Self-assembly of a novel macrotricyclic Pd(II) metallocage encapsulating a nitrate ion

Article abstract of DOI:10.1039/b104853h

Complexation of the ligand 1 with Pd(NO₃)₂ leads to the self-assembly of a very stable M₂L₄ type macrotricyclic cage that encapsulates a nitrate ion inside its cavity.

Full text of DOI:10.1039/b104853h

Self-assembly of a novel macrotricyclic Pd(II) metallocage
encapsulating a nitrate ion†
Dillip K. Chand, Kumar Biradha and Makoto Fujita*
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University and CREST,
Japan Science and Technology Corporation (JST), Chikusaku, Nagoya 464-8603, Japan.
E-mail: mfujita@apchem.nagoya-u.ac.jp
Received (in Cambridge, UK) 4th June 2001, Accepted 13th July 2001
First published as an Advance Article on the web 15th August 2001
Complexation of the ligand 1 with Pd(NO3)2 leads to the
self-assembly of a very stable M2L4 type macrotricyclic cage
that encapsulates a nitrate ion inside its cavity.
Construction of discrete and well-defined molecular archi-
tectures using organic ligands and cleverly selected metal ions
through the self-assembly route has received much attention
during the last decade.1 Desired species having predetermined
structural and functional properties can be obtained by simply
mixing the participating components under suitable conditions.2
The self-assembled structures formed via a metal-driven self-
assembly path can be classified logically under MxLy types
(where M and L denote metal ions and ligands involved,
respectively) with varying values of x and y. While previously
reported cage structures are mostly formulated as M3L2,3
M6L4,4 or M4L6,5 there is a scarcity in the series of M2L4 type
cages6 in the literature. Atwood and coworkers have reported an
early example of a M2L4 cage6a where two octahedral copper(II
)
ions are bridged by four units of a bidentate ligand. While all the
four equatorial positions of each copper ions are coordinated
with the terminal pyridyl groups of the ligand strands, the axial
positions are occupied by water molecules. Dinuclear copper(II
)
complexes of the M2L4 family having twisted structures and
including chloride6c and perchlorate6d anions in between the
metal centers are also reported. Another analogus structure is a
dinuclear Pd(II) cage,6b with inclusion of hexafluorophosphate
ion inside the cavity. However, in none of the cases the
complexation reaction leading to the cage was directly mon-
itored using NMR spectroscopy unlike our study in this work.
Herein, we report the self-assembly of a novel macrotricyclic
Scheme 1
dried in vacuo to obtain the complex [(Pd)2(1)4(NO3)4] 2§ as a
white solid in 74% yield (Scheme 1).
1
Ligand 1 and complex 2 were characterised by H and 13C
NMR. All the peaks were further completely assigned using H–
H COSY and C–H COSY techniques. The signals for complex
2 as compared to ligand 1 in its proton NMR spectrum,
particularly for pya and pyb protons, were remarkably down-
field shifted due to the complexation. A simple pattern of the
spectrum suggests the formation of a discrete species (Fig. 1).
Similarly 13C NMR data also supports the assumed structure.¶
All the findings indicate that the four arms in complex 2 are
equivalent and that four-fold symmetry axes pass through the
metal centers in solution.
The same complexation reaction was also carried out in
DMSO-d6 and the solution was directly monitored by NMR
spectroscopic means without isolating the complex. The
spectrum obtained matched exactly that of the isolated complex.
No peaks other than due to complex 2 were observed which
establishes the quantitative self-assembly of 2. When the metal
cage molecule from ligand 1 and Pd(NO3)2, the pivotal
positions of the cage being occupied by Pd(II) ions. A nitrate ion
is included inside the cavity in a cascade fashion bridging both
the metal centers.
Ligand 1 was synthesized by Suzuki coupling of 1,3-bis(3-
bromophenyl)benzene with 2-(4-pyridyl)-4,4,5,5-tetramethyl-
1,3-dioxaborolane7 using K3PO4 as base and Pd(PPh3)4 as
catalyst.‡ The ligand 1 was mixed with Pd(NO3)2 at a ratio of
2+1 in DMSO and stirred at 90 °C for 1 day. Subsequently,
addition of diethyl ether precipitated a pale yellow powder
which was separated by filtration, washed with MeOH, and
† Electronic supplementary information (ESI) available: Crystallography
section; Figs. S1–8: 1H, 13C, H–H and C–H COSY NMR spectra for 1 and
2. See http://www.rsc.org/suppdata/cc/b1/b104853h/
1652
Chem. Commun., 2001, 1652–1653
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104853h

Fig. 3 Linear chain in the crystal structure of 2. Three DMSO molecules are
trapped in each of the intermolecular cavities. Key: palladium (magenta);
nitrogen (blue); oxygen (red); sulfur (yellow); carbon (gray). The disorders
of the nitrate anions and DMSO molecules are not shown.
D. K. C. thanks the Japan Society for the Promotion of
Science (JSPS) for a postdoctoral fellowship.
Fig. 1 1H NMR spectra of (a) ligand 1 and (b) macrotricyclic cage 2 (500
MHz, DMSO-d6, 25 °C, TMS as an external standard).
Notes and references
‡ Ligand 1: a mixture of 1,3-bis(3-bromophenyl)benzene (0.3881 g, 1.0
mmol), 2-(4-pyridyl)-4,4,5,5-tetramethyl-1,3-dioxaborolane (0.5126 g, 2.5
mmol), K3PO4 (0.7429 g, 3.5 mmol), and Pd(PPh3)4 (0.1155 g, 0.1 mmol),
was refluxed in 1,4-dioxane (25 mL) for 2 days at 100 °C under an argon
atmosphere. After usual aqueous work-up the residue was chromatographed
over silica gel to obtain a white soild, which was recrystallized from
methanol affording 1 as white needles in 65% yield. dH (500 MHz, DMSO-
d6,TMS): 9.17 (d, 4H, a), 8.66(s, 2H, c), 8.65 (s, 1H, g), 8.41(d, 2H, d), 8.36
(d, 4H, b), 8.32–8.34 (m, 4H, f & h), 8.16 (t, 2H, e), 8.13 (t, 1H, i).dC (125
MHz, DMSO-d6,TMS): 151.04 (a), 147.92 (Cq), 142.04(Cq), 141.51(Cq),
138.84(Cq), 130.70(e), 130.47(i), 128.86(d), 127.41(h), 126.97(f),
126.64(g), 126.45(c), 122.47(b); mp 192–193 °C, Anal. Calc. for
C28H20N2·0.3CH3OH: C, 86.12; H, 5.44; N, 7.09. Found: C, 86.17; H, 5.16,
N, 7.07%.
§ Cage 2: dH (500 MHz, DMSO-d6,TMS): 10.18 (d, 16H, a), 9.20(s, 8H, c),
9.04 (d, 16H, b), 8.96(s, 4H, g), 8.53(d, 8H, d), 8.44 (d, 8H, f), 8.27 (d, 8H,
h), 8.19 (t, 8H, e), 8.06 (t, 4H, i). dC (125 MHz, DMSO-d6,TMS): 152.39 (a),
151.17 (Cq), 141.77(Cq), 140.58(Cq), 135.71(Cq), 131.12(e), 130.74(i),
130.25(f), 127.28(h), 127.19(c & d), 126.88(g), 125.61(b); mp decomp. at
289 °C), Anal. Calc. for C112H80N12O12Pd2·6(CH3)2SO: C, 60.36; H, 4.74;
N, 6.81. Found: C, 60.12; H, 4.59, N, 6.80%.
¶ In the 13C NMR spectrum of 2, instead of 13 peaks only 12 peaks were
observed. Here the signals of carbons c and d (see Fig. 1 for nomenclature)
overlapped with each other as confirmed by the C–H COSY spectrum.
Fig. 2 Representation of [(Pd)2(1)4(NO3)]3+ in the crystal structure of 2 (Pd:
ball mode and others: cylinder mode). The disorder of the nitrate anion is not
shown. Key: palladium (magenta); nitrogen (blue); oxygen (red); carbon
(gray).
salt was added in a lesser amount than the required stoichio-
metry for the complexation, all the signals of complex 2 were
still observed, without any indication of any impurity, along
with additional signals corresponding only to the uncomplexed
free ligand. Also, by adding excess of the metal salt, no other
new structures were suggestive from the NMR pattern. All these
findings support the remarkable thermodynamic stability of 2.
Finally, the structure of complex 2 was determined unambi-
guously from an X-ray diffraction study.∑ Needle-shaped
crystals, suitable for X-ray diffraction analysis, were obtained in
2 days by layering diethyl ether over a solution of 2 in DMSO.
A perspective view of the molecule is shown in Fig. 2. The
crystal structure consists of the complexed cation
[(Pd)2(1)4(NO3)]3+, three nitrate anions, nine DMSO and two
diethyl ether molecules. Each Pd(II) has a square planar
geometry with Pd–N bond distances in the range
2.024(7)–2.030(6) Å. The size of the 3-D cavity is ca. 11.1 3
10.2 3 8.4 Å which is defined by the arms of the rectangular
array formed from four hydrogen centers, Hg (see Fig. 1 for
labelling and Fig. 2 for comparison) and Pd–Pd separation. The
cavity size after correcting for the van der Waals radius of the H
centers is 8.7 3 7.8 3 6.0 Å . While three out of four nitrate ions
stay outside of the cavity, one nitrate ion is encapsulated inside
by ionic interactions, the Pd–O distance being 3.135(7) Å.
Analysis of crystal packing displayed the extension of
intermolecular interactions in a linear manner, where three
DMSO molecules lie in the intermolecular cavity formed in
between two consecutive [(Pd)2(1)4(NO3)]3+ units (Fig. 3). The
Pd–Pd axes of adjacent cages adopt a perpendicular geometry to
each other making a hydrophobic pocket.
∑ Crystal data for 2: C112H80N12O12Pd2·9(CH3)2SO·2(C2H5)2O, M
=
2850.07, monoclinic, space group C2/c, a = 23.510(4), b = 22.229(3), c =
27.394(5) Å, b = 104.216(3)°, U = 13878(4) Å3, T = 193 K, Z = 4, Dc
= 1.364 g cm23, l = 0.71073 Å, 36208 reflections measured, 12216
unique (Rint = 0.2039) which were used in all calcutations. R1 = 0.0766
and wR2 = 0.1676. Two of the nitrates and seven DMSO molecules were
disordered.
CCDC reference number 164966. See http://www.rsc.org/suppdata/cc/
b1/b104853h/ for crystallographic data in CIF or other electronic format.
1 Templating, Self-Assembly and Self-Organization, Exec. ed. J.-P. Sauv-
age and M. W. Hosseini, Comprehensive Supramolecular Chemistry, ed.
J.-M. Lehn (Chair), Pergamon Press, Oxford, 1995, vol. 9.
2 M. Fujita, K. Umemoto, M. Yoshizawa, N. Fujita, T. Kusukawa and K.
Biradha, Chem. Commun., 2001, 509; S. Leininger, B. Olenyuk and P. J.
Stang, Chem. Rev., 2000, 100, 853; R. W. Saalfrank, E. Uller, B.
Demleitner and I. Bernt, Struct. Bonding (Berlin), 2000, 96, 149; D. L.
Caulder and K. N. Raymond, Acc. Chem. Res., 1999, 32, 975.
3 M. Fujita, S. Nagao and K. Ogura, J. Am. Chem. Soc., 1995, 117,
1649.
4 M. Fujita, D. Oguro, M. Miyazawa, H. Oka, K. Yamaguchi and K. Ogura,
Nature, 1995, 378, 469; P. J. Stang, B. Olenyuk, D. C. Muddiman and
R. D. Smith, Organometallics, 1997, 16, 3094; C. M. Hartshorn and P. J.
Steel, Chem. Commun., 1997, 541.
5 R. W. Saalfrank, A. Stark, K. Peters and H. G. Von Schnering, Angew.
Chem., Int. Ed. Engl., 1988, 27, 851; T. Beissel, R. E. Powers and K. N.
Raymond, Angew. Chem., Int. Ed. Engl., 1996, 35, 1084.
6 (a) L. J. Barbour, G. William Orr and J. L. Atwood, Nature, 1998, 393,
671; (b) D. A. McMorran and P. J. Steel, Angew. Chem., Int. Ed., 1998,
37, 3295; (c) T. D. Owens, F. J. Hollander, A. G. Oliver and J. A. Ellman,
J. Am. Chem. Soc., 2001, 123, 1539; (d) C.-Y. Su, Y.-P. Cai, C.-L. Chen,
H.-X. Zhang and B.-S. Kang, J. Chem. Soc., Dalton Trans., 2001, 359.
7 C. Coudret, Synth. Commun., 1996, 26, 3543.
Chem. Commun., 2001, 1652–1653
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