Dangling arms: A tetrahedral supramolecular host with partially encapsulated guests

Article abstract of DOI:10.1002/anie.200502209

A protruding tail: A guest composed of a cationic sandwich complex, an alkyl chain, and a sulfonate anionic group is partially incorporated within a host [Ga₄L₆]^12- cluster (see scheme). The cationic head group is quickly incorporated into the cavity of the host, while the sulfonate unit at the other end of the chain is not, as the alkyl sulfonate tail protrudes through an opening in a triangular face of the tetrahedral cluster. (Figure Presented).

Full text of DOI:10.1002/anie.200502209

Angewandte
Chemie
Host–Guest Systems
DOI: 10.1002/anie.200502209
Dangling Arms: A Tetrahedral Supramolecular
Host with Partially Encapsulated Guests**
Bryan E. F. Tiedemann and Kenneth N. Raymond*
Supramolecular chemistry takes advantage of self-assembly
to prepare large, discrete structures from relatively simple
subunits. Using metal–ligand interactions, p stacking, and/or
hydrogen bonds to link the various subunits together, many
elegant examples of supramolecular assemblies have been
synthesized which have been the subject of some excellent
review articles.[1–5] Some examples of the utilization of host–
guest properties of such clusters to modify guest reactivity
include work done by Fujita,[6,7] Reek,[8–10] and Rebek.[11–14]
The rational design and properties of self-assembling tetra-
hedral M4L6 supramolecular clusters (M = AlIII, FeIII, GaIII,
TiIV;
H4L = 1,5-bis(2,3-dihydroxybenzamido)naphthalene)
have been described by Raymond and co-workers, with
structural features illustrated in Figure 1.[15–19] These chiral
assemblies can encapsulate a variety of lipophilic monoca-
3
tionic molecules as guests within the 350–500-Š cavity.[20–22]
While encapsulated, guests can undergo reactions—both
stoichiometric and catalytic—with significant rate enhance-
ment and improved product selectivity in some cases.[23–25] The
catalytic cycle proposed to explain these results requires the
substrate to enter the host and react inside the cluster and
then the product to exit. For polymer formation, the product
must continually exit the host while the cluster remains intact.
Can this be achieved using the M4L6 assembly?
[*] B. E. F. Tiedemann, Prof. Dr. K. N. Raymond
Department of Chemistry
University of California
Berkeley, CA 94720-1460 (USA)
Fax: (+1)510-486-5283
E-mail: raymond@socrates.berkeley.edu
[**] This research is supported by NSF grant CHE-0317011. We thank
the following for their assistance: Dr. Dorothea Fiedler and Dr.
Georg Seeber for stimulating discussions, Matthew N. Stavis and
Dennis H. Leung for aid with the graphics, Dr. Ulla N. Andersen at
the Mass Spectrometry Facility, and Don Harris at Waters
Corporation for high-resolution electrospray mass spectrometry.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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cationic sandwich complex at the phenyl ligand, and the other
bound to a sulfonate anion.
Addition of a stoichiometric amount of RuCn to K12-
[Ga4L6] in D2O led to the encapsulation of the ruthenium
1
head group for all chain lengths (n = 4–10). The H NMR
spectra of the resulting host–guest complexes display signals
for the Cp and phenyl rings of the guest shifted to significantly
higher fields relative to the values observed in the absence of
the host (Figure 2). Such an upfield shift is a diagnostic
Figure 1. Left: Schematic structure of the M4L6 tetrahedral cluster
which illustrates the structure of L4À and its coordination to the metal-
ion vertices. Right: Illustration of the host–guest complex
[Et4NꢀGa4L6]11À based on the X-ray structure coordinates, with a Et4N+
guest shown in blue.
Recently, it was observed that [Ga4L6]12À and [Ti4L6]8À
hosts facilitate guest exchange at essentially the same rate,
thus demonstrating that the guest-exchange mechanism does
not require a ligand-dissociation step. Small openings exist in
the triangular faces of the host, and concerted cluster
distortion is proposed as a means to enlarge these gaps for
guests to pass through.[26] If a stable host–guest complex was
synthesized with part of the guest protruding from the host
through this opening, the aperture would have to expand to
accommodate the steric bulk introduced by the protrusion.
Such a system is analogous to the transition state proposed for
nondissociative guest exchange. Rebek and co-workers have
reported examples of linear molecules “partially” encapsu-
lated in the hydrophobic pocket of an open-ended cavitand,[27]
but no examples have been reported for “closed” self-
assembled hosts, such as the M4L6 cluster. Herein, the
synthesis and characterization of [RuCnꢀGa4L6]12À (RuCn =
[CpRu{h6-C6H5(CH2)nSO3}], n = 4, 6, 8, 10; Cp = cyclopenta-
dienyl) is reported in the solution state: a series of host–guest
complexes with linear “arms” protruding from the host
interior.
Figure 2. Portions of the 1H NMRspectra (D 2O, 500 MHz) of the
host–guest complexes [RuCnꢀGa4L6]12À. Guest resonance assignments
follow the labeling scheme illustrated for RuC4 (top), with the same
numbering pattern used for all chain lengths. Cp denotes the 5H
singlet from the Ru-bound cyclopentadiene ring. Interior protons are
highlighted with red labels, and signals that integrate to two protons
are identified by subscripted labels.
With the [Ga4L6]12À ion as the host, a suitable guest
compound has to be designed with an appendage capable of
protruding through the opening of the host into the bulk
solvent. Such a guest should have three regions: a lipophilic
monocationic head group which exhibits sufficient binding
stability for encapsulation, a linear chain of variable length to
protrude through the aperture, and a hydrophilic anionic end
group to stabilize the arm in aqueous solution. The sandwich
complex [CpRu(h6-C6H6)]+ is known to be encapsulated by
the [Ga4L6]12À ion,[28] and was chosen as the cationic head
group for RuCn (Scheme 1). This series of compounds
features linear alkyl chains, with one end bound to the
feature observed for encapsulated guest protons and is caused
by the magnetic shielding from the naphthalene groups
surrounding the cavity of the [Ga4L6]12À ion.[16,18,20] Further-
more, the two sets of mirror-related phenyl protons become
diastereotopic upon encapsulation of the sandwich complex
by the host because of the chiral environment of the cavity.
Diastereotopic splitting is also observed for most, but not
all, geminal methylene resonances, accompanied by varying
upfield shifts. With the aid of 2D COSYand/or TOCSY NMR
spectroscopic analysis, all chain protons were fully assigned
(Figure 2). Methylene carbon atoms are numbered sequen-
tially along the chain and begin with the carbon atom bound
to the sandwich complex. For RuC4, the geminal methylene
protons on C1–C3 become diastereotopic upon encapsulation
by the [Ga4L6]12À ion, and diastereotopic geminal methylene
protons are observed for C1–C5 for encapsulated RuC6,
RuC8, and RuC10. Upfield shifts tend to increase for protons
closer to the head group. In comparison, several methylene
protons remain enantiotopic and show little, if any, upfield
Scheme 1. Structural formula of RuCn (n=4, 6, 8, and 10).
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shift. The signal from the methylene group adjacent to the
sulfonate moiety is unsplit and unshifted in all four
1
[RuCnꢀGa4L6]12À host–guest spectra. These H NMR obser-
vations indicate that part of the alkyl chain bound to the
cationic sandwich complex resides within the chiral host and
the rest of that alkyl chain lies outside the cavity with the
terminal sulfonate group. At least one methylene group is
found outside the host in all four systems. Thus, the [Ga4L6]12À
cluster encapsulates only part of the RuCn zwitterion.
The signals for the 72 host protons are typically observed
as six sets of 12 protons (Figure 3a), as the catechol and
naphthalene ring edges are interrelated in point group T, each
Figure 4. The 2D NOESY NMRspectrum (D 2O, 500 MHz) of the
[RuC6ꢀGa4L6]12À system shows cross peaks between host resonances
(horizontal axis) and guest resonances (vertical axis). Resonances
from naphthyl protons that border the aperture, ortho and meta to the
amide nitrogen atom, are highlighted in red. The catechol resonance
highlighted in blue shows cross peaks with signals from exterior
methylene protons. nap=naphthyl, cat=catechol.
nances of the guest and catechol proton signals of the host,
thus confirming that the cationic head group is buried deep
within the host cavity, near the naphthyl ring walls. Similar 2D
NOESY NMR spectroscopic observations have been
reported for other guests within the [Ga4L6]12À cluster, such
as Et4N+ and [CpRu(h6-C6H6)]+ ions.[18,28]
Figure 3. Portions of the 1H NMRspectra (D 2O) that show the host
resonances observed for a) point group T typically observed for most
host–guest complexes and b) [RuC10ꢀGa4L6]12À characteristic of C3
symmetry.
Cross peaks are also observed between several host
resonances and most, if not all, methylene groups on the alkyl
chain. The first six to eight methylene groups all show cross
peaks with the same two host resonances—one doublet and
one triplet, which are highlighted in Figure 4. According to
molecular-modeling studies (Figure 5) and 2D COSY NMR
spectra, these signals originate from two adjacent naphtha-
with three adjacent nonequivalent protons. In the presence of
the protruding arm, however, these resonances are split into
24 sets of three protons (Figure 3b). This observation
indicates that the overall symmetry is reduced from point
group T for the host alone to C3 upon encapsulation of RuCn.
Such a symmetry reduction is expected to occur when the
alkyl sulfonate arm of the guest protrudes through the
opening in a triangular face of the tetrahedron. This action
breaks the ligand twofold symmetry, but retains the C3 axis
running from a Gaiii vertex through the aperture of interest.
The six ligands that span the edges of the tetrahedron
separate into two chemically nonequivalent groups: three
“base” ligands that surround the protruding arm and three
“side” ligands connected to the Gaiii vertex opposite the
protrusion, with 12 different protons for each. According to
2D COSY NMR spectroscopy, the 24 host signals originate
from four sets of catechol protons and four sets of naphthyl
protons (see the Supporting Information).
The 2D NOESY NMR spectra of the [RuCnꢀGa4L6]12À
system provide additional information about the structures of
the host–guest complexes. Figure 4 shows the spectrum of the
[RuC6ꢀGa4L6]12À system with focus on the cross section
between the host and guest resonances. The phenyl and Cp
signals of the guest show strong correlations with signals from
three out of the four sets of host naphthalene protons. No
cross peaks are observed between the phenyl or Cp reso-
Figure 5. MM3 minimized structural model of the [RuC10ꢀGa4L6]12À
system
Angew. Chem. Int. Ed. 2006, 45, 83 –86
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85

Communications
lene protons, located ortho and meta to the amide nitrogen
such as linear polymerizations, with the product extending out
of the host cavity as it forms.
atom. Because of the orientation of the naphthyl ring, the
para proton is directed away from the alkyl chain and strong
cross peaks are not observed with the guest. Relative
distances between the host and guest protons in the
[RuC6ꢀGa4L6]12À system were determined from NOE inter-
action growth rates and show these two protons are the closest
to the protruding alkyl chain on the C3-related edges of the
three naphthalene rings that surround the aperture. These
hydrogen atoms are directed away from the cluster center and
can be used as boundary markers to distinguish the host
interior from the exterior. Methylene groups 4 and 5 are
closest to the boundary, whereas methylene group 1 is the
farthest. This difference suggests that the two naphthyl
protons are situated between the C4 and C5 of the alkyl
chain, with C4 on the interior side and C5 on the exterior side
of the cavity boundary. C1, immediately adjacent to the
encapsulated cationic head group, lies deep within the cluster.
Future NOE interaction growth studies will address the
specific conformations of the alkyl chain for guests with
different chain lengths.
Received: June 23, 2005
Published online: November 22, 2005
Keywords: host–guest systems · sandwich complexes ·
.
supramolecular chemistry · zwitterions
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This study shows that the [Ga4L6]12À tetrahedron remains
intact upon incorporation of the cationic head of a zwitterion,
while the link to the anionic tail passes through one of the
small openings at the centers of the triangular faces. This
process is fully consistent with the nondissociative guest-
exchange mechanism recently described.[26] If a guest can
extend out of the cavity, new reactions may be envisioned,
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