Enhancement of catalytic reactivity of zinc(II) complex by a cyclotriveratrylene-capped structure

Article abstract of DOI:10.1016/j.jorganchem.2012.01.006

Cyclotriveratrylene (CTV)-capped zinc(II) complex is a covalent molecular capsule that is constructed from a CTV host unit, a tris(2-aminoethylamine) (tren) ligand, and three rigid phenyl spacers. A direct comparison between CTV-capped zinc(II) complex an

Full text of DOI:10.1016/j.jorganchem.2012.01.006

Journal of Organometallic Chemistry 706-707 (2012) 26e29
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Enhancement of catalytic reactivity of zinc(II) complex by a cyclotriveratrylene-
capped structure
,
b
b
b
b
Yoshimasa Makita a , Keisuke Ikeda , Kazuya Sugimoto , Tomoyuki Fujita , Tomofumi Danno ,
*
Karan Bobuatong c, Masahiro Ehara c, Shin-ichi Fujiwara a, Akiya Ogawa b
a Department of Chemistry, Osaka Dental University, 8-1 Kuzuhahanazono-cho, Hirakata, Osaka 573-1121, Japan
b Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan
c Institute for Molecular Science, Okazaki 444-8585, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Cyclotriveratrylene (CTV)-capped zinc(II) complex is a covalent molecular capsule that is constructed
from a CTV host unit, a tris(2-aminoethylamine) (tren) ligand, and three rigid phenyl spacers. A direct
comparison between CTV-capped zinc(II) complex and CTV-uncapped zinc(II) complex based on X-ray
crystal structures and the hydrolysis of various activated alkyl carbonates revealed that the 23-
membered cap of the complex enhanced its catalytic reactivity.
Received 26 October 2011
Received in revised form
22 December 2011
Accepted 10 January 2012
A cyclotriveratrylene-capped structure enhances the catalytic reactivity of zinc complex.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Zinc
Hemicryptophane
Hydrolysis
Cyclotriveratrylene
1. Introduction
Zn(OAc)2$2 using kinetic studies of MPC hydrolysis revealed that
the cage structure enhanced the catalytic activity of the hydrolysis.
Constructing an artificial cavity around the active sites of
enzyme models is of fundamental importance for understanding
and designing artificial catalysts [1]. Molecular capsules [2e4],
which have well-defined artificial cavities, have been studied
widely for the enzymatic reactions that they are involved in such as
substrate recognition [5e7] and protecting reactive intermediates
[8e13]. However, a few examples of molecular capsules that
contain a transition metal complex within the internal space for
receptors [14e17] and catalysis [18e25] have been reported.
Hemicryptophanes are covalent molecular capsules constructed
from a CTV host unit and they are responsible for dissymmetry at
the molecular cavity level [26e39]. We recently synthesized hem-
icryptophane 1 composed of a CTV, a tren ligand, and three rigid
phenyl spacers, which provide an appropriate pocket within the
cavity, as shown in Fig. 1 [40,41]. We also synthesized its zinc(II)
complex [42e45], which contained a zinc(II) cation. The apical
ligand exchange site was present within the cavity due to the rigid
spacers, as can be seen by X-ray crystal structure analysis in Fig. 2.
The CTV-capped Zn(OAc)2$1 was examined with regard to its ability
to catalyze the hydrolysis of methyl para-nitrophenyl carbonate
In this paper, we report on a direct comparison between the
crystal structure of Zn(OAc)2$1 and Zn(OAc)2$2 and a study of
Zn(OAc)2$1 catalyzed hydrolysis of MPC and various activated alkyl
carbonate. We shed light on the catalytic reactivity of the zinc(II)
complex within the cavity of the complex.
2. Materials and methods
2.1. Synthesis and X-ray crystallography
Zn(OAc)2$1 and Zn(OAc)2$2 were synthesized by using the
following procedure [40]. Single crystals of Zn(OAc)2$2 were
obtained by slow evaporation from a solution of chloroform. The
crystallographic data and the X-ray crystal structure of Zn(OAc)2$2
are shown in Table 1 and Fig. 3, respectively. Propyl 4-nitrophenyl
carbonate 4 [46], isopropyl 4-nitrophenyl carbonate 7 [47], and
tert-butyl 4-nitrophenyl carbonate 8 [48] were synthesized by the
known procedure. The other chemicals that were used in this study
were reagent grade and used without further purification.
(MPC).
A direct comparison between 1 and the uncapped
2.2. Kinetic studies
The rate constants were estimated from kinetic runs that were
monitored over a one- or two-day(s) period and were determined
* Corresponding author.
E-mail address: makita@cc.osaka-dent.ac.jp (Y. Makita).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.01.006

Y. Makita et al. / Journal of Organometallic Chemistry 706-707 (2012) 26e29
27
Table 1
Crystallographic data of uncapped Zn(OAc)2$2.
Empirical formula
Formula weight
Crystal system
Space group
Z
C124 H156 N16 O17 Zn4
2404.21
triclinic
P-1
1
a (Å)
b (Å)
c (Å)
11.0746(3)
11.5315(3)
24.7665(6)
82.1600(7)
81.6450(7)
81.1200(7)
0.085
a
b
g
m
(ꢁ)
(ꢁ)
Fig. 1. Structure of hemicryptophane 1 and tris[2-(benzylamino)ethyl]amine 2.
(ꢁ)
(MoK
a
) mmꢀ1
R1
wR2
0.0645
0.1924
1.055
from the method of initial rates. All the kinetic experiments were
run at least twice. The obtained rate constants were reproducible to
within 10% with margins of error that were considered acceptable
for the purposes of the present study. The complexation reaction of
zinc ion and ligand 1 or 2 were completed within 5 min by 1H NMR
experiments. These reactions were examined using alkyl carbonate
Goodness of fit (GOF) for F2
were first-order reactions for MPC (Fig. 3). The hydrolysis reaction
proceeded slowly in solvents such as CDCl3, CD3CN, d6-acetone, and
d8-THF. A 1:1 mixture of CDCl3 and DMSO-d6 solvent accelerated
the reaction. DMSO-d6 solvent was the best choice for the hydro-
lysis probably because of the solubility of the zinc catalyst and D2O
in the solvent. Counter anion effects were examined for DMSO-d6
solvent and the same reaction conditions. The reactions were first-
order for MPC with Zn(OTf)2 and Zn(ClO4)2, the same as for
Zn(OAc)2, and the reactions rates with ligand 1 were higher than
with ligand 2.
Kinetic studies of various alkyl carbonate hydrolysis were per-
formed and the results are summarized in Table 3. The reaction rate
of MPC 3 hydrolysis in the presence of Zn(OAc)2$1 was higher than
that for Zn(OAc)2$2 (Table 3 entries 1and 2). Despite the benzyl
group being bigger than the size of the cavity, the reaction rates for
4 hydrolysis in the presence of Zn(OAc)2$1 or 2 were lower than
those for 5. The benzyl and p- nitrophenoxy groups were bigger
than the 23-membered cap over the Zn(OAc)2$1 cavity. The
hosteguest complexes Zn(OAc)2$1 and 3, 4 or 5 were not detected
by NMR spectroscopy at 295 K. These results indicate that no
encapsulation of these alkyl groups in the cavity occurred
throughout the course of the reaction. For the hydrolysis of 6 and 7
in the presence of Zn(OAc)2$1 and 2, respectively, the reaction rates
for Zn(OAc)2$1 and 2 (kligand 1/kligand 2) were bigger than the
hydrolysis rates for 3, 4, and 5 (Table 3 entry 7e10). In the case of
hydrolysis 8 in the presence of Zn(OAc)2$1 and 2, the reaction rates
were slower, indicating that the t-Bu group hindered access to the
inner catalytic zinc site. The mechanism of the hydrolysis by a zinc
tren complex required the active participation of zinc-coordinated
water molecule [43]. DFT gas-phase structure optimization and
frequency calculations were performed at the B3LYP/6-31G level of
(51
mmol), excess N,N-diisopropylethylamine (255 mmol), D2O
(510 mmol), Zn(X)2 (5.4 mmol), 1 (5.4 mmol), and DMSO-d6 (0.60 mL).
Each reaction was monitored by using 1H NMR at 295 K.
3. Results and discussion
3.1. X-ray crystallography
The X-ray crystallographic analysis showed that Zn(OAc)2$2
crystallized in the triclinic space group P1 and had a C3-symmetric
conformational structure. The zinc ion was surrounded by four
nitrogen atoms and an acetate anion in a slightly distorted trigonal
bipyramidal configuration. The three benzyl groups adopted
equatorial positions to shield themselves from the acetate anion.
The tertiary amine nitrogen N(1) and the acetate oxygen O(1) were
located at apical positions, and the three secondary amine nitrogen
atoms N(2), N(3) and N(4) were located at equatorial positions. The
ZneN average distances for the trigonal base of the pyramid were
2.105 Å. The tertiary nitrogen atom was more loosely bound to the
metal ion [d(Zn(1)eN(1)) ¼ 2.289 Å] than was the acetate anion
[d(Zn(1)eO(1)) ¼ 1.965 Å] as well as Zn(OAc)2$1 ([d(Zn(1)e
N(1)) ¼ 2.150 Å] and [d(Zn(1)eO(1)) ¼ 1.957 Å]). The bond angle
for Zn(OAc)2$2 [O(1)ꢀZn(1)ꢀN(1) ¼ 165.84ꢁ] was more wide than
the bond angle for Zn(OAc)2$1 [O(1)ꢀZn(1)ꢀN(1) ¼ 151.77ꢁ] due to
the caged structure of Zn(OAc)2$1.
3.2. Kinetic studies of MPC hydrolysis
Table 2 summarizes the observed rate constants (kobs) for the
solvents and zinc(II) salts for the hydrolysis of MPC. The reactions
Fig. 3. X-ray crystal structure of uncapped Zn(OAc)2$2. All the hydrogen atoms and
solvents of crystallization are omitted for clarity. Selected bond lengths (Å) and angles
(ꢁ): Zn(1)ꢀN(1) 2.289(3); Zn(1)ꢀN(2) 2.107(3); Zn(1)ꢀN(3) 2.100(3); Zn(1)ꢀN(4)
2.108(3); Zn(1)ꢀO(1) 1.965(6); O(1)ꢀZn(1)ꢀN(1) 165.84(18); N(1)ꢀZn(1)ꢀN(2)
79.58(11); N(1)ꢀZn(1)ꢀN(3) 81.03(11); and N(1)ꢀZn(1)ꢀN(4) 80.71(11).
Fig. 2. X-ray crystal structure of CTV-capped (II) Zn(OAc)2$1 [40]. All the hydrogen
atoms and solvents of crystallization are omitted for clarity.

28
Y. Makita et al. / Journal of Organometallic Chemistry 706-707 (2012) 26e29
Table 2
solvent access owing to the inhibition of DMSO coordination with
Solvent and counter anion effects.a
the zinc complex in the cavity. We reported recently on CTV-
capped azaphosphatrane where the reactivity of its endohedral
proton was highly suppressed as a result of being protected by the
CTV-capped structure [50]. These differences in reactivity within
the cavity caused difference in covalent PeN bonds and coordinate
ZneN bonds and difference in the size of the phosphorous atom
and zinc ion, respectively.
Entry
Ligand
X
Solvent
kobs(10ꢀ3 hꢀ1
)
4. Conclusions
1
2
3
4
5
6
7
8
1
1
1
1
1
1
2
1
2
1
2
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OTf
CDCl3
CD3CN
d6-acetone
d8-THF
CDCl3 þ DMSO-d6
DMSO-d6
DMSO-d6
DMSO-d6
DMSO-d6
DMSO-d6
DMSO-d6
5.1
9.1
5.9
9.9
16
35
16
33
16
In this paper, we reported a direct comparison of the crystal
structures of 1 and 2, and hydrolysis reactions of various activated
alkyl carbonates that were catalyzed by 1. Kinetic studies on MPC
hydrolysis catalyzed by 1 and 2 were also performed. A direct
comparison between 1 and 2 revealed that the 23-membered cap
enhanced the catalytic activity.
9
10
11
OTf
ClO4
ClO4
37
15
Acknowledgments
a
Conditions: 85 mM MPC, 5.0 equivalents of EtN(i-Pr)2, 10 equivalents of D2O, and
0.1 equivalent of catalyst in DMSO-d6 at 295 K.
We thank Mr. S. Katao for his support in carrying out the X-ray
crystal structure analysis at the Nara Institute of Science and
Technology under the Kyoto Advanced Nanotechnology Network
program. This work was supported by a Grant-in-Aid for Young
Scientists (B) (23792302).
Table 3
Zinc-catalyzed hydrolysis of various alkyl carbonates.a
Appendix. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2012.01.
006.
Entry
Substrate
Ligand
kobs(10ꢀ3 hꢀ1
)
kligand 1/kligand 2
1
2
3
4
5
6
7
8
3
3
4
4
5
5
6
6
7
7
8
8
1
2
1
2
1
2
1
2
1
2
1
2
35
16
18
10
27
15
6.2
2.4
9.2
2.6
4.4
5.4
2.2
1.8
1.8
2.6
3.5
0.8
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