Preparation and single crystal structure of a dirhenium(I) complex of a bent-core mesogen

Article abstract of DOI:10.1080/15421406.2011.581121

Dinuclear complexes of ReI are prepared by functionalising a symmetric, bent-core mesogen containing two imine groups. The resulting complex is characterised by single crystal X-ray diffraction and presents the expected geometry, but unfortunately the int

Full text of DOI:10.1080/15421406.2011.581121

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Preparation and Single Crystal Structure
of a Dirhenium(I) Complex of a Bent-
Core Mesogen
Deborah M. Huck a , Laurent Omnès a , Mark E. Light b & Duncan W.
Bruce a
a Department of Chemistry , University of York , Heslington, York
YO10 5DD (UK)
b EPSRC Crystallographic Service, Department of Chemistry ,
University of Southampton , Southampton So17 1BJ (UK)
Published online: 07 Oct 2011.
To cite this article: Deborah M. Huck , Laurent Omnès , Mark E. Light & Duncan W. Bruce (2011)
Preparation and Single Crystal Structure of a Dirhenium(I) Complex of a Bent-Core Mesogen, Molecular
Crystals and Liquid Crystals, 549:1, 19-28, DOI: 10.1080/15421406.2011.581121
To link to this article: http://dx.doi.org/10.1080/15421406.2011.581121
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Mol. Cryst. Liq. Cryst., Vol. 549: pp. 19–28, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.581121
Preparation and Single Crystal Structure of a
Dirhenium(I) Complex of a Bent-Core Mesogen
1
1
`
DEBORAH M. HUCK, LAURENT OMNES,
MARK E. LIGHT,2 AND DUNCAN W. BRUCE1,∗
1Department of Chemistry, University of York, Heslington,
York YO10 5DD (UK)
2EPSRC Crystallographic Service, Department of Chemistry, University
of Southampton, Southampton So17 1BJ (UK)
Dinuclear complexes of ReI are prepared by functionalising a symmetric, bent-core
mesogen containing two imine groups. The resulting complex is characterised by single
crystal X-ray diffraction and presents the expected geometry, but unfortunately the
introduction of the two Re(CO)4 moieties suppresses the mesomorphism of the parent
ligand.
Introduction
Since the report of spontaneous symmetry breaking in bent-core mesogens in 19951 and the
identification of the origin of the chirality soon afterwards, there has been a great deal of
interest in the preparation and properties of large families of bent-core mesogens. Arguably,
bent-core mesogens were reported in the early part of the 20th century by Vorla¨nder, but
more systematic work originated from papers by Matsunaga and co-workers in the early
1990s.2 An insightful review into bent-core materials was published in 2006 by Reddy and
Tschierscke,3 while their implication in the biaxial nematic phase forms part of a recent
critical review by Tschierscke and Photinos.4
Of the variables in bent-core materials, the literature shows that the compounds are
sensitive (of course) to their chemical composition and subtle changes thereof (e.g. the
direction of esters or imine functions), to the angle at the ‘elbow’ of the compound and
to dipole moment. Indeed, the combination of bend angle and dipole moment are crucial
factors in discussions of the NB phase of bent-core materials.5
However, another factor that can be of significance is shown by some of our work on
mesogenic terpyridines. We prepared examples (Figure 1) of 5,5ꢀ-disubstituted terpyridines
(4) using inverse electron demand Diels-Alder chemistry with 1,2,4-triazine substrates (3)
prepared from hydrozone oximes (1) and pyridine-2,6-dicarboxaldehyde (2). Compounds 5,
which are derivatives of 4, were found to show one of the SmCaPA or SmCsPF (B2) phases,
as optical microscopy showed them to exist in a chiral mesophase. However, compound 6,
which is also obtained from 3, showed an entirely different behaviour, exhibiting both a
SmC and a N phase. While there was a small difference in the bend angle between 5 and
∗Corresponding author. E-mail: duncan.bruce@york.ac.uk
19

20
D. M. Huck et al.
Figure 1. Synthetic route to mesomorphic, bent-core terpyridines 5 and 6.
6, it is likely somehow that ‘filling in’ of the elbow is a significant factor in changing the
mesomorphism.6
Such effects can be seen in other ways, too, so that long-chain derivatives of 4 are
not mesomorphic, while on complexation to ZnII the resulting material shows a columnar
mesophase.7 Similarly, related hexacatenar 1,3-dipyridylbenzenes (7) are not liquid crys-
talline, while their chloroplatinum(II) complexes (8) are.8 Here, it seems that rigidification
(from complexation) and the introduction of a dipole moment (reorientation of the pyridine
nitrogen atoms on complexation) contribute to the induction of mesomorphism. However,
it is possible for rigidification and even dipole moment to work in different ways for,

Preparation and Single Crystal Structure
21
as stated, long-chain analogues of 4 are not liquid crystalline and yet, as five-ring, bent-
core molecules, mesomorphism could reasonably be expected. However, as we reported,
preparation of the ester derivatives, 5, leads to materials showing one of the chiral B2
modifications. Addition of the ester function adds both a dipole moment and flexibility and
mesomorphism is induced.
In considering the incorporation of metals, the study of metallomesogens in the last
25 years has shown the possibilities offered by different metal-ligand combinations, and
these are collected in several reviews.9 Many metals have been used in this way and all
major areas of the periodic table are represented. For example, they are known to affect
linear polarisability and birefringence, to introduce paramagnetic properties, to modulate
photophysical and mesophase properties due to intermolecular interactions, to be emissive
and to act as templates for heterogeneous catalysts.10 Therefore, it is of interest to consider
the possibilities offered if a metal were incorporated into a bent-core structure.
Results and Discussion
Metallomesogenic, bent-core materials are little studied. Abe et al.11 reported 4,4’-
disubstituted derivatives of metal-salen complexes, but the core of these materials is too
small to be considered alongside classical bent-core materials. Similarly, Pucci et al.12
reported on zinc(II) complexes (10) of dimeric salicylaldimato ligands linked by a flexible
spacer (Figure 2). Sepelj et al.13 had reported the mesomorphic properties of the isolated
ˇ
ligands (9) and had shown that when the number of linking methylene groups, m, was
an even number, the compounds behaved as conventional mesogens showing SmC and N
phases, while when m was odd, the behaviour changed markedly and Colr and Ncol phases
were seen—behaviour often associated with bent-core materials.
Figure 2. Flexibly linked salicylaldimino ligands (9) and their complexes with ZnII (10).

22
D. M. Huck et al.
Figure 3. Proposed arrangement of Zn salicylaldimine complexes in the SmCintercal (B6) phase.
Reproduced with permission from reference 12; copyright Wiley-VCH Verlag GmbH & Co. KGaA.
When complexed to tetrahedral ZnII, it was proposed that the flexible linker was held in
a fly-over arrangement and that the whole complex had an overall bent structure, consistent
with the observation of a SmCintercal phase, Figure 3 (also known as a B6 phase), although
the extensive hysteresis at the clearing point does give some cause for concern in attributing
the thermal behaviour.12
Some time ago, we had shown that the Group 7 elements Mn and Re could react with
benzylideneanilines as their [MMe(CO)5] complexes to give ortho-metallated tetracarbonyl
complexes that were mesomorphic (Figure 4). Design of these complexes required that the
Figure 4. Three- and four-ring orthometallated imine complexes of MnI and ReI.

Preparation and Single Crystal Structure
23
imine ligands were sufficiently anisotropic to compensate for the destabilising effect of
the [M(CO)4] moiety, so that four-ring imine ligands led to complexes with enantiotropic
mesomorphism, while three-ring ligands resulted in monotropic behaviour.14
Imine links are also a common feature of bent-core mesogens, and so in the light of the
discussion above, it was of interest to attempt the preparation of bent-core, imine-containing
materials and to investigate the possible ortho-metallation using rhenium (which, in general,
gives more thermally stable complexes compared to manganese). The ligand identified is
compound 12, whose preparation is shown in Scheme 1.
Scheme 1. Preparation of the target complexes. i) DCC/DMAP/CH2Cl2, rt, N2; ii) H+ (cat.),
toluene/rt/N2; iii) toluene (anhyd.), N2, reflux.
While the ligand is a known material prepared by esterification of resorcinol with
the appropriate carboxylic acid,15 in this work we obtained the material in a different
way in order to allow for subsequent convergent synthesis of analogues. Thus, resorcinol
was esterified with benzaldehyde-4-carboxylic acid using a DCC/DMAP coupling to give
aldehyde 11, which was then condensed with hexyloxyaniline under acid catalysis to give

24
D. M. Huck et al.
target ligand 12. We were able to confirm the published mesomorphism of 12 as:
Cr • 144 • B4 • 159 • B1 • 174 • Iso
Ligand 12 was then doubly metallated by reaction with [ReMe(CO)5] (itself from
[Re2(CO)10] by reaction with Na/Hg and then MeI16) in anhydrous toluene under re-
flux and the yellow/orange product was obtained by separation from excess of ligand using
column chromatography in dichloromethane. Crystallisation was achieved from the same
solvent by layering hexane on top and gave the product, 13, in 69% yield for this last step.
Crystal and Molecular Structure of 13
Careful crystallisation of 13 from CH2Cl2/hexane yielded crystals that were analysed using
¯
single-crystal X-ray crystallography. The complex crystallised in the P1space group with
an R-factor that is acceptable, particularly given the fact that the data were acquired at
ambient temperature. Essential crystallographic data are collected in Table 1.
Table 1. Crystallographic data for 13
CCDC Deposition Number
Empirical formula
806600
C54H46N2O14Re2
1319.33
Formula weight/g mol−1
Temperature/K
293(2)
Wavelength/Å
0.71073
Crystal system
Triclinic
Space group
P-1
Unit cell dimensions (Å)/angles (◦)
a = 13.6293(10) Å
b = 14.9561(12) Å
c = 16.6900(18) Å
2773.3(4)
2
1.580
4.424
1292
α = 111.817(5)◦
β = 91.913(4)◦
γ = 115.516(3)◦
Volume/Å3
Z
Density (calculated)/Mg m−3
Absorption coefficient/mm−1
F(000)
Crystal size/mm3
0.20 × 0.10 × 0.05
θ range for data collection/◦
Index ranges
2.92 − 24.71
−16 ≤ h ≤ 16, −17 ≤ k ≤ 16, −19 ≤ l ≤ 19
23336
Reflections collected
Independent reflections
Completeness to θ = 24.71◦
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [F2 > 2σ(F2)]
R indices (all data)
9284 [Rint = 0.1141]
98.2%
Multiscan, SORTAV
0.8091 and 0.4715
Full-matrix least-squares on F2
9284/266/627
0.937
R1 = 0.0623, wR2 = 0.1279
R1 = 0.1711, wR2 = 0.1687
1.363 and −0.951
Largest diff. peak and hole/e Å−3

Preparation and Single Crystal Structure
25
Figure 5. Three views of the molecular structure of complex 13. Views A and B are general views,
while view C shows short intermolecular contacts.
The structure (Figure 5) shows clearly the incorporation of two ReI centres and, within
the bounds of statistical factors, the two centres are identical in terms of bond lengths
and angles. The rings of the complex are not co-planar. Thus, the central benzene ring
makes angles of 38.11◦ and 33.82◦ with the adjacent rings to which it is joined via ester
functionality and these are in opposite senses so that one Re centre appears above the
plane of the central resorcinol ring, while the other appears below it. The two rings that are
involved in the metallation make a mutual angle of 47.10◦ at Re(1) and 43.86◦ at Re(2).
Statistically, there is no change in the Re–CO bond lengths that depends on whether the
carbonyl is trans to nitrogen or carbon.
In terms of intermolecular contacts at the level of 0.1 Å less than the sum of the van
der Waals’ radii, contacts (3.277 Å) are identified between the carbonyl oxygen of an ester
function and an imino hydrogen, although it is difficult to say with any certainty if these

26
D. M. Huck et al.
are in any way structure directing. This interaction is shown in Figure 5C. In the pair of
molecules shown in this figure, the two benzene rings in the centre of the figure, one from
each complex, are formally parallel with a plane-to-plane separation of 3.407 Å, although
there is no evidence for intermolecular π-π interactions. These planes do not perpetuate in
the structure.
Thermal Behaviour
Unfortunately, complex 13 proved not to be mesomorphic and heating the crystalline solid
resulted in a melting event to an isotropic fluid at 98 ◦C. Cooling this liquid either slowly or
quickly showed no evidence of a birefringent texture, even under mechanical disturbance,
although the high viscosity of the fluid at lower temperatures suggested the formation of a
glass.
Thus, the disruption of the already reduced anisotropy of the ligand (most mesogens
can be approximated to be C∞v, while bent-core mesogens are approximated as C2v) by
the inclusion of two, octahedral centres is incommensurate with forming an organised,
fluid structure. At the present time it is unclear whether preparation of an unsymmetric
ligand containing a single metallated octahedral rhenium centre would support meso-
morphism or whether a much more extended structure would be necessary. However, it
is also not entirely clear whether any disruption to the bent-core motif would be toler-
ated, for example by the introduction of a square-planar metal centre such as that which
may be obtained using PdII or PtII, marking this area as one required detailed further
investigation.
Experimental
X-Ray Structure Determination
The diffractometer was a Nonius Kappa with CCD area detector, using ϕ scans and ω
scans to fill the asymmetric unit sphere. The cell determination used DirAx software,17
while data collection used Collect software.18 Data reduction and cell refinement employed
Denzo,19 while absorption corrections used SORTAV.20 Structure solution and refinement
used SHELXS9721 and SHELXL97,22 respectively.
Compound 11
1,3-Dicyclohexylcarbodiimide (DCC) (6.71 g, 32.7 mmol) was dissolved in CH2Cl2
(100 cm3) to give a colourless solution. To this, a solution of resorcinol (3.00 g, 27.2 mmol)
dissolved in THF (22 cm3) was added followed by 4-formylbenzoic acid (7.55 g, 50.3 mmol)
giving an off-white milky solution due to the incomplete dissolution of the benzaldehyde.
Finally 4-(N,N-dimethylamino)pyridine (DMAP) (0.79 g, 6.1 mmol) was added giving a
slightly exothermic reaction. The mixture was stirred at room temperature under nitrogen
for 15 h giving a cloudy white precipitate, which was recovered by filtration. The product
was extracted into hot DCM in which the excess starting materials are insoluble and then
crystallised from THF to give the product 11 in a yield of 36.5% (3.71 g, 9.9 mmol).
Synthesis of Compound 12
Compound 11 (0.50 g, 1.30 mmol) and distilled 4-hexyloxyaniline (0.52 g, 2.7 mmol) were
placed in a flask along with toluene (100 cm3) in which the 4-hexyloxyaniline dissolved

Preparation and Single Crystal Structure
27
straight away to give a brown solution. The mixture was heated gently for 10 minutes to
dissolve 11 and acetic acid (glacial, 2 drops) was added. The mixture was stirred at room
temperature under nitrogen for 15 hours. After cooling in the fridge, an off-white solid
precipitated and was recovered by filtration and then crystallised from DCM and ethanol in
a 4:1 ratio (30 cm3). A second crystallisation was performed from the same solvent giving
compound 12 in a yield of 11.7% (0.11 g, 0.15 mmol).
CHN % Expected (% Obtained) C: 76.2 (75.8) H: 6.7 (6.7) N: 3.9 (3.6)
Synthesis of Compound 13
Compound 12 (0.093 g, 0.13 mmol) and methylpentacarbonylrhenium(I) (0.88 g, 0.26
mmol) were dissolved in anhydrous toluene (20 cm3) and heated at reflux under N2 for 15 h.
The solvent of the resulting brown solution was evaporated off leaving a yellow/orange solid.
1H-NMR spectroscopy indicated the presence of excess 12. A chromatography column
using CH2Cl2 was performed with the first band being the product. This was further
purified by precipitation where the solid was dissolved in CH2Cl2 (1 cm3) and a layer of
hexane (3 cm3) was added. The two layers were left to mix slowly and were placed in the
fridge for 3 d to ensure full precipitation. Orange needle-like crystals of compound 13 were
filtered off in a yield of 69.2% (0.12g, 0.09 mmol). It is accepted that the N analysis below
is outside of normal limits, but both C and H are within and all other data are consistent
with a pure material.
1H NMR (CDCl3) δ: 0.84–0.93 (6H, m, H1), 1.22–1.52 (8H, m, H2, H3) 1.76–1.81 (8H,
m, H4, H5), 4.00 (4H, t, 3JHH = 8.3 Hz, H6), 6.97 (4H, AAꢀXXꢀ system, J = 7.92 Hz, H7),
7.20–7.28 (7H, m, H8, H14, H15), 7.53 (1H, m, H13), 7.8 (2H, d, 3JHH = 7.97 Hz, H10), 7.96
(2H, dd, 4JHH = 1.90 Hz, 3JHH = 7.97 Hz, H11), 8.64 (2H, s, H9), 7.74 (2H, d, 4JHH = 1.90
Hz, H12).
% Expected (% Obtained) C: 49.2 (49.0) H: 2.9 (3.2) N: 3.9 (3.0)
Acknowledgements
We are grateful to the EU and NEDO for funding (LO) and H.C. Stark for a generous gift
of ammonium tetraoxorhenate(VII).
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