Selective retention of methanol over ethanol by a cyclen-based cryptand/copper(II) complex

Article abstract of DOI:10.1021/ic4021417

A cyclen-based cryptand (2) was prepared in a two-step synthesis from dioxocyclen. When a Cu(CF₃SO₃)₂ complex with 2 was prepared in methanol, the 2/Cu(CF₃SO₃)₂ complex incorporated one methanol in the cavity formed by the diethyleneoxy unit and the NH group of the cyclen. When prepared in ethanol, the 2/Cu(CF ₃SO₃)₂ complex similarly incorporated one ethanol. Cold electrospray ionization mass spectrometry (CSI-MS) of the 2/Cu(CF₃SO₃)₂/alcohol complexes selectively retains methanol over ethanol under CSI-MS conditions.

Full text of DOI:10.1021/ic4021417

Communication
pubs.acs.org/IC
Selective Retention of Methanol over Ethanol by a Cyclen-Based
Cryptand/Copper(II) Complex
Yoichi Habata,*,†,‡ Mari Ikeda,† Ajay K. Sah,†,§ Kanae Noto,‡ and Shunsuke Kuwahara†,‡
†Department of Chemistry, Faculty of Science, and ‡Research Center for Materials with Integrated Properties, Toho University, 2-2-1
Miyama, Funabashi, Chiba 274-8510, Japan
S
* Supporting Information
2,1-diyloxy)]bis[3-(chloromethyl)benzene] (6) with 1,4,7,10-
ABSTRACT: A cyclen-based cryptand (2) was prepared
in a two-step synthesis from dioxocyclen. When a
Cu(CF3SO3)2 complex with 2 was prepared in methanol,
the 2/Cu(CF3SO3)2 complex incorporated one methanol
in the cavity formed by the diethyleneoxy unit and the NH
group of the cyclen. When prepared in ethanol, the 2/
Cu(CF3SO3)2 complex similarly incorporated one ethanol.
Cold electrospray ionization mass spectrometry (CSI-MS)
of the 2/Cu(CF3SO3)2/alcohol complexes selectively
retains methanol over ethanol under CSI-MS conditions.
tetraazacyclododecane-2,6-dione (7). 3 was finally reduced with
DIBAL-H to give 2. The structures of 2 and 3 were confirmed by
1H and 13C NMR, fast atom bombardment mass spectrometry,
elemental analysis, and X-ray crystallography. As shown in Figure
2, the diethyleneoxy unit of 2 adopts a twisted conformation.
yclic polyamines, such as cyclam (14-membered cyclic
C
polyamine) and cyclen (12-membered cyclic polyamine),
can potentially bind heavy-metal ions and exhibit host−guest
interactions.1 Other research groups have added to these basic
cyclic polyamines and have reported the syntheses of bicyclic,2a−e
tricyclic,2f,g and tetracyclic2f compounds derived from cyclam
and cyclen. During our efforts to functionalize cyclen, we found
that quadruple- and double-armed cyclens with aromatic side
arms behave like insectivorous plants (e.g., Venus flytrap) when
forming complexes with Ag+ ions.3 We called these quadruple-
and double-armed cyclens “argentivorous molecules”. To extend
this research, we have prepared a cyclen-based cryptand (2) that
incorporates a bridging diethyleneoxy unit between two aromatic
side arms at the 1 and 7 positions of the cyclen ring (Figure 1).
Figure 2. Ellipsoid plot of 2. Hydrogen atoms and the solvent
acetonitrile have been omitted for clarity.
We prepared the 2/Cu(CF3SO3)2 complex using methanol
and ethanol as solvents. Structures of a methanol-containing
Cu(CF3SO3)2 complex [2/Cu(CF3SO3)2/MeOH] and an
ethanol-containing Cu(CF3SO3)2 complex [2/Cu(CF3SO3)2/
EtOH] were confirmed by elemental analysis and X-ray
crystallography. Elemental analysis revealed that one molecule
of methanol or ethanol was contained in each complex. Figures 3
Figure 1. Double-armed cyclen (1) and cyclen-based cryptand (2).
The new cryptand has two binding moieties, a cyclen and a
diethyleneoxy unit. We expected that the conformation of the
diethyleneoxy unit would allow inclusion of a neutral guest, such
as an alcohol, when the cyclen moiety has formed a complex with
a metal ion. Here, we report the first example of a complex
employing the cyclen-based cryptand, which selectively retains
methanol over ethanol.
Figure 3. Ball and stick plot (a) and skeletal drawing (b) of the 2/
Cu(CF3SO3)2/MeOH complex. Hydrogen atoms and the non-
coordinating CF3SO3 anion have been omitted from the ball and stick
plot for clarity. Figure S7 in the SI shows an ORTEP diagram with the
hydrogen atoms of 2/Cu(CF3SO3)2/MeOH.
The new cryptand 2 was prepared in a two-step synthesis from
dioxocyclen (7; see the Supporting Information, SI). The bicyclic
precursor 3 was prepared by the reaction of 1,1′-[oxybis(ethane-
Received: August 21, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic4021417 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
and S7 in the SI show the structure of 2/Cu(CF3SO3)2/MeOH.
In this complex, Cu1 is five-coordinated by the four nitrogen
atoms of the cyclen and one oxygen atom of the CF3SO3 anion.
The Cu1−N1, Cu1−N2, Cu1−N3, and Cu1−N4 bonds are
1.9879(16)−2.0713(16) Å, and the Cu1−O4 bond is
2.2121(13) Å. The Cu1−N (N1, N2, N3, and N4) distances
are comparable with those of the copper(II) complex with a
double-armed cyclen previously reported.3b The diethyleneoxy
unit of the complex is not disordered, and the oxygen atom
(O10) of the methanol forms hydrogen bonds with the three
oxygen atoms (O1, O2, and O3) of the diethyleneoxy unit and
with one nitrogen atom (N2) of the cyclen. As shown in Figure
3b, the O10(methanol)−O1, O10−O2, and O10−3 distances
are in the range 3.10−3.23 Å, and the O10−N2 distance is 2.91 Å.
These O−O and O−N distances are comparable with the O−O
and O−N hydrogen-bond distances in complexes of 18-crown-
64a−d and diaza-18-crown-64e−g derivatives. The methanol
oxygen O10 is above the mean plane of the bonding atoms
(green square, O1, O3, and N2) at a distance of 1.34 Å.
To see if the copper(II) complex with 2 can retain methanol
and ethanol, CSI-MS of a mixture of 2 and Cu(CF3SO3)2 was
measured in methanol or ethanol at 298 K. Figure 5 (top) shows
When ethanol was used as a solvent to prepare the
Cu(CF3SO3)2 complex, an ethanol-containing Cu(CF3SO3)2
complex [2/Cu(CF3SO3)2/EtOH] was obtained. As shown in
Figures 4a and S7 in the SI, Cu1 is again five-coordinate, with
Figure 5. CSI-MS (298 K) of a mixture of 2 and Cu(CF3SO3)2 in
methanol (top) and ethanol (bottom). [2] = [Cu(CF3SO3)2] = 100
μmol/L.
CSI-MS of a mixture of 2 and Cu(CF3SO3)2 in methanol. The
− +
fragment ion peak arising for [2 + Cu2+ + CF3SO3 ] was
−
observed at m/z 666, and the peak for [2 + Cu2+ + CF3SO3
+
MeOH + 2]+ was observed at m/z 700.5 The fragment ion peak
patterns agree with theoretical distributions. On the other hand,
no fragment ion peak for [2 + Cu2+ + CF3SO3− + EtOH + 2]+
(m/z 714) was observed in the corresponding ethanol solution
(Figure 5 bottom). These results suggest that the 2/Cu-
(CF3SO3)2/alcohol complex is more stable with methanol than
ethanol under CSI-MS conditions at 298 K.
Figure 4. Ball and stick plot (a) and skeletal drawing (b) of 2/
Cu(CF3SO3)2/EtOH. Hydrogen atoms and one CF3SO3 anion have
been omitted in the ball and stick plot for clarity. Figure S7 in the SI
shows an ORTEP diagram with the hydrogen atoms of 2/Cu-
(CF3SO3)2/EtOH.
Thermogravimetric differential thermal analysis (TG-DTA) of
the 2/Cu(CF3SO3)2 complexes containing methanol or ethanol
were carried out in order to compare the binding of methanol
and ethanol to the 2/Cu(CF3SO3)2 complex. As shown in
Figures S7 and S8 in the SI, the weight losses at 180 °C in 2/
Cu(CF3SO3)2/MeOH and 2/Cu(CF3SO3)2/EtOH are 3.88 and
5.56%, respectively. These weight losses correspond to one
molecule of methanol (calcd 3.78%) and ethanol (calcd 5.34%).
The weight loss at 120 °C, however, is ca. 0.5% in 2/
Cu(CF3SO3)2/EtOH, while in 2/Cu(CF3SO3)2/MeOH, it is
less than 0.1%. These results suggest that 2/Cu(CF3SO3)2/
EtOH loses alcohol molecules at a lower temperature than 2/
Cu(CF3SO3)2/MeOH. The TG-DTA measurements therefore
indicate that 2/Cu(CF3SO3)2 retains more strongly to methanol
than ethanol.
four nitrogen atoms of the cyclen and one oxygen atom of the
CF3SO3 anion. The Cu1−N (N1, N2, N3, and N4) distances are
similar to those of 2/Cu(CF3SO3)2/MeOH and are in the range
1.988(6)−2.079(6) Å, and the Cu1−O4 distance is 2.195(5) Å.
Interestingly, the ethanol molecule is encapsulated in the cavity
formed by the three oxygen atoms (O1, O2, and O3) of the
diethyleneoxy unit and the nitrogen atom (N2) of the cyclen.
The distance (1.65 Å) between the O20(ethanol) and the mean
plane of the ligand (O1, O3, and N2) is longer than that in 2/
Cu(CF3SO3)2/MeOH. In addition, some disorder is observed in
the ethanol molecule [see the CIF file of 2/Cu(CF3SO3)2/EtOH
in the SI]. These results indicate that the interaction between
ethanol and the 2/Cu(CF3SO3)2 complex is weaker than that
between methanol and the 2/Cu(CF3SO3)2 complex. The 2/
Cu(CF3SO3)2 complex therefore distinguishes the methyl and
ethyl groups by the difference in steric bulk. Lippard and co-
workers have reported that a dicopper(II) complex with a
dinucleating hexaimidazole ligand includes one MeOH mole-
cule.4h The example differs from 2/Cu(CF3SO3)2/MeOH
because the MeOH molecule is bound by hydrogen bonding
in our case.
In order to estimate the binding energies of the 2/CuII/MeOH
and 2/CuII/EtOH complexes, the single-point-energy calcu-
lations for the X-ray structures were conducted using density
functional theory (DFT; B3LYP/6-31G* and EDF2/6-31G*)
methods with dual basis sets.6 As shown in Table 1, the binding
II
II
energies ΔEMeOH and ΔEEtOH (=E2/Cu /ROH − E2/Cu − EROH) for
the 2/CuII/MeOH and 2/CuII/EtOH complexes are ca. −22 and
B
dx.doi.org/10.1021/ic4021417 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
II
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Table 1. Binding Energies (ΔEROH = E2/Cu /ROH − E2/Cu
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ΔEEtOH) between 2/CuII/MeOH and 2/CuII/EtOH
−
−
E
compd
ΔEROH (kcal/mol)
B3LYP/6-31G*
EDF2/6-31G*
2/CuII/MeOH
−21.69 (ΔEMeOH
−18.68 (ΔEEtOH
)
2/CuII/EtOH
)
ΔΔE (=ΔEMeOH − ΔEEtOH
)
)
−3.01
2/CuII/MeOH
−23.06 (ΔEMeOH
−17.61 (ΔEEtOH
)
2/CuII/EtOH
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ΔΔE (=ΔEMeOH − ΔEEtOH
−5.45
−18 kcal/mol, respectively. These energies are greater than those
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reported by Hossain et al.7 This is due to the four hydrogen
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where T = 298 K).
In conclusion, we have demonstrated that the Cu(CF3SO3)2
complex with the cyclen-based cryptand, 2, retains methanol but
not ethanol in the CSI-MS conditions. This is the first instance
for the preference between alcohols by a metal complex including
a cryptand.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthesis and spectral data of compounds 2−6, X-ray structures
of 2 and 3, crystal data for 2/Cu(CF3SO3)2/MeOH and 2/
Cu(CF3SO3)2/EtOH, crystallographic data of 2/Cu(CF3SO3)2/
MeOH and 2/Cu(CF3SO3)2/EtOH in CIF format, TG-DTA
analysis of 2/Cu(CF3SO3)2/MeOH and 2/Cu(CF3SO3)2/
EtOH, and DFT calculations on 2/CuII/MeOH and 2/CuII/
EtOH complexes. This material is available free of charge via the
Internet at http://pubs.acs.org.
(3) (a) Habata, Y.; Ikeda, M.; Yamada, S.; Takahashi, H.; Ueno, S.;
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Taniguchi, A.; Ikeda, M.; Hiraoka, T.; Matsuyama, N.; Otsuka, S.;
Kuwahara, S. Inorg. Chem. 2013, 52, 2542−2549. (c) Habata, Y.; Oyama,
Y.; Ikeda, M.; Kuwahara, S. Dalton Trans. 2013, 42, 8212−8217.
(d) Habata, Y.; Okeda, Y.; Ikeda, M.; Kuwahara, S. Org. Biomol. Chem.
2013, 11, 4265−4270.
AUTHOR INFORMATION
Corresponding Author
*E-mail: habata@chem.sci.toho-u.ac.jp.
■
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Present Address
§Department of Chemistry, Birla Institute of Technology and
Science, Pilani, Rajasthan 333031, India.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Professors Masatoshi Hasegawa and Junichi
Ishi and also Yuki Hoshino for the TG-DTA measurements. This
research was supported by Grants-in-Aid (08026969 and
11011761), a High-Tech Research Center project (2005−
2009), and the Supported Program for Strategic Research
Foundation at Private Universities (2012−2016) from the
Ministry of Education, Culture, Sports, Science and Technology
of Japan for Y.H.
−
(5) The fragment ion peak arising from [2 + Cu2+ + CF3SO3
+
2H2O]+ was observed at m/z 702. This fragment ion peak is overlapped
with the fragment ion peak arising from [2 + Cu2+ + CF3SO3− + MeOH
+2]+ at m/z 700.
(6) Spartan 10; Wavefunction Inc.: Irvine, CA.
(7) Hossain, M. A.; Saeed, M. A.; Gryn’ova, G.; Powell, D. R.;
Leszczynski, J. CrystEngComm 2010, 12, 4042 The authors used DFT at
the M052x/6-31+G(d) level of theory to calculate the binding energies.
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dx.doi.org/10.1021/ic4021417 | Inorg. Chem. XXXX, XXX, XXX−XXX