Binuclear mesogenic copper(I) isocyanide complexes with an unusual inorganic core formed by two tetrahedra sharing an edge

Article abstract of DOI:10.1021/ic025703r

The preparation of binuclear mesogenic copper(I) isocyanide complexes [CuX(CNR)2]2 (X = halogen; R = C₆H₄C₆H₄- OC₁₀H₂₁ (L^A), C₆H₄COOC₆H₄

Full text of DOI:10.1021/ic025703r

Inorg. Chem. 2002, 41, 5754−5759
Binuclear Mesogenic Copper(I) Isocyanide Complexes with an Unusual
Inorganic Core Formed by Two Tetrahedra Sharing an Edge
Mohamed Benouazzane, Silverio Coco, and Pablo Espinet*
Departamento de Qu´ımica Inorga´nica, Facultad de Ciencias, UniVersidad de Valladolid,
E-47005 Valladolid, Spain
Joaqu´ın Barbera´
Departamento de Qu´ımica Orga´nica, Facultad de Ciencias-ICMA,
UniVersidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain
Received May 8, 2002
The preparation of binuclear mesogenic copper(I) isocyanide complexes [CuX(CNR)2]2 (X ) halogen; R ) C H C H -
6
4 6 4
OC H (LA), C H COOC H OC H2n+1 (LB), C H (3,4,5-OC H2n+1)3 (LC)) with an unusual tetrahedral core is described.
10 21
6
4
6
4
n
6
2
n
The copper complexes with LA are not liquid crystals, but the Cu−LB complexes show SmA mesophases and the
Cu−LC derivatives display hexagonal columnar mesophases, some of them at room temperature. The relationship
between the molecular structure of the complexes and their thermal behavior is discussed.
Introduction
Many liquid crystals with metal-containing molecules
(metallomesogens) have been synthesized, and molecular
design and synthesis of mesogenic metal complexes with
new structures or improved physical properties constitute an
active research area.1-10 The most popular molecular design
to build liquid crystals is that confining the core and the
chains of the molecule near a plane. So aromatic rings linked
by conjugated spacers that give rise to rodlike molecules
(Figure 1a) are very common structures in organic liquid
crystals. The same idea is often applied to metallomesogens,
although these can produce liquid crystals with less ap-
propriate molecular shapes. Complexes based on square
* To whom correspondence should be addressed. E-mail: espinet@
qi.uva.es.
(1) Espinet, P.; Esteruelas, M. A.; Oro, L. A.; Serrano, J. L.; Sola, E.
Coord. Chem. ReV. 1992, 117, 215.
(2) Bruce, D. W. In Inorganic Materials, 2nd ed.; Bruce D. W., D. O’Hare,
D., Eds.; Wiley: Chichester, U.K., 1996; Chapter 8.
(3) Hudson, S. A.; Maitlis, P. M. Chem. ReV. 1993, 93, 861.
(4) Metallomesogens; Serrano, J. L., Ed.; VCH: Weinheim, Germany,
1996.
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(6) Giroud-Godquin, A. M. Coord. Chem. ReV. 1998, 180, 1481.
(7) Donnio, B.; Bruce, D. W. Struct. Bonding 1999, 95 (Liquid Crystals
II), 193.
Figure 1. Typical liquid crystal molecular shapes: (a) organic rodlike
molecules; (b) metallomesogens based on square planar coordination around
the metal. The shape of the molecules prepared in this paper (c) can be
considered atypical.
planar coordination around the metal are particularly suited.
The dimer shown in Figure 1b represents a particularly
successful structural type that has given rise to hundreds of
metallomesogens,4,7 but the plethora of ligands and metals
that can be used in this field has provided a rich structural
diversity of metal complexes showing liquid crystal behavior.
These include apparently unfavorable geometries such as
(8) Espinet, P. Gold Bull. 1999, 32, 127.
(9) Collison, S.; Bruce, D. W. In Transition Metals in Supramolecular
Chemistry; Sauvage, J. P., Ed.; Wiley: Chichester, U.K., 1999; Chapter
7.
(10) Bruce, D. W. AdV. Inorg. Chem. 2001, 52, 151.
5754 Inorganic Chemistry, Vol. 41, No. 22, 2002
10.1021/ic025703r CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/10/2002

Binuclear Mesogenic Copper(I) Isocyanide Complexes
tetrahedral nickel(II)11-13 and Cu(I) complexes,14 some
octahedral derivatives of several transition metals,15-20
distorted square-pyramidal complexes of iron21-23 and va-
nadyl,24 a few Co, Cu, Zn, and Fe trigonal-bipyramidal
complexes,25,26 and zirconium square-antiprismatic com-
pounds.27
num(II) derivatives,40-43 and trigonal bipyramidal iron(0)
complexes.26 In this paper we describe the preparation and
liquid crystal behavior of binuclear copper(I) isocyanide
complexes [CuX(CNR)2]2 (X ) halogen; R ) C6H4C6H4-
OC10H21 (LA), C6H4COOC6H4OCnH2n+1 (LB), C6H2(3,4,5-
OCnH2n+1)3 (LC)).
A novel promising structural metallocore for metallome-
sogens might be that represented in Figure 1c, since it sets
the four coordination positions for the promesogenic ligands
in a plane. A structure of this type has been determined by
X-ray diffraction for binuclear copper isocyanide complexes
[CuX(CNR)2]2 (X ) halogen; R ) alkyl or aryl group).28
We decided to explore this structural motif because it appears
in the chemistry of a number of transition metals and might
open a rather wide field for future work. Moreover, isocya-
nide is a particularly appropriate ligand to prepare liquid
crystals, since it gives very stable complexes with many
transition metals in different oxidation states. The diversity
of liquid crystal based on isocyanide metal complexes
includes linear copper(I),29 silver(I),30 and gold(I)31-39 com-
pounds, cis and trans square-planar palladium(II) and plati-
Experimental Section
Materials and Reagents. Literature methods were used to
33 C6H4COOC6H4OCnH2n+1
prepare CNR (R ) C6H4C6H4OCnH2n+1, ,
41
41
C6H4OOCC6H4OCnH2n+1
,
C6H2(3,4,5-OCnH2n+1)3; n ) 4, 6, 8,
10, 12)37 and CuCl.44 Instrumentation was as described elsewhere.26
XRD measurements were performed with a pinhole camera
(Anton-Paar) operating with a point-focused Ni-filtered Cu KR
beam. The sample was held in Lindemann glass capillaries (1 mm
diameter) and heated, when necessary, with a variable-temperature
attachment. The patterns were collected on flat photographic film.
The capillary axis and the film are perpendicular to the X-ray beam.
Spacings are obtained via Bragg’s law.
Only examples procedures are described, as the syntheses were
similar for the rest of the compounds. Yields, IR, and analytical
data are given for all the cooper complexes.
Preparationof[CuX(CNR)2]2[X)Cl,Br,I;R)C6H4C6H4OCnH2n+1,
C6H4COOC6H4OCnH2n+1, C6H2(3,4,5-OCnH2n+1)3]. To a di-
chloromethane (40 mL) suspension of CuX (X ) Cl, Br, I) (0.5
mmol), under an atmosphere of nitrogen, was added 2 equiv of the
corresponding isocyanide (1.1 mmol). After being stirred for 1 h,
the resulting solution was filtered in air and hexane (10 mL) was
added. The solution was concentrated, which afforded the corre-
sponding compounds as white solids, except the chloro and bromo
derivatives that appear as yellow solids.
(11) Griesar, K.; Galyametdinov, Y.; Athanassopoulou, M.; Ouchinnikov,
I.; Haase, W. AdV. Mater. 1994, 6, 381.
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1982, 2, 241.
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1
Yields, IR, analytical data, and representative H NMR follow
below. When 1H NMR are not given, these are practically identical
to those provided for similar complexes (except for the intensity
of the multiplet comprising the undefined hydrogens of the alkoxy
chains, which is in each case proportional to their number).
R ) C6H4C6H4OC10H21 (Cu-LA). X ) Cl. Yield: 92%. IR
(21) Haase, W.; Griesar, K.; Iskander, M. F.; Galyametdinov, Y. Mol. Cryst.
Liq. Cryst., Sect. C 1993, 171, 1.
(Nujol) [ν(CtN)/cm-1]: 2136 s, 2161 s. H NMR (CDCl3): δ1
1
(22) Galyametdinov, Y.; Ivanovo, G.; Griesar, K.; Prosvirin, A.; Ouchin-
nikov, I.; Haase, W. AdV. Mater. 1992, 3, 115.
7.63, δ2 7.72, AA′XX′ spin system (3J1,2 + J1,2′ ) 8.6 Hz), δ3
5
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7.63, δ4 7.12, AA′XX′ spin system (3J3,4 + J3,4′ ) 8.7 Hz), 4.14
5
1995, 173.
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1996, 6, 349.
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Chem., Int. Ed. Engl. 1996, 35, 1858.
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130.
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2001, 11, 1740.
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2002, 12, 691.
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Chem. 1995, 5, 687.
(t, J ) 6.5 Hz, OCH), 2.00-1.01 (m, 19H, alkoxy chain). Anal.
Calcd for C92H116Cl2Cu2N4O4: C, 71.76; H, 7.59; N, 3.64. Found:
C, 71.47; H, 7.53; N, 3.84.
X ) Br. Yield: 61%. IR (Nujol) [ν(CtN)/cm-1]: 2153 s. Anal.
Calcd for C92H116Br2Cu2N4O4: C, 67.84; H, 7.18; N, 3.44. Found:
C, 67.91; H, 7.23; N, 3.54.
X ) I. Yield: 71%. IR (Nujol) [ν(CtN)/cm-1]: 2151 s. Anal.
Calcd for C92H116Cu2I2N4O4: C, 64.14; H, 6.79; N, 3.25. Found:
C, 64.09; H, 6.74; N, 3.54.
R ) C6H4COOC6H4OCnH2n+1 (Cu-LB). X ) Cl. n ) 4.
Yield: 69%. IR (Nujol) [ν(CtN)/cm-1]: 2146 s, 2165 s. 1H NMR
(CDCl3): δ1 7.60, δ2 8.25, AA′XX′ spin system (3J1,2 + J1,2′
)
5
8.5 Hz), δ3 7.09, δ4 6.92, AA′XX′ spin system (3J3,4 + 5J3,4′ ) 9.0
Hz), 3.96 (t, J ) 6.5 Hz, OCH2), 1.79-0.95 (m, 7H, alkoxy chain).
Anal. Calcd for C72H64Cl2Cu2N4O12: C, 62.70; H, 4.97; N, 4.06.
Found: C, 62.57; H, 5.05; N, 4.01.
(33) Benouazzane, M.; Coco, S.; Espinet, P.; Mart´ın-AÄ lvarez, J. M. J. Mater.
Chem. 1995, 5, 441.
(34) Coco, S.; Espinet, P.; Falaga´n, S.; Mart´ın-AÄ lvarez, J. M. New. J. Chem.
1995, 19, 959.
(35) Alejos, P.; Coco, S.; Espinet, P. New. J. Chem. 1995, 19, 799.
(36) Bayo´n, R.; Coco, S.; Espinet, P.; Ferna´ndez-Mayordomo, C.; Mart´ın-
AÄ lvarez, J. M. Inorg. Chem. 1997, 36, 2329.
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1994, 4, 859.
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Chem. 1997, 7, 19.
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Chem. 1999, 9, 2327.
(39) Omenat, A.; Serrano, J. L.; Sierra, T.; Amabilino, D. B.; Minquet,
M.; Ramos, E.; Veciana, J. J. Mater. Chem. 1999, 9, 2301.
(42) Coco, S.; D´ıez-Expo´sito, F.; Espinet, P.; Ferna´ndez-Mayordomo, C.;
Mart´ın-AÄ lvarez, J. M.; Levelut, A. M. Chem. Mater. 1998, 10, 3666.
(43) Wang, S.; Mayr, A.; Cheung, K. K. J. Mater. Chem. 1998, 8, 1561.
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Inorganic Chemistry, Vol. 41, No. 22, 2002 5755

Benouazzane et al.
n ) 6. Yield: 58%. IR (Nujol) [ν(CtN)/cm-1]: 2146 s, 2165
s. Anal. Calcd for C80H80Cl2Cu2N4O12: C, 64.42; H, 5.68; N, 3.76.
Found: C, 64.69; H, 5.87; N, 3.65.
Scheme 1
n ) 8. Yield: 64%. IR (Nujol) [ν(CtN)/cm-1]: 2146 s, 2165
s. Anal. Calcd for C88H96Cl2Cu2N4O12: C, 65.91; H, 6.29; N, 3.49.
Found: C, 65.91; H, 6.35; N, 3.36.
n ) 10. Yield: 60%. IR (Nujol) [ν(CtN)/cm-1]: 2147 s, 2166
s. Anal. Calcd for C96H112Cl2Cu2N4O12: C, 67.20; H, 6.81; N, 3.27.
Found: C, 67.50; H, 7.05; N, 3.16.
n ) 12. Yield: 85%. IR (Nujol) [ν(CtN)/cm-1]: 2147 s, 2165
s. Anal. Calcd for C104H128Cl2Cu2N4O12: C, 68.33; H, 7.28; N, 3.07.
Found: C, 68.69; H, 7.62; N, 2.97.
X ) Br. n ) 4. Yield: 62%. IR (Nujol) [ν(CtN)/cm-1]: 2145
s, 2158 s. Anal. Calcd for C72H64Br2Cu2N4O12: C, 58.90; H, 4.67;
N, 3.82. Found: C, 59.14; H, 4.88; N, 3.82.
n ) 12. Yield: 50%. IR (Nujol) [ν(CtN)/cm-1]: 2142 s, 2158
s. Anal. Calcd for C104H128Br2Cu2N4O12: C, 65.16; H, 6.94; N,
2.92. Found: C, 65.11; H, 6.89; N, 3.08.
X ) I. n ) 4. Yield: 61%. IR (Nujol) [ν(CtN)/cm-1]: 2136 s,
2158 s. Anal. Calcd for C72H64Cu2I2N4O12: C, 55.37; H, 4.39; N,
3.59. Found: C, 55.48; H, 4.42; N, 3.46.
n ) 12. Yield: 50%. IR (Nujol) [ν(CtN)/cm-1]: 2136 s, 2159
s. Anal. Calcd for C104H128Cu2I2N4O12: C, 62.11; H, 6.62; N, 2.79.
Found: C, 62.26; H, 6.59; N, 3.00.
R ) C6H2(3,4,5-OCnH2n+1)3 (Cu-LC). X ) Cl. n ) 4.
Yield: 83%. IR (Nujol) [ν(CtN)/cm-1]: 2158 s. 1H NMR
(CDCl3): δ 6.64 (s, J ) 2H, C6H2), 3.93 (m, 6H, OCH2), 1.83-
0.92 (m, 21H, alkoxy chains). Anal. Calcd for C76H116Cl2-
Cu2N4O12: C, 61.86; H, 7.92; N, 3.80. Found: C, 61.56; H, 7.71;
N, 3.79.
n ) 6. Yield: 58%. IR (Nujol) [ν(CtN)/cm-1]: 2170 s. Anal.
Calcd for C100H164Cl2Cu2N4O12: C, 66.27; H, 9.12; N, 3.09.
Found: C, 66.55; H, 8.87; N, 3.29.
n ) 8. Yield: 65%. IR (Nujol) [ν(CtN)/cm-1]: 2158 s. Anal.
Calcd for C124H212Cl2Cu2N4O12: C, 69.30; H, 9.94; N, 2.61.
Found: C, 69.12; H, 9.77; N, 2.99.
n ) 10. Yield: 58%. IR (Nujol) [ν(CtN)/cm-1]: 2171 s. Anal.
Calcd for C148H260Cl2Cu2N4O12: C, 71.51; H, 10.54; N, 2.25.
Found: C, 71.58; H, 10.32; N, 2.57.
n ) 12. Yield: 63%. IR (Nujol) [ν(CtN)/cm-1]: 2168 s. Anal.
Calcd for C172H308Cl2Cu2N4O12: C, 73.20; H, 11.00; N, 1.99.
Found: C, 72.91; H, 10.89; N, 2.01.
X ) Br. n ) 4. Yield: 87%. IR (Nujol) [ν(CtN)/cm-1]: 2152
s. Anal. Calcd for C76H116Br2Cu2N4O12: C, 58.34; H, 7.47; N, 3.58.
Found: C, 58.43; H, 7.32; N, 3.53.
Their IR spectra show two ν(CtN) absorptions (D2h, B2u
+ B3u) for the isocyanide groups, at higher wavenumbers
(ca. 60 cm-1) than for the free isocyanide, as reported for
similar p-tolyl isocyanide complexes.28 Some show only one
ν(CtN) absorption, possibly due to overlapping of two
expected bands.
The 1H NMR spectra of the metal complexes are all very
similar for each family. LA and LB derivatives, show, as
expected, four somewhat distorted “pseudodoublets” (a
deceptively simple pattern for the AA′XX′ spin systems) for
the two phenyl groups present in the molecule. In addition,
the first methylene group of the alkoxy chain is observed as
a triplet at 3.9-4.1 ppm. The remaining chain hydrogens
1
appear in the range 0.8-1.8 ppm. The H NMR spectra of
LC complexes show one signal from aromatic protons,
corresponding to the two equivalent protons present in the
molecule.
Mesogenic Behavior. The free isocyanides LA and LB
display nematic and/or smectic A phases in the range 60-
90 °C.40 However, only the Cu derivatives with LB are
mesogenic. The copper derivatives of LA look unpromising;
those prepared were not liquid crystals, and more systematic
variation of the chain was not undertaken. On the contrary,
although the free LC isocyanides are not liquid crystals, all
their copper isocyanide complexes are mesogenic.
The optical, thermal, and thermodynamic data of com-
plexes are collected in Table 1. The Cu-LB derivatives
display smectic A (SmA) mesophases, identified in optical
microscopy by their typical myelinic and homeotropic
textures that reorganize to produce fan-shaped focal-conic
areas at temperatures close to the clearing point. When the
length of the chain increases, the melting points of the chloro
Cu-LB derivatives do not change significantly, but the
clearing temperatures decrease narrowing the range of SmA
mesophase. The substitution of Cl by Br or I produces little
variation in the melting points, but the mesogenic ranges
cannot be compared because the clearing temperatures are,
in fact, decomposition temperatures.
n ) 12. Yield: 75%. IR (Nujol) [ν(CtN)/cm-1]: 2159 s. Anal.
Calcd for C172H308Br2Cu2N4O12: C, 70.96; H, 10.66; N, 1.92.
Found: C, 70.72; H, 10.37; N, 1.93.
X ) I. n ) 4. Yield: 83%. IR (Nujol) [ν(CtN)/cm-1]: 2154 s.
Anal. Calcd for C76H116Cu2I2N4O12: C, 55.03; H, 7.05; N, 3.38.
Found: C, 55.32; H, 6.82; N, 3.57.
n ) 12. Yield: 76%. IR (Nujol) [ν(CtN)/cm-1]: 2161 s. Anal.
Calcd for C172H308Cu2I2N4O12: C, 68.74; H, 10.33; N, 1.86.
Found: C, 68.97; H, 10.03; N, 1.66.
Results and Discussion
Synthesis and Structural Characterization. Binuclear
complexes [CuX(CNR)2]2 (X ) halogen; R ) C6H4C6H4-
OC10H21 (LA), C6H4COOC6H4OCnH2n+1 (LB), C6H2(3,4,5-
OCnH2n+1)3 (LC)) were prepared according to Scheme 1. C,
H, and N analyses for the complexes, yields, and relevant
IR data are given in the Experimental Section.
When 3,4,5-trialkoxyphenyl isocyanides are used, all the
copper complexes prepared, Cu-LC, display liquid crystal
behavior, some of them at room temperature. The derivatives
5756 Inorganic Chemistry, Vol. 41, No. 22, 2002

Binuclear Mesogenic Copper(I) Isocyanide Complexes
Table 1. Optical, Thermal, and Thermodynamic Data for Complexes
[Cu2(µ-X)2(CNR)4]
R
n
X
transitiona
tempb (°C)
∆Hb (kJ/mol)
LA
10
Cl
C f C′
C′ f I
C f I
C f I
105.5
167.2
164.7
167.8
184.9
196.1
267 (dec)c
187.4
265 (dec)c
185.8
270 (dec)c
179.4
236 (dec)c
189.7
195 (dec)c
138.3
191.4
199 (dec)c
99.2
157.6
185.9
196 (dec)c
109.9
188.5
194 (dec)c
146.6
159.4
177.5
199 (dec)c
76.5d
2.8
96.1
75.1
102.2
23.2
24.3
LA
LA
LB
10
10
4
Br
I
Cl
C f C′
C′ f SmA
SmA f I
C f SmA
SmA f I
C f SmA
SmA f I
C f SmA
SmA f I
C f SmA
SmA f I
C f C′
C′ f SmA
SmA f I
C f C′
C′ f C′′
C′′ f SmA
SmA f I
C f C′
C′ f SmA
SmA f I
C f C′
C′ f C′′
C′′ f SmA
SmA f I
C f I
I f Colh
C f Colh
Colh f I
Colr f Colh
Colh f I
Colh f I
C f Colh
Colh f I
C f I
LB
LB
LB
LB
LB
6
8
Cl
Cl
Cl
Cl
Br
57.2
44.2
45.7
45.8
10
12
4
Figure 2. DSC scans of [CuCl(CNC6H2(3,4,5-OC12H25)3)2]2: (a) first
heating; (b) first cooling; (c) second heating.
-19.3
73.6
note that the mesophase parameter increases with the chain
length, as expected, but it does not vary, within the
experimental error, upon changing the halogen atom. This
suggests that the molecule thickness is determined essentially
by the bulky trialkoxyphenyl groups. The three as-obtained
compounds with dodecyloxy chains are crystalline, but upon
cooling of the molten samples, the Colh mesophase persists
at room and below room temperature for X ) Cl and needs
cooling to lower temperature to produce crystals. A second
DSC heating scan produces the crystal-mesophase transition
below room temperature and with a lower enthalpy exchange,
as shown in Figure 2. This suggests that a different,
metastable crystalline form (C) has been formed on cooling
from the mesophase, different from the originally formed
upon crystallization from the solution (C′). On the other hand,
the chloro complex with octyloxy chains shows, in addition
to a Colh, a second mesophase at room temperature both in
the original complex and in a previously molten sample. The
X-ray patterns from this new phase can be indexed in a
rectangular columnar structure with lattice constants a ) 48.5
Å and b ) 28 Å and two columns/unit cell. It is worth noting
that the transition from the high-temperature mesophase to
the low-temperature mesophase in this compound seems to
produce only a slight distortion in the columnar arrangement,
as a hexagonal lattice with a ) 27.7 Å is equivalent to a
C-centered rectangular lattice with a ) 48 Å, b ) 27.7 Å
(a/b ) x3), and two molecules/unit cell.
LB
12
Br
26.7
46.4
58.7
LB
LB
4
I
I
7.1
74.4
12
3.6
10.6
52.8
LC
LC
LC
4
6
8
Cl
Cl
Cl
36.4
-5.2
5.5
7.6
18.1
7.8
12.0
48.8
14.1
24.3
-2.1
16.6
8.5
5.8
4.3
17.9
1.4
45.9
10.9
66.7
29.2
83.9
56.3
78.0
77.0
13.9
LC
LC
10
12
Cl
Cl
72.9
LC
LC
4
Br
Br
78.9d
I f Colh
C f C′
C′ f Colh
Colh f I
C f C′
70.2
9.6
27.0
12
75.2
LC
LC
4
I
I
74.7d
C′ f I
88.1d
I f Colh
C f Colh
Colh f I
51.9
13.7
82.2
12
a Key: C, crystal; Sm, smectic; Colh, hexagonal columnar; Colr,
rectangular columnar; I, isotropic liquid. b Data referred to the second DSC
cycle starting from the crystal. Temperature data are at peak onset.
c Microscopy data. d Data referred to the first DSC cycle.
Discussion
with n ) 4 are only monotropic. The optical textures, when
viewed with a polarizing microscope on cooling from the
isotropic melt, suggest columnar hexagonal phases and
display linear birefringent defects, large areas of uniform
extinction, and fan domains.42 In fact the high-temperature
X-ray patterns of [CuCl(CNC6H2(3,4,5-OC8H17)3)2]2 and of
the three compounds [CuX(CNC6H2(3,4,5-OC12H25)3)2]2 (X
) Cl, Br, I) are consistent with a columnar hexagonal
mesophase. The small-angle region contains a set of three
or four reflections in a reciprocal spacing ratio 1:x3:x4:
x7. The hexagonal lattice constant deduced from the patterns
is 27.7 Å at 65 °C for [CuCl(CNC6H2(3,4-OC8H17)3)2]2 and
The molecular structure of these compounds consists of
two [CuCl2(CNR)2] tetrahedral copper centers sharing an
edge that contains the two bridging halogens. This structure
produces a planar arrangement of the other four coordination
positions, which accommodate the four isocyanide groups.
The two bridging halogens lay above and below this plane,
as represented in Figure 3.28
In the 3,4,5-trialkoxyphenyl isocyanide derivatives, 12
alkyl chains are situated around the central core of the
molecule, producing a disklike structure (Figure 4a) suitable
to give rise to a columnar packing and display mesogenic
behavior, as indeed observed. However, when the isocyanide
used contains only one alkyl chain, calamitic mesophases
are observed. This result can be understood considering that
the arrangement beyond the double tetrahedral core can
o
33.5 ( 0.5 Å at 42 C for the three compounds with
dodecyloxy chains. From density estimations it is deduced
that the columnar structure is generated by single molecules
stacked at a mean distance of about 5 Å. It is interesting to
Inorganic Chemistry, Vol. 41, No. 22, 2002 5757

Benouazzane et al.
like copper derivatives, which show calamitic mesophases,
produces a decrease in the transition temperatures in the order
Cl > Br > I for n ) 4 and Br > Cl > I for n ) 12. In the
LC discotic-like complexes, which display columnar meso-
phases, the variation is the contrary, I > Br > Cl. To suggest
a rationale for the observed thermal behavior, one can look
at these molecules as initially planar, with two halogeno
substituents located in the core of the molecule protruding
out of the plane and the aryl and chain atoms filling the space
close to that plane in such a way that an efficient molecular
packing is achieved. Each X-M-isocyanide moiety provides
a complex multipolar leg of these four-legged molecules,
with electrostatic results very difficult to predict. However,
the experimental results can be understood assuming that
the balance of charge interactions between X-M-isocyanide
moieties decreases in the order I > Br > Cl. Thus, the
substitution of the chloro atoms by bromo or iodo would
cause mainly two opposite effects: (i) a perturbation of the
molecular distribution, which might increase the breadth of
the molecule in the direction the halogens are protruding,
affording lower transition temperatures for the heavier
halogen; (ii) a decrease of the balance of charge interactions
between X-M-isocyanide, leading to higher transition
temperatures for the heavier the halogen. The variation of
transition temperatures versus halogen will be determined
by the predominant factor.
Figure 3. Structural core of the complexes [CuX(CNR)2]2.
Experimental evidence in calamitic organic liquid crystals
(considered as a cylinders) shows that a decrease of
intermolecular forces always predominates when the lateral
substituent is bulky enough to increase and determine the
breadth of the molecule.45 Considering the X-ray structure
determined for [CuCl(CNC6H4CH3-p)2]2,28 it is easy to
conclude that in our monoalkylphenyl isocyanide copper
derivatives (LB) the breadth of the molecule should be
determined by the size of the halogen. Consequently, lower
transition temperatures should be obtained for the larger halo
ligands, as observed. However, when the length of the alkoxy
chain is increased, one might expect that the melt of the
chains, not perfectly stretched since they have to fill the space
between the parallel directions determined by the aryl rings,
will increase the breadth of the molecule so as to make the
size of the halogen unimportant for Cl and Br but still
determinant for X ) I. This may explain why the trend
observed for n ) 12 is Br > Cl > I.
Figure 4. Space-filling model of the complexes [CuCl(CNR)2]2 [(a) R )
C6H2(3,4,5-OC2H5)3; (b) C6H4COOC6H4OC2H5; (c) C6H4C6H4OC2H5],
showing the structural variations introduced by the isocyanide.
become calamitic for isocyanide ligands that can adopt an
adequate conformation. This is the case of the ligands LB,
which contain an ester moiety and can acquire a conforma-
tion where the second ring and its chain in the para position
become aligned by pairs (Figure 4b). This conformation leads
to the better space filling and intermolecular interactions.
Thus, the molecular structure becomes calamitic, especially
for long chain, and it is not unexpected that LB copper
derivatives show liquid crystal behavior.
Similarly, in trialkoxyphenyl isocyanide derivatives LC,
the high number of alkoxy chains makes very improbable
that they (and their aryl rings) are reasonably confined close
to the plane. Consequently, the breadth of the molecule will
be determined essentially by the packing of the nonplanar
trialkoxyphenyl groups and should be the same for all the
halogens, as obtained by X-ray data. In this case the molecule
with the best multicharge interactions should produce, as
observed, the higher transition temperatures.
On the contrary, for biphenyl isocyanide derivatives, the
rigidity of biphenyl group does not permit one to obtain a
conformation with a dominant cylindrical shape (Figure 4c).
Only from the oxygen of the alkoxy chain, parallel confor-
mation of the ligands suitable for mesogenic behavior
becomes possible, but it is clear from Figure 3c that at this
point the width of the molecule is too wide and the chains
are too far away and would not fill the space efficiently,
except if they curl. Consistently, the copper biphenyl
isocyanide compounds are not liquid crystals.
In summary, the binuclear copper(I) isocyanide complexes
prepared exhibit mesogenic behavior and provide the first
(45) Gray, G. W. In Liquid Crystals & Plastic Crystals; Gray, G. W.,
Finally, it is interesting to analyze the effect of the halogen
atoms. The substitution of Cl by Br and I in the LB calamitic-
Winsor, P. A., Eds.; John Wiley: New York, 1974; Vol. 1, Chapter
4.
5758 Inorganic Chemistry, Vol. 41, No. 22, 2002

Binuclear Mesogenic Copper(I) Isocyanide Complexes
examples of liquid crystals with a core formed by two
tetrahedra sharing an edge. The shape of the molecule, which
is mainly dominated by the organic ligand, permits a control
of the mesogenic behavior, while the length of the chains
and the use of different halogens provide a fine-tuning of
the thermal properties.
Acknowledgment. We thank the Direccio´n General de
Investigacio´n (Project MAT2002-00562) and the Junta de
Castilla y Leo´n (Projects VA15/00B and VA050/02) for
financial support.
IC025703R
Inorganic Chemistry, Vol. 41, No. 22, 2002 5759