Encapsulation of a guest molecule in a strained form: An extended 36-membered dodecanuclear manganese metallamacrocycle that accommodates a cyclooctane in the S4 symmetry conformation

Article abstract of DOI:10.1039/b607675k

An extended 36-membered dodecanuclear manganese metallamacrocycle with S₁₂ symmetry has been synthesized using the ligand N-cyclohexanoylsalicylhydrazide (H₃chxshz) by a self-assembly that accommodates a cyclooctane of conformational

Full text of DOI:10.1039/b607675k

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Encapsulation of a guest molecule in a strained form: an extended
36-membered dodecanuclear manganese metallamacrocycle that
accommodates a cyclooctane in the S4 symmetry conformation{
Rohith P. John, Jaejoon Park, Dohyun Moon, Kyungjin Lee and Myoung Soo Lah*
Received (in Cambridge, UK) 31st May 2006, Accepted 11th July 2006
First published as an Advance Article on the web 31st July 2006
DOI: 10.1039/b607675k
An extended 36-membered dodecanuclear manganese metalla-
macrocycle with S12 symmetry has been synthesized using the
ligand N-cyclohexanoylsalicylhydrazide (H3chxshz) by a self-
assembly that accommodates a cyclooctane of conformation-
ally strained S4 symmetry in its hydrophobic cavity.
Metallamacrocycles are an interesting class of compounds because
of their diverse molecular architectures,1 their utilization as motifs
for coordination networks2 and their potential as host systems in
guest inclusion chemistry.3 Diaza-bridged macrocycles4–9—termed
metalladiazamacrocycles4a—have received considerable interest
because of their potential as secondary building blocks for the
construction of two- or three-dimensional network structures.2c–e
Fig. 1 Schematic diagram of the ligand H3chxshz with carbon atom
labelling for the N-terminal group.
These compounds can be readily assembled using the trianionic
pentadentate ligand N-acylsalicylhydrazide with a metal ion such
as manganese,4,5 iron,5–7 cobalt8 or gallium.7 We recently reported
a dodecanuclear manganese metallamacrocycle with pentadentate
ligand N-trans-2-pentenoylsalicylhydrazide that could offer a
geometric restriction at the N-terminal position of the ligand.4a
Even though the metalladiazamacrocycle is a 36-membered ring
system, ring puckering led to the macrocycle containing no
meaningful inner cavity for the accommodation of a guest.
To prepare metalladiazamacrocycles with a large inner cavity,
we need to use a ligand with both a sterically rigid and bulky
N-terminal group to prevent ring puckering. During our
investigations, by modifying the N-terminal group to influence
the size and nuclearity of metallamacrocycles, we identified that a
cyclohexyl group would lead to a new type of dodecanuclear
structure with a large enough hydrophobic inner cavity to
accommodate small organic molecules. Even though several
metallamacrocyclic systems with a central cavity have been
reported, most of them are for anion or cation sequestration.3a–e
The inclusion of a hydrophobic organic molecule within the cavity
of a metallamacrocycle is very rare.3f
The ligand N-cyclohexanoylsalicylhydrazide (H3chxshz) (Fig. 1)
was synthesized by a procedure reported previously.4 Dark brown
single crystals of [Mn12(chxshz)12(MeOH)12] (1) were obtained by
slow diffusion of manganese(II) acetate into a solution of
H3chxshz.{ The ligand binds to the metal in a back-to-back
fashion, resulting in a dodecanuclear cyclic structure with
…
…
successive metal centers in an alternating
LDLDLD -type
configuration, where the metallamacrocycle that lies at the
crystallographic S4 axis in P42/n space group has three Mn–ligand
moieties as an asymmetric unit (Fig. 2 and Fig. S1{). This
alternating configuration of metal centers drives the extended
In this work we report a new 36-membered dodecanuclear
manganese metalladiazamacrocycle, obtained by the use of steric
restraint at the N-terminal position of the ligand. We also discuss
the recognition of small hydrophobic organic molecules, such as
benzene, cyclohexane and cyclooctane, and their behavior within
the cavity.
Department of Chemistry and Applied Chemistry, Hanyang University,
Ansan, Kyunggi-do, 426-791 Korea. E-mail: mslah@hanyang.ac.kr;
Fax: +82 31 407 3863; Tel: +82 31 400 5496
{ Electronic supplementary information (ESI) available: Experimental
procedures and crystallographic details for the structure determination.
See DOI: 10.1039/b607675k
Fig. 2 Schematic diagram of a 36-membered dodecanuclear manganese
metalladiazamacrocycle with L and D configurations for the alternating
manganese centers.
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structure to close at some point, resulting in a cyclic system. As all
the N-cyclohexyl groups point to the inner core of the macrocycle,
close contact interactions occur between alternate N-cyclohexyl
groups. The size of the N-terminal group and its geometrical
disposition determines the extent of close contact interactions. The
choice of cyclohexyl as the N-terminal group offers directionally-
rigid a, b, c and d carbon atoms, the arrangement of which leads
to the formation of a dodecanuclear metallamacrocycle in an
extended conformation. The adoption of a sequence of chiralities
by a certain kind of macrocycle is triggered by the tendency of the
system to choose the lowest energy macrocycle. In the case of the
N-terminal group N-trans-2-pentenoyl, a puckered dodecanuclear
system of S6 symmetry is the choice for the lowest energy
macrocycle, where the chirality of the macrocycle varies in a
…
…
LLDDLLDD -type configuration. In the present instance, the
preferred system is the extended dodecanuclear structure of S12
symmetry. This is due to the sterically rigid orientation of the two
b-carbons, which the former lacks. The extended dodecanuclear
metallamacrocycle system leaves a large enough hydrophobic inner
cavity of the same S12 symmetry (the cavity dimensions are y4 s
portal diameter, y7 s inner diameter and y6 s height) for the
encapsulation of small organic molecules.
The crystal structure of 1 shows the presence of a disordered
solvent molecule in the cavity, where the methyl group of the
disordered methanol is in close contact with the N-cyclohexyl
group of the ligand (Fig. S1{). This prompted us to probe the
hydrophobic guest inclusion behavior of the macrocycle. As a
follow-up, we tried to insert benzene into the cavity. The structural
analyses of the isolated crystals, either prepared with benzene as a
co-solvent or from the addition of benzene to the mother liquor
after the growth of the crystals, did not show any evidence for the
accommodation of benzene molecules in the inner cavity.
However, in the case of cyclohexane as guest, we were able to
isolate statically disordered cyclohexane trapped in the inner cavity
of the dodecanuclear metallamacrocycle, regardless of the method
of insertion (Fig. S2{). Even though the size of a cyclohexane
molecule approximately matches that of the cavity, the guest’s S6
symmetry does not match the cavity’s S12 or S4 symmetry. The
symmetry mismatch explains the static disorder of the captured
cyclohexane within the cavity.
Fig. 3 Top: The CPK diagram of [Mn12(chxshz)12(MeOH)12][cyclo-
octane. The encapsulated cyclooctane is shown in purple. Bottom: A ball-
and-stick diagram of a side view. The guest at the center of the
metallamacrocycle (in purple) represents one of two disordered cyclo-
octane sites in a strained S4 symmetry conformation. Mn (green), N (blue),
O (red) and C (gray).
cyclooctane molecule in two different half-occupancy sites, both in
their strained S4 symmetry conformations (Fig S3{). This happens
because the strained S4 symmetry conformation of the guest
matches one of the allowed cavity symmetries.
The guest encapsulation has a small effect on the overall host
framework geometry. When the local geometry around the metal
centers and the overall features of the macrocycle, such as the
adjacent Mn1–Mn2 distance, the next-adjacent Mn1–Mn3 dis-
tance, the inversion symmetry-related Mn–Mn distances and the
Mn1–Mn2–Mn3 angle, were compared, the differences in the
average values were found to be within experimental error ranges
(Table 1 and Fig. S4{). However, the cavity dimensions of the
metallamacrocycles changed, depending on the size and shape of
the guest molecule. When a cyclohexane was accommodated,
small increases in portal diameter and height of the cavity were
observed but were within experimental error range (Table 1).
However, when the cyclooctane was encapsulated in the cavity in a
To investigate the shape or symmetry recognition of the host for
guest molecules, we tried to insert a molecule of cyclooctane as a
guest into the macrocyclic cavity, and were able to isolate crystals
that comfortably accommodated it (Fig. 3). Even though the
symmetry of cyclooctane in its most stable conformation is the
extended S8 symmetry, the cavity accommodates a disordered
Table 1 Comparisons of the geometrical features of the metalladiazamacrocycles with guest molecules
1
1[cyclohexane
1[cyclooctane
Mn1–Mn2/sa
Mn1–Mn3/sb
4.84(3)
8.90(7)
17.78(3)
134(2)
y4.1(1)
y6.0(2)
4.85(4)
8.94(7)
17.86(10)
134(2)
y4.2(1)
y6.1(1)
4.83(4)
8.91(6)
17.80(9)
134(2)
y4.4(2)
y6.4(1)
Mn–Mn/sc
Mn1–Mn2–Mn3/u
Portal diameter of cavity/sd
Cavity height/se
a
b
c
The distance between adjacent metal centers in the ring system. The distance between next-adjacent metal centers in the ring system. The
distance between inversion symmetry-related metal centers in the ring system. The portal of the cavity was formed of the six C6 symmetry-
d
e
related c-hydrogen atoms of the N-terminal cyclohexyl groups. The height of the cavity was calculated from the distance between the close-
lying c-carbons (S12 symmetry-related c carbons) of cyclohexyl groups located on opposite sides of the portal cavity.
3700 | Chem. Commun., 2006, 3699–3701
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For crystallographic data in CIF or other electronic format see DOI:
10.1039/b607675k
strained S4 symmetry conformation, marginal increases both in the
portal diameter and the height of the cavity were observed. The
increase in the cavity dimensions were achieved by modulation of
the rather flexible N-cyclohexyl groups of the ligands.
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In this study, we were able to prepare a metallamacrocycle with
a large enough hydrophobic cavity to accommodate small organic
molecules. The encapsulation of guest molecules of sizes larger
than the portal diameter suggests that the portal has some degree
of flexibility. In addition, the cavity distinguishes guest molecules
based on their size and shape/symmetry. The failure to observe
benzene encapsulation might be due to the smaller size of the guest
compared to the cavity size. Cyclohexane, of matching size, could
be accommodated in a chair conformation with disorder, even
though the symmetry of the guest did not match that of the cavity.
Meanwhile, cyclooctane could be encapsulated only in its
conformationally-strained S4 symmetry conformation.
This work was supported by KRF (KRF-2005-070-C00068),
ITEP (M1–0213–03–0004), KOSEF (R01-2005-000-10490-0) and
CBMH. The authors also acknowledge PAL for beam line use
(2005-3063-10).
Notes and references
{ Crystal data for [Mn12(chxshz)12(MeOH)12]?12(DMF)?2(H2O)?
4.5(MeOH) (1): Mn12C220.5H334N36O66.5, M = 5212.54 g mol21, tetragonal,
space group P42/n, a = b = 27.0406(3), c = 18.7658(3) s, V = 13721.4(3) s3,
T = 2173(2) uC, Z = 2, m(synchrotron, l = 0.82657 s) = 0.592 mm21
,
55375 reflections measured, 7501 unique (Rint = 0.0355) which were used in
all calculations. The final R1 was 0.0874 for observed data with I . 2s(I).
Structure refinement following modification of the data with the
SQUEEZE routine in PLATON:10 R1 = 0.0548 (I . 2s(I)), wR2 =
0.1777, GOF = 1.091, max./min. residual electron density 0.693/
20.420 e s23. CCDC 298633.
Crystal data for 1[cyclohexane: Mn12C186H240N24O48
, M =
4239.30 g mol21, tetragonal, space group P42/n, a = b = 27.189(3), c =
18.904(4) s, V = 13974(3) s3, T = 2173(2) uC, Z = 2, m(Mo-Ka,
l = 0.71073 s) = 0.580 mm21, 67612 reflections measured, 16082 unique
(Rint = 0.0683). The final R1 was 0.1403 for observed data with I . 2s(I).
Structure refinement following modification of the data with the
SQUEEZE routine in PLATON:10 R1 = 0.0853 (I . 2s(I)), wR2 =
0.1957, GOF = 1.057, max./min. residual electron density 0.796/
20.617 e s23. CCDC 298634.
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Crystal data for 1[cyclooctane: Mn12C188H244N24O48
, M =
4267.35 g mol21, tetragonal, space group P42/n, a = b = 27.073(2), c =
18.909(3) s, V = 13860(3) s3, T = 2173(2) uC, Z = 2, m(Mo-Ka,
l = 0.71073 s) = 0.586 mm21, 71393 reflections measured, 16854 unique
(Rint = 0.0745). The final R1 was 0.1438 for observed data with I . 2s(I).
Structure refinement following modification of the data with the
SQUEEZE routine in PLATON:10 R1 = 0.0948 (I . 2s(I)), wR2 =
0.2394, GOF = 1.061, max./min. residual electron density 0.873/
20.711 e s23. CCDC 298635.
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