Calamitic organometallic liquid crystals with terminal metal. Syntheses and liquid crystal properties of dicarbonylrhodium(I) β-diketonate complexes

Article abstract of DOI:10.1016/S0022-328X(97)00653-0

A series of novel organometallic complexes based on γ-substituted β-diketone ligands with terminal metal Rh(I) have been prepared by reaction of the ligands with [Rh(CO)₂(μ-Cl)]₂. The mesomorphism of the ligands and complexes has been investigated using DSC and polarizing microscope. It is found that non-mesogenic ligands with n=7, 8, 9, 10, 11 can form liquid crystalline phase by direct coordination to metal. The effect of the terminal carbon number on the mesomorphism has also been discussed.

Full text of DOI:10.1016/S0022-328X(97)00653-0

Journal of Organometallic Chemistry 557 (1998) 157–161
Calamitic organometallic liquid crystals with terminal metal. Syntheses
and liquid crystal properties of dicarbonylrhodium(I) i-diketonate
complexes
Wen Wan, Wen-Jie Guang, Ke-Qin Zhao, Wei-Zhong Zheng, Liang-Fu Zhang *
Chengdu Institute of Organic Chemistry, Academia Sinica, Chengdu, 610041, China
Received 26 March 1997; received in revised form 1 September 1997
Abstract
A series of novel organometallic complexes based on k-substituted i-diketone ligands with terminal metal Rh(I) have been
prepared by reaction of the ligands with [Rh(CO)2(v-Cl)]2. The mesomorphism of the ligands and complexes has been investigated
using DSC and polarizing microscope. It is found that non-mesogenic ligands with n=7, 8, 9, 10, 11 can form liquid crystalline
phase by direct coordination to metal. The effect of the terminal carbon number on the mesomorphism has also been discussed.
© 1998 Elsevier Science S.A. All rights reserved.
Keywords: Organometal; Complex; Liquid crystal; i-diketone
Then we report here the results of the synthesis and
liquid-crystalline properties of a new family of terminal
metalloheterocyclic complexes: dicarbonylrhodium(I)
i-diketonate complexes.
The synthetic route of the title complexes is outlined
in Scheme 1.
Owing to the d–y and y–y* interaction between
metal and carbon monoxides, we can expect that these
complexes have not only good conjugation structure,
but also have great dipole moments.
Rod-like molecules containing rigid core have tradi-
tionally been regarded as the most suitable geometry to
give rise to mesogenic behavior [1]. The synthesis of
organometallic liquid crystals containing transition-
metal atom as rigid core is currently in progress [2–4]
owing to the special combination of liquid-crystalline
properties with their unusual electro-optic and magnetic
properties, which can offer several potential
applications.
So far, the metal atom of organometallic liquid crys-
tals is most situated at the center of the molecules
[5–7]. Such molecules have always central symmetry,
which will be disadvantageous to further enhancement
of electro-optic response. It is well known that 1,3-dis-
ubstituted i-diketonate complexes of Cu(II) and Ni(II)
give rise mostly to discotic liquid crystals, which often
show the structures of central symmetry. However, the
mesomorphism of rod-like k-substituted i-diketonate
complexes with terminal metal atom has never been
reported, whose structure is non-centrosymmetrical.
1. Results and discussion
1.1. Synthesis and characterization
The ligands were prepared by reaction of 4-(4%-
alkoxybenzoxy)benzoate with 3-chloro-2,4-pentane-
dione in dry DMF as shown in Scheme 1. The
complexes were obtained with good yields from the
reaction of the ligands with v-dichlorotetracarbonyl
dirhodium under the excess of barium carbonate in
acetone.
* Corresponding author.
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(97)00653-0

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W. Wan et al. / Journal of Organometallic Chemistry 557 (1998) 157–161
Scheme 1.
The elemental analyses, IR, and 1H-NMR of the
matic ring. Those results lead to the shielding effect of
i-diketonate on the proton.
complexes are consistent with the above proposed
structures.
1H-NMR studies of the complex and ligand with
n=10 are shown in Table 1.
The free ligands of k-substituted acetylacetone have
two stretching vibration frequencies at 1720 and 1740
cm−1, which are respectively assigned to the w(CꢀO) and
w(COO). While, the stretching vibration frequencies of the
carbonyl group in the corresponding complexes are
lowered to 1590–1580 cm−1 except for the stretching
The stability of the complexes was studied by ther-
mogravimetric analysis and none of the complexes ap-
peared weight loss until 230°C.
1.2. Mesogenic beha6ior
vibration band of ester group still around 1740 cm−1
,
It is shown that the carbonyl and CꢀC bond in the
conjugated chelated ring possess double bond and sin-
gle bond characters respectively, accounting for the
large frequency shifts of CꢀC bond and carbonyl group
absorption after the chelation of ligand with metal [8,9].
We also find that there is no appreciable shift with
phenyl group(B4 cm−1) between the ligands and the
complexes. The infrared spectra of the i-diketonate
All of the described complexes can form liquid crys-
tals. The complexes and the ligands show mesogenic
behaviors which obey the rules observed in classical
rod-like organometallomesogens and organic com-
pounds: the longer the terminal chain, the lower the
melting points and the wider the liquid-crystalline
phase. The transition temperature and enthalpies were
determined by DSC and the structure of the mesomor-
phism of the ligands and complexes has been affirmed
on the basis of the optical textures.
complexes appear absorption 2030 and 2005 cm−1
,
which are the characteristic of the carbon monoxide
stretching region.
1
According to the H-NMR data of the ligands and
1.2.1. The ligand
the complexes, it is found that the proton chemical shift
values and coupling constants of complexes are a little
lower than those of the free ligands. We think that the
results are presumably due to the electronic delocaliza-
tion in the organometallic complexes. One is the aid of
the interaction of metal atom with CO ligands through
the d–y and y–y* action. The other is the effect of the
great electronic field on the i-diketonate quasi-aro-
The ligands with n=7, 8, 9, 10, 11 have no mesomor-
phism. While, the ligands with n=12, 14 show enan-
tiotropic mesophase. The transition temperatures and
enthalpies are shown in Table 2. The transition temper-
atures of ligands versus the terminal alkoxyl group are
displayed in Fig. 1.
One of the textures observed with a polarizing micro-
scope, between crossed polarizers, is shown in Fig. 2.

W. Wan et al. / Journal of Organometallic Chemistry 557 (1998) 157–161
159
Table 1
The 1H-NMR spectra of ligand and complex with n=10 in CDCl3
l (ppm) (multiplicities)
a
b
c
J (Hz)
H1, H1%
H2, H2%
H3, H3%
H4, H4%
H12
H14
Ligand
Complex
6.95(d)
6.90(d)
8.12 (d)
8.08 (d)
7.35(d)
7.38(d)
8.22(d)
8.20(d)
4.05(t)
3.98(t)
2.08(s)
2.05(s)
5.70(s)
8.0
7.8
8.4
8.8
1.2.2. The complex
and all the complexes have higher transition tempera-
tures than the corresponding ligands. It can be ob-
served that the clearing point temperatures of the
complexes obey an odd–even effect.
The mesophases of the complexes are identified by
their polarizing optical textures as smectic C phase. It is
found that the transition temperature ranges of com-
plexes with n=7, 8, 9, 10, 11 become wider with the
increase of the terminal carbon number. However, these
complexes show high transition temperature and a nar-
row mesophase range. Interestingly, the complex with
n=14 gives a much lower melting point at 89°C and
shows a wider mesophase range to 42°C. We are sur-
prised to find that the complex with n=12 only gives
monotropic mesomorphism phase. It is found that the
clearing point temperatures of the complexes become
lower with the increase of the terminal carbon number
In comparison with the non-mesogenic free ligands
(n=7, 8, 9, 10, 11), it is found that all of the corre-
sponding rhodium complexes show liquid-crystalline
phases. To the best of our knowledge, the mesogenic
complexes previously described contained a liquid crys-
tal ligand or, alternatively involved significant struc-
tural changes upon coordination of the organic ligand.
Our results and recent reports [10,11] show that the new
organometallic liquid crystals can be obtained by direct
coordination of a single molecule of non mesogenic
ligand to metal. These materials represent further exam-
Table 2
Optical, thermal and thermodynamic data for the ligands
n
Transition
Tempa, °C
DHa, (kJ mol−1
)
7
8
9
C–I
C–I
C–I
C–I
C–I
83.4
88.4
91.0
88.3
79.8
18.08
32.29
21.71
27.21
18.74
10
11
12
C–SA
S*–Sc
Sc–I
51.9
61.3
88.7
7.02
1.11
18.65
14
C–Sc
Sc–I
53.9
68.8
22.32
8.02
Fig. 1. Transition temperatures as a function of the alkoxy chain
length for the ligands.
a DSC data from first scan.

160
W. Wan et al. / Journal of Organometallic Chemistry 557 (1998) 157–161
Fig. 2. Texture of the ligand (n=14) under cross polarizers. 60°C, on
cooling run. ×220 Sc phase.
Fig. 3. Transition temperatures as a function of the alkoxy chain
length for the complexes.
ples of mesomorphic complexes derived from non-
mesogenic ligands.
2. Experimental section
The transition temperatures and enthalpies of the
complexes determined by DSC are summarized in
Table 3. The transition temperatures of complexes ver-
sus the terminal alkoxyl group are displayed in Fig. 3.
One of the textures observed with a polarizing optical
microscope is shown in Fig. 4.
2.1. Techniques
1
The H-NMR spectra were recorded on 300 MHz
Brucker AC-P300 spectrometer, using CDCl3 as solvent
and TMS as internal standard. The IR spectra (nujol)
were performed on NICOLET FT-MX-IE spectrome-
ter. Elemental analyses were obtained from CARLO
ERBA-1106 microanalyzer. The thermotropic behav-
iors were determined by DSC using Perkin–Elmer 7
series analysis system operated at a scanning rate of
10°C min−1. The apparatus was calibrated with indium
(156.6°C, 28.4 J g−1) and tin (231.1°C 60.5 J g−1). The
textures of mesophase were observed under ORTH-
LUX-II POLBK Polarizing optical microscope with a
hot stage (self made) and TDA temperature controller.
Table 3
Optical, thermal and thermodynamic data for the complexes
n
Transitiona,
Tempb, °C
DHb, (KJ mol−1
)
7
C–C%
C%–Sc
Sc–I
151.0
166.3
169.8
6.95
4.64
0.72
8
9
C–Sc
–Sc–I
158.7
170.5
25.87
1.85
C–C%
C%–Sc
Sc–I
149.2
156.2
171.6
2.47
10.94
6.49
10
11
12
14
C–C%
C%–Sc
Sc–I
143.2
149.7
164.7
3.28
3.41
1.74
C–C%
C%–Sc
Sc–I
143.6
152.9
166.9
5.33
6.44
3.20
C–I
I–N
N–C
148.0
141.0c
126.5c
44.83
C–C%
C%–Sc
Sc–I
59.80
88.93
130.56
1.67
11.00
20.04
a C,C%, crystalline phase; 1, isotropic liquid.
b DSC data from first scan.
Fig. 4. Texture of the complex (n=12) under cross polarizers. 130°C,
on cooling run. ×220, nematic phase.
c Optical microscopy data.

W. Wan et al. / Journal of Organometallic Chemistry 557 (1998) 157–161
161
compounds were characterized by IR and 1H-NMR
techniques without significant difference within the
same series
Table 4
The elemental analyses (percent) of the ligands and the complexes
(calculated values in parenthesis) and yields
Compounds
Ligand
n
C (%)
H (%)
Yields (%)
7
8
9
68.88 (68.72)
69.43 (69.23)
69.57 (69.71)
6.71 (6.61)
6.90 (6.84)
7.24 (7.05)
7.43 (7.26)
7.31 (7.45)
7.70 (7.63)
7.93 (7.97)
58
63
47
50
54
59
51
3. Conclusion
A new series of calamitic organometallomesogens of
i-diketonate complexes with terminal metal Rh(I) have
been synthesized and characterized. Interestingly, the
results reported in the paper show that non-mesogenic
rod-like ligands of i-diketone can form liquid crys-
talline phases by direct coordination to metal Rh(I).
10 70.28 (70.16)
11 70.58 (70.59)
12 70.78 (70.99)
14 71.52 (71.74)
Complex
7
8
9
54.80 (54.90)
55.79 (55.59)
56.21 (56.25)
4.80 (4.74)
4.96 (4.95)
5.27 (5.16)
5.51 (535)
5.76 (5.54)
5.98 (5.72)
6.23 (6.06)
82
87
83
78
89
91
88
10 56.79 (56.88)
11 57.23 (57.48)
12 57.89 (58.06)
14 59.30 (59.15)
Acknowledgements
This work was supported by the National Natural
Science Foundation. (2927064, 29673041)
2.2. Synthesis
2.2.1. Synthesis of the ligands
References
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and distilled. The residue was chromatographed on
silica gel using ethyl acetate: petroleum ether (60–90°C)
(1:2) as eluant. The products were purified by recrystal-
lization from ethanol, yields 50–60%.
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2.2.2. Synthesis of the complexes
A mixture of acetone solutions of ligand (1 mmol)
and v-dichlorotetracarbonyl dirhodium [Rh(CO)2Cl]2
[16] (0.5 mmol) with excess of solid barium carbonate
was stirred under N2 at room temperature for 30 min
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was chromatographed on silica gel using chloroform as
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