Tricyclic pseudocryptand formed by reaction of FeII with tripodand bearing triazacrown ether

Article abstract of DOI:10.1016/j.tetlet.2006.03.084

A novel protonated tricyclic pseudocryptand possessing a triazacrown moiety has been synthesized and characterized by X-ray crystallographic analysis. The protonated host exhibited a unique binding behavior toward alkaline earth metal ions.

Full text of DOI:10.1016/j.tetlet.2006.03.084

Tetrahedron Letters 47 (2006) 3541–3544
Tricyclic pseudocryptand formed by reaction of FeII with
tripodand bearing triazacrown ether
Tatsuya Nabeshima,* Yasushi Tanaka, Toshiyuki Saiki, Shigehisa Akine,
Chusaku Ikeda and Soichi Sato
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
Received 21 January 2006; revised 13 March 2006; accepted 15 March 2006
Abstract—A novel protonated tricyclic pseudocryptand possessing a triazacrown moiety has been synthesized and characterized by
X-ray crystallographic analysis. The protonated host exhibited a unique binding behavior toward alkaline earth metal ions.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Pseudomacrocyclic and pseudomacrobicyclic com-
pounds possess a cavity as an effective binding site for
guests.1 The cyclic framework is formed by complexa-
tion of a suitable metal ion with the corresponding lin-
ear or branched compound bearing a ligating moiety
at each terminal. The formation of the cyclic structure
causes a dramatic change in the binding affinity to a
guest. Thus, these compounds have often been utilized
for artificial allosteric systems.1,2 For example, we have
recently achieved large positive and negative allosteric
effects on ion recognition by using a pseudocryptand.2
A series of pseudocryptands are of great interest and
importance because a strong and versatile guest-binding
mode is expected.2,3 However, only a few examples of
pseudocryptand as a bicyclic host bearing polyether
chains have been reported so far.4 In addition, a tricyclic
pseudocryptand consisting of polyether chains has never
been synthesized, although such would exhibit unique
binding properties compared to those of bicyclic
pseudocryptands. In this letter, we report the synthesis
and X-ray crystal structure of a novel tricyclic pseudo-
cryptand 1ÆFeIIÆH+ possessing a triazacrown moiety.
The bipyridine moieties of the precursor 1 for 1ÆFeIIÆH+
were expected to react with an FeII ion very smoothly
and quantitatively as seen in the nitrogen-pivot pseudo-
cryptand.2 Furthermore, the triazacrown ether may
provide an additional binding affinity toward metal cat-
ions and proton (Fig. 1).
The synthetic route to 1 is shown in Scheme 1. Bromide
35 was treated with diol 26 in the presence of NaH to
give 4 in 63% yield. Tosylate 5 was obtained from 4 in
41% yield. The reaction of 5 with triazacyclonon-
aneÆ3HCl (TACNÆ3HCl)7 and Na2CO3 afforded 1 as a
yellow oil in 40% yield. The host 1 was characterized
1
by H NMR, 13C NMR, DEPT, H,H-COSY, HMQC,
and HMBC. ESI-MS also confirmed the structure.
The host 1 readily reacted with an FeII ion to give the
corresponding octahedral red complex. A new MLCT
absorption (518 nm, CHCl3/MeOH = 1:1) characteristic
of a bipyridine–FeII octahedral complex appeared upon
the addition of FeII. UV–vis spectroscopic titration
suggested quantitative formation of the 1:1 complex
(Fig. 2). The red complex 1ÆFeII was isolable in 75%
yield as a pure HBF4 salt by the reaction of 1 and
Fe(BF4)2Æ6H2O and subsequent treatment with HBF4.
ESI-MS and elemental analysis suꢀpported the proton-
ated structure of 1ÆFeII bearing BF4 as a counter anion.
In addition, 19F NMR spectroscopy strongly suggested
ꢀ
that the host contains three BF4 anions by using
1,2,4,5-tetrafluorobenzene as an internal standard for
determining the content of the fluorine nucleus. Free
1ÆFeII was not obtained by neutralization with NaOH
due to dissociation of the FeII ion from the complex.
X-ray crystallographic analysis confirmed the formula
of the protonated pseudocryptand (Fig. 3).8 The asym-
metric unit consists of one 1ÆFeIIÆH+ cation, three
Keywords: Pseudocryptand; Fe complex; Bipyridine; Triazacrown
ether; Supramolecule.
*
Corresponding author. Tel./fax: +81 29 853 4507; e-mail:
nabesima@chem.tsukuba.ac.jp
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.03.084

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T. Nabeshima et al. / Tetrahedron Letters 47 (2006) 3541–3544
O
O
O
N
N
FeII
N
N
N
N
FeII
N
N
O
O
O
O
O
O
N
N
N
N
Binding cavity
FeII
: octahedral complex
Figure 1. Formation of tricyclic pseudocryptand.
NaH
+
HO
O
O
OH
N
N
Br
HO
O
O
O
N
N
3
2
4
63%
TsCl
TsO
O
O
O
N
N
O
NaOH
5
41%
O
O
N
N
H
N
N
5
N
• 3HCl
O
O
O
O
O
O
N
N
N
N
N
Na2CO3
N
H
H
N
1
40%
Scheme 1. Synthesis of tripodand 1.
BF4ꢀ, and four chloroform molecules. The iron atom
has a six-coordinate octahedral environment bonded
to six nitrogen donors of the three bipyridine moieties
proton of the TACN moiety. The hydrogen atom H1
was located in a difference Fourier map and its coordi-
nates were refined with an isotropic displacement
˚
˚
(Fe–N distances: 1.956–1.978 A). Three polyether
parameter. The N1–H1 distance 1.16(8) A undoubtedly
chains assemble to form a triple helix ranging from the
FeII(bpy)3 moiety to N1 in the TACN moiety. Each
chain winds around the Fe1–N1 axis of the molecule
with a twist angle of 153–168ꢁ,9 which is slightly smaller
than that of the macrobicyclic analog.2 The TACN ring
(N1–N2–N3) sits at an angle of 60ꢁ with respect to the
mean plane of the FeII(bpy)3 cap (C37–C38–C39).
Unlike the macrobicyclic analog, the cavity of 1ÆFeIIÆH+
shrinks and collapses in such a way that one of the
polyether chains (O4–C31–C32–O7) fills up the concave
part. This structural deformation can be attributed to
intramolecular hydrogen bonds involving the NH
indicates that the proton is localized on N1 among the
three nitrogen atoms of TACN as seen in the case of
Me3TACN10 or bipyridine-substituted TACN.3c The
NH proton forms hydrogen bonds to the two other
nitrogen donors (N2 and N3) of the TACN moiety
˚
˚
(1.8–2.0 A) as well as ether oxygen O1 (2.4 A), resulting
in the pinched cavity.
In contrast to the solid state, the structure of 1ÆFeIIÆH+
in solution is much more complicated. While 19F
NMR indicated a mono-protonated structure (vide
supra), 1H NMR showed that 1ÆFeIIÆH+ consists of

T. Nabeshima et al. / Tetrahedron Letters 47 (2006) 3541–3544
3543
Figure 2. Absorption spectral changes of 1 upon addition of Fe(BF4)2Æ6H2O in CHCl3/MeOH = 1:1. [1] = 5.0 · 10ꢀ5 M, 0 6 ([FeII]/[1]) 6 4.
supported the formation of pseudocryptand with an
1
octahedral bipyridine–FeII complex. H NMR titration
in CD3CN suggested the quantative 1:1 complexation,
as seen in the UV–vis experiment. Definite structural
determination could not be carried out due to complex-
ity of the spectra. However, these are presumably iso-
mers in which the conformations of the protonated
TACN moiety are different. While the structures are un-
clear, this interesting solvent effect on the distribution of
the isomers has never been observed in other pseudo-
cryptands. The structural change would be utilized for
controlling recognition ability toward guests.
Binding ability of the protonated host to cations in
1
CD3CN was preliminarily examined by H NMR spec-
troscopy (Fig. 4). The host was considered not to inter-
act with Na+, K+, Rb+, and Cs+ at all because these
metal ions did not result in any spectral change of the
host. In contrast, Mg2+ and Ca2+ caused considerable
spectral changes, which are indicative of 1:1 complexa-
tion because excess metal ions did not affect the NMR
spectra meaningfully.12 Interestingly, the spectrum of
the host was not influenced by Ba2+ and tetrabutyl-
ammonium ions. These results suggest that the cavity
of the protonated host acts as a selective binding site
for cationic guests. The binding mode is not clear but
the proton in the positively charged TACN ring may
contribute to the selectivity.12
Figure 3. X-ray crystal structure of 1ÆFeIIÆH+. Hydrogen atoms are
omitted for clarity except for that of the NH group. Selected
interatomic distances, Fe1–N4 1.956(4), Fe1–N5 1.969(4), Fe1–N6
1.965(4), Fe1–N7 1.978(4), Fe1–N8 1.976(4), Fe1–N9 1.969(4), N1–H1
˚
We have here synthesized and clarified the crystal struc-
ture of tricyclic pseudocryptand 1ÆFeIIÆH+ as a novel
protonated host, which exhibited a significantly unique
binding behavior to cations. This new framework might
be applied not only to a new ion recognition system
responding to the solvent but also to sensing of solvent
polarity. We are currently studying the detailed struc-
ture of the protonated host in solution and the binding
mode on the complexation with alkaline earth metal
ions.
1.16(8), N2–H1 1.80(8), N3–H1 2.00(8), O1–H1 2.39(8) A.
two isomers with C3 symmetry. Ratios of the isomers
significantly depend on the solvent system used in
the NMR measurement. In CD3CN, the ratio is >4:1
but the content of CDCl3 changed the ratio dramati-
cally (1:1 in CDCl3/CD3CN = 90:10; 2:3 in CDCl3/
CD3CN = 95:5). The 1H NMR and H,H-COSY spectra

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T. Nabeshima et al. / Tetrahedron Letters 47 (2006) 3541–3544
Figure 4. 400 MHz 1H NMR spectra of (a) 1ÆFeIIÆH+ (2.0 · 10ꢀ3 M) in CD3CN in the presence of 1.0 equiv of (b) Mg(ClO4)2, (c) Ca(ClO4)2, and (d)
Ba(ClO4)2.
4. (a) Nabeshima, T.; Inaba, T.; Sagae, T.; Furukawa, N.
Tetrahedron Lett. 1990, 31, 3919–3922; (b) Saiki, T.;
Iwabuchi, J.; Akine, S.; Nabeshima, T. Tetrahedron Lett.
2004, 45, 7007–7010; (c) Nabeshima, T.; Saiki, T.; Iwabu-
chi, J.; Akine, S. J. Am. Chem. Soc. 2005, 127, 5507–5511.
5. Heck, R.; Dumarcay, F.; Marsura, A. Chem. Eur. J. 2002,
8, 2438–2445.
Acknowledgments
This research was supported by Grants-in-Aid for Scien-
tific Research from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan.
6. Lapouyade, R.; Morand, J.-P. J. Chem. Soc., Chem.
Commun. 1987, 223–224.
Supplementary data
7. Buttafava, A.; Fabbrizzi, L.; Perotti, A.; Poggi, A.; Poli,
G.; Seghi, B. Inorg. Chem. 1986, 25, 1456–1461.
8. Crystallographic data for 1ÆFeIIÆH+ÆðBF4ꢀÞ3Æ4CHCl3:
C76H86B3Cl12F12FeN9O9 (2011.22), red prism (0.4 · 0.3 ·
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2006.03.084.
0.25 mm3), triclinic, P1, a = 13.912(6), b = 16.804(7),
ꢀ
˚
c = 20.084(9) A, a = 84.609(15)ꢁ, b = 82.051(15)ꢁ, c =
71.580(13)ꢁ, V = 4405(3) A , Z = 2, Dcalc = 1.516 g cmꢀ3
,
3
˚
˚
References and notes
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`
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constants could not be determined due to the complex
spectral changes.