A zinc(II)-included hemicryptophane: Facile synthesis, characterization, and catalytic activity

Article abstract of DOI:10.1021/ic100725j

A zinc(II)-included hemicryptophane, which has a zinc(II) center embedded in the cavity, was synthesized and characterized. The catalytic activity of the hemicryptophane was tested in the hydrolysis of methyl para-nitrophenyl carbonate (MPC). A direct com

Full text of DOI:10.1021/ic100725j

7220 Inorg. Chem. 2010, 49, 7220–7222
DOI: 10.1021/ic100725j
A Zinc(II)-Included Hemicryptophane: Facile Synthesis, Characterization,
and Catalytic Activity
Yoshimasa Makita,*,† Kazuya Sugimoto,‡ Kenta Furuyoshi,‡ Keisuke Ikeda,‡ Shin-ichi Fujiwara,†
Tsutomu Shin-ike,† and Akiya Ogawa‡
†Department of Chemistry, Osaka Dental University, 8-1 Kuzuhahanazono-cho, Hirakata, Osaka, 573-1121
Japan, and ‡Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University,
Gakuen-cho 1-1, Nakaku, Sakai, Osaka 599-8531, Japan
Received April 15, 2010
A zinc(II)-included hemicryptophane, which has a zinc(II) center
embedded in the cavity, was synthesized and characterized. The
catalytic activity of the hemicryptophane was tested in the hydro-
lysis of methyl para-nitrophenyl carbonate (MPC). A direct com-
parison between the hemicryptophane and the model complex,
which lacks a cavity, demonstrated that the cage structure enhan-
ced the catalytic activity.
of the artificial enzyme.2 However, a few examples of molec-
ular capsules that contain a transition metal complex within
the internal space for recognition and catalysis have been
reported.3-5 In this study, we selected hemicryptophane,
which has been reported by Dutasta,4-6 to construct such
an internal functionalized molecular capsule catalyst. Hemi-
cryptophane is composed of a catalytic site and a suitable
binding pocket for a particular substrate in the internal space.
The catalytic site and the binding pocket are connected by the
rigid spacer.
The synthetic pathway that led to the preparation of
hemicryptophane 2 is outlined in Scheme 1. (Rac)-tri-
(benzaldehyde)-substituted CTV 1 was furnished by a known
procedure.6 A solution of CTV 1 and tris(2-aminoethyl)-
amine (tren) in a diluted acetonitrile and methanol mixture
(3:7) was refluxed, and then sodium tetrahydroborate was
added to the solution. Hemicryptophane 2 was obtained in
97% yield from 1.
The single crystals of 2 were obtained by slow evaporation
from an acetonitrile solution. The X-ray crystal structure of 2
is shown in Figure 1. The crystal structure of 2 exhibits the
preorganized cavity by the CTV and the tren moieties. An
acetonitrile molecule is encapsulated in the cavity and is
Much of the inspiration for the design of supramolecular
catalysis arises from the observation and understanding of
the enzyme catalysis.1 Molecular capsules which provide
precise internal spaces have been created as the active site
*To whom correspondence should be addressed. Fax: þ81-72-864-3162;
Tel: þ81-72-864-3162; E-mail: e-mail makita@cc.osaka-dent.ac.jp.
(1) (a) Kirby, A. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 707.
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113. (c) Sanders, J. K. M. Chem.;Eur. J. 1998, 4, 1378. (d) van Leeuwen, P. W.
N. M. Supramolecular Catalysis; Wiley-VCH: Weinheim, Germany, 2008.
(2) (a) Cram, D. J.; Cram, J. M. Container Molecules and their Guests;
RSC: Cambridge, U. K., 1994. (b) Jasat, A.; Sherman, J. C. Chem. Rev. 1999, 99,
931. (b) Warmuth, R.; Yoon, J. Acc. Chem. Res. 2001, 34, 95. (c) Koblenz, T. S.;
Wassenaar, J.; Reek, J. N. H. Chem. Soc. Rev. 2008, 37, 247. (d) Brotin, T.;
Dutasta, J.-P. Chem. Rev. 2009, 109, 88. (e) Yoshizawa, M.; Klosterman, J. K.;
Fujita, M. Angew. Chem., Int. Ed. 2009, 48, 3418.
(4) (a) Gautier, A.; Mulatier, J.-C.; Crassous, J.; Dutasta, J.-P. Org. Lett.
2005, 7, 1207. (b) Martinez, A.; Robert, V.; Gornitzka, H.; Dutasta, J.-P. Chem.;
Eur. J. 2010, 16, 520.
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23, 545. (b) Raytchev, P. D.; Perraud, O.; Aronica, C.; Martinez, A.; Dutasta, J.-P.
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ꢀ ꢁ
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Chem. 2005, 3, 2371. (h) Kuil, M.; Soltner, T.; van Leeuwen, P. W. N. M.; Reek,
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13274.
(8) Crystal data of 2: C57H67N7O8, Mw = 978.20, monoclinic, space
˚
˚
˚
group P21/c; a = 15.8289(3) A, b = 18.2279(3) A, c = 17.9070(3) A, R =
3
˚
90.0000°, β = 92.8142(7) °, γ = 90.0000°, V = 5160.45(16) A ; Z = 4; μ =
0.085 mm-1; R1 = 0.0392, wR2 = 0.1017; goodness-of-fit (GOF) on F2
=
1.042. CCDC 771645. Crystal data of 3: C55H66N4O13Zn, Mw = 1056.53,
˚
˚
ꢀ ꢁ
P. W. N. M.; Reek, J. N. H. Chem. Commun. 2006, 1700. (m) Seneque, O.;
triclinic, space group P1; a = 11.1164(5) A, b = 15.0225(6) A, c =
˚
Rager, M.-N.; Giorgi, M.; Reinaud, O. J. Am. Chem. Soc. 2000, 122, 6183. (n)
Darbost, U.; Rager, M.-N.; Petit, S.; Jabin, I.; Reinaud, O. J. Am. Chem. Soc.
2005, 127, 8517.
17.5824(8) A, R = 64.3547(10) °, β = 78.9111(12) °, γ = 81.7640(12) °,
V = 2591.55(20) A ; Z = 2; μ = 0.543 mm-1; R1 = 0.0681, wR2 = 0.2441;
3
˚
goodness-of-fit (GOF) on F2 = 1.128. CCDC 771644.
r
pubs.acs.org/IC
Published on Web 07/21/2010
2010 American Chemical Society

Communication
Inorganic Chemistry, Vol. 49, No. 16, 2010 7221
˚
stabilized through CH-π interactions [d(C CdC)=3.8 A]
NMR spectrum of 2 indicates that 2 is a C3-symmetrical
conformation in CDCl3 solution (Figure 2).
The addition of zinc acetate into the acetonitrile solution of
2 resulted in the formation of zinc(II)-included hemicrypto-
phane 3 in 86% yield.
3 3 3
˚
and by a hydrogen bond [d(N(4) N(5)) = 3.296 A]. The
3 3 3
a
Scheme 1.
Slow diffusion of diethyl ether in an acetonitrile solution
of 3 resulted in crystals suitable for X-ray crystal structure
analysis. The X-ray crystal structure of 3 is shown in Figure 3.
The zinc(II) center is coordinated to the N4 core of the tren
cap and to the acetate anion. The zinc(II)-included hemi-
cryptophane 3 has three 23-membered entrances into the
cavity. The acetate anion is embedded in the cavity. The
Zn-N average distance for the trigonal base of the pyramid
a(i) Tris(2-aminoethyl)amine, p-TsOH, then NaBH4, 97%.
is 2.217 A, and the tertiary N atom capping thesystem ismore
˚
Figure 1. Crystal structures of 2 CH3CN (left, side view; right, top view). The location of the hydrogen atoms of the amino groups and the acetonitrile
3
˚
molecule was calculated. Other hydrogen atoms and solvents of crystallization were omitted for clarity. Selected bond lengths (A): N(2)-N(5), 3.455(2);
N(3)-N(5), 3.787; N(4)-N(5), 3.296(2).
Figure 2. 1H NMR spectra (400 MHz, 295 K). (a) 2 in CDCl3. (b) 3 in DMSO-d6.
Figure 3. Crystal structures of 3 (left, side view; right, top view). All hydrogen atoms and solvents of crystallization were omitted for clarity. Selected bond
˚
lengths (A) and angles (deg): Zn(1)-N(1), 2.150(4); Zn(1)-N(2), 2.292(3); Zn(1)-N(3), 2.102(3); Zn(1)-N(4), 2.258(3); Zn(1)-O(1), 1.957(3);
N(1)-Zn(1)-N(2), 79.21(15); N(1)-Zn(1)-N(3), 87.13(16); N(1)-Zn(1)-N(4), 80.35(15).

7222 Inorganic Chemistry, Vol. 49, No. 16, 2010
Makita et al.
Table 1. Kinetic Data of the Hydrolysis of MPCa
entry
catalyst
kobs(10-3 h-1
)
kobs/kuncat
1
2
3
4
5
none
AcOK
Zn(OAc)2
4
3
0.3
0.5
13
16
35
1
1.7
43
53
117
a Conditions: 85 μM MPC, 5.0 equiv of EtN(i-Pr)2, 10 equiv of D2O,
and 0.1 equiv of catalyst in DMSO-d6 at 295 K.
Figure 4. Structures of a zinc(II)-included hemicryptophane 3 and
zinc(II) tris[2-(benzylamino)ethyl]amine complex 4.
zinc(II) acetate or zinc(II) tris[2-(benzylamino)ethyl]amine
complex 4 (entries 3 and 4). It is noteworthy that the reaction
rate of 3 was higher than that of 4. This result indicates that
the cage structure of a CTV moiety definitely enhanced the
catalytic activity. The host-guest complex of 3 and MPC was
not detected by NMR spectroscopy at 295 K, probably
because the methyl carbonate moiety in MPC is smaller than
the entrance of the cavity.
Scheme 2. Hydrolysis of MPC
We have demonstrated facile synthesis and characteriza-
tion of a hemicryptophane 2 having rigid phenyl spacers and
its zinc complex 3. Kinetic studies of the MPC hydrolysis
were performed. A direct comparison between the zinc(II)-
included hemicryptophane 3 and the model complex 4, which
lacks a cavity, demonstrated that the cage structure enhanced
the catalytic activity.
˚
loosely bound to the metal ion [d(Zn(1)-N(1) = 2.150 A] than
˚
the acetate anion [d(Zn(1)-O(1)) = 1.957 A]. The zinc(II)-
included hemicryptophane 3 was characterized in solution as
well (Figure 4). The profile is remarkably simple in DMSO
solution, attesting to a C3-symmetrical conformation. The
1
peaks of two acetate anions are indistinguishable in the H
NMR spectrum at 295 K. This result indicates that zinc(II)-
included hemicryptophane 3 contains the zinc(II) ion in the
cavity in solution as well as in the solid state.
Acknowledgment. We thank Mr. S. Katao for his sup-
port for measuring X-ray crystal structure analysis and
Mr. F. Asanoma for his support for measuring NMR
at Nara Institute of Science and Technology under the
Kyoto-Advanced Nanotechnology Network program.
We thank Dr. H. Ijyuin at Kanagawa University for her
support for measuring ESI-MS. This work was supported
by Grant-in-Aid for Young Scientists (B) (21791957).
Kinetic studies of the MPC hydrolysis were performed
(Scheme 2). The reactions were carried out in DMSO-d6 with
an excess amount of EtN(i-Pr)2 and D2O and 10 mol % of the
zinc(II) catalyst and were monitored by 1H NMR at 295K.
The reaction was found to be first-order in MPC, and the
observed rate constants (kobs) are summarized in Table 1.
Carbonate hydrolysis without a catalyst and with potassium
acetate were slow under these conditions (entries 1 and 2).
The reaction rates were slightly increased by the presence of
Supporting Information Available: Synthetic details, X-ray
crystal data, and kinetic experiments. This material is available
free of charge via the Internet at http://pubs.acs.org.