Group 12 Metal Complexes of an 18-Membered N2O2S2 Macrocycle Incorporating Two Pyridines: First Examples of an Infinite Mercury(I) Complex and a Dumbbell-Shaped Heteronuclear Complex with a Macrocyclic Ligand

Article abstract of DOI:10.1021/acs.inorgchem.6b01583

Homo- and heteronuclear group 12 metal (Zn²⁺, Cd²⁺, and Hg²⁺) complexes 1-6 containing a newly designed 18-membered N₂O₂S₂ macrocycle incorporating two pyridine subunits (L) were prepared a

Full text of DOI:10.1021/acs.inorgchem.6b01583

Article
pubs.acs.org/IC
Group 12 Metal Complexes of an 18-Membered N2O2S2 Macrocycle
Incorporating Two Pyridines: First Examples of an Infinite Mercury(I)
Complex and a Dumbbell-Shaped Heteronuclear Complex with a
Macrocyclic Ligand
Sujin Seo, Huiyeong Ju, Seulgi Kim, In-Hyeok Park, Eunji Lee,* and Shim Sung Lee*
Department of Chemistry and Research Institute of Natural Sciences, Gyeongsang National University, Jinju 52828, South Korea
S
* Supporting Information
ABSTRACT: Homo- and heteronuclear group 12 metal
(Zn2+, Cd2+, and Hg2+) complexes 1−6 containing a newly
designed 18-membered N2O2S2 macrocycle incorporating two
pyridine subunits (L) were prepared and structurally
characterized. The individual complexes isolated exhibit
unusual stoichiometries, geometries, oxidation states, and
structural topologies and include an infinite mercurous
complex and a heteronuclear dumbbell-shaped complex.
Both the Zn(II) complex [Zn(L)][ZnBr4] (1) and the Cd(II)
complex [Cd(L)Br2] (2) are mononuclear, with the metal ion
located inside the macrocyclic cavity. The six-coordinated
Zn(II) center in 1 adopts an octahedral geometry and is
shielded from the anion and solvent by the strongly bound
macrocycle. The Cd(II) center in 2 is seven-coordinate, being bound equatorially to two N donors, two O donors, and a S donor
from the macrocycle and axially to two bromide ions on opposite sides of the macrocyclic plane, adopting a pentagonal-
bipyramidal geometry. In the Hg(II) complexations, the configuration adopted by the macrocycle L shows a dependence of the
nuclearity on the anion used. When mercury(II) bromide was used, the dinuclear complex [HgII (L)Br4] (3) was obtained, while
2
the reaction with mercury(II) nitrate afforded the unexpected Hg(I) complex {[HgI2(L)](NO3)2}n (4) with a one-dimensional
polymeric structure. In heterometallic complexation experiments, one-pot reaction of L with a mixture of ZnBr2 and CdBr2·4H2O
resulted in the stepwise isolation of two pure solubility-dependent Cd(II) complexes (2 and 5), including the half-dumbbell-type
complex [Cd(L)(μ-Br)(CdBr3)] (5), while a mixture of CdBr2·4H2O and HgBr2 yielded the heterometallic bis(macrocycle)
product [(CdL)2(μ-Hg2Br6)](Hg2Br6) (6). This is the first example of a heteronuclear dumbbell-shaped complex in which two
terminal macrocyclic Cd(II) complexes are linked by a hexabromodimercury(II) cluster via Cd−Br−Hg bonds. The
heterometallic dumbbell 6 can be considered as a good example of competition and collaboration between Cd(II) and Hg(II) ions
because its formation is associated with the higher coordination affinity of Cd(II) toward the macrocycle and the formation of the
(Hg2Br6)2− cluster, which links the two endocyclic Cd(II) complexes directly. Both NMR titration and comparative NMR data
indicate a relatively higher coordination affinity of Cd(II) toward the macrocycle than occurs for Hg(II), in parallel to the
situation observed in the solid state.
have been reported by both us7 and other groups.8 For
INTRODUCTION
■
example, a series of dumbbells of type [(LAg)−L′−(AgL)] in
Macrocyclic complexes with a wide range of structures have
been reported over the last four decades.1 The structural
diversity of such complexes often arises from connection of the
macrocyclic building blocks via metal centers or coligands using
a variety of bridging arrangements.2−6 As a result, discrete
macrocyclic complexes involve not only 1:1 (metal:ligand)
complexes with typical endocyclic complexation but also
extended oligomeric analogues such as sandwich,2 club
sandwich,3 double decker,4 cyclic oligomer,5 and dumbbell6−8
shaped structures displaying unusual stoichiometries.
which two endocyclic silver(I)−L complex units are inter-
connected by organic coligands of type L′ [L′ = 1,4-
diazabicyclo[2.2.2]octane (dabco), 4,4-bipyridine (bpy), or
1,4-bis(4-pyridyl)piperazine (bpp)] have been described.7a
A
bpy-linked diruthenium(II) dumbbell complex, [{Ru(12S4)-
Cl}2(μ-bpy)] (12S4 = 1,4,7,10-tetrathiacyclododecane), has
also been reported by the Grant group.8a
The divalent ions of group 12 metals (Zn, Cd, and Hg) are
d
10 systems that cover coordination numbers ranging from 2 to
The formation of dumbbells has been achieved by employing
two endocyclic complex units and one linker unit (Chart 1).6−8
Several macrocyclic dumbbells linked with organic coligands
Received: July 2, 2016
© XXXX American Chemical Society
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Chart 1. Dumbbell-Shaped Macrocyclic Complexes Linked
with (a) Organic Coligands, (b) Homonuclear Clusters, and
(c) Heteronuclear Clusters
8 with linear to dodecahedral geometries. Unlike its lighter
group 12 neighbors zinc and cadmium, mercury commonly
exists as a divalent ion but can also exist as the monovalent ion
Hg22+, which is a dimer via a Hg−Hg bond (2.5 Å). Generally,
the stability of Hg(I) and the formation of its complexes might
be associated with disproportionation to give Hg(0) and
Hg(II). In most mercury complexes, however, the metal centers
have a +2 oxidation state because of the lower reactivity and
associated with its two pyridine domains: the NS2 donor set
approximates a soft-base domain, while the NO2 donor set
approximates a hard-base domain. For both donor sets, the
presence of the pyridine N donor is expected to locate inside
the cavity. Resulting from our interest in ditopic mixed-donor
macrocycles, herein we report the synthesis and structural
aspects of L bearing two pyridine N donors as well as two S and
two O donors, together with its complexation behavior toward
group 12 metal ions. In view of the binding affinity to the metal
ions with respect to their soft or semisoft base nature, Br− was
selected as a counteranion. In the case of Hg(II) complexations,
2+
poor purity of the Hg(I) complexes. Only four Hg2
complexes of macrocycles have been structurally characterized
to date,9−12 and thus, the coordination chemistry of Hg(I)
complexes with macrocyclic ligands has not been extensively
explored.
both Br− and NO3 were employed.
−
In our previous work, we found that homonuclear Hg(II)6a
and Cd(II)6b dumbbells linked by corresponding halide clusters
(see Chart 1b) can be formed in one pot-reactions. Our
continuing interest in new types of oligomeric macrocyclic
complexes prompted us to investigate the possibility of
fabricating dumbbell-shaped heteronuclear complexes in
which one metal ion (M) is located inside each macrocyclic
cavity and a cluster involving another metal ion (M′) links the
two terminal macrocyclic complex units (see Chart 1c).
Despite their spectroscopically silent nature, the complex-
ation of group 12 metal ions with new ligands has been a
popular area of research because of potential biochemical and
environmental applications. In general, the coordination
chemistry of group 12 metal ions has been conducted using a
wide variety of ligand types incorporating N, S, and/or O
donors.13 In particular, there has been considerable effort
directed toward investigating the interaction of group 12 metal
ions with various mixed-donor organic ligands, often
incorporating pyridyl nitrogen(s) as coordination site(s). The
incorporation of versatile pyridine coordination sites into an
oxathia macrocycle scaffold may have beneficial effects with
respect to coordination mode and stability toward particular
metal ions. For example, (i) the endocyclic coordination mode
may be favored, (ii) the complex stability may be increased, and
(iii) the complexes formed might act as metalloligands for the
preparation of oligomeric species.
We prepared six complexes (1−6) employing the ditopic
macrocycle L, including one heteronuclear species. As
mentioned above, an exciting feature of the results is the
formation of a dumbbell-shaped heteronuclear complex. A
further intriguing result is the isolation of a Hg(I) complex that
shows an infinite structure and forms via Hg(II) reduction. To
the best of our knowledge, the observed formation of a
dumbbell-shaped heteronuclear complex and a polymeric
Hg(I) complex has not been reported previously. Related
solution studies including 1H NMR titrations are also reported.
RESULTS AND DISCUSSION
■
Synthesis of the N2O2S2 Macrocycle L. The bimolecular
cyclization of dialcohol−ditosylate coupling enables the syn-
thesis of macrocycles via C−O bond formation.14a,b Thus, the
target macrocycle L was synthesized by a coupling reaction
between ditosylate 8 and dialcohol 9 in the presence of sodium
hydride in anhydrous tetrahydrofuran (THF) (20% yield;
Scheme 1). Compounds 8 and 9 were prepared using known
procedures.14c,d The NMR spectra of L (Figures S1 and S2) are
in accord with its predicted structure.
Zn(II) and Cd(II) Complexes of L (1, M = Zn; 2, M = Cd).
Both a zinc(II) bromide complex, 1, and a cadmium(II)
bromide complex, 2, were obtained by reaction of the
corresponding metal bromides with L in methanol/dichloro-
methane at room temperature. The yields of the crystalline
products were 50−60%. Single-crystal X-ray diffraction (SC-
XRD) analysis revealed that both products are mononuclear,
In this work, we report the synthesis of a ditopic 18-
membered macrocycle L that provides two different donor sets
Scheme 1. Synthesis of L
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with the metal ion located inside the macrocyclic cavity (Figure
1). The Zn(II) complex 1 (Figure 1a) crystallizes in the
over Cd1−N1 [2.359(2) Å] is observed. The results suggest
that hard/soft acid/base (HSAB) considerations with respect to
complex formation might be overridden by steric or other
geometrical factors associated with complex formation in this
case.
Anion-Dependent Mercury Complexes (3 and 4). In
the investigation of Hg(II) complexation with L, two salts
(bromide and nitrate) were used to examine the effect of the
anion on the coodination behavior. As shown in Scheme 2, the
Scheme 2. Mercury Complexes with Different Anions
Figure 1. Crystal structures of (a) the zinc(II) bromide complex
[Zn(L)][ZnBr4] (1) showing a distorted octahedral geometry and (b)
the cadmium(II) bromide complex [Cd(L)Br2] (2) showing a
distorted pentagonal-bipyramidal geometry (also see Figures S3 and
S4). The [ZnBr4]2− anion in 1 has been omitted.
monoclinic space group P21/n with the formula [Zn(L)]-
[ZnBr4]; the Cd(II) complex 2 (Figure 1b) also crystallizes in
the monoclinic space group P21/n but has the formula
[Cd(L)Br2]. Even though the two complexes share some
common features, it is still of interest to compare their
structures.
anion dependence is reflected not only in the observed
topologies but also in the oxidation states of the metal centers.
For example, bromide forms the anion-coordinated discrete
dinuclear Hg(II) complex 3, while nitrate results in the
formation of the unprecended one-dimensional (1D) Hg(I)
(Hg22+) coordination polymer 4.
With respect to their coordination geometries, the structural
differences between 1 and 2 mainly reflect the effect of anion
coordination on the conformational arrangement of L. In 1, for
example, the Zn(II) center is six-coordinate, being bound to all
of the donors in the macrocycle, which adopts a bent and
twisted conformation. The coordination geometry in 1 can be
described as a distorted octahedron, with the two O and two S
donors from the macrocycle defining the square plane and the
axial positions occupied by the two pyridine nitrogen atoms
[∠N1−Zn1−N2 = 169.4(2)°]. Hence, the metal center in 1 is
effectively shielded from the anion and solvent by the bound
macrocycle. This preferred anion/solvent-noncoordinated
structure in 1 is in keeping with the coordination abilities of
the anion and solvent being weaker than that of the macrocycle
in this case. In contrast, the Cd(II) center in 2 is seven-
coordinate, being bound to two pyridine nitrogen donors, two
oxygen donors, and one sulfur donor from the macrocycle as
well as two bromide ions on opposite sides of the macrocyclic
plane. Thus, the coordination geometry can be described as a
distorted pentagonal bipyramid with the five donors of L
defining the pentagonal plane and the axial positions occupied
by the two bromide ions [∠Br1−Cd1−Br2 = 173.3(2)°].
Unlike 1, in 2 one exo-oriented sulfur donor (S2) remains
uncoordinated, with a Cd1···S2 distance of 4.58 Å. As Cd(II) is
a soft acid and sulfur is a soft base, it was expected that both
sulfur donors (S1 and S2) would bind to the Cd(II) center.
This apparently anomalous behavior may be due to the
preferred formation of the seven-coordinate geometry and the
stronger coordination ability of the Br− ion toward the Cd(II)
center, leading to a puckered conformation of the macrocycle
with one sulfur donor uncoordinated. As predicted, the two
pyridine nitrogen atoms in 1 bind strongly to the metal center
[Zn1−N1 2.073(1), Zn1−N2 2.115(1)]. In 2, however, an
unexpected longer bond distance for Cd1−N2 [2.564(2) Å]
When mercury(II) bromide was reacted with L in
acetonitrile/dichloromethane, a colorless precipitate was
isolated. The precipitate was dissolved in dimethylformamide
(DMF), and vapor diffusion of diethyl ether into this solution
afforded crystalline 3. The X-ray crystal structure of 3 shows an
interesting dinuclear complex of type [Hg2(L)Br4] (Figure 2a).
Since there is an imposed inversion center present, the
asymmetric unit contains one Hg(II) atom, half a molecule
of L, and two bromide ions. Each Hg(II) center above or below
the macrocyclic cavity is five-coordinate, being bound to one
pyridine N atom and two O or two S donors in compositional
disorder (50:50). The coordination sphere is completed by two
bromide ions and adopts a distorted square-pyramidal
geometry (Figure 2b), with the three donors from L and Br1
forming the square plane while Br2 occupies the apex position.
The separation of the two aromatic rings in adjacent
macrocycles is inside the range expected for π−π stacking
interactions (centroid-to-centroid distance = 4.00 Å; Figure 2c).
Typically, macrocyclic ligands surround one metal ion inside
the cavity to form endocyclic mononuclear complexes.
However, some larger macrocycles ring sizes greater than 22
atoms sometimes can bind two metal ions in the cavity
simutaneously. Our group15 and other groups16 have reported
dinuclear macrocyclic complexes with 28−40-membered
macrocycles synthesized via [2 + 2] cyclization reactions.
Some larger Schiff base macrocycles obtained via [2 + 2]
cyclization reactions also yield dinuclear complexes.17 Con-
sequently, the two Hg(II) ions in 3 are located slightly above
the chair-shaped macrocyclic plane of L to form half-sandwich-
type complexes because the cavity is too small to accommodate
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Figure 2. Crystal structure of the mercury(II) bromide complex [HgII (L)Br4] (3): (a) general view of the dinuclear mercury(II) bromide complex;
2
(b) coordination environment around the Hg1 atom showing a distorted square-pyramidal arragement; (c) interligand π−π stacking interactions
(dashed lines).
Figure 3. Crystal structure of the mercury(I) nitrate complex {[HgI2(L)](NO3)2}n (4): (a) 1D polymeric structure linked with dimercury(I) ions
(HgI−HgI); (b) schematic representation of the 1D linkage; (c) coordination environment around the dimercury(I) ion showing a distorted
trigonal-prismatic arragement. Noncoordinated anions have been omitted.
two metal ions simultaneously. The distance between the two
Hg(II) centers is 5.83 Å.
N(O1/S1)(O2/S2) donor sets from two adjacent macrocycles
to yield an overall metal coordination geometry of six. The
resulting coordination environment of the mercurous dimer ion
can be described as a distorted trigonal prism (Figure 3c). The
two trigonal faces are defined by N1A−S2A/O2A−O1A/S1A
and N1−O1/S1−O2/S2. The macrocycle adopts a twisted and
elongated conformation to accommodate two Hg(I) atoms on
different sides. The pyridine nitrogen atom also binds strongly
to the Hg(I) atom [Hg1−N1 2.294(2) Å]. The nitrate anions
in 4 remain uncoordinated, with the closest distance between
the Hg and O atoms being 3.0297(2) Å (Figure S5). In this
case, no Hg(II) product was recrystallized. As we understand it,
complex 4 is the first example of a polymeric Hg(I) macrocyclic
complex.
Practically, most of the reported mercury complexes of
macrocycles have Hg in the +2 oxidation state. Only four Hg(I)
complexes of macrocycles have been reported to date, and all of
them have discrete structures (Chart 2).9−12 Syntheses of the
sandwichlike symmetric [Hg2(18-crown-6)2(H2O)2]2+ com-
plex9 (Chart 2a) and asymmetric [Hg2(18-crown-6)(15-
crown-5)(H2O)]2+ complex10 (Chart 2b) have been performed
Upon reaction of mercury(II) nitrate with L in acetonitrile/
dichloromethane, a colorless precipitate was obtained. Accord-
ing to the elemental analysis and electrospray ionization (ESI)
mass spectrum (Figure S7), the reaction produced a mixture of
Hg(I) and Hg(II) compounds. Therefore, recrystallization of
the impure solid product from DMF/ethyl ether yielded X-ray-
quality crystalline 4. The X-ray analysis revealed that 4 is a
Hg(I) complex with the formula {[HgI2(L)](NO3)2}n that
adopts an infinite 1D structure in which Hg+−Hg+ dimers
(Hg22+) link the macrocycles, leading to the formation of a
polymeric chain (Figure 3a). In 4, the Hg22+ ions are present on
opposite sides of the macrocycle. The Hg1−Hg1A bond
distance [2.519(2) Å] is comparable to the corresponding
distance in HgI2(NO3)2 [2.508(2) Å]12 and other reported
complexes of this type, showing a 2.49−2.56 Å range,18 but
much shorter than the metallic Hg−Hg distance of 3.00 Å.
Again, the two S and two O atoms in L are disordered (50:50).
The asymmetric unit in the complex part in 4 contains half of L
and one Hg(I) atom. The Hg(I) center coordinates to two
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Chart 2. Hg(I) Complexes of Macrocycles Reported Previously9−12
Scheme 3. Complexes Obtained from the Reaction of L with Metal Bromide Mixtures
from a Hg(II) salt over elemental mercury via disproportiona-
tion. The Sykes group11 also reported a sandwich-type
mercurous complex of 1,8-anthraquinone-18-trithiacrown-5
with the coordination to six sulfur donors from the two
macrocycles via the reduction of Hg(II) in DMF (Chart 2c).
through six nitrogen donors, with the remaining two soft
selenium donors being uncoordinated (Chart 2d).
The Baldwin group19 reported a Hg(I) complex of a tripodal
nitrogen ligand isolated from the reaction with a Hg(II) salt via
comproportionation of Hg(II) and Hg(0) to give Hg22+. In a
range of studies,20 however, the serendipitous formation of
Hg22+ complexes from Hg(II) has been attributed to the role of
the solvent or ligand as the possible reducing agent. Recently,
The Singh group12 reported a rare complex of a Hg2 ion
2+
trapped inside a 28-membered N6Se2 macrocycle bound
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the Zhang group20e isolated a Hg(I) complex of a pentadentate
thioether ligand during an assembly reaction with a Hg(II) salt
due to the reductive activity of the thioether ligand used.
Therefore, the formation of the Hg(I) complex in this work
also seems to be attributed to the reductive activity for the
thioether ligand L.
bromo atom (Br1), resulting in a distorted monocapped
trigonal prism geometry (Figure 4b). As mentioned above,
complexes 2 and 5 were isolated sequentially from the same
reaction flask, and the observed fractional crystallization is due
to the lower solubility of 5 than 2. Once again, Zn(II) is
excluded in the formation of 5, probably due to the higher
coordination affinity of L and the anion toward Cd(II) than
Zn(II).
The isolation of 4 in this work is somewhat unusual since
mercury(II) nitrate was the preferred metal ion source without
addition of elemental mercury as the reductant. In an alternate
approach to prepare 4 directly by using mercury(I) nitrate
under otherwise identical conditions, a mixture of Hg(I) and
Hg(II) products as well as elemental mercury was obtained and
confirmed from the ESI-MS and other data (see the elemental
mercury in Figure S8 and the ESI-MS spectrum in Figure S9).
Complexation with Mixed Metal Bromides. As an
extension of the above homonuclear zinc, cadmium, and
mercury complexes, we proceeded to the preparation of the
related heteronuclear complexes of L with extended structures,
including the dumbbell type. Three possible cases of mixed
metal bromides were employed in the reactions with L. As
shown in Scheme 3, the reaction of L with Cd/Hg afforded a
dumbbell-shaped complex, while Zn/Cd complexation resulted
in the isolation of two solubility-dependent Cd(II) complexes
with different stoichiometries, including the half-dumbbell-
shaped complex 5. In the case of Zn/Hg, no pure solid product
was isolated under the conditions employed.
Dumbbell-Shaped Heteronuclear [Cd(II)/Hg(II)] Com-
plex 6. When 1 equiv of HgBr2 plus 1 equiv of CdBr2·4H2O in
methanol was used in a one-pot reaction with L in
dichloromethane, a colorless crystalline product 6 was obtained.
Very interestingly, X-ray analysis revealed that 6 is a dumbbell-
shaped heteronuclear complex with the formula [(CdL)2(μ-
Hg2Br6)](Hg2Br6) (Figure 5a). In 6, the Cd(II) occupies the
cavity of L to form a cationic complex (CdL)2+, and two such
adjacent cationic units are linked by one anionic
hexabromodimercury(II) cluster, (μ-Hg2Br6)2−, to generate
the dumbbell-like bis(macrocycle) cationic complex
[(CdL)2(μ-Hg2Br6)]2+. One anionic hexabromodimercury(II)
cluster exists separately (see Figure S6). The Cd(II) center is
seven-coordinate, being bound to all six donors from L in a
twisted conformation, with the remaining coordination site
occupied by one bridging Br atom with a Cd1−Br1 distance of
2.769(1) Å, which is slightly longer than the corresponding
bond distance found in the half-dumbbell analogue 5 [Cd1−
Br1 2.696(1) Å; Figure 4a]. The coordination geometry of the
Cd(II) atom is a distorted monocapped trigonal prism, in
which one O atom (O2) caps the one distorted rectangular
plane (N1−Br1−N2−S2) (Figure 5b). In 6, the distance
between the two Cd(II) centers in the macrocyclic cavities is
11.90 Å.
Previously, our group reported three examples of cluster-
linked homonuclear dumbbell-type macrocyclic complexes
(Chart 1b): [(CuLCl)2(μ-CuCl4)] (L = 18-membered NS4
macrocycle) (Chart 3a),21 [(HgL)2(μ-Hg2Br6)] (L = 20-
membered NO2S3 macrocycle) (Chart 3b),6a and [(CdL)2(μ-
Cd4I12)] (L= 20-membered NO2S3 macrocycle) (Chart 3c).6b
However, our subsequent efforts to prepare heteronuclear
dumbbell complexes by employing diverse types of macro-
cycles, including thiamacrocycles, were not successful. For
example, when we employed 1,10-dithia-18-crown-6
(DT18C6) in an attempted assembly reaction with selected
metal ion pairs including Cd(II)/Hg(II), a discrete product was
obtained with three separated components, namely, [CdI-
(DT18C6)]2[Hg2I6] (Chart 3d).22 Thus, 6 obtained in the
present work appears to be the first characterized example of an
heteronuclear dumbbell complex (Chart 1c) based on a
macrocyclic host ligand.
The reaction of L with a mixture of ZnBr2 and CdBr2
afforded two products (2 and 5) depending on the separation
process. For instance, a colorless precipitate was obtained from
the reaction mixture. This was dissolved in DMF, and following
ether vapor diffusion, a crystalline product was obtained. X-ray
analysis revealed that this product was 2, which was already
obtained via the direct reaction of L with CdBr2 (see Figure
1b), indicating preferential coordination of Cd(II) over Zn(II).
After 2 was separated from the mother liquid, as outlined in
Scheme 3, slow evaporation of the filtrate afforded a colorless
crystalline product. X-ray analysis confirmed this product to be
a new dicadmium(II) complex with the formula [Cd(L)(μ-
Br)(CdBr3)] (5) (Figure 4a). Interestingly, 5 is a dinuclear
complex in which one macrocyclic monocadmium(II) unit and
2−
one CdBr4 are linked via a Cd1−Br1 bond [2.696(2) Å],
resulting in the formation of the half-dumbbell structure. The
endocyclic coordinated Cd1 atom is seven-coordinate, being
bound to all six donors from the macrocycle in a twisted
conformation; the coordination sphere is completed by one
The formation of this heteronuclear dumbbell is more
complicated than that for the corresponding homonuclear ones.
Thus, the preferred formation of the dumbbell-type hetero-
nuclear species in this work might be considered as a
meaningful example of competition and collaboration between
two group 12 species. In the competition, Cd(II) occupied the
macrocyclic cavity over Hg(II), suggesting relatively higher
coordination affinity for Cd(II) to the macrocyclic host
compared with Hg(II). In the collaboration, the two macro-
cyclic Cd(II) complex units are linked by Hg(II) via the
bridging counterions to generate the heterometallic species via
Cd−Br−Hg bonds directly.
Figure 4. Crystal structure of the dicadmium(II) bromide complex
[Cd(L)(μ-Br)(CdBr3)] (5): (a) general view of the half-dumbbell-
shaped complex; (b) coordination environment of Cd1 showing a
distorted monocapped trigonal prism arrangement.
NMR Titrations To Investigate Cd(II) and Hg(II)
Complexation. For the further understanding of the above
F
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Figure 5. Crystal structure of the heteronuclear Cd(II)/Hg(II) bromide complex {[Cd(L)]2(μ-Hg2Br6)}[Hg2Br6] (6): (a) general view of the
dumbbell-shaped bis(macrocycle) complex; (b) coordination environment of Cd1 showing a distorted monocapped trigonal prism arragement. The
separated anionic [Hg2Br6]2− part has been omitted.
Chart 3
Figure 6. NMR titrations of L with (a) cadmium(II) and (b) mercury(II) (as bromides) in CD3OD.
heteronuclear dumbbell complex that forms via competition
and collaboration between Cd(II) and Hg(II), the correspond-
ing complexation studies in solution were also performed. H
cadmium(II) bromide and mercury(II) bromide with L were
performed in CD3OD. In both titrations, it was possible to
observe the cation-induced chemical shift changes for the
macrocycle in which all of the proton resonances shifted
1
NMR titration experiments to follow the interaction of
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Figure 7. 1H NMR spectra of the aromatic region for L in CD3OD via stepwise addition of (A) cadmium(II) followed by mercury(II) (as bromides)
or (B) mercury(II) followed by cadmium(II) (as bromides), [L] = 3 mM. (C) Proposed complexation equilibria between the corresponding species
in solution.
downfield (Figures S10 and S11), indicating complex
formation, with the ligand exchange rate being fast on the
NMR time scale. The magnitude of the chemical shift change
(Δδ, ppm) of each resonance peak was measured as a function
of the mole ratio (M/L) (Figure 6). The titration curves show
that the complexation behaviors of Cd(II) (Figure 6a) and
Hg(II) (Figure 6b) are quite different.
The magnitudes of the Cd(II)-induced chemical shift
changes for the proton peaks of the macrocycle (0.25−0.60
ppm) are much larger than those for Hg(II) (below 0.08 ppm)
without exception. Clearly, this evidence indicates that Cd(II)
interacts more strongly with the macrocycle than Hg(II) does
in CD3OD solution. In case of the Cd(II) titration, the
complexation process is seen to occur via several steps to give
two or more species, including a 1:1 (metal:L) complex, that
coexist depending on the mole ratio. In contrast, the Hg(II)
titration curves exhibit no significant shifts above a mole ratio of
2.0, indicating the formation of the stable complex with a 2:1
(metal:L) stoichiometry.
In the Cd(II) titration, the complexation-induced shifts
follow the order H1 ≫ H4 > H2 > H3 (see Figure 6a for each
methylene proton). Notably, the observed largest downfield
shift for H1 between the S donor and the pyridine group is in
accord with strong binding of Cd(II) to the S donors, in
keeping with the presence of a cooperative effect between Cd−
S and Cd−N bonds in solution. The second largest shift was
observed for H4 between the O donor and the pyridine group.
The difference between these shifts follows that based on
HSAB considerations, as Cd(II) and the S donor are a soft acid
and soft base, respectively, but the O donor is a hard base. In
the Hg(II) titration, as mentioned above, the Hg(II)-induced
chemical shift changes are smaller than those for Cd(II), with
the complexation-induced shift changes for the protons, H1 >
H2 ≫ H3, H4 differing to some extent from those observed in
the Cd(II) titration (Figure 6b). In the Hg(II) case, the Δδ
values for H1 and H2 are larger than those for H3 and H4,
suggesting that Hg(II) is more thiaphilic (softer) than Cd(II).
In the UV titration for the L−Cd(II) complexation (Figure
S12), two isosbestic points (235 and 262 nm) were found to
H
DOI: 10.1021/acs.inorgchem.6b01583
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Inorganic Chemistry
Article
show the multiple complexation equilibria, similar to the NMR
titration results, indicating the existence of several stoichio-
metric complexed species. However, calculations of the stability
constants were impossible with these data because of the large
uncertainty.
most strongly bound to L, as found in the competition
reactions in solution.
CONCLUSION
■
The newly designed ditopic macrocycle L was synthesized, and
its complexation with group 12 metals was investigated in both
the solid and solution states. Both zinc(II) and cadmium(II)
bromide complexes feature an endocyclic mononuclear species,
but the former adopts an octahedral coordination geometry
without anion coordination while the latter yields a pentagonal-
bipyramidal geometry with anion coordination at the axial sites.
Unlike its smaller neighbors, mercury(II) bromide forms a
dinuclear macrocyclic complex. When mercury(II) nitrate was
used under the same conditions, an unexpected Hg(I) complex
with a one-dimensional polymeric structure was isolated as a
first example of such a structure. Mixed metal bromide
complexation reactions were also carried out, leading to new
heterometallic species. A mixture of ZnBr2 and CdBr2·4H2O
resulted in the isolation of two Cd(II) complexes with different
solubilities, including a half-dumbbell-type monomeric species,
while a mixture of CdBr2·4H2O and HgBr2 yielded the first
example of a heteronuclear dumbbell-type complex in which
two macrocyclic Cd(II) complexes are linked by a
hexabromodimercury(II) cluster. The isolation of the dumbbell
complex 6 provides an elegant example of the role of
competition and collaboration between Cd(II) and Hg(II) ions
in the formation of this unusual heteronuclear complex.
Comparative NMR Study of Competitive Reactions of
Cd(II) and Hg(II) toward L. The relative reactivity of Cd(II)
and Hg(II) toward L in solution is a primary issue in the
formation of the dumbbell-shaped complex. Therefore,
competitive reactions of these two metal ions toward L were
1
investigated by H NMR spectroscopy. The signals of the
aromatic protons in the macrocycle (Ha−Hd) are well-resolved
and readily identified (see the spectrum given in Figure 7A-a).
As shown in Figure 7A-b, the addition of 1.0 equiv of Cd(II)
causes downfield shifts for Ha−Hd of 0.15−0.3 ppm and
indicates 1:1 complexation with fast ligand exchange occurring
on the NMR time scale. In this case, the chemical shift changes
for Ha and Hb (marked in blue in Figure 7) near the NS2 donor
domain are larger than those for Hc and Hd (marked in red)
near the NO2 donor domain, indicating that Cd(II) favors
binding to the S donors rather than to the O donors. However,
further addition of Hg(II) (1.0 or 2.0 equiv) led to no
significant chemical shift changes (Figure 7A-c,d), suggesting
that the 1:1 Cd(II) complex formed earlier is maintained and
no further reaction occurs (as proposed in Figure 7C, clockwise
direction).
Comparative NMR experiments for the above competition
reaction were also performed in the reverse order of salt
addition [i.e., Hg(II) followed by Cd(II)]. As shown in Figure
7B-a,b, the Hg(II) complexation proceeds by two steps. First,
the addition of 1.0 equiv of Hg(II) causes downfield shifts for
Ha−Hd (0.01−0.03 ppm). Upon addition of another 1.0 equiv
of Hg(II), further downfield shifts of each peak were observed
(0.02−0.05 ppm), in keeping with the formation of a
dimercury(II) species, [Hg2L]4+ (Figure 7C). Upon addition
of 1.0 equiv of Cd(II), much larger downfield shifts (0.2−0.3
ppm) occur, and the spectral pattern in Figure 7B-d becomes
the same as that shown in Figure 7A-d, suggesting that the
respective reactions finally reach the formation of the
monocadmium(I) species [CdL]2+ (Figure 7C, anticlockwise
direction, starting from L). Once again, this result agrees with
the titration data, which show that Cd(II) has a higher affinity
for L than Hg(II) does. Also, this is in agreement with the
results obtained for the solid state.
EXPERIMENTAL SECTION
■
General. All of the chemicals and solvents were of reagent grade
and were used without further purification. NMR spectra were
recorded on a Bruker 300 spectrometer (300 MHz). The FT-IR
spectra were measured with a Nicolet iS10 spectrometer. The ESI
mass spectra were obtained on a Thermo Scientific LCQ Fleet
spectrometer. Elemental analyses were carried out on a Thermo
Scientific Flash 2000 series elemental analyzer.
Synthesis and Characterization of L. The precursor diol 9 (1.00
g, 3.87 mmol) was dissolved in anhydrous THF (15 mL), and the
solution was added dropwise to a stirred suspension of NaH (0.23 g,
9.67 mmol) in anhydrous THF (15 mL) under nitrogen. After
addition of the diol at room temperature, the mixture was refluxed for
1 h and then cooled to 0 °C. 2,6-Bis(tosyloxymethyl)pyridine (8)
(1.73 g, 3.87 mmol) was dissolved in anhydrous THF (30 mL), and
the solution was added slowly dropwise to the reaction mixture. The
reaction mixture was stirred rapidly for 48 h at room temperature and
then evaporated. The residue was partitioned between water and
dichloromethane. The aqueous phase was separated and extracted with
two further portions of dichloromethane. The combined organic
phases were dried with anhydrous sodium sulfate and then evaporated
to dryness. Flash column chromatography (SiO2; 3:7 dichloro-
methane/ethyl acetate) afforded the product as a white solid in 35%
Similarly, an NMR competition study of Zn(II) and Cd(II)
was also performed (Figure S13). As shown in Figure S13A, the
addition of 1.0 equiv of Cd(II) caused downfield shifts for all of
the aromatic peaks with line broadening. However, further
addition of Zn(II) (1−2 equiv) led to no significant chemical
shift changes, indicating the higher affinity of L for Cd(II) than
Zn(II). The above competition reaction performed in the
reverse order of salt addition (Figure S13B) also showed
stronger Cd(II) complexation.
1
yield. Mp: 86−87 °C. H NMR (300 MHz, CDCl3): δ 7.69 (t, 1H,
Ar), 7.59 (t, 1H, Ar), 7.28 (d, 2H, Ar), 7.23 (d, 2H, Ar), 4.66 (s, 4H,
ArCH2O), 3.78 (s, 4H, ArCH2S), 3.71 (t, 4H, SCH2CH2O), 2.68 (t,
4H, SCH2CH2O). 13C NMR (75 MHz, CDCl3): δ 158.19, 157.69,
137.61, 137.23, 121.40, 120.97, 73.39, 69.50, 37.47, 30.28. IR (KBr
pellet): 3058, 2912, 2865, 1585, 1451, 1358, 1285, 1221, 1153, 1082,
1027, 989, 934, 831, 815, 756, 724, 696 cm−1. Anal. Calcd for
[C18H22N2O2S2]: C, 59.64; H, 6.12; N, 7.73; S, 17.69%. Found: C,
59.73; H, 6.17; N, 7.40; S, 17.33%. Mass spectrum (ESI): m/z 363.20
[L]+.
1
In addition, the H NMR spectra of complexes 1, 2, and 3
were added together with that of L in CD3OD (Figure S14).
The peaks of the complexes show general downfield shifts
compared with those of free ligand L. In all of the complexes,
the induced chemical shift of H1 (between the pyridine and the
sulfur donor) is greater than those of other protons because of
the higher affinity of the NS2 donor set toward the metal center.
The order of the magnitudes of the induced chemical shift
changes of H1 for the complexes is Cd (2, 0.24 ppm) > Hg (3,
0.09 ppm) > Zn (1, 0.03 ppm), in accord with Cd(II) being
Preparation of [Zn(L)][ZnBr4] (1). Zinc(II) bromide (9.4 mg,
0.042 mmol) in methanol (1 mL) was added to a solution of L (10.1
mg, 0.028 mmol) in dichloromethane (1 mL). Slow evaporation of the
solution afforded 1 as a colorless crystalline product suitable for X-ray
analysis. Mp: 224−225 °C (decomp.) IR (KBr pellet): 3058, 3027,
I
DOI: 10.1021/acs.inorgchem.6b01583
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2920, 2889, 1656, 1600, 1573, 1545, 1457, 1408, 1360, 1291, 1188,
1163, 1045, 1022, 951, 930, 803, 701 cm−1. Anal. Calcd for
[C18H22Zn2Br4N2O2S2]: C, 26.60; H, 2.73; N, 3.45; S, 7.89%.
Found: C, 26.68; H, 2.73; N, 3.35; S, 7.99%. Mass spectrum (ESI):
m/z 214.08 [Zn(L)]2+.
carried out using the SHELXTL package.24 In all cases, all non-
hydrogen atoms were refined anisotropically and all hydrogen atoms
were placed in idealized positions and refined isotropically in a riding
manner along with their respective parent atoms. Relevant crystal data
collection and refinement data for the crystal structures of 1−6 are
summarized in Table S1.
Preparation of [Cd(L)Br2] (2). In a glass tube, CdBr2·4H2O (14.2
mg, 0.041 mmol) in methanol (1 mL) was layered onto a solution of L
(10.1 mg, 0.028 mmol) in dichloromethane (1 mL), and 2 was
obtained as an X-ray-quality colorless crystalline product. Mp: 229−
230 °C (decomp). IR (KBr pellet): 3064, 2921, 2863, 1655, 1599,
1455, 1420, 1368, 1293, 1169, 1086, 1003, 806, 756, 660 cm−1. Anal.
Calcd for [C18H22CdBr2N2O2S2]: C, 34.06; H, 3.49; N, 4.41; S,
10.10%. Found: C, 34.13; H, 3.48; N, 4.53; S, 10.30%. Mass spectrum
(ESI): m/z 555.00 [Cd(L)Br]+.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01583.
NMR spectra, additional crystal structures, ESI-MS
spectra, NMR titrations, and crystal data (PDF)
Crystallographic data for 1−6 (CIF)
Preparation of [HgII (L)Br4] (3). Mercury(II) bromide (14.9 mg,
2
0.041 mmol) in acetonitrile (1 mL) was added to a solution of L (10.1
mg, 0.028 mmol) in dichloromethane (1 mL). The colorless
precipitate obtained was filtered and dissolved in DMF. Vapor
diffusion of diethyl ether into the DMF solution afforded 3 as a
colorless crystalline product suitable for X-ray analysis. Mp: 182−183
°C (decomp.). IR (KBr pellet): 3067, 2924, 1743, 1653, 1598, 1575,
1452, 1398, 1369, 1238, 1166, 1078, 1005, 805, 757, 674 cm−1. Anal.
Calcd for [C18H22Hg2Br4N2O2S2]: C, 19.96; H, 2.05; N, 2.59; S,
5.92%. Found: C, 20.37; H, 2.04; N, 2.58; S, 5.99%. Mass spectrum
(ESI): m/z 643.17 [HgII(L)Br]+, 282.25 [HgII(L)]2+.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: rubiyi7@naver.com.
*E-mail: sslee@gnu.ac.kr.
Notes
The authors declare no competing financial interest.
Preparation of {[HgI2(L)](NO3)2}n (4). Hg(NO3)2·H2O (14.2 mg,
0.041 mmol) in acetonitrile (1 mL) was added to a solution of L (10.2
mg, 0.028 mmol) in dichloromethane (1 mL). A colorless precipitate
was formed in 60% yield after 1 h of stirring. The precipitate was
filtered off, washed with diethyl ether, and dried in air. For the further
purification, the precipitate was dissolved in DMF and filtered to
remove the insoluble impurity. Vapor diffusion of diethyl ether into
the DMF solution afforded 4 as a colorless crystalline product suitable
for X-ray analysis. Yield: 20%. Mp: 141−142 °C. IR (KBr pellet):
3022, 2922, 2860, 1599, 1578, 1480, 1440, 1383, 1301, 1133, 1106,
1088, 1033, 1004, 933, 813, 791, 710 cm−1. Anal. Calcd for
[C18H22Hg2N4O8S2]·H2O·CH3CN: C, 25.37; H, 2.87; N, 7.40%.
Found: C, 25.08; H, 2.61; N, 7.42%.
Preparation of [Cd(L)(μ-Br)(CdBr3)] (5). A mixture of zinc(II)
bromide (9.4 mg, 0.042 mmol) and cadmium(II) bromide (14.4 mg,
0.042 mmol) in methanol (1 mL) was added to a solution of L (10.2
mg, 0.028 mmol) in dichloromethane (1 mL). A colorless precipitate
formed after 1 h of stirring was isolated by filtration (40% yield). The
precipitate was recrystallized by dissolution in DMF followed by vapor
diffusion of diethyl ether to afford single crystals that were confirmed
by SC-XRD to be compound 2. From slow evaporation of the filtrate
for 2 days, the colorless crystalline product 5 was obtained. Mp: 220−
221 °C (decomp.). IR (KBr pellet): 2923, 2854, 1657, 1601, 1575,
1546, 1453, 1418, 1367, 1284, 1258, 1163, 1084, 1046, 1010, 807, 790,
736 cm−1. Anal. Calcd for [C18H22Cd2Br4N2O2S2]: C, 23.84; H, 2.44;
N, 3.09; S, 7.07%. Found: C, 23.94; H, 2.45; N, 3.05; S, 6.98%. Mass
spectrum (ESI): m/z 238.17 [Cd(L)]2+, 555.17 [Cd(L)Br]+.
Preparation of {[Cd(L)]2(μ-Hg2Br6)}(Hg2Br6) (6). CdBr2·4H2O
(28.5 mg, 0.083 mmol) and mercury(II) bromide (30.1 mg, 0.083
mmol) in methanol (1 mL) were layered onto a solution of L (10.2
mg, 0.028 mmol) in dichloromethane (1 mL), and 6 was obtained as
an X-ray-quality colorless crystalline product. Mp: 189−190 °C
(decomp.). IR (KBr pellet): 3068, 2923, 2876, 1601, 1574, 1451, 1406,
1364, 1283, 1239, 1195, 1164, 1085, 1049, 1009, 880, 791, 736, 671
cm−1. Anal. Calcd for [C18H22Cd1Hg2Br6N2O2S2]: C, 15.95; H, 1.64;
N, 2.07%. Found: C, 16.07; H, 1.90; N, 2.23%. Mass spectrum (ESI):
m/z 555.08 [Cd(L)Br]+, 643.17 [HgII(L)Br]+.
X-ray Crystallographic Analysis. All of the data were collected
on a Bruker SMART APEX II ULTRA diffractometer equipped with
graphite monochromatized Mo Kα radiation (λ = 0.71073 Å)
generated by a rotating anode. The cell parameters for the compounds
were obtained from least-squares refinement of the spot (from 36
collected frames). Data collection, data reduction, and semiempirical
absorption corrections were carried out using the APEX2 software
package.23 All of the calculations for the structure determination were
ACKNOWLEDGMENTS
This work was supported by the NRF (2012R1A4A1027750),
South Korea.
■
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