Synthesis, structure and complexation ability of novel metalloreceptors containing two pincer complexes of palladium(II)

Article abstract of DOI:10.1002/ejic.200400508

Metalloreceptors 1a and 1b were synthesized by palladation of 42- and 54-membered crown ethers that have two SCS-pincer ligand moieties. The X-ray crystallographic analysis reveals that the metalloreceptor 1b has a folded structure in which the two pincer-Pd moieties are stacked in a parallel manner. The metalloreceptor 1b recognizes the aromatic bidentate guests 4,4′-bipyridine, pyrazine, and pyrimidine and binds to them in a bridging fashion to form the corresponding bimacrocyclic 1:1 complex. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400508

SHORT COMMUNICATION
Synthesis, Structure and Complexation Ability of Novel Metalloreceptors
Containing Two Pincer Complexes of Palladium(II
)
Tatsuya Nabeshima,*[a] Daisuke Nishida,[a] Shigehisa Akine,[a] and Toshiyuki Saiki[a]
Keywords: Host-guest systems / Pincer ligands / Palladium / Metalloreceptors / Crown ethers
Metalloreceptors 1a and 1b were synthesized by palladation
of 42- and 54-membered crown ethers that have two SCS-
pincer ligand moieties. The X-ray crystallographic analysis
reveals that the metalloreceptor 1b has a folded structure in
which the two pincer-Pd moieties are stacked in a parallel
manner. The metalloreceptor 1b recognizes the aromatic bi-
dentate guests 4,4Ј-bipyridine, pyrazine, and pyrimidine and
binds to them in a bridging fashion to form the corresponding
bimacrocyclic 1:1 complex.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Metalloreceptors have recently attracted much attention
in supramolecular chemistry because the physical and
chemical properties of the metal center can be applied in
guest binding, sensing, and regulation of molecular func-
tions.[1] In particular, the coordination of various ligands to
metals is very useful for the formation of a stable supramol-
ecular system.[2Ϫ6] Combination of the coordinate bonding
and other intermolecular interactions provides a strong and
selective recognition.
Complexes between Pd and pincer ligands are useful as
they provide a nice coordination site for host-guest interac-
tion.[7] For example, crown ethers containing a Pd center
inside the ring and molecular recognition have been
reported.[8Ϫ10] Cooperative binding of the Pd unit and the
Scheme 1. Recognition of bidentate guests in a bridging fashion
crown ether moiety to nucleobases such as cytosine and ad- and the following tosylation gave 6a, 6b. The ditosylates 6a
enine was observed in this system.[11,12] However, a host and 6b were then treated with 4 under high dilution con-
molecule containing several pincer complexes as a binding ditions to give crown ethers 7a and 7b in 42% and 45%
site has not been synthesized although this type of host is yield,
very interesting as a multi-point receptor.
Here we report the synthesis, structure and molecular
respectively.
Metallation
[Pd(CH3CN)4](BF4)2 yielded metallohosts 1a, 1b.
All spectroscopic data are consistent with the formula of
of
7
with
[16]
recognition ability of large macrocyclic hosts 1, which con- 1 (see Exp. Sect.). Palladation of 7 caused broadening of
tain two pincer complexes of PdII. The metallohosts are ex- the 1H NMR signals, probably due to the enhanced rigidity
pected to bind bidentate guests of different sizes in the cav- of the structure, which restricts the free rotation of the
ity because the framework of these hosts is flexible so as to methylene group. On metallation, downfield shifts of the
cause induced-fit recognition (Scheme 1).
signals for the SCH2CH2 protons (e.g. δ ϭ 2.60 for 7b, δ ϭ
Metallohosts 1 containing the two Pd centers were pre- 3.32 for 1b, Figure 1) and the benzyl protons is observed.
pared according to Scheme 2. Diols 2a[13] and 2b[14] were This is elucidated by the decreased electron-density of these
treated with p-toluenesulfonyl chloride to afford 3a and 3b protons. ESI-MS for 1b gave signals at m/z ϭ 614.1
in 80% and 63% yield, respectively. Reaction of dithiol 4[15] (C52H70O12S4Pd2, [1bϪ2BF4Ϫ2CH3CN]2ϩ) and m/z ϭ
with 3a, 3b in the presence of K2CO3 afforded diols 5a, 5b, 1247.2 (C52H70FO12S4Pd2, [1bϪ2BF4Ϫ2CH3CNϩF]ϩ),
(the FϪ anion was obtained from decomposition of the
Ϫ
BF4 anion during the ESI-MS measurements). In ad-
[a]
Department of Chemistry, University of Tsukuba,
dition, the X-ray crystallographic analysis clarified the
structure of 1b (Figure 2). The two complex moieties exhibit
square-planar geometry at Pd, with three donor atoms pro-
vided by the rigid SCS chelate. The PdϪS distances are in
Tsukuba, Ibaraki 305-8571, Japan
Fax: (internat.) ϩ81-29-853-6503
E-mail: nabesima@chem.tsukuba.ac.jp
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Eur. J. Inorg. Chem. 2004, 3779Ϫ3782
DOI: 10.1002/ejic.200400508
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3779

T. Nabeshima, D. Nishida, S. Akine, T. Saiki
SHORT COMMUNICATION
Scheme 2. Synthesis of metallohosts 1
˚
˚
the range between 2.334 A and 2.286 A. The two aromatic
rings, which bind the palladium atoms, are stacked. The
distance between the mean planes of the benzene rings
˚
(3.4 A) suggests the existence of π-π interactions. Further-
more, the palladium atoms are located in closely proximity
of the sulfur atoms. The distance between Pd2ϪS2 (3.371
˚
A) is shorter than the sum of van der Waals radii for sulfur
˚
and palladium (3.43 A). Two CH3CN molecules coordinate
to Pd, and the Pd1ϪN1 and Pd2ϪN2 distances are
˚
˚
2.105 A and 2.092 A, respectively.
Figure 2. Crystal structure of 1b; thermal ellipsoids are drawn at
30% probability level; the minor component of the disordered
atoms, counter anions, and hydrogen atoms are omitted for clarity
The interaction between 1b and 4,4Ј-bipyridine was
1
evaluated by H NMR spectroscopy (Figure 3). Signals of
free 1b disappear when 1.0 equiv. of 4,4Ј-bipyridine is ad-
ded, and no change is observed in the presence of excess
ligand. This result supports the 1:1 complexation between
1b and 4,4Ј-bipyridine in a bridging fashion as illustrated
in Scheme 1. ESI-MS also supports the 1:1 complexation.
A mixture of 1b and 4,4Ј-bipyridine gives a signal at m/z ϭ
692.1 (calcd. for C62H78N2O12S4Pd2, [1bϪ2BF4Ϫ
2CH3CNϩ4,4Ј-bipyridine]2ϩ). In addition, the absorbance
of 1b at 304 nm increases until 1.0 equivalent of 4,4Ј-bipyri-
dine is added (Figure 4). All the experiments described
above indicate the 1:1 complexation between 1b and 4,4Ј-
Figure 1. 1H NMR spectral changes of 7b upon palladation
(400 MHz, CDCl3); SCH2CH2O and benzyl protons are indicated
with open and filled circles, respectively
3780
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2004, 3779Ϫ3782

Novel Metalloreceptors Containing Two Pincer Complexes of Palladium(II)
SHORT COMMUNICATION
crown ether 7a (271 mg, 0.322 mmol) in acetonitrile (20 mL), and
the mixture was stirred for 2 h at room temperature. The mixture
was refluxed for 13 h, and then the solvent was evaporated. The
residue was dissolved in dichloromethane (20 mL) and filtered. The
filtrate was concentrated and dried in vacuo to afford 1a (215 mg,
51%) as a yellow solid, which was further purified by recrystalliza-
tion from acetonitrile to give pale yellow crystals. 1H NMR
(400 MHz, CD3CN): δ ϭ 3.30 (br. s, 8 H), 3.80 (t, J ϭ 4.2 Hz, 8
H), 3.90 (br. t, 8 H), 4.12 (t, J ϭ 4.2 Hz, 8 H), 4.29 (br. t, 8 H),
6.77 (d, J ϭ 7.6 Hz, 4 H), 6.90Ϫ6.99 (m, 10 H) ppm. 13C NMR
(100 MHz, CD3CN): δ ϭ 40.00, 46.78 (br.), 69.20, 70.54 (ϫ2,
CH2), 114.78, 122.39, 123.96, 126.65 (CH), 149.49, 151.65 (br.), 156
(br., C) ppm. ESI-MS: m/z found 1071.1 [1aϪ2BF4Ϫ
2CH3CNϩF]ϩ. C48H60B2F8N2O8Pd2S4: calcd. C 44.09, H 4.62, N
2.14; found C 43.78, H 4.57, N 2.28.
bipyridine. Pyrazine and pyrimidine as bidentate ligands
also give rise to 1:1 complexation, as seen by 1H NMR
spectroscopic experiments, although the distances involving
the ligating nitrogen atoms of pyrazine and pyrimidine are
much shorter than that of 4,4Ј-bipyridine.
Synthesis of 1b: 1b was prepared from 7b (319 mg, 0.315 mmol)
and [Pd(CH3CN)4](BF4)2 (302 mg, 0.680 mmol) in 63% yield by a
procedure similar to that for 1a. 1b: yellow oil. 1H NMR
(400 MHz, CDCl3): δ ϭ 3.32 (br. s, 8 H), 3.67Ϫ3.69 (m, 8 H),
3.73Ϫ3.75 (m, 8 H), 3.86Ϫ3.89 (m, 8 H), 3.90Ϫ3.92 (m, 8 H), 4.16
(t, J ϭ 4.0 Hz, 8 H), 4.18 (br., 8 H), 6.79 (d, J ϭ 7.0 Hz, 4 H), 6.85
(t, J ϭ 7.0 Hz, 2 H), 6.92 (s, 8 H) ppm. 13C NMR (100 MHz,
CD3CN): δ ϭ 39.68, 46.35, 69.40, 70.50 (ϫ 2), 71.29, 71.38 (CH2),
115.09, 122.40, 124.01, 126.65 (CH), 149.62, 151.72, 156.59 (C)
ppm. ESI-MS: m/z found 1247.2 [1bϪ2BF4Ϫ2CH3CNϩF]ϩ.
C56H76B2F8N2O12Pd2S4·CHCl3: calcd. C 42.70, H 4.84, N 1.75;
found C 42.30, H 5.06, N 2.08.
1
Figure 3. H NMR spectral changes of 1b upon addition of 4,4Ј-
bipyridine (400 MHz, CDCl3, [1b] ϭ 2.0 m); signals for free and
complexed 1b are indicated with open and filled circles, respectively
X-ray Crystallographic Analysis of 1b:[20] Pale yellow crystals (0.3
ϫ 0.15 ϫ 0.1 mm) were obtained after oily 1b was allowed to stand
at room temperature. C56H76B2F8N2O12Pd2S4, M ϭ 1483.85.
Intensity data were collected at 130 K on a Rigaku R-AXIS Rapid
˚
diffractometer with Mo-Kα radiation (λ ϭ 0.71069 A). Reflection
data were corrected for Lorentz and polarization factors, and for
absorption using the multi-scan method. Crystal system, triclinic,
˚
˚
˚
3
a ϭ 13.2731(12) A, b ϭ 14.4368(16) A, c ϭ 18.1120(19) A, α ϭ
˚
71.267(4)°, β ϭ 71.424(3)°, γ ϭ 86.580(3)°, V ϭ 3112.0(6) A , space
group P1, Z ϭ 2, µ(Mo-Kα) ϭ 0.797 mmϪ1, Dcalcd. ϭ 1.584 g/cm3,
¯
2θmax ϭ 52°, collected reflections 25108, unique reflections 12053
(Rint ϭ 0.0719). The structure was solved by Patterson methods
(DIRDIF-99)[21] and refined by full-matrix least-squares on F2
using SHELXL 97.[22] The non-hydrogen atoms were refined aniso-
tropically except for the minor component of disordered sulfur
atoms (S4 and S6). Hydrogen atoms were idealized by using the
riding models. Occupancy factor involving the disordered sulfur
atoms was refined to be 0.888(3) for S3 and S5, and 0.112(3) for
S4 and S6. R1 ϭ 0.0618 [I Ͼ 2σ(I)], wR2 ϭ 0.1537 (all data), GOF
(F2) ϭ 1.036 for 786 parameters.
Figure 4. UV/Vis spectral changes of 1b upon addition of 4,4Ј-
bipyridine (CHCl3, [1b] ϭ 1.0 ϫ 10Ϫ5 )
Complexation of 1b with these bidentate ligands should
give two small cavities which are almost half as large as that
of 1b (Scheme 1). The change in cavity size is very interest-
ing, because this strategy may be most useful to regulate
recognition ability of artificial receptors.[17Ϫ19]
Acknowledgments
In conclusion, we have prepared metallohosts 1 bearing
two Pd centers inside the cavity, and studied the structure
This work was supported by Grants-in-Aid for Scientific Research
from the Ministry of Education, Culture, Sports, Science, and Tech-
nology, Japan.
1
and guest-binding ability of 1b in detail by H, 13C NMR
spectroscopy or X-ray crystallographic analysis. We are cur-
rently investigating this bridging strategy to control molecu-
lar recognition.
[1]
J. -M. Lehn, Supramolecular Chemistry, Concepts and Perspec-
tives, VCH: Weinheim, 1995.
[2]
T. Nabeshima, Coord. Chem. Rev. 1996, 148, 151Ϫ169.
[3]
Experimental Section
T. Nabeshima, S. Akine, T. Saiki, Rev. Heteroat. Chem. 2000,
22, 219Ϫ239.
D. L. Caulder, K. N. Raymond, J. Chem. Soc., Dalton Trans.
[4]
Synthesis of 1a: A solution of [Pd(CH3CN)4](BF4)2 (302 mg,
0.680 mmol) in acetonitrile (10 mL) was added to a solution of
1999, 1185Ϫ1200.
Eur. J. Inorg. Chem. 2004, 3779Ϫ3782
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3781

T. Nabeshima, D. Nishida, S. Akine, T. Saiki
SHORT COMMUNICATION
[5]
[17]
[18]
[19]
[20]
M. Albrecht, Chem. Rev. 2001, 101, 3457Ϫ3497.
T. Nabeshima, Y. Yoshihira, T. Saiki, S. Akine, E. Horn, J. Am.
T. Nabeshima, H. Furusawa, Y. Yano, Angew. Chem. Int. Ed.
Engl. 1994, 33, 1750Ϫ1751.
T. Nabeshima, D. Nishida, Tetrahedron Lett. 2002, 43,
5719Ϫ5722.
[6]
Chem. Soc. 2003, 125, 28Ϫ29.
[7]
M. Albrecht, G. van Koten, Angew. Chem. Int. Ed. 2001, 40,
3750Ϫ3781.
T. Nabeshima, D. Nishida, T. Saiki, Tetrahedron 2003, 59,
[8]
J. E. Kickham, S. J. Loeb, J. Chem. Soc., Chem. Commun.
639Ϫ647.
1993, 1848Ϫ1850.
CCDC-241251 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
at http://www.ccdc.cam.ac.uk/conts/retrieving.html [or from
the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; Fax: (internat.) ϩ44-1223-336-033;
E-mail: deposit@ccdc.cam.ac.uk].
P. T. Beurskens, G. Beurskens, R. de Gelder, S. Garcia-Granda,
R. O. Gould, R. Israel, J. M. M. Smits, The DIRDIF-99 pro-
gram system, Crystallography Laboratory, University of
Nijmegen, The Netherlands, 1999.
[9]
J. E. Kickham, S. J. Loeb, Inorg. Chem. 1994, 33, 4351Ϫ4359.
[10]
J. E. Kickham, S. J. Loeb, Inorg. Chem. 1995, 34, 5656Ϫ5665.
J. E. Kickham, S. J. Loeb, S. L. Murphy, J. Am. Chem. Soc.
[11]
1993, 115, 7031Ϫ7032.
[12]
J. E. Kickham, S. J. Loeb, Organometallics 1995, 14,
[21]
[22]
3584Ϫ3587.
[13]
S. Kopolow, T. E. Hogen Esch, J. Smid, Macromolecules 1973,
6, 133Ϫ142.
S. Shinkai, K. Inuzuka, O. Miyazaki, O. Manabe, J. Am. Chem.
[14]
Soc. 1985, 107, 3950Ϫ3955.
T. Sato, K. Nishiyama, A. Morita, Y. Iitaka, Bull. Chem. Soc.
G. M. Sheldrick, SHELXL 97, Program for crystal structure
refinement, University of Göttingen, Germany, 1997.
Received June 11, 2004
[15]
Jpn. 1985, 58, 2366Ϫ2369.
R. F. Schramm, B. B. Wayland, J. Chem. Soc., Chem. Commun.
[16]
1968, 898Ϫ899.
3782
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2004, 3779Ϫ3782