Metallosupramolecules of pillar[5]-bis-trithiacrown including a mercury(ii) iodide ion-triplet complex

Article abstract of DOI:10.1039/d0cc03902k

A combination of pillar[5]-bis-trithiacrown (L) and mercury(ii) halides afforded a monomer complex (Cl-form), a 1-D coordination polymer (Br-form) and a supramolecular ion-triplet complex [(I·Hg·I)@L] (I-form). In the ion-triplet complex, the host encapsu

Full text of DOI:10.1039/d0cc03902k

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Metallosupramolecules of pillar[5]-bis-trithiacrown including a
mercury(II) iodide ion-triplet complex†
Mingyeong Shina, Sujin Seoa, In-Hyeok Parka, Eunji Leea,b, Yoichi Habatab and Shim Sung Lee*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A combination of pillar[5]-bis-trithiacrown (L) and mercury(II) supramolecular ion-triplet complex of calcium(II) ion, [Cl-
halides afforded
a a
monomer complex (Cl--form), 1-D ∙Ca2+∙Cl-@macrocycle], with a 30-membered [2+2] amide-
coordination polymer (Br--form) and a supramolecular ion-triplet macrocycle by ESI-mass in solution.5b However no further
complex [(I∙Hg∙I)@L] (I--form). In the ion-triplet complex, the host examples of the formation or isolation on supramolecular ion-
encapsulates (I--Hg2+-I-) entity via Hg2+∙∙∙π and C-H∙∙∙I- interactions, triplet complexes of metal ions both in solution and the solid
reflecting geometrical complementarity.
state have been reported so far.
In our experience, endocyclic and exocyclic coordination
modes of some thiamacrocyclic complexes are known to be
controlled by several factors that include their anion
coordination abilities.6,7 Pillar[n]arenes are a relatively new
category of macrocyclic receptors and their inclusion
complexes with organic guests such as diamines and dinitriles
to yield polyrotaxane and pseudo-rotaxane structures have
been widely studied in recent years.8
Macrocyclic hosts have played a pivotal role in supramolecular
chemistry especially because of their ability to exhibit selective
recognition of particular cation (C+)1,2 or anion (A-)2,3 guests.
The latter recognition is typically more challenging due to the
intrinsic nature of many anions including their larger sizes,
different shapes and often higher polarizabilities. A number of
hetero-di or poly-topic receptors incorporating one or more
macrocycle units have been shown to exhibit concurrent
cation and anion recognition to form inclusive or endocyclic
(guest-in-cavity) ion-pair complexes.4 Common motifs of such
ion-pair complexes involve, as shown in Fig. 1, (a) contact and
(b) solvent-separated arrangement of types [C+∙A-@receptor]
and [C+∙solvent∙A-@receptor], respectively.3 In principle, the
formation of corresponding ion-triplet complexes exhibiting [A-
∙C2+∙A-@receptor] or [C+∙A2-∙C+@receptor] arrangements
appeared possible under appropriate conditions (Fig. 1c).
In marked contrast to the big advances in pillar[n]arene
chemistry
involving
organic
guests,9
their
metallosupramolecular derivatives are rare except for works
from our10 and two other groups.11 In a prior study our
modification of a pillar[5]arene to form a pseudo[1]catenane-
type pillar[5]-mono-thiacrown showed a chiral inversion upon
mercury(II) complexation under anion-coordination control.10a
We have also prepared a di-armed pillar[5]arene-based two-
dimensional silver(I) poly-pseudo-rotaxane in which the same
dinitrile guest both threads and crosslinks.10b The Huang group
have reported ion-pair recognition of pyridinium iodide12 and
silver trifluoroacetate11b using a peralkylated pillar[5]arene. In
this work, we have shown that it is possible to isolate an ion-
triplet complex by employing pillar[5]-bis-trithiacrown (L).
(a)
(b)
(c)
Fig. 1 Schematic representation of (a) a contact ion-pair complex, (b) a
solvent-separated ion-pair complex and (c)
complex with poly-topic macrocycles.
a contact ion-triplet
Indeed, the Lüning group have shown the formation of a
In our results, the coordination modes of the mercury(II)
halide complexes of are anion-dependent. For example,
L
L
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forms a mononuclear complex and an infinite type complex via lie outside the cavity have different coordination
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DOI: 10.1039/D0CC03902K
an exo-coordination with chloride and bromide, respectively. environments. First, the terminal Hg1 atom is three-
Surprisingly, mercury(II) iodide led to the formation of an ion- coordinate, being bound to one S atom from one trithiacrown
triplet complex (Fig. 1c) as the first example isolated in the loop and two Br atoms. Second, each of Hg2, Hg3, Hg5 and
solid state to the best of our knowledge. The details are Hg6A atoms is four-coordinated by two S atoms from one
discussed below.
trithiacrown loop and two Br atoms to form a distorted
The pillar[5]-bis-trithiacrown
L
was synthesised by tetrahedral coordination geometry. The Hg4 atom is five-
bicyclisation employing pillar[5]arene tetrabromide and the coordinate, being bound to four Br atoms and one S atom to
required dithiol (yield 30%, Scheme S1, Figs. S1 and S2). The yield a three-way node. An acetonitrile molecule occupies the
tetrabromide precursor was obtained employing a mixture of cavity of each
dibromide, 1,4-dimethoxybenzene, paraformaldehyde and In , two (CH3CN@
BF3∙O(C2H5)2.10b The crystal structure of
was determined and linked by a linear Hg3-Br-Hg4-Br-Hg6A segment via Hg-S bonds
L.
2
L) units which locate up and down are
L
shown to include one n-hexane molecule in its central cavity [2.577(5)-2.763(5) Å] to form a C-shaped dimer arrangement
(Figs. S3, S4 and Table S1). Both trithiacrown loops (O-S-S-S-O) (Fig. 3b). Individual dimers are further linked via a Hg4-S9 bond
in
repulsive interaction between adjacent S donors.
Reaction of with HgCl2 in dichloromethane/methanol
afforded a colourless crystalline product . The X-ray analysis
revealed that crystallises in the monoclinic space group Cc
with Z = 4 (Tables S1 and S2). The structure features an
exocyclic mononuclear arrangement of type [HgCl2(CH2Cl2@ )]
L show t-t-t-t (t = trans) torsion arrangements reflecting the [2.734(4) Å] to give a 1-D polymeric structure.
L
1
1
L
(Fig. 2 and Fig. S5). The Hg1 atom, with a distorted tetrahedral
geometry [82.41(12)-126.39(14)°], is bonded to two S atoms
[Hg1-S2 2.611(4), Hg1-S3 2.714(3) Å], with its coordination
environment completed by two Cl atoms [Hg1-Cl1 2.363(4),
Hg1-Cl2 2.378(4) Å]. Four S donors remain uncoordinated, with
a dichloromethane molecule occupying the cavity of
L
.
(a)
In , the nonsymmetrical binding of the Hg1 atom induces
1
the large conformational change of the O1-S1-S2-S3-O2 loop
segment from t-t-t-t to g-t-g-g (g = gauche, t = trans) torsion
arrangements, remaining that of the O3-S4-S5-S6-O4 loop
segment (t-t-t-t) unchanged. Unlike oxygen-bearing crown
ethers, thiamacrocycles so often show a repulsive interaction
between adjacent sulfur donors, stabilizing a trans torsion
arrangement which tends to lead to adoption of an exo-
coordination mode. 6,7,13
(b)
Fig.
3
The exo-coordinated mercury(II) bromide complex
[Hg6Br12(CH3CN@L)2]n (2): (a) the 1-D polymeric structure (the
asymmetric unit is shown with the labelling) and (b) the connectivity
pattern showing the linkage of the C-shaped dimers (terminal Br atoms
are omitted). Symmetry operation A: -1+x, -y, -0.5+z.
When
HgI2
was
reacted
with
L
in
dichloromethane/methanol, a pale yellow product
3
was
isolated. In marked contrast to the chlorido and bromido
complexes, is an endocyclic ion-triplet complex with the
formula *I∙Hg∙I@ ] ( ) which crystallises in the monoclinic
space group P21/c with Z = 4 (Fig. 4a, Figs. S6 and S7, Tables S1
and S4). In
, surprisingly, one (I--Hg2+-I-) entity locates inside
the cavity of . Although some ion-triplets organic cation
complexes of types [(R1R2-NH2 )2∙SO4 @bis(calix[6]arene)] and
[(Cl-∙R4N+∙Cl-)@bis(calix[4]pyrrole] have been reported,14 no
solid ion-triplet metal complexes of such type have been
reported so far.
3
L
3
Fig.
2
The exo-coordinated mercury(II) chloride complex
)] ( ).
[HgCl2(CH2Cl2@
L
1
3
L
Reaction of
L
with HgBr2 in chloroform/acetonitrile yielded
which crystallizes in the monoclinic
+
2-
the colourless complex
space group Pc with Z = 2 (Table S1 and S3). This product
features an exocyclic one-dimensional (1-D) polymeric
arrangement of formula [Hg6Br12(CH3CN@L)2]n (Fig. 3a). The
2
asymmetric unit contains one formula unit. The
crystallographically independent six Hg atoms (Hg1-Hg6) that
2 | J. Name., 2012, 00, 1-3
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(a)
(b)
(c)
As mentioned above,
products with mercury(II) salts. Thus the formation of
L
prefers to form exo-coordinated of such a condition being met with six interactions present for
raises each I- on forming the ion-triplet complex. The linear (I--Hg2+-I-)
3
some questions. (i) What is the driving force to stabilize the entity in
mercury(II) ion in the centre of the cavity? (ii) How is the cavity of
3
is intimately associated with the peanut-shaped
L
giving rise to an excellent geometric and electronic
iodide ion stabilized inside the crown loop? (iii) How is the complementary via the multiple cation-π interactions and H-
liner (I--Hg2+-I-) entity formed? Each of these aspects are now bonds. Thus it not surprising how the linear arrays of HgI2 form
discussed.
because the solid structure of the yellow polymorph of HgI2, as
First, some metal ions including K+, Cs+, Ag+ and Hg2+ mentioned, is almost linear (178.3°)18b and the linear HgI2
interact with aromatic carbons via cation-π interactions, with guest from [··I-Hg-I···I-Hg-I··] columns found in the CuII/HgII-
such interactions being well established for calixarenes.15,16 MOF (MOF = metal-organic framework) is also stabilised by
With respect to this, the Hg2+ centre in
such cation∙∙∙π interactions (3.82-4.38 Å, dashed lines in Fig.
4b) to form a pentagonal planar array. Additionally, two I- ions chloroform/methanol, the formation constants (log K) for the
occupy axial sites to yield an overall pentagonal bipyramidal complexations of with mercury(II) halides were obtained
geometry [Hg1-I1 2.5642(6), Hg1-I2 2.5555(6) Å, I1-Hg1-I2 (Table 1 and Figs. S8-S10). The log K values (for 1:1 complexes)
3
is stabilized by five weak coordination of four iodide ions in the 3-D framework.21
Based on the UV-vis titration experiments in
L
178.30(2)°].
tend to be smaller for larger anions because the larger halide
Second, the larger size and softer nature of the I- ion may ions as a softer base can bind to mercury(II) centre (soft acid)
play an advantageous role in the confined space. When large more tightly hence reduce the complex stability. The log K
numbers of weak H-bonds act cooperatively, quite often they value for the iodide system is, however, larger than expected
play an important role in stabilizing supramolecular structures. one because the extra stabilization of the ion-triplet via the
Thus, each I- ion in
3
is found to interact with five H atom from supramolecular interactions probably compensate such effect
different O-CH2 and O-CH3 groups via C-H···I- H-bonds,17 with in part. In the bromide system, the ligand
binds to two
the distances and angles being 3.2132(0)-3.5277(0) Å and mercury(II) ions via two steps but the 1:1 (M ) complex is
more stable than the 2:1 complex (M2L) which is formed above
L
L
133.193(2)-157.987(2)° (Fig. 4c, Table S4).
Third is the question of how the linear (I--Hg2+-I-) entity the 1 equivalent of HgBr2.
might form. Mercury(II) iodide is known to crystallize in several
Table 1. Formation constants (log K) for the complexations of L with
polymorphs including linear, tetrahedral and tetragonal
structures depending on the preparation process.18 In
principle, the interaction of solvent-separated ion-pair is
primarily electrostatic, but mercury(II) iodide is quite soluble in
common organic solvents including methanol19 and is present
partially as non-dissociated ion-triplet due to the covalent
character.20 As mentioned, each I- ion is bound to a bridging μ-
Hg2+ centre to yield a pentagonal pyramidal geometry (Fig. 4c).
Since synthetic hosts for halide ions favour two to six binding
interactions,2,3 each I- ion in Fig. 4c represent a good example
mercury(II) halides in chloroform/methanol (1:2.5, v/v) at 298 Ka
reactions
products
formation constants
HgCl2 + L
Hg(L)Cl2
log K11
5.94 ± 0.003
HgBr2 + L
2 HgBr2 + L
Hg(L)Br2
Hg2(L)Br4
log K11
log K21
4.46 ± 0.006
8.18 ± 0.002
HgI2 + L
Hg(L)I2
log K11
5.10 ± 0.003
aUV-vis titration method using the HypSpec software.22
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8. (a) T. Ogoshi, S. Kanai, S. Fujinami, T.-A. Yamagishi and Y.
In summary, we report here the isolation of the first solid
state example of a supramolecular ion-triplet complex
incorporating a hetero-tritopic macrocycle. To achieve this, the
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Nakamoto, J. Am. Chem. Soc., 2008, 130, 5022-5023; (b) X. Shu, S.
DOI: 10.1039/D0CC03902K
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pillar[5]-bis-trithiacrown
L was synthesised and its anion-
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dependent mercury(II) halide complexes exhibiting different
topological arrangements were isolated. In the chlorido and
bromido complexes, exo-coordination via Hg-S bonds occurs,
with the corresponding anions binding only to the cation (in
different mutual configurations). In the iodido complex,
however, the mode of interaction of this anion is totally
different; it not only interacts with the metal cation but also
with the ligand
L to form the unique contact ion-triplet
complex. Consequently, the formation of the ion-triplet
complex reflects that the multiple Hg2+∙∙∙π interactions
stabilize the overall structure, with the concerted C-H∙∙∙I- H-
bonds being geometrically and electronically well matched to
satisfy the mutual complementarity required for binding in the
confined central space in L. A further investigation of the
potential application of triplet complexes of the present type
for the detection and removal of toxic heavy metal species is in
progress.
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This work was supported National Research Foundation
(NRF)
of
South
Korea
(2017R1A4A1014595
and
2019R1A2C1002075). We are thankful to Prof. Naoki Hirayama
(Toho University) for the helpful discussions on fitting data.
Conflicts of interest
There are no conflicts to declare.
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Graphical abstract
In an extension of supramolecular ion-pair complexes, first isolation of a supramolecular ion-triplet complex in the
solid state is reported. A combination of pillar[5]-bis-trithiacrown and HgI2 afforded a contact ion-triplet complex
in which the host encapsulates the (I--Hg2+-I-) entity with the supramolecular interactions that include Hg2+∙∙∙π and
C-H∙∙∙I- H-bond interactions, reflecting geometrical complementarity.
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