knowledge, the host NMR signals’ up
feld shifts are connected with the increasing of electron
density in the ligand’s conjugated system which could owe
to supramolecular interactions such as π–π stacking with
the electron-rich guests (Rizzuto et al. 2016). Meanwhile,
after adding pyrene molecules into the host solution, the
most up feld shift (Δδ = -0.01 ppm) is very slight, which
suggests the weakness of π-π stacking interactions. Other
This can be explained with steric hindrance of the
methyl groups attached in the pyridine ring of 1, which
interferes the complementary arrangement of tetraphe-
nylborate anions with peripheral apertures formed by the
ligand’s grooves along the edges of the tetrahedral cage.
So the tetraphenylborate guest could only have chance
to approach the vertexes of the cage structure, and uses
its electron-rich phenyl rings as the acceptors of relative
week C–H…π interactions, while H atoms (Ha, Hb, Hc and
He) from pyridine ring coordinated with positive-charged
metal center act as donors.
Table 1 Summary changes of chemical shift value of protons in cage ligand during the host–guest interaction
Host−only
+Pyrene
+Naphthalene-1,5-diol
+Pyren-1-ol
+NaBPh4
+Phenanthrene
+Benzene-1,3,5-triol
Ha
Hb
Hc
Hd
He
Hf
Hg
Hh
Hi
2.79
8.11
7.59
7.01
9.00
5.18
5.79
7.45
7.18
2.78 (−0.01)
8.10 (−0.01)
7.58 (−0.01)
7.01 (0)
8.99 (−0.01)
5.19 (0.01)
5.78 (−0.01)
7.46 (0.01)
7.19 (0.01)
2.79 (0)
2.78 (−0.01)
Covered**
Covered**
7.01 (0)
8.99 (−0.01)
5.19 (0.01)
5.78 (−0.01)
7.45 (0)
2.71 (−0.08)
8.06 (−0.05)
7.51 (−0.08)
Covered**
8.94 (−0.06)
5.18 (0)
5.75 (−0.04)
7.45 (0)
7.13 (−0.05)
2.78 (−0.01)
8.11(0)
7.58 (−0.01)
7.01(0)
8.99 (−0.01)
5.19(0.01)
5.78 (−0.01)
7.45 (0)
2.78 (−0.01)
8.11 (0)
7.58 (−0.01)
Covered**
9.00 (0)
5.19 (0.01)
5.78 (−0.01)
7.44 (−0.01)
7.18 (0)
8.10 (−0.01)
7.57 (−0.02)
Covered**
9.00 (0)
5.18 (0)
5.79 (0)
7.44 (−0.01)
7.17 (−0.01)
7.18 (0)
7.18 (0)
*All signals were calibrated base on the CD3CN solution peak δ=1.94 ppm
**Covered=signals covered by the peaks of guest, cannot be recognized clearly
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Chemical Papers
1
by H NMR and ESI-MS measurements. The host–guest
investigations by tracing shifts of host 1H NMR signals show
that non-planar tetraphenylborate anion has moderate infu-
ence on the host complex than the planar aromatic guests
due to the extra electronic density. And the methyl groups
attached in the pyridine ring hinder the relative larger tet-
raphenylborate anion from further interactions within the
peripheral apertures of the cage.
N
(a)
G
N
CH3
N
r
o
o
v
e
CH3
O
N
N
N
Groove
h
0
O
O
f
0
Supplementary Information The online version contains supplemen-
i
g
-0.05
-0.04
tary material available at https://doi.org/10.1007/s11696-021-01685-w.
Acknowledgements This work has been funded by the College Stu-
dents’ Innovation and Open Experimentation Fund Project, NJTECH
(2021DC0472 & 2021DC0474).
N
N
e
-0.06
a H C
0.08
3
Declarations
covered d
b
-0.05
Conflict of interest The authors declare that they have no confict of
c
interest.
-0.08
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