Alkanoates Series: Monotropic Polymorphism and Mesomorphism

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New Advances in the 1D Coordination Polymer Copper(II) Alkanoates Series: Monotropic Polymorphism and Mesomorphism Miguel Ramos Riesco, Francisco Javier Martínez-Casado, José A. Rodriguez Cheda, M. Isabel Redondo Yélamos, Iván Da Silva, Tomás S. Plivelic, Sol López-Andrés, and Paolo Ferloni Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00182 • Publication Date (Web): 11 Mar 2015 Downloaded from http://pubs.acs.org on March 20, 2015

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Crystal Growth & Design

New Advances in the 1D Coordination Polymer Copper(II) Alkanoates Series: Monotropic Polymorphism and Mesomorphism Miguel Ramos Riesco,† Francisco J. Martínez-Casado,*‡ José A. Rodríguez Cheda,† M. Isabel Redondo Yélamos,† Iván da Silva,§ Tomás S. Plivelic,‡ Sol López-Andrés,┴ and Paolo Ferloni║ †

Departamento de Química Física I, Facultad Ciencias Químicas, and

┴

Departamento de

Cristalografía y Mineralogía, Facultad Ciencias Geológicas, Universidad Complutense, 28040 Madrid, Spain. ‡

MAX IV Laboratory - Lund University. Ole Römers väg 1. 223 63, Lund, Sweden.

§

ISIS Facility, Rutherford Appleton Laboratory, Harwell Oxford, Didcot OX11 0QX, United

Kingdom ║

Dip. Chimica, Sezione di Chimica Fisica, Universitá di Pavia, Viale Taramelli 16, 27100

Pavia, Italy. *Corresponding author. Email: [email protected] KEYWORDS: polymorphism, monotropism; columnar discotic liquid crystals, developable domains, paddle-wheels, copper(II) alkanoates, MOFs; 1D coordination polymers; PXRD and SCXRD; modulated-DSC; TGA; FTIR; polarizing light microscopy; SEM; Pyrolysis.

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ABSTRACT

The polymorphism in the copper(II) alkanoates, recently discovered for one member, has been thoroughly studied for the whole series, from 3 to 16 C atoms. Three polymorphic phases have been found, all of them sharing the same molecular unit, the paddle-wheel, which grows forming a 1D coordination polymer or catena. The three polymorphs are defined by a different packing of these catenae and a specific arrangement of the alkyl chains. Ten new crystal structures of those compounds have been solved by high resolution powder diffraction and presented in this manuscript.

The polymorphism in this series has been found to be monotropic and is responsible for the complex thermal behavior observed. The most characteristic feature, the endothermicexothermic effect, has been explained for the first time in these compounds by a combination of data from differential scanning calorimetry (in normal and modulated modes), powder Xray diffraction and Fourier transform infrared spectroscopy. These techniques, together with small angle X-ray scattering and optical microscopy, were used to analyze the hexagonal columnar discotic liquid crystal phase of copper(II) alkanoates. Thus, new information has been found in the packing and stacking of the discs formed by the paddle-wheel units, also maintained in the mesophase.

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1. INTRODUCTION

Copper(II) alkanoates have been studied for a long time by many authors, 1-7 using a great variety of techniques (DCS1, DTA 8, FTIR 9- 12, Raman9,13 and NMR8 spectroscopies, dilatometry14, conductance 15, EXAFS,3,4 and single crystal 16-25 and powder1,7 X-ray diffraction). Moreover, there are several reviews discussing general aspects of the copper(II) alkanoate series. Among them, the ones on metal-containing liquid crystals by GuiroudGodquin, 26 Donnio 27, or Polishchuk et al. 28 are very interesting. In another important article, by Mrovzinski’s 29 et al., a molecular magnetism study on copper alkanoates has been carried out oriented to modeling electrical, magnetic and optical properties in this family of compounds. Besides their behavior as surfactants because of their amphiphilic nature, their interest could be summarized as: 1) they are mesogens of columnar discotic liquid crystals (CDLC), 2) form adducts as a consequence of the particular coordination of copper (II) ion,25,29,30-34 3) are precursors of structures of organometalic compounds with biological or pharmaceutical applications, 35-37 4) can be used as templates or nanoreactors 38-42, in the synthesis of Cu and CuO nanoparticles of different shape, etc. Most of the metal soaps (or alkanoates), (Me(Cn)x from now on, where Me is the metallic cation (+x), and n the total number of C atoms of the carboxylate), are crystalline 2D coordination polymers in the crystal phase, showing a bilayered arrangement: 43- 46 an ionic layer (formed by the metal cations and the carboxylate anions), and a lipidic layer (van der Waals interactions between the -CH3 chain ends). However, copper(II)16-25 alkanoates, as well as rhodium(II)7,27 and ruthenium(II) 47 alkanoates, present a different arrangement, even when they maintain a similar bilayer structure, at least in most of their polymorphs. They 3



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present binuclear units, formed by the copper(II) and carboxylate ions, and designed in the literature as Chinese lantern 48 or more recently paddle-wheels,20,33 that can be represented as discs (see Figure 1a). These units are coordinated (a double Cu···O – Cu···O coordination bond) forming a catena (Figure 1b), which is a 1D coordination polymer. These fiber-like catenae are attracted one to another by Van der Waals forces. A very recent study on the copper(II) decanoate-decanoic acid phase diagram has allowed to discover a new polymorph of the copper(II) salt and helped to relate it with the other polymorphs, based on the different stacking of these catenae.25

Cu-O (coordination polymer) Cu-O C-O

h

C-C

a) Catena 1

Polymorph A

Polymorph B

Catena 2

Polymorph C

b)

c)

Figure 1. a) Paddle-wheel unit (disc) indicating its thickness (h); b) 1D coordination catenae (1 and 2 are similar, rotated 180º with respect to the axis of the discs), indicating the tilt direction of the discs; c) schematic representation of the three polymorphs of Cu(Cn)2 based in the catenae orientation and indicating the unit cell.

Three different polymorphs have been discovered so far for the members of the copper(II) alkanoates series (A, B and C).25 The three of them present the same molecular units, paddle4


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wheels catenae, the main difference being the packing of these catenae, apart from the different orientation of the alkyl chains in the polymorphs. Thus, polymorphs A and C are bilayered (or lamellar), whilst B presents a two-by-two interdigitated alkyl chains structure. Polymorphs A and B show the same orientation for all the catenae, and C presents two alternating catenae with opposite orientation (see Figure 1c). However, as it will be discussed later, not all the members present all the polymorphic phases. Thus, polymorph A has been only observed in members with n ≤ 10, polymorph B in even members with 8 ≥ n ≥ 14, and polymorph C in members with n ≥ 9. Thus, Cu(C10)2 is the only member with the three polymorphic phases, which are represented in Figure 2.

Polymorph A

Polymorph B

Polymorph C

Figure 2. The three polymorphs of Cu(C10)2, A (from this work), B (from reference 24), and C (from reference 25), where the different orientation and packing of the paddle-wheel catenae and the alkyl chains can be observed. The H atoms are omitted for clarity.

Recently, selective formation of CuO and Cu(OH)2 nanoparticles has been observed from lamellar and interdigitated polymorphs of Cu(C10)2, respectively,38 proving the relevance of these polymorphic structures. 5



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The thermal behavior of these compounds is very complicated and depends on the obtaining method of the samples. For instance, by recrystallization in different solvents or by cooling from the liquid crystal phase, mixtures of polymorphs and/or low-crystallinity effects may be obtained. In fact, an “endoexo” effect is found in the thermograms, designated in the literature as premelting or oscillations for the last 35 years,1b which is here explained for the first time as a structural relaxation (endo), followed by a cold crystallization (exo). Moreover, the relationship between polymorphs is monotropic, so the metastable polymorphs melt (totally or partially) before converting into the corresponding stable polymorphs. The mesogenicity of these compounds has been widely studied and is very characteristic.26,49 The liquid crystal phase appears on members of the series with n ≥ 4. The structure of this phase, studied by XRD,1 is hexagonal columnar discotic (HCDLC or simply CDLC, here on), with the exception of Cu(C4)2, which is tetragonal.1e The same paddle-wheel units (or discs) described above for the room temperature crystal are maintained in the CDLC phase,1,50 showing the same bond lengths (observed by EXAFS3,4 and PDF analysis25). It has been proved that the discs in the CDLC phase are not tilted (again with the exception of Cu(C4)2), in contrast to the crystal phase. Thus, the discs remain, as a core, forming the fiber-like 1D coordination polymer and the alkyl chains are melted. This phase decomposes before the transition to isotropic liquid. In this work, a thermal, spectroscopic, and structural study was carried out for the members of the Cu(Cn)2 series (up to Cu(C16)2). We also report the structural characteristics of this series, explaining the monotropic polymorphism. The structures of the new polymorphic phases are also provided (ten new structures solved by XRD), and the relationship between them is also discussed in depth, as well as the studies of the CDLC phase, using several techniques (such as DSC, X-ray scattering, FTIR, and optical microscopy). 6


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2. EXPERIMENTAL 2.1 Sample preparation Several methods have been employed to synthesize the different members of the Cu(Cn)2 series, depending on the alkyl chain length of the ligands, which make the corresponding alkanoic acids soluble or not. However, the recrystallization in different solvents clearly affects to the formation of different polymorphs with 8 ≤n ≤14. A general method to obtain either short or large copper(II) alkanoates consisted in the dropwise addition of alkanoic acid (Fluka, >99.0%) to a hot suspension of basic copper(II) carbonate (Fluka, >95%) in excess, in absolute ethanol, to which some mL of de-ionized water is also added.17,51 After 2-4 days, when evolution of CO2 is no more apparent, the hot solution is filtered to remove the excess of carbonate. Then, the solvent is evaporated to reduce its volume to one half, allowed to cool down to room temperature, and kept the flasks for 2-3 days at 273 K, obtaining the first crystallization of the salts. For members with n < 10, a specific method was used consisting in the direct reaction between the corresponding organic acid and grinded copper(II) oxide suspended in hot ethanol (Fluka, > 99.8%) heating at reflux, for several hours.10 One new method39 was still used to synthesize members with 9 ≤ n, consisting in the addition of CuCl2 (Aldrich, 99%) to a solution of the corresponding potassium or sodium alkanoate in deionized water. In this second method the samples were repeatedly washed with cold water and acetone. In the rest of the cases, a subsequent recrystallization was carried out using different solvents, such as heptane, ethanol, and benzene, among others. Their different polarity is responsible 7



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for the polymorph or mixture of polymorphs formation, as well as for the crystallinity of them. Since polymorph B of copper(II) decanoate24 (Cu(C10)2–B, from now on) had been obtained in ethanol and benzene, these two solvents were chosen in order to find this polymorph B in shorter and longer members of the series. Crystals suitable for single crystal XRD could be obtained by slow evaporation from ethanolic solutions (for members with n ≤ 10), although their single crystal quality decreased rapidly with the longer alkyl chains.16-24 For long members (n ≥ 10), a recent method was developed to obtain crystals: slow cooling of copper(II) alkanoates dissolved in the corresponding alkanoic acid (HCn).25 In Table 1, the different polymorphs obtained by different methods and solvents used are summarized. In some cases a mixture of polymorphs was formed, as in the cases of Cu(C8)2 in benzene, Cu(C9)2 in water, or cooling from the melt in the even members with n ≥ 8. The polymorphic phase C of Cu(Cn)2 with n ≥ 9 was also obtained by a thermal treatment consisting in heating until the end temperature of the endoexo effect. This method, explained later in the sample thermal behavior (paragraph 3.2), is the only one that allows obtaining Cu(C9)2-C. Table 1. Polymorphs obtained by the different method used (recrystallization in solvents and cooling from the liquid crystal phase). Method

n≤7

Cu(C8)2

Cu(C9)2

Cu(C10)2

Odd members n ≥ 11 C C C C*

Even members n ≥ 12 C C C C*

A A A+C C Water A A+B A B Benzene A A A B Ethanol A A A A* Heptane Corresponding A A A C C C alcanoic acid§ Thermal C C C C treatment# Cooling from the A A+B A* A*+B* C* C*+B melt (10 K·min-1) * Phases with low crystallinity. § Recrystallization in the alkanoic acid with the same C atoms. E.g., decanoic acid for Cu(C10)2.25 # Heating up to the end of the endoexo effect.

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No impurities of free acid (the most common one in these salts) have been detected by FTIR in any of the cases. The purities of the synthesized salts, determined by DSC in the fusion to the liquid crystal phase, are summarized in the Supporting Information (Table SI.7). 2.2. Powder X-ray Diffraction (PXRD) and Small Angle X-ray Scattering (SAXS) High resolution powder diffraction (HRPD) measurements were performed at SpLine beamline (BM25A) of the Spanish CRG at the European Synchrotron Radiation Facility (ESRF, Grenoble) in two different experiments: a) for Cu(C9)2–A and Cu(C11)–C, and b) for members from Cu(C12)2 to Cu(C16)2 (polymorphs C), with a fixed wavelength of 0.8266 and 0.7754 Å, respectively. All the measurements were carried out at room temperature. The powdered samples were placed inside a 0.7 mm-diameter glass capillaries, which were rotated during exposure. Data collection was done in a 2θ-step scan mode with 0.015º step and 3-5 sec. acquisition time per point. Cu(C9)2–C, Cu(C10)2–A, and the mixture of polymorphs of Cu(C8)2 (polymorphs A and B) were measured at room temperature in transmission mode in a Panalytical X’Pert PRO diffractometer equipped with a “hybrid monochromator” (a combination of a parabolic mirror and a Ge220 channel cut monochromator) and a fast RTMS detector X’Celerator (Cu Kα1 radiation, 1.54056 Å, 45 kV, 40 mA). The measurement ranges of 2θ were from 2º to 90º (for Cu(C9)2 and Cu(C10)2) and from 2º to 60º (for Cu(C8)2), and the step size 0.017º. The samples were prepared in 0.5 mm-diameter glass capillaries, and rotated during exposure. SAXS measurements as a function of temperature were performed at the I911-4 SAXS beamline of the Swedish Synchrotron MAX IV Laboratory MAX-lab synchrotron beamline I911-4 (SAXS The scattering vector q range analyzed was from 0.1 to 6.5 nm-1 (q = (4π / λ) sin (θ), where λ = 0.91 Å and 2θ the scattering angle). The sample- detector distance was 9



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calibrated with silver behenate. The data were collected in a bidimensional detector (PILATUS 1M, Dectris). A LINKAM hot stage model THMS600 was used to control the temperature of the sample. Data reduction was done using the software bli911-4. 52 Routine XRD measurements as a function of temperature were carried out in reflection mode with a Panalytical X’Pert PRO MPD X-ray diffractometer with vertical goniometer θ/θ and RTMS X’Celerator detector equipped with a high-temperature camera Anton Paar HTK1200 (Cu Kα1 radiation, 1.54056 Å, 45 kV, 40 mA, Ni filter). In-situ XRPD scans were measured in inert atmosphere (N2(g)) from 2θ = 2º-40º at several temperatures in the range from 298 to 393 K during heating and cooling, after programming every temperature change at a rate of 3 K·min-1. The Rietveld refinements and fitting were carried out using the FullProf 53,54 program. 2.3. Differential Scanning Calorimetry A TA Instruments modulated-DSC, Model Q20, connected to a RCS cooling unit, was used in the normal mode to register all the thermograms at a heating rate of 5K·min-1 (except those for the endoexo effect study, where the heat-only mode was used), in the temperature range from 200 to 600 K. Tightly sealed aluminum volatile pans were used, in dry nitrogen, flowing at 50.0 mL·min-1. A MT5 Mettler microbalance was used to weigh the samples, ranging between 3 and 10 mg (with an error of ±0.001 mg). The calorimeter was calibrated for temperature using standard samples of In and Sn, supplied by TA Instruments (purity >99.999% and >99.9%, respectively), and of benzoic acid (purity >99.97%), supplied by the former NBS (lot 39i), and for enthalpy with the In and Sn standards already described.

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Onset temperatures were read in all the experiments, with the exception of the solubilization process, in which the intersection of the Indium slope (moved to the maximum of the peak) and the base line is read. 55,56 The temperature precision obtained was ± 0.1 K. High pressure volatile pans, specially prepared to resist internal pressures ≤ 30 bars (provided by Seiko Inst. Inc.), were also used to register the decomposition process shown by all of the members at the liquid crystal phase, previous to the second fusion or clearing point. 2.4. FTIR spectroscopy. Infrared spectra of samples in KBr pellets were recorded using a Perkin-Elmer Spectrum100 FTIR spectrometer at a resolution of 2 cm-1. A commercial variable temperature cell, SPECAC VTL-2, adapted for solid samples, was employed to obtain IR spectra of the heated samples. 3. RESULTS AND DISCUSSION 3.1. Polymorphism: polymorphs A, B and C. Crystal structures The known structures, prior to the discovery of polymorphism in the Cu(Cn)2 series,24 are isostructural and correspond to the members from Cu(C3)2 to Cu(C8)2,16-22 and Cu(C10)2 (solved by Lomer et al,23 which present some particularities that are discussed at the end of this paragraph). They all present the structure of polymorph A. Ten new structures have been solved in this work for medium and long members of the Cu(Cn)2 series, from Cu(C8)2 to Cu(C16)2, including the three known polymorphic phases: A, B and C. Thus, the structures of Cu(C9)2–A and Cu(C10)2–A, Cu(C8)2–B, and Cu(C9)2–C to Cu(C16)2–C (with the exception of Cu(C10)2, recently published)25 are presented here. All the crystallographic data and

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fittings obtained by Rietveld refinement are given in the Supporting Information, as well as the cif files. A crystalline mixture of polymorphs (A and B, in 75.0% and 25.0% in mass per cent, respectively), obtained by recrystallization in benzene, was used to solve the structure of Cu(C8)2-B. The mixture was refined by the Rietveld method, using the known structure of Cu(C8)2-A22 and a model based in Cu(C10)2-B24 (Figure 3). The mixture was analyzed by SEM and the different morphology of the crystals of both polymorphs was observed (see SI).

60000 Yobs Ycalc Yobs-Ycalc Bragg peaks polymorph A Bragg peaks polymorph B

50000

5000 4000 3000

Intensity / a.u.

40000

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30000

2000 1000 0

20000 -1000 10

20

10000

30

40

2Θ / º

0 -10000 0

10

20

30

40

50

2Θº

Figure 3. Rietveld refinement for the mixture of Cu(C8)2-B and Cu(C8)2-A, showing the excellent agreement between the observed and calculated diffraction patterns, their difference, and Bragg positions (in orange and green, respectively).

Cu(C9)2-A was solved by single crystal XRD (see Supporting Information (SI), for details) with poor statistics, due to the bad quality of the crystals. However, the solution was used in the powder XRD measurements and properly solved by Rietveld refinement. This structure was also used as a model for Cu(C10)2-A (obtained by recrystallization in heptane, with low 12


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crystallinity), which was refined using rigid bodies constraints for the alkyl chains. The structures of the polymorphic phase C for Cu(C9)2 and Cu(C11)2-Cu(C16)2 were solved by Simulated Annealing (with the Fullprof program),53 using rigid bodies constraints for the alkyl chains, and refined by the Rietveld method (see SI). The three polymorphs are centrosymmetric and present a common asymmetric unit, formed by one Cu atom and two carboxylates. These two carboxylates are different: one of them with all-trans conformation of the C atoms of the alkyl chain and another one with a gauche effect in the alpha carbon. The paddle-wheel shows two asymmetric units. There is one cell parameter that is similar in the three polymorphs, which corresponds to the coordination axis of the catenae (with a value of 5.0-5.2 Å). Moreover, the paddle-wheels (discs) show tilt angles ranging from 25.5-30.6º, and the same thickness (4.52 ± 0.07 Å), calculated for all the known compounds in their polymorphic phase (see SI). Polymorph A has a triclinic unit cell (space group P-1) and shows a lamellar arrangement of same-oriented 1D catenae, which grow in the plane of the carboxylate with the all-trans alkyl chain. There is a characteristic angle between the axes of both alkyl chains with respect to the perpendicular plane of the Cu-O catenae of about 20-25º, depending on the compounds. Members from Cu(C3)2 to Cu(C10)2 show this polymorphic phase. The structure of Cu(C9)2A is shown in Figure 4.

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a)

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b)

c)

Figure 4. Structure of Cu(C9)2-A: a) asymmetric unit (the semi-transparent atoms have been generated by symmetry operations); b) angle formed by the two alkyl chains, perpendicular to the plane containing the catenae (021); c) lamellar packing along the a axis, showing the planes containing the C atoms (parallel).

Polymorph B presents a structure with interdigitated same-oriented 1D catenae, in a triclinic unit cell (see Figure 5). Unlike polymorph A, this catenae grow in the plane formed by the carboxylate with the gauche effect. The angle between the two alkyl chains is lower (5-15º) than in the previous case. This polymorph has been only solved for Cu(C8)2 and Cu(C10)2, but was also detected for Cu(C12)2 and Cu(C14)2 (see section 3.2). It is worth noting that the pseudo-hexagonal packing of the catenae in this polymorph is very similar to the one found in the CDLC phase (which is hexagonal). This resemblance may explain the partial formation of this phase when cooling from the liquid crystal phase.

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a)

b)

c)

Figure 5. Structure of Cu(C8)2-B: a) asymmetric unit (the semi-transparent atoms have been generated by symmetry operations); b) angle formed by the two alkyl chains, perpendicular to the plane containing the catenae (013); c) interdigitated packing along the a axis (pseudo-hexagonal arrangement).

The third polymorph observed in the Cu(Cn)2 series, C, appears in the members with n ≥ 9 (see Figure 6). It presents a monoclinic unit cell (space group C2/c) with 4 paddle-wheel units in the unit cell (Z = 8). The orientation of the catenae is alternate, as explained in the introduction (see Figures 1 and 2c), with a lamellar structure, as polymorph A, and growing also in the plane of the all-trans carboxylates. In this case, the angle between the alkyl chains is negligible, and the planes formed by the trans C-atoms of the two chains show a herringbone arrangement (internal orientational order), 57 with an angle of about 45º, unlike the other two polymorphs, where those plane are almost parallel (< 5º).

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a)

c)

b)

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d)

Figure 6. Structure of Cu(C11)2-C: a) asymmetric unit (the semi-transparent atoms have been generated by symmetry operations); b) projection of the plane containing the catenae (20-1), showing the negligible angle formed by the two alkyl chains; c) intersection of the planes containing the C atoms of the alkyl chains, showing the herringbone structure; d) lamellar packing along the b axis (the two alternating orientations of the catenae cannot be seen in this projection, see Fig. 2c).

The alkyl chain packing, parallel (polymorph C) and crossed (polymorph A), makes these polymorphs to show different 00l d-spacings, although both are lamellar. Thus, the linear relationship of this spacing with respect to the number of C atoms shows a step from polymorph A to C (see SI, section 5). This can be explained by the closer resemblance of polymorphs C (longer members) to hydrocarbons: as the number of carbons increases, the lipidic layer becomes more similar to paraffins, 58- 60 where the alkyl chains are parallel to each other and form a herringbone structure (internal orientational order). The structure of Cu(C10)2 solved by Lomer et al.23 obtained by the gel diffusion method with silica gel and aqueous solutions of potassium decanoate and copper(II) sulphate, does not correspond to any of these three polymorphic forms. However, it combines characteristics of 16


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the three of them: a) it is bilayered and presents a triclinic unit cell with a paddle-wheel unit (like A), but significantly different cell parameters; b) the 1D catenae is formed in the plane with the Cu atoms and the carboxylates with the gauche effect (like B); c) the axes of the two alkyl chains are parallel to each other (like C), although there is not a herringbone structure in this case. Although many attempts were carried out to obtain this structure, using the mentioned method and recrystallization in other solvents, it has not been possible to get it in this work. However, it is possibly another polymorphic phase of Cu(C10)2. 3.2. Thermal study. The endoexo effect The presence of polymorphism makes the thermal behavior of the compounds in this series very complex. Moreover, this polymorphism was proved to be monotropic, because the polymorphs do not show a reversible transition when they convert into another polymorphic form, or melt directly to the CDLC phase. Combined XRD and DSC data have allowed studying the stability of the polymorphs and their thermal parameters, and characterizing the phases in every thermodynamic and kinetic process. The stable polymorphic forms are found to be A for members with n ≤ 9, and C for n ≥ 10. Polymorph B is metastable for all the compounds that present this phase. A thorough thermal analysis study, by DSC and TGA, was done on all of the synthesized copper(II) alkanoate members. In Figure 7 (a and b), the DSC thermograms for the first heating of the thermal stable polymorph and second heating (after cooling from the CDLD phase) are shown, respectively. For more details, see SI (sections 6-8), where tables (SI.5, SI.6) with all the thermal data of each compound studied and figures of the DSC (pyrolysis, Figure SI.5) and TGA (Figure SI.6) thermograms are shown.

17



n=7

n=7 n=8 n=9 n = 10

n=8 n=9 n = 10

n = 11

n = 11 n = 12 n = 13 n = 14 n = 15 n = 16

(dQ/dt)/ a.u.

(dQ/dt)/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

n = 12 n = 13 n = 14 n = 15 n = 16

Cu(Cn)21st heating

Cu(Cn)2 2nd heating

ENDO

340

Page 18 of 39

ENDO

360

380

T/K

400

a)

340

360

380

T/K

400

b)

Figure 7. DSC thermograms at heating rate of 5 K·min-1: a) 1st heating of the stable polymorphs (A for n ≤ 9, and C for n ≥ 10); b) 2nd heating (after cooling at 10 K·min-1).

The transition temperatures are shown in Figure 8 for all the studied compounds. It is worth noting that all the members of the Cu(Cn)2 series decompose before reaching the isotropic liquid phase. The pyrolysis temperatures from the mesophase for all the members of the series, was measured by DSC (see SI, section 6). All of the members present a fusion on heating from a crystal phase to the CDLC phase (at a practically constant temperature for members above n > 8), with the exception of i) Cu(C3)2, that simply decomposes directly from the crystal, and ii) Cu(C4)2, which is the only member presenting two solid-to-solid enantiotropic transitions.

18


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560 540 520 500

fusion polymorph A fusion polymorph B fusion polymorph C T endo effect T exo effect T solid-solid transitions Cu(C3)2 and Cu(C4)2 T pyrolisis

480

T/K

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


460 440 420 400 380 360 2

4

6

8

10

12

14

16

18

Cu(Cn)2

Figure 8. Temperatures for the specific processes observed for the copper(II) alkanoates. All the transition temperatures were read on thermograms registered at 5 K·min-1. Cu(C3)2 suffers pyrolysis from the solid state.10

The total enthalpies (ΔtotalH) and entropies (ΔtotalS) are the sum of those functions are in the so called step-wise melting process (see SI, section 7, for more details). From these values, the contributions of the CH2 groups to the melting process can be estimated. The calculated CH2 entropic contribution is much lower than the theoretical value. That is, the alkyl chains in the CDLC are not yet completely melted. Thus, the rigid core of the paddle-wheels could include the contiguous C atoms (α, β, …) to the carboxylate one, still presenting the same conformation as in the crystal. The endoexo effect. Remarkable differences can be observed in the DSC thermograms for members with n ≥ 8 between the first and the second heatings, as the two peaks observed in the fusion of Cu(C8)2 and, mainly, the endoexo effect (n ≥ 9). This clearly points to a difference in the existing (or co-existing) phases and/or a lower crystallinity between the samples obtained by recrystallization (corresponding to the 1st heating) and the ones cooled from the melt (2nd heating). In fact, both effects are responsible of the different behavior. Moreover, both effects are also responsible of the depressions in the melting point and in the 19



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enthalpy of transition, found in all of the copper(II) alkanoates with endoexo effect, which are particle-size depending (nano or microsize). This effect had been already studied not only experimentally61,62 but also by modelling. 63- 67 A detailed study, particularized for Cu(C9)2, is shown in the SI (section 9). In the case of Cu(C8)2, the presence of two endothermic peaks in the thermograms corresponds to the fusion of the mixture of polymorphs formed on cooling the samples from the liquid crystal phase, similarly to what happens with the samples obtained from recrystallization in benzene (note that polymorph B melts 6 K below the melting point of polymorph A). On the other hand, the nature of the endoexo effect observed from n ≥ 9 is more intricate. One clear example is the Cu(C9)2, whose behavior was comparatively analyzed by DSC and XRD (see Figure 9). Thus, the crystalline sample obtained by recrystallization (in most of the solvents used) corresponds to polymorph A and shows a fusion in the first heating (i). When cooling from the CDLC phase at 10 K·min-1, the phase shows the same reflections by XRD (same structure), but with much broader peaks, indicating less crystallinity (smaller and more defective crystals). This phase is polymorph A* (ii), and presents an endoexo effect on heating, which indicates the formation of another polymorph after the melting (endothermic) and crystallization (exothermic) process: polymorph A* melts at a lower temperature than polymorph A due to the lower crystallinity, and the metastable liquid crystal phase formed crystallizes to polymorph C, which melts at a higher temperature. This new polymorphic phase can thus be obtained by holding the temperature at 370.7 K, after the endoexo process (iii), and cooling to RT. After this thermal treatment, polymorph C is obtained. It presents a different structure than the initial phase (see Figure 9b), and it shows only a fusion to the CDLC phase on heating (iv). FTIR measurements of this polymorph C, for Cu(C14)2, have 20


Page 21 of 39

been recorded as a function of temperature and no change in the bands was observed until the CDLC is reached (see reference 9, and SI for more details).

i)

CDLC

Polymorph A

dQ/dT (a.u.)

Polymorph C ii)

2ndheating: Polymorph A*

iii)

2ndheating: Polymorph A*

iv)

CDLC

Polymorph C

CDLC

ENDO 320

340

360

380

400

T/K

a) Polymorph A Polymorph A* Polymorph C

3.4

3.6

3.8

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


i)

ii)

iv) 5

10

15

20

2Θ / º

25

b)

Figure 9. a) DSC thermograms (recorded at 5K·min-1) showing the thermal behavior of Cu(C9)2: i) 1st heating: polymorph A, ii) 2nd heating: polymorph A*, iii) 2nd or next heating of any polymorph up to 370.7 K and waiting for a few minutes, iv) polymorph C obtained after cooling from 370.7 K. b) Diffractograms at room temperature of the corresponding phases of Cu(C9)2.

However, the endoexo effect depends on the phases that are present for a given compound, apart from the thermal treatment and heating rate (due to its kinetic nature). In this sense, the members of the Cu(Cn)2 series can be sorted in six groups (summarized in Table 2), according to the different polymorphs that they present. Moreover, the phase or mixture of 21



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Page 22 of 39

phases formed after cooling from the CDLC phase determine the thermal behavior of the compounds. Table 2. Polymorphs observed in the Cu(Cn)2 series, stable polymorphs, and phases and processes observed and in the endoexo effect. Group

I

II

III

IV

V

VI

Members

n≤7

Cu(C8)2

Cu(C9)2

Cu(C10)2

n = 12, 14

n = 11, 13 and n ≥ 15

Polymorphs observed§

A

A, B

A, C

A, B, C

C, B

C

Endoexo effect†

-

-

A*C

A*+B*C

C*+BC

C*C

§ Stable polymorph in bold. * Phases with low crystallinity. † Scheme of the transformation (in two steps for groups IV and V).

A thorough diffraction study as a function of temperature was carried out to analyze in detail the endoexo effect, specially in the cases where polymorph B is also present after cooling from the melt. The most complex case occurs for Cu(C10)2, where the three polymorphs are observed in a second heating. The thermogram and the thermodiffractograms (3D and 2Dcontour plot) of the second heating of Cu(C10)2 are shown in Figure 10(a-c). The different processes and phases involved are the following: 1) initially, a mixture of polymorphs A* and B* (both with low crystallinity) is observed; 2) polymorph A* starts to melt, and a rearrangement of polymorph B* is detected; 3) polymorph C starts to crystallize (from A, which disappears completely), and polymorph B is completely ordered until the end of this step, when it starts to melt; 4) polymorph B melts to CDLC, so the coexisting phases are polymorph C and CDLC; 5) crystallization: the corresponding part of CDLC phase crystallizes to polymorph C; 6) finally, polymorph C melts to liquid crystal.

22


Page 23 of 39

2

1

dQ/dT / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3 4

5

6

polymorph A fusion

polymorph B fusion ENDO

polymorph C fusion 350

360

370

380

T/ K

b)

390

a)

c)

Figure 10. a) DSC thermogram (recorded at 5K·min-1), corresponding to the second heating (after melting) of Cu(C10)2. 3D (b) and 2D (c) representation of the thermodiffractograms for low q values, corresponding to the second heating of Cu(C10)2; The peaks corresponding to polymorph B are indicated with black arrows for clarity.

It is worth noting that polymorph B has been only solved for Cu(C8)2 and Cu(C10)2, but it has been detected in Cu(C12)2 and Cu(C14)2 (group V), after cooling from the CDLC phase. Thus, the presence of this polymorph has been observed in samples obtained by cooling from the CDLC phase, as it shown in Figure 11, with significantly less amount of this phase for Cu(C14)2. Their thermograms show a slightly larger endoexo effect in comparison with those for odd members with long alkyl chains. 23



In groups V and VI, there is no evidence of the existence of polymorph A. A less crystalline form of polymorph C is found instead, from recrystallization in heptane or by cooling from the liquid crystal. The thermal behavior and existing phases for the other groups listed in Table 2 (showing the thermodiffractograms) are explained in the SI (section 11).

Intensity / a.u.

0.01

Cu(C12)2 recrystallized Cu(C12)2 cooled from CDLC polymorph B

1E-3

1E-4 1

2

3

4

5

6

a)

q / nm-1 Cu(C14)2 recrystallized Cu(C14)2 cooled from CDLC

0.01

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 39

polymorph B 1E-3

1E-4 1

2

3

4

5

6

b)

-1

q / nm

Figure 11. Diffraction patterns of Cu(C12)2 (a) and Cu(C14)2 (b) of samples recrystallized in heptane and after cooling from the CDLC phase, showing the existence of polymorph B in these compounds.

The mixture of polymorphs in Cu(C14)2 was also analyzed by FTIR. At room temperature, the carboxylic region of the infrared spectra of the sample quenched from the CDLC (Figure 10a) shows a characteristic pattern with the asymmetric and symmetric carboxylate stretching bands centered at 1585 cm-1 and the 1471 cm-1 respectively. When heating the sample these bands shift to 1583 and 1467 cm-1 (Figure 10b), that correspond to the values observed in the crystal of Cu(C14)2-C (Figure 10c). The rest of bands present in this region are coincident in

24


Page 25 of 39

the two samples. This fact confirms that the polymorphs present in the quenched Cu(C14)2 transform in the polymorph C after the endoexo effect.

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c) b)

a)

1700

1600

1500

1400

1300

-1

wavenumbers / cm

Figure 12. FTIR spectra in the carboxylic region for CuC14 quenched from the CDLC a) 298 K, b) at 368 K (after the endoexo effect) and c) Cu(C14)2 polymorph C at 368 K.

Final considerations on the nature of the endoexo effect in copper(II) alkanoates. It is very well known that some linear polymers (e.g., PET, 68-70 PEN,68 PS 71 or textile fibers 72,73) present a similar endoexo effect, which is associated with a glass transition separately at lower temperature or hidden underneath the endoexo peaks, described in the literature as conformational relaxation- cold crystallization. This glass transition can be detected by modulated-DSC. This experiment was carried out for Cu(C9)2 and Cu(C14)2 (see SI, section 9) and no glass transition was detected, observing a melting process instead. In conclusion, the endoexo effect in the Cu(Cn)2 series consists in an endothermic reversing fusion of the metastable polymorph or mixture of polymorphs, with low crystallinity (small and defective crystals), followed by an exothermic nonreversing cold crystallization.

25



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In the samples presenting polymorphism, no reversible transition from one polymorph to another has been observed in the same heating. So all the polymorphs observed should be considered monotropic 3.3 Hexagonal Columnar Discotic Liquid crystal phase (CDLC) As it has been studied for many years,1-7,9 the Cu(Cn)2 compounds present a CDLC phase. The paddle-wheel structure is maintained in the CDLC and presents the same bonding lengths than in the crystal phase, as it was observed by EXAFS3,4 and by PDF analysis.25 This phase shows a hexagonal arrangement in the columns packing, formed by the stacking of paddle-wheels, and where the alkyl chains are melted (disordered) (see Figure 13).

3

1

4

2

Figure 13. Representation of the structure of the columnar discotic liquid crystal phase (CDLC), showing the molecules/discs (1), hexagonal packing (2), honeycomb domain (3) and developable domain (4). Crystalline domain thickness: H = h×(25-30).

Calculation of the stacking period of discs in the CDLC phase. In the diffraction patterns of the CDLC phase, a diffuse band at 4.50 Å is observed, which corresponds to the c parameter of the hexagonal unit cell (see Figure 14a). The profile matching of the patterns with a 26


Page 27 of 39

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hexagonal cell gave a value for c of 4.49±0.13 Å. The intercolumnar distance D (which corresponds to the a parameter of the hexagonal unit cell) was also calculated from the diffraction profile matching in this phase for all the members of the series. Since the paddlewheel units are still present in this phase and the discs are perpendicular to the c axis of the hexagonal cell, the stacking period of the columnar mesophase (h), that is, the thickness of the discs formed by the paddle-wheel units in the mesophase, is constant for all the members. Thus, the unit cell volume and, therefore, the area of the hexagons (since h does not vary) show a linear relationship with the number of C atoms, and the value of (dD2/dn) can be obtained, which is also constant. From this data, h can be estimated, as described in reference 74 (see SI, section 12, for details). A value of h = 4.41±0.06 Å was thus obtained. This h value is slightly smaller than that calculated by Abied et al.1b (4.68 Å), but is in fair agreement with the c parameter obtained from the X-ray diffraction data. On the other hand, the disc thickness calculated for the different polymorphic crystals (at RT) presents an average value of 4.52 ± 0.07 Å (see SI. 12, Table SI.9), which is also similar to the one in the CDLC phase.

27



(100)

Intensity / a.u.

(001)

16

18

20

22

24

2Θ / º

(110) (200) 10

20

30

40

2Θ / º

CDLC crystal

0.01

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 39

(100) 1E-3

(110)

1E-4

0.1

1

10

q / nm-1

Figure 14. a) Powder pattern of Cu(C15)2 in the CDLC phase (green arrow indicating the diffuse band corresponding to the (001) plane); b) SAXS curves for the crystalline and CDLC phases of Cu(C15)2 (blue arrow indicating the presence of a characteristic distance in the CDLC phase).

SAXS measurements in the CDLC phase. The liquid crystal phase was studied by SAXS, for the first time, in order to explore the low q range and detect characteristic distances larger than the presented on the hexagonal packing. The crystal and the CDLC phase show a different power law in the small-angle scattering curves. Furthermore a broad peak is only detected in the liquid crystalline phase (see Figure 14b). The center of the peak is around 0.50 nm-1, which corresponds to a characteristic distance of 12.5 nm (125 Å). A similar peak is observed for all Cu(Cn)2 series in the CDLC phase (see Figure SI.18) The peaks of powder pattern in the liquid crystal phase (Figure 14a) show that the hexagonal structure presents long range order in the plane of the hexagons, and not in the stacking direction of discs (axis c), as in a honeycomb arrangement presented in Figure 13-3. 28


Page 29 of 39

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However, it is well known that the discs are randomly stacked one respect to the other (maintaining the 1D coordination), and the distance of 125 Å could correspond to the range of the liquid crystalline order (H) of these discs (around 25-30 discs) (Figure 13). The texture in the CDLC observed by optical microscopy consists in developable domains (Figure 13-4), which in fact are honeycomb structures which director axis (in this case, the direction of the 1D coordination polymer) is bended and grow in spiral. 75,76 The same distance (H) has been observed in the highly concentrated mixtures of the decanoic acid-copper(II) decanoate phase diagram,25 where the same developable domains are formed in the CDLC phase (see Figure SI.18). 4. CONCLUSIONS The polymorphism in the members of the Cu(Cn)2 series and the columnar discotic liquid crystal phase have been thoroughly described in this work combining results from several techniques, such as XRD, DSC, FTIR, SAXS and polarized light microscopy. Copper(II) alkanoates present a common core structure, the paddle wheel, which extends forming a 1D coordination polymer (catenae). Diverse arrangements of these 1D catenae define the different polymorphic crystal phases. Three of them have been identified (A, B and C), being A for members with n

Alkanoates Series: Monotropic Polymorphism and Mesomorphism

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