Influence of N-Alkyl Substituents and Counterions on the Structural and Mesomorphic Properties of Guanidinium Salts: Experiment and Quantum Chemical Calculations

Article abstract of DOI:10.1002/cphc.201000444

A series of N-4-(4'-alkoxybiphenyl)-N',N',N",N"-tetramethylguanidinium salts was synthesized with varying alkoxy chain lengths and additional N-alkyl substituents, each with a number of different counterions. X-ray crystal-structure analyses of 1b I, 1b PF₆, 2a I, and 4a I reveal bilayer structures in the solid state and, for the 1b and 1b PF₆ salts, a hydrogen-bond-type connectivity between the guanidinium N-H group and the anion is found. For the N-alkyl homologues 2a I and 4a I the anion is still oriented close to the head group, although at a larger distance. Ion pairs are present also in solution, as demonstrated by ¹H NMR: the N-H chemical shift shows a good linear correlation with the radius, and hence the hardness, of the anion. The intramolecular conformational flexibility of 1b I, 2b I, 3b I, and 4b I was studied by temperature-dependent ¹H NMR spectroscopy and discrete activation barriers were determined for rotations about each of the three C-N partial double bonds of the guanidinium core. The relative heights of the individual barriers change between the N-H and the N-alkylguanidinium salts. A fourth barrier is observed for the rotation about the N-biphenyl bond. DFT calculations of charge densities show that the positive charge resides primarily on the central carbon atom. Rotational barriers were calculated for N'-substituted 2-amino-1,3-dimethylimidazolidinium cations as models, and are in qualitatively good agreement with the NMR data. Mesomorphic properties were studied by differential-scanning calorimetry, polarizing optical microscopy, and X-ray diffraction (WAXS/SAXS). All liquid-crystalline guanidinium salts exhibit smectic A mesophases. Clearing temperatures show a linear correlation with the anionic radius. Substitution of the N-H group with methyl, ethyl, or propyl results in decreasing mesophase widths and a concomitant shrinkage of the layer spacings.The anion makes the difference: The synthesis of various guanidinium salts with varying alkoxy chain lengths and additional N-alkyl substituents is reported (see picture). Mesomorphic properties are studied by differential-scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. All liquid-crystalline guanidinium salts exhibit smectic A mesophases.

Full text of DOI:10.1002/cphc.201000444

DOI: 10.1002/cphc.201000444
Influence of N-Alkyl Substituents and Counterions on the
Structural and Mesomorphic Properties of Guanidinium
Salts: Experiment and Quantum Chemical Calculations
Martin Butschies,[b] Sven Sauer,[b] Elena Kessler,[a] Hans-Ullrich Siehl,*[a] Birgit Claasen,[b]
Peter Fischer,[b] Wolfgang Frey,[b] and Sabine Laschat*[b]
A series of N-4-(4’-alkoxybiphenyl)-N’,N’,N”,N“-tetramethylguani-
dinium salts was synthesized with varying alkoxy chain lengths
and additional N-alkyl substituents, each with a number of dif-
ferent counterions. X-ray crystal-structure analyses of 1b I,
1b PF6, 2a I, and 4a I reveal bilayer structures in the solid state
and, for the 1b and 1b PF6 salts, a hydrogen-bond-type con-
nectivity between the guanidinium N-H group and the anion is
found. For the N-alkyl homologues 2a I and 4a I the anion is
still oriented close to the head group, although at a larger dis-
tance. Ion pairs are present also in solution, as demonstrated
guanidinium core. The relative heights of the individual barri-
ers change between the N-H and the N-alkylguanidinium salts.
A fourth barrier is observed for the rotation about the Nꢀbi-
phenyl bond. DFT calculations of charge densities show that
the positive charge resides primarily on the central carbon
atom. Rotational barriers were calculated for N’-substituted 2-
amino-1,3-dimethylimidazolidinium cations as models, and are
in qualitatively good agreement with the NMR data. Mesomor-
phic properties were studied by differential-scanning calorime-
try, polarizing optical microscopy, and X-ray diffraction (WAXS/
SAXS). All liquid-crystalline guanidinium salts exhibit smectic A
mesophases. Clearing temperatures show a linear correlation
with the anionic radius. Substitution of the N-H group with
methyl, ethyl, or propyl results in decreasing mesophase
widths and a concomitant shrinkage of the layer spacings.
1
by H NMR: the N-H chemical shift shows a good linear correla-
tion with the radius, and hence the hardness, of the anion. The
intramolecular conformational flexibility of 1b I, 2b I, 3b I, and
1
4b I was studied by temperature-dependent H NMR spectros-
copy and discrete activation barriers were determined for rota-
tions about each of the three CꢀN partial double bonds of the
1. Introduction
Based on a seminal observation by Knight and Shaw that long-
chain alkyl pyridinium halides display liquid-crystalline proper-
ties,[1] and more detailed investigations on the mesophase
type by Bruce, Maitlis, Seddon and Holbrey,[2,3] ionic liquid crys-
tals have gained increasing interest among material scien-
tists.[4] Several groups have studied counterion effects on mes-
omorphic properties[5] and correlated, for example, the liquid-
crystalline behavior of pyridinium salts with the formation of
charge-transfer complexes between the pyridinium cation and
a counterion.[6,7] However, the effect of the anion on the struc-
ture–property relationship in ionic liquid crystals requires fur-
ther investigation.
all charge distribution in the guanidinium moiety. Combined
dynamic NMR investigations and charge-density calculations
should provide a deeper insight into the relation between in-
tramolecular organization and mesomorphic, that is, intermo-
lecular properties of guanidinium salts. A comparative study of
the N-H guanidinium salts 1X and the corresponding N-alkyl
homologues 2–4I might even provide a probe for assessing
ionic-liquid-crystal properties in general. We report the results
herein.
Recently, we have reported that changing the anion of the
N-4-(4’-dodecyloxybiphenyl)-N’,N’,N”,N“-tetramethylguanidinium
salt 1b Cl affects the stability of the smectic A (SmA) meso-
phases more critically than variations in the alkoxy chain
length R1 (Scheme 1).[8,9] Bruce et al. have observed the same
behavior for silver stilbazole complexes.[10] Crystallographic
data for 1b Cl show a tilted bilayer structure in the solid state
as well as a hydrogen-bond-type connectivity between the
chloride anion and the guanidinium N-H.[8a] Hence, it was ex-
pected that N-alkylation might lead to a decisive change in the
mesomorphic properties because hydrogen bonding is no
longer possible, and also because the electronic (+I) and steric
effects of the different alkyl substituents will change the over-
[a] E. Kessler, Prof. Dr. H.-U. Siehl
Institut fꢀr Organische Chemie I
Universitꢁt Ulm
Albert Einstein Allee 11d-89069 Ulm (Germany)
Fax: +(49)731-502-2787
E-mail: ullrich.siehl@uni-ulm.de
[b] M. Butschies, Dr. S. Sauer, Dr. B. Claasen, Dr. P. Fischer, Dr. W. Frey,
Prof. Dr. S. Laschat
Institut fꢀr Organische Chemie
Universitꢁt Stuttgart
Pfaffenwaldring 55d-70569 Stuttgart (Germany)
Fax: (+49)711-68564285
E-mail: sabine.laschat@oc.uni-stuttgart.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201000444.
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Structural and Mesomorphic Properties of Guanidinium Salts
Scheme 1. Calamitic guanidinium salts with various anions 1X and guanidi-
nium iodides 2–4I with a modified head group.
2. Results and Discussion
2.1. Experimental
Synthesis (Scheme 2) and analytical characterization of the
compounds 1c,d,f,g Cl and 1a,d,e X (X¼Cl; see Scheme 1),
2–4I, 7, 8c,d,f,g and 9c,d,f,g are described in the Supporting
Information. Synthetic details for the compounds 1a,b,e Cl and
1b X (X¼Cl), 5, 6, 8a,b,e, 9a,b,e have been reported previous-
ly[8] and these compounds are included here for comparative
investigations. Tridecyl bromide,[11a] (S)-1-bromo-3,7-dimethyl-
octane,[11b–e] 1-bromo-3,7,11,15-tetramethylhexadecane,[11b,c,f–h]
and tetramethylchloroamidinium chloride[12] were prepared ac-
cording to literature procedures.
Scheme 2. Synthesis overview (for details on alkyl chain length and yields
see the Supporting Information).
2.2. Synthesis of Guanidinium Salts
see Table S1 in the Supporting Information). Mixed-anion gua-
nidinium salts 1ClxI1ꢀx were obtained by incomplete salt ex-
change.
The synthesis of N-4-(4’-alkoxybiphenyl)-N’,N’,N”,N“-tetramethyl-
guanidinium chlorides 1Cl (Scheme 2) started from 4-nitrobi-
phenyl-4’-yl benzoate 5, available in two steps from 4-hydroxy-
biphenyl by following our previously published procedure.[8]
Saponification of 5 with KOH, rather than with K2CO3, to obtain
the alcohol 6 as reported,[8] yielded the corresponding potassi-
um biphenolate 7 in 93% yield, which was then directly react-
ed with alkyl bromides in refluxing acetonitrile to give the
ethers 8 in 71–94% yield. Catalytic hydrogenation of the nitro
group (85–95%), treatment of amines 9[13] with tetramethyl-
chloroamidinium chloride[8,12] in the presence of NEt3, and sub-
sequent neutralization with NaOH afforded the guanidinium
chlorides 1Cl in an overall yield of 86–97%. The chlorides 1Cl
were then submitted to salt metathesis with NaI, NaBr, NaBF4,
KPF6, KSCN, and KBPh4, respectively, in MeCN to give the corre-
sponding guanidinium salts 1X (X¼Cl) (Scheme 2, for yields
The corresponding pentaalkylarylguanidinium iodides 2–4I
were obtained by reaction of the chlorides 1Cl with methyl,
ethyl, and n-propyl iodide, respectively, in acetonitrile at room
temperature or 1008C in the presence of K2CO3.
2.3. Single-Crystal X-Ray Diffraction Studies
Suitable single crystals could be grown for compounds 1b I,
1b PF6, 2a I, and 4a I. The corresponding X-ray structures of
the single molecules and packing diagrams are presented in
Figures 1–4.[14]
All packing diagrams displayed bilayer structures with inter-
digitated alkyl chains as reported before,[8] but also revealed in-
teresting interactions between the guanidinium cations and
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H.-U. Siehl, S. Laschat et al.
the anions. Hydrogen-bond-type connectivities were detected
in the solid state for 1b Cl[8a] and 1b I (Figure 1), with ‘hydro-
gen-bond’ distances of d=225pm for (N-1)ꢀH···Cl and 270 pm
for (N-1)ꢀH···I. The hexafluorophosphate 1b PF6 (Figure S1 in
the Supporting Information) features a bifurcated hydrogen
bond with distances of d=244/231pm for (N-1)ꢀH···F2PF4.
For the N,N’,N’,N’’,N’’-pentaalkylguanidinium salts 2–4I, hy-
drogen-bond formation is no longer feasible, yet some weak
interactions still remain between the guanidinium head group
and the anions. An atomic distance of d=317pm was deter-
mined for (C-3)ꢀH···I in the case of 2a I (Figure 2) and of d=
297pm for (C-8)ꢀH···I for 4a I (Figure S2 in the Supporting In-
formation). The anion thus is located close to the guanidinium
head group, regardless of whether the guanidinium moiety is
N-protonated or N-alkylated.
With such detailed information available about the structural
organization of the N-alkylguanidinium salts in the crystalline
state (Figure 2 and Figure S2), it appeared interesting to com-
pare the intramolecular organization (relative orientation, con-
formation) of the individual, single cations in solution using
1H NMR spectroscopy. As in the case of the X-ray diffraction
studies, the N-H guanidinium salts were included for compari-
son.
Figure 1. a) X-ray structure of 1b I in the solid state (numbering of atoms
given here is used only for the X-ray diffraction studies); b) Packing diagram
for 1b I.
2.4. 1H NMR Studies
The first observation was that the anion exchange reactions of
1
the salts 1Cl were easy to monitor by H NMR since the N-H
proton resonance shifts upfield in a significant manner upon
replacement of chloride by other anions. Typical spectra of
1a X are shown in Figure 3. The following tendency of d(N-H) is
preserved in all four series 1a X, 1b X,[15] 1d X, and 1e X (X=
Cl, Br, I, BF4, PF6, SCN), regardless of the alkyl chain lengths R1:
ꢀ
d(N-H) Clꢀ >Brꢀ >SCNꢀ >Iꢀ >BF4ꢀ >PF6
.
All other signals
appear virtually unchanged upon anion exchange.
Figure 2. a) X-ray structure of 2a I in the solid state (numbering of atoms
given here is used only for the X-ray diffraction studies); b) Packing diagram
for 2a I.
1
Figure 3. Low-field section of the H NMR spectra (300 MHz, CDCl3, 296K,
c=45mmolLꢀ1) of the guanidinium salts 1a X with different anions X.
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Structural and Mesomorphic Properties of Guanidinium Salts
1
Figure 5. Temperature-dependent H NMR spectral traces (500 MHz, CD2Cl2,
expansion 2.4–3.6ppm) for the N-H guanidinium iodide 1b I.[21]
Figure 4. Correlation of the N-H chemical shifts for 1b X and 1e X (d [ppm],
CDCl3, 296K, c=15.4mmolLꢀ1) with the anionic radii.
The observed upfield shift shows a fairly good linear correla-
tion with the anionic radius[16] (Figure 4; tetraphenylborates
1b,e BPh4 were not included because of the large additional
anisotropic effect of the phenyl rings). This finding indicates
that the anion is positioned quite close to the N-H group of
the guanidinium ion, presumably by hydrogen bonding (also
in solution), and is consistent with the X-ray structures of, for
example, 1b I and 1b PF6 (Figure 1 and Figure S1 in the Sup-
porting Information). These results are in good agreement
with previous work by Seddon and others on imidazolium
salts, which revealed the position of the anion to be controlled
by hydrogen bonds rather than Coulomb attractions, that is,
by close proximity of the anion to the area of greatest positive
charge.[17] To strengthen this point, we investigated whether
there is a pronounced solvent and/or temperature effect on
the N-H chemical shift, and hence on the hydrogen-bond for-
mation within the ion pairs. The N-H signal of 1b Cl is in fact
shifted upfield by up to 2ppm with increasing solvent polarity
(CDCl3 to DMSO, see Figure S3 in the Supporting Information).
The a-OCH2 signal, on the other hand, stays more or less con-
stant. This deshielding effect in less polar solvents was previ-
ously observed by Saielli with octyl viologen salts.[18] Tempera-
ture, in contrast, affects the N-H signal much less. From ꢀ508C
to +508C, the N-H resonance of 1b Cl in CDCl3 is shifted
downfield by only ~0.5ppm (Figure S4 in the Supporting Infor-
mation).
Scheme 3. Numbering scheme for the topologically different positions in
the compounds 1b I–4b I (the numbering given here differs from that em-
ployed for the X-ray diffraction studies, but is maintained consistently from
here on).
(Figure 5; for characterization of the positions, see Scheme 3).
At 300 K, these four lines have coalesced into one single, but
still rather broad resonance. The N-alkyl homologues 2b I, 3b I,
and 4b I display the same spectral pattern, with the final coa-
lescence, however, shifted to higher temperatures by ~40 K to
338K compared to 1b I (1H NMR spectra of 2b I at various tem-
peratures are shown in Figure 6). The different sites are equili-
brated by rotation around the three partial double bonds be-
tween C1 and N2, N3, and N4, respectively. These intramolecular
1
For mixed guanidinium salts in CDCl3, for example
dynamics were evaluated by temperature-dependent H NMR
1a Cl
0.25I0.75, one single signal was observed for the N-H proton,
spectroscopy.[19]
with the average chemical shift corresponding to the molar
ratio (Figure S5 in the Supporting Information). Even though
hydrogen bonds seem to fix the anion at a certain distance to
the guanidinium cation, there still is a rapid exchange between
the different halide ions. The same behavior is found even in
DMSO.
Calculating the free energy of activation, DGc‡, for any given
dynamic process requires precise values for the coalescence
temperature (Tc) and the line separation at this temperature
(dnc).[20] In the case of the N-H compound 1b I, for instance,
spectra were therefore recorded within 18-intervals from 237–
248K to exactly determine the coalescence point for the first
pair of N-CH3 signals (~2.60/3.07ppm) at 247K (see Figure S7
in the Supporting Information for the spectral traces). The ex-
perimentally not accessible dnc value has to be obtained from
a dn versus T plot. All guanidinium salts studied herein dis-
played a small to negligible temperature dependence of dn.
The N-H guanidinium salt 1b I and the N-alkyl homologues
2b I, 3b I, and 4b I also display a pronounced conformational
1
mobility. At 196K, the H NMR spectrum (500 MHz, CD2Cl2) of
1b I shows four discrete signals for the four topologically dif-
ferent N-CH3 groups which are spread over almost 1ppm
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H.-U. Siehl, S. Laschat et al.
Table 2. Coalescence temperatures TC, frequency separations dnc, rate
‡
constants k, and enthalpy barriers DGC for the guanidinium iodides 1b I,
2b I, 3b I, and 4b I (Scheme 3, for details see the Supporting Informa-
tion).
rotational proc.
TC [K]
dnc [Hz]
k [Hz]
DGC‡ [kJmolꢀ1
]
1b I
2b I
3b I
4b I
2
3
4
4
4
2, 3
4
4
2, 3
4
247
271
277
231
245
324
240
257
320
232
249
317
224
187
223
81
350
138
109
389
135
93
498
415
495
180
778
307
242
864
300
206
853
318
47
52
53
46
46
64
46
47
63
46
47
63
1
4
2, 3
384
143
Figure 6. Temperature-dependent H NMR spectral traces (500 MHz, 196–
296K in CD2Cl2, 296–343K in Cl2CDꢀCDCl2, expansion 2.4–3.6ppm, not all
traces are shown) for the N-methylguanidinium iodide 2b I.
Nevertheless, dnc values extrapolated to Tc were always used
for the calculations (Table 2).
ipso carbon C1’ of the phenyl ring is 306pm (Table 1). One hy-
drogen atom of the 4S-CH3 group is located 354pm above the
center of the aromatic ring, and thus deep within the shielding
cone of the ring current. Upon raising the temperature, the
4S-CH3 resonance (2.597ppm) coalesces with the N-CH3 signal
Already at 250 K, the signal of C2’/C6’-H, in the ortho position
of the N3 aryl ring and high-field part of an AA’XX’ spin system,
appears severely broadened relative to the signals of the
C3’/C5’-H (low-field half of the AA’XX’ system) and also to those
of the second aryl ring of the 4’-decyloxybiphenyl moiety. At
186K, this signal is split into two distinct and widely separated
resonances. Thus, rotation around the C1’ꢀN3 bond can likewise
be frozen out. For all four guanidinium salts 1b I–4b I, this ro-
tation 1 (Scheme 3) has the lowest energy barrier. The frequen-
cy separation dn and the coalescence temperature both
cannot be determined with the same precision as for the other
‡
at 3.073ppm for 1b I (TC =247K, DGC =47kJmolꢀ1). Coales-
cence for the other two methyl signals at 3.124 and 3.494ppm,
respectively, was observed at 271K (DGC‡ =52kJmolꢀ1). The
final coalescence of the two broad merged resonances could
be fixed at 277K. All coalescence temperatures and the enthal-
py barriers, calculated from the coalescence data, are summar-
ized in Table 2.
If the first coalescence was due to rotation 4 around the
C1ꢀN3 partial double bond, this would exchange positions 4S-
CH3 and 2S-CH3 as well as 4A-CH3 and 2A-CH3. Different coales-
cence temperatures are expected for these two exchange pro-
cesses since the frequency separations dn for the two pairs of
lines involved differ sufficiently (Table 2). The free activation
barriers, however, calculated from the coalescence data, should
be identical because the same intramolecular motion mutually
interchanges positions 4S/2S and 4A/2A, respectively. Since this
is not the case, the first intramolecular motion detected upon
raising the temperature from 196K must be assigned to rota-
tion 2 around C1ꢀN4, followed by that around C1ꢀN2. In the
last stage, rotation 4 around C1ꢀN3 finally equilibrates all four
N-CH3 positions. With the first dynamic process fixed as rota-
tion 2, involving 4S- and 4A-CH3, the resonance at 3.073ppm
can be assigned unequivocally to 4A-CH3.
‡
dynamic processes. The calculated activation barrier, DGC , is
~35kJmolꢀ1, that is, 10–15kJmolꢀ1 lower than for the other ro-
tational processes (Table 2).
One of the methyl resonances appears shifted upfield by
ꢁ0.5ppm for all four guanidinium iodides 1b I–4b I. It can be
assigned to 4S-CH3. In the crystal structure of 1b I (see
Figure 1) this methyl group is located directly above the aro-
matic ring. The distance between the 4S-CH3 carbon and the
Table 1. Selected structural parameters from the X-ray diffraction analy-
ses of the guanidinium iodides 1b I, 2a I, and 4a I (numbering as given in
Scheme 3, not as in the ORTEP plots).
1a I
H
2b I
CaH3
4b I
CaH2CH2CH3
R2
For the N-methylguanidinium iodide 2b I (Figure 6), 4S-CH3
displays an even more pronounced upfield shift, and hence
must dip even deeper into the aromatic shielding cone. Coa-
lescence links this resonance to the N-methyl signal at
3.272ppm, corresponding to a frequency separation dn of
almost 360 Hz. The second pair of coalescing N-CH3 signals is
RMS deviation[a] [pm]
dihedral angle[b] (+) [8]
for C4SN4C4A
0.87
0.16
0.00
29.3
28.6
34.9
306
354
31.9
30.0
46.9
299
28.2
33.6
48.2
297
for C2SN2C2A
for C1’N3H/Ca
distance[c] [pm]
distance[d] [pm]
366/351
319/351
‡
more closely spaced (dn ~80 Hz). Nevertheless, identical DGC
[a] RMS deviation from coplanarity for the central guanidinium core
(N2N3N4C1). [b] Twist of the three amino groups relative to the guanidini-
um core plane. [c] Distance between 4S-CH3 and the ipso aryl carbon
atom C1’. [d] Distance between closest/two closest 4S-CH3 hydrogens and
the center of the aryl ring.
values were calculated for both exchange processes. This is ex-
pected if rotation 4 around the C1ꢀN3 bond now is the intra-
molecular motion with the lowest activation barrier. The N-
ethyl and N-propyl homologues, 3b I and 4b I, show compara-
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Structural and Mesomorphic Properties of Guanidinium Salts
‡
ble line patterns, and almost identical DGC values were calcu-
2.5. Quantum Chemical Calculations
lated for all three coalescence phenomena (Table 2).
The drastic change in intramolecular dynamics between the
N-H and the N-alkyl derivatives can be rationalized by closer in-
spection of the crystal structures of 1b I, 2a I, and 4a I (Fig-
ures 1,2 and Figure S2 in the Supporting Information). The
RMS deviation from coplanarity for the central C1N2N3N4
moiety is <1pm for the N-H and <0.2pm for the N-alkylguani-
dinium salts (Table 1). The molecular core thus can be consid-
ered as perfectly planar. Overlap between the three nitrogen
lone pairs and the central carbon C1, that is, delocalization of
the positive charge, and the concomitant partial double-bond
character of the individual C1ꢀN bonds thus depends only
upon the twist of the three amino moieties relative to the gua-
nidinium plane. As the values for the dihedral angles in Table 1
show, both dimethylamino groups are twisted uniformly by
~308 in the crystalline state. For the more bulky N-arylamino
group, the dihedral angle is 358 in the N-H compound.
With an additional alkyl substituent at N3, that is, for 2a I
and 4a I, however, the N-alkyl-N-arylamino group is twisted by
almost 508 (Table 1), severely reducing the pp-overlap between
C1 and N3 (cos2 function) and thus the partial double-bond
character of this bond. Consequently, the corresponding inter-
nal motion now has the lowest activation barrier even though
a much bulkier group has to be rotated around the C1ꢀN3
bond. With the N-alkyl-N-arylamino moiety contributing less to
the stabilization of the positive charge, the electronic demand
upon the two dimethylamino groups is increased concomitant-
ly. The barrier for rotation around the C1ꢀN4 and C1ꢀN2 bonds,
which now are topologically equivalent, is in fact raised by
>10kJmolꢀ1 for all three N-alkylguanidinium iodides relative
to the N-H compound.
Quantum chemical calculations were expected to provide a
more detailed understanding of charge distribution, conjuga-
tion effects, and intramolecular dynamics in this type of guani-
dinium cations. The Gaussian 03 program suite[25,26] was used
for all calculations. The DFT hybrid functional B3LYP[27] and the
standard 6-31G(d,p) basic set were used for all calculations. We
calculated the optimized structures for the isolated guanidini-
um cations 1b, 2a, and 4a (Tables S11–S13 in the Supporting
Information) for a direct comparison with the crystal structure
data for the corresponding iodides 1b I, 2a I, and 4a I. General-
ly, good agreement was found between experimental and cal-
culated CꢀC and CꢀN bond lengths (Drmax =j6j pm) and bond
angles (Ddmax =j2j8). The dihedral angles C1ꢀN3ꢀC1’ꢀ
C2’/6’, for instance, are ~408, ~508, and ~458 for the calculated
structures 1b, 2a, 4a, respectively. The corresponding X-ray dif-
fraction values are 278, 348, and 368. The agreement of the tor-
sion angle for the calculated isolated molecule and the experi-
mental data for the solid state is quite reasonable since this di-
hedral angle comprises two intramolecular motions, the twist
of the N3R2R3 amino group around the C1ꢀN3 bond with re-
spect to the guanidinium core plane, and the torsion of the bi-
phenyl moiety around the N3ꢀC1’ bond relative to the
C1’N3H(Ca) plane.
For all further calculations, the long alkoxy chain R1 in the bi-
phenyl moiety was replaced by an OMe group to minimize
computational costs. The following set of model guanidinium
cations was used for the quantum chemical studies
(Scheme 4): the N-4-(4’-methoxybiphenyl)-N’,N’,N’’,N“-tetrame-
The internal dynamics of protonated guanidines and espe-
cially of N,N’,N’,N’’,N’’-pentamethylguanidinium salts have been
firmly established by the fundamental work of Kessler and
Leibfritz.[22a–b] These authors have also demonstrated that for
both classes of compounds the topological interchanges are
due to distinct rotational processes around the three CꢀN par-
tial double bonds rather than to nitrogen inversion or a depro-
tonation/reprotonation sequence. With the higher chemical
shift dispersion at 11.74Tesla compared to 1.408Tesla, we eval-
uated even the lowest of the three coalescence processes (see
Table 2), and thus verified that the two lowest coalescence pro-
cesses in, for example, the pentamethylarylguanidinium iodide
2b I are linked to identical energy barriers.[23]
This constitutes direct proof that in the N-alkyl, other than in
the corresponding N-H guanidinium salts, rotation around the
‡
C1ꢀN3 bond indeed has the lowest barrier. The DGC values for
Scheme 4. Model structures 10–20 used for quantum chemical calculations.
the three coalescence processes, and hence all rotational barri-
ers, remain unaffected within the error limit[24] of ꢂ0.5kJmolꢀ1
(Table 2) when the size of the N3-alkyl substituent R2, and
hence steric interaction with the aryl group, is increased from
methyl to propyl. For the pentamethylarylguanidinium salts,
Kessler and Leibfritz have found that two ortho methyl groups,
on the other hand, already lead to a dramatic increase in this
barrier.[22a–b]
thylguanidinium cation (10) and the homologous cations with
an additional N-methyl, N-ethyl, and N-propyl substituent (11–
13). As smaller models, the N-4-(4’-methoxyphenyl)guanidini-
um cations, 15 and 16, and the penta- and hexamethylguanidi-
nium cations, 18 and 19, as well as the corresponding neutral
guanidines 14, 17, and 20 were included for comparison. Only
small deviations between the X-ray structural data and compa-
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3757

H.-U. Siehl, S. Laschat et al.
rable calculated bond lengths (Drmax =j5j pm) and bond
angles (Ddmax =j10j8) were observed in all the model struc-
tures.
of the N(Me)2 groups and the guanidinum main plane is f=
358. The coordinates for all minimum structures 1b, 2a, 4a,
and 10–20 are collected in Tables S14–S40 in the Supporting
Information.
The minimum structures for the guanidinium cations 1b, 2a,
4a, 10–13, 15, 16, 18, and 19 have a twisted right- or left-
handed propeller-type arrangement of the planar NR2 groups
around the central carbon C1. Optimum resonance stabilization
of the cationic center C1 would require a close to coplanar ar-
rangement whereas repulsive steric interaction between alkyl
and aryl substituents induces torsion of the NR2 group plane
relative to the guanidinium core plane (C1N2N3N4). Table 3
The calculated interplane torsion angle q of the substituted
aryl ring of the biphenyl substituent with respect to the
C1’N3R2 plane is q=448 for the guanidinium cations 1b and 10,
similar to the experimentally determined angle for the crystal-
line salt 1b I of q=378. For 2a and 11 a torsion angle of q=
548 is calculated as compared to the experimentally measured
angle of q=408 in 2a I. For the guanidinium cations 4a, 12,
and 13 the alkyl substituent R3 (Et, n-Pr) can adopt different
conformations which lead to minimum structures of similar
energy but different interplane torsion angles q [for 13 for ex-
ample, 13a (R3 =anti-n-Pr, q=468) and 13b (R3 =gauche-n-Pr,
q=448, DE=+6kJmolꢀ1) see the Supporting Information]. A
torsion angle of q ꢃ 45ꢄ is calculated for 4a, 12, and 13 with
an alkyl-group conformation analogous to that experimentally
observed for the solid guanidinium salt 4a I , where q=308.
Atomic charges for the guanidinium cations 10–13, 15, 16,
18, and 19 were calculated using the natural bond orbital
(NBO) method (Table 4).[28] In all eight guanidinium cations, an
identical positive charge (0.70) is localized at the central
carbon C1 (Table 4a). The charge distribution is identical for the
three N-H guanidinium cations 10, 15, 18, and thus independ-
ent of the N3-substituent, 4’-methoxybiphenyl, 4-methoxy-
phenyl, or methyl. The negative charge density at the secon-
dary nitrogen N3 is higher than that at the dimethyl-substitut-
ed nitrogens N2 and N4 and is equal for all nitrogens of the
hexasubstituted guanidinium cations 11–13, 16, and 19.
Group charges for the structural fragments, the guanidinium
core G and the different R3 substituents (4’-methoxybiphenyl,
4-methoxyphenyl, methyl), are the same for all eight guanidini-
um cations (Table 4b). The total charge for the guanidinium
moiety G is constant, independent of the N3 substitution. This
implies that the N3-aryl and N3-Me substituents have the same
effect, and thus there is no evidence for delocalization of the
Table 3. Torsional angles
f between the guanidinium core plane
(C1N2N3N4) and the planes of the three amino functions, and torsional
angles q between the plane of the amino group N3R2R3 and the plane
0
0
0
(C6 C1 C2 ) of the aryl substituent.
Structure
f [8]
q [8]
N3ꢀR2
N2(Me)2
N3C1’R2“
N4(Me)2
1b I
1b
10
2a I
2a
11
12
4a I
4a
13a
13b
H
H
H
29
31
30
30
35
35
36
34
36
35
36
35
30
29
47
37
37
40
48
42
38
42
29
33
32
32
34
34
33
28
33
35
33
37
44
44
40
54
54
44
30
45
46
44
Me
Me
Me
Et
n-Pr
n-Pr
n-Pr
n-Pr
shows a comparison of corresponding measured and calculat-
ed interplane angles f. The calculated interplane angles f be-
tween the NR2-group planes and the C1N2N3N4 plane of the
central guanidinium moiety vary between 29–428. Calculated
and experimental diffraction angles f for the two N(Me)2
groups are in fairly good agreement. The calculated torsion
angles for the cations are marginally larger (f~2–58) than
those determined experimentally
for the corresponding salts. The
plane of the secondary imino/
amino group (N3R2Caryl, R2 =H) of
1b and 10 is twisted ~308 from
the guanidinium main plane,
somewhat less than measured in
the crystal structure 1b I. This in-
terplane angle increases with in-
creasing steric demand of the
substituent R2 (2a, 11, R2 =Me,
f=378; 12, R2 =Et, f=408; 4a
and 13b, R2 =n-Pr, f=428). Ex-
perimentally, this angle increases
from 1b I (R2 =H, f=358) to 2a I
(R2 =Me, f=478) and 4a I (R2 =
n-Pr, f=488). For the hexameth-
Table 4. NBO charge distribution in the guanidinium cations 10, 11, 15, 16, 18, and 19: 4a) for the guanidini-
um core atoms, 4b) for the two structural fragments G (total guanidinium core C1N2N3N4) and R3 (aryl/methyl
substituent at N3), and 4c) for C1 and the three individual amino groups ( S=total charge).
10
11
12
13
15
16
18
19
4a)
C1
+0.70
ꢀ0.44
ꢀ0.61
ꢀ0.43
+1.78
+1.00
+0.70
ꢀ0.44
ꢀ0.44
ꢀ0.43
+1.61
+1.00
+0.70
ꢀ0.44
ꢀ0.44
ꢀ0.43
+1.61
+1.00
+0.70
ꢀ0.44
ꢀ0.45
ꢀ0.42
+1.61
+1.00
+0.70
ꢀ0.43
ꢀ0.61
ꢀ0.43
+1.77
+1.00
+0.70
ꢀ0.44
ꢀ0.44
ꢀ0.43
+1.61
+1.00
+0.70
ꢀ0.43
ꢀ0.61
ꢀ0.44
+1.78
+1.00
+0.70
ꢀ0.43
ꢀ0.43
ꢀ0.43
+1.59
+1.00
N2
N3
N4
other
ꢀ
4b)
4c)
G
+0.75
+0.25
+1.00
+0.74
+0.26
+1.00
+0.75
+0.25
+1.00
+0.75
+0.25
+1.00
+0.74
+0.26
+1.00
+0.74
+0.26
+1.00
+0.73
+0.27
+1.00
+0.73
+0.27
+1.00
R3
ꢀ
C1
+0.70
+0.10
+0.09
+0.11
+1.00
+0.70
+0.09
+0.10
+0.11
+1.00
+0.70
+0.09
+0.10
+0.11
+1.00
+0.70
+0.09
+0.09
+0.12
+1.00
+0.70
+0.10
+0.10
+0.09
+1.00
+0.70
+0.09
+0.10
+0.10
+1.00
+0.70
+0.11
+0.10
+0.11
+1.00
+0.70
+0.10
+0.10
+0.10
+1.00
N2(Me)2
N3(R2,R3)
N4(Me)2
ꢀ
yl-substituted
guanidinium
cation 19 the calculated inter-
plane angle between the plane
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ChemPhysChem 2010, 11, 3752 – 3765

Structural and Mesomorphic Properties of Guanidinium Salts
positive charge in the N3-aryl p-system. The group charges for
the central carbon C1 and the three groups N2(Me)2, N3(R2,R3),
and N4(Me)2 confirm equal and predominant localization of the
positive charge at carbon C1 independent of different N-sub-
stituents (Table 4c).
state structures were obtained by calculations scanning the di-
hedral angle. Transition state structures were confirmed to
have imaginary frequencies only for the torsional vibration as-
sociated with the corresponding C1ꢀN3 bond rotation. Intrinsic
reaction coordinate (IRC) calculations proved that the transi-
tion state structures for the C1ꢀN3 bond rotation connect the
corresponding minima. The calculated rotational barriers for
the C1ꢀN3 bond rotation in 22–29 are listed in Table 6 (see also
Scheme 5).
In the guanidines 14, 17, and 20 calculated for comparison,
the imino-nitrogen N3 carries slightly more negative charge
than the amino-nitrogens N2 and N4, independent of aryl or
methyl substitution (see Table 5a). The NBO group charges for
Table 5. NBO charge distribution in the guanidines 14, 17, and 20: 5a)
for the guanidine core atoms, 5b) for C1 and the three individual amino/
imino groups (S=total charge).
Table 6. Calculated barriers of the C1ꢀN3 bond rotation for the imidazol-
idinium cations 22–29 (for numbering refer to Scheme 5).
imidazolidinium
R2
R3
R4
DG‡ [kJ mol-1]
14
17
20
22
23
24
25
26
27
28
29
H
H
Me
Me
H
H
Me
Me
Me
anisyl
Me
anisyl
Me
anisyl
Me
H
H
H
H
Me
Me
Me
Me
63
71
63
76
34
39
24
29
5a)
C1
+0.64
ꢀ0.48
ꢀ0.57
ꢀ0.50
+1.91
+1.00
+0.64
ꢀ0.48
ꢀ0.57
ꢀ0.50
+1.91
+1.00
+0.62
ꢀ0.49
ꢀ0.57
ꢀ0.52
+1.96
+1.00
N2
N3
N4
other
ꢀ
anisyl
5b)
C1
+0.64
ꢀ0.05
ꢀ0.52
ꢀ0.07
ꢂ0.00
+0.64
ꢀ0.06
ꢀ0.50
ꢀ0.08
ꢂ0.00
+0.62
ꢀ0.09
ꢀ0.42
ꢀ0.11
ꢂ0.00
N2(Me)2
N3R3
N4(Me)2
ꢀ
The minimum structures of the
N2,N4-H imidazolidinium ions 22–
25 have a coplanar arrangement of
the N2C1N4 plane of the imidazoli-
dine ring and the planar exocyclic
amino group (R2N3R3 plane). This
allows p–n-orbital p-type conjuga-
tive interaction between the for-
mally positively charged C1 and all
three planar nitrogen atoms. The
anisyl ring in the model structures
23, 25, 27, and 29 is oriented
almost perpendicular to the imidazolidin ring, thus p-aryl reso-
nance delocalization of the positive charge at C1 is lacking. In
the transition state structures for C1ꢀN3 bond rotation (TS-22–
TS-25) the exocyclic amino group N3 is no longer planar (sp2
bond angle sum 3608) but pyramidalized (338–3468) towards
sp3-hybridization (3278). The R2N3R3 plane is orthogonal to the
imidazolidine ring plane (N2C1N4). Thus overlap between the
vacant p-orbital at C1 and the n-orbital at N3 is interrupted in
the transition state for C1ꢀN3 bond rotation. The barriers for
C1ꢀN3 bond rotation vary between 63 and 76kJmolꢀ1 for 22–
25 (Table 6).
The N,N-dimethylimidazolidinium model structures 26 and
27 are sterically more congested due to methyl substitution of
the nitrogen of the imidazolidine amino group. The exocyclic
secondary amino group (R2 =H) yet adopts in the ground state
an almost coplanar orientation (7–108) relative to the N2C1N4
plane. In the transition state structures TS-26 and TS-27, the
nitrogen is pyramidalized (bond angle sum 3488 and 3458),
and the R2N3R3 plane and the ring plane are almost orthogonal
(87–838). Structures 28 and 29 have a bulkier, disubstituted
exocyclic amino group (R2 =Me) and the minimum structures
of 28 and 29 no longer adopt a coplanar conformation. The
the central carbon C1 and the three amino/imino substituents
N2(Me)2, N3R3, and N4(Me)2 show strong Cd+ꢀNdꢀ polarization
of the imino double bond C1=N3 (Table 5b). The C1ꢀN2 and
C1ꢀN4 bond lengths (140 pm) calculated for 14, 17, and 20 are
slightly shorter than a CꢀN single bond (147pm in MeꢀNH2).
The imino bond C1ꢀN3 (129pm) is close to the length of a
C=N double bond (127pm in Me=NH). The similar calculated
NBO charge distribution (Table 5b) for 14, 17, and 20 suggests
no significant conjugation of the 4’-methoxy(bi)phenyl sub-
stituent at N3 with the guanidine core.
Scheme 5.
The larger calculated negative charge density at the imino-
nitrogen atom N3 compared to the amine-nitrogen atoms N2
and N3 in 14, 17, 20 is in accord with earlier findings that N3 is
the preferred protonation site in guanidines.[22]
2.6. Modeling Rotational Barriers
Guanidinium cations in principle can show bond rotations
around the three different [C1]+ꢀN bonds impeding calculation
of individual rotational barriers. Thus, we have chosen the
2-amino-substituted
imidazolidinium
cations
22–29 with different substituents R2 and R3 at the nitrogen N3
as computational models. These structures have fewer degrees
of freedom and allow one to single out specific rotational bar-
riers around the [C1]+ꢀN3(R2R3) bond which then can be com-
pared with the experimentally determined rotational barriers
in the guanidinium salts 1b I and 2b I. Stationary points, the
energy minimum structures 22–29 and transition state struc-
tures TS-22–TS-29, were verified by analysis of vibrational fre-
quencies. Starting geometries for the optimization of transition
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H.-U. Siehl, S. Laschat et al.
R2N3R3 plane and the imidazolidinium ring plane are twisted
(~368). This reduces n–p–p conjugation in the ground state.
Accordingly, ~10kJmolꢀ1 lower barriers for rotation around the
[C1]+ꢀN3 bond are calculated for 28 (24kJmolꢀ1) and 29
(29kJmolꢀ1) as compared to 26 and 27. The transition state
structures TS-28 and TS-29 have a pyramidalized exocyclic ni-
trogen N3 (bond angle sum 3508 and 3568) and the interplane
angle between the R2N3R3 and the N2C1N4 planes is close to
908. Some conformational effects of the anisyl group in TS-29
are subject to further studies.
upon cooling. No recrystallization peak was identified because
compound 1a Cl presumably undergoes a steady transition
into the glassy state. The corresponding glass transition at
338C and a clearing transition at 1938C were observed during
the second heating. Compound 1a Cl thus displays enantio-
tropic mesomorphism. All N-H guanidinium chlorides 1Cl
showed similar behavior, irrespective of the R1 chain length
(Figure 8). It appears noteworthy that the guanidinium chlor-
ides 1f Cl and 1g Cl, with branched side chains, displayed in-
creased mesophase widths, combined with decreasing first
melting temperatures, compared to the derivatives with corre-
sponding linear side chains, 1h Cl (R1 =C8H17)[8] and 1e Cl.
The model calculations of a barrier of about 10kJmolꢀ1
lower for rotation around the [C1]+ꢀN3 bond for 28 and 29 as
compared to 26 and 27, respectively, are in very good agree-
ment with the barrier of ~7kJmolꢀ1 lower in energy deter-
1
mined experimentally by H NMR spectroscopy for the guanidi-
nium salt 2b I (R2 =Me, DGC =46kJmolꢀ1) compared to 1b I
‡
(R2 =H, DGC‡ =53kJmolꢀ1). Explorative calculations using other
DFT functionals which include dispersion terms such as
X3LYP[29] and B97D[30] give within ꢂ3kJmolꢀ1 the same results
as B3LYP. Further computational studies on the structure and
dynamics of this type of guanidinium cations are in progress.
2.7. Mesomorphic Properties
The mesomorphic properties of the guanidinium salts 1X and
2–4I were studied by differential-scanning calorimetry (DSC),
polarizing optical microscopy (POM), and X-ray diffraction
(WAXS, SAXS).
Figure 8. Phase behavior of guanidinium chlorides 1Cl with varying chain
length (heating/cooling rate 10Kminꢀ1; data for 1h Cl are taken from ref. [8];
compound 1f Cl and 1g Cl can be found in the Supporting Information).
A possible influence of the anion on the mesomorphic prop-
erties was studied next for four different linear alkyl chain
lengths [C10 (a), C12 (b), C14 (d), and C16 (e)]. The temperature
range of the SmA phase is similar for the guanidinium chlor-
ides, bromides, and iodides, with a trend towards an increasing
mesophase width with longer chain length (Figure 9). The two
tetraphenylborates 1b,e BPh4 were non-mesomorphic, on the
other hand, the bulky anion completely suppressing a SmA
double-layer organization. For the tetrafluoroborate, hexafluor-
ophosphate, and thiocyanate salts, the situation appeared less
clear-cut. For the derivatives 1a,b X with a C10- and C12-alkyl
chain, respectively, these counterions induced a considerable
shrinking of the mesophase width relative to that for the corre-
sponding halides. In the C14-alkyl chain series, however, the tet-
rafluoroborate salt 1d BF4 behaves like the halide salts, with
the SmA phase ranging over 188K, whereas the SmA phases
of the corresponding hexafluorophosphate 1d PF6 and thiocya-
nate 1d SCN span only 61 and 77K, respectively. For the guani-
dinium salts 1e X, the hexadecyl chain determines the meso-
phase properties and all salts show SmA phase behavior over a
wide temperature range.
Figure 7. DSC curves of guanidinium chloride 1a Cl during first and second
heating (bottom and middle traces) and first cooling (upper trace), heating/
cooling rate 10Kminꢀ1 [31]
.
A typical DSC curve for 1a Cl is shown in Figure 7. At 608C,
an exothermal crystal-to-crystal transition was observed, but
without any attempt for a more detailed investigation (addi-
tional crystal-to-crystal transitions were observed during the
first heating cycle for some derivatives; for details see Ta-
bles S3–S10 in the Supporting Information). A melting transi-
tion at 1018C (onset temperature) and a clearing transition at
1948C were detected upon first heating. The transition from
the isotropic into the smectic A phase was observed at 1948C
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Structural and Mesomorphic Properties of Guanidinium Salts
the corresponding graph in
Figure 11, clearing points can in
principle be derived directly
1
from H NMR shift data.
As the last parameter, we
studied the influence of N-alkyla-
tion on the mesomorphic prop-
erties of the guanidinium iodides
2–4I. The data in Figure 12 clear-
ly reveal that the presence of
the NꢀH moiety in the guanidi-
nium head group is a crucial
factor for mesophase stability.
The SmA phase of 1a I with a C10
chain, for example, ranges from
Figure 9. Phase behavior of the guanidinium salts 1a,b,d,e X depending on the different anions X.
Figure 11. Correlation between clearing temperature T and N-H chemical
shifts (300 MHz, 296K, CDCl3, c=15.4mmolLꢀ1) for 1b X and mixed guanidi-
nium salts 1b ClxI1ꢀx with different chloride/iodide ratios.
Figure 10. Correlation of clearing temperature T with the anionic radius for
the guanidinium salts 1a,b,d,e X (data for 1SCN salts were omitted for clari-
ty, but can be found in the Supporting Information).
The clearing temperatures in fact display a linear correlation
with the anionic radius for the salts 1a,b,d,e X.[16] As Figure 10
shows, they decrease with increasing anionic radius; for the
bulkiest anion tetraphenylborate, the SmA phase is lost com-
pletely. Presumably due to the ellipsoid structure of the thio-
cyanate anion, the clearing temperatures of the salts 1SCN
were always lower than those for the other anions.
Since the N-H chemical shift was found to be anion-depen-
dent as well, clearing points might possibly be deduced from
chemical shift data or, in other words, phase transition temper-
1
atures could be obtained by H NMR. Clearing points were de-
termined by DSC for a series of guanidinium salts 1b X and
1b ClxI1ꢀx with a varying chloride/iodide ratio to probe this hy-
pothesis. Figure S6 (Supporting Information) shows that the
clearing points indeed decrease with a higher molar ratio of
iodide. When all clearing temperatures T were plotted against
dN-H for the different anions and chloride/iodide ratios, a fairly
good direct, linear relationship was obtained (Figure 11). With
Figure 12. Phase-transition temperatures for the N-methyl-, N-ethyl-, and N-
propylguanidinium iodides 2–4I depending on the alkoxy chain length R1.
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H.-U. Siehl, S. Laschat et al.
9 to 1418C (132K), for 1e I with a C16 chain from 41 to 2378C
(196K). In contrast, the N-methylguanidinium iodides 2a,b I
with the shorter C10- and C12-chain lengths are nonmesomor-
phic, and for the salt 2e I with a C16 chain the mesophase
range is reduced to 114K (143–2578C). This destabilizing effect
appears even more pronounced for the N-ethyl- and N-propyl-
guanidinium iodides 3I and 4I. It should be noted that
Hunt[32a] and Zahn et al.[32b] have independently studied by
computational methods the counterintuitive increase of melt-
ing points and viscosity upon methylation of C-2 in imdiazoli-
um ionic liquids. They propose that the loss of hydrogen-bond
connectivity leads to a reduced free movement of the corre-
sponding anion relative to the imidazolium cation. The reduc-
tion in entropy results in a greater ordering, thus raising the
melting points. Presumably, similar effects are operative in our
case.
Figure 14. WAXS experiment for 3d I at 1908C with the corresponding dif-
fraction pattern.
The mesophases of the guanidinium salts 1X and 2–4I were
studied by polarizing optical microscopy (POM) and displayed
spontaneous homeotropic alignment over large areas during
cooling from the isotropic melt to the smectic mesophase,
giving rise to the homeotropic textures typical for SmA
phases.[33] Representative POM textures are shown in Figure 13.
of all N-H salts 1X,[8] thus can unequivocally be characterized
as SmA phases.
If one compares the layer spacings, obtained by X-ray dif-
fraction studies for the N-alkylguanidinium iodides 2d I–4d I in
the liquid-crystalline phase with the calculated length of the
molecules 2d–4d (~3100 pm for an all-trans conformation),[34]
an analogous bilayer structure with strongly interdigitated
alkyl chains becomes apparent. The smectic layer spacing of
the bilayers was also temperature-dependent and decreased
with increasing temperature (Figure 15). This effect was found
Figure 13. a) SmA texture of 1d Cl at 2428C with large, homeotropically
aligned areas; b) Bꢁtonette texture of 1f Cl at 1508C; c) focal conic texture
of 1d SCN at 1758C with the homeotropic alignment disturbed around the
edges of air bubbles; d) SmA texture of 2b I at 2158C (all with magnification
ꢂ200 and a cooling rate of 30 Kminꢀ1).
Figure 15. Temperature dependence of the d001 layer spacing for 1d I–4d I
obtained from SAXS experiments.
2.8. X-Ray Powder Diffraction Studies
In a previous publication,[8] we have proposed a smectic bilayer
for the guanidinium salts 1Cl, similar to the packing situation
in the corresponding single crystals. However, one should not
draw too great parallels between X-ray information from the
solid state and the organization in the liquid-crystal mesophas-
es. The X-ray measurements for compounds 2d I–4d I likewise
displayed characteristic diffraction patterns with a strong fun-
damental diffraction peak (001) typical for smectic mesophases
(shown in Figure 14 for 3d I). In conjunction with the POM re-
sults, the mesophases of the N-alkyl salts 2–4I, as well as those
to be much more pronounced for the N-H compound 1d I
than for the N-alkyl homologues 2d I–4d I.
For a quantitative comparison of the guanidinium salts 1d I–
4d I, despite the observed temperature dependence, layer
spacings were compared at a reduced temperature (Tred =0.95
Tiso). These reduced layer spacings dred showed a general
shrinking in the order 2d I>1d I>3d I>4d I (Table 7), keeping
in mind that there was no common mesophase range at a
given temperature. Just before the clearing point, the layer
3762
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ChemPhysChem 2010, 11, 3752 – 3765

Structural and Mesomorphic Properties of Guanidinium Salts
The influence of the N-alkyl substituent is also manifest in in-
tramolecular processes in solution, as demonstrated by varia-
ble-temperature NMR studies. A number of rotational degrees
of freedom exist within the guanidinium moiety. In all N-H gua-
nidinium structures, the two dimethylamino functions have
the lowest barriers of rotation, with the N(Me)2 group syn to
the N3-aryl substituent even lower than with the N(Me)2 group
in anti position. In the N-alkylguanidinium iodides 2b I, 3b I,
and 4b I, in contrast, the lowest rotational barrier is found for
the C1ꢀN3 bond, and hence the smallest partial double-bond
character is derived for this C1ꢀN3 bond. The inversion of the
relative heights of these three rotational barriers can be de-
duced directly from the crystal structure data and is confirmed
by calculations for imidazolidinium model cations. In order to
get insight into the electronic (and dynamic) behavior of gua-
nidinium salts, NBO charge distributions were calculated. The
results reveal that the replacement of the mesogenic p-alkoxy-
biphenly moiety by p-alkoxyphenly or even methyl at the gua-
nidinium core does not change the charge density particulary
on atom N3, whereas the replacement of N-H by N-Me, N-Et,
and N-nPr results in a decreased charge density at N3. Thus,
the computional results gave no indication for a delocalization
of the positive charge in various N3-aryl p-systems and there-
fore the NꢀC aryl bond behaves like a spacer between the cat-
ionic head group and the mesogenic biaryl substituent.
Table 7. Reduced layer spacings dred of the guanidinium salts 1d I–4d I.
compound
Tiso [8C]
Tred [8C]
dred [pm]
1d I
2d I
3d I
4d I
210
232
187
138
200
220
178
131
4170
4240
4090
4060
spacing dred is smaller for 1d I than for 2d I even though at
lower temperatures compound 1d I displayed a much larger
layer spacing. The polarity of the polar guanidinium head
group decreases with increasing N-alkyl chain length; ionic re-
pulsion between the guanidinium moieties thus is reduced, re-
sulting in an overall shrinkage of the layer spacings.
3. Conclusions
For the penta-substituted guanidinium salts 1X, a direct hydro-
gen-bond-type connectivity was found in the crystalline solid
state between the anion and the N-H group. This ion-pair con-
tact is conserved even in solution. The N-H resonance is shifted
to higher field by ~4ppm (in dilute solution) when the anion
ꢀ
is changed from Clꢀ (12ppm) to PF6 (8ppm). For the mixed-
anion guanidinium salts 1b ClxI1ꢀx , the N-H proton chemical
shift shows a linear correlation with the chloride/iodide molar
ratio. These findings clearly demonstrate that the anion cannot
be located directly above the central carbon atom C1 of the
guanidinium functionality where NBO calculations predict the
highest positive charge (+0.7).[35] Rather, the anion must be
oriented along the axis of the NꢀH bond, just as found in the
solid state. The hydrogen bonding of the anion to N-H guani-
dinium cations in 1X also directly affects their liquid-crystalline
properties. The observed anion effects on hydrogen bonding
in solution and in the solid state, as well as their stabilizing
effect on the liquid-crystalline properties, agree well with previ-
ous experimental and theoretical studies by Seddon,[17]
Bruce,[10] and others working on imidazolium and pyridinium
ionic liquid crystals. Thus, large anions destabilize the meso-
phases as they reduce the anisotropy. Comparison with the lit-
erature cited above also shows the general trend that the
anion effect overrules the side-chain effect with regard to mes-
ophase stabilities. Along these lines it was demonstrated that
Besides the extensively investigated pyridinium and imidazo-
lium salts, guanidinium salts constitute an alternative class of
thermotropic ionic liquid crystals. Their properties can be ad-
justed by several structural parameters, thus opening a wide
field for future applications.
Acknowledgements
Financial support by the Bundesministerium fꢀr Bildung und For-
schung (BMBF-Verbundprojekt Nr. 01 RI 05177), the Studienstif-
tung des Deutschen Volkes (fellowship for M. Butschies), the Deut-
sche Forschungsgemeinschaft, the Ministerium fꢀr Wissenschaft,
Forschung und Kunst des Landes Baden-Wꢀrttemberg, and the
Fonds der Chemischen Industrie is gratefully acknowledged. We
thank the referees for valuable suggestions.
Keywords: charge
distribution
·
density
functional
1
calculations · internal dynamics · liquid crystals · mesophases
the anion-dependent H NMR chemical shift of the N-H signal
could be correlated with the clearing transition temperature
even for guanidinium salts with a mixture of anions. In other
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Received: June 1, 2010
Revised: October 6, 2010
Published online on November 17, 2010
ChemPhysChem 2010, 11, 3752 – 3765
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
3765