Coordination framework hosts consisting of 4-pyridyl-substituted carboxylic acid (PCA) dimers and 1D chains of Ni2+ and SCN-: A rational structural extension toward coordination framework hosts with large rectangular cavities

Article abstract of DOI:10.1021/ic0605367

For the expansion of a rectangular cavity (RC) defined by two isonicotinic acid (isoH) dimers as bridging ligands and two SCN bridges, we conducted a structural extension based on the elongation of the bridging ligands by the replacement of isoH with longer 4-pyridyl-substituted carboxylic acid (PCA). For this purpose, the following three PCAs have been employed: trans-3-(4-pyridyl) propenoic acid (acrylH), 4-(4-pyridyl)benzoic acid (pybenH), and trans-3-(4-(4-pyridyl)phenyl)propenoic acid (pppeH). Self-assembly of Ni ²⁺, SCN^-, and each of four PCAs involving isoH, acrylH, pybenH, and pppeH in the presence of an aromatic guest gave four inclusion compounds formulated as [Ni(SCN)₂(isoH)₂]·1/ 2(benz[a]anthracene) (1), [Ni(SCN)₂(acrylH)₂]·1/ 2(benz[a]anthracene) (2), [Ni(SCN)₂(pybenH)₂] ·(pyrene) (3), and [Ni(SCN)₂(pppeH)₂] _3/2·(benz[a]anthracene) (4). X-ray crystal structural determination of 1-4 revealed that the proposed structural extension was successful. Their crystal structures are layered structures of two-dimensional (2D) grid-type coordination frameworks (2D host layers) framed with bridging ligands of the corresponding PCA dimers and 1D chains consisting of Ni ²⁺ ions and μ_1,3-SCN^- ions. The lengths of the PCA dimers are 12.269(5) A (isoH dimer), 16.890(4) A (acrylH dimer), 20.89(2) A (pybenH dimer), 25.387(3) A (pppeH dimer A), and 25.527-(4) A (pppeH dimer B). Each 2D host layer has RCs defined by the two corresponding PCA dimers and the two SCN bridges. The dimensions of RCs are expanded in proportion to the increase in the lengths of the PCA dimers: 29.52 x 5.60-7.20 A² (4) > 24.95 x 5.46-7.38 A² (3) > 20.88 x 5.49-7.25 A² (2) > 16.41 x 5.53-7.43 A² (1). These expansions reflect the number of aromatic guests that can be included in RCs. RC of 1 include only one molecule of benz[a]anthracene, whereas RCs of 3 or 4 includes two molecules of pyrene or benz[a]anthracene, respectively. Comparison of the lengths between the PCA dimers and 4,4′-bipyridine-type ligands demonstrated that a design strategy - the preparation of a bridging ligand through self-assembly of two PCAs - is both efficient and particularly suitable for the preparation of very long bridging ligands.

Full text of DOI:10.1021/ic0605367

Inorg. Chem. 2006, 45, 9233−9244
Coordination Framework Hosts Consisting of 4-Pyridyl-Substituted
+
-
Carboxylic Acid (PCA) Dimers and 1D Chains of Ni2 and SCN : A
Rational Structural Extension toward Coordination Framework Hosts
with Large Rectangular Cavities
Ryo Sekiya,*,† Shin-ichi Nishikiori,† and Katsuyuki Ogura‡
Department of Basic Science, Graduate School of Arts and Sciences, The UniVersity of Tokyo,
3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan, and Department of Applied Chemistry and
Biotechnology, Faculty of Engineering, Chiba UniVersity, 1-33 Yayoi-cho, Inage-ku,
Chiba 263-8522, Japan
Received March 29, 2006
For the expansion of a rectangular cavity (RC) defined by two isonicotinic acid (isoH) dimers as bridging ligands
and two SCN bridges, we conducted a structural extension based on the elongation of the bridging ligands by the
replacement of isoH with longer 4-pyridyl-substituted carboxylic acid (PCA). For this purpose, the following three
PCAs have been employed: trans-3-(4-pyridyl)propenoic acid (acrylH), 4-(4-pyridyl)benzoic acid (pybenH), and
trans-3-(4-(4-pyridyl)phenyl)propenoic acid (pppeH). Self-assembly of Ni2+, SCN-, and each of four PCAs involving
isoH, acrylH, pybenH, and pppeH in the presence of an aromatic guest gave four inclusion compounds formulated
as [Ni(SCN)2(isoH)2]
‚
1/2(benz[a]anthracene) (1), [Ni(SCN)2(acrylH)2]
‚
1/2(benz[a]anthracene) (2), [Ni(SCN)2(pybenH)2]
‚
(pyrene) (3), and [Ni(SCN)2(pppeH)2]3
‚
(benz[a]anthracene) (4). X-ray crystal structural determination of 1
−4 revealed
/2
that the proposed structural extension was successful. Their crystal structures are layered structures of two-dimensional
(2D) grid-type coordination frameworks (2D host layers) framed with bridging ligands of the corresponding PCA
+
dimers and 1D chains consisting of Ni2 ions and
µ
1,3-SCN- ions. The lengths of the PCA dimers are 12.269(5)
Å (isoH dimer), 16.890(4) Å (acrylH dimer), 20.89(2) Å (pybenH dimer), 25.387(3) Å (pppeH dimer A), and 25.527-
(4) Å (pppeH dimer B). Each 2D host layer has RCs defined by the two corresponding PCA dimers and the two
SCN bridges. The dimensions of RCs are expanded in proportion to the increase in the lengths of the PCA dimers:
29.52
× 5.60− × 5.46− × 5.49− × 5.53−
7.20 Å2 (4) > 24.95 7.38 Å2 (3) > 20.88 7.25 Å2 (2) > 16.41 7.43 Å2 (1).
These expansions reflect the number of aromatic guests that can be included in RCs. RC of 1 include only one
molecule of benz[a]anthracene, whereas RCs of 3 or 4 includes two molecules of pyrene or benz[a]anthracene,
respectively. Comparison of the lengths between the PCA dimers and 4,4
that a design strategy the preparation of a bridging ligand through self-assembly of two PCAs
and particularly suitable for the preparation of very long bridging ligands.
′
-bipyridine-type ligands demonstrated
−
−is both efficient
Introduction
because of its strong propensity for the formation of highly
directional and relatively strong hydrogen bonding with its
counterpart.1-3 This propensity originates from self-comple-
mentary pairwise O-H/O hydrogen bonding, denoted by
R22(8) graph set 4, and a substantially large ∆H of formation
compared with single hydrogen bonding.1,5 A well-known
example of this structural equivalence is a benzoic acid dimer
and p-terphenyl.6 As shown in Figure 1, the benzoic acid
dimer is a supramolecular analogue of p-terphenyl because
Self-assembly of two components with the assistance of
highly directional and strong non-covalent interaction gives
the corresponding homodimer, which is a supramolecular
analogue of a single molecule with almost identical molecular
structure. The carboxyl functional group has been most
frequently employed as a connecting unit of two components
* To whom correspondence should be addressed. Phone: 81-3-5454-
6569. Fax: 81-3-5454-6569. E-mail: csekiya@mail.ecc.u-tokyo.ac.jp.
† The University of Tokyo.
(1) Jeffery, G. A. An Introduction To Hydrogen Bonding; Oxford
‡ Chiba University.
University Press: New York, 1997.
10.1021/ic0605367 CCC: $33.50
Published on Web 10/12/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 23, 2006 9233

Sekiya et al.
Scheme 1
Figure 1.
of the structural equivalence between the R22(8) ring system
of the benzoic acid dimer and the 1,4-disubstituted phenyl
ring of p-terphenyl.
From a structural perspective, this equivalence means that
carboxylic acid dimers (CADs) can be substituted for their
single molecular counterparts. This design strategy is par-
ticularly advantageous when their single molecular counter-
parts have so long and/or large molecular structures that they
exhibit poor solubility toward organic solvents and/or water.
(Poor solubility originates mainly from their large molecular
weight and extensive molecular surface, which strengthen
intermolecular interaction, e.g., van der Waals interaction,
between molecules and prevent themselves from dissolving
into solvents.) A recent work reported by Nangia and co-
workers demonstrated the usefulness of this design strategy
with 4-(triphenylmethyl)benzoic acid.7 In that study, they
employed this monocarboxylic acid on the basis of notions
that (1) its corresponding CAD acts as a supramolecular
analogue of 4,4′′-bis(triphenylmethyl)-p-terphenyl, a wheel-
and-axle class of a host compound,8 and (2) 4,4′′-bis-
(triphenylmethyl)-p-terphenyl is expected to exhibit low
solubility toward organic solvents originating from its
polyaromatic backbone. Indeed, 4-(triphenylmethyl)benzoic
acid self-assembles to form the corresponding CAD acting
as a wheel-and-axle host compound. The resulting CAD
forms inclusion compounds with several aromatic solvents.
Our design strategy is to employ CAD as a bridging ligand
of a metal-organic coordination framework.9 The motivation
to this is based on the following backgrounds. (1) Most of
bridging ligands employed in metal-organic coordination
frameworks are single molecules,10 and thus the employment
of CAD as a bridging ligand is of structural interest. (2)
Inclusion of large organic molecules requires large cavities
whose size must be larger than those of target guests, and
thus long bridging ligands, e.g., long 4,4′-bipyridine-type
ligands, are required. However, because of the reason
mentioned above, such ligands are estimated to exhibit poor
solubility toward organic solvents and/or water. Recently,
we reported new inclusion compounds characterized by
isonicotinic acid (isoH) dimer as a bridging ligands (isoH
system).9 In that study, we successfully exemplified that self-
assembly of small components of isoH, Ni2+, and SCN- in
the presence of an aromatic guest gives a two-dimensional
(2D) grid-type coordination framework (2D host layer), with
relatively large rectangular cavities (RCs) defined by two
isoH dimers and two SCN bridges, as schematically shown
in Scheme 1. In that structure, the isoH dimers act as bridging
ligands with more than 12 Å in length, being a supramo-
lecular analogue of a 4,4′-bipyridine-type ligand of 1,4-bis-
(4-pyridyl)benzene. The employment of an ambidentate
anionic ligand of SCN- is based on both prevention of an
interpenetrating network by shortening the width of the RC
and cancellation of positive charge on metal centers.
(2) For recent articles see: (a) George, S.; Goldberg, I. Cryst. Growth
Des. 2006, 6, 755-762. (b) Jayaraman, A.; Balasubramaniam, B.;
Valiyaveettil, S. Cryst. Growth Des. 2004, 4, 1403-1409. (c) Gdaniec,
M.; Jankowski, W.; Milewska, M. J.; Połon˜ski, T. Angew. Chem., Int.
Ed. 2003, 42, 3903-3906. (d) Qin, Z.; Jennings, M. C.; Puddephatt,
R. J. Inorg. Chem. 2002, 41, 5174-5186. (e) Tang, G.; Zhu, H.-G.;
Liang, B.-H.; Chen, X.-M. J. Chem. Soc., Dalton Trans. 2001, 580-
585. (f) Gianneschi, N. C.; Tiekink, E. R. T.; Rendina, L. M. J. Am.
Chem. Soc. 2000, 122, 8474-8479. (g) Kolotuchin, S. V.; Thiessen,
P. A.; Fenlon, E. E.; Wilson, S. R.; Loweth, C. J.; Zimmerman, S. C.
ChemsEur. J. 1999, 5, 2537-2547. (h) Holy´, P.; Za´vada, J.; C´ısaøova´,
I.; Podlaha, J. Angew. Chem., Int. Ed. 1999, 38, 381-383. (i) Beketov,
K.; Weber, E.; Seidel, J.; Ko¨hnke, K.; Makhkamov, K.; Ibragimov,
B. Chem. Commun. 1999, 91-92. (j) Aakero¨y, C. B.; Beatty, A. M.;
Leinen, D. S. Angew. Chem., Int. Ed. 1999, 38, 1815-1819. (k)
Ashton, P. R.; Collins, A. N.; Fyfe, M. C. T.; Menzer, S.; Stoddart, J.
F.; Williams, D. J. Angew. Chem., Int. Ed. 1998, 36, 735-739.
(3) For reviews see: (a) Braga, D.; Maini, L.; Polito, M.; Tagliavini, E.;
Grepioni, F. Coord. Chem. ReV. 2003, 246, 53-71. (b) Beatty, A. M.
Coord. Chem. ReV. 2003, 246, 131-143. (c) Braga, D.; Grepioni, F.;
Desiraju, G. R. Chem. ReV. 1998, 98, 1375-1405. (d) Braga, D.;
Grepioni, F.; Sabatino, P.; Desiraju, G. R. Organometallics 1994, 13,
3532-3543. (e) Leiserowitz, L. Acta Crystallogr., Sect. B 1976, 32,
775-802.
The fact that the dimension of the RC is defined by the
length of the isoH dimer led us to anticipate that the
replacement of isoH with a longer 4-pyridyl-substituted
carboxylic acid (PCA), a structural analogue of isoH, would
expand its dimension. For this purpose, we have employed
the following three PCAs: trans-3-(4-pyridyl)propenoic acid
(acrylH), 4-(4-pyridyl)benzoic acid (pybenH), and trans-3-
(4-(4-pyridyl)pheny)propenoic acid (pppeH), as shown in
(4) (a) Bernstein, J.; Davis, R. E.; Shimoni, L.; Chang, N.-L. Angew.
Chem., Int. Ed. 1995, 34, 1555-1573. (b) Etter, M. C. Acc. Chem.
Res. 1990, 23, 120-126.
(5) Gavezzoti, A.; Filippini, G. J. Phys. Chem. 1994, 98, 4831-4837.
(6) (a) Desiraju, G. R. Chem. Commun. 1997, 1475-1482. (b) Desiraju,
G. R. Angew. Chem. Int. Ed. 1995, 34, 2311-2327.
(9) Sekiya, R.; Nishikiori, S.; Ogura, K. J. Am. Chem. Soc. 2004, 126,
16587-16600.
(10) For reviews see: (a) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem.,
Int. Ed. 2004, 43, 2334-2375. (b) Eddaoudi, M.; Moler, D. B.; Li,
H.; Chen, B.; Reineke, T. M.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem.
Res. 2001, 34, 319-330. (c) Zaworotko, M. J. Chem. Commun. 2001,
1-9.
(7) Jetti, R. K. R.; Xue, F.; Mak, T. C. W.; Nangia, A. J. Chem. Soc.,
Perkin Trans. 2 2000, 1223-1232.
(8) Toda, F. In ComprehensiVe Supramolecular Chemistry; MacNicol, D.
D., Toda, F., Bishio, R., Eds.; Pergamon: Oxford, U.K., 1996; Vol.
6, pp 305-350.
9234 Inorganic Chemistry, Vol. 45, No. 23, 2006

4-Pyridyl-Substituted Carboxylic Acid Dimers
Scheme 2
layer was acidified with concentrated HCl; the resulting white
precipitate was collected, washed with CH3CN, and recrystallized
from pyridine to afford 4-(4-pyridyl)benzoic acid (1.72 g, 8.63
mmol) in 85% yield. Mp: >330 °C (dec). IR (KBr, νmax): 2431,
1700, 1604, 1403, 1317, 1296, 1180, 1073, 1006, 828, 733, 742,
1
701, 668, 519 cm-1. H NMR (DMSO-d6, 500 MHz): δ 13.13
(br. s, 1H), 8.67 (dd, 2H, 1.53 Hz, 4.57 Hz), 8.06 (dd, 2H, 1.83
Hz, 6.72 Hz), 7.92 (dd, 2H, 1.83 Hz, 6.72 Hz), 7.76 (dd, 2H, 1.53
Hz, 4.57 Hz). 13C NMR (DMSO-d6, 126 MHz): δ 166.90, 150.33,
145.96, 141.24, 131.26, 130.08, 127.11, 121.46. MS (EI, 70 eV,
m/z): 199 (M+), 182 (OH), 154 (CO2H). Anal. Calcd for C12H9-
NO2: C, 72.35; H, 4.55; N, 7.03. Found: C, 72.26; H, 4.58; N,
6.83.
trans-4-(2-Carboxyethenyl)benzeneboronic Acid. Malonic acid
(0.72 g, 6.92 mmol) was added to a degassed solution of
p-formylbenzeneboronic acid (0.50 g, 3.33 mmol) in 30 mL of
pyridine at 25 °C and 0.1 cm3 of piperidine was then added to the
solution. The mixture was refluxed under N2 for 13 h. The solvent
was removed under reduced pressure. The residue was taken up in
water that was acidified with concentrated HCl; the resulting white
precipitate was collected, washed with water, and dried under
reduced pressure to afford trans-4-(2-carboxyethenyl)benzenebo-
ronic acid (0.59 g, 3.07 mmol) in 92% yield. Mp: 194-195 °C.
IR (KBr, νmax): 3346, 2518, 1659, 1511, 1411, 1339, 1311, 1268,
Scheme 2. Their corresponding CADs can act as very long
bridging ligands, some of which are more than 20 Å in
length. Self-assembly of Ni2+, SCN-, and each of three PCAs
would result in the formation of 2D host layers whose
fundamental architectures are almost identical to that of the
starting host system but have expanded RCs. In principle,
resulting RCs would include much larger molecules that
cannot be included in RC of the isoH system and the number
of the guests that can be included in the resulting RCs would
by increase two or more. Therefore, the proposed structural
extension is a fundamental and important investigation aimed
at the construction of new host systems.
In the present study, we will describe a successful
structural extension from the isoH system and the usefulness
of our design strategy through both introduction of four
inclusion compounds and comparison of the resulting PCA
dimers with 4,4′-bipyridine-type ligands employed in metal-
organic coordination frameworks. The four inclusion com-
pounds reported herein are [Ni(SCN)2(isoH)2]‚1/2(benz[a]-
anthracene) (1) (isoH system), [Ni(SCN)2(acrylH)2]‚1/2-
(benz[a]anthracene) (2) (acrylH system), [Ni(SCN)2(pybenH)2]‚
(pyrene) (3) (pybenH system), and [Ni(SCN)2(pppeH)2]3/2‚
(benz[a]anthracene) (4) (pppeH system).
1
1151, 1061, 975, 876, 786, 676 cm-1. H NMR (DMSO-d6, 500
MHz) δ 12.42 (br. s, 1H), 8.16 (br. s, 2H), 7.80 (d, 2H, 8.09 Hz),
7.62 (d, 2H, 8.09 Hz), 7.57 (d, 2H, 16.17 Hz), 6.54 (d, 2H, 16.17
Hz). 13C NMR (DMSO-d6, 126 MHz) δ 167.61, 143.87, 136.39,
135.61, 134.55, 127.14, 119.67. MS (EI, 70 eV, m/z): 147
(-B(OH)2), 131, (-B(OH)2, -OH), 103 (-B(OH)2, -CO2H).
Anal. Calcd for C9H9BO4: C, 56.31; H, 4.72. Found: C, 56.07; H,
4.82.
trans-3-(4-(4-Pyridyl)phenyl)propenoic Acid (pppeH). Pd-
(PPh3)4 (61 mg, 5.3 × 10-2 mmol) was added to a degassed solution
of (E)-4-(2-carboxylethenyl)benzeneboronic acid (1.02 g, 5.31
mmol), 4-bromopyridine hydrochloride (1.05 g, 5.40 mmol), and
sodium carbonate (1.82 g, 17.2 mmol) in 80 mL of a mixed solution
(1:1 CH3CN: H2O) at 25 °C. The mixture was refluxed under N2
for 20 h. The hot suspension was filtered. The filtrate was
concentrated to about half of the original volume and then washed
with CH2Cl2. The aqueous layer was acidified with concentrated
HCl; the resulting white precipitate was collected, washed with CH3-
CN, and recrystallized from pyridine to afford desired product (0.96
g, 4.26 mmol) in 80% yield. Mp: >330 °C (dec). IR (KBr, ν max):
2363, 1923, 1700, 1636, 1605, 1408, 1194, 1064, 1019, 1007, 814,
684, 607, 521, 496 cm-1. 1H NMR (DMSO-d6, 500 MHz) δ 12.48
(br. s, 1H), 8.64 (dd, 2H, 1.53 Hz, 4.58 Hz), 7.85 (m, 4H), 7.75
(dd, 2H, 1.53 Hz, 4.58 Hz), 7.64 (d, 1H, 16.17 Hz), 6.62 (d, 1H,
16.17 Hz). 13C NMR (DMSO-d6, 126 MHz): δ 167.48, 150.30,
146.04, 143.36, 138.51, 135.11, 129.00, 127.27, 121.11, 120.12.
MS (EI, 70 eV, m/z): 225 (M+), 208 (-OH), 180 (-CO2H); Anal.
Calcd for C14H11NO2: C, 74.65; H, 4.92; N, 6.22. Found: C, 74.55;
H, 4.73; N, 6.01.
Experimental Section
General Methods. Unless stated otherwise, reagents and solvents
were purchased from commercial sources and used directly.
Synthesis of pybenH, trans-4-(2-carboxyethenyl)benzeneboronic
acid, and pppeH was conducted under a dry nitrogen atmosphere.
Melting points were determined using a Laboratory Devices Inc.
melting point apparatus and are uncorrected. 1H NMR spectra and
13C NMR spectra were recorded on a JEOL R-500. All infrared
spectra were measured on a JASCO FT/IR-350 spectrometer by
the KBr disk method at room temperature. Mass spectra were
measured on a JEOL JMS-600H machine under electron impact at
70 eV.
Synthesis. 4-(4-Pyridyl)benzoic Acid (pybenH).11 Pd(PPh3)4
(0.30 g, 0.26 mmol) was added to a degassed solution of
carboxybenzene boronic acid (1.70 g, 10.2 mmol), 4-bromopyridine
hydrochloride (2.01 g, 10.3 mmol), and sodium carbonate (2.10 g,
19.8 mmol) in 120 mL of a mixed solution (1:1 CH3CN:H2O) at
25 °C. The mixture was refluxed under N2 for 22 h. The hot
suspension was filtered. The filtrate was concentrated to about half
of the original volume and then washed with CH2Cl2. The aqueous
[Ni(SCN)2(isoH)2]‚1/2(benz[a]anthracene) (1). To an acetoni-
trile solution (120 mL) in which NiCl2‚6H2O (0.12 g, 0.51 mmol)
was suspended was added KSCN (0.10 g, 1.03 mmol) with vigorous
stirring, and the mixture was refluxed for 1 h. After the precipitated
KCl was filtered off, isonicotinic acid (0.12 g, 0.97 mmol) was
added to the solution. The solution was left for 24 h at room
temperature, and then benz[a]anthracene was added. Green crystals
of 1 were obtained after slow evaporation of the solvent over a
period of a few weeks at room temperature. The crystals were
collected, washed with acetonitrile, and dried in the air. Mp: >330
(11) PybenH was synthesized by a modified procedure based on a literature
method. See: Gong, Y.; Pauls, H. W. Synlett 2000, 6, 829-831.
Inorganic Chemistry, Vol. 45, No. 23, 2006 9235

Sekiya et al.
Scheme 3
°C (dec). Anal. Calcd for C23H16N4O4S2Ni: C, 51.61; H, 3.02; N,
10.47. Found: C, 51.59; H, 3.06; N, 10.39.
[Ni(SCN)2(acrylH)2]‚1/2(benz[a]anthracene) (2). To methanol
solution (100 mL) in which NiCl2‚6H2O (0.18 g, 0.76 mmol) was
suspended was added KSCN (0.15 g, 1.54 mmol) was added with
vigorous stirring, and the mixture was refluxed for 1 h. After the
precipitated KCl was filtered off, trans-3-(4-pyridyl)propenoic acid
(0.30 g, 1.51 mmol) was added to the solution. The solution was
left for 24 h at room temperature, and benz[a]anthracene was then
added. Green crystals of 2 were obtained after slow evaporation of
the solvent over a period of a few weeks at room temperature. The
crystals were collected, washed with methanol, and dried in the
air. Mp: >330 °C (dec). Anal. Calcd for C27H20N4O4S2Ni: C,
55.21; H, 3.41; N, 9.54. Found: C, 55.31; H, 3.51; N, 9.48.
[Ni(SCN)2(pybenH)2]‚(pyrene) (3). To a methanol solution (200
mL) in which NiCl2‚6H2O (0.18 g, 0.76 mmol) was suspended was
added KSCN (0.15 g, 1.54 mmol) with vigorous stirring, and the
mixture was refluxed for 1 h. After the precipitated KCl was filtered
off, 4-(4-pyridyl)benzoic acid (0.30 g, 1.51 mmol) was added to
the solution. The solution was left for 24 h at room temperature,
and pyrene was then added. Green crystals of 3 were obtained after
slow evaporation of the solvent over a period of a few weeks at
room temperature. The crystals were collected, washed with
methanol, and dried in the air. Mp: >330 °C (dec). Anal. Calcd
for C42H28N4O4S2Ni: C, 65.04; H, 3.05; N, 7.23. Found: C, 65.04;
H, 3.71; N, 7.06.
Scheme 4
[Ni(SCN)2(pppeH)2]3/2‚(benz[a]anthracene) (4). To a methanol
solution (100 mL) in which NiCl2‚6H2O (0.07 g, 0.30 mmol) was
suspended was added KSCN (0.06 g, 0.62 mmol) with vigorous
stirring, and the mixture was refluxed for 1 h. After the precipitated
KCl was filtered off, trans-3-(4-(4-pyridyl)phenyl)propenoic acid
(0.13 g, 0.58 mmol) was dissolved. The solution was left for 24 h
at room temperature, and benz[a]anthracene was then added to the
solution. Green crystals of 4 were obtained after slow evaporation
of the solvent over a period of a few weeks at room temperature.
The crystals were collected, washed with methanol, and dried in
92% isolated yield. No corresponding cis-isomer was obtained.
Subsequent Pd-catalyzed cross-coupling reactions of 4-bromopy-
ridine hydrochloride with the boronic acid intermediate in the
presence of Na2CO3 as base and catalytic amount of Pd(PPh3)4 in
1:1 CH3CN:H2O solution provided pppeH in 80% isolated yield.
Preparation of Inclusion Compounds. Diffraction-quality green
crystals of 1 were prepared by slow evaporation at room temperature
of an acetonitrile solution containing Ni2+, SCN-, isoH, and benz-
[a]anthracene (Scheme 4). Green crystals of 2-4 were prepared
from methanol solutions by methods almost identical to that of 1.
IR spectra of 1-4 exhibited strong and sharp peaks characteristic
of carboxyl functional group and SCN- ion as intermetallic bridge
at 1684-1706 and 2101-2120 cm-1, respectively. Broad absorption
bands with some weak peaks in the range of 2300-3200 cm-1
suggested the formation of hydrogen bonding between the carboxyl
functional groups of the corresponding PCAs. The IR spectra of
1-4 are shown in Figures S1-S4, respectively, of the Supporting
Information. All inclusion compounds are air-stable and insoluble
in common organic solvents except for polar ones such as DMF
and DMSO.
X-ray Crystallography. X-ray crystal structure determination
of 1-4 revealed their compositions to be [Ni(SCN)2(isoH)2]‚1/2-
(benz[a]anthracene), [Ni(SCN)2(acrylH)2]‚1/2(benz[a]anthracene),
[Ni(SCN)2(pybenH)2]‚(pyrene), and [Ni(SCN)2(pppeH)2]3/2‚(benz[a]-
anthracene), respectively. All inclusion compounds belong to the
triclinic crystal system with space group P1h (No. 2). Detailed
crystallographic data and their host:guest ratios (host-to-guest
stoichiometric ratios) are listed in Table 1. Neither solvent of
crystallization nor counterion was found in the crystal structures
of 1-4. Because the carboxyl hydrogen atoms of PCAs could not
be found in difference maps, the hydrogen atoms were placed at
the air. Mp: >330 °C (dec). Anal. Calcd for C63H45N6O6S3Ni1.5
:
C, 64.88; H, 3.89; N, 7.21. Found: C, 64.68; H, 3.95; N, 7.03.
X-ray Crystal Structure Determination. The intensity data of
1-4 were collected on a Rigaku RAXIS-RAPID imaging plate
area detector using graphite-monochromatized Mo KR radiation
(λ /Å ) 0.71069). The crystal structures were solved by the direct
method using the SHELXS-97 program 12 and refined by successive
differential Fourier syntheses and full-matrix least-squares proce-
dures using the SHELXL-97 program.13 Anisotropic thermal factors
were applied to all non-hydrogen atoms. All hydrogen atoms were
generated geometrically.
Synthesis
Synthesis of Building Blocks. PybenH11 and pppeH were
synthesized as outlined in Scheme 3. PybenH was synthesized by
a Pd-catalyzed cross-coupling reaction of 4-bromopyridine hydro-
chloride with 4-carboxybenzeneboronic acid in the presence of Na2-
CO3 as base and a catalytic amount of Pd(PPh3)4 in 1:1 CH3CN:
H2O solution in 85% isolated yield. PppeH was synthesized from
p-formylbenzeneboronic acid in two steps. Knoevenagel condensa-
tion of p-formylbenzeneboronic acid with malonic acid in the
presence of piperidine in pyridine solution provided trans-4-(2-
carboxyethenyl)benzeneboronic acid (boronic acid intermediate) in
(12) Sheldrick, G. M. SHELXS-97: Program for X-ray Crystal Structure
Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(13) Sheldrick, G. M. SHELXL-97: Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
9236 Inorganic Chemistry, Vol. 45, No. 23, 2006

4-Pyridyl-Substituted Carboxylic Acid Dimers
Table 1. Crystallographic Data for the Inclusion Compounds of 1-4
1
2
3
4
formula
aromatic guest
host-to-guest ratioa
fw
C23H16N4O4S2Ni
benz[a]anthracene
2:1
C27H20N4O4S2Ni
benz[a]anthracene
2:1
C23H16N4O4S2Ni
pyrene
1:1
C63H45N6O6S3Ni1.5
benz[a]anthracene
3:2
535.2
587.3
755.5
1166.3
T (K)
298
123
183
153
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V(Å3)
triclinic
P1h (No. 2)
10.025(2)
11.0678(9)
17.963(2)
114.955(7)
50.81(2)
131.83(1)
1151(3)
2
1.54
1.063
60
5997
3910
391
0.0663
0.1745
1.103
triclinic
P1h (No. 2)
9.7107(8)
10.982(1)
20.646(2)
95.132(3)
35.560(4)
90.949(3)
1269(1)
2
1.54
0.971
60
6827
4388
427
0.0608
0.1712
1.000
triclinic
P1h (No. 2)
10.69(1)
10.926(7)
26.82(2)
115.63(7)
45.8(1)
128.98(7)
1747(9)
2
1.47
0.726
60
8683
2332
481
0.0695
0.2083
0.782
triclinic
P1h (No. 2)
10.9741(4)
16.7548(4)
33.6402(9)
120.456(3)
37.446(1)
134.530(5)
2665(2)
2
1.45
0.714
60
15107
9608
799
0.0484
0.1309
Z
F
calcd (g cm-3
)
µ (Mo KR) (mm-1
2θmax (deg)
no. of reflns obsdb
no. of reflns usedc
params
)
R1d
wR2e
GOF
0.957
a Host-to-guest stoichiometric ratio. b The number of observed reflections. c The number of reflections used in the structure determination. d R1 ) (∑||Fo|
- |Fc||)/∑|Fo|. e wR2 ) [{∑(w(Fo - Fc )2)}/∑(w(Fo )2)]1/2
.
2
2
2
Figure 2. (a) Framework with van der Waals surface of the 2D host layer of 1 spreading over the bc plane. Benz[a]anthracene guests are omitted for
clarity. Color scheme: gray (carbon), white (hydrogen), blue (nitrogen), red (oxygen), yellow (sulfur), green (nickel). (b) Coordination environment around
Ni(1) and Ni(2) in 1 (50% probability level ellipsoids). Symmetry code used to generate equivalent atoms: (i) - x, 1 - y, 1 - z.
calculated positions adjacent to the oxygen atom with the longer
carbon-to-oxygen bond length.
environments around metal centers with a major atom
numbering scheme are shown in panels a14 and b of Figure
4, respectively. (The corresponding coordination frameworks
and coordination environments of 1 and 2 are shown in
Figures 2 and 3, respectively.) The asymmetric unit contains
two Ni2+ ions (Ni(1) and Ni(2)) with octahedral coordination
geometries, two SCN- ions, two pybenH ligands, and a
pyrene guest. (The asymmetric units of 1 and 2 contain half
of a benz[a]anthracene guest.) The Ni2+ ions lie on crystal-
lographic inversion centers. Figure 4b shows that four
equatorial coordination sites of each Ni2+ ion are occupied
by two nitrogen atoms from SCN- ions and two sulfur atoms
from SCN- ions in trans configurations. The SCN- ions act
as µ1,3-intermetallic bridges and doubly connect adjacent Ni2+
ions in antiparallel fashion with an Ni(1)/Ni(2) separation
of 5.5643(3) Å to build up an 1D chain propagating along
the b axis. (The corresponding Ni/Ni separations found in 1
and 2 are 5.5339(5) and 5.4908(5) Å, respectively.) Two
pybenH ligands are bonded to the remaining axial coordina-
In the following crystal structure descriptions, axis settings are
used for easy structural comparison among 1-4. 1D chains
propagate along the b axis. Ni2+-PCA/PCA-Ni2+ linkages propa-
gate along the [012] axis for 1-3 and the [023] axis for 4. Two-
dimensional host layers spread over the bc plane and are stacked
parallel to the bc plane.
Description of the Crystal Structures
General. The crystal structures of 1-4 are characterized
by layered structures of 2D host layers composed of Ni2+,
SCN-, and the corresponding PCA ligands. Their architec-
tures are generally identical to each other. However, some
differences were found between 4 and the others. Thus, we
will describe first the structure of the 2D host layer of 3, as
a representative of 1-3, and then that of the 2D host layer
of 4. After that, we will describe the crystal structures of
1-4.
Architecture of the 2D Host Layer of 3. The coordination
framework with van der Waals surface of the 2D host layer
formed in 3 and the ORTEP drawing of the coordination
(14) Merritt, E. A.; Bacon, D. J. Raster3D: Photorealistic Molecular
Graphics; Methods Enzymol. 1997, 277, 505-524.
Inorganic Chemistry, Vol. 45, No. 23, 2006 9237

Sekiya et al.
Figure 3. (a) Framework with van der Waals surface of the 2D host layer of 2 spreading over the bc plane. Benz[a]anthracene guests are omitted for
clarity. Color scheme: gray (carbon), white (hydrogen), blue (nitrogen), red (oxygen), yellow (sulfur), green (nickel). (b) The coordination environment
around Ni(1) and Ni(2) in 2 (50% probability level ellipsoids). Symmetry code used to generate equivalent atoms: (i) - x, 1 - y, 1 - z.
Figure 4. Framework with van der Waals surface of the 2D host layer of 3 spreading over the bc plane. Pyrene guests are omitted for clarity. Color
scheme: gray (carbon), white (hydrogen), blue (nitrogen), red (oxygen), yellow (sulfur), green (nickel). (b) Coordination environment around Ni(1) and
Ni(2) in 3 (50% probability level ellipsoids). Symmetry code used to generate equivalent atoms: (i) - x, 1 - y, 1 - z.
Table 2. Selected Bond Distances (Å) and Angles (deg) for 1a
Table 3. Selected Bond Distances (Å) and Angles (deg) for 2a
Ni(1)-N(1)
2.033(4)
2.516(1)
2.133(4)
90.7(1)
89.6(1)
93.9(1)
Ni(1)-N(11)
2.139(4)
2.036(3)
2.530(1)
92.5(1)
89.7(1)
89.1(1)
Ni(1)-N(1)
2.022(3)
2.5425(9)
2.116(3)
90.4(1)
89.09(8)
92.95(9)
Ni(1)-N(11)
2.110(3)
2.035(3)
2.507(1)
93.71(9)
90.8(1)
Ni(1)-S(2)
Ni(2)-N(2)
Ni(1)-S(2)
Ni(2)-N(2)
Ni(2)-N(21)i
N(1)-Ni(1)-N(11)
N(11)-Ni(1)-S(2)
N(2)-Ni(2)-S(1)
Ni(2)-S(1)
Ni(2)-N(21)i
N(1)-Ni(1)-N(11)
N(11)-Ni(1)-S(2)
N(2)-Ni(2)-S(1)
Ni(2)-S(1)
N(1)-Ni(1)-S(2)
N(2)-Ni(2)-N(21)i
N(21)i-Ni(2)-S(2)
N(1)-Ni(1)-S(2)
N(2)-Ni(2)-N(21)i
N(21)i -Ni(2)-S(2)
89.40(8)
a Symmetry code used to generate equivalent atoms: (i) - x, 1 - y,
1 - z.
a Symmetry code used to generate equivalent atoms: (i) - x, 1 - y,
1 - z.
Table 4. Selected Bond Distances (Å) and Angles (deg) for 3a
tion sites of each Ni2+ ion via pyridyl nitrogen atoms. Neither
coordination of the carboxyl oxygen of a pybenH ligand to
a metal center nor deprotonation of the carboxyl hydrogen
from a pybenH ligand was found. Both phenomena are
common in other systems where a PCA ligand, in most cases
isoH, was employed as a building block.2j,15 The bond lengths
and angles of the coordination spheres of the Ni2+ ions are
compiled in Table 4. (The corresponding bond lengths and
angles of 1 and 2 are compiled in Tables 2 and 3,
respectively.)
Each pybenH ligand forms a pybenH dimer with a pybenH
ligand of an adjacent 1D chain through self-complementary
pairwise O-H/O hydrogen bonding (R22(8) graph set) with
O/O distances in the range 2.593(7)-2.661(7) Å, giving rise
to the 2D host layer spreading over the bc plane. (The
corresponding O/O distances found in 1 and 2 are in the
ranges 2.605(8)-2.654(7) and 2.626(4)-2.623(4) Å, respec-
Ni(1)-N(1)
2.077(7)
2.515(5)
2.124(6)
90.6(3)
87.3(2)
93.4(2)
Ni(1)-N(11)
2.118(6)
2.013(7)
2.587(7)
91.1(2)
89.5(2)
89.6(2)
Ni(1)-S(2)
Ni(2)-N(2)
Ni(2)-N(31)i
N(1)-Ni(1)-N(11)
N(11)-Ni(1)-S(2)
N(2)-Ni(2)-S(1)
Ni(2)-S(1)
N(1)-Ni(1)-S(2)
N(2)-Ni(2)-N(31)i
N(31)i -Ni(2)-S(2)
a Symmetry code used to generate equivalent atom: (i) - x, 1 - y,
1 - z.
tively.) The pybenH dimers connect Ni2+ ions with a Ni/Ni
separation of 24.95(2) Å. (The corresponding Ni/Ni separa-
tions of 1 and 2 are 16.414(2) and 20.884(2) Å, respectively.)
The Ni2+-pybenH/pybenH-Ni2+ linkages propagate along
the [012] axis. Figure 4a shows that the 2D host layers have
RCs defined by two pybenH dimers and two SCN bridges.
Architecture of the 2D host layer of 4. The coordination
framework with van der Waals surface of the 2D host layer
formed in 4 and the ORTEP drawing of the coordination
environments around metal centers with a major atom
numbering scheme are shown in panels a14 and b of Figure
(15) MacGillivary, L. R.; Groeneman, R. H.; Atwood, J. L. J. Am. Chem.
Soc. 1998, 120, 2676-2677.
9238 Inorganic Chemistry, Vol. 45, No. 23, 2006

4-Pyridyl-Substituted Carboxylic Acid Dimers
Figure 5. Framework with van der Waals surface of the 2D host layer of 4 spreading over the bc plane. Benz[a]anthracene guests have been omitted for
clarity. Color scheme: gray (carbon), white (hydrogen), blue (nitrogen), red (oxygen), yellow (sulfur), green (nickel). (b) Coordination environment around
Ni(1), Ni(2), Ni(2)iii in 4 (50% probability level ellipsoids). Symmetry codes used to generate equivalent atoms: (i) - x, 1 - y, 1 - z; (ii) x, y, -1 + z; (iii)
- x, 1 - y, - z.
Table 5. Selected Bond Distances (Å) and Angles (deg) for 4a
atom of pppeH dimer B and a least-squares plane of an
aromatic ring of adjacent pppeH dimer A) is 2.91 Å. Inter-
chain Ni2+/Ni2+ separations subjected by pppeH dimer A
and pppeH dimer B are 29.521(1) and 29.728(1) Å, respec-
tively. The latter is slightly longer than the former by 0.2 Å.
The Ni2+-pppeH/pppeH-Ni2+ linkages propagate along the
[023] axis. Figure 5a shows that 2D host layers have RCs
defined by two pppeH A dimers and two SCN bridges.
Ni(1)-N(1)
2.057(2)
2.5218(7) Ni(2)-N(2)
Ni(1)-N(11)
2.085(2)
2.020(2)
2.118(2)
2.5146(8)
Ni(1)-S(2)
Ni(2)-N(3)
2.034(2)
2.123(2)
2.5601(8)
91.39(8)
91.39(8)
93.74(9)
81.99(6)
88.40(9)
97.17(6)
Ni(2)-N(51)ii
Ni(2)-S(1)
Ni(2)-N(31)i
Ni(2)-S(3)iii
N(1)-Ni(1)-N(11)
N(11)-Ni(1)-S(2)
N(2)-Ni(2)-N(51)ii
N(2)-Ni(2)-S(3)iii
N(3)-Ni(2)-N(51)ii
N(3)-Ni(2)-S(3)iii
N(1)-Ni(1)-S(2)
92.32(6)
N(2)-Ni(2)-N(31)i 91.17(9)
N(2)-Ni(2)-S(1)
90.62(6)
N(3)-Ni(2)-N(31)i 86.63(8)
N(3)-Ni(2)-S(1)
90.26(6)
Layered Structures. The crystal structures viewed down
along the [012] axis (1-3) and the [023] axis (4) are shown
in Figure 6a-d, respectively. The 2D host layers of 1-4
are stacked parallel to the bc plane to give the layered
structures. 1-4 are classified into two structural types
according to relative locations between RCs of adjacent
layers. The one is such that RCs of one layer are isolated
from those of adjacent layers and the other is such that RCs
of one layer adjoin those of adjacent layers to give 1D
channels propagating along the stacking direction (a axis).
1, 3, and 4 belong to the former type, and 2 belongs to the
latter. Aromatic guests of either benz[a]anthracene (1, 2, 4)
or pyrene (3) are included in RCs (1, 3, 4) or the 1D channels
(2) with their aromatic planes parallel to those of the PCA
dimers.
N(31)i-Ni(2)-S(1) 91.13(6)
N(51)ii-Ni(2)-S(1) 92.79(6)
N(31)i-Ni(2)-S(3)iii 90.56(6)
N(51)ii-Ni(2)-S(3)iii 86.19(6)
a Symmetry codes used to generate equivalent atoms: (i) - x, 1 - y,
1 - z; (ii) x, y, -1 + z; (iii) - x, 1 - y, - z.
5, respectively. The asymmetric unit contains two Ni2+ ions
(Ni(1) and Ni(2)) with octahedral coordination geometries,
three SCN- ions, three pppeH ligands, and a benz[a]-
anthracene guest. Ni(1) lies on a crystallographic inversion
center and Ni(2) does not. Figure 5b shows that the
coordination environments around the metal centers are
identical to those of 1-3. One-dimensional chains consisting
of Ni2+ ions and µ1,3-intermetallic bridges of SCN- ions
propagate along the b axis with Ni(1)/Ni(2) and Ni(2)/
Ni(2)iii separations of 5.6029(3) and 5.5959(7) Å, respec-
tively. Two pppeH ligands are bonded to the two axial
coordination sites of each Ni2+ ion via pyridyl nitrogen
atoms. The bond lengths and angles of the coordination
spheres of the Ni2+ ions are compiled in Table 5. Each pppeH
ligand forms a pppeH dimer with a pppeH ligand of an
adjacent 1D chain through self-complementary pairwise
O-H/O hydrogen bonding with O/O distances in the range
2.585(6)-2.64(2) Å, giving rise to the 2D host layer
spreading over the bc plane.
In contrast to 1-3, two types of pppeH dimer were
identified in the 2D host layers. Figure 5a shows that the
one (pppeH dimer A) defines a boundary of a rectangular
cavity and the other (pppeH dimer B) is sandwiched by two
pppeH dimer A. The aromatic planes of pppeH dimer B are
almost perpendicular (66.1-76.8°) to those of an adjacent
pppeH dimer A. C-H/π interaction was identified between
them. Its average distance (a distance between a hydrogen
The crystal structures of 1-4 viewed down along the b
axis are shown in Figure 7a-d, respectively. The 2D host
layers are inclined by ca. 59 (1), 35 (2), 53 (3), and 44° (4)
with respect to the ab planes of the respective crystal
structures. Among them, 35° of 2 is substantially small
compared to the others. As a result, the guest-accessible
volume (GAV) of 2 is significantly reduced, regardless of
the expansion of its potential GAV (vide infra).
Figure 7a-d shows the packing situations of aromatic
guests in RCs (1, 3, 4) and in the 1D channel (2). In 1, each
RC contains one benz[a]anthracene guest. In 3 or 4, each
RC contains two pyrene or two benz[a]anthracene guests,
respectively. In contrast, in 2, each benz[a]anthracene guest
penetrates two adjacent RCs such that the central moiety of
a benz[a]anthracene guest is positioned between layers and
its terminal moieties occupy the upper and the lower spaces
of adjacent RCs. In 1 and 2, the benz[a]anthracene guests
Inorganic Chemistry, Vol. 45, No. 23, 2006 9239

Sekiya et al.
Figure 6. (a) Crystal structure of 1 viewed along the [012] axis. (b) Crystal structure of 2 viewed along the [012] axis. (c) Crystal structure of 3 viewed
along the [012] axis. (d) Crystal structure of 4 viewed along the [023] axis. Hydrogen atoms are omitted for clarity.
Figure 7. Crystal structures of 1-4 viewed along the b axis. (a) Crystal structure of 1. The benz[a]anthracene guests are disordered over two positions.
(b) Crystal structure of 2. The benz[a]anthracene guests are disordered over two positions. (c) Crystal structure of 3. Each rectangular cavity contains two
pyrene guests. (d) Crystal structure of 4. Each rectangular cavity contains two benz[a]anthracene guests. Dotted lines show hydrogen bonds. Hydrogen
atoms are omitted for clarity.
are disordered over two positions because they lie on
crystallographic inversion centers.
12.269(5) (isoH dimer), 16.890(4) (acrylH dimer), 20.89(4)
(pybenH dimer), 25.387(3) (pppeH dimer A), and 25.527-
(4) Å (pppeH dimer B). The PCA dimers are elongated by
almost twice the length of a hydrocarbon unit introduced
between the pyridyl ring and the carboxyl functional group
of each of three PCAs. The introduction of an ethenyl unit
(from isoH to acrylH and from pybenH to pppeH) elongates
the resulting bridging ligands by ca. 4.6 Å and that of a
phenyl unit (from isoH to pybenH and from acrylH to pppeH)
by ca. 8.5 Å. Of the three PCA dimers, pppeH dimer A and
B (entry 4 and 5) amount to being twice as long as the isoH
Structures of 4-Pyridyl-Substituted Carboxylic Acid
Dimers
From the crystal structures of 1-4, it is clear that the
proposed structural extension is successful. In all cases, PCAs
form the corresponding PCA dimers, acting as long bridging
ligands. The lengths (interatomic distances between the
pyridyl nitrogen atoms of the PCA dimers) of the PCA
dimers and the Ni/Ni separations subjected by them are listed
in Table 6 (entry 1-5). The lengths of the PCA dimers are
9240 Inorganic Chemistry, Vol. 45, No. 23, 2006

4-Pyridyl-Substituted Carboxylic Acid Dimers
Table 6. Lengths (Å) and Metal-Metal Separations (Å) of the PCA Dimers and 4,4′-Bipyridine-Type Ligandsa
a Lengths (Å) and metal-metal separations (Å) of 4,4′-bipyridine-type ligands were calculated by the PLATON program package. See ref 27. b An interatomic
distance (Å) between the pyridyl nitrogen atoms of a bridging ligand. c An interatomic distance (Å) between the metal centers linked by a bridging ligand.
d trans,trans-1,4-Bis(2-(4-pyridyl)ethenyl)benzene acts as a monodentate ligand.
dimer (entry 1). The Ni/Ni separations are also elongated
with the elongation of the PCA dimers. The Ni/Ni separations
are 16.414(1) (isoH dimer), 20.884(2) (acrylH dimer), 24.95-
(2) (pybenH dimer), 29.521(1) (pppeH dimer A), and 29.728-
(1) Å (pppeH dimer B). The Ni/Ni separations of the latter
two amount to 30 Å.
Figures 8 and S5 (see the Supporting Information) show
the front and side views of the PCA dimers found in 1-4,
respectively. The isoH and the acrylH dimers adopt a slightly
twisted arrangement of their constituent hydrocarbon units:
the dihedral angles between the pyridyl rings and the central
R22(8) ring systems are 3.7(2) and 11.7(2)° for the isoH
Inorganic Chemistry, Vol. 45, No. 23, 2006 9241

Sekiya et al.
Figure 8. ORTEP drawing of the front views of the PCA dimers formed in 1-4 (50% probability level ellipsoid): (a) isoH dimer; (b) acrylH dimer; (c)
pybenH dimer; (d) pppeH dimer A and B. The central moiety of pppeH dimer A is disordered over two positions. The site occupancy factor of its major
component is 0.744(6) at 153 K.
dimer and 5.2(2) and 16.8(2)° for the acrylH dimer. The
pybenH and the pppeH dimers adopt twisted structures about
Cpyridyl-Cphenyl bonds, originating from steric repulsion
between hydrogen atoms on the ortho carbon atoms of
adjacent rings: the corresponding dihedral angles are 14.1-
(3) and 25.1(3)° for the pybenH dimer, 23.5(1) and 21.0(1)°
for pppeH dimer A, and 21.7(3)° for pppeH dimer B. The
central moieties of the pybenH and pppeH dimers (the
phenyl-CO2H/CO2H-phenyl moiety of the pybenH dimer
and the phenylethenyl-CO2H/CO2H-phenylethenyl moi-
eties of the pppeH dimers) adopt slightly twisted structures:
the dihedral angles between the phenyl rings and the central
R22(8) ring systems are almost identical to those observed in
the previous two PCA dimers.
16). The latter one is a very long 4,4′-bipyridine-type ligand
almost 20 Å in length and is much longer than most of the
4,4′-bipyridine-type ligands reported to date. To the best of
our knowledge, the pppeH dimer is one of the longest
bridging ligands employed in the metal-organic coordination
framework. The pybenH dimer (entry 3), which is shorter
than the pppeH dimers by ca. 4.6 Å (entry 4 and 5), is
comparable to this very long 4,4′-bipyridine-type ligand.
Although the acrylH dimer is the shortest PCA dimer of the
three (entry 2), it bears favorable comparison with the 4,4′-
bis(4-pyridyl)biphenyl18 (15.756(3) Å; entry 9), trans,trans-
1,4-bis(2-(4-pyridyl)ethenyl)benzene19 (16.097(7) Å; entry
12), and 1,4-bis(4-pyridylethynyl)benzene22 (16.516(4) Å;
entry 15) ligands. In addition to these three PCA dimers,
the isoH dimer (entry 1), the fundamental PCA dimer, is
comparable to the 1,4-bis(4-pyridyl)benzene18 (11.398(3) Å;
entry 8) and bis(4-pyridyl)butadiyne21 (12.212(5) Å; entry
The lengths of 4,4′-bipyridine-type ligands employed in
metal-organic coordination frameworks and metal/metal
separations subjected by them are listed in Table 6 (entry
6-16). At a first glance, the pybenH and pppeH dimers are
substantially long bridging ligands compared with the 4,4′-
bipyridine-type ligands. The pppeH dimers (entry 4 and 5),
which are the longest PCA dimers reported herein, are three-
and-a-half times as long as a 4,4′-bipyridine ligand17 (7.129-
(16) Carlucci, L.; Ciani, G.; Gramaccioli, A.; Proserpio, D. M.; Rizzato,
S. CrystEngComm 2000, 29, 1-10.
(17) Biradha, K.; Domasevitch, K. V.; Moulton, B.; Seward, C.; Zaworotko,
M. J. Chem. Commun. 1999, 1327-1328.
(18) Biradha, K.; Fujita, M. Chem. Commun. 2001, 15-16.
(19) Banfi, Stefano.; Carlucci, L.; Caruso, E.; Ciani, G.; Proserpio, D. M.
J. Chem. Soc., Dalton Trans. 2002, 2714-2721.
(20) Dong, Y.-B.; Layland, R. C.; Smith, M. D.; Pscirer, N. G.; Bunz, U.
H. F.; Loye, H.-C. Inorg. Chem. 1999, 38, 3056-3060.
(21) Zaman, M. B.; Udachin, K. A.; Ripmeester, J. A. CrystEngComm 2002,
4, 613-617.
(6) Å) (entry 7), which is in the most widespread use in
15,17,23
metal-organic coordination frameworks
and other
supramolecular systems 24, and are much longer than the 4,4′-
bis(4-pyridylmethoxy)biphenyl ligand22 (19.776(18) Å; entry
(22) Zaman, M. B.; Udachin, K.; Ripmeester, J. A.; Smith, M. D.; Loye,
H.-C. Inorg. Chem. 2005, 44, 5047-4059.
9242 Inorganic Chemistry, Vol. 45, No. 23, 2006

4-Pyridyl-Substituted Carboxylic Acid Dimers
Figure 9. ORTEP drawing of the front views of the rectangular cavities formed in 1-4 (50% probability level ellipsoid): (a) rectangular cavity of 1 (16.41
× 5.53-7.43 Å2); (b) rectangular cavity of 2 (20.88 × 5.49-7.25 Å2); (c) rectangular cavity of 3 (24.95 × 5.46-7.38 Å2); (d) rectangular cavity of 4 (29.52
× 5.60-7.20 Å2). Hydrogen atoms are omitted for clarity.
14) ligands. These comparisons exemplify that the design
strategy, the preparation of bridging ligands through self-
assembly of two PCAs with the assistance of CAD formation,
is both efficient and particularly suitable for the preparation
of very long bridging ligands.
crystalline state has been reported by several groups26.
Detailed analysis will be reported elsewhere. On the other
hand, no such disorder was identified in pppeH dimer B.
Most likely, the C-H/π interaction between pppeH dimer
B and pppeH dimer A fixes its orientation.
Easy preparation of the PCA dimers is also a notable
feature of this design strategy. The PCA dimers consist of
two corresponding PCAs which are either commercially
available (isoH and acrylH) or synthesized in one (pybenH)
or two steps (pppeH). Despite their easy preparation, the
resulting PCA dimers act as very long bridging ligands, some
of which are well more than 20 Å in length. These two
features, that is, their considerable lengths and easy prepara-
tion, are advantageous over their single molecular counter-
parts in the construction of metal-organic coordination
frameworks.
Interestingly, the ethenyl units and the central R22(8) ring
system of the pppeH dimer A is disordered over two
positions. The site occupancy factor of its major component
is 0.744(6) at 153 K. This disorder might indicate that the
central moiety of pppeH dimer A, that is, the ethenyl-CO2H/
CO2H-ethenyl moiety, undergoes a rotation like the pedal
motion of a bicycle.25 This type of dynamic disorder in the
Structures of The Rectangular Cavities. The elongation
of the bridging ligands successfully expanded the cavity
dimensions of 2-4 without the formation of an interpenetrat-
ing network. Figure 9 shows that the architectures of RCs
formed in 1-4 are very similar to each other. They have
spindle-like shapes. The widths of RCs vary from ca. 5.53
(1), 5.49 (2), 5.46 (3), and 5.60 Å (4) (these values are in
agreement with the Ni/Ni separations subjected by the SCN
bridges) to ca. 7.43 (1), 7.25 (2), 7.38 (3), and 7.20 Å (4)
(these values are in agreement with O/O distances between
the PCA dimers). The heights of RCs are ca. 16.41 (1), 20.99
(2), 24.95 (3), and 29.52 Å (4), which are in agreement with
the Ni/Ni separations subjected by the respective PCA
dimers. Therefore, the cavity dimensions of 1-4 are 16.41
× 5.53-7.43, 20.88 × 5.49-7.25, 24.95 × 5.46-7.38, and
29.52× 5.60-7.20 Å2, respectively.
(24) For recent articles (pure organic systems) see: (a) Varughese, S.;
Pedireddi, V. R. Chem.sEur. J. 2006, 12, 1597-1609. (b) Santis,
A.; Formi, A.; Liantonio, R.; Metrangolo, P.; Pilati, T.; Resnati, G.
Chem.sEur. J. 2003, 9, 3974-3983. (c) MacGillivray, L. R.; Reid,
J. L.; Ripmeester, J. A. Chem. Commun. 2001, 1034-1034.
(25) (a) Harada, J.; Ogawa, K. J. Am. Chem. Soc. 2001, 123, 10884-10888.
(b) Harada, J.; Ogawa, K.; Tomoda, S. Acta Crystallogr., Sect. B. 1997,
53, 662-672.
(23) For recent articles (metal-organic coordination frameworks) see: (a)
Ohmori, O.; Fujita, M. Chem. Commun. 2004, 1586-1587. (b) Kitaura,
R.; Seki, K.; Akiyama, G.; Kitagawa, S. Angew. Chem., Int. Ed. 2003,
42, 428-431. (c) Noro, S.; Kitagawa, S.; Kondo, M.; Seki, K. Angew.
Chem., Int. Ed. 2000, 39, 2081-2084. (d) Subramanian, S.; Zaworotko,
M. J. Angew. Chem., Int. Ed. 1995, 34, 2127-2129. (e) Fujita, M.;
Kwon, Y. J.; Washizu, S.; Ogura, K. J. Am. Chem. Soc. 1994, 116,
1151-1152.
(26) (a) Ohba, S.; Hosomi, H.; Ito, T. J. Am. Chem. Soc. 2001, 123, 6349-
6352. (b) Natarajan, A.; Mague, J.T.; Venkatesan, K.; Ramamurthy,
V. Org. Lett. 2005, 7, 1895-1898.
Inorganic Chemistry, Vol. 45, No. 23, 2006 9243

Sekiya et al.
Figure 10. Molecular structures of the four PCA dimers, bipy, and its analogues (n ) 0-5, m ) 0, 1).
GAVs of 1-4 calculated by the PLATON program
package27 are 390.6, 380.7, 663.6, and 743.0 Å3, respectively.
In comparison to GAV of 1, GAVs of 3 and 4 are expanded
by 273 and 352 Å3, respectively. In particular, 743.0 Å3 of
4 amounts to being about twice as large as 390.6 Å3 of 1.
The fact that GAV of 2 is the smallest one of the four has
a close relation with its crystal structure. As mentioned above,
the 2D host layers are inclined by ca. 34.5° with respect to
the ab plane, so that the upper and the lower spaces of RCs
of one layer are partly preoccupied by the 1D chains of
adjacent layers. As a result, GAV of 2 becomes small
regardless of the expansion of its potential GAV.
The expansion successfully increases the number of guests
that can be included in RCs. The RC of 1 contains only one
molecule of benz[a]anthracene, whereas the RC of 3 or 4
contains two molecules of pyrene or benz[a]anthracene,
respectively. These results indicate that aromatic guests
whose sizes are, in principle, up to twice as large as pyrene
or benz[a]anthracene should be included in the RC of 3 or
4, respectively. A detailed survey of inclusion properties is
currently in progress.
4-Pyridyl-Substituted Carboxylic Acid Dimers: Supra-
molecular Analogues of 4,4′-Bipyridine-Type Ligands.
From the structures of the PCA dimers found in 1-4, it
is evident that they can be regarded as being supramolecular
analogues of 4,4′-bipyridine-type ligands. Figure 10 shows
the molecular structures of the four PCA dimers, bipy, and
its analogues (n ) 1-5, m ) 0, 1). Considering the lengths
of the PCA dimers, the isoH, acrylH, pybenH, and pppeH
dimers are supramolecular analogues of 1,4-bis(4-pyridyl)-
benzene (n ) 1), trans,trans-1,4-bis(2-(4-pyridyl)ethenyl)-
benzene (and 4,4′-bis(4-pyridyl)biphenyl (n ) 2)), 4,4′′-bis(4-
pyridyl)-p-terphenyl (n ) 3), and trans,trans-1,4-bis(2-(4-
(4-pyridyl)phenyl)ethenyl)benzene (and 4,4′′′-bis(4-pyridyl)-
quaterphenyl (n ) 4)), respectively. The three former ligands
have been employed in metal-organic coordination frame-
works,20,21,25,26 whereas the three latter have not. This is
probably closely related to their solubility toward solvents.
On the other hand, the pybenH dimer and the pppeH dimers
could be employed as bridging ligands of the metal-organic
coordination frameworks despite their lengths of well more
than 20 Å. This is because they are not single molecules but
consist of two corresponding PCAs.
Conclusion
In conclusion, the proposed structural extension was
successful. These expansion increases the number of aromatic
guests that can be included in the rectangular cavities (RCs)
from one molecule of benz[a]anthracene (1) to two molecules
of pyrene (3) or benz[a]anthracene (4). The 4-pyridyl-
substituted carboxylic acid (PCA) dimers can be regarded
as being supramolecular analogues of 4,4′-bipyridine-type
ligands because of their structural equivalence. Comparison
of the lengths between the PCA dimers and 4,4′-bipyridine-
type ligands demonstrated that a design strategy, that is, the
preparation of a bridging ligands though self-assembly of
two PCAs with assistance of carboxylic acid dimer formation,
is both efficient and particularly suitable for the preparation
of very long bridging ligands.
Acknowledgment. We are grateful to professor Mat-
sushita for permission to use a Rigaku RAXIS-RAPID
imaging plate area detector. This work was supported by a
Grant-in-Aid for Scientific Research (17750122) from the
Ministry of Education, Culture, Sports, Science, and Tech-
nology of Japan.
Supporting Information Available: X-ray crystallographic data
in CIF format for 1-4; IR spectra of 1-4; side views of the PCA
dimers (PDF). This material is available free of charge via Internet
at http://pubs.acs.org.
(27) Spek, A. L. PLATON; University of Glasgow: Glasgow, Scotland,
1998.
IC0605367
9244 Inorganic Chemistry, Vol. 45, No. 23, 2006