Lanthanide(III) alkanoates are salts of alkanoic acids (fatty acids). They are also known as “lanthanide soaps” and their general formula is [R(CnH2nþ1COO)3].
The first examples of lanthanide salts of the higher alkanoic acids were described by Mehrotra and coworkers (Mehrotra et al., 1966; Misra et al., 1963a), although shorter homologues have been reported much earlier (Wolff, 1905). The lanthanide(III) alkanoates can be prepared by a metathesis reaction between the corresponding sodium alkanoate and the lanthanide salt (nitrate or chloride) in ethanol/water solution:
3CnH2nþ1COONaþRX3!R Cð nH2nþ1COOÞ3þ3NaX: ð14Þ The metal soaps can be purified by recrystallization from 1-pentanol. The pH of the solution used for synthesis is of importance, because the alkanoic
[R(OH)x(CnH2nþ1)3x] are formed at high pH values (Skrylev et al., 1980).
The experimental procedures had to be adapted for the short chain homolo- gues, because of too high a solubility of the compounds in the solvents used for the synthesis of the long-chain compounds. Butyrates were obtained by reaction between the corresponding lanthanide(III) hydroxide and butyric acid in a 1:3 molar ratio (Binnemans et al., 2000c) Another synthetic method is the reaction between a lanthanide(III) isopropoxide and butanoic acid in a 1:3 molar ratio in benzene (Hasan et al., 1968). Typically, lanthanide soaps with short chain lengths are obtained as dihydrates, those with an intermediate chain length as mono- or hemihydrates, and the homologues with a long chain length as anhydrous compounds. The water of crystallization is lost upon heating before the melting point is reached or upon melting, so that com- pounds obtained from cooling of the melt are always anhydrous. Anhydrous soaps can be synthesized by reaction between the alkanoic acid and anhydrous lanthanide(III) chloride in dry toluene (Misra et al., 1963b). By changing the stoichiometric ratio, it is possible to obtain mixed compounds of the type [RClx(CnH2nþ1COO)3x].Misra et al. (1987)prepared lanthanide(III) alkano- ates with three different alkyl chains within one and the same compound by stepwise substitution of the isopropoxide groups in lanthanide(III) isopropox- ides. The long chain homologues are insoluble in water and have a low solu- bility in all organic solvents at room temperature. They are soluble at elevated temperatures in the highern-alcohols (1-butanol, 1-pentanol,. . .) and in aro- matic solvents (benzene, toluene,. . .). For crystallization, then-alcohols are preferable over the aromatic solvents, because in the former solvents an easier filterable precipitate is obtained, whereas the use of aromatic solvents leads to gel formation. The best filterable precipitates are obtained in 1-pentanol—
ethanol mixtures. The solubility of the short-chain homologues is much higher than that of the compounds with long alkyl chains. Data on scandium(III) alkanoates are scarce (Rai and Parashar, 1979). Perfluorinated alkanoic acids also form metal soaps with trivalent lanthanides (Fan et al., 1988).
In the solid state, the structure of the lanthanide(III) alkanoates can be described as an infinite ionic sheet of lanthanide ions and carboxylate groups, separated by a bilayer of alkyl chains (Binnemans et al., 2000c; Jongen et al., 2001a,b,c; Marques et al., 1998). The alkyl chains are in the all-transconfor- mation and perpendicular to the ionic layer. It is possible to calculate the d-spacing using the formula (Marques et al., 1998)
dcalc¼2dCHþ2ðn1ÞdCCsin55þ2dCOþ2rR3þ, ð15Þ where n is the total number of carbon atoms in the chain, dC–H¼1.09 A˚, dC–C¼1.54 A˚ anddC–O¼1.36 A˚.rR3þ is the radius of the trivalent lanthanide ion. The coordination mode of the carboxylate groups in the mesomorphic lanthanide alkanoates is not known in detail yet, although some structural fea- tures can be derived from the crystal structures of lanthanum(III) butyrate
hydrate (Jongen et al., 2001e) and neodymium(III) butyrate hydrate (Binne- mans et al., 2000c). For these compounds it was observed that different types of lanthanide-carboxylate coordination modes are present. Although it is in principle possible to deduce the type of lanthanide-carboxylate bonding by infrared spectroscopy, these studies do not give an unambiguous answer, probably due to the presence of multiple coordination modes. The lantha- nide(III) alkanoates have a polymeric structure. Although only three carboxyl- ate groups are sufficient to balance the tripositive charge of the R3þion, this only results in six coordinate complexes if three bidentate carboxylate groups bind to one lanthanide ion. The coordination sphere of the lanthanide(III) ion is unsaturated in this situation. The lanthanide(III) ion can saturate its coordi- nation sphere by allowing the carboxylate groups to act as bridges between two lanthanide(III) ions and by sharing of an oxygen atom between two lan- thanide ions. Also coordination of water can help to saturate the coordination sphere of the lanthanide(III) ion. Structural changes in the metal-carboxylate network are possible over the lanthanide series because the large ions in the beginning of the lanthanide series typically have higher coordination numbers than the small ions at the end of the series.
Mesomorphism of the lanthanide(III) soaps was first observed by Burrows and coworkers in a series of cerium(III) alkanoates (Marques et al., 1998).
These authors found by polarized optical microscopy and by DSC that these compounds exhibit a viscous mesophase which was tentatively assigned as a SmA-like mesophase. Without high-temperature XRD it is difficult to deter- mine the mesophase type unambiguously, because the optical textures of these compounds are not diagnostic. It was observed that the thermal behavior was complex, in the sense that the second heating run of the DSC differed mark- edly from the first heating run. Especially theDHvalues for two heating runs were markedly different. On the basis of the transition enthalpies, it was con- cluded that electrostatic interactions in the polar region (due to the essentially ionic character of the metal-carboxylate bond) play an important role in the melting process. In the series of homologous cerium(III) alkanoates, each CH2 group contributes for 2.5 kJ mol1 to the total melting enthalpy. This value is lower than the 3.8 kJ mol1associated with complete fusion of ali- phatic chains in their fully crystalline state (Seurin et al., 1981). An analysis of the corresponding entropy data showed that the chains melt incompletely, so that molecular aggregates are present in the melt. The mesophase of lantha- num(III) tetradecanoate was identified as a smectic A phase byBinnemans et al. (1999b)via XRD studies. Shortly thereafter, an in-depth study of the thermal behavior of the lanthanide(III) alkanoates was started by Jongen and Binnemans. Both metal ion and the alkyl chain were varied in a system- atic way. This work revealed that of the series of the lanthanide(III) dode- canoates, only the lanthanum(III), cerium(III), praseodymium(III) and neodymium(III) compounds form a mesophase, but not samarium(III) dodecanoate and the dodecanoates of the heavier lanthanides (Fig. 56;
Binnemans et al., 2000d). It was also observed that the change ind-spacing at the crystal-to-mesophase transition increased over the lanthanide series. The alkyl chain length was found to have an influence on the thermal behavior.
For the neodymium(III) alkanoates, a mesophase was observed for the homol- ogous series [Nd(C4H9COO)3]–[Nd(C14H29COO)3], but not for neodymium (III) hexadecanoate and the homologues with longer alkyl chains (Binnemans et al., 2000c). The compounds with short alkyl chains exhibit two meso- phases: a smectic A phase at high temperature and an unidentified high- ordered smectic phase M at lower temperatures. The mesophase behavior of the alkanoates of lanthanum(III) (Jongen et al., 2001b), cerium(III) (Jongen et al., 2001c), and praseodymium(III) (Jongen et al., 2001a) is very compara- ble to the mesophase behavior of the neodymium(III) alkanoates, except that these compounds also form a mesophase for the hexadecanoates and the higher homologues. A typical feature for the lanthanide(III) alkanoates is the large supercooling for the clearing temperature. For conventional LCs, the onset temperature of the clearing point in the heating run of a DSC ther- mogram coincides with the onset temperature of the clearing point in the cool- ing run. The difference can be more than 10C for the lanthanide(III) alkanoates. It is not easy to determine whether some of the lanthanide(III) alkanoates form a mesophase or not. In contrast to many other types of LCs, it is very difficult to obtain well-developed optical textures for the lantha- nide soaps. These compounds have a very strong tendency to align homeotropi- cally between two microscope glass slides. As a result, the sample appears black
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 80
100 120 140 160
Crystal
SmA Isotropic liquid
Temperature (°C)
FIGURE 56 Phase diagram of the lanthanide(III) dodecanoates (drawn using the data inTable 2 ofBinnemans et al., 2000d).
between crossed polarizers and no optical texture can be observed. Sometimes birefringence can be induced in the samples by pressing with a needle on top of the cover glass slide or by shearing the two glass slides, but often this is only a transient phenomenon. As soon as the pressure or shear force are relieved, the birefringence disappears. It is not evident whether the disappearance of the bire- fringence is due to the absence of a mesophase in the sample or to the fact that the sample is homeotropically realigned. Therefore, it is not surprising that conflicting results appear in the literature. Whereas Binnemans and Jongen have not obtained evidence for a mesophase in europium(III) dodecanoate, a smectic A phase was reported by other authors for europium(III) alkanoates with longer alkyl chains (Li et al., 2005a). The latter authors state that the higher atomic number of the lanthanide, the longer the alkyl chains of their alkanoates has to be in order to form a mesophase on heating. On the other hand,Binnemans et al. (2000c)observed for the neodymium(III) alkanoates a narrowing of the mesophase stability range with increasing chain lengths and eventually a disap- pearance of the mesophase. It is evident that further work is needed to solve this issue. In a very interesting paper,Corkery (2008)considers the phase sequence crystalline (Cr)—crystalline smectic rotor phase (CrB)—partially molten-chain smectic (SmA1)—molten-chain smectic (SmA2)—ordered melt for the lantha- num(III), cerium(III), praseodymium(III) and neodymium(III) alkanoates with long alkyl chains (C12H25, C14H29, C16H33, C18H37), and the phase sequence crystalline (Cr)—crystalline smectic rotor phase (CrB)—conformationally dis- ordered smectic rotor phase (SmB)—ordered melt for the samarium(III), ter- bium(III), holmium(III), thulium(III) and ytterbium(III) alkanoates. The SmA1 phase corresponds to the M phase observed by Binnemans et al.
(2000c)for the neodymium(III) alkanoates and the SmA2 corresponds to the SmA phase reported by Jongen and Binnemans for the lanthanide(III) alkano- ates. The lanthanum(III) soaps show a rotator phase CrB over a narrow temper- ature range between the crystalline state and the SmA1 phase. This Cr–CrB transition can only be observed in the DSC thermogram as a small skewing of the melting-peak to lower temperatures. The temperature existence region for the CrB phase becomes larger for the alkanoates of the heavier lanthanides.
The lanthanum(III), cerium(III) and terbium(III) salts of a C17fatty acid with a methyl group in the mid chain are room-temperature LCs, and they exhibit a hexagonal columnar mesophase (Corkery, 2004). The low melting points of the compounds are attributed to packing frustration of the long alkyl chains due to mid-chain methyl group. The corresponding lutetium(III) compound does not show liquid-crystalline behavior at room temperature, but birefrin- gence can be induced by a shear force.Corkery (2008)gives a detailed interpre- tation of the thermal phenomena observed for lanthanide soaps. He draws an analogy between lanthanide soaps and metal-organic frameworks (MOFs) and considers the anhydrous and hydrated lanthanide soaps as 2D MOFs at room temperature. During heating, the alkyl chains and the MOF head groups
increases, and the compounds are transformed to either a 2D smectic rotor- phase MOF, 2D liquid-crystalline smectic MOFs, or 1D liquid-crystalline smectic MOFs. Hydrated lanthanide soap samples show a significantly higher heat of fusion than samples obtained by crystallization from the melt. This is due to the release of coordinated water. Moreover the hydrated samples also show a higher crystallinity. The thermal and hydration history of the lantha- nide(III) alkanoates does not seems to influence the first order transition tem- peratures, but rather the heat changes and the temperature range of the second order (or weakly first order) transitions that proceed the main first order tran- sition (premelting effects). The escape of water molecules during the heating process softens the polar layer parts, induces disorder in the hydrocarbon chains and causes the transitions to occur over a wider temperature range in the hydrated lanthanide soaps in comparison with the anhydrous salts.Corkery (2008)presents a structural model to explain why a smectic A1 phase can only be formed by large lanthanide ions and not by small ones. The SmA1 phase is considered as a 2D MOF and can be formed by polymerization of 1D MOFs.
The interfacial area of the chains defines the distance that the metal- carboxylate bonds must bridge for polymerization of the 1D MOFs into 2D MOFs. The model predicts that the R–O distance must be greater than about 2.64 A˚. Because the Nd–O distances in neodymium(III) butyrate monohydrate are between 2.42 and 2.66 A˚ (Binnemans et al., 2000c), it can be understood why neodymium(III) alkanoates are borderline cases for the occurrence of the SmA1 phase. The SmA2 phase is assumed to consist of 1D MOFs. Transi- tion temperatures of the lanthanide(III) alkanoates are summarized in Tables 17–21. Although samarium(III) dodecanoate and the dodecanoates of the heavier lanthanides do not exhibit a mesophase, it was found that a meso- phase could be induced by preparing binary mixtures of these compounds with lanthanum(III) dodecanoate (or with cerium(III), praseodymium(III) or neo- dymium(III) dodecanoates) (Jongen et al., 2001d). The mole fraction of lan- thanum(III) dodecanoate required to induce the mesophase increased over the lanthanide series. Whereas 4 mol.% of lanthanum(III) dodecanoate was sufficient to induce mesomorphism in europium(III) dodecanoate, this amount increased to 48 mol.% for ytterbium(III) dodecanoate. A melt of the higher homologues of the lanthanide(III) alkanoates can be easily supercooled to a transparent glass, although the glass samples tend to crystallize after a period of 1 year or longer.Corkery and Martin (1999)studied the luminescence of vitrified europium(III) dodecanoate. Binnemans et al. (2001a) determined the Judd-Ofelt intensity parameters of glasses of lanthanide(III) octadecano- ates. The lanthanum(III) salts of 4-hexyloxybenzoic acid, 4-octyloxybenzoic acid and 4-nonyloxybenzoic acid form a smectic A phase, whereas the corresponding holmium(III) complexes are not liquid-crystalline (Jongen et al., 2003). For the lanthanide(III) salts of 4-hexyloxybenzoic acid, only those of lanthanum(III) and praseodymium(III) show a mesophase (SmA)
(Jongen et al., 2004). The corresponding compounds of the heavier lantha- nides (R¼Nd, Sm, Eu) are not liquid-crystalline.Singh et al. (2005)investi- gated the thermal properties of a series of lanthanide(III) complexes of 4-alkoxybenzoates, with different chains lengths (CnH2nþ1,n¼6, 8, 10, 12, 16) and lanthanide ions (R¼La, Pr, Nd, Eu, Gd, Tb, Dy). Most of the complexes were found to exhibit a smectic A phase. Anhydrous and hydrated samples of lanthanide(III) phytanates show both thermotropic and lyotropic mesomorphism (Conn et al., 2010). Phytanic acid (¼3,7,11,15-tetramethyl hexadecanoic acid) has an isoprenoid-type of hydrocarbon chain with four methyl substituents along its saturated backbone. Several of the hydrated salts form a hexagonal columnar phase at room temperature.
TABLE 17 Thermal Behavior of Lanthanum(III) Alkanoates Compound Temperatures (C) Reference
[La(C3H7COO)3] Cr 188.4 I Jongen et al. (2001b) [La(C4H9COO)3] Cr 93.8 M 139.2 SmA 190.2 I Jongen et al. (2001b) [La(C5H11COO)3] Cr 94.4 M 137.9 SmA 189.9 I Jongen et al. (2001b) [La(C6H13COO)3] Cr 97.7 M 133.5 SmA 189.3 I Jongen et al. (2001b) [La(C7H15COO)3] Cr 94.6 M 122.4 SmA 175.9 I Jongen et al. (2001b) [La(C8H17COO)3] Cr 98.1 M 121.0 SmA 179.8 I Jongen et al. (2001b) [La(C9H19COO)3] Cr 95.6 M 122.1 SmA 172.3 I Jongen et al. (2001b) [La(C10H21COO)3] Cr 106.9 SmA 165.9 I Jongen et al. (2001b) [La(C11H23COO)3] Cr 110.0 SmA 160.6 I Jongen et al. (2001b) [La(C12H25COO)3] Cr 116.4 SmA 154.6 I Jongen et al. (2001b) [La(C13H27COO)3] Cr 120.3 SmA 157.1 I Jongen et al. (2001b) [La(C14H29COO)3] Cr 121.4 SmA 157.0 I Jongen et al. (2001b) [La(C15H31COO)3] Cr 123.4 SmA 148.2 I Jongen et al. (2001b) [La(C16H33COO)3] Cr 123.9 SmA 149.7 I Jongen et al. (2001b) [La(C17H35COO)3] Cr 125.9 SmA 148.0 I Jongen et al. (2001b) [La(C18H37COO)3] Cr 126.7 SmA 126.7 I Jongen et al. (2001b) [La(C19H39COO)3] Cr 140.1 SmA 140.1 I Jongen et al. (2001b) [La(C20H41COO)3] Cr 128.1 SmA 143.6 I Jongen et al. (2001b) Cr, crystalline phase; M, unidentified smectic phase; SmA, smectic A phase; I, isotropic liquid.