Novel liquid-crystalline phases with layarlike organization

Article abstract of DOI:10.1002/1521-3773(20021104)41:21<4031::AID-ANIE4031>3.0.CO;2-5

Competition between microsegregation and rigidity in bolaamphiphilic biphenyl derivatives bearing lateral semiperfluorinated alkyl chains leads to the formation of novel smectic liquid-crystalline phases, designated SmA_b and Lam_A, in

Full text of DOI:10.1002/1521-3773(20021104)41:21<4031::AID-ANIE4031>3.0.CO;2-5

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[11] The NMR spectra for 1a¥1a and 1b¥1b were measured at 298 K, as the
complexes are kinetically stable on the chemical shift timescale at this
temperature. In contrast, it was necessary to record the NMR spectra
of (þ)-2a¥(À)-2a and (þ)-2b¥(À)-2b at 263 K to achieve kinetic
stability. We attempted to determine the thermodynamic stability of
and mobility on molecular, supramolecular, and macroscopic
levels. The mobility enables them to respond to different
external stimuli by changing their configuration, which in turn
determines their importance in biological systems and tech-
nical applications.[1] In conventional LC materials rod- or
disklike anisometric rigid units, which provide long-range
orientational order, are connected to flexible alkyl chains,
which are largely responsible for positional order and
mobility. This design principle leads to the well-known
nematic, smectic, and columnar LC phases.[2] A characteristic
feature of such LC materials is a molecular topology in which
rigid cores and flexible chains are connected in such a manner
that the parallel organization of the rigid segments and the
segregation of the incompatible molecular parts enhance each
other.
Exciting new mesophase morphologies can be realized if
rodlike (calamitic) rigid units are combined with two incom-
patible subunits in a competitive and nonlinear manner.[3,4]
Examples are bolaamphiphilic biphenyl derivatives carrying a
long lateral alkyl chain.[4] In such bolaamphiphiles, the
strongest attractive forces (hydrogen bonding) act at the
two termini of a rigid calamitic core. The lateral alkyl chains
represent a third group of units that are incompatible with
both the polar terminal diol groups[5] and the rigid biphenyl
cores. As a consequence, each of these units segregates into its
own subspace, and a series of unusual columnar mesophases
results. They are built up by networks of cylinders, formed by
ribbons of the biphenyl cores which are held together by
ribbons of hydrogen-bonding networks (see Figure 1). The
1
these aggregates by H NMR dilution experiments in CDCl3, but did
not observe any changes in chemical shift upon dilution to 100 mm.
Isothermal titration calorimetry yielded thermodynamic parameters
for dimerization (C6H6/DMSO 95:5, 298 K, 1a¥1a: Ka ¼ 5550 ꢀ
1230mÀ1, DH ¼ À15.7 ꢀ 0.9 kJmolÀ1).
[12] CCDC-187956 (1a¥1a) and CCDC-187957 ((þ)-2b¥(À)-2b) contains
the supplementary crystallographic data for this paper. These data can
be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrie-
ving.html (or from the Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge CB21EZ, UK; fax: (þ 44)1223-336-033; or
deposit@ccdc.cam.ac.uk).
[13] For a review on H-bond geometry, see: R. Taylor, O. Kennard, Acc.
Chem. Res. 1984, 17, 320 326. For 1a¥1a, the two external amide
¼
protons are strongly H-bonded to the internal amide C O groups with
À
typical separations (N¥¥¥O 2.937 ä, H¥¥¥O 2.088 ä) and angles (N
H¥¥¥O 162.08), whereas the structural data indicates that the internal
À
amide N H groups may benefit from weaker interactions with the
¼
ureidyl C O moiety (N¥¥¥O 2.909 ä, H¥¥¥O 2.240 ä; N-H¥¥¥O 132.68).
For (þ)-2b¥(À)-2b, the two external amide protons H-bonded to the
internal amide C ¼ O groups have typical geometries (N¥¥¥O 2.860 ä,
H¥¥¥O 2.014 ä; N-H¥¥¥O 160.88), whereas the internal amide protons
clearly do not benefit from additional interactions (N¥¥¥O 3.296 ä,
H¥¥¥O 2.683 ä).
[14] Selected reviews: J.-M. Lehn, Chem. Eur. J. 1999, 5, 2455 2463; P.
Timmerman, D. N. Reinhoudt, Adv. Mater. 1999, 11, 71 74; S. J.
Rowan, S. J. Cantrill, G. R. L. Cousins, J. K. M. Sanders, J. F. Stoddart,
Angew. Chem. 2002, 114, 938 993; Angew. Chem. Int. Ed. 2002, 41,
899 952.
[15] The observed self-sorting phenomenon can be explained based on
either kinetic or thermodynamic arguments. The lifetimes of the
kinetically more stable dimers determined by 1H EXSY NMR
measurements (1a¥1a: 13.6 sÀ1; 1b¥1b: 6.3 sÀ1) indicate that a fast
exchange occurs, which rules out a kinetically controlled process and
implies a thermodynamic preference for self-sorting in this system.
Novel Liquid-Crystalline Phases with Layerlike
Organization**
Xiao Hong Cheng, Malay Kumar Das,
Siegmar Diele, and Carsten Tschierske*
Investigation of the driving forces of molecular self-
organization is one of the most exciting and most rapidly
growing areas of chemical research. In this respect, ordered
structures with liquid-crystalline (LC) properties are of
special interest, because they are reversibly formed under
thermodynamic equilibrium conditions and combine order
[*] Prof. Dr. C. Tschierske, Dr. X. H. Cheng
Institute of Organic Chemistry
Martin-Luther-University Halle-Wittenberg
Kurt-Mothes-Str. 2, 06120 Halle (Germany)
Fax : (þ 49)345-55-27223
E-mail: tschierske@chemie.uni-halle.de
Dr. M. K. Das, Dr. S. Diele
Institute of Physical Chemistry
Martin-Luther-University Halle-Wittenberg
[**] This work was supported by the Deutsche Forschungsgemeinschaft,
the Fonds der Chemischen Industrie, and the European Commission
(RTN LCDD).
Figure 1. Molecular organization of compounds 1 in their mesophases in
dependence on the molecular structure.[4]
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interior of these cylinders is filled with columns of the
microsegregated fluid lateral chains. Figure 1 gives an over-
view of the obtained mesophase structures. First, the meso-
phase type depends on the length of the lateral chain, that is,
their space requirement. Second, it depends on the space
available for the chains within the cylinders, which is
determined by the length of the bolaamphiphilic core.[4] The
question arose what would happen if the ratio of the size of
the nonpolar chains and the length of the bolaamphiphilic
core is shifted further, so that the formation of cylinders
becomes disfavored.
Here we report novel mesophases obtained with the new
mesogenic block molecules 2 4. They have the same funda-
mental structure as compounds of the type 1, but replacement
of one of the terminal 2,3-dihydroxypropoxy groups by a
single OH group makes the bolaamphiphilic core shorter and
reduces the number of possible hydrogen bonds. Additionally,
the lateral alkyl chains are replaced by semiperfluorinated
chains,[6] which require much more space than related hydro-
carbon chains.[7] Moreover, the stronger segregation[8] of such
fluorinated chains from the polar and rigid molecular parts
(PR parts) increases the mesophase stability and thus
compensates for the mesophase destabilization that results
from the reduced cohesive forces between the bolaamphi-
philic cores (fewer hydrogen bonds). Both structural varia-
tions lead to a dramatic change in self-organization in
comparison to compounds 1.
Scheme 1. Synthesis of 2 4. a) RFI, [Pd(PPh3)4];[9] b) LiAlH4, Et2O;
c) HBr, AcOH, DMSO;[10] d) BrCH2CH CH2, K2CO3, MeCN; e) N-
¼
methylmorpholine N-oxide, cat. OsO4, acetone, H2O;[11] f) 2,2-dimethoxy-
propane, Py¥TosOH; g) (4-BnO)C6H4B(OH)2, [Pd(PPh3)4], NaHCO3,
glyme;[12] h) 10% HCl, EtOH; i) H2, Pd/C, EtOAc. RF ¼ CnF2nþ1 (n ¼ 8,
10, 12).
which is often found in conventional LC materials below an
SmA phase. In samples with a fanlike texture only a
significant change in the birefringence can be observed at
the phase transition. The color changes from yellow through
red, violet, blue, and green to yellow again, but the fans do not
break or change their shape (Figure 2d). These textural
features were predicted for biaxial SmA phases (SmAb,
McMillan phase)[14] and were recently observed experimen-
tally for such a mesophase.[15] The transition to the third
mesophase at 1528C can be seen by the transformtion of the
schlieren texture into a paramorphotic mosaiclike texture
(Figure 2h),[16] whereas no change is observed in the regions
with fan-shaped texture. Here only the color changes, from
deep yellow to red (Figure 2g). This suggests that the layer
structure itself remains nearly unchanged in all three phases,
whereas the order within the layers changes dramatically. This
is additionally confirmed by X-ray scattering. The X-ray
diffraction pattern does not change at the phase transitions
between the three distinct phases. As shown in Figure 3 it is
characterized by diffuse scattering in the wide-angle region,
These compounds were synthesized as shown in
Scheme 1.[13] The products were investigated by differential
scanning calorimetry (DSC), polarized light microscopy, and
X-ray diffraction of well-aligned samples. The observed
mesophases and their transition temperatures are collected
in Table 1.
Compound 3 has the richest polymorphism and will be
discussed in more detail. Three distinct mesophases can be
detected by polarized light optical microscopy (see Figure 2).
The high-temperature mesophase behaves like a conventional
SmA phase, characterized by a typi-
cal fan texture (Figure 2a) which can
Table 1. Transition temperatures T [8C] and corresponding enthalpy values DH [kJmolÀ1] (lower lines) of
compounds 2 4.[a]
easily be aligned homeotropically.
These homeotropically aligned areas
are optically isotropic and appear
completely dark between crossed po-
larizers (Figure 2b). On cooling, at
1538C a schlieren texture appears in
these regions (Figure 2e) and indi-
cates the transition to a new optically
biaxial mesophase. A characteristic
feature of this schlieren texture is the
absence of four-brush disclinations.
This excludes an SmC-like organiza-
tion in which the molecules are tilted
with respect to the layer planes and
Comp
n
Cr
LamA
SmAb
SmA
IS
*
*
*
2
8
118
139
[0.8][b]
135[c]
[37.2]
154[c]
[18.6]
[6.0]
156
[1.7]
188
*
*
*
*
*
*
*
*
3
4
10
12
152
153
[2.0][d]
*
(
142)
[2.0]
[a] Abbreviations: Cr¼ crystalline solid state; LamA ¼ laminated smectic A phase, see Figure 2i;[20]
SmAb ¼ biaxial smectic A phase, see Figure 2 f; SmA ¼ smectic A phase, see Figure 2c; Iso ¼ isotropic
liquid phase. Transition temperatures and enthalpies were determined by DSC (Perkin-Elmer DSC-7, first
heating scan, rate: 10 KminÀ1) and confirmed by polarized light optical microscopy. Values in parentheses
refer to monotropic (metastable) phases which were determined in the second heating scan. [b] Only
partial crystallization, glassy solidification at Tg ¼ 188C. [c] Multiple melting and Cr Cr transitions were
observed. [d] Not resolved.
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Figure 2. Textures of the mesophases of compound 3, as observed between
crossed polarizers, and suggested models of the organization of the
molecules in the distinct mesophases. a) Fan-shaped texture of the SmA
phase at 1558C. b) Homeotropically aligned SmA phase at 1558C.
c) Organization of the molecules in the SmA phase. d) Fan-shaped texture
of the SmAb phase at 152.58C. e) Schlieren texture of the SmAb phase at
152.58C. f) Organization of the molecules in the SmAb phase. g) Fan-
shaped texture of the LamA phase at 1408C. h) Paramorphotic mosaic
texture at 1458C. i) Organization of the molecules in the LamA phase.[20]
Figure 3. X-ray diffraction pattern of an aligned sample of compound 3
a) in the SmA phase at 1558C, b) in the SmAb phase at 152.58C, and c) in
the LamA phase at 1448C.
all conventional smectic phases, the calamitic biphenyl cores
are organized parallel to the layer planes. The PR layers are
separated by the nonpolar layers of the lateral chains, which
are strongly disordered (liquidlike). This special organization
should be provided 1) by the unique topology of connection of
the nonpolar chains in a lateral position at the rodlike cores,
2) by the parallel alignment of the biphenyl cores, and 3) by
the cohesive hydrogen bonds at the termini of the biphenyl
cores, which stabilize the PR layers. The experimentally
determined layer distance requires that the thickness of the
PR sublayers correspond on average to about three parallel
aligned biphenyl cores. According to X-ray scattering and
optical investigations, the layer structure is maintained in all
three mesophases. Therefore, the phase transitions should
result from a reorganization of the biphenyl cores within the
PR layers.
Let us at first consider the biaxial smectic phase SmAb
shown in Figure 2 f. Here, the biphenyl cores should adopt
long-range orientational order within the PR sublayers, and
the individual layers are orientationally correlated with each
other, so that optical biaxiality of the bulk sample results.
Hence, this biaxial smectic phase is built up by a regular
sequence of thin layers of the ordered PR cores, separated by
the layers of the lateral chains. Since only orientational order
of the PR cores exists within the layers, this phase can be
regarded as a nematic phase which is laminated parallel to the
long axis of the calamitic molecules.
which indicates the fluid state of this mesophase. Additionally,
the small-angle region contains three sharp equidistant
reflections that indicate the presence of a well-defined layer
structure. The layer thickness is d ¼ 3.41 nm at T¼ 1378C and
is nearly temperature independent. No change of the layer
thickness can be detected at the phase transitions. However,
the layer spacing significantly increases with elongation of the
lateral alkyl chain (2: d ¼ 3.0 nm, 4: d ¼ 3.8 nm). The most
remarkable feature of the X-ray diffraction pattern of well-
oriented samples shown in Figure 3[17] is that the diffuse wide-
angle scattering forms a ring (D ¼ 0.49 nm) with a maximum
intensity at the meridian, that is, in the same direction as the
layer reflection. This diffraction pattern is completely differ-
ent from those of conventional smectic A phases, for which
crescentlike halos are located perpendicular to the direction
of the layer reflections.
The microscopic observations together with the peculiar-
ities of the X-ray pattern led to the models of the mesophase
structures shown in Figure 2c, f, and i. Accordingly, the layer
structure should be the result of segregation of the nonpolar
lateral chains from the bolaamphiphilic PR parts (biphenyl
units and terminal diol groups). The layers are formed parallel
to the molecular long axes, and this means that, in contrast to
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At the transition to the high-temperature mesophase SmA
either the long-range parallel alignment of the rigid cores
within the PR layers or the orientational correlation between
adjacent layers may be lost. It is, however, most likely that the
two processes occur simultaneously. This means that the
formation of a long-range orientational order within the
individual layers at the transition from the uniaxial SmA to
the biaxial SmAb phase is accompanied by the formation of a
long-range orientational correlation between adjacent layers.
Hence, the phase structure of this SmA phase is related to
those of conventional SmA phases, with the difference that in
this new phase the biphenyl cores should be aligned on
average parallel to the layer planes (see Figure 2c).[18]
of the classical 3D mesophases. Hence, it could be expected
that a diversity of different higher-order structures related to
those known from conventional smectic phases could also be
obtained within the layers of these laminated mesophases.
Additional new mesophases could be expected at the
transition between the columnar phases of compounds 1 and
the smectic phases of 2 4, and as a result of the positional
correlation between adjacent layers of the laminated smectic
phases, and this would lead to numerous novel mesophases
with two- and three-dimensional organization.[24]
Received: June 24, 2002 [Z19590]
The transition from the SmAb phase to the low-temperature
phase is characterized by a reduction in the fluidity, as
indicated by the transition from a schlieren texture to a
paramorphotic mosaiclike texture. This means that additional
order should occur at this phase transition. In analogy to the
phase sequence Iso-N-SmA, which is often observed in
conventional LC systems on decreasing the temperature, it
can be assumed that in the low-temperature mesophase of 3
the bolaamphiphilic cores adopt a positional order within the
PR sublayers. This results from the segregation of the
hydrogen-bonding networks from the aromatic cores which
leads to a smecticlike organization within these sublayers (see
Figure 2i). Hence, microsegregation occurs in two distinct
directions: Segregation of the nonpolar chains from the PR
cores leads to the ™bulk∫ layer structure, and segregation of
polar and aromatic subunits within the PR layers gives rise to
an additional periodicity within these sublayers which occurs
parallel to the layer planes. As the wide-angle scattering
remains diffuse in all three mesophases, no additional order
between the aromatic cores (e.g., SmB-like) or within the
sublayers of the lateral chains should occur in this low-
temperature mesophase. In other words, this mesophase can
be regarded as a fluid smectic phase which is laminated
parallel to the molecular long axes by the fluid layers of the
lateral chains. Because we have no particular indication for a
tilted arrangement of the molecules within these quasi-2D
smectic sublayers[19] the simplest possibility for a laminated
SmA structure (LamA)[20] is suggested for this mesophase
(Figure 2i). Surprisingly, however, the additional repeat
distance within the PR sublayers, which is expected to occur
perpendicular to the layer reflection at the equator of the two-
dimensional X-ray pattern, cannot be found in the X-ray
diffraction pattern. This could be explained by assuming that
the electron-density modulation within the 2D smectic sub-
layers is low;[21] hence, the intensity of the corresponding
reflections is very low. Additionally, there is no positional
correlation between adjacent layers, and this means that there
is only an orientational correlation of adjacent layers, but the
individual layers can still slide with respect to each other. This
phase structure has some similarities to the sliding colum-
nar[22] and lamellar columnar mesophases.[23]
[1] J.-M. Lehn, Supramolecular Chemistry, VCH, Weinheim, 1995.
[2] D. Demus, J. Goodby, G. W. Gray, H. W. Spiess, V. Vill, Handbook of
Liquid Crystals, Wiley-VCH, Weinheim, 1998.
[3] J. A. Schrˆter, C. Tschierske, M. Wittenberg, J. H. Wendorff, J. Am.
Chem. Soc. 1998, 120, 10669.
[4] M. Kˆlbel, T. Beyersdorff, X. H. Cheng, C. Tschierske, J. Kain, S.
Diele, J. Am. Chem. Soc. 2001, 123, 6809.
[5] C. Tschierske, Prog. Polym. Sci. 1996, 21, 775.
[6] Selected examples of LC materials with fluorinated chains: a) F.
Guittard, E. Taffin de Givenchy, S. Geribaldi, A. Cambon, J. Fluorine
Chem. 1999, 100, 85; b) H. T. Nguyen, G. Sigaud, M. F. Achard, F.
Hardouin, R. J. Twieg, K. Betterton, Liq. Cryst. 1991, 10, 389; c) T.
Doi, Y. Sakurai, A. Tamatani, S. Takenaka, S. Kusabashi, Y. Nishihata,
H. Terauchi, J. Mater. Chem. 1991, 1, 169; d) S. V. Arehart, C. Pugh, J.
Am. Chem. Soc. 1997, 119, 3027; e) S. Pensec, F.-G. Tournilhac, P.
Bassoul, C. Durliat, J. Phys. Chem. 1998, 102, 52; f) S. Diele, D. Lose,
H. Kruth, G. Pelzl, F. Guittard, A. Cambon, Liq. Cryst. 1996, 21, 603;
g) U. Dahn, C. Erdelen, H. Ringsdorf, R. Festag, J. H. Wendorff, P. A.
Heiney, N. C. Maliszewskyj, Liq. Cryst. 1995, 19, 759; h) V. Percec, G.
Johansson, G. Ungar, J. Zhou, J. Am. Chem. Soc. 1996, 118, 9855; i) A.
Pegenau, X.-H. Cheng, C. Tschierske, P. Gˆring, S. Diele, New. J.
Chem. 1999, 23, 465; k) M.-A. Guillevic, D.-W. Bruce, Liq. Cryst. 2000,
27, 153.
[7] a) X. H. Cheng, C. Tschierske, P. Gˆring, S. Diele, Angew. Chem.
2000, 112, 605; Angew. Chem. Int. Ed. 2000, 39, 592; b) X. H. Cheng,
M. K. Das, S. Diele, C. Tschierske, Langmuir 2002, 18, 6521.
[8] C. Tschierske, J. Mater. Chem. 2002, 11, 2647.
[9] Q.-Y Chen, Z. Y. Yang, C.-X. Zhao, Z. M. Qiu, J. Chem. Soc. Perkin
Trans. 1 1988, 563.
[10] G. Majetch, R. Hicks, S. Reister, J. Org. Chem. 1997, 62, 4321.
[11] V. Van Rheenen, D. Y. Cha, W. M. Hartley, Org. Synth. 1979, 58, 43.
[12] a) N. Miyaura, T. Yanagi, A. Suzuki, Synth. Commun. 1981, 11, 513;
b) M. Hird, G. W. Gray, K. J. Toyne, Mol. Cryst. Liq. Cryst. 1991, 206,
187.
[13] All analytical data are in accordance with the proposed structures; for
1
example, 3: H NMR (400 MHz; [D6]DMSO): d ¼ 9.36 (s, 1H, OH),
7.33 (d, 3J(H,H) ¼ 8.6 Hz, 2H, ArH), 7.28 (m, 2H, ArH), 6.91 (d,
3J(H,H) ¼ 8.4 Hz, 1H, ArH), 6.77 (d, 3J(H,H) ¼ 8.6 Hz, 2H, ArH),
4.84 (brs, 1H, CHOH), 4.58 (m, 1H, CH2OH), 3.76 3.49 (m, 3H,
3
ArCH2OCH)), 3.46 (m, 2H, CH2OH), 2.60 (t, 2H, J(H,H) ¼ 7.2 Hz,
CH2Ar), 2.08 (2H, CH2CF2), 1.75 (m, 2H, CH2); 13C NMR (100 MHz;
[D6]DMSO): d ¼ 156.6, 155.6, 132.7, 131.1 129.4, 127.4, 127.3, 125.1,
115.6, 112.1, 70.0, 69.5, 62.7, 29.8, 20.1; 19F NMR (188 MHz;
[D6]DMSO): d ¼ À82.46 (3F, CF3), À114.40 (2F, CH2CF2), À122.74
(m, 12F, CF2), À123.91 (2F, CF2CF2CF3), À127.32 (2F, CF2CF3);
elemental analysis (%) calcd for C28H21O4Si21: C 40.99, H 2.58; found:
C 40.83, H 2.99.
[14] H. R. Brand, P. E. Cladis, H. Pleiner, Macromolecules 1992, 25, 7223.
[15] T. Hegmann, J. Kain, G. Pelzl, S. Diele, C. Tschierske, Angew. Chem.
2001, 113, 911; Angew. Chem. Int. Ed. 2001, 40, 887.
[16] The texture of the LamA phase of compound 2 which occurs directly
from the isotropic liquid state is not a paramorphotic texture and
therefore typical mosaic and spherulitic textures can be observed.
[17] The alignment is obtained by spreading the samples on a glass
substrate, without applying external electric or magnetic fields.
These results show that the competitive combination of
microsegregation and rigidity is an appropriate way to obtain
exciting new mesophase morphologies that are quite distinct
from conventional mesophases. The individual sheets of these
lamellar arrangements can be regarded as quasi-2D analogues
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[18] A related phase structure should be present in the SmA phases of a
TTF derivative with four branched lateral chains (R. A. Bissell, N.
Boden, R. J. Bushby, C. W. G. Fishwick, E. Holland, B. Movaghar, G.
Ungar, Chem. Commun. 1998, 113.) and was suggested for 1,4,5,8-
tetraalkyl-substituted derivatives of anthracene and anthraquinone (S.
Norvez, F. G. Tournilhac, P. Bassoul, P. Hersonn, Chem. Mater. 2001,
13, 2552).
[19] In this case, which would correspond to laminated SmC or SmCA
structures, the aromatic cores would be tilted with respect to the
direction of the periodicity within the aromatic sublayers, but parallel
to the layer planes of the bulk organization.
N
N
N
N
Mes
Mes
Cl
Mes
Mes
Cl
Ru
PR3
Ru
Ph
Cl
Cl
O
1 (R = Cy)
2 (R = Ph)
4
3 (R = [p-CF3C6H4])
N
N
[20] To distinguish such phases from conventional smectic phases, in which
the calamitic molecules are arranged perpendicular to or tilted with
respect to the layer planes, it is proposed to designate such phases as
laminated mesophases (Lam), whereby a subscript designates the
organization within the ordered sublayers (LamA ¼ laminated SmA
phase, see Figure 2i). In principle, the SmAb and SmA phases of 2 4
can alternatively be described as a laminated nematic phase (LamN)
and laminated isotropic phases (LamIso), respectively, but the phase
symmetry of these phases is the same as in conventional SmAb and
SmA phases, so that this-well established nomenclature should still be
used in these cases.
[21] a) F. Hardouin, H. T. Nguyen, M. F. Achard, A. M. Levelut, J. Phys.
Lett. 1982, 3, L-327; b) B. I. Ostrovskii, Liq. Cryst. 1993, 14, 131.
[22] T. C. Lubensky, C. S. O©Hern in Slow Dynamics in Complex Systems:
Eighth Tohwa University International Symposium, (Ed.: M. Tokuya-
ma, I. Oppenheim), The American Institute of Physics, Woodbury,
NY, 1999, p. 105.
[23] a) A. M. Levelut, M. Ghedini, R. Bartolino, F. P. Nicoletta, F.
Rustichelli, J. Phys. (Paris) 1989, 50, 113; b) R. Ziessel, L. Douce,
A. El-ghayoury, A. Harriman, A. Skoulios, Angew. Chem. 2000, 112,
1549; Angew. Chem. Int. Ed. 2000, 39, 1489.
[24] A crystalline SmG-like organization of aromatic cores was proposed
for the aromatic layers of 1,4,5,8-tetraalkyl-substituted derivatives of
anthracene and anthraquinone.[18b]
Mes
Mes
Cl
N
Cl
Ru
Ph
R
N
R
5 (R = H)
6 (R = 3-Br)
7 (R = 4-Ph)
Scheme 1. Ruthenium-based olefin metathesis catalysts. Mes ¼ 2,4,6-tri-
methylphenyl.
such as olefin cross metathesis (CM) with directly function-
alized olefins.[4] For example, acrylonitrile CM has only been
successful with Schrock©s arylimido molybdenum alkylidene
catalyst[5] and the ether-tethered ruthenium alkylidene deriv-
ative [(H2IMes)(Cl)2Ru CH(o-iPrOC6H4)] (4).[6,7] Phos-
¼
phane-ligated ruthenium catalysts have given poor results
for this transformation,[3a,5c,6c,8,9] except for one report of
efficient CM between purified acrylonitrile and 1-decene
mediated by 1.[10] We have determined that dissociation rates
of ligands are related to catalyst efficiency during CM with
acrylonitrile. On this basis, we have developed a new, highly
efficient ruthenium complex to perform acrylonitrile CM with
unpurified acrylonitrile; this catalyst is the fastest initiator of
any ruthenium-based catalyst reported to date.[11]
A Practical and Highly Active Ruthenium-
Based Catalyst that Effects the Cross
Metathesis of Acrylonitrile**
Previous studies have shown that precatalysts of the type
[L2X2Ru CHR] initiate by dissociating one L-type ligand
¼
before entering the catalytic cycle (Scheme 2);[12] in com-
plexes 1 3, L is a phosphane (PR3), and in complex 4, L is a
tethered ether ligand (iPrO). Importantly, complexes 1 4 all
provide the same propagating species (A and B) after a single
turnover.[13] If either A or B is trapped by L, dissociation of L
must occur before catalysis can continue. The relative affinity
of A and/or B for the olefin in preference to L (i.e., favoring
propagation) controls how long these species remain in the
catalytic cycle. Consequently, the differences in activity
between catalysts 1 4 depend on their rates of initiation and
rebinding of L, both of which can be tuned by the nature of
Jennifer A. Love, John P. Morgan, Tina M. Trnka,
and Robert H. Grubbs*
The N-heterocyclic carbene-coordinated ruthenium ben-
¼
zylidene complex [(H2IMes)(PCy3)(Cl)2Ru CHPh] (1) is a
highly active catalyst for a wide variety of olefin-metathesis
reactions,[1] including those with sterically demanding[2] and
electron-deficient olefins (Scheme 1).[3] In spite of recent
advances, there are several processes that remain challenging,
16]
L.[12b,14
[*] Prof. R. H. Grubbs, Dr. J. A. Love, J. P. Morgan, T. M. Trnka
Arnold and Mabel Beckman Laboratory of Chemical Synthesis
Division of Chemistry and Chemical Engineering
California Institute of Technology
Complexes 1 4 are all active for a variety of metathesis
processes, such as CM, ring-closing metathesis (RCM), and
ring-opening-metathesis polymerization (ROMP). However,
the situation involving acrylonitrile CM is more complex; CM
between acrylonitrile and allylbenzene proceeds efficiently
with 4 (68% yield) but not with 1 3 (21%, 35%, and 36%
yields, respectively; Table 1).[17] Clearly, this difference cannot
simply be an issue of precatalyst initiation; 2 and 4 have
roughly the same initiation rates and initiation is faster with 3
than 1, 2, or 4 Table 2. The difference is also not a result of
Pasadena, CA 91125 (USA)
Fax : (þ 1)626-564-9297
E-mail: rhg@its.caltech.edu
[**] This work was funded by the National Science Foundation. J.A.L.
acknowledges the National Institutes of Health for a postdoctoral
fellowship, and T.M.T. acknowledges the Department of Defense for a
NDSEG graduate fellowship.
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2002, 41, No. 21
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0044-8249/02/4121-4035 $ 20.00+.50/0
4035