Implementing liquid-crystalline properties in single-stranded dinuclear lanthanide helicates

Article abstract of DOI:10.1002/ejic.201300176

The connection of flexible protodendritic wedges to the bis-tridentate rigid polyaromatic ligand L1 provides amphiphilic receptors L5 and L6; their reduced affinities for complexing trivalent lanthanides (Ln = La, Y, Lu) in organic solvent (by fifteen orders of magnitude!) prevent the formation of the expected dinuclear triple-stranded helicates [Ln₂(Lk) ₃]⁶⁺. This limitation could be turned into an advantage because L1 or L6 can be treated with [Ln(hfac)₃] (Hhfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione) to give neutral single-stranded [Ln₂(Lk)(hfac)₆] complexes with no trace of higher-order helicates. Whereas ligands L1 and L5 are not liquid crystals, L6 can be melted above room temperature (41°C) to give a nematic mesophase, and its associated dinuclear helical complex [Y₂(L6)(hfac)₆] self-organises at the same temperature into a fluidic smectic mesophase. The lipophilic dendritic ligand L6 selectively reacts with trivalent yttrium hexafluoroacetylacetonate (hfac) to give the liquid-crystalline single-stranded dinuclear helicate [Y₂(L6)(hfac)₆], which self-organises into an SmA mesophase. Copyright

Full text of DOI:10.1002/ejic.201300176

FULL PAPER
DOI:10.1002/ejic.201300176
Implementing Liquid-Crystalline Properties in Single-
Stranded Dinuclear Lanthanide Helicates
Emmanuel Terazzi,*[a] Amir Zaïm,[a] Bernard Bocquet,[a]
Johan Varin,[a] Laure Guénée,[b] Thibault Dutronc,[a]
Jean-François Lemonnier,[a] Sébastien Floquet,[c] Emmanuel Cadot,[c]
Benoît Heinrich,[d] Bertrand Donnio,[d] and Claude Piguet*[a]
COVER PICTURE
Keywords: Liquid crystals / Metallomesogens / Polynuclear complexes / Amphiphiles / Helical structures / Lanthanides
The connection of flexible protodendritic wedges to the bis-
tridentate rigid polyaromatic ligand L1 provides amphiphilic
receptors L5 and L6; their reduced affinities for complexing
trivalent lanthanides (Ln = La, Y, Lu) in organic solvent (by
fifteen orders of magnitude!) prevent the formation of the ex-
pected dinuclear triple-stranded helicates [Ln2(Lk)3]6+. This
limitation could be turned into an advantage because L1 or
L6 can be treated with [Ln(hfac)3] (Hhfac = 1,1,1,5,5,5-hexa-
fluoro-2,4-pentanedione) to give neutral single-stranded
[Ln2(Lk)(hfac)6] complexes with no trace of higher-order
helicates. Whereas ligands L1 and L5 are not liquid crystals,
L6 can be melted above room temperature (41 °C) to give a
nematic mesophase, and its associated dinuclear helical com-
plex [Y2(L6)(hfac)6] self-organises at the same temperature
into a fluidic smectic mesophase.
metallic cations with [Xe]4fn configurations.[8] Roughly
speaking, most thermotropic liquid crystals (i.e., liquid
crystals that self-organise with temperature) are produced
by connecting long and flexible aliphatic chains to various
rigid polarisable aromatic cores.[9] Optimisation of the inter-
molecular interactions in the crystalline state results in a
microsegregation process, in which the rigid cores are
packed together, whereas the less polarisable flexible alkyl
chains fill the residual voids of the structure.[10] Upon heat-
ing, the decorrelation between the flexible alkyl chains pro-
duces a molten continuum (assigned to the melting pro-
cess), with the residual interactions between aromatics en-
suring the stability of the liquid-crystalline phase. At higher
temperature, the interactions between the aromatic units be-
come smaller than the thermal energy and a classical liquid
is obtained (assigned to the isotropisation process). This
standard two-step melting model satisfyingly predicts the
thermal behaviour of low-molecular-weight amphiphilic or-
ganic molecules that possess considerable anisometric
shapes.[10,11] In metallomesogens, the isotropic three-dimen-
sional expansion brought by the coordination spheres of
the metals usually disrupts the microsegregation and desta-
bilises the liquid-crystalline phase. Except for complexes
with linear or square-planar arrangements that are reminis-
cent of the molecular shapes found in purely organic liquid
crystals,[7,12] common coordination geometries are difficult
to combine with the structural anisotropy required for in-
ducing mesomorphism and liquid-crystalline phases are not
formed. In this context, only a few metallomesogens con-
tain a tetrahedral[13] or an octahedral metal centre,[14] and
Introduction
For a long time, lanthanide supramolecular chemistry fo-
cused on the design and on the thermodynamic rationalis-
ation of discrete sophisticated polynuclear molecular archi-
tectures such as helicates,[1] cages,[2] clusters[3] and grids.[4]
Owing to the remarkable magnetic and optical properties
associated with the 4f-centred open-shell electronic configu-
rations, these compounds were approached for overcoming
modern technological limitations, but the exploitation of
their intrinsic properties for lighting, sensing and switching
further requires specific patterns gained by the self-organis-
ation of amphiphilic receptors connected to the inorganic
cores.[5,6] When the organic coating induces liquid-crystal-
line properties, the resulting metal-containing liquid crys-
tals are referred to as metallomesogens,[7] or more specifi-
cally, as lanthanidomesogens for those that incorporate
[a] Department of Inorganic and Analytical Chemistry, University
of Geneva,
30 quai E. Ansermet, 1211 Geneva 4, Switzerland
E-mail: Emmanuel.Terazzi@unige.ch
Claude.Piguet@unige.ch
Homepage: http://www.unige.ch/sciences/chiam/piguet/
Welcome.html
[b] Laboratory of Crystallography, University of Geneva,
24 quai E. Ansermet, 1211 Geneva 4, Switzerland
[c] Institut Lavoisier de Versailles, UMR 8180,
University of Versailles Saint-Quentin,
45 av. des Etats-Unis, 78035 Versailles, France
[d] Institut de Physique et Chimie des Matériaux de Strasbourg
(IPCMS), UMR 7504 (CNRS – Université de Strasbourg),
23 rue du Loes, BP43, 67034 Strasbourg cedex 2, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300176.
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those with coordination number (CN) 7–12, typical of large The conditions required for the preparation of stable di-
lanthanide cations, have remained challenging for some nuclear lanthanidomesogens are explored together with
time.[5,15] Pioneering work dedicated to fullerodendri- their liquid-crystalline properties.
mers[16] established that mesomorphism could be induced
when the bulky spherical cores were coated with divergent
Results and Discussion
polarised dendritic architectures, a strategy that led to the
preparation of a unique discotic dinuclear lanthanidomeso-
gen.[15d] To the best of our knowledge, there is no other
report of multinuclear mesomorphic analogues despite a
rich catalogue of polynuclear linear triple-stranded helicates
such as [Ln2(L1)3]6+, [Ln3(L2)3]9+ and [Ln4(L3)3]12+, the cy-
Synthesis of Ligands Lk (k = 5, 6) and Formation of the
Triple-Stranded Complexes [Ln2(Lk)3(CF3SO3)6] (Ln = La,
Y, Lu; k = 5, 6) in Solution
Alkylation of intermediate 11 with 15 gave the target li-
lindrical rigid cores of which make them ideal for the design gand L5, whereas esterification with 16[15i] yielded L6
of calamitic metallomesogens.[1]
(Scheme 1 and Appendix 1 in the Supporting Information).
However, by following this strategy, lipophilic dinuclear
The five different 1H NMR spectroscopic signals de-
helicates with d-block metals [Cu2(L4n)2]2+ (n = a, b, c) tected for the aromatic protons Hb–Hf in Lk (k = 5, 6),
have been shown to self-organise into columnar meso- combined with the observation of a singlet for the protons
phases.[13b,17] Interestingly, the connection of the long lipo- Ha and of a quartet for the protons Hm, confirmed the
philic and diverging alkyl chains to the cylindrical core existence of dynamically averaged C2v symmetries for the
drastically limited the stability of these complexes in solu- free ligands in solution (see Scheme 1 for numbering). The
tion, an observation that might explain the paucity of lack of a nuclear Overhauser enhancement (NOE) effect be-
helical scaffolds in metallomesogens. To extend this ap- tween the ethyl residues of the benzimidazole rings and the
proach to magnetically and optically active 4f-block cat- protons connected at the 3-positions of the pyridine rings
ions, we connect here two different lipophilic protomeso- agreed with a standard trans arrangement of the N-donor
morphic dendrons perpendicularly to the helical axis in 11 atoms borne by the adjacent aromatic rings. This arrange-
through ether links (L5) or ester bonds (L6) (Scheme 1). ment was confirmed by analysis of the solid-state structure
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Scheme 1. Synthesis and structures of bis-tridentate ligands L5 and L6, derived from 11 (with numbering scheme for NMR spectra).
of intermediate 10 (Appendix 1 in the Supporting Infor- for the diastereomeric methylene protons of the ethyl resi-
mation). Reaction of the bis-tridentate ligand L6 (3 equiv.) dues (Hm), whereas the singlet (A2 spin system) observed
with [Y(CF3SO3)3]·4H2O (1 equiv.) in CD2Cl2 provided a for the enantiotopic methylene protons (Ha) of the di-
temperature-independent broad 1H NMR spectrum (Fig- phenylmethane spacer points to the existence of three two-
ure S2a in the Supporting Information) that indicates weak fold axes perpendicular to the threefold axis in line with the
interactions between the binding unit and the central dia- regular D3-symmetrical triple-helical arrangement of the ex-
magnetic Y3+ cation, which leads to the formation of intri- pected triple-stranded helicate [Y2(L5)3]6+ (Figure 1, a).
cate mixtures of complexes in intermediate exchange on the Upon addition of increasing amounts of CD2Cl2, the sig-
NMR spectroscopic timescale. The use of more polar sol- nals of the complex were broadened because of partial li-
vents (CDCl3, CD3NO2, [D8]THF; Figure S2b) did not im- gand dissociation and slow exchange processes (Figure 1b–
prove the situation and the ESI-MS spectrum of the colour- h and Figure S3 in the Supporting Information), a trend
less mixture only showed the presence of the protonated that was confirmed by the decreasing ratio of the peaks
free ligand [L6 + H]+. Since the bis-tridentate ligand L1 is detected by ESI-MS spectra for [Y2(L5)3(CF3SO3)n](6–n)+
known to produce very stable triple-helical complexes and [L5 + H]+, respectively. We conclude that the reduced
L1
[Ln2(L1)3](CF3SO3)6 in acetonitrile [log(β ≅ 25)],[18] lipophilicity in L5 limits the solvation of the free ligand to
2,3
this deleterious effect can be tentatively assigned to the con- such an extent that [Y2(L5)3]6+ can be obtained in polar
siderable lipophilicity of L6, which is efficiently solvated in organic solvents at millimolar concentrations [see Equilib-
organic solvents. Upon reaction of the less lipophilic ligand rium (1) below].
L5 (3 equiv.) with [Y(CF3SO3)3]·4H2O (1 equiv.) in CD2Cl2/
If we take the integral Igh of the 1H NMR spectral singlet
CD3CN = 1:4, the six metal-shifted 1H NMR spectroscopic recorded for the peripheral aromatic protons Hg,Hh as a
signals observed for the aromatic protons Hb–Hh were di- probe for calibrating the total ligand concentration in each
agnostic for overall threefold symmetry.[19] The helical ar- mixture (|L5|tot = |L5| + 3|[Y2(L5)3]|), the stability constants
Ln,L5
rangement of the strands was substantiated by the detection
β
for Equilibrium (1) can be estimated by integration
2,3
of pseudo-sextets [ABX3 spin systems for which 2J = 2·(3J)]
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Figure 1. 1H NMR spectra with numbering scheme of the diamagnetic triple-stranded helicate [Y2(L5)3]6+ at different VCD Cl /VCD CN
2
2
3
volume; * = CDHCl2 (total ligand concentration |L5|Tot ≅ 0.01 m, 298 K).
of the signals recorded for proton b (Ib) in each solvent the Supporting Information).[18] These thermodynamic data
mixture (Table 1).
unambiguously demonstrate that the efficient solvation of
ligand Lk (k = 5, 6) in organic solvents severely limits their
affinity for trivalent lanthanides, but the presence of a sig-
Ln,L5
3L5 + 2Ln(CF3SO3)3 h [Ln2(L5)3](CF3SO3)6 β
(1)
2,3
The stability constants estimated for [Ln2(L5)3](CF3- nificant amount of CH2Cl2 is crucial for the solubilisation
SO3)6 are more than fifteen orders of magnitude smaller of these ligand. These two conflicting requirements prevent
than those found for the parent nonlipophilic complexes the isolation of pure triple-stranded lipophilic helical lan-
[Ln2(L1)3](CF3SO3)6 in pure acetonitrile (Figures S4–S9 in thanide complexes, which results from the successive
Ln,L5
1
Table 1. Experimental cumulative formation constants log(β
) obtained by H NMR spectroscopy according to Equilibrium (1) (Ln
2,3
= La, Y and Lu; total concentration of ligand |L5|tot = 0.01 m).[a]
La,L5
Y,L5
Lu,L5
VCD Cl /VCD CN
log(β
)
log(β
)
log(β
2,3
)
2,3
2,3
2
2
3
[La2(L5)3](CF3SO3)6
[Y2(L5)3](CF3SO3)6
[Lu2(L5)3](CF3SO3)6
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
8.8(1)
8.3(1)
7.4(1)
7.1(2)
6.4(4)
5(1)
10.1(1)
9.8(2)
9.6(2)
9.1(1)
8.9(1)
8.3(1)
7.3(2)
7.1(3)
9.5(2)
8.4(1)
7.7(1)
7.1(2)
6.5(4)
4.4(4)
5.9(8)
[b]
–
[b]
[b]
–
–
[a] The standard deviations are calculated for integration uncertainties of 5%. [b] The signals of the complexes are too weak to be
detected.
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fixation of three tridentate binding unit about each metal. of the equatorial plane and the third roughly perpendicular
However, this drawback can be turned into an advantage (81.4–84.1°) to the latter plane. This results in pseudo-C2v
with the lipophilic ligand L6, which possesses the structural symmetry around the lutetium ion, an arrangement that is
criteria for designing lanthanidomesogens. Indeed, the re- compatible with the description of the nine-coordinate co-
placement of poorly coordinating triflate anions with hexa- ordination spheres as distorted mono-capped square anti-
fluoroacetylacetonate (hfac) anions should strictly limit the prisms with N1 (Lu1) or N5 (Lu2) occupying the capping
affinity of the central metal to a single tridentate binding positions (Figure S10 and Tables S7–S8 in the Supporting
unit, as found in [Ln(Lk)(hfac)3] (k = 7, 8),[20] thus leading Information). The average distances Lu1–O = 2.34(5) Å,
to the formation of the single-stranded [Ln2(L6)(hfac)6] Lu1–N = 2.477(4) Å, Lu2–O = 2.35(5) Å and Lu2–N =
complexes with no possible mixing with higher-order 2.502(8) Å (Table S5 in the Supporting Information) are
helicates.
close to those found in the mononuclear analogues
[Lu(Lk)(hfac)3] (k = 7, 8),[20] an observation that is corrob-
orated by the calculated bond valences (Table S9 in the Sup-
porting Information).[21] The helicity of the diphenylmeth-
ane spacer, defined as a five-atom crooked line (C15–C14–
C20–C21–C27) amounts to H = 0.55,[22] a value that is in
line with H = 0.76 reported for the analogous single-
stranded complex [Eu2(L1)(NO3)6(H2O)2].[23]
Monitoring the titration of L6 with [Y(hfac)3(diglyme)]
in CDCl3 by using 1H NMR spectroscopy showed the step-
wise formation of the complexes [Y(L6)(hfac)3] and
[Y2(L6)(hfac)6] according to Equilibria (2) and (3) (Fig-
ures S11–S17 in the Supporting Information). The broad-
Synthesis and Structure of the Single-Stranded Complexes
[Ln2(Lk)(hfac)6] (k = 1, 6; Ln = Y, Lu)
Reaction of the C2v-symmetrical bis-tridentate ligand L1
(1 equiv.) with [Lu(hfac)3(diglyme)] (2 equiv.) in acetonitrile
followed by the slow diffusion of diethyl ether yielded small
colourless crystals suitable for X-ray analysis (Tables S4–
S6 in the Supporting Information). No standard hydrogen
bonds or intermolecular stacking interactions could be
found, and the molecular structure confirmed the coordina-
tion of one LuIII ion to each cis–cis tridentate segment of
ligand L1, thus leading to an intramolecular intermetallic
Lu···Lu separation of 12.533(1) Å (Figure 2). Each LuIII
cation is almost held within the equatorial plane defined by
the three donor atoms of each tridentate binding unit (O1,
N1, N3 for Lu1 and O2, N5, N7 for Lu2; deviations of the
Lu atom from the planes: 0.009 and 0.018 Å, respectively).
Each coordination sphere is completed with three didentate
1
ened H NMR spectroscopic signals implied the operation
of intermolecular exchange processes with an intermediate
rate on the NMR spectroscopic timescale but the concomi-
tant observation of a singlet for the protons of the three
complexed hfac anions indicated fast intramolecular ex-
change between axial and meridional positions in
[Y(L6)(hfac)6] and [Y2(L6)(hfac)6].
Y(hfac)3,L6
L6 + [Y(hfac)3(diglyme)] h [Y(L6)(hfac)3] + diglyme β
1,1
(2)
Y(hfac)3,L6
L6 + 2[Y(hfac)3(diglyme)] h [Y2(L6)(hfac)6] + 2diglyme β
2,1
(3)
1
A thorough analysis of the H NMR spectroscopic sig-
hfac anions, with two of them being located on both sides nals for the YIII/L6 = 2.0 ratio in chloroform revealed the
Figure 2. Perspective view with numbering scheme of the molecular structure of [Lu2(L1)(hfac)6]. Ellipsoids are represented at 50%
probability level. The labels for hfac and the hydrogen atoms are omitted for clarity.
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1
Figure 3. H NMR spectrum with numbering scheme of the diamagnetic complex [Y2(L6)(hfac)6] (CDCl3, 298 K); * = CHCl3.
existence of a single complex in solution with a pattern of
five signals for the aromatic protons Hb–Hf (twofold sym-
metry) and the presence of a singlet for the enantiotopic
methylene protons Ha (planar symmetry) that are diagnos-
tic of an average C2v-symmetrical arrangement of the coor-
dinated ligand strand in [Y2(L6)(hfac)6] (Figure 3).
Nonlinear least-squares fit[24] of the dynamically
averaged 1H NMR spectroscopic titration data provided
complete sets of chemical shifts for [Y(L6)(hfac)6] and
[Y2(L6)(hfac)6] (Table S10 in the Supporting Information)
Y(hfac)3,L6
together with cumulative stability constants log[β
]
1,1
= 6(2) and log[βY2,(1hfac) ,L6] = 11(2), which agreed with
3
log[βL1,n1(hfac) ,Lk]Ն5.7 estimated for the mononuclear com-
3
plexes [Ln(Lk)(hfac)3] (k = 7, 8; Ln = La, Eu, Lu, Y) in
CDCl3.[20] The pure single-stranded dinuclear [Y2(L6)-
(hfac)6]·29H2O complex could be eventually obtained by
treating L6 (1 equiv.) with [Y(hfac)3(diglyme)] (2 equiv.) in
CHCl3 followed by precipitation with hexane (Table S1 in
the Supporting Information).
Liquid-Crystalline Organisation of Ligand L6 and Its
Complex [Y2(L6)(hfac)6]
The thermal behaviours of ligand L6 and its yttrium
complex [Y2(L6)(hfac)6] were studied by a combination of
polarised optical microscopy (POM), thermogravimetric
analysis (TGA) and differential scanning calorimetry
(DSC) measurements (Figure 4, Table 2 and Figures S18–
20 in the Supporting Information). Temperature-dependent
POM observations (20–250 °C temperature range) showed
the formation of fluid birefringent textures for L6 and
[Y2(L6)(hfac)6] upon heating (Figure S18) that are typical
of the occurrence of liquid-crystalline organisations,
Figure 4. (a) DSC trace of the ligand L6 at 5 °Cmin–1; (b) DSC
trace of complex [Y2(L6)(hfac)6] at 5 °Cmin–1 (second heating and
whereas ligand L5 did not show any birefringence in solid cooling).
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or in liquid states. The optical defects observed in the li- tering, that, along POM observation, corroborated the in-
quid-crystalline texture of ligand L6 were typical of a ne- duction of the nematic liquid-crystalline phase. The dif-
matic phase (N) (Figure S18a), whereas the observation of fractogram collected at 120 °C for [Y2(L6)(hfac)6], taken as
homeotropic zones for [Y2(L6)(hfac)6] suggested the forma- a representative example, showed two sharp small-angle re-
tion of a more ordered smectic A (SmA) phase (Fig- flections in the ratio 1:2, which was characteristic of 1D
ure S18b).
lamellar ordering (the reflections were indexed with the 001
and 002 Miller indices), and the position of which gave a
weakly temperature-dependent periodicity of d = 105 Å.
This periodicity in [Y2(L6)(hfac)6] is shorter (i) than the
total length of L6 in its extended conformation (estimated
to be about 130 Å; see Figure S26 in the Supporting Infor-
mation) and (ii) than the periodicity previously reported for
the SmA phases (d ≈ 120 Å) produced by the related mono-
nuclear dendritic complexes [Ln2(L9)(NO3)6] and
[Ln2(L10)(NO3)6] (see Figure S26).[15i–15j]
The abnormal intensity profile displayed by the two re-
flections (Figures S22–S24 in the Supporting Information)
with a relatively weak (001) reflection with respect to (002)
involves the alternation of several high-electronic density
sublayers, hence the central metallic fragment and the
branching and peripheral mesogens, with low-electronic
Table 2. Temperatures, enthalpy and entropy changes of the phase
transitions observed for ligands Lk (k = 5, 6) and complex
[Y2(L6)(hfac)6].
Compound
Phase
Transition temp. T
ΔH
ΔS
sequence[a]
[°C]
[kJmol–1] [Jmol–1 K–1]
L5
G Ǟ I
G Ǟ N
N Ǟ I
–30
41
134
40
–
–
16.2
–
–
–
39.8
–
L6[b]
[Y2(L6)(hfac)6][b] G Ǟ SmA
SmA Ǟ I
172
15.8
35.5
[a] G = glassy state, N = nematic phase, SmA = smectic A phase,
I = isotropic fluid. [b] The liquid-crystalline phases were identified
from their optical textures by POM and from small angle (SA)
XRD studies. Temperatures are given for the onset of the peaks
observed during the second heating processes.
DSC measurements confirmed these observations. Li- density sublayers associated with the C10 aliphatic spacers
gand L5 transformed directly through a low-temperature (Figure 5). The relative intensity ratio indeed depends on
glass transition into an isotropic liquid, which was then the respective thickness ratios of these sublayers and from
stable up to 350 °C (Table 2 and Figure S19 in the Support- the sharpness of the various interfaces (i.e., the quality of
ing Information). For ligand L6 and its dinuclear complex the microsegregation). A rough estimation of the volume of
[Y2(L6)(hfac)6], glass transitions (centred around 40 °C for the rodlike yttrium complex at 120 °C, obtained by addition
both compounds) led to the mesophases, which were then of partial volume {Vmol = V(L6) + 2V([Y(hfac)3]) ≈ 6200 +
transformed into isotropic liquid through first-order transi- 1800 = 8000 Å2},[25] gives a molecular area of Amol = Vmol
/
tions at higher temperature (Figure 4, Table 2). After the d = 76 Å2 (e.g., a value that corresponds to an area per
first heating, during which co-crystallised molecules of cyanobiphenyl of 38 Å2), which is significantly larger than
water were lost (Figure S20 in the Supporting Information), the cross-section of an aliphatic chain (22.5 Å2). Altogether,
these two mesomorphic compounds showed a good thermal the shorter lamellar periodicity and the larger molecular
stability (i.e., no weight loss is observed by TGA and the area can be traced back to a considerable tilt angle of the
complex remained unchanged by POM) together with a re- peripheral and internal mesogens and consequently to the
versible thermal behaviour over several heating/cooling cy- undulation of the layers at the mesogen–mesogen interface.
cles (5 °Cmin–1 under N2).
A possible model for the supramolecular organisation of
Temperature-dependent SAXRD patterns were recorded the complexes into a lamellar structure would consist of
for L6 and [Y2(L6)(hfac)6] in the 20–200 °C range (Fig- their lateral arrangement primarily induced by the segrega-
ures S21–25 in the Supporting Information). In both cases, tion of the chelating part to form the central sublayer. On
the presence of a large and diffuse signal at approximately either side of that sublayer are first disposed a mixed stra-
4.5 Å, which was associated with the liquidlike lateral or- tum including aliphatic spacers and branching mesogen,
dering of the molten chains and mesogenic cores, confirmed followed by terminal cyanobiphenyls. The large surface area
the fluidlike nature of the mesophases. No sharp small- is compensated by large undulations of the layer, which is
angle reflection could be detected for L6, only a broad scat- equivalent at the molecular level to an average important
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tilt angle of the mesogens to reach the 38 Å2 target area for mind, the limited stability of multistranded complexes can
a terminal cyanobiphenyl.
be exploited for the alternative selective preparation of the
dinuclear single-stranded complex [Y2(L6)(hfac)6] in the ab-
sence of higher-order helicates. The rigid metallic core mod-
elled by [Y2(L1)(hfac)6] (helicity index = 0.55) has a cylin-
drical shape with a 50–80 Å2 cross-section and 25 Å length
(Figure S36 in the Supporting Information). Its dendritic
lipophilic version found in [Y2(L6)(hfac)6] self-organises
just above room temperature to give a SmA-type liquid-
crystalline phase. To the best of our knowledge,
[Y2(L6)(hfac)6] is the first reported rodlike dinuclear lan-
thanidomesogen.
Experimental Section
General: Chemicals were purchased from Acros, Alfa Aesar and
Aldrich, and used without further purification unless otherwise
stated. The trifluoromethanesulfonate [Ln(CF3SO3)3]·xH2O[29] and
hexafluoroacetylacetonate [Ln(hfac)3(diglyme)][30] salts were pre-
pared from the corresponding oxide (Aldrich, 99.99%). The Ln
contents of solid salts were determined by complexometric ti-
trations with Titriplex III (Merck) in the presence of urotropine
and xylene orange.[31] Acetonitrile and dichloromethane were dis-
tilled from calcium hydride. Methanol was distilled from
Figure 5. Proposed SmA-like organisation for [Y2(L6)(hfac)6]. The
red cylinders are the metallic cores of the complexes, the blue lines
are the dendritic spacer and the black rectangles are the terminal
cyanobiphenyl mesogens.
Further insight into the organisation of the cyanobi-
phenyl units at the interface can be gained by monitoring
the CϵN stretching vibration (νCN) during the crystal–li-
quid to isotropic liquid phase transitions.[26] Interestingly, Mg(OCH3)2. Silica gel plates Merck 60 F254 were used for thin-
layer chromatography (TLC) and Fluka silica gel 60 (0.04–
0.063 mm) was used for preparative column chromatography.
νCN is particularly sensitive to changes in intermolecular
CN···CN interactions that occur at the phase transitions[27]
but the temperature dependence (20–200 °C) of both fre-
Preparation of L5: Anhydrous K2CO3 (0.12 g, 0.87 mmol) and a
quency (νCN [cm–1]) and integrated intensities (ICN in the catalytic amount of KI (50 mg) were added to a solution of 11
2200 to 2250 cm–1 range)[28] of the CN stretching band in
[Y2(L6)(hfac)6] during the isotropisation process shows no
(0.10 g, 0.15 mmol) and 15 (0.24 g, 0.35 mmol) in DMF (20 mL).
The reaction mixture was stirred at 60 °C for 24 h under a nitrogen
atmosphere. The reaction mixture was evaporated to dryness and
abrupt variation (Figures S27 and S28 in the Supporting
the resulting oily mixture was partitioned between CH2Cl2/half-
Information) except during the first heating, in which
saturated NaCl (100 mL:100 mL). The organic phase was sepa-
cocrystallised water molecules are expulsed (Figures S29–
rated, dried with anhydrous MgSO4, filtered and the solvents were
32 in the Supporting Information). We conclude that, as
evaporated to dryness. The product was purified by column
expected from the large molecular area estimated at the
chromatography [silica gel (50 g), CH2Cl2/MeOH 100:0, 99.5:0.5,
interface, the terminal cyano groups are not significantly
involved in the stabilisation of the SmA phase in
99:1 and 98.5:1.5] and dried for 24 h under vacuum at 80 °C to
yield L5 (0.170 g, 8.68ϫ10–5 mol, 60%) as a white oil that solidi-
[Y2(L6)(hfac)6]. On the contrary, a significant and abrupt fied after several days. 1H NMR (400 MHz, CDCl3): δ = 0.90 (t,
3J = 6.8 Hz, 12 H), 1.10 (t, 3J = 7.1 Hz, 6 H), 1.23–1.52 (m, 112
change of νCN was measured for ligand L6 at the NǞI
3
3
H), 1.72–1.89 (m, 16 H), 3.38 (q, J = 7.1 Hz, 4 H), 3.61 (q, J =
transition (Figures S33 and S34 in the Supporting Infor-
mation). The weaker constraints in the nematic phase of L6
make the CN groups of the ligand free enough to interact
with neighbouring groups, thus contributing to the stabili-
sation of the mesophase.
3
7.1 Hz, 4 H), 3.92 (t, J = 6.6 Hz, 12 H), 4.12–4.12 (br., 4 H), 4.30
(s, 2 H), 4.77 (q, 3J = 6.8 Hz, 4 H), 6.08 (s, 6 H), 7.07 (d, 4J =
2.0 Hz, 2 H), 7.24 (d, 3J = 8.0 Hz, 2 H), 7.37 (d, 3J = 8.0 Hz, 2 H),
7.72 (s, 2 H), 7.89 (s, 2 H) ppm. ESI-MS (CH2Cl2/MeOH, 9:1): m/z
= 960.5 [M + 2H]2+, 1919.9 [M + H]+. C121H192N8O10 (1918.90):
calcd. C 75.61, H 9.95, N 8.39; found C 75.74, H 10.09, N 8.34.
Preparation of L6: 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide
hydrochloride (EDCI·HCl; 0.033 g, 1.74ϫ10–4 mol), and 4-dimeth-
ylaminopyridine (DMAP; catalytic) were added to a solution of 16
(0.145 g, 9.58ϫ10–5 mol) in CH2Cl2 (40 mL) under an inert atmo-
sphere. The solution was stirred at room temp. for 5 min and com-
pound 10 (0.030 g, 4.36ϫ10–5 mol) was added. The reaction mix-
ture was heated at reflux for 12 h [the progression of the reaction
was monitored by TLC (CH2Cl2/MeOH, 98:2)]. The reaction mix-
ture was then diluted to 100 mL with CH2Cl2, washed with a half-
saturated aqueous solution of NaCl (3ϫ50 mL), dried with anhy-
drous Na2SO4, filtered and the solvents evaporated to dryness. The
crude product was purified by column chromatography [silica gel
Conclusion
The drastic decrease in stability previously reported for
lipophilic d-block liquid-crystalline helicates [Cu2(L4n)2]2+
(n = a, b, c)[17] is strictly mirrored by f-block helicates. The
special anchoring of the dendrimeric residues perpendicular
to the helical axis excludes the operation of significant re-
pulsive interstrand steric effects, and the instability of the
lipophilic complexes [Ln2(L5)3](CF3SO3)6 (Ln = La, Y, Lu)
and [Y2(L6)3](CF3SO3)6 in CD2Cl2/CD3CN can be unam-
biguously assigned to the efficient solvation of the lipophilic
receptors Lk (k = 5, 6) in organic solvents. With this in (75 g), CH2Cl2/MeOH 100:0, 99.5:0.5, 99:1]. Finally, the product
Eur. J. Inorg. Chem. 2013, 3323–3333
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FULL PAPER
was dissolved in CH2Cl2 (2 mL) and this solution was then poured
into MeOH (50 mL). The precipitate was filtered using a mem-
brane (45 μm porosity), washed with MeOH and dried under vac-
uum at 80 °C for 12 h to yield L6 (0.099 g, 2.70ϫ10–5 mol, 62%)
as a white solid. 1H NMR (400 MHz, CDCl3): δ = 1.15 (t, 3J =
tures (from 20 to 160 °C), for which the sample temperature was
controlled within 0.05 °C. (2) An image plate. The cell parameters
were calculated from the position of the reflection at the smallest
Bragg angle, which was in all cases the most intense. Periodicities
up to 150 Å could be measured, and the sample temperature was
controlled within a temperature range of 0.3 °C. The exposure
times were varied from 2 to 6 h, depending on the specific reflec-
tions sought (weaker reflections clearly taking longer exposure
times). (3) An SAXS system from Molecular Metrology equipped
3
7.0 Hz, 6 H), 1.31 (t, J = 7.1 Hz, 6 H), 1.33–1.54 (m, 78 H), 1.75–
3
3
1.88 (m, 24 H), 3.44 (q, J = 7.0 Hz, 4 H), 3.64 (q, J = 7.1 Hz, 4
3
3
H), 4.06 (t, J = 6.5 Hz, 8 H), 4.07 (t, J = 6.5 Hz, 4 H), 4.30 (s, 2
3
3
H), 4.37 (t, J = 6.8 Hz, 8 H), 4.39 (t, J = 6.8 Hz, 4 H), 4.81 (q,
3J = 7.2 Hz, 4 H), 7.00 (d, J = 9.1 Hz, 8 H),7.01 (t, J = 9.1 Hz, with a Cu-Kα1 Bede microsource conditioned with confocal Max-
3
3
4 H), 7.26 (dd, 3J = 8.5 Hz, 4J = 1.5 Hz, 2 H), 7.34 (d, 3J = 8.9 Hz,
FluxTM optics and a two-dimensional multiwire gas detector. A
modified temperature stage from Linkham was used to control the
temperatures. The computational analysis was performed with
3
4
8 H), 7.39 (d, J = 8.5 Hz, 2 H), 7.58 (d, J = 2.2 Hz, 2 H), 7.65
(d, 3J = 9.1 Hz, 8 H), 7.69–7.72 (m, 10 H), 7.74–7.77 (m, 8 H), 8.08
(d, 4J = 1.5 Hz, 4 H), 8.17 (d, 3J = 9.1 Hz, 4 H), 8.18 (d, 3J = MatLab-based open-source software from Molecular Metrology.
9.1 Hz, 8 H), 8.21 (d, 3J = 8.8 Hz, 4 H), 8.28 (d, 3J = 8.8 Hz, 4 H),
8.38 (d, 4J = 2.2 Hz, 2 H), 8.61 (t, 4J = 1.5 Hz, 2 H) ppm.
ESI-MS (CH2Cl2/MeOH, 9:1): m/z 3681.4 [M
H]+.
X-ray Crystallography: Summary of crystal data, intensity measure-
ments and structure refinements for 10 and [Lu2(L1)(hfac)6] are
collected in Tables S2 and S4 (in the Supporting Information).
Crystals were mounted on a quartz fibre with protection oil. Cell
dimensions and intensities were measured at 180 K with a Stoe
IPDS diffractometer with graphite-monochromated Mo-Kα radia-
tion (λ = 0.71073 Å) for 10 and at 180 K with a Supernova (Ag-
ilent) diffractometer using mirror-monochromated Cu-Kα radiation
(λ = 1.5418 Å) for [Lu2(L1)(hfac)6]. Data were corrected for Lo-
rentz and polarisation effects and for absorption. The structure was
solved by direct methods (SIR97)[32] or SHELXS;[33] all other cal-
culation were performed with SHELXL97[33] and ORTEP[34] soft-
ware.
=
+
C225H232N12O36·18.01, H2O: calcd. C 67.48, H 6.75, N 4.20; found
C 67.48, H 6.75, N 4.20. The hydration was confirmed by TGA
(Figure S20a in the Supporting Information).
Preparation of Complex [Y2(L6)(hfac)6]: Ligand L6 (30 mg,
8.15ϫ10–6 mol; 1 equiv.) was treated with anhydrous [Y(hfac)3(di-
glyme)] (2.0 equiv.) in CHCl3 (2 mL) at room temperature. After 1
h stirring, hexane (4 mL) was added and a white precipitate
formed. The suspension was centrifuged (5000 rpm, 10 min) and
the solvent was removed. This procedure was repeated twice. The
white solid was dried under vacuum at 80 °C for 12 h, thereby
quantitatively yielding the expected [Y2(L6)(hfac)6] complex
(Table S1 in the Supporting Information).
Spectroscopic and Analytical Measurements: 1H and 13C NMR
spectra were recorded at 298 K with a Bruker Avance 400 MHz
spectrometer. Chemical shifts are given in ppm with respect to
TMS. Pneumatically assisted electrospray (ESI-MS) mass spectra
were recorded from 10–4 m solutions with an Applied Biosystems
API 150EX LC/MS System equipped with a Turbo Ionspray
source. Elemental analyses were performed by K. L. Buchwalder
of the University of Geneva. The variable-temperature FTIR spec-
tra were recorded with an IRTF Nicolet iS10 spectrometer in dif-
fuse reflectance mode by using a high-temperature diffuse reflec-
tance environmental chamber. The samples were diluted into a KBr
matrix and the resulting mixtures that contained about 10% of
compound were ground before being heated at 200 °C for a few
minutes. After cooling to room temperature, the FTIR spectra were
recorded in the 20–200 °C and 200–20 °C temperature ranges by
using a heating or cooling rate of 2 °Cmin–1. The spectra were re-
corded with a resolution of 0.4 cm–1. TGA was performed with a
Seiko TG/DTA 320 thermogravimetric balance (under N2). DSC
traces were obtained with a Mettler Toledo DSC1 Star Systems
CCDC-905224 (for 10) and -905225 (for [Lu2(L1)(hfac)6]) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Comments on the Crystal Structure of 10: Precursory ligand 10 crys-
tallised in a monoclinic system (space group C2/c) with one inde-
pendent molecule in the asymmetric unit (Z = 8). There was one
additional nitromethane solvent molecule. Atoms C17, C16 and
C26 were disordered and refined over two sites with occupancy
factors of 0.52/0.48, 0.61/0.39 and 0.47/0.53, respectively, and with
constrained isotropic displacement parameters. The positions of
hydrogen atoms were calculated and refined with restraints on
bond lengths and bond angles. Intermolecular π stacking were ob-
served (1) between planar aromatic benzimidazole rings related by
an inversion centre (d = 3.615Å, shift distance = 0.863Å) and (2)
between a phenyl ring and a benzimidazole ring (d = 3.67Å, angle
between planes = 4.69°). There were three intermolecular π-stack-
ing interactions per molecule (Figure S1b in the Supporting Infor-
mation).
differential scanning calorimeter from
3 to 5 mg samples
(5 °Cmin–1 under N2). Characterisation of the mesophases was per-
formed with a Leitz OrthoplanPol polarising microscope with a
Leitz LL 20ϫ/0.40 polarising objective and equipped with a Lin-
kam THMS 600 variable-temperature stage. The SAXRD patterns
were obtained with three different experimental setups, and in all
cases the crude powder was filled in Lindemann capillaries that
were 1 mm in diameter: (1) A STOE STADI P transmission powder
diffractometer system using a focused monochromatic Cu-Kα1
Comments on the Crystal Structure of [Lu2(L1)(hfac)6]: Crystallisa-
tion of complex [Lu2(L1)(hfac)6] in the triclinic P1 space group
¯
gave small colourless prismatic crystals. The measured small crystal
is a two nonmerohedral twin (the ratio of the twin components is
0.47:0.53). No solvate molecules were found in the structure. Some
CF3 groups of the coordinated hfac counterions were disordered
(rotation about the threefold axes) and therefore displayed large
anisotropic ellipsoids. The three most disordered CF3 groups were
refined by splitting each F atom on two sites with restraints on
bond lengths and on bond angles. Their displacement parameters
were refined isotropically and constrained to be equal. Occupancy
factors were refined and converge to values of around 0.5. Hydro-
gen atoms were calculated and refined with restraints on bond
lengths and bond angles.
beam obtained from
a curved germanium monochromator
(Johann-type) and collected on a curved image-plate position-sensi-
tive detector. A calibration with silicon and copper laurate stan-
dards for high- and low-angle domains, respectively, was performed
preliminarily. Sample capillaries were placed in the high-tempera-
ture attachment for measurements in the range of desired tempera-
Eur. J. Inorg. Chem. 2013, 3323–3333
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FULL PAPER
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Acknowledgments
Financial support from the Swiss National Science Foundation is
gratefully acknowledged. S. F. acknowledges the Centre National
de la Recherche Scientifique (CNRS), the Ministère de l’Enseigne-
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Received: February 4, 2013
Published Online: April 22, 2013
Eur. J. Inorg. Chem. 2013, 3323–3333
3333
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim