Design and self-assembly of variform organometallic macrocycle with terminal imidazole-based bridging ligands utilizing joints twist and rotation

Article abstract of DOI:10.1039/c3dt52724g

Organometallic macrocycles based on bridge ligands with terminal imidazole groups show the formation of various patterns. The end imidazolyl finishes the conjugated system on the back bone and can freely twist or rotate just like the joints of a human body such as the knee and wrist.

Full text of DOI:10.1039/c3dt52724g

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Design and self-assembly of variform
organometallic macrocycle with terminal
imidazole-based bridging ligands utilizing joints
twist and rotation†
Cite this: Dalton Trans., 2014, 43,
2356
Received 30th September 2013,
Accepted 3rd October 2013
DOI: 10.1039/c3dt52724g
Tong Wu, Yue-Jian Lin and Guo-Xin Jin*
www.rsc.org/dalton
Organometallic macrocycles based on bridge ligands with terminal
imidazole groups show the formation of various patterns. The end
imidazolyl finishes the conjugated system on the back bone and
can freely twist or rotate just like the joints of a human body such
as the knee and wrist.
In recent decades, organometallic macrocycles have attracted
significant attention due to their tuneable cavities/radius,
shape recognition and complex interpenetration.1 The cage
with a large and suitable cavity can encapsulate different guest
molecules with the aim of drug targeted delivery or fluo-
rescence labeling.2 Changing the size of the spacers to tune
the distance between two functional groups such as ethenyl
can make the photochemical [2 + 2] cycloadditions easily
occur under ultraviolet irradiation.3 It’s crucial that even a
slight change in the shape of their backbones can make their
relevant properties convert dramatically.
Scheme 1 Two quite different imidazole-based bridging ligands.
Following this idea, we have designed a series of bridge
ligands with a terminal imidazole group as a flexible knuckle.
In this type of ligand, the imidazolyl can rid itself of backbone
π-conjugate system control and rotate freely. Its rotation by a
large margin also can drive the adjacent aromatic ring to
revolve and twist. This kind of motion is considered as the
mechanical arm behavior.4 In this system, they need to drive
upper arm swing with the help of the torsional moment from
their wrists and lower arms. Based on similar theories and
principles, we expect to find a flexible mode to satisfy the con-
figuration requirements of organometallic macrocycles.
tetranuclear complexes that bear the BiBzIm(2,2′-bisbenzimi-
dazole) ligand, [Cp*4M4(p-DiImBz)2(BiBzIm)2](OTf)4 (2a: M =
Ir, 2b: M = Rh) were designed and synthesized. Then the
binuclear macrocycle that bears the BiBzIm or tetraacetyl-
ethane ligand, formulated as [Cp*2M2(cis,cis-L2)(BiBzIm)]-
(OTf)2 (3a: M = Ir, 3b: M = Rh) and [Cp*2M2(cis,cis-L2)(tetraace-
tylethane)](OTf)2 (4a: M = Ir, 4b: M = Rh) was prepared as
orange crystals via 1a′ or 1b′ with four equivalents of AgOTf
and subsequent reaction with cis,cis-L2 (cis,cis-1,4-bis(diimid-
azolylbenzene)diethenylbenzene) in methanol at room temp-
erature (ratio = 1 : 1) (Scheme 2).
As shown in Scheme 1, two kinds of bridge ligand L1 and L2
were chosen as our experimental subject. First, we just made a
small change to the normal linear bridge ligands, the
p-DiImBz (p-diimidazolylbenzene) was employed and the
The 1H NMR spectrum of 2b showed four singlets at δ =
1.87, 6.67, 7.60, 7.17 and 5.71 ppm in a 60 : 8 : 4 : 4 : 4 intensity
ratio, due to Cp*, centra-phenyl and imidazolyl protons,
respectively. In addition, the structure was determined by
single crystal X-ray analysis. According to the crystal structure
in Fig. 1, the cation 2b reveals that this complex possesses a
standard rhombus structure in the top view bridge by a
p-DiImBz ligand between two spacers of BiBzIm with dimen-
sions of 9.8777(6) × 11.8306(8) Å (S4-Fig. 2†). That’s quite
different from other macrocycles reported in our previous
research which are mostly rectangles.5 The key point is that
Shanghai Key Laboratory of Molecular Catalysis and Innovative Material,
Department of Chemistry, Fudan University, Shanghai, 200433, P. R. China.
E-mail: gxjin@fudan.edu.cn; Fax: (+86)-21-65643776
†Electronic supplementary information (ESI) available: Experimental details and
crystal structure determination. CCDC 948275 (2b), 948276(3a), 948277 (3b), and
948278 (4b). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3dt52724g
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Scheme 2 Synthesis of 2a–4b (The guest anions in 4a, 4b are OTf−).
How about ligands with longer and folded backbones such as
cis,cis-L2?
The cis,cis-L2, as shown in Scheme 1, has a length of about
24.0 Å which can give the terminal imidazolyl groups more
room to rotate and occurs as a cooperative effect of their adja-
cent benzene ring due to the mechanical behavior we have
mentioned. Because of this behavior, they can’t self-assemble
to form the tetranuclear complex but the binuclear complex
instead. After two different kinds of spacers were employed to
this system, 3a–b and 4a–b were obtained as shown in
Scheme 2.
The cation in Fig. 2 reveals that complex 3b possesses a
D-shape structure in the side view bridged by cis,cis-L2 and
clamped the narrow spacer tightly. The backbone atoms N5,
N7, C18, C21, C24, C25, C32, C33, C34 and C37 are located in
the same plane. Based on this plane, the imidazole rings
rotate 65.041° and 70.614°, the side-benzenes rings rotate
35.981° and 33.733°, the centra-benzene ring rotates 40.737°.
The two side-benzene rings form an angle of 68.605°. From
the space-filling cation, we know there is scarcely room for
cavity and guest molecules in its arm and the distance
between the inner side hydrogen atoms on the two benzene
Fig. 1 Cation structure of 2b with thermal ellipsoids drawn at the 30%
level. Hydrogen atoms are omitted for clarity. Selected distances (Å) and
angles (°): Rh(1)–N(1) 2.140(2), Rh(1)–N(2) 2.163(2), Rh(1)–N(5) 2.115(3),
N(6)–C(15) 1.472(12), Rh(2)–N(3) 2.157(2), Rh(2)–N(4) 2.170(2), Rh(2)–
N(8A) 2.121(3), N(5)–Rh(1)–N(1) 86.76(10), N(5)–Rh(1)–N(2) 85.75(10),
C(21)–N(5)–Rh(1) 124.9(2), C(23)–N(5)–Rh(1) 128.5(2), N(8A)–Rh(2)–N(3)
85.48(10), N(8A)–Rh(2)–N(4) 87.99(10), C(24)–N(8)–Rh(2A) 125.1(8),
C(26)–N(8)–Rh(2A) 126.8(5). The symmetry codes are A: −x, −y + 2, −z + 1.
the p-DiImBz ligands have their imidazolyl groups twisted to
60.0° and 32.3° respectively when they play the role of building
blocks here. That is a dramatic trend of deformation but not
revolutionary, because they are limited by the linear backbone.
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Fig. 2 Cation structure of 3b with thermal ellipsoids drawn at the 30%
level (Rh orange; N blue; C gray). Hydrogen atoms are omitted for
clarity. Selected distances (Å) and angles (°): Rh(1)–N(1) 2.149(4), Rh(1)–
N(2) 2.190(4), Rh(1)–N(5) 2.101(4), N(6)–C(18) 1.442(6), C(24)–C(25)
1.338(8), Rh(2)–N(3) 2.163(4), Rh(2)–N(4) 2.126(4), Rh(2)–N(8) 2.104(4),
N(7)–C(37) 1.438(7), C(32)–C(33) 1.323(8), N(5)–Rh(1)–N(1) 87.29(16),
N(5)–Rh(1)–N(2) 85.58(16), C(25)–C(24)–C(21) 129.2(6), C(24)–C(25)–
C(26) 128.7(6), N(8)–Rh(2)–N(3) 85.81(16), N(8)–Rh(2)–N(4) 87.62(16),
C(33)–C(32)–C(29) 129.0(6), C(32)–C(33)–C(34) 127.8(6). The symmetry
codes are A: −x, −y + 2, −z + 1.
Fig. 3 Cation structure of 4b with thermal ellipsoids drawn at the 30%
level (Rh orange; N blue; C gray; O red; F green, S yellow). Hydrogen
atoms are omitted for clarity. Selected distances (Å) and angles (°): Rh
(1)–O(1) 2.069(8), Rh(1)–O(2) 2.065(7), Rh(1)–N(1) 2.082(8), N(2)–C(14)
1.400(13), C(9)–C(10) 1.299(17), O(1)–C(2) 1.267(10), O(2)–C(4) 1.270(9),
O(1)–Rh(1)–N(1) 84.0(3), O(2)–Rh(1)–N(1) 86.1(3), O(2)–Rh(1)–O(1)
88.7(3), C(9)–C(10)–C(11) 128.2(11), C(10)–C(9)–C(7) 131.2(12), C(17)–
N(1)–Rh(1) 125.3(7), C(19)–N(1)–Rh(1) 131.1(8). The symmetry codes are
A: −x + 1/2, −y + 3/2, z.
rings is 2.7189(2) Å, a very strong hydrogen bond intramolecu-
lar interaction. This status is called closed-mode.
The cation in Fig. 3 shows that complex 4b possesses a
spiral D-shape in the stereoscopic view with two OTf anions in
its cavity. The cation lies about a two fold axis. The backbone imidazole rings open and make the cis,cis-L2 distorted as part
atoms C(6), C(7), C(8), C(9), C(6A), C(7A), C(8A) and C(9A) are of a spring. The two planes passed by two diacetylethane
located in the same plane. Based on this plane, the olefin groups are crossed at an angle of 87.265°.
bonds twist for 14.965°, the side-benzene rings rotate 66.130°,
Here the complex 4b encapsulates two OTf anions in the
the imidazole rings rotate 34.033°. The two side-benzene rings cavity. Different to other aromatic guests, there is no π⋯π
form an angle of 7.416° nearly parallel to each other which stacking interaction to make them insert into the cavity, but
like two doors open to encapsulate the anion guest from two
there are more than six hydrogen bonds around each anion
sides which is called open-mode as shown in the space-filling (two of them come from cp* for 2.6274(118) and 2.6570(154) Å,
cation of Fig. 3. In the 19F NMR spectrum, complex 3b has one comes from the imidazole ring for 2.8589(138) Å, three of
only one signal at −78.19 ppm (free OTf −). But the complex 4b
them come from the centra-benzene ring for 2.9334(120),
shows three signals: −78.19 (free OTf−), −77.89 and −78.74 3.3330(173) and 2.4107(114) Å and the last one comes from
(OTf− encapsulated) ppm as evidence of structural stability in the methyl group of the tetraacetylethane ligand for
solution.
2.5322(189) Å as shown in ESI-S4-Fig. 4†).6 Two factors
Compared with the cation 3b, the distance of the two cooperate with each other to form this host–guest system: the
rhodium atoms are stretched from 5.5895(7) Å to 8.2353(14) Å long spacer lets the benzene rings rotate followed by the imi-
and the shape of the spacer is more like a pillar rather than a
dazole rings to form an open-mode cavity like an open door;
narrow fence. This ligand also can rotate itself to satisfy the hydrogen bonds between the cp* fragments and hydrogen
angle of coordination site. That is why it can prop the two atoms with the guest molecules catch the anions tightly.
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In summary, we have described a type of bridge ligand with monochromated Mo Kα radiation (λ = 0.71073 Å). All the data
terminal imidazolyl groups as building blocks of organometal- were collected at room temperature using the ω scan tech-
lic macrocycles, which occurs through mechanical arm nique. These structures were solved by direct methods, using
behaviour to control the self-assembly for closed-mode or Fourier techniques, and refined on F2 by a full-matrix least-
open-mode structures with the help of different spacers. The squares method. All the calculations were carried out with the
shape of the open-mode complex looks like a double-faced SHELXTL program.8
bowl which encapsulated the two anions embedded in each
2b: in asymmetric unit of this data, the diimidazole ligand
side, respectively. This system provides us with a new concept was disordered and it was divided into two parts (60 : 40 for the
to build various structures to enhance the corresponding benzene ring and 52 : 48 for imidazole ring). 4 ISOR, 2 DFIX
properties.
and 1 FLAT instructions were used to restrain the ligand so that
there were 28 restraints in the data.
3a: in asymmetric unit of 3a, there were disordered solvents
(three diethyl ether and four methanol molecules) which could
not be restrained properly. Therefore, the SQUEEZE algorithm
Experimental section
All manipulations were performed under an atmosphere of was used to omit them. One triflate anion was disordered and
nitrogen using standard Schlenk techniques. Solvents were it was divided into two parts (48 : 52). O4, O6, O4′ and O6′ were
dried and deoxygenated by MBraun Solvent Purification refined isotropically because of NPD and other non-hydrogen
System and collected just before use. [Cp*MCl2]2(M = Ir/Rh)7a atoms were refined isotropically. 5 ISOR and 12 DFIX instruc-
tetraacetylethane7b and 2,2′-bisbenzimidazole7c were prepared tions were used so that there were 42 restraints in the data.9
according to literature methods. The cis,cis-L2 was prepared as
3b: in asymmetric unit of 3b, there were disordered solvents
described in ESI S1.†6 IR spectra were recorded on a Nicolet (three diethyl ether and four methanol molecules) which could
AVATAR-360 IR spectrometer, elemental analyses were carried not be restrained properly. Therefore, the SQUEEZE algorithm
out on an Elementar III Vario EI Analyzer. 1H-NMR spectra was used to omit them. C64, C66 and C68 were refined isotro-
were obtained on a Bruker DMX-500 or a Bruker DMX-400 pically because of NPD and other non-hydrogen atoms were
spectrometer in DMSO-d6 solution.
refined anisotropically. Two ISOR and four DFIX instructions
Synthesis of 2a, 2b: to a solution of [Cp*MCl(μ-Cl)]2(M = Ir/Rh) were used to restrain the anions and solvents so that there
(40.0 mg/30.0 mg, 0.05 mmol) in dry CH3OH (10 mL) was were 16 restraints in the data.
added p-DiImBz (10.5 mg, 0.05 mmol) at room temperature.
4b: in the asymmetric unit of this data, there was a dis-
After vigorous stirring for about 4 h, AgOTf (51.2 mg, ordered water molecule which could not be restrained prop-
0.20 mmol) was added to the solution for 5 h. Finally, 2,2′-bis- erly. Therefore, the SQUEEZE algorithm was used to omit it.
benzimidazole (11.7 mg, 0.05 mmol) was added into the solu- 9 DFIX, 4 ISOR and 1 DELU instructions were used to restrain
tion with vigorous stirring for 5 h. After the reaction was the Cp* fragment and triflate anion so that there were 35
completed, the solution was filtered to remove undissolved restraints in the data.
compounds. The pure products were obtained by recrystalliza-
tion through the diffusion of ether into the filtrate, giving the
crystals of 2a (light yellow, 46.44 mg, 65%) and 2b (bright
orange, 44.34 mg, 71%).
Acknowledgements
Synthesis of 3a, 3b, 4a and 4b: [Cp*MCl(μ-Cl)]2(M = Ir/Rh) This work was supported by the National Science Foundation
(20.0 mg/15.0 mg, 0.025 mmol) was dissolved in dry CH3OH of China (91122017, 21374019), the Program for Changjiang
(10 mL) then cis,cis-L2 (10.4 mg, 0.025 mmol) was added at Scholars and Innovative Research Team in University
room temperature. After vigorous stirring for about 4 h, AgOTf (IRT1117) and the Shanghai Science and Technology Commit-
(25.6 mg, 0.10 mmol) was added to a suspension for 5 h, fol- tee (13JC1400600, 13DZ2275200).
lowed by adding 2,2′-bisbenzimidazole (5.85 mg, 0.025 mmol)/
tetraacetylethane (5.0 mg, 0.025 mmol) into the solution and
vigorous stirring for 5 h. After the reaction was complete, the
solution was filtered to remove undissolved compounds. The
Notes and references
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(yellow, 34.74 mg, 83%), 3b (orange, 33.27 mg, 89%), 4a
(yellow, 27.2 mg, 68%) and 4a (orange, 26.99 mg, 76%).
The characterization data of complex 2a–4b are shown in
ESI-Table 1.†6
All single crystals were immersed in the mother liquor and
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Bruker SMART APEX system. All the determinations of unit
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