Synthesis, characterization and molecular recognition of a bis-platinum terpyridine dimer

Article abstract of DOI:10.1039/b716459a

A novel bis-platinum(ii) terpyridine-based macrocycle has been quantitatively obtained by self-assembly; the Pt(ii) host binds neutral planar and electron-rich aromatic guests with good selectivity in DMSO. The Royal Society of Chemistry.

Full text of DOI:10.1039/b716459a

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Synthesis, characterization and molecular recognition of a bis-platinum
terpyridine dimerw
Robert Trokowski, Shigehisa Akine and Tatsuya Nabeshima*
Received (in Cambridge, UK) 25th October 2007, Accepted 17th December 2007
First published as an Advance Article on the web 17th January 2008
DOI: 10.1039/b716459a
A novel bis-platinum(II) terpyridine-based macrocycle has been
quantitatively obtained by self-assembly; the Pt(II) host binds
neutral planar and electron-rich aromatic guests with good
selectivity in DMSO.
several tenths of a ppm were observed, with particularly
significant downfield shifts of the terminal pyridyl protons
Ha (Dd = 0.61 ppm) and Hb (Dd = 0.86 ppm) (Fig. 1).
Additional support for the proposed structure of self-assembly
was provided by an ESI-MS analysis of 1 as a tetrafluorobo-
rate salt in MeOH/CHCl3. The prominent peak at m/z =
328.56 was attributed to the 4+ ion.
Pd(II) and Pt(II)1–3 are widely used in molecular self-assembly,
which has emerged as one of the most efficient methods to
create new materials with controlled physical and chemical
properties.4–6 The number of examples of Pt(II) self-assembly
with conformationally-flexible N-ligands compared to that of
Pd(II) analogs remain limited.7,8 This can be attributed to the
general inertness and the high thermodynamic stability of
Pt–N bonds. One way to overcome this limitation is to design
self-assembly systems from terpyridyl Pt(II) complexes. These
have been shown to facilitate the displacement of the fourth
ligand as a consequence of p-back-bonding from the metal to
the empty p*-orbitals of the pyridine sub-units.9 Reasoning
that a cavity with planar parallel-disposed building units
would offer enhanced binding and selectivity for planar aro-
matic guests, molecular receptors consisting of two terpyridine
units present an attractive approach towards application in
molecular recognition. Bosnich and co-workers10 have re-
ported molecular recognition using a rigid spacer-chelator
bearing cofacial terpyridyl Pd(II) and Pt(II) complexes. These
receptors interact with 9-methylanthracene, not only inside,
but also outside the cleft recognition site to give 1 : 1 and 1 : 2
host–guest complexes. For other neutral aromatic guests, gels
were formed, indicating extensive self-aggregation.
Host 1 possesses a cavity that permits the recognition of flat
aromatic guests. 1H NMR titration experiments of 1 with
benzene and naphthalene derivatives were carried out at 2 mM
in DMSO-d6 (Fig. 2). Plots of the chemical shift of Hc vs. the
concentration of guests produced isotherms that nicely fitted a
1 : 1 host–guest model to yield binding constants, Ka. The
NOE cross-correlation peaks in the NOESY spectrum
between naphthalene-2,6-diol and 1 cavity protons demon-
strate that the guest exclusively locates inside the cavity of 1.
It is noteworthy that the more electron-rich aromatic
compounds work better as a guest (Table 1).
The UV/vis spectrum of the host–guest complex of 1 with
naphthalene-2,6-diol is dominated by strong absorption bands
in the region 230–420 nm. The broad and weak charge-transfer
absorption, reaching up to 650 nm, with a maximum extinc-
tion coefficient of 860 MÀ1 cmÀ1, is responsible for the red
color of the host–guest complex.
Red needle-like single crystals of 1 with naphthalene-2,6-
diol as a guest were obtained by means of the vapor diffusion
of benzene into a solution of 1 and naphthalene-2,6-diol in
DMSO. The X-ray structure of the complex is depicted in
Fig. 3.11 The least-square planes of the two terpyridine rings lie
parallel to each other with a 6.67 A distance, enabling
In this Communication, we describe novel Pt-host 1, which
was quantitatively formed by the simple self-assembly of 5.
Stable metallohost 1 exhibits a unique and selective binding
behavior towards electron-rich aromatic compounds with a
1 : 1 stoichiometry.
Synthesis of 5, the precursor of 1, is shown in Scheme 1. The
bis-platinum complex 1 was quantitatively formed upon addi-
tion of 1.85 equiv. of Pt(cod)(BF4)2 in acetone to a solution of
5 in acetonitrile/chloroform and isolated as a yellow solid.
1H NMR spectra in DMSO-d6 revealed selective dimer
formation, in which the two assembled ligands are equivalent.
In the platinum complex, for the most part, downfield shifts of
Graduate School of Pure and Applied Sciences, University of
Tsukuba, Tennodai 1-1-1 Tsukuba, Ibaraki 305-8571, Japan.
E-mail: nabesima@chem.tsukuba.ac.jp
w Electronic supplementary information (ESI) available: Experimental
details of 1H NMR and UV-vis titration, ESI-MS analysis of 1, and
NOESY spectrum of the 1:naphthalene-2,6-diol complex. See DOI:
10.1039/b716459a
Scheme 1 Synthesis of 1. Reagents and conditions: (a) 1,3-dibromo-
benzene, Pd(PPh3)4, K2CO3, dioxane, 56%; (b) 4,4,5,5-tetramethyl-
2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3,2-dioxaborolane,
Pd(dppf)Cl2, dppf, KOAc, dioxane, 94%; (c) 3, Pd(dppf)Cl2, dppf,
K2CO3, dioxane, 33%; (d) Pt(cod)(BF4)2, CH3CN, CHCl3,
CH3COCH3, 99%.
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Fig. 3 X-Ray structure of 1 with naphthalene-2,6-diol as a guest; for
clarity, one part of 1 is shown in a capped-stick representation, and the
second part in a space-filling representation: top view (left), side view
(right); platinum is colored green, nitrogen colored blue and oxygen
colored red.
Fig. 1 1H NMR spectra of 1 and 5 (400 MHz, [1] = [5] = 4 mM,
DMSO-d6, 25 1C).
Since the simple efficient synthetic route and promising recog-
nition properties of 1 warrant further scrutiny, these results
will be reported in due course.
This work was supported by Grants-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports and
Technology, Japan.
Notes and references
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853.
Fig. 2 1H NMR spectra upon the addition of naphthalene-2,6-diol to
1 (400 MHz, [1] = 2 mM, DMSO-d6, 25 1C).
5 J. S. Siegel, Science, 1996, 271, 949.
6 M. W. Cooke and G. S. Hanan, Chem. Soc. Rev., 2007, 36, 1466.
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9 R. Romeo, M. R. Plutino, L. M. Scolaro, S. Stoccoro and G.
Minghetti, Inorg. Chem., 2000, 39, 4749.
Table 1 Association constants for various aromatic guests with 1a
10 J. D. Crowley, I. M. Steele and B. Bosnich, Inorg. Chem., 2005, 44,
2989.
Ka/MÀ1
Guest
11 Crystal data for 1 Á naphthalene-2,6-diol Á 6DMSO Á 5H2O: formula
Naphthalene-2,6-diol
Naphthalene-2,3-diol
Benzene-1,4-diol
Benzene-1,3-diol
Naphthalene-2-ol
Benzene-1,2-diol
Pyrene
2-Chloronaphthalene
1,4-Dichlorobenzene
1,2-Dichlorobenzene
710 Æ 30
630 Æ 30
460 Æ 13
300 Æ 8
230 Æ 3
200 Æ 11
83 Æ 1
C86H106B4F16N8O13Pt2S6, formula weight 2381.50, crystal system
ꢀ
triclinic, space group P1, a = 10.382(4), b = 17.061(7), c =
18.049(7) A, a = 114.721(16), b = 91.774(13), g = 101.924(14)1, V
= 2816.3(19) A3, Z = 1, reflections measured 22 301, unique
reflections 9888 (Rint = 0.1062), temperature = 120 K, R1 (I 4
2s(I)) = 0.0796, wR2 (all data) = 0.2052.22 CCDC 665387. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b716459a.
80 Æ 3
16 Æ 1
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14 Æ 1
a
1
Conditions: H NMR 400 MHz, [1] = 2 mM, DMSO-d6, 25 1C.
p-stacking interactions with the guest.12–15 Both 1 and the
naphthalene-2,6-diol molecule lie about the same inversion
centre at (0.5, 0.5, 0.5) in the crystal. The dihedral angle
between the plane of the naphthalene ring and the mean
least-square plane of the terpyridyl moiety is 4.67 Æ 0.191.
The Pt–N distances in the terpyridine are in excellent
agreement with those previously reported for terpyridyl Pt(II)
complexes.16–21
15 C. G. Claessens and J. F. Stoddart, J. Phys. Org. Chem., 1997, 10,
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In summary, novel bis-platinum dimer 1, as a metallohost,
was quantitatively synthesized in a very facile self-assembling
way. The size of the cavity enables efficient p-stacking inter-
actions with neutral planar and electron-rich aromatic guests.
21 A. Chernega, A. S. Droz, K. Prout, T. Vilaivan, G. W. Weaver and
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¨
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This journal is The Royal Society of Chemistry 2008
890 | Chem. Commun., 2008, 889–890