Unusual Recognition and Separation of Hydrated Metal Sulfates [M2(μ-SO4)2(H2O)n, M = ZnII, CdII, CoII, MnII] by a Ditopic Receptor

Article abstract of DOI:10.1021/acs.inorgchem.6b00176

A ditopic receptor L1, having metal binding bis(2-picolyl) donor and anion binding urea group, is synthesized and explored toward metal sulfate recognition via formation of dinuclear assembly, (L1)₂M₂(SO₄)₂. Mass spectrometric analysis, ¹H-DOSY NMR, and crystal structure analysis reveal the existence of a dinuclear assembly of MSO₄ with two units of L1. ¹H NMR study reveals significant downfield chemical shift of -NH protons of urea moiety of L1 selectively with metal sulfates (e.g., ZnSO₄, CdSO₄) due to second-sphere interactions of sulfate with the urea moiety. Variable-temperature ¹H NMR studies suggest the presence of intramolecular hydrogen bonding interaction toward metal sulfate recognition in solution state, whereas intermolecular H-bonding interactions are observed in solid state. In contrast, anions in their tetrabutylammonium salts fail to interact with the urea -NH probably due to poor acidity of the tertiary butyl urea group of L1. Metal sulfate binding selectivity in solution is further supported by isothermal titration calorimetric studies of L1 with different Zn salts in dimethyl sulfoxide (DMSO), where a binding affinity is observed for ZnSO₄ (K_a = 1.23 × 10⁶), which is 30- to 50-fold higher than other Zn salts having other counteranions in DMSO. Sulfate salts of Cd^II/Co^II also exhibit binding constants in the order of ～1 × 10⁶ as in the case of ZnSO₄. Positive role of the urea unit in the selectivity is confirmed by studying a model ligand L2, which is devoid of anion recognition urea unit. Structural characterization of four MSO₄ [M = Zn^II, Cd^II, Co^II, Mn^II] complexes of L1, that is, complex 1, [(L1)₂(Zn)₂(μ-SO₄)₂]; complex 2, [(L1)₂(H₂O)₂(Cd)₂(μ-SO₄)₂]; complex 3, [(L1)₂(H₂O)₂(Co)₂(μ-SO₄)₂]; and complex 4, [(L1)₂(H₂O)₂(Mn)₂(μ-SO₄)₂], reveal the formation of sulfate-bridged eight-membered crownlike binuclear complexes, similar to one of the concentration-dependent dimeric forms of MSO₄ as observed in solid state. Finally, L1 is found to be highly efficient in removing ZnSO₄ from both aqueous and semiaqueous medium as complex 1 in the presence of other competing Zn^II salts via precipitation through crystallization. Powder X-ray diffraction analysis has also confirmed bulk purity of complex 1 obtained from the above competitive crystallization experiment.

Full text of DOI:10.1021/acs.inorgchem.6b00176

Article
pubs.acs.org/IC
Unusual Recognition and Separation of Hydrated Metal Sulfates
[M2(μ-SO4)2(H2O)n, M = ZnII, CdII, CoII, MnII] by a Ditopic Receptor
Tamal Kanti Ghosh, Ranjan Dutta, and Pradyut Ghosh*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Kolkata
700032, India
S
* Supporting Information
ABSTRACT: A ditopic receptor L1, having metal binding bis(2-
picolyl) donor and anion binding urea group, is synthesized and
explored toward metal sulfate recognition via formation of
dinuclear assembly, (L1)2M2(SO4)2. Mass spectrometric analysis,
1H-DOSY NMR, and crystal structure analysis reveal the
existence of a dinuclear assembly of MSO4 with two units of
L1. 1H NMR study reveals significant downfield chemical shift of
−NH protons of urea moiety of L1 selectively with metal sulfates
(e.g., ZnSO4, CdSO4) due to second-sphere interactions of sulfate
1
with the urea moiety. Variable-temperature H NMR studies
suggest the presence of intramolecular hydrogen bonding
interaction toward metal sulfate recognition in solution state,
whereas intermolecular H-bonding interactions are observed in solid state. In contrast, anions in their tetrabutylammonium salts
fail to interact with the urea −NH probably due to poor acidity of the tertiary butyl urea group of L1. Metal sulfate binding
selectivity in solution is further supported by isothermal titration calorimetric studies of L1 with different Zn salts in dimethyl
sulfoxide (DMSO), where a binding affinity is observed for ZnSO4 (Ka = 1.23 × 106), which is 30- to 50-fold higher than other
Zn salts having other counteranions in DMSO. Sulfate salts of CdII/CoII also exhibit binding constants in the order of ∼1 × 106
as in the case of ZnSO4. Positive role of the urea unit in the selectivity is confirmed by studying a model ligand L2, which is
devoid of anion recognition urea unit. Structural characterization of four MSO4 [M = ZnII, CdII, CoII, MnII] complexes of L1, that
is, complex 1, [(L1)2(Zn)2(μ-SO4)2]; complex 2, [(L1)2(H2O)2(Cd)2(μ-SO4)2]; complex 3, [(L1)2(H2O)2(Co)2(μ-SO4)2]; and
complex 4, [(L1)2(H2O)2(Mn)2(μ-SO4)2], reveal the formation of sulfate-bridged eight-membered crownlike binuclear
complexes, similar to one of the concentration-dependent dimeric forms of MSO4 as observed in solid state. Finally, L1 is found
to be highly efficient in removing ZnSO4 from both aqueous and semiaqueous medium as complex 1 in the presence of other
competing ZnII salts via precipitation through crystallization. Powder X-ray diffraction analysis has also confirmed bulk purity of
complex 1 obtained from the above competitive crystallization experiment.
2000 mg/L.7 During wastewater and nuclear waste treatment,
removal of metal sulfates is also very problematic as well as a
INTRODUCTION
■
Uncontrolled release of heavy metal salts, particularly in the
form of metal sulfates, into the environment due to
industrialization and urbanization has posed a great concern
challenging task due to high solvation energy associated with
these metal sulfates.8−10
worldwide.1 Unlike organic pollutants, heavy metal salts do not
degrade into harmless end products.2 The presence of heavy
metal ions is problematic to many life forms due to their
toxicity.3 According to the World Health Organization, among
the 10 heavy metals that have been assigned as major public
concern, cadmium(II) is ranked almost at the top position of
the table,4 and the sulfate form of this metal is extremely
carcinogenic and very toxic for lungs.5 Zinc(II) sulfate, the
lighter congener of cadmium(II), is also considered a toxic
heavy metal.5,6 Intake of MnSO4 results in many health
problems, and hydrated CoSO4 is considered as a carcinogenic
compound. Many industrial wastewaters, in particular, those
associated with mining, contain high concentrations of
transition metal sulfates that exceed the secondary drinking
water standard of 250 mg/L and are in the range from 250 to
During the past decade or so liquid−liquid extraction,
precipitation, and crystallization techniques have been utilized
for the extraction of ion pairs of alkali metal salts from solution
using suitable ditopic and dual-host receptors.11−16 However,
Custelcean et al. had first introduced the concept of selective
crystallization for sulfate separation by metal−organic frame-
works.17−19 Later, several other groups,20−30 including
ours,31−33 have utilized such metal ion-assisted self-assembly
processes for selective separation of sulfate via second-sphere
coordination. In this strategy, metal ion and sulfate sit in their
separate and individual binding pockets by disrupting the
contact ion-pair structural features of metal sulfates, which is
Received: January 23, 2016
© XXXX American Chemical Society
A
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one of the concentration-dependent forms of metal sulfate. In
fact, single-crystal X-ray structures of MSO4·xH2O [M = ZnII
and MnII] show the existence of ion-pair bridged dimers
[M2(μ-SO4)2(H2O)n] in solid state.34,35 Thus, an alternative
strategy for metal sulfate recognition and separation would be
to trap such dimeric species of metal sulfates, which might be
advantageous in terms of overcoming high hydration energy
problem associated with the recognition and separation of such
metal salts in aqueous environment. Herein, we show selective
ZnSO4·7H2O and CoSO4·6H2O are subjected to ESI(-ve)
mass spectrometric analysis, characteristic molecular ion peaks
at m/z = 1002.20 and m/z = 992.21 are observed, which
correnpond to binuclear [Zn2(L1)2(SO4)2]− and
[Co2(L1)2(SO4)2]− complexes, respectively (Figure 1). The
experimental isotope distribution patterns of both the
complexes match well with those calculated on the basis of
natural abundances. Thus, in all these cases, probably two L1
trap the preformed sulfate-bridged dimeric unit in solution and
that even exists in the gas phase. However, other monomeric
nonbridged ZnII salts such as Zn(ClO4)2·6H2O, Zn(CF3SO3)2,
and ZnCl2·4H2O show characteristic peaks that correspond to
the mononuclear complexes of ZnII (Figure 2). This suggests
different structural motif for complexes of L1 with MSO4 as
compared to other mononuclear complexes of ZnII salts.
1H Nuclear Magnetic Resonance Experiments. As L1
possesses anion binding urea moiety, hence to gain insight into
the mode of interaction between the ditopic receptor L1 and
various anions, we performed 1H NMR titrations with different
anions having tetrabutylammonium (TBA) countercation in
deuterated dimethyl sulfoxide (DMSO-d6) at 298 K. L1 does
not interact with any of the anions irrespective of their basicity
2−
recognition and separation of hydrated metal sulfates as SO4
bridged dimeric form [(M)2(μ-SO4)2][M = ZnII/CdII/CoII/
MnII] by a simple ditopic receptor, L1. To the best of our
knowledge this represents the first report on the recognition of
metal sulfates as bridged dimer in solution.
RESULT AND DISCUSSION
■
Designing Aspects of Ligands. Recognition of anion
generally occurs via interaction of guest anions with the acidic
hydrogen of receptors. Binding of metal ion in suitable part of
the ligand can also alter the electronic and geometrical
parameters of the ligand, which introduces selectivity among
the investigated anions.36−38 Dimeric form of metal sulfates,
that is, M2(SO4)2(H2O)x, are found to be stabilized by
tridentate metal chelating unit along with some assistance
from anion binding unit. Keeping these aspects in mind, we
prepared ligand L1 having bis(2-picolyl) amine as metal
binding unit and pendant tertiary-butyl urea group as anion
recognizing unit (Chart 1). Thus, binding of metal could
1
(Figure S10). H NMR of a mixture of L1 (5.69 mM) and
TBAHSO4 (61.28 mM) in DMSO-d6 showed negligible change
in chemical shifts of the NH protons of urea moiety.
Furthermore, addition of (TBA)2SO4 (64.23 mM) into L1
(6.28 mM) also did not alter the peak positions of the urea
moiety. This silent behavior of the receptor L1 toward anions
can be can be attributed toward combination of (i)
intermolecular H bonding between the urea moieties of
adjacent L1 units and (ii) weak acidity of NH protons, which
make L1 reluctant toward H bonding interaction with anions.
This indicates L1 as a poor anion receptor.
Chart 1. Synthesis of L1 and Numbering Scheme Used for
1H NMR Spectral Assignment
However, 1H NMR analysis of L1 in the presence of
equivalent amount of ZnSO4 (Figure 3j) shows significant
downfield shifts of NH protons (Δδ = 0.65 ppm for NHb and
Δδ = 0.52 ppm for NHa) of urea unit along with usual shifts of
pyridyl protons due to binding of ZnII with bis(2-picolyl) unit
2−
(Figure 3). This indicates the binding of SO4 with NH
protons of L1. Also, NH protons of L1 bind with SO42− unit of
CdSO4 as evident from considerably large downfield shift (Δδ
= 0.75 ppm for NHb and Δδ = 0.60 ppm for NHa) of NH
protons of urea unit along with usual shifts of pyridyl ring
protons due to binding of CdII with bis(2-picolyl) unit (Figure
1
3k). Interestingly, H NMR analysis of L1 in the presence of 1
equiv of other ZnII salts having various counter-anions such as
−
−
−
−
Cl−, Br−, NO3 CH3CO2 , ClO4 , CO32−, or H2PO4 (Figure
influence the binding of the anion counterpart in such a way
that selectivity among the investigated anions can be
introduced through dissimilar binding with the pendant urea
moiety. To investigate the positive role of urea group toward
anion selectivity in L1, modified ligand L2 is synthesized
(Chart 2), which lacks the anion binding urea moiety having
the same metal binding motif.
Mass Spectrometric Analysis. First evidence on the
trapping of dinuclear sulfate salts of ZnII and CoII are obtained
from electrospray ionization mass spectrometry (ESI-MS)
study. When solution of equimolar mixture of L1 and
3) show practically no change in resonance positions of NH
1
protons at 298 K in DMSO-d6. Careful analysis of H NMR
spectra suggest that most of the ZnII salts react with L1 without
any observable chemical shift change of the -NH protons of the
urea moiety except in the cases of Zn(CF3SO3)2, Zn(ClO4)2,
and Zn(NO3)2. Slight and same extent of downfield shift
(exactly 0.18 and 0.1 ppm for NHb and NHa, respectivly, in all
the cases) of N−H protons for these three metal salts (Figure
3g−i) can be attributed to the polarization of electron density
from -NH protons induced by metal coordination of −CO
group39 adjacent to N−H protons without having any second-
sphere interaction between the anionic counterpart of metal
salts and -NH protons, which is later confirmed by single-
crystal X-ray diffraction studies (Figure S18). All these data
Chart 2. Synthetic Route Towards Preparation of L2
2−
confirmed that among all the salts, SO4 salts are only
recognized by L1 through second-sphere interaction.
B
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Figure 1. Experimental (upper) and simulated (lower) ESI-MS(-ve) spectra of (a) complex 1 and (b) complex 3.
Figure 2. ESI-MS(+ve) spectra of monomeric complexes of (a) Zn(OTf)2 complex of L1, (b) ZnCl2 complex of L1, and (c) Zn(ClO4)2 complex of
L1.
1
1H NMR spectral pattern is observed in case of CdSO4 like that
Inspired by the above results H NMR titration experiments
are performed between L1 with ZnSO4 to evaluate the binding
stoichiometry. During the course of titration NHb and NHa
protons of free L1 at 5.76 and 5.63 ppm gradually diminish
with concomitant increase in intensity of new set of -NH peaks
at 6.32 and 6.04 ppm, respectively (Figure 4a). After the
addition of nearly 1 equiv of ZnSO4, complete disappearance of
NHb and NHa protons are found along with generation of new
set of NH peaks, which suggest, for example, that 1:1 could be
2:2 binding stoichiometry between L1 and ZnSO4 in solution.
The actual stoichiometry in solution is determined later by 1H-
DOSY experiment (Figure 8 and Table 2). Also, identical
binding stoichiometry between L1 and ZnSO4 is found by
monitoring C5−H1 peak in Figure 4a as the intensity of peak at
δ = 8.47 ppm gradually decreases along with concomitant
increase in intensity of a peak at δ = 8.98 ppm, which saturates
after addition of nearly 1 equiv of ZnSO4 solution (Figure 4a).
To further corroborate the binding stoichiometry of L1 with
of ZnSO4. In the course of titration with CdSO4 in DMSO-d6,
NHa and NHb protons of free L1 at 5.63 and 5.75 ppm
diminish gradually, and a new set of N−H peaks originate at
6.20 and 6.44 ppm, respectively. After the addition of nearly 1
equiv of CdSO4 solution, saturation is attained, which
confirmed that 1:1 could be 2:2 binding stoichiometry between
L1 and CdSO4 in solution similar to the case of ZnSO4. The
actual stoichiometry in solution is later confirmed by 1H-DOSY
experiment (Figure 8 and Table 2). To elaborate the
importance of preorganization of MSO4 through dimerization
during its recognition process, binding affinity of complex 5,
[(L1)Zn(CF3SO3)2], toward anions having TBA countercation
1
is monitored by H NMR experiments (Figure S15). Addition
of DMSO-d6 solution of anions into a DMSO-d6 solution of
complex 5 resulted in upfield shift of the NH protons, and after
saturation the -NH proton peak positions match exactly with
the NH peak positions of free L1. This indicates that none of
the anion binds with the urea moiety. Instead, some of the
2−
1
other SO4 salts, we performed H NMR titration experiment
of L1 with CdSO4·8/3 H2O in DMSO-d6 (Figure 4b). Similar
anions (e.g., Cl−, H2PO4 , OAc−, etc.) bind with the metal
−
C
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Figure 3. 1H NMR of (a) free L1 (5.81 mM) and changes in resonance position after mixing 1 equiv of each (b) ZnCO3, (c) Zn(CH3CO2)2·4H2O,
(d) ZnCl2·xH2O, (e) ZnBr2, (f) Zn(H2PO4)2, (g) Zn(CF3SO3)2, (h) Zn(ClO4)2·6H2O, (i) Zn(NO3)2·6H2O, (j) ZnSO4·7H2O, (k) 3CdSO4·8H2O
in DMSO-d6 at 298 K.
Figure 4. (a) 1H NMR titration profile of L1 (3.2 mM) upon gradual addition of 0.15 equiv of ZnSO4 (31.25 mM) solution in DMSO-d6 at 298 K in
300 MHz. Even after addition of 0.9 equiv of ZnSO4, parent peaks could be observed in the baseline, which diminishes after addition of nearly 1.05
1
equiv of ZnSO4 solution. (b) H NMR titration profile of L1 (2.8 mM) upon gradual addition of 0.12 equiv of CdSO4 (35.6 mM) solution in
DMSO-d6 at 298 K in 300 MHz. Saturation is observed after addition of 0.96 equiv of CdSO4 solution.
center, and hence the metal-bound −CO unit gets expelled
and becomes pendant urea unit as in free L1. Hence, complex 5
does not show any binding with any anions having TBA
countercation as in case of free L1. As after formation of
complex 5 preorganization is not possible, hence any of the
in the same solvent at 298 K. ITC studies reveal that L1 does
not bind with any of the anions as observed from the 1H NMR
studies (Figure S12a). Interestingly, when L1 is titrated with
ZnSO4·7H2O in DMSO at 298 K a smooth and clear
exothermic titration profile is obtained, and subsequent fitting
to a one-site binding profile provided access to the association
constant (Ka), enthalpy change (ΔH), entropy change (ΔS),
and free energy change (ΔG) of the binding process. The
thermodynamic parameters obtained from the above exper-
imental data showed Ka = 1.23 × 106 M−1, ΔS = −25.7 J mol−1
deg−1, and ΔH = −42.6 kJ mol−1. However, it should be
mentioned that the thermodynamic parameters obtained have
contributions from both first- (binding of Zn2+ with bis(2-
picolyl) amine moiety of L1) and second-sphere (binding of
2−
anions including SO4 is not being recognized by urea unit in
2−
complex 5. Recognition of SO4 via urea -NH protons only
with ion-pair of metal sulfates indicates cooperative binding of
metal sulfates by L1.
Isothermal Titration Calorimetric Studies. The solution-
state binding affinity of L1 with various anions is performed by
isothermal titration calorimetric (ITC) experiments. In a typical
ITC experiment, solution of respective anion as its TBA salt in
HPLC-grade DMSO is titrated with a solution of receptor L1
D
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Figure 5. ITC plot in DMSO at 298 K for the addition of a solution of (a) ZnSO4·7H2O (0.685 mM) to a solution of L1 (0.099 mM) and (b)
Zn(ClO4)2 (1.285 mM) to a solution of L1 (0.1087 mM). (upper) The heat pulses experimentally observed in each titration. (lower) The respective
time integrals translating as the heat evolved for each aliquot and its coherence to the one-site binding model.
Figure 6. ITC plot in DMSO at 298 K for the addition of a solution of (a) ZnSO4 (0.931 mM) to a solution of L2 (0.124 mM), (b) Zn(CF3SO3)2
(0.792 mM) to a solution of L1 (0.099 mM), and (c) Zn(NO3)2 (0.9208 mM) to a solution of L1 (0.099 mM). (upper) The heat pulses
experimentally observed in each titration. (lower) The respective time integrals translating as the heat evolved for each aliquot and its coherence to
the one-site binding model.
SO42− with urea moiety of L1) coordination of ZnSO4 with L1.
Table 1. Thermodynamic Parameters Obtained from
To get insight into the anion recognition via urea moiety or to
get an estimate of the second-sphere binding constant (binding
of SO4 with urea moiety of L1), we must eliminate the first-
Isothermal Titration Calorimetric Experiments in Dimethyl
Sulfoxide at 298 K
2−
host
guest
Ka (mol−1
)
ΔH (kJ/mol) ΔS (kJ/mol/deg)
sphere binding constant. To get the value of only the first-
sphere binding constant, L1 is titrated with Zn2+ salts having
noninteracting counteranions (e.g., Zn(ClO4)2, Zn(CF3SO3)2).
In both cases we obtained similar binding constants (Ka = 5.34
× 104 and Ka = 7.18 × 104 for Zn(CF3SO3)2 and Zn(ClO4)2,
respectively; Figures 5b and 6 and Table 1), which is
significantly smaller than that of ZnSO4. Similarly, other ZnII
salts, for example, ZnCl2 and Zn(NO3)2, show binding
constants Ka in the order of ∼1 × 104 M−1, which is similar
to that of Zn(ClO4)2 and Zn(CF3SO3)2 but much smaller as
compared to that of ZnSO4 (Table 1 and Figures 5, 6, and
S12). Binding constant values obtained in these cases are
mainly a manifestation of primary-sphere binding only. From
the binding constant values obatained from the ITC measure-
a
L1
L1
L1
L1
L1
L1
L1
L1
L2
TBAHSO4
n.d.
n.d.
n.d.
Zn(ClO4)2·6H2O
Zn(CF3SO3)2
Zn(NO3)2·6H2O
ZnCl2·xH2O
ZnSO4·7H2O
CoSO4·6H2O
CdSO4·6H2O
ZnSO4·7H2O
7.18 × 104
5.34 × 104
6.96 × 104
5.27 × 104
1.23 × 106
1.58 × 106
7.49 × 106
2.4 × 104
−20.6
−20.4
−23.7
−20.6
−42.6
−58.9
−39.4
−33.9
24.3
22.6
13.6
21.7
−25.7
−78.54
−0.1
−29.1
a
None detected indicated by n.d.
ments, the order of binding affinity of metal salts toward L1 is
as follows: CdSO4 > CoSO4 > ZnSO4 > Zn(ClO4)2
>
E
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Figure 7. ITC plot in DMSO at 298 K for the addition of a solution of (a) CoSO4·6H2O (0.7425 mM) to a solution of L1 (0.099 mM) and (b)
CdSO4·6H2O (0.8695 mM) to a solution of L1 (0.1087 mM). (upper) The heat pulses experimentally observed in each titration. (lower) The
respective time integrals translating as the heat evolved for each aliquot and its coherence to one-binding model.
1
Figure 8. H DOSY NMR of (a) complex 1 and (b) complex 2 in DMSO-d6 at 298 K.
Zn(NO3)2 > Zn(CF3SO3)2 > ZnCl2. Thus, the enhanced
participation of urea moiety toward selective recognition of
MSO4 in case of L1.
−
binding constant (KZnSO4/KZnX2 ≈ 30−50, where X = ClO4 /
−
−
CF3SO3 /NO3 /Cl−) in cases of ZnSO4 with L1 indicates
participation of different mode of binding for sulfate salts with
L1 through preorganization as well as the role of urea group in
stabilizing the dimeric unit through second-sphere H-bonding
interaction.
In the course of titration of L1 with ZnSO4, CdSO4, CoSO4,
and MnSO4 the entropy values decrease (ΔS = −ve), whereas
entropy values increase (ΔS = +ve) in cases of other Zn salts
(Table 1). This clearly suggests more ordering of systems in
cases of binding of MSO4 through the formation of sulfate-
bridged dimer [M2(μ-SO4)2(H2O)n] with L1 when compared
with nonbridged mononuclear hydrated form of other ZnII
salts. Decrease in values of entropy in solution in cases of
MSO4 and L1 has contrbution of both (i) formation of dimeric
unit from monomer and (ii) second-sphere binding between
To assertain the metal sulfate selectivety of L1 we also
performed ITC experiments with CdSO4·8/3 H2O and CoSO4·
6H2O with L1 in DMSO, and the binding constants are found
to be 7.49 × 106 and 1.58 × 106 M−1, respectivly (Table 1 and
Figure 7), which is quite similar to the value of ZnSO4 but
much larger as compared to the values obtained in case of other
ZnII salts.
To confirm the positive role of urea moiety we synthesized a
control receptor L2, having similar bis(2-picolyl)amine moiety
but lacks any anion binding group (Chart 2). ITC experiment
of L2 with ZnSO4 (Figure 6) under similar experimental
condition shows binding constant Ka = 2.4 × 104 M−1 (Table
1), which is similar to the cases of L1 and ZnX2 and much
smaller than the case of L1 and ZnSO4. This confirms the
2−
the pendant urea moiety and bridged SO4 unit. The binding
process of MSO4 with L1 is such a highly enthalpy driven
(highly exothermic) process that it supersedes the unfavorable
negative or nearly zero contribution from entropy to produce
high negative value of Gibbs free energy change (ΔG = −ve),
which is reflected in high binding constant of MSO4 with L1. In
the course of titration of L2 and ZnSO4, decrease in entropy
value is probably due to the formation of SO42− bridged dimer;
however, ordering of system due to H bonding is not possible
here. All these data support the existence of dimeric unit in
F
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solution in case of metal sulfates. In cases of other ZnII salts, the
binding process is both moderately entropy and enthalpy
driven with lower values of Gibbs free energy change, that is,
lower binding constant values.
and Table 2). From Table 2 it is evident that the sizes of both
the complexes in solution are significantly larger than that of
L1. Interestingly, the molecular diameters of both the
complexes in solution match quite well with the calculated
diameters from the crystal structures for 2:2 complexes between
L1 and ZnSO4/CdSO4. This suggests the formation of 2:2
complexes between L1 and ZnSO4/CdSO4 in solution. All the
peaks in the spectra of both the complexes show similar
diffusion coefficients, which in turn suggests the formation of a
single 2:2 complex in solution. Furthermore, the diffusion
coefficient of equivalent amount of L1 and ZnSO4 in DMSO-d6
solution is found to be 1.72 × 10−10 m2/s, which is similar to
that of complex 1 (Figure S25). Thus, we conclude that L1
It is a known fact that adding even a small amount of water
to DMSO can significantly weaken the interaction between
hydrogen bond donors and anions. So, to find the sustainability
of selectivity of L1 toward MSO4, we performed ITC
measurements in 10% H2O/DMSO binary solvent mixture.
ITC results showed highest binding constant for ZnSO4 (Ka =
4.86 × 106) with L1, which is ∼20−30 times higher as
compared to other ZnII salts (Ka ≈ 1 × 104; Figure S24).
To confirm the binding stoichiomentry between MSO4 (M =
ZnII, CdII) and L1 in solution we performed two-dimensional
diffusion-ordered spectroscopy (DOSY), which has become an
important technique for studying the self-assembly of supra-
1
forms 2:2 complex with MSO4 in solution as supported by H
DOSY experiments. The similar values of molecular diameter
also suggest that ZnSO4/CdSO4 exists as contact ion pair in
solution.
molecular systems in solution.40−42 Figure 8 shows H DOSY
1
NMR of complexes 1 and 2 in DMSO-d6. The hydrodynamic
radius (r) of the complexes is calculated from the following
Stokes−Einstein relation using experimentally obtained dif-
fusion coefficients at 298 K for L1 (Table 2).
Single-Crystal X-ray Studies. Finally, we ascertained the
existence of the dimeric [M2(μ-SO4)2] unit in MSO4 complexes
of L1 by single-crystal X-ray diffraction studies. Block-shaped
crystals of complex 1, [Zn2(L1)2(μ-SO4)2]; complex 2,
[Cd2(L1)2(μ-SO4)2(H2O)2]; complex 3, [Co2(L1)2(μ-
SO4)2(H2O)2]; and complex 4, [Mn2(L1)2(μ-SO4)2(H2O)2]
are obtained from the mixture of L1 and ZnSO4·7H2O, CdSO4·
8/3H2O, CoSO4·6H2O, and MnSO4·6H2O, respectively, both
from DMSO/H2O (3:1 v/v) binary solvent system and pure
water. Crystallographic details of complexes 1−4 are given in
Table 3. Interestingly, all the structures reveal that dimeric
metal sulfate cluster M2(μ-SO4)2(H2O)m [m = 0 and 2 for M =
ZnII and CdII/CoII/MnII, respectively] is trapped between two
units of L1. In all these complexes ligated water molecules of
the dinuclear sulfate-bridged complex M2(SO4)2(H2O)x are
displaced from the primary coordination sphere in the course of
complexation according to the coordination requirement of
respective metal ions, keeping the bridged unit intact. Crystals
of complex 1 are isolated by the slow evaporation of DMSO/
H2O (3:1) solution of L1 and ZnSO4·7H2O. Crystal structure
analysis reveals the presence of two pentacoordinated ZnII
having trigonal bipyramidal (TBP) geometry. The five
coordination sites of each ZnII are occupied by three nitrogen
atoms (N1, N2, and N3) of bis(2-picolyl) unit and two oxygen
atoms (O2 and O3) of two bridging SO42−. One oxygen atom
KT
hydrodynamic radius r =
6πηD
where D = diffusion coefficient; k = Boltzmann constant
(1.3807 × 10−23 m2 kg S−2 K−1); T = temperature in kelvin; η =
viscosity coefficient of the solution (1.991 × 10−2 g cm−1 S−1
for DMSO); r = hydrodynamic radius of the molecular sphere.
Assuming spherical shape of the crystal structure we
determined molecular diameter of both the complexes by
averaging the dimensions of major and minor axes (Figure 9
of the bridging SO4 (O2) is coordinated to the ZnII in the
2−
Figure 9. Space-filling models from single-crystal X-ray structures of
(a) complex 1, major axis: C11−H11···C10−H10 = 13.3 Å; minor
axis: C14−H14A···C14−H14B = 13.2 Å and (b) complex 2, major
axis: C19−H19A···C19−H19B = 17.4 Å, minor axis: C5−H5···C6−
H6 = 10.8 Å. Blue: nitrogen, Green: hydrogen, Red: oxygen, and Gray:
carbon.
equatorial plane along with N1 and N3, whereas the other two
axial positions are occupied by one tertiary nitrogen atom (N2)
2−
of bis(2-picolyl) unit and one oxygen atom (O3) of SO4
group (Figure 10). The index of trigonality (τ) value is
measured as 0.61, which falls between the ideal τ value for a
perfect square pyramidal geometry (τ = 0) and TBP geometry
(τ = 1). All the bond distances and bond angles are provided in
Table S1. Two SO42− anions act as bidentate ligand and bridge
two Zn2+ centers by forming an eight-membered zigzag ring
exactly similar to the structure of molecular crownlike structure
of S8 (Figure S13). Coordination environment of SO42− shows
Table 2. Diffusion Coefficient (D) and Hydrodynamic
Diameter (2r) in DMSO-d6 at 298 K
2−
two oxygens (O2 and O3) of SO4 anions are coordinated to
2r, Å
ZnII, where the equatorial Zn(1)−O(2) bond has smaller bond
length (1.961 Å) as compared to the axial Zn(1)−O(3) bond
(2.054 Å) as expected from the TBP geometry. All the bond
angles fall in the region between 106° and 130°, which are quite
common with distorted TBP coordination environment. The
two equatorial bond lengths of Zn(1)−N(1) and Zn(1)−N(3)
are 2.048 and 2.089 Å, respectively, which is in agreement with
the tridentate ligands containing pyridyl units (2.02−2.09 Å),
D,
1× 10−10
calculated (from
crystal structure)
experimental (from
compound
m2 s−1
diffusion coefficient)
L1
2.63
1.69
1.65
1.72
8.1 Å
complex 1
complex 2
13.25 Å
14.1 Å
13.18 Å
13.5 Å
13.12 Å
1:1 mixture of L1
and ZnSO4
G
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Table 3. Crystal Data and Refinement Details of Complexes 1−4
compound
chemical formula
complex 1
complex 2
complex 3
complex 4
C38H54N10O12S2Zn2
1037.77
C42H78Cd2N10O18S4
1364.18
monoclinic
16.1563(12)
11.0458(8)
16.3922(12)
90.00
C42H66Co2N10O18S4
1245.15
monoclinic
16.0294(11)
10.8894(8)
16.4207(12)
90.00
C38 H54 Mn2N10O14S2
1048.91
formula mass
crystal system
triclinic
monoclinic
13.439(2)
10.8861(17)
19.870(3)
90
a/Å
9.1081(5)
b/Å
9.5838(5)
c/Å
14.8352(8)
78.6128(12)
73.7441(11)
66.1009(11)
1131.38(11)
150(2)
α/deg
β/deg
104.037(2)
90.00
104.5296(16)
90.00
109.719(4)
90
γ/deg
unit cell volume/Å3
2838.0(4)
150(2)
2774.6(3)
150(2)
2736.5(8)
150(2)
temperature/K
space group
P1
P2(1)/n
2
P2(1)/n
2
P2(1)/n
2
̅
no. of formula units per unit cell, Z
1
radiation type
Mo Kα
1.223
10 841
3945
Mo Kα
0.973
Mo Kα
Mo Kα
absorption coefficient, μ/mm−1
no. of reflections measured
no. of independent reflections
Rint
0.826
32 918
32 372
23 492
4417
4993
4865
0.0234
0.0301
0.0927
0.0325
0.0950
1.070
0.0226
0.0409
0.0581
0.1038
0.3126
0.1246
0.3233
1.153
final R1 values (I > 2σ(I))
final wR(F2) values (I > 2σ(I))
final R1 values (all data)
final wR(F2) values (all data)
goodness of fit on F2
0.0190
0.0356
0.0510
0.1243
0.0198
0.0438
0.0515
0.1363
1.090
1.094
Figure 10. Ball-and-stick representation of crystal structures of (a) complex 1, [(L1)2Zn2(SO4)2], and (b) complex 2, [(L1)2Cd2(SO4)2(H2O)2].
Hydrogen atoms, except those of the urea group, are omitted for clarity.
Figure 11. Ball and stick representation of crystal structures of (a) Complex 3, [(L1)2Co2(SO4)2] and (b) Complex 4, [(L1)2Mn2(SO4)2(H2O)2].
Hydrogen atoms, except those of the urea group, are omitted for clarity.
and the other axial Zn(1)−N(3) bond length (2.264 Å) is
slightly higher as observed with TBP geometry. One of the
coordinated oxygen atoms of SO42− group having longer ZnII−
O (Zn1−O3) bond length (2.054 Å) is involved in strong
intermolecular hydrogen bonding interaction with the N−H
group (N5−H5) of urea moiety, while other oxygen atom (O2)
having stronger interaction with the metal center is reluctant in
hydrogen bonding with the urea group (Figure S14).
Similarly, we obtained the crystals of complex 2
[Cd2(L1)2(SO4)2(H2O)2] by slow evaporation of equimolar
mixture of L1 and CdSO4·8/3 H2O in DMSO/H2O (3:1 v/v)
binary solvent system and pure water. Crystal structure of 2
H
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Figure 12. Change in chemical shifts of NHa protons in (a) complex 1 and (b) complex 2 upon change in temperature in DMSO-d6.
shows two symmetrical CdII centers in a distorted octahedral
environment, coordinated by N3O3-type ligand motif in a facial
geometrical arrangement (Figure 10). Two CdII centers are
the coordination of three nitrogen atoms (N4, N5, N3) from
bis(2-picolyl) unit, and the other face is formed by two oxygen
2−
atoms (O3, O6) from two bridging SO4 anion and one
2−
connected to each other via two bridging SO4 anions. In the
oxygen atom from coordinated H2O molecule. All the bond
distances and bond angles are provided in Table S1. The crystal
structure of complex 4 also shows intermolecular H-bonding
facial-type geometry one face is formed by the coordination of
three nitrogen atoms (N1, N2, and N3) from bis(2-picolyl)
unit, and the other face is formed by two oxygen atoms (O5,
O2) from two bridging SO42− anion and one oxygen atom from
coordinated H2O molecule. The other two noncoordinated
2−
interaction between SO4 unit and urea moiety from another
unit (Figure S17). We also obtained the crystals of complex 5
by diffusing a solution of di-isopropyl ether into acetonitrile
solution of 1:1 mixture of L1 and Zn(OTf)2. Crystal structure
analysis of complex 5 shows ZnII ion is in a distorted TBP
geometry in which the three coordination sites are occupied by
the thee nitrogen atoms of bis(2-picolyl) unit (N1, N2, and
N3) and the remaining two positions are occupied by the
carbonyl oxygen atom (O1) and one water molecule (O8;
Figure S18). In the crystal structure N2, N3 from the bis(2-
picolyl) unit and along with the carbonyl oxygen, O1
coordinates to the central ZnII in the equatorial plane. All the
bond angles in the equatorial plane range within 108°−128°,
which are relatively close to the ideal TBP geometry, whereas
the axial O8−Zn1−N1 bond angle is measured as 167°, which
deviates nearly 13° from the ideal structure, and the index of
trigonality (τ) value is calculated as 0.68, which falls between
the ideal τ value for a perfect square pyramidal geometry (τ =
0) and TBP geometry (τ = 1). All the bond distances and bond
2−
oxygen atoms (O3 and O4) of SO4 anion are involved in
intermolecular hydrogen bonding interaction with the -NH
group of urea moiety (N4−H4···O3 and N5−H5···O4). Both
of the hydrogen bond lengths (2.095 and 2.119 Å, respectively,
for N4−H4···O3 and N5−H5···O4) in complex 2 are much
shorter as compared to that of complex 1 (2.185 Å N5−H5···
O3; Figure S15). Similarly, we also got the crystal structure of
complex 3 via slow evaporation of DMSO/H2O (3:1) binary
solvent mixture and pure H2O solution of L1 and CoSO4·
6H2O. Crystal structure reveals the presence of two sym-
metrical CoII units in a distorted octahedral geometry with
N3O3-type donor ligands (Figure 11). Two CoII centers are
2−
connected to each other via two bridging SO4 anions in this
facial type of octahedral geometry in which one of the faces is
formed by the coordination of three nitrogen atoms (N1, N2,
N3) of bis(2-picolyl) unit, while the other face contains two
oxygen atoms (O2 and SO42−) and one oxygen atom of
coordinated H2O molecule (O3). In the square plane the four
bond angles range from 78° to 100°, which is in quite
acceptable range for a distorted octahedral geometry. Nearly
same bond distance of Co1−N3 and Co1−N1 suggests that the
two pyridyl nitrogen along with O2 and O3 are positioned in
the square plane and that the other two atoms (N2 and O5) are
placed in the axial position. In the course of secondary sphere
interaction37,38,43 as contradictory with ZnSO4 here the other
−
angles are provided in Table S1. The CF3SO3 counteranions
in complex 5 are in weak hydrogen bonding interactions with
the urea −NH protons. The lengthening of C15−O1 bond
(1.28 Å) in complex 5 as compared to the free ligand L1 (1.23
Å) is in agreement with −CO coordination to the central
ZnII atom.
Intramolecular Versus Intermolecular Hydrogen
Bonding. It is important to mention here that in solid state
all the sulfate complexes show intermolecular H-bonding
2−
2−
two noncoordinated oxygen atom of SO4 anion are involved
interaction between SO4 unit and pendant −NH group of
in intermolecular hydrogen bonding interaction with the two
N−H (N4−H4···O4 and N5−H5···O6) protons of urea moiety
in which the hydrogen bond distances are much shorter as
compared to those of complex 1 (Figure S16). Crystals of
complex 4 [Mn2(L1)2(SO4)2(H2O)2] are obtained by slow
evaporation of equimolar mixture of L1 and MnSO4·6H2O in
DMF/H2O (3:1 v/v) binary solvent system and pure water.
Crystal structure of 4 shows two symmetrical MnII centers in a
distorted octahedral environment, coordinated by N3O3-type
ligand motif in a facial geometrical arrangement (Figure 11).
Two MnII centers are connected to each other via two bridging
SO42− anions. In the facial-type geometry one face is formed by
urea moiety as evident from the crystal structure analysis
(Figures S14 and S15). However, variable-temperature 1H
NMR spectra (Figure 12) and dilution experiments (Figures
S26 and S27) confirmed the existence of intramolecular H
bonding in these complexes in solution. In temperature-
dependent studies, in case of complex 1, when changes in
chemical shift of the urea protons are plotted against
temperature, negative slope value of −3.6 × 10−3 K−1 is
obtained, which is in acceptable range for intramolecular H
bonding in solution in highly polar solvent such as DMSO.44
Also, much lower value of negative slope, that is, −1 × 10−3, is
obtained in case of complex 2, which also supports intra-
I
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molecular H bonding in solution. These results suggest that
although in solid state these compounds show intermolecular H
bonding, but in solution state they show preference for
intramolecular H bonding. This is not surprising at all because
in solid state crystal packing is much more important than
minor contribution obtained from entropic factors (which
favors intramolecular H bonding) and thus we observe different
H-bonding mode in solution and solid state.
competitive crystallization experiment in H2O, which gives
values of C 45.19%, H 5.28%, and N 13.18% that matches quite
well with the theoretically calculated C, H, and N values of
complex 1. The elemental analysis of the crystal obtained from
competitive crystallization experiment in DMSO/H2O mixture
did not match well with the calculated one most probably
because of presence of residual high-boiling DMSO molecules,
which are not easily removable. We calculated the separation
efficiency of L1 by weighing the isolated complex obtained
from mixture of L1 and equivalent amount of several ZnII salts.
L1 (200 mg, 0.6 mmol) is found to selectively precipitate out
complex 1 (360 mg, 0.35 mmol) with 58% yield from DMSO/
H2O solvent mixture and 65% yield from pure H2O.
Selective Trapping and Removal of MSO4 from
Aqueous and Semiaqueous Medium. Inspired by the
high degree of affinity of L1 toward MSO4 in solution we
proceeded to investigate the ion-pair selectivity and separation
properites of L1 in both DMSO/H2O binary mixture and pure
water. L1 quantitaively forms complex with ZnSO4 in 100%
water (Figures S20 and S23) and is capable of removing ZnSO4
via precipitation through crystallization. We performed a
competitive crystallization experiment where equimolar
amounts of several competing ZnII salts [e.g., ZnCl2, ZnBr2,
Zn(NO3)2, Zn(ClO4)2, Zn(OTf)2, Zn(OAc)2, ZnCO3, and
ZnSO4] and L1 in 100% water and 3:1 DMSO/H2O showed
that only complex 1 crystallizes from solution that is confirmed
CONCLUSION
■
In conclusion, a ditopic receptor having tridentate metal
chelating site and a pendant urea moiety as anion recognition
element shows selective trapping and separation of metal
sulfate as sulfate-bridged bimetallic ion-pair from aqueous and
semiaqueous solutions. Recognition of metal sulfates in
1
solution is depicted by H NMR and further supported by
1
by H NMR studies (Figures 13 and S21), crystal structure
thermodynamic parameters obtained from ITC studies. 2:2
binding stoichiometry between L1 and ZnSO4/CdSO4 in
solution is confirmed by 1H DOSY NMR spectroscopy. Ligand
also shows potential ability for removal of metal sulfates from
aqueous medium in the presence of several competing salts as
dimeric assembly of metal sulfate with L1, which is confirmed
analysis, and finally by PXRD experiments of the isolated
crystals.
1
by H NMR, powder XRD, and single-crystal X-ray diffraction
analysis. The existence of ZnSO4/CdSO4 as ion pair in solution
is confirmed by comparing the molecular diameters obtained
1
from H DOSY NMR with the diameter of crystal structure,
where ZnSO4/CdSO4 exists as ion pair with two units of L1.
This represents unique examples of metal sulfate recognition as
dimeric bonded ion-pair structure, which is one of the
concentration-dependent forms of metal sulfate in solution.
Our approach of recognizing metal sulfates in this unusual form
could motivate the development of a new generation of anion-
bridged metal salt receptors. Presently, we are working on such
ditopic systems using the above approach to develop various
metal sulfate receptors in aqueous/semiaqueous media.
Figure 13. Comparison of 1H NMR spectra of (a) DMSO-d6 solution
EXPERIMENTAL SECTION
■
1
of crystals of complex 1 and (b) H NMR in DMSO-d6 of crystals
Materials. All reactions were performed in argon gas atmosphere
followed by workup at ambient conditions. Dichloromethane (DCM)
and acetonitrile were dried over CaH2 and collected before use.
HPLC-grade DMSO was purchased from Spectrochem Pvt. Ltd.,
India, and used for complexation, crystallization, and ITC studies.
Deuterated solvents, tetrabutylammonium salts of F−, Cl−, Br−, I−,
obtained from mixture of L1 with several ZnII salts from DMSO/H2O
solvent mixture. Both spectra were recorded in 300 MHz at 298 K.
This selectivity pattern in solution is well-understood when
mixture of equimolar several ZnII salts and L1 in either DMSO-
d6 or D2O showed a 1H NMR spectrum similar to the 1H NMR
of complex 1 in either DMSO-d6 or D2O, respectively (Figures
14 and S22). This result indicates that L1 selectively binds with
ZnSO4 in solution among all other ZnII salts and crystallizes as
complex 1 from both aqueous and semiaqueous solution.
Finally, we performed powder XRD experiments to
determine the nature and bulk purity of the isolated complexes
(in the form of crystals) from mixture of several Zn salts and L1
in DMSO/H2O solvent mixture and pure H2O. PXRD patterns
of of the isolated crystals from above two solvent systems show
similar diffraction patterns with the simulated pattern obtained
from the single-crystal X-ray data of complex 1 (Figure 15).
This result also suggests that only complex 1 is selectively
separated from mixture with high degree of bulk purity. We also
checked elemental analysis of the isolated product from
−
−
−
−
−
AcO−, BzO−, ClO4 , NO3 , HCO3 , HO−, HSO4 , H2PO4 , and zinc
triflate were purchased from Sigma-Aldrich and were used as received.
ZnSO4·7H2O, CdSO4·8/3H2O, and CoSO4·6H2O were purchased
from Merck India Pvt. Ltd. Caution! Although we experienced no
difficulties with the perchlorate salts, these should be regarded as
potentially explosive and handled with care.
Methods. ESI-MS experiments were performed with a Waters
QtoF Model YA 263 mass spectrometer in positive/negative ESI
mode. All the samples for mass spectrometry were prepared by
dissolving the compounds in methanol having following concen-
trations (i) 2.1 × 10−6 (M) for L1, (ii) 3.3 × 10−6 (M) for complex 1,
(iii) 1.4 × 10−6 (M) for complex 3, (iv) 2.5 × 10−6 (M) for
(L1)Zn(ClO4)2 complex, (v) 1.8 × 10−6 (M) for (L1)Zn(CF3SO3)2
complex, and (vi) 2.3 × 10−6 (M) for (L1)ZnCl2 complex. 1H and 13
C
experiments were performed on FT-NMR Bruker DPX 500/300 MHz
NMR spectrometer. Elemental analysis was performed on PerkinElmer
2500 series II elemental analyzer, PerkinElmer, USA. Chemical shifts
J
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Figure 14. Selective formation of ZnII sulfate complex in solution from mixture of several competing Zn(II) salts at 298 K in D2O. 1H NMR of (a)
L1 in D2O, (b) 1:1 mixture of L1 and ZnSO4 in D2O, (c) 1:1 mixture of L1 and various ZnII salts in D2O, and (d) crystals obtained as precipitate
from solution of 1:1 mixture of L1 and various ZnII salts in D2O.
subtracted from the corresponding titration to remove any effect from
the heats of dilution from the titrant.
Diffusion Measurements. All 1H DOSY experiments were
performed on a Bruker AVII 500 MHz spectrometer. Data analyses
were performed using tools within TOPSPIN 2.1 software. Sample
volumes were 450 μL, and the concentration of the samples were 5.2
mM for complex 1 and 5.5 mM for complex 2. All the experiments
were performed in DMSO-d6 solvent. Diffusion coefficients and
hydrodynamic radii are correlated theoretically by the Stokes−Einstein
relation.
X-ray Crystallographic Refinement Details. Crystals suitable
for single-crystal X-ray diffraction studies for complexes 1−5 were
selected from the mother liquor and immersed in paratone oil and
then mounted on the tip of a glass fiber and cemented using epoxy
resin. Intensity data for the all the crystals were collected using Mo Kα
(λ = 0.7107 Å) radiation on a Bruker SMART APEX II diffractometer
equipped with CCD area detector at 150 K. The data integration and
reduction were processed with SAINT software45 provided with the
Figure 15. Comparative of PXRD patterns of (a) crystals obtained
software package of SMART APEX II. An empirical absorption
from mixture of L1 with several ZnII salts in H2O (experimental), (b)
crystals obtained from mixture of L1 with several ZnII salts from
DMSO/H2O solvent mixture (experimental), and (c) complex 1
(simulated).
correction was applied to the collected reflections with SADABS.46
The structures were solved by direct methods using SHELXL47 and
were refined on F2 by the full-matrix least-squares technique using the
SHELXL-9748 program package. Graphics were generated using
PLATON49 and MERCURY 2.3.50 Non-hydrogen atoms were refined
anisotropically until convergence was reached.
for 1H and 13C NMR were reported in parts per million, calibrated to
the residual solvent peak set, with coupling constants reported in hertz.
1H NMR titration was performed on 300 MHz Bruker DPX NMR
spectrometer.
Isothermal Titration Calorimetric Studies. The ITC experiments
were performed with a MicroCal VP-ITC instrument. The titrations
were performed at 298 K in HPLC-grade DMSO solvent. A solution
of host in DMSO was placed in the measuring cell. This solution was
then titrated with 30/60 injections of the respective guest solution (5/
10 μL) that was prepared in DMSO. An interval of 220 s was allowed
between each injection, and the stirring speed was set at 329 rpm. The
obtained data were processed by using Origin 7.0 software that was
supplied with the instrument and was fitted to a one-site binding
model. A blank titration of plain solvent was also performed and
Syntheses. Synthesis of [tert-Butyl 2-(bis(pyridin-2-ylmethyl)-
amino)ethylcarbamate] (L1″). The ligand was synthesized following
the literature procedure.51
Synthesis of [1-(2-(Bis(pyridin-2-ylmethyl)amino)ethyl)-3-tert-bu-
tylurea] (L1). In a 100 mL round-bottom flux 625 mg (1.82 mmol) of
L1″ was dissolved in 20 mL of dry DCM and a solution of 2 mL
trifluoroacetic acid in 10 mL dry DCM was added dropwise via a
dropping funnel into the previously mentioned solution at 0 °C. The
resulting solution was allowed to stir at room temperature for 6 h. The
solution was evaporated to get an oily product, which was used in the
next step without further purification. The oily product (1.8 mmol)
was dissolved in dry DCM, and dry triethylamine (2.53 mL, 18.2
mmol) was added into it; the resulting solution was stirred for 10−15
min, and after that 220 μL (1.9 mmol) of tertiarybutyl isocyanate in
K
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dry DCM was added dropwise via a pressure-equalizing funnel into the
stirring solution at 0 °C. The reaction mixture was allowed to stir
overnight, and then the solvent was evaporated. After addition of water
into the crude solution it was extracted with DCM for 2−3 times. The
overall DCM was collected and washed with brine solution and then
dried with Na2SO4. The DCM part was filtered and evaporated to get
an oily product, which after washing with hexane for several time
yielded solid 398 mg L1 in 65% yield. Anal. Calcd for C19H27N5O
(MW 341.4) C, 66.83; H, 7.97; N, 20.51; found: C, 66.73; H, 7.93; N,
20.45%; ESI MS C19H28N5O calcd, m/z = 342.22; found, m/z =
342.15. ESI MS C19H27N5ONa calcd, m/z = 364.21; found, m/z =
364.11. ESI MS C19H27N5OK calcd, m/z = 380.18; found, m/z =
380.09. 1H NMR (300 MHz, DMSO-d6): δ = 8.476−8.462 (d, 2H, H1,
J = 4.2 Hz), δ = 7.777−7.720 (t, 2H, H3, J = 8.5 Hz), δ = 7.598−7.572
(d, 2H, H2, J = 7.8 Hz), δ = 7.261−7.217(m, 2H, H4), δ = 5.766 (s,
1H, Hb), δ = 5.649−5.612 (t, 1H, Ha, J = 5.4 Hz), δ = 3.739 (s, 4H,
H5), δ = 3.135−3.074 (q, 2H, H7, J = 6.3 Hz), δ = 1.121 (s, 9H, H8)
ppm, 13C NMR (75 MHz, DMSO-d6): 159.81, 157.76, 149.15, 136.90,
123.10, 122.54, 60.15, 54.36 49.40, 37.51, 29.82 ppm.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00176.
■
S
Spectral characterization of each compound, solution-
1
state H NMR studies, ITC experiments, truncated S8
view of complexes, hydrogen bonding pattern in
complexes 1 and 2, ORTEP views of L1 and other
complexes, selected bond distances and angles for
complexes 1−5, hydrogen bonding data for complexes
1−4, 1H NMR of selectivity and separation studies.
(PDF)
(CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: icpg@iacs.res.in.
Notes
■
Synthesis of Complex 1. Into a clear solution of ligand L1 (40 mg,
0.12 mmol) in DMSO, a solution of ZnSO4·7H2O (34 mg, 0.118
mmol) in water was added and stirred for 1 h yielding a clear solution.
This solution was kept for crystallization, and after 2−3 d crystals
suitable for X-ray diffraction were obtained from the solution. Anal.
Calcd for C38H54N10O10S2Zn2 (M.W: 1005.80) C, 45.38; H, 5.41; N,
13.3. Found: C, 45.05; H, 5.33; N, 12.96%. ESI MS(-ve)
C38H54N10O10S2Zn2 calcd, m/z = 1002.2049; found, m/z =
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.G. gratefully acknowledges the Council for Scientific and
Industrial Research (CSIR), New Delhi, India (Grant No.
01(2715)/13/EMR-II), for financial support. T.K.G. acknowl-
edges CSIR for SRF.
1
1002.1024. H NMR (300 MHz, DMSO-d6): δ = 8.99−9.01 (d, 2H,
J = 6 Hz), δ = 8.06−8.00 (t, 2H, J = 9 Hz), δ = 7.54−7.48 (m, 4H), δ
= 6.44 (bs, 1H,), δ = 6.20 (bs, 1H), δ = 4.28−4.23 (d, 2H, J = 15 Hz),
δ = 4.05−4.00 (d, 2H, J = 15 Hz), δ = 2.93 (bs, 2H), δ = 2.84 (bs, 2H)
ppm.
REFERENCES
■
Synthesis of Complex 2. Into a solution of ligand L1 (40 mg, 0.12
mmol) in DMSO, a solution of CdSO4·8/3 H2O (90.24 mg, 0.117
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suitable for X-ray diffraction were obtained from the solution. Anal.
Calcd for C38H54Cd2N10O10S2 (MW 1099.84) C, 41.50; H, 4.95; N,
12.74. Found: C, 41.25; H, 4.93; N, 12.96%. ESI MS(-ve)
C38H52Cd2N10O10S2 calcd, m/z = 1100.1527; found, m/z =
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9.37%. ESI MS (+ve) C22H31F3N5O4SZn calcd, m/z = 582.13; found,
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1
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