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The structures of the crystalline phase and columnar mesophase of rhodium (II) heptanoate and of its binary mixture with copper (II) heptanoate probed by EXAFS
M. Inb-Elhaj, D. Guillon, A. Skoulios, P. Maldivi, A. Giroud-Godquin, J.-C.
Marchon
To cite this version:
M. Inb-Elhaj, D. Guillon, A. Skoulios, P. Maldivi, A. Giroud-Godquin, et al.. The structures of the crystalline phase and columnar mesophase of rhodium (II) heptanoate and of its binary mixture with copper (II) heptanoate probed by EXAFS. Journal de Physique II, EDP Sciences, 1992, 2 (12), pp.2237-2253. �10.1051/jp2:1992263�. �jpa-00247801�
Classification Physics Abstracts
61.30
The structures of the crystalline phase and columnar
mesophase of rhodium (II) heptanoate and of its binary mixture
with copper (II) heptanoate probed by EXAFS
M. Ibn-Elhaj (I), D. Guillon (I), A. Skoulios (I), P. Maldivi (2), A.M. Giroud-
Godquin (2) and J.-C. Marchon (2)
1') Groupe des Mat£riaux Organiques, Institut de Physique et Chimie des Matdriaux de
Strasbourg, ICS, 6 rue Boussingault, 67083 Strasbourg Cedex, France
(2) Laboratoire de Chimie de Coordination (*), DRFMC/SESAM, Centre d'Etudes Nucl£aires de Grenoble, B-P- 85X, 38041 Grenoble Cedex, France
(Received J9 May J992, accepted in final form 4 September J992)
Abstract. EXAFS was used to investigate the local structure of the polar spines of rhodium (II) soaps in the columnar liquid crystalline state. It was also used to ascertain the degree of blending of
the cores in binary mixtures of rhodium (II) and copper (II) soaps. For the pure rhodium soaps, the
columns are shown to result from the stacking of binuclear metal-metal bonded dirhodium
tetracarboxylate units bonded to one another by apical ligation of the metal atom of each complex
with one of the oxygen atoms of the adjacent molecule. Mixtures of rhodium (II) and copper (II) soaps give a hexagonal columnar mesophase in which pure rhodium and pure copper columns are
randomly distributed.
1. Introduction.
Rhodium (II) complexes of fatty acids, also known as rhodium soaps, are binuclear metal- metal bonded compounds of the general formula : [CH~(CH~)~_~ CO~]4Rh~. They are
crystalline at room temperature and columnar liquid crystalline at temperatures above ca.
100 °C [1, 2]. Their lamellar packing in the solid state is exemplified by the crystal structure of rhodium (II) butyrate (n
= 4 [3], in which planes of polar dirhodium tetracarboxylate groups
are separated by double layers of extended aliphatic chains. Within the planes, the polar cores
of the molecules are stacked in rows lying parallel to one another. The stacking of the cores is obtained by apical ligation of the metal atoms with the oxygen atoms of the neighboring cores (Fig. I). On the other hand, the columnar liquid crystalline structure consists of columns of
polar cores, stacked with a period of about 4.6 h, and surrounded with the aliphatic chains in a disordered conformation. The columns are parallel to one another and laterally packed
(*) (URA l194 CNRS).
Ii M /°
o o
/°
M ?°
o o
~lil ~/~
~/~ M ~?~
Fig. I. Apical coordination (dashed line) of the metal atoris of
two neighboring dimetaLtetracarboxy-
late polar heads. Substituents of the carboxylate groups are not shown.
according to a two-dimensional hexagonal lattice. Thus, the thermotropic behaviour and structural features of rhodium (II) soaps are very similar to those previously described for copper (II) soaps [4, 5].
The present EXAFS investigation was undertaken with a dual purpose. The first one was to
analyze in some detail the columnar liquid crystalline architecture of rhodium soaps, by determining the local structure of the polar spines around the rhodium atoms. The latter was
expected to be similar to that of the crystalline phase, as in the case of copper analogs [6, 7] but
subject to experimental confirmation. The second purpose was related to the mesomorphic
behaviour of binary mixtures. It is well known that blends of structurally similar discotic mesogens yield columnar mesophases which look homogeneous by hot stage microscopy, and
observation of the textures shown by mixtures is the basis of a ready classification of
mesogens. However, whether individual mesogens are randomly stacked in columns or
segregated in random columns is unknown. We thought that the liquid crystalline phase of mixtures of rhodium and copper soaps could be a « textbook example » of such systems, and that EXAFS at the rhodium and copper edges would be uniquely suited for ascertaining the
degree of blending at the molecular level. Attention was restricted to the study of the
heptanoate (n=7) soaps. Their transition temperatures from the crystalline into the
mesomorphic phase are 93 °C for C7Cu [5] and l16 °C for C~Rh [3].
2. Experimental.
2.I SAMPLE PREPARATION. Soaps were synthesized according to previously described
methods [1, 5]. The mixtures were prepared by dissolving the two soaps together in boiling heptane and recrystallizing them by cooling down to room temperature the precipitate
obtained was filtered, dried under vacuum, and controlled by elemental analysis. Samples (about 50mg) were pressed (under a pressure of about 5 x10~ g,cm~~) into pellets of
dimensions 28 x 5 x 0.5 mm, which were carefully checked for homogeneous thickness and absence of cracks or holes. The pellets were then placed in tight boron nitride cells with thin
(0,I mm) windows. The cells were inserted within the heating unit designed for X-ray
absorption measurements [7]. The temperature was regulated within I °C.
2.2 DATA COLLECTION. The EXAFS experiments were run on beamline « EXAFS I » at LURE (Orsay). The experimental setup used has been described elsewhere [8]. The X-ray
beam was obtained from the DCI storage ring under the usual running conditions : 1.85 GeV, a maximum stored current of 250 mA, and a (331)-Si channel-cut monochromator. Data were collected in transmission mode ; the direct beam and scattered beam intensities were measured
with ionization chambers. Energy resolution was estimated to be about 0.5eV for the
experiments performed at the copper edge, and 4 eV for the experiments performed at the rhodium edge. The energy calibrations were monitored using a copper foil and a sample of rhodium acetate. The energy scanning was obtained through rotation of the monochromator.
Some typical absorption spectra, obtained at the rhodium edge, are shown in figure 2.
2.3 DATA ANALYSIS. The EXAFS spectra were analyzed by classical procedures [8], using
programs provided by Bonnin et al. [9]. In a first step, the absorption pi before the edge was
fitted with a law of the type a + b* E~, where c was of the order of 2 and the atomic absorption
o.05
-s
~
°~-o.05~
-o.15
2 9 16
k( I ~)
a)
o.05
-s
~
~
°~
-o.05
-0.15
2 9 16
k( 1~)
b)
Fig. 2. K~edge rhodium EXAFS absorption spectra of (a) C7Rh in the crystalline phase at 20 °C, (b) C7Rh in the columnar mesophase at 120 °C, (c) C7Rh/C7Cu mixture (lo : I molar concentration) in the
crystalline phase at 20°C, (d) C7Rh/C7Cu mixture (10:1 molar concentration) in the columnar mesophase at 120 °C.
o.05
-s
~
~
°~
-o,05
-o.15
2 9 16
k( K~)
C)
o.05
-s
~
~
°~
-o.05
-o.15
2 9 16
k ( I )
d) Fig. 2 (continued).
coefficient Ho was fitted with a spline function. The EXAFS signal corresponding to the atom shells around the metal atoms was then extracted from the experimental spectra using the
following normalization relationship :
x (E) = ((~c ~c1) ~lo)/~lo
The energy edge value has been defined as being the energy corresponding to the maximum of p (E derivative. For the Fourier transform of the Rh EXAFS spectra, a Kaiser-Bessel window (with T = 2) between 4.2 and 14.4 h~
was used together with a k~ weighting factor, for the Cu EXAFS spectra, the Fourier transform was obtained with a Hamming window between 2.5
and 13.4 h-I and
a k~ weighting factor. Three main peaks (Pi, P~ and P~) were analyzed in each radial distribution to obtain the filtered EXAFS spectrum of the corresponding shell. In the case of Rh, this radial distribution was filtered between 1.35 and 1.90 h (Pi), between 1.90 and 2.76 A (P~), and between 2.76 and 3.25 A (P~) in the case of copper, it was filtered
between 1.17 and 1.96 (Pi), between 1.96 and 2.58 A (P~), and between 2.64 and 3,19 A
(P~).
In a second step, the values of backscattering amplitudes and phases were deduced using the
known crystallographic data of the two reference compounds, C~Rh [10] and C~CU [11],
whose EXAFS spectra were also recorded for that very purpose under the same experimental
conditions. In a third and final step, these amplitudes and phases were used to determine the local distribution of scatterers around the metal atoms, as well as their radial distance
therefrom, for C7Rh, C7Cu, and for their binary mixtures. Relevant interatomic distances for the two reference compounds are shown in table1.
Table I. Radial distances of atoms A jkom metal atoms as determined crystallographically for C~Rh, 2H~O [10] and C~CU [11].
Atom* Rh-A (A) Shell Cu-A (A) Shell
Oi
O~ 2.04 Pi 1,96
O~
O~ P~
O 2.31 2.22
M;~~~~ 2.38 2.59
P~
Cl P~
C2 2.89 2,85
C3 C4
05
06 3.08 3.10
07
08 P~ P~
Minter 3'37 3.26
* Atom numbering is shown in figure 4.
3. Results and discussion.
3.I RHODIUM HEPTANOATE. Absorption measurements of C~Rh, as well as of the C~Rh,
2 H20 reference, were carried out at the K-edge of rhodium (Eo
= 23 220 eV). Endowed with
a large number of electrons, rhodium atoms are powerful backscatterers, with a maximum of
JOURNAL DE PHYSIQUE U -T 2, N' <2, DECEMBER 1992 83
scattering amplitude at k
m [2 m/h~(E Eo)]~~~
= [0.262513(E Eo)]~~~
= lo A~ [12]. As
the rhodium content of the compounds is rather large, multiple scattering is also very
important. In order to be in a position to use the classical EXAFS treatment of the data [13],
care was taken in our work to only consider energies km 4 h~
~, where multiple scattering is weak. Anyway, the essential of the EXAFS information regarding heavy atoms is found in this
region of large wave vectors.
For the crystalline phase of C~Rh, as well as for the reference compound C~Rh, 2 H~O, EXAFS spectra were registered at 20 and 70 °C. The corresponding pseudo-radial distributions
are shown in figure 3. In the case of C~Rh, three peaks are clearly present: Pi,
P~ and P~, centered at 1.66, 2.2i and 3.07 A respectively. The same peaks are also observed for C~Rh, 2 H~O at the same temperature. The interpretation of these spectra was carried out in two independent ways. The first uses the theoretical values of backscattering amplitudes and
phases given by McKale [14] ; the second consists in using the crystallographic data at hand, in
extracting experimental values of backscattering parameters from the recorded spectra of C2Rh, 2 H20, and in transferring them to C~Rh. Both methods gave essentially identical
results. Thus, it was possible to assign peak Pi to the four equatorial oxygen atoms
(Oi, O~, O~, 04) (see Fig. 4) to assign peak P~ to the axial oxygen atom (O), and to the second rhodium atom (Rh~~~~) within the binuclear complex containing the central Rh atom, and to the
four carbon atoms of the carboxylic groups (Ci, C~, C~, C4); finally, to assign peak
P~ to the four remaining oxygen atoms of the binuclear complex (05, O~, O~, O~) and to the rhodium atom (Rh,~~~~) of the adjacent binuclear complex (see Tab.1).
360 400
P2 p2
270 Pl 300 Pl
180 200
~o p3
i~~ p3
0 0
0 3 6 9 0 3 6 9
l~( I R( I)
a) b)
Fig. 3. Pseudo-radial atomic distributions around the rhodium atoms at 20 °C (a) C2Rh, 2 H20, and (b) C7Rh.
The peaks so identified were properly filtered and Fourier-transformed back into « exper- imental » EXAFS spectra corresponding separately to the three atomic shells. The experimental spectra were then compared with the theoretical ones calculated using the general EXAFS
theory [13] (Fig. 5). The adjustable parameters used in the theoretical calculations are : the number (N) of given atoms, their radial distance (R) to the central rhodium atom, the deviation (Am of the Debye-Waller effect of C~Rh with respect to C~Rh, 2 H~O, the energy
shift (AEo) of the K-edge the corresponding reliability factor is AX(k)= I(Xexp-
,' ,
4 j
3
j 2
j I
i j
t
i i j
i i
,'
7 6
Fig. 4. Labelling of atoms around the central M rhodium atom (see also Fig, I).
Table II. EXAFS parameters of C~Rh in the crystalline state at 20 and 70 °C (parameters
are defined in the text). Labelling of atoms is shown in figure 4: O~~~~~=Oi, 02,
°3' °4 °axiat " ° ~hintra ~ ~il ~
" ~l' ~2' ~3' ~4' °
" °4' °6' °7' °8' ~~inter
" ~i3.
T Atoms N R Au AEO Ax(k)
("C) (A) (A) (eil~ x10S
Oequat 4.0 2.04 0.018 -4,18 .29
O~j~j 0.8 2.32 0.015 -3,20
20 Rhj~~~~ .0 2.33 0.025 -3.89 3.46
C 4.1 2.90 0.079 3.45
O 4.2 3,07 0.054 -2.30 5.32
Rh;~~~~ l .0 3.38 0,001 -5.88
Oequat 4,1 2,04 0.051 .08 0,88
O~;~j ,0 2.33 0.020 -8.01
70 Rh;~~~~ l.0 2.34 0.100 4.83 11.17
C 4.0 2,90 0.070 -5.42
O 4,0 3.07 0.068 -2.71 2.06
Rh;~~~ l ,0 3,36 0.04o -4.03
Xth)~/I(Xexp)~. It is important to note that the number of independent points N~~~
=
2 A(k) A(R )tar
= 6 for this reason, only two of the four theoretical parameters were allowed to vary during the fits, so that the number of fit parameters never exceeded the number of
independent data points. The final adjusted values are reported in table II. They are in fair
o.03
-~
ji~ °.°~
-0.02
3 7 11 15
k( 1 ')
a) o.ois
-0.010
5 7 ii 15
k 1 ')
b) o.oos
-o.oos
3 7 15
k ~~~)
C) Fig. 5. Comparison of
« experimental » (points) and calculated (lines) EXAFS spectra for C7Rh at 20 °C (a) shell P~, (b) shell P~, and (c) shell P~.
agreement with those expected from the crystal structures of C2Rh, 2 H20 and C4Rh. It is clear, therefore, that the binuclear complexes observed with crystalline C2Rh, 2 H20 and
C4Rh do also exist in crystalline C~Rh and are also connected with one another through the
apical coordination of the rhodium atoms shown in figure I.
For the columnar mesomorphic phase, EXAFS spectra of both C7Rh and C2Rh, 2 H20
were registered at the same temperature (120 °C) to minimize possible differences in Debye-
Waller effect. Figure 6 shows pseudo-radial atomic distributions around the central rhodium atom for the two compounds. The three first peaks of C7Rh, namely Pi, P~ and P~, centered at 1.66, 2.21 and 3.01A respectively, exactly correspond to those of C2Rh,
2 H~O ; in the absence of direct crystallographic data for this hydrate at 120 °C, these peaks
were assigned to the same atomic shells as above. The experimental spectra of C7Rh (obtained by Fourier-transform of the peaks properly filtered) were compared with the theoretically [13]
calculated ones (Fig. 7). The adjustable parameters used in the calculations are reported in
table III. They are perfectly consistent with those found in the crystalline state : the binuclear unit present in the crystal also exists in the mesomorphic state and is connected to its
neighbours through the apical coordination shown in figure I, with a distance of two successive homologous rhodium atoms (Mi M~ in Fig. 4) of 5.24 A. It is clear, therefore,
that the columns in the mesomorphic phase result in the stacking of such binuclear complexes.
360 400
P~ P2
270 p~ 300
pi
180 200
90 p3 loo
0 0
0 3 6 9 0 3 6 9
RI I RI I
a) b)
Fig. 6. Pseudo~radial atomic distributions around the rhodium atoms at 120 °C (a) C2Rh, 2 H20, and (b) C7Rh.
In the crystalline state, the stacking involves a regular shift of the binuclear complexes along
the same direction by half the diagonal of the « square » formed by the equatorial oxygen
atoms (Fig. 8a ; see also Fig. I). This is why the stacking period along the columnar axis is of
5.2 A while the thickness of each individual complex is of only 4.64 A, as estimated from the distance of Mi to O (Mi O
m Mi M + MO, see Fig. 4). It is of interest to note that, in the case of the columnar mesophase, the stacking period (4.6 A
as measured by X~rays [I]) is precisely equal to this thickness. This suggests that the stacked binuclear cores are shifted relative to their neighbours in such a way as to ensure the perpendicular orientation of the complexes with respect to the columnar axis. It can be surmised that the shift takes place in a regular manner,
according to a 4~fold helicoidal symmetry axis as shown in figure 8b, rather than randomly.
o.03
-~
ji~ °.°~
-0.02
5 7 ii 15
k( 1 ')
a)
o.ois
-0.o10
5 7 11 15
k( 1 ')
b)
o.oos
-0.005
5 7 11 15
k( t')
c)
Fig. 7. Comparison of« experimental » (points) and theoretically simulated (lines) EXAFS spectra for
C7Rh at 120 °C : (a) shell Pj, (b) shell P~, and (c) shell P~.
Table III. EXAFS parameters of C7Rh in the columnar state at 120 °C (parameters are defined in the text). Labelling of atoms is shown in figure 4: O~~~~~=Oi, O~, 03,
°4 °ax<al
"
° ~hintra " ~iI C
= Cl' ~2' ~3' ~4 °
" °4' °6' °7' °8' ~hinter " ~i3.
T Atoms N R Au AE~ AX(k)
(C) (A) (A) (eil~ x10S
Oequat 4.0 2.05 0,030 -4.09 4.06
O~w .0 2.30 0.009 -2.81
120 Rh;~~~~ l .0 2.34 0.007 -3.62 2.33
C 4.0 2.88 0,133 3.38
O 4.1 3.08 0,058 -0.30
Rhj~~~~ .0 3,38 0.004 ,54 9,89
a) b)
Fig. 8. - chematic representation of the columnar stacking of the binuclear complexes in the crystal
(a) in
the columnar (b). h'= 5.21h is the ntracolumnar stacking
Experimental confirmation of this assumption probably would be difficult since the extent of helicoidal ordering is possibly limited.
3.2 COPPER HEPTANOATE. We have published earlier an EXAFS investigation of several
copper (II) soaps [6, 7]. In the present work, C~CU was studied in much more detail than in the past, following the same methodological procedure as for rhodium soaps, the only difference
being in the fact that the axial oxygen atom (O) is contained in the Pi shell and not in the P~ one as for C~Rh. The results obtained (Tab. IV) are in perfect agreement with those reported
previously.
Table IV. EXAFS parameters of C7Cu in the crystalline (20 °C) and columnar (120 °C)
state (parameters are defined in the text). Labelling of atoms is shown in figure 4:
°equat ~ °l' °2' °3' 04 °axial ~ ° ~Uintra
~ ~il ~
~ ~l' ~2' ~3' ~4 °
" °4' °6'
°7' °8 ~Uinter ~ ~i3.
T Atoms N R Au AE~ Ax(k)
("C) (A) (A) jeil~ x10S
Oequat 4.0 .99 0.040 -1 .05 0.50
O~;~j .0 2,19 0,005 -0.68
20 Cu;~~~~ 0.9 2.62 0.006 8.00 5.26
C 4.0 2.84 0.008 1.00
O 4.0 3.10 0,008 1,19 2.45
Cu;~~~~ l.0 3,26 0.085 -0.05
Oequat 4.I .97 0.040 -1 .05 0.20
O~;~j .0 2.22 0.015 -0.18
120 Cu;~~~~ l .0 2.61 0.056 4.82 2,24
C 3,9 2.86 0.001 -1.69
O 4.0 3.10 0,000 5,20 0,65
Cuj~~~~ 1.0 3.26 0.082 -0.78
3.3 MIXTURES oF C~Rh WITH C~CU. An equimolar mechanical mixture of C~Rh and
Ciicu was first studied using X-ray diffraction. At room temperature, this mixture is formed of the coexisting crystals of the two soaps, as evidenced by the presence in the diffraction
pattems of the Bragg reflections corresponding to the individual lamellar structure of the two
species (with spacings of 19.3 and 29. I A, respectively). At high temperatures, above the transition into the columnar mesophase, the two soaps melt separately to produce two distinct columnar phases, characterized by intercolumnar distances of 15.8 and 19.0 A respectively,
which in the course of time rapidly evolve to merge into a common columnar phase (Fig. 9)
