Self-assembly of metal-organic hybrid nanoscopic rectangles

Article abstract of DOI:10.1039/b617513a

The combination of an amide containing a linear ligand (L1) and an organometallic molecular "clip" (clip-1) leads to the self-assembly of a Pt₄ nanoscopic framework representing the first example of a Pt-based molecular rectangle (rectangle-1)

Full text of DOI:10.1039/b617513a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Self-assembly of metal–organic hybrid nanoscopic rectangles†‡§
Sushobhan Ghosh and Partha Sarathi Mukherjee*
Received 30th November 2006, Accepted 27th March 2007
First published as an Advance Article on the web 16th April 2007
DOI: 10.1039/b617513a
The combination of an amide containing a linear ligand (L1) and an organometallic molecular “clip”
(
clip-1) leads to the self-assembly of a Pt nanoscopic framework representing the first example of a
4
Pt-based molecular rectangle (rectangle-1) incorporating amide functionality. A complementary
approach was also followed to prepare a Pd(II)-based molecular rectangle (rectangle-2) by reaction of a
donor organic rigid clip (clip-2) and trans-(PEt
3
)
2
Pd(OTf) as the linear metal acceptor (L2). The
2
Pd(II)-rectangle was characterised by multinuclear NMR and ESI-mass spectroscopy.
Introduction
hydrogen bonding of the amide group make this a useful functional
group to incorporate into such assemblies. One problem inherent
in the design of lower symmetry discrete self-assembled species
containing non-symmetric ligands is the possibility of formation
of several isomeric products. So, it was of interest to us to
see whether the self-assembly of a non-symmetric donor with
a suitable Pt(II) linker would afford self-selection for a single
isomer. We report here the incorporation of amide functionality
into a Pt(II) nanoscopic molecular rectangle (rectangle-1) via self-
assembly of an organometallic “clip” (clip-1) and a non-symmetric
amide ligand (L1) (Scheme 1). To the best of our knowledge
rectangle-1 represents the first example of a Pt(II) based molecular
rectangle with amide functionality.
Self-assembly of nanoscopic assemblies of finite shape by a
1–2
directional bonding approach has received special attention
by chemists since the discovery of the metallasupramolecular
square in 1990. The interesting aspect of this process is the
rational design of structures of various shapes, geometries and
symmetries. Square planar Pd(II) and Pt(II) have long been among
the favourite metal ions used for this purpose because of their rigid
coordination environment. In a majority of the cases, symmetrical
dipyridyl ligands have been used with cis-protected metal centers
to generate molecular squares, triangles, and rhomboids with
very limited examples of molecular rectangles since designing
3
rectangles needs a special kind of “clip” type ligand. Several
Self-assembly by a directional bonding approach may also allow
us to obtain the same geometry by a complementary approach.
However, this kind of effort in coordination driven self-assembly
is rare. An example of such a kind of self-assembled geometry
functional groups/moieties like porphyrin, carborane and cal-
4
ixarene have been incorporated into self-assemblies. We have
recently introduced amide and ester functionalities into 3D Pd(II)
5
cages. Amide functionality has proved to useful in self-assembly
5d
can be found in the synthesis of truncated tetrahedra. Herein,
through hydrogen bonding, and they have obvious relevance to
natural systems. For example, amide linked catenanes, rotaxanes,
and knots have been prepared. Hence, the known patterns of
we describe the complementary approach (Scheme 2) of the above
self-assembly (rectangle-1), i.e. the use of a purely organic “clip”
(
clip-2) in conjunction with a metal based linear acceptor (L2) to
obtain a new molecular rectangle (rectangle-2) of Pd(II).
Department of Inorganic & Physical Chemistry, Indian Institute of Science,
Bangalore, 560012, India. E-mail: psm@ipc.iisc.ernet.in; Fax: +91 80
2
†
‡
3601552; Tel: +91 80 22932827
Experimental
The HTML version of this article has been enhanced with colour images.
This work is dedicated to Prof. Nirmalendu Ray Chaudhuri on the
occasion of his 65th birthday.
Synthesis
§
Electronic supplementary information (ESI) available: Crystallographic
parameters and detail of the bond parameters and NMR of rectangle-2.
L1 and 1,8-bis(4-pyridylethynyl)anthracene (clip-2) were prepared
as previously reported. The elemental analyses were performed
6
See DOI: 10.1039/b617513a
Scheme 1 Self-assembly of rectangle-1 and its alternative isomeric product rectangle-1a.
542 | Dalton Trans., 2007, 2542–2546
2
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Scheme 2 Self-assembly of rectangle-2 and its polymeric analogue.
using a Thermo Finnigan Flash EA 1112 CHNSO analyzer. NMR
spectra were recorded on a Varian Unity 300 spectrometer. The
mass spectrum was obtained using an ESI source.
rectangle-1. To a 3-mL acetone solution containing 11.6 mg
(0.010 mmol) of 1,8-bis[trans-Pt(PEt (NO )]anthracene (clip-
1), a methanol solution of 2.00 mg (2 mL) of L1 (0.01 mmol)
3
)
2
3
was added dropwise with continuous stirring (5 min). The light-
1
1
,8-bis[trans-Pt(PEt
Schlenk flask was charged under nitrogen with 1,8-
dichloroanthracene (247 mg, 1.0 mmol) and Pt(PEt (1.67 g,
.5 mmol). Freshly distilled toluene (40 mL) was added to
3
)
2
(NO
3
)]anthracene (clip-1). A 100 mL
yellow solution was stirred for another 30 min. H NMR (CDCl
3
,
3
(
H
00 MHz): d 10.89 (s broad, 2H, CO–NH), 9.65 (s, 2H, H
d, 4H, Ha-Py), 9.28 (d, 4H, Ha-Py), 8.81 (d, 4H, Hb-Py), 8.73 (d, 4H,
b-Py), 8.23 (s, 2H, H10), 7.77 (d, 4H, H4,5), 7.01 (m, 4H, H3,6),
9
), 9.34
)
3 4
2
3
1
the flask under nitrogen by syringe, and the resulting orange–
red solution was stirred for 24 h at 110 C in an oil bath.
The solvent was removed in vacuo and the residue was stirred
1.57 (m, 48H, PCH
2
CH
, 121.4 MHz): 11.56 (s), 11.40 (s). Anal. Calcd for
Pt : C, 43.16; H, 5.65; N, 5.13% Found: C, 43.43;
3
), 1.01 (m, 72H, PCH
2
CH
3
). P{1H}
◦
NMR (CDCl
C
3
98
H
154
N
10
O
14
P
8
4
with hot methanol (10 mL). Light yellow microcrystalline 1,8-
H, 5.88; N, 5.39%. Yield: (11.20 mg, 82.7%).
bis[trans-Pt(PEt
a refrigerator for 2 h. To a stirred solution of 1,8-bis[trans-
Pt(PEt Cl]anthracene (332 mg, 0.30 mmol) in acetone (20 mL),
was added AgNO (103 mg, 0.60 mmol). The reaction was stirred
overnight in the dark, the mixture was filtered through a bed of
Celite to remove AgCl and the filtrate evaporated to dryness under
reduced pressure. The crude product was taken up in 5 mL of hot
ethanol and filtered. The hot filtrate was kept in the refrigerator
3
)
2
Cl]anthracene was obtained upon cooling in
rectangle-2. To a 2-mL dichloromethane solution contain-
3
)
2
ing 12.8 mg (0.02 mmol) of trans-[(PEt
3
)
2
Pd(CF
3
SO ) ] (L2), a
3
2
dichloromethane solution of 7.6 mg (2 mL) of clip-2 (0.02 mmol)
was added dropwise with continuous stirring (1 h). The orange–
yellow solution was stirred for another 30 min. The product was
3
isolated as microcrystals upon diffusing ether into the solution
1
of the product. H NMR (CD
3
OD, 300 MHz): d 9.12(2H, s,
overnight to obtain yellow microcrystalline pure 1,8-bis[trans-
anthracene H
7.99 (4H, d, anthracene H3,6); 7.6–7.9 (14H, m, anthracene H4,5,10
and Py-b); 2.24 (24H, q, CH -ethyl); 1.5 (36H, CH -ethyl) ppm.
OD, 121.4 MHz): 34.5 ppm. Anal. Calcd for
: C 49.34%; H 4.5%; N 2.74%. Found: C,
9
); 8.85 (8H, d, Py-a); 8.35 (4H, d, anthracene H2,7);
◦
Pt(PEt
3
)
2
(NO
3
)]anthracene (clip-1). (Yield = 85%). mp 259
C
3
1
1
(
dec). P{1H} NMR (acetone-d
6
, 121.4 MHz): d 14.5 (s). H
2
3
3
1
NMR (acetone-d
6
, 300 MHz): d 9.51 (s, 1H), 8.22 (s, 1H), 7.62 (m,
P{1H} NMR (CD
3
4
C
6
H), 7.15 (m, 2H), 1.65 (m, 24H), 1.03 (m, 36H). Anal. Calcd for
Pt : C, 39.24; H, 5.89; N, 2.41. Found: C, 39.50;H,
.12; N, 2.68. ESI-MS (acetone): 520 Da (M ).
Pd
2
C
84
H
92
P
4
S
4
O
12
F
12
N
4
H
68
N
2
O
6
P
4
2
49.71; H, 4.88; N, 2.40%. Yield: (16.30 mg, 80.5%).
38
2
+
X-Ray data collection and refinements
trans-(PEt
of Pd(COD)Cl
dichloromethane (20 ml), 1M solution of PEt
THF was added by syringe. The color of the solution changed
from orange to yellow and this solution was stirred for another
3
)
2
Pd(CF
3
SO
3
)
2
(L2). To
mmol] in dry degassed
[2 ml, 2 mmol] in
a
stirred solution
2
[285.42 mg,
1
A light yellow plate shaped crystal 0.38 × 0.28 × 0.13 mm in
size was mounted on a glass fiber with traces of viscous oil and
then transferred to a Bruker SMART APEX CCD diffractometer
3
˚
equipped with Mo K radiation (k = 0.71073 A). The structure
2
h and then the solvent was completely removed under vacuum.
It was further kept under vacuum for another 3 h to remove all
the volatiles, and trans-(PEt PdCl was isolated as a greenish
yellow solid. Yield = 88%. Analysis calculated for PdC12
C, 34.95; H, 7.28. Found: C, 35.25; H, 7.39. To the stirred
solution of Pd(PEt Cl [163 mg, 0.39 mmol] in dry degassed
was solved by a combination of direct methods and heavy atoms
7
using SIR 97. All of the non-hydrogen atoms were refined
3
)
2
2
with anisotropic displacement coefficients. Hydrogen atoms were
assigned isotropic displacement coefficients U(H) = 1.2U(C) or
1.5U(C-methyl), and their coordinates were allowed to ride on
H
30Cl
2
2
P ,
8
3
)
2
2
their respective carbons using SHELXL97. The weighting scheme
2
2
2
dichloromethane (20 ml) silver triflate [200.4 mg. 0.80 mmol] was
added and the mixture was stirred for 12 h under nitrogen in
the absence of light. The white solid was filtered through Celite
and the filtrate was concentrated to 2 mL. Diethyl ether was
added to the concentrated filtrate to isolate the product as a white
employed was w = 1/[ (F
o
) + (0.1205P) + 442.5332P] where
2
2
P = (F
o
+ 2F
c
)/3. The refinement converged to R1 = 0.0823,
wR2 = 0.2179. The maximum D/r in the final cycle of the least-
squares was 0.001, and the residual peaks on the final difference-
−
3
˚
Fourier map ranged from −4.435 to 3.203 e A . The nitrate anions
were found to be disordered. The disorder about the N2–C20–
O1 moiety as well as that of the pyridine N3 was successfully
interpreted as two orientations of the group. The two pyridine rings
(50% occupancy each) have been isotropically refined restraining
precipitate. Yield: 90%. Analysis calcd for PdC14
H
30
S
2
P
2
O
6
6
F : C,
3
1
2
1
1
6.21; H, 4.68. Found: C, 26.58; H, 4.86. P{1H} NMR (CDCl
3
,
1
21.4 MHz): d 44 ppm (s). H NMR (CDCl
3
, 300 MHz): 1.49 (m,
2H, PCH
2
CH
3
), 1.04 (m, 18H, PCH
2
CH ) ppm.
3
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Dalton Trans., 2007, 2542–2546 | 2543

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the geometry. The ethyl groups of the phosphines also revealed
some disorder and terminal methyl carbons were isotropically
refined in order to avoid prolate ellipsoids. The data were treated
Table 1 Crystal data and structure refinement for rectangle-1
Compound
Empirical
formula
1
C
98 154 10 14
H N O
9
with the program SQUEEZE which is an effective cure for the
8 4
P Pt
disordered solvent syndrome in crystal structure refinement.
CCDC reference number 623169.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b617513a
FW
T/K
2724.43
150(1)
Monoclinic
C2/c
40.6123(13)
14.7952(4)
20.0041(7)
90.00
98.1700(10)
90.00
11897.8(6)
4
4.853
0.71073
0.2179
0.0823
Crystal system
Space group
a/A
b/A˚
˚
c/A˚
◦
a/
b/
Results and discussion
◦
◦
c /
Synthesis and NMR
3
V/A˚
Z
l(Mo-Ka)/mm
k/A˚
The organometallic diplatinum(II) “clip” (clip-1) was prepared
from the reaction of 1,8-anthracenedichloride and freshly pre-
−1
pared Pt(PEt
3
)
4
in an inert atmosphere using dry toluene as
R
R
w
the solvent followed by nitration with AgNO
3
. Ligand L1 was
D/Mg m−
3
1.521
6
prepared using a published procedure. The 1 : 1 stoichiometric
combination of an acetone solution of clip-1 with a methanol
solution of the linear linker L1 rapidly produced a light-yellow
2
2
2
2
2
1/2
R = R ꢀ F
o
| − | F
c
ꢀ/R | F
o
|; R
w
= [R{w(F
o
− F
c
) }/R{w(F
o
) }]
.
3
1
solution. P{1H} NMR analysis of the product indicated the
formation of a species (rectangle-1) by the appearance of two
1
95
closely-spaced sharp singlet with concomitant Pt satellites. The
upfield shift of the signals near 5 ppm relative to the clip indicated
3
1
ligand to Pt coordination. In this case, the P NMR data are
insufficient for distinguishing the product rectangle-1 from its
isomeric relative rectangle-1a. The rectangle-1 has an inversion
centre at the centre of the molecule. Consequently, it has only
one type of H
9
and H10 anthracene proton nuclei, while isomer
1
rectangle-1a has two types. The H NMR spectrum of the product
also established the formation of rectangle-1 as a single product.
Finally, ESI mass spectrometry established the formation of the
3
+
2
+ 2 adduct by the appearance of a peak corresponding to M
in the mass spectrum. In a complementary approach (Scheme 2),
a Pd(II) based molecular rectangle (rectangle-2) was also prepared
using a purely organic donor clip (clip-2) and a Pd(II) containing
linear acceptor trans-(PEt
bis(4-pyridylethynyl)anthracene] was synthesized by reaction of
,8-diethynylanthracene and 4-bromopyridine in the presence of
3
)
2
Pd(OTf) (L2). The ligand clip-2 [8-
2
Fig. 1 ORTEP (30% probability) of the centrosymmetric rectangle-1 (Et
groups not shown and only one of the two orientations of the NH–CO
ꢁ
1
moiety is depicted). The symmetry operation invoked by the prime ( )
ꢁ
Pd(II) catalyst. The self-assembly of rectangle-2 was monitored by
character in the N3 label is (1 − x, 1 − y, 1 − z).
multinuclear NMR. An upfield shift of 10 ppm of the phosphorus
3
1
peak and the appearance of a single peak in the P NMR spectrum
indicated the formation of a single product. Shifts for the proton
signals were also found as usual due to complex formation.
◦
square planar with a C(1)–Pt(1)–P(2) angle of 90.2(4) , N(1)–
◦
◦
Pt(1)–P(2) of 90.5(3) ; P(1)–Pt(1)–P(2) angle of 170.15(17) and
◦
C(1)–Pt(1)–N(1) angle of 179.3(5) . Especially interesting is the
However, a similar approach using trans-(PEt
3
)
2
Pt(OTf) yielded
2
packing pattern of this complex in the solid state. The rectangles
are packed in layers, which form long channels of rectangular
a mixture of intractable products.
˚
shape of approximately 16.5 A diameter as shown in Fig. 2.
The distance between the H10 of the anthracene moieties of the
rectangle measured the length of the channel. The ethyl groups
of the phosphine have not been taken into account with regard
to any occupation/blocking of the channels. The repeating unit
Structure analysis
X-Ray crystallography unambiguously established the nanoscopic
rectangular shape of the product rectangle-1. Single crystals grew
at ambient temperature by vapor diffusion of diethyl ether into
a concentrated acetone solution of the complex over 7 days.
The molecular structure of rectangle-1 is shown in Fig. 1.
Crystallographic data and selected bond parameters are given
in Tables 1 and 2, respectively. The exterior length of the
˚
distance between each stacked macrocycle is 11.56 A. After
several attempts, we were able to obtain single crystals though the
quality was not very good. However, the data set was consistent
with the formation of a 2 + 2 rectangle (not the alternative
polymeric product) and proper connectivity of the linkers was
also established by NMR and ESI. The dihedral angle between the
˚
rectangle is approximately 23 A, while the internal cavity measures
approximately 18 A. The geometry around each Pt is close to
◦
˚
extreme rings of the anthracene moiety = 8.2(7) and the dihedral
2
544 | Dalton Trans., 2007, 2542–2546
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◦
Table 2 Selected bond lengths ( A˚ ) and angles ( ) of rectangle-1
Pt(1)–C(1)
Pt(1)–P(1)
Pt(2)–P(4)
2.026(14)
2.320(4)
2.320(4)
Pt(1)–N(1)
Pt(2)–C(11)
Pt(2)–P(3)
2.114(10)
2.009(14)
2.333(4)
Pt(1)–P(2)
Pt(2)–N(3)
2.303(4)
2.132(11)
a
C(1)–Pt(1)–P(1)
C(1)–Pt(1)–P(2)
N(1)–Pt(1)–P(1)
C(11)–Pt(2)–P(4)
C(11)–Pt(2)–P(3)
88.5(4)
90.2(4)
90.7(3)
88.6(4)
88.3(4)
C(1)–Pt(1)–N(1)
N(1)–Pt(1)–P(2)
P(2)–Pt(1)–P(1)
P(4)–Pt(2)–P(3)
179.3(5)
90.5(3)
170.15(17)
169.20(14)
a
Symmetry transformations used to generate equivalent atoms: 1 −x + 1, −y + 1, −z + 1.
is a potential H-bond donor as well as acceptor. Barbituric acid
and urea were used as guest molecules to see whether rectangle-
1
could function as a potential host for these amide containing
organic molecules. However, no host–guest interaction was found.
This is obviously due to the fact that each rectangular channel in
the solid state of rectangle-1 is already occupied by two nitrate
ions through strong H-bonding.
UV-properties of rectangle-2. Upon formation, the rectangle-2
assumes an orange color. The absorption spectrum of this complex
is shown in Fig. 3. The absorbance in the electronic spectrum
exhibits a near-UV transition. The absorbance per building block
clip-2 increases upon formation of rectangle-2, specifically the
anthracene-based absorbance, centered around 265 nm increases
significantly. The anthracene-based absorbance around 265 nm is
also red shifted compared to the starting clip-2.
Fig. 2 Packing diagram of rectangle-1; trapped nitrate anions are shown
by CPK model. Coordinated PEt
clarity. Medium balls represent Pt.
3
ligands are omitted for the sake of
◦
angle between py-N(1)–py-N(3) rings = 38.8(8) , py-N(1)–py-
◦
N(3a) rings = 40.8(8) . The Pt(1) and Pt(2) are displaced slightly
˚
by 0.1, 0.07 A respectively from their mean coordination plane
towards the other metal. The coordination planes [N(1)–C(1)–
P(1)–P(2)] and [N(3)–C(11)–P(3)–P(4)] present slight tetrahedral
˚
distortions (+, −0.1 A)
The width, as defined by the distance between the metals, is
˚ ˚
5.40 A, while the distance between amide nitrogen atoms is 5.72 A.
Each rectangular ensemble hosted a pair of disordered nitrate
anions through strong hydrogen bonding by two amide N–H
protons.
Fig. 3 Absorption spectra of rectangle-2 (17 lM) and building block
clip-2 (34 lM).
ESI mass spectroscopy proved to be an indispensable tool
for establishing even large supramolecules. Finally, electrospray
In conclusion, we report the first nanoscopic Pt(II) based
molecular rectangle incorporating amide functionality using a
linear non-symmetric amide containing a bridging ligand. Despite
the possibility of forming multiple products L1 prefers to self-
assemble predominantly into one isomeric species. Following a
complementary approach, a Pd(II) based molecular rectangle was
prepared using a rigid organic clip (clip-2) and a Pd(II) containing
ꢁ
mass spectroscopy clearly confirmed the M
PEt Pd(OTf)
2
L
2
composition [M =
(
3
)
2
2
] for rectangle-2 with a molecular weight of 2043.0
Da despite the possibility of forming 1D chains (Scheme 2). The
ESI-mass spectrum of rectangle-2 showed a signal corresponding
3
+
to the consecutive loss of triflate counterions, [M–3CF
3
SO
3
]
4
+
and [M–4CF
3
SO
3
] . Several attempts failed to obtain diffractable
quality single crystals for rectangle-2 and thus energy-minimized
simulations were employed to understand the probable shape and
size of this complex. The MM2 energy minimized calculation
yielded a rectangular shape with an internal length and width
linear acceptor trans-(Et
3
P)
2
Pd(OTf) . Despite the possibility of
2
forming a 1D zig-zag chain, the macrocycle rectangle-2 was
formed exclusively. To the best of our knowledge rectangle-2 is
the first Pd(II) based rectangle prepared via a directional bonding
approach.
˚
˚
of 18.76 A and 4.5 A, respectively (see ESI§). Amide functionality
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Acknowledgements
M. Ravikanth, Inorg. Chim. Acta, 2005, 358, 2671; (h) S. Y. Chang, M. C.
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(
i) J. A. Whiteford, P. J. Stang and S. D. Huang, Inorg. Chem., 1998, 37,
595.
We thank the Council for the Scientific and Industrial Research
5
(
CSIR), New Delhi, India, for financial support. Authors specially
3
4
M. Bala, P. Thanasekaran, T. Rajendran, R. T. Liao, Y. H. Liu, G. H.
Lee, S. M. Peng, S. Rajagopal and K. L. Lu, Inorg. Chem., 2003, 42,
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Yoshimara, F. Tani, Y. Naruta and S. Shinkai, Chem. Lett., 1998, 587;
thank Prof. Ennio Zangrando for scientific discussion and inputs
on the structure solution.
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