Rare earth metals form intermetallic compounds with most of the metallic and semi-metallic elements in the periodic table. Rogl, Phase equilibria in ternary and higher order systems with rare earth elements and silicon 1.
HISTORICAL INTRODUCTION
The present chapter focuses on the most recent achievements (up to the early beginning of 2013) in the synthesis of rare-earth borohydride complexes designed for the polymerization of polar monomers. SYNTHESIS OF RARE-EARTH BORONHYDRIDE COMPLEXES The general aspects of the synthesis and characterization of rare-earth boron hy-.
SYNTHESIS OF RARE-EARTH BOROHYDRIDE COMPLEXES The general aspects of the synthesis and characterization of rare-earth borohy-
Amidinates and guanidinates are among the most popular ligands in postmetallocene rare earth element chemistry (Edelmann et al., 2002). The reaction of RCl3(Ln¼Y, Yb) with sodium salts of b-diketiminate gave rare earth bis(b-diketiminate) chlorides.
RARE-EARTH BOROHYDRIDE INITIATORS IN THE POLYMERIZATION OF POLAR MONOMERS
The molar mass of the polymer synthesized from 1N increased as the yield (Mn to 39,000 g mol1 with Mw=Mnca:1:4) was fairly well controlled. No further characterization of the PHB (chain ends, thermal transition temperature of isotactic PHB) was reported. The series of bis(phosphinimino)methane complexes of the rare earth elements developed by Roesky and colleagues were first evaluated in the ROP of CL of the alkoxide initiators prepared in situ (Gamer et al., 2007) (Table 1).
Quite good control of the molar mass (assuming two growing polymer chains per metal center) and dispersity values was obtained. Thus, the initial step of the polymerization of TMC promoted by [Sm(BH4)3(THF)3] (1Sm) involves substitution of the coordinated THF with the first three incoming carbonate molecules to give [Sm(BH4)3(TMC)3] (i) (Schedule 33). The activities of the rare earth borohydrides studied in the ROP of polar monomers (under the reported operating conditions, i.e. reaction times not necessarily optimized), including lactones, lactides, N-carboxyanhydrides and carbonates can be estimated from data collected in tables 1-4.
All rare earth borohydride initiators, whether or not in the presence of an alkylating agent and regardless of the solvent, were (compared to the ROP of cyclic esters, vide supra) generally poorly active in the polymerization of MMA (non-optimized TOF values over generally 120 molMMAmolrare-earthh1) with one claimed exception (11,120 molMMAmolSch1, Jian et al., 2010).
COMPUTATIONAL STUDIES: DFT INPUT INTO THE MECHANISM OF THE POLYMERIZATION OF POLAR
Isotactic formation of PMMA due to capture of BH3 from the surface allowing the formation of the enolate [(@SiO)La(BH4)]. Increased reactivity due to the presence of the silica surface Possible blocking of BH3 from the surface. Indeed, the first step involves the attack of the borohydride ligand that is decoordinated by.
Moreover, the mechanism involves only two steps, as in the DFT study of the ROP of CL by the trivalent derivatives (Barros et al., 2008a). It is noteworthy that the formation of the enolate complex is determined exergonically, because the capture of the BH3 is ensured by the nitrogen of the bis(phosphinimino)methanide ligand, as similarly observed in the ROP of TMC (Guillaume et al., 2012).
CONCLUSIONS AND OUTLOOKS
For example, in the specific case of borohydride catalysts, it has been unequivocally shown theoretically that the main problem in targeting well-defined polymers is capturing the formed BH3 molecule. When the BH3 portion is captured by the carbonyl group, only α,o-dihydroxytelechelic polymers are obtained in the case of cyclic monomers, while poor activity is obtained with MMA. On the other hand, if the BH3 molecule can be efficiently captured by the solvent, ancillary ligand or surface, a greater diversity of polymers and, consequently, of polymer properties can be accessed.
Likewise, theoretical methods are able to accurately account for polymer tacticity and the steric and electronic effects that control selectivity. The authors gratefully acknowledge the collaborators whose names appear in the following references for their contributions to this research.
INTRODUCTION
In this chapter, the concept of "liquid" as one of the three states of matter is first defined and the following points are discussed: the general theory of liquids, structural characteristics and properties of molten salts, and methodology of structural analyses. The current state of the art of works on structures and properties of rare-earth molten salts is outlined. In contrast, particles in liquids are mobile and come into contact easily; thus, interparticle interactions are very difficult to model, which has consequently slowed progress in the solid-state physics of fluids.
A legitimate theory of liquid emerged in the 1930s by taking advantage of the analysis of atomic arrangement in liquids by X-ray diffraction (XRD). The main differences between solids and liquids are that solids have short order as well as long order, but liquids have only the latter order; furthermore, liquids retain mobility but keep their density constant thanks to the cohesion between the constituent particles.
MOLTEN SALTS AS LIQUIDS
The fraction of packing in the closest packed crystals is estimated at about 3/4 and that in liquids at about 1/2; this disparity is a fundamental factor leading to fluidity for liquids. The attractive force between molecules in molecular liquids is well expressed by a Van der Waals potential which varies as the inverse sixth power of the intermolecular spacer.
DEFINITIONS OF MOLTEN SALTS
TYPICAL STRUCTURES OF MOLTEN SALTS
Direct Fourier transformation of the scattered intensity leads to the pdf or RDF only for the systems consisting of monatomic liquids. Distancermax, which gives the first maximum inD(r), is taken as the mean radius of the first shell. As for method C, it is usually difficult to define the position of the second peak and to extrapolate the coordination number.
For large cations, on the other hand, significant penetration of the first shell by like ions results in an asymmetric first peaking(s). As a consequence, the evaluation of the nearest neighbor coordination number should be based on the integration of g(r) orgþ(r) up to the intersection of forgþ(r) and gg(r), or up to the first point wheregþ(r) ) approaches unity from the origin.
CRYSTAL STRUCTURES OF RARE-EARTH HALIDES
The crystals of LaCl3to GdCl3 have hexagonal UCl3-type structure, and those of YCl3and DyCl3to LuCl3 possess monoclinic AlCl3-type TABLE 9 Interatomic distances (Din nm) and coordination numbers (CN) of molten metal halides of trivalent cations (Iwadaate)—Iwadaate (Iwadaate) d. Only TbCl3 crystallizes in an orthorhombic PuBr3-type structure, and it transforms into a hexagonal high-temperature phase above 520C. Since the ionic radius of Clis is larger than that of F, the coordination number of Claround is a rare-earth ion not as large as that of F.
As given in Table 11, crystal structures of rare-earth tribromides at ambient temperature and pressure are classified into three types. The crystals of LaBr3to PrBr3 are of UCl3 type; that of NdBr3to EuBr3 has a PuBr3-type structure; and finally ScBr3, YBr3 and GdBr3 to LuBr3 possess BiI3-type structure.
DENSITY OF RARE-EARTH HALIDES IN SOLID AND MOLTEN STATES
The molar volumes of rare earth halides in the solid and molten state are estimated from their respective density data, which leads to volume changes upon melting. For molten pure rare-earth halides, a limited number of data are available (Janz et al., 1968; Janz et al., 1975), as there are many problems regarding their reliability due to hygroscopicity of rare-earth chlorides and their reactions with water content to produce oxychlorides at elevated temperatures as well as maintaining a constant temperature in the long furnace.
Empirical density equations for molten rare earth trihalides are shown in Table 12, from which the densities at 1100 K are estimated for comparison by interpolation or extrapolation. Much data has been collected so far on the density of molten mixtures of rare earth trihalides with alkali halides; are listed in Tables 13–21 as a linear function of temperature, rm¼a–bT.
STRUCTURES OF RARE-EARTH HALIDE MELTS
The ND pattern of molten NdCl3 shows a relatively faint and broad structure in the region of the first sharp diffraction peak, FSDP (Saboungi et al., 1990). In the liquid, the R–Cl distances are similar to the sum of the ion radii and the results are consistent with the existence of distorted RCl63 octahedral units. The data is discussed in terms of the possible species formed in the melt mixtures.
Two years later, the vibrational modes and structure of LaCl3–CsCl melts were reported by Zissi et al. The same technique was used to measure the absorption spectra of the hypersensitive transition Ho(5G6 5I8) in different chloride coordination environments (Chrissanthopoulos and Papatheodorou, 2008). In conjunction with Raman spectroscopic data (Iwadate et al., 1998), it was confirmed that octahedral complex ions of the type SmCl63 exist in the melt.
The boundary distance of the first coordination shell (LDFCS) that defines the size of the shell is defined in the following ways; it corresponds to (1) the second point at which gSm–.
CONCLUDING REMARKS
The values of CNs are directly influenced by the definitions adopted for CN, the range of integration and whether or not penetration of the coordination shell of Cl-Cl pairs into that of La-Cl pairs occurs. The simulated PDFs of molten LaCl3 and molten YCl3 suggest the importance of the integration range in the evaluation of the nearest neighbor CN (Hutchinson et al., 1999), as illustrated in Fig. In (C), molecular models are given for YCl3, with different interionic distances Y–CI, Y–Y and Cl–Cl corresponding to the different peaks (bold lines). Reproduced with permission from Hutchinson et al.
Therefore, the author thinks that integration of the first coordination shell of La-Cl pairs to the point where the first coordination shells of La-Cl pairs and Cl-Cl pairs intersect is a better procedure. As for higher CNs like 7 or 8, obtained by extending the integration range to the first minimum of the pdf, these can be explained without appeal to the existence of [LaCl7]4- or [LaCl8]5- species.
![FIGURE 38 Partial pair distribution functions for (A) LaCl 3 . [The inset shows the effect of including cation polarization on g LaCl (r).] (B) TbCl 3 and (C) YCl 3 with peak positions displayed.](https://thumb-ap.123doks.com/thumbv2/123dok/10272235.0/197.648.99.548.435.811/figure-partial-distribution-functions-including-polarization-positions-displayed.webp)
