Formation of a four-bladed waterwheel-type chloro-bridged dicopper(ii) complex with dithiamacrocycle: Via double exo-coordination

Article abstract of DOI:10.1039/c9dt04372a

A combination of O₃S₂-macrocycles incorporating different sulfur-to-sulfur separations (S-(CH₂)_n-S, L¹: N = 2, L²: N = 3) and copper(ii) nitrate afforded new types of both monocopper(ii) an

Full text of DOI:10.1039/c9dt04372a

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Formation of a four-bladed waterwheel-type
chloro-bridged dicopper(II) complex with
Cite this: DOI: 10.1039/c9dt04372a
dithiamacrocycle via double exo-coordination†
Received 12th November 2019,
Accepted 17th December 2019
a
Seulgi Kim, a In-Hyeok Park, *b Han-Byeol Choi, a Huiyeong Ju,
Eunji Lee, a Tun Seng Herng, b,c Jun Ding, c Jong Hwa Jung a and
DOI: 10.1039/c9dt04372a
a
Shim Sung Lee
*
rsc.li/dalton
A combination of O3S2-macrocycles incorporating different sulfur- undergo both endocyclic and exocyclic coordination in which
to-sulfur separations (S-(CH2)n-S, L1: n = 2, L2: n = 3) and copper(II) the single-ring macrocycle binds to two metal ions in different
nitrate afforded new types of both monocopper(II) and dicopper(II) ways.5 In this study, we report an example of another macro-
complexes, respectively. L1 gave
[Cu2(L1)2(NO3)4]n (1) based on
a
1D coordination polymer cyclic ligand coordination mode that differs from types A–D
a
convergent exo-coordination which gives rise to a new type of dinuclear complex involving
mode while L2 resulted in the formation of a divergent exo/exo-co- “double” exo-coordination.
ordinated dicopper(II) complex, [Cu2(L2ox)4(μ-Cl)](NO3)4 (2), whose
Although exo-coordination occurs less frequently in macro-
shape resembles a four-bladed waterwheel in which in situ oxi- cyclic complexes, it quite often occurs in thiamacrocycles upon
dized macrocycles (L2ox) act as the blades and a CuII-(μ-Cl)-CuII binding soft metal ions.5 Furthermore, exo-coordination is a
entity corresponds to the axle shaft. The chloro-bridging ligand is promising tool for linking macrocycles in diverse ways for
derived from the dichloromethane solvent and its arrangement is the construction of highly ordered discrete metallosupra-
held together by C–H⋯Cl− H-bonds. Compound 2 shows a weak molecules5b,6 and coordination polymers.5a,7 The incorpor-
antiferromagnetic property via the CuII-(μ-Cl)-CuII entity.
ation of a specific binding subunit into a ligand design pro-
vides an approach toward the development of new supramole-
cular coordination compounds and metal–organic nano-
materials.8 Recently, we have reported the use of bis-O2S2-
macrocycle isomers that incorporate U-shaped and W-shaped
exo-coordination binding sites, respectively, for the controlled
formation of CunIn (n = 2 or 4) clusters.3a
In this work, we propose a rational approach for construct-
ing exo/exo-type dinuclear copper(II) complexes (Fig. 2b) that
represent a new dinuclear type that is different from types
A–D. In particular, unlike paramagnetic mononuclear copper(II)
complexes, the dinuclear copper(II) complexes bridged by
small ligands tend to exhibit antiferromagnetic coupling.9
Dinuclear complexes are of special interest across diverse areas
that include biomimetic catalysts, allosteric behaviours and
magnetic materials.1 Thus, various approaches have been
devoted to the development of ligating systems capable of
binding two metal ions in defined sites. In particular, the
dinucleating process is well exemplified by the formation of
dinuclear macrocyclic complexes via self-assembly and/or
molecular recognition.2–5
Common motifs in the design of dinucleating macrocycles
involve, as shown in Fig. 1, a large macrocycle capable of
binding two metal ions in its cavity (type A),1c,d,2 two linked
macrocycles sharing a spiro-centre (type B)3 and two linked
macrocycles connected by a spacer unit (type C).4 Unlike types
A–C that give rise to endocyclic metal coordination, type D can
aDepartment of Chemistry and Research Institute of Natural Science,
Gyeongsang National University, Jinju 52828, South Korea. E-mail: sslee@gnu.ac.kr
bDepartment of Chemistry, National University of Singapore, Singapore 117543,
Singapore. E-mail: pihghost@nate.com
cDepartment of Materials Science and Engineering, National University of Singapore,
Singapore 119260, Singapore
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, NMR, IR, PXRD patterns, and crystal structure. CCDC 1898671 (L1),
1898672 (L2), 1898673 (1), and 1898674 (2). For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/C9DT04372A
Fig. 1 Four types of dinuclear macrocyclic complexes.
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Fig. 2 Two copper(II) complexes 1 and 2 isolated in this work showing
(a) convergent exo-mode and (b) divergent exo/exo-modes, respect-
ively, depending on the size of (–CH2–)n chain linking the sulfur donors.
Fig. 3 Copper(II) nitrate complex of L1, [Cu2(L1)2(NO3)4]n (1): (a) 1D poly-
meric structure, (b) local coordination environments of the Cu1 and
Cu2 atoms and (c) comparison of the S1⋯S2 distances (Å) in free and
bound L1.
Toward this goal, we employed two macrocycles incorporating
different –S–(CH2)n–S– segments, L1: n = 2 and L2: n = 3,
because the alteration of such binding sites might induce the
formation of exo-coordination-based complexes exhibiting
either a convergent mode or a divergent mode.10
Indeed, L1 resulted in the formation of a one-dimensional
(1D) coordination polymer involving convergent exo-coordi- copper atoms (Cu1 and Cu2) have different coordination
nation (Fig. 2a). Surprisingly, the resulting product 2 upon environments. The Cu1 atom is six-coordinate, being bound to
using the L2 ligand was a waterwheel-like dicopper(II) complex four sulfur donors from two different L1 ligands to form a
based on divergent exo/exo-coordination which has CuII– square plane. The two axial sites are occupied by the bridging
Osulfoxide bonds by the oxidation of L2 to L2 (Fig. 2b and c). nitrate ions, linking the Cu1 atom and Cu2 atoms. The Cu2
ox
Furthermore, the chlorido ligand in the mono-chloro-bridged atom is four-coordinate, trans square planar, being bound to
dicopper(II) complex 2 showing an antiferromagnetic superex- two bridging nitrate ions and two terminal monodentate
change9 is derived from the decomposition of the dichloro- nitrate ions. The preferred formation of 1 accords well with L1
methane solvent. Both isolated products appear to be the first incorporating a shorter sulfur-to-sulfur distance to form a stable
examples of such species yet reported. The details of our inves- 5-membered chelate ring involving
a
convergent exo-
tigation are described below. coordination mode (Fig. 2a), leading to a bis(macrocycle)
The macrocyclic ligands L1 and L2 were each synthesized monocopper(II) complex unit. The open sites in the copper
using a coupling reaction between dichloride 5 and the coordination sphere of the Cu1 atom allow nitrate coordination
required dithiols in the presence of cesium carbonate under which also links another copper atom (Cu2) to ultimately afford
high dilution conditions (Scheme S1†). The intermediate com- the 1D chain. The S1⋯S2 distance in 1 (3.2380(8) Å) is shorter
pounds 3–5 were prepared using known procedures.11 Single than that in the free ligand L1 (4.3857(1) Å) due to the twisting
crystals of L1 and L2 were obtained by slow evaporation from of the torsion angle between the two S atoms from a trans to a
dichloromethane and their solid structures were determined gauche arrangement upon complexation (Fig. 2c).
by single crystal X-ray analysis (Table S1,† see later).
Product 2 crystallizes in the monoclinic space group I4/m
The crystalline copper(II) complexes 1 and 2 were obtained with the formula [Cu2(μ-Cl)(L2ox)4]Cl(NO3)2 showing a 2 : 4
by the reaction of one equivalent of Cu(NO3)2·3H2O with L1 (metal-to-ligand) stoichiometry (Fig. 4a). As mentioned, the
and L2 in dichloromethane/acetonitrile, respectively. In obtain- overall structure of 2 resembles a four-bladed waterwheel
ing 1, slow evaporation of the reaction mixture afforded the (Fig. 4b). Furthermore, the double exo-coordinated dicopper(II)
product as dark blue crystals in 40% yield. In the case of 2, system coupled with the multiple events arising from the
slow evaporation of the reaction mixture over 3 weeks afforded ligand oxidation (L2ox) (Fig. 2c) and the solvent decomposition
a small number of blue crystals (yield ∼2%). A comparison of provides a pathway to this unique complex. Thus, 2 incorpor-
the PXRD patterns for 1 and 2 with the corresponding simu- ates two copper(II) centres linked by one chloride ion (μ-Cl−) to
lated data confirmed the phase purity of each product form a central shaft or axle of type CuII-(μ-Cl)-CuII. The centro-
(Fig. S7†).
symmetric complex has a C4 rotation axis, with an inversion
Product 1 crystallizes in the monoclinic space group C2/c centre at the chloro-bridge. The chloro-bridged dicopper(II)
with the formula [Cu2(L1)2(NO3)4]n by adopting a 1D polymeric shaft and four L2 ligands are linked by Cu1–O3 bonds
ox
structure (Fig. 3a). The asymmetric unit for 1 contains half of (1.945(3) Å) in the divergent exo/exo-coordination mode to
the formula unit. In 1, the two crystallographically different form the four-bladed waterwheel-like structure.
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reaction of copper(II) nitrate with 1,10-dithia-18-crown-6,
forming a polymeric product in which the air-oxidation of
sulfur donors to disulfoxides had taken place.13b Since in this
past example and in the present case we found no evidence
for the reduction of copper(II), we conclude that L2 was
ox
generated by air-oxidation. Among the 380 hits found for
–SvO–CuII in the Cambridge Structural Database,14 almost
all correspond to solvato complexes of dimethylsulfoxide
(DMSO), with complexes related to other sulfoxide ligands
being rare.
Second, we were puzzled by the origins of the chloride ion
that bridges the two copper(II) centres in 2. In other reactions
of the present general type, species arising from decomposed
solvent molecules have been shown to sometimes participate
in product formation as auxiliary ligating components.15 For
example, we recently reported isostructural dichloro-bridged
dicopper(II) bis(macrocycle) complexes of the type [LCu-(μ-Cl)2-
CuL]X2 (L: N3O2-macrocycle, X: NO3 or ClO4) whose chlorido
ligands are derived from the decomposition of the dichloro-
methane solvent.15b Similarly, this appears to be the source of
the chloro-bridge in 2. Compared with the di-chloro-bridged
dicopper(II) entity, mono-chloro-bridged dicopper(II) entities as
found in 2 are less common.16 The preferred formation of this
linear mono-chloro-bridged dicopper(II) entity in 2 may reflect
the confined environment in which it resides.
Fig. 4 Crystal structure of [Cu2(μ-Cl)(L2ox)4]Cl(NO3)2 (2): (a) view of the
2 : 4 (metal-to-ligand) stoichiometric complex, (b) schematic presen-
tation of the four-bladed waterwheel complex, (c) chloro-shared double
square pyramidal geometry and (d) comparison of the interatomic dis-
tances (Å) before and after complexation.
Third is the question of how such a supra-structure could
Each copper(II) centre in 2 is five-coordinate being bound to be formed. If it is supposed that four L2 ligands were to coordi-
four oxygen atoms (Cu1–O3 1.945(3) Å) from SvO groups nate to the copper(II) centres directly via Cu–S bonds to form
arising from four different L2 ligands together with a μ-Cl− such a highly ordered structure, then difficulties would almost
ox
group (Cu1–Cl1 2.4699(19) Å) to yield a square pyramidal geo- certainly be encountered in overcoming the steric hindrance
metry for each centre, with the apical position occupied by the between adjacent bulky macrocycles. Instead, L2 binds via
ox
bridging chloride ligand (Fig. 4c); the τ value is 0.11 (where τ = Cu–OvS bonds13b and this will ameliorate any such steric hin-
0 corresponds to a perfect square pyramid, while τ = 1 corres- drance, allowing the waterwheel product to form.
ponds to a perfect trigonal bipyramid).12 The conformation Furthermore, the chloro-bridge in 2 was found to interact with
of the S1–C–C–C–S2 linkage in the free L2 shows a g–t–g four CH2 groups from four different ligands via C–H⋯Cl−
(g = gauche; t = trans) arrangement which is then changed to a bonds, with the C⋯Cl− distances and C–H⋯Cl− angles being
g–g–g arrangement upon complexation (Fig. 4d). In 2, the 2.9684(15)
SvO⋯OvS distance (5.3491(29) Å) is larger than that of the Fig. S8a†).17 Considering that synthetic hosts for halide ions
S⋯S distance (3.8463(21) Å) (Fig. 4d).
normally employ between 2 and 6 binding interactions, the
Å and 150.04°, respectively (dashed lines in
Meanwhile, the formation of 2 raises the following ques- octahedral geometry of the chloride ion in 2 (Fig. S8a†) rep-
tions. (i) How is the in situ oxidized macrocycle (L2ox) formed? resents a good example of the use of the highest coordination
(ii) What is the source of the chloro-bridge? (iii) What is the number to achieve maximum stabilization.
mechanism for the assembly of this highly ordered supra-
structure from its individual components? Each of these other (larger) halide ions by addition of the corresponding
aspects will now be discussed.
sodium(I) salts were not effective, probably because this would
Our attempts to prepare analogous products incorporating
First, in forming the waterwheel structure from the reaction lead to the formation of less favourable H-bonds and, also,
mixture of Cu(NO3)2·3H2O and L2, clear sulfoxidation of L2 has because of the limited space available to occupy the larger
occurred in situ to afford L2ox. The observed sulfoxidation of L2 halide ions. Weaker H-bonds, including C–H⋯N and C–H⋯O
was initially confirmed by the IR spectrum of 2 which shows a interactions, quite often play an important role in stabilizing
new peak at 1009 cm−1 assigned to the νSvO stretching mode supramolecular structures when large numbers may act in
(Fig. S6†).13 It appears that the sulfoxidation might occur via concert.17a,18 In 2, the presence of three C–H⋯O interligand
one of the two routes: oxidation by copper(II) or by air. In this H-bonds were confirmed [C–H11b⋯O2F 2.7433(14) Å (144.70°),
respect, it is noted that, on standing in air, the reaction C–H11bE⋯O2B 2.7433(14) Å (144.70°), C–H12b⋯O1F 2.464(1) Å
mixture maintained its blue colour over about 3 weeks; during (171.98°)] (Fig. S8b†). The linking of four macrocycles via
this time a few blue single crystals of 2 grew in the mixture. It twelve H-bonds stabilizes the overall structure to yield a
is relevant to the above that we have previously reported the pseudo-cage, with all these H-bonds being geometrically and
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electronically well matched to satisfy the mutual complemen-
tary conditions required for the pseudo-cage formation.
Conclusions
Although the procedure for 2 was reproducible, the low In summary, our approach toward the development of an exo/
yield (∼2%) was probably due to the chloride being the limit- exo-type dinuclear macrocyclic complex led to the formation of
ing reagent; it appeared to be present from the decomposition an unusual waterwheel-like tetra(macrocycle) dicopper(II)
of the dichloromethane solvent,15a,b since no chloride was complex. In this aspect, the supra-structured product isolated
added to the reaction solution. When an excess amount of was much more interesting than initially expected.
NaCl was added to the same reaction mixture, it resulted in Furthermore, the formation of the waterwheel structure high-
the formation of crystalline 2 in higher yield (∼10%) after one lights the decisive roles of the individual components in deter-
week. However, an attempt for the sulfoxidation of L2 to L2
by H2O2 was ineffective in increasing the yield.19
mining the final structural outcome. For example, the chloride
ox
ion is bound by two copper(II) to form a linear CuII-(μ-Cl)-CuII
As mentioned, dinuclear ligand-bridged paramagnetic axis and the supramolecular arrangement is held together by
metal complexes often exhibit weak antiferromagnetic coup- Cu–OvS coordination bonds in addition to strategically posi-
ling, sometimes giving them a diamagnetic nature.10 Magnetic tioned CH⋯Cl− interactions. Furthermore, the “opening up”
susceptibility (χM) measurements for 1 and 2 were carried out that occurs with the use of four disulfoxide ligands of type L2
ox
(Fig. 5). The χMT values for 1 slightly decrease with decreasing minimizes the steric repulsion between macrocyclic units by the
temperature over 20–300 K and then increase below 20 K, indi- Cu–OvS bonding in forming the waterwheel-like dicopper(II)
cating the presence of weak ferromagnetic coupling, with a complex 2. In this, each component is precisely matched to
coupling constant (J) of +1.12 cm−1. The experimental data satisfy the divergent exo/exo-coordination mode required for
were fitted by using the PHI20 program by means of an isotro- the generation of the observed highly ordered assembly. The
pic spin Hamiltonian (SH) accounting for the exchange coup- restricted structural conformation of the waterwheel-like
ling (Heisenberg–Dirac–van Vleck Hamiltonian) and zero-field dicopper(II) complex by its 1H-NMR spectrum has proved poss-
splitting (ZFC) for the two interacting S = 1/2 centers. The best- ible due to the presence of antiferromagnetic coupling
fitting for experimental data leads to the parameters: J = between two copper(II) centres. The present work not only rep-
+1.12 cm−1, TIP = 5 × 10−4 cm3 mol−1 and g = 2.03. The solid resents a step forward in achieving the construction and
line in Fig. 5 corresponds to the best theoretical fit. The corres- characterization of a new exo/exo-type dinuclear macrocyclic
ponding plot for 2 shows that the χMT values are nearly con- complex but may also serve as an inspiration and guide for the
stant over 25–300 K but sharply decrease below 25 K, indicat- design and development of new sulfur-extended supramolecu-
ing the presence of weak antiferromagnetic coupling (J = lar materials in the future.
−0.64 cm−1) between the two copper(II) atoms via the linear
chloro-bridge.
The 1H-NMR spectrum of 2 using DMSO-d6 was observed to Conflicts of interest
occur in the normal (diamagnetic) region due to the antiferro-
magnetic coupling (Fig. S9†), indicating that the waterwheel
There are no conflicts to declare.
structure remains intact and is in keeping with complexation
occurring through both sulfoxides of L2ox. However, the 1D
Acknowledgements
polymeric structure of 1 was found to dissociate in DMSO-d6 to
yield free L1 in accord with the DMSO-d6 sulfoxide group
This work was supported by National Research Foundation of
Korea (2019R1A2C1002075 and 2017R1A4A1014595).
showing higher affinity towards copper(II) relative to that of the
thioether sulfurs in L1.
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