DOI: 10.1002/anie.201100193
Container Molecules
A Self-Assembled M8L6 Cubic Cage that Selectively Encapsulates
Large Aromatic Guests**
Wenjing Meng, Boris Breiner, Kari Rissanen, John D. Thoburn, Jack K. Clegg, and
Jonathan R. Nitschke*
Biological encapsulants such as ferritin,[1] lumazine syn-
thase,[2] and viral capsids[3] achieve their selective separation
and sequestration of substrates by providing: 1) a guest
microenvironment isolated from the surroundings, 2) favor-
able interactions complementing a size and shape match with
the encapsulated guests, and 3) sufficient flexibility to allow
guests to be incorporated and released.[4] These biological
hosts self-assemble from multiple copies of identical protein
subunits, the symmetries and connection properties[5] of which
dictate the hollow polyhedral structures of the encapsulant. In
order to create abiological molecular systems that are capable
of expressing functions of similar complexity to biological
systems and to explore new applications of synthetic hosts,[6]
there is a need to create synthetic capsules capable of tightly
and selectively binding large substrates.
similar, which can render their separation difficult. The higher
fullerenes represent particularly attractive targets because
their potential applications[10] remain difficult to explore
because of the challenges associated with their separation,
despite recent advances.[11]
Employing principles of geometric analysis,[5] we deter-
mined that combination of the C4-symmetric tetrakis-biden-
tate ligand shown in Figure 1 with the C3-symmetric iron(II)
tris(pyridylimine) center would result in the formation of an
O-symmetric cubic structure of general formula M8L6, in
which the corners of the cube are defined by the metal centers
and the faces by the ligands (Figure 1). This cage represents
the first example of a new class of closed-face metallo-
supramolecular cubic hosts to be synthesized. In order to
provide favorable binding sites for our target guests we
incorporated porphyrin moieties, which have previously been
demonstrated to interact with large aromatic mole-
cules,[11a–c,12] into our design. This design also provides for
small pore sizes and the potential to create new chemical
functionality through the introduction of different metal ions
into the centers of the N4 macrocycle and by substituting these
metalsꢀ axial ligands. We chose to employ labile iron(II)
centers with pyridylimine ligands as chelating agents to allow
for the formation of the ligand in situ through the subcompo-
nent self-assembly approach.[13]
The reaction between tetrakis(4-aminophenyl)porphyrin
(H2-tapp), 2-formylpyridine, and iron(II) trifluoromethane-
sulfonate (triflate, OTfÀ) in DMF produced cage [H2-
1]·16OTf (Figure 1) as the uniquely observed product, as
verified by NMR spectroscopy (Figure 3b), electrospray mass
spectrometry (ESI-MS), and elemental analysis. Substitution
of nickel(II) tetrakis(4-aminophenyl)porphyrin (Ni-tapp) or
zinc(II) tetrakis(4-aminophenyl)porphyrin (Zn-tapp) for H2-
tapp under identical conditions yielded the nickel-containing
(Ni-1) and zinc-containing (Zn-1) congeners of H2-1 (Figur-
es S2a and S3a in the Supporting Information), respectively,
suggesting the formation of such capsules to be a general
feature of tetrakis(4-aminophenyl) porphyrins (Figure 1).
Vapor diffusion of diethyl ether into a DMF/acetonitrile
solution of Ni-1 resulted in the isolation of block-shaped dark
purple crystals. Single-crystal X-ray diffraction revealed a
solid-state structure (Figure 2) consistent with the O-sym-
metric NMR spectra recorded in solution.
Taking inspiration from natural systems[1–3] and from
other previously reported metal–organic capsules,[7] we report
the design and synthesis of a series of metallo-supramolecular
cage molecules capable of selectively encapsulating large
aromatic guests. The necessary features to achieve this
function are: 1) small pore sizes to isolate guests from the
environment,[8] 2) large cavity sizes to ensure sufficient
volume for the guests of interest, 3) enough flexibility and
lability to allow guests to enter and exit the host, and
4) regions of the cage walls rich in p-electron density to
provide favorable interactions with targeted guests.[9] The
selective encapsulation of large aromatic molecules is an
attractive goal since their physicochemical properties are
[*] W. Meng, Dr. B. Breiner, Prof. J. D. Thoburn, Dr. J. K. Clegg,
Dr. J. R. Nitschke
University of Cambridge, Department of Chemistry
Lensfield Road, Cambridge, CB2 1EW (UK)
E-mail: jrn34@cam.ac.uk
Prof. K. Rissanen
Department of Chemistry, Nanoscience Center
University of Jyvꢀskylꢀ
P.O. Box 35, 40014 JYU (Finland)
Prof. J. D. Thoburn
Department of Chemistry, Randolph-Macon College
Ashland, VA 23005 (USA)
[**] This work was supported by the Walters-Kundert Charitable Trust,
EPSRC, the U.S. Army Research Office, the Academy of Finland
(projects 212588 and 218325), and the Marie Curie IIF Scheme of
the 7th EU Framework Program. We thank the EPSRC Mass
Spectrometry Service at Swansea for conducting FT-ICR MS
experiments.
Each face of Ni-1 is covered by one porphyrin ligand and
each corner is defined by a six-coordinate low-spin FeII ion.
All of the FeII centers within each cage adopt the same L or D
configuration; both enantiomers of Ni-1 are present in the
crystal lattice. The Ni–Ni distance between opposite faces is
15 ꢁ, and the internal cavity volume is 1340 ꢁ3 (Figure S2e).
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2011, 50, 3479 –3483
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3479