Samarium cations

A high degree of stereoselectivity was achieved in reductive radical cyclizations with Coordination of the oxime function (e.g. 108) with samarium cation seems to play an important role, since the identical reaction with a tributyltin hydride/radical initiator system produces poor stereoselectivity (equation 79). ... 

As an aside, it should also be noted that a highly efficient deoxygenation of tartrate diesters via a samarium iodide-induced electron transfer process allows direct conversion to 539. Ethylene glycol, presumably due to its modest acidity and strong coordinating ability with the samarium cation, provides the best results [174]. 

Although rare-earth ions are mosdy trivalent, lanthanides can exist in the divalent or tetravalent state when the electronic configuration is close to the stable empty, half-fUed, or completely fiUed sheUs. Thus samarium, europium, thuUum, and ytterbium can exist as divalent cations in certain environments. On the other hand, tetravalent cerium, praseodymium, and terbium are found, even as oxides where trivalent and tetravalent states often coexist. The stabili2ation of the different valence states for particular rare earths is sometimes used for separation from the other trivalent lanthanides. The chemicals properties of the di- and tetravalent ions are significantly different. 

Figures 20A and B show the PL spectra, recorded at 290 K, at 600 nm, and as a function of pressure, for Cs9(SmW10O36) and SmWi0O36-LDH, respectively (Park et al., 2002). For the sake of comparison, the line shapes are normalized and displaced along the vertical axis. In both cases, the peak position is red-shifted by 4—5 nm when the hydrostatic pressure increases from 1 bar to 61 kbar. It was shown that the red-shift from A to A lies solely in the deformation of the samarium complexes by the uniaxial stress exerted by the host layers, whereas the shift from B to B is also influenced by the change in the cation environment. Under the same conditions, B is not at the same position for the non-intercalated (HN (n -b u t y 1) 3) 9 (SmW10O3e) and Cs9(SmWi0O36) compounds (Park et al., 2002). Thus only peak A is available to measure the unixial stress. This observation can be used to determine the uniaxial stress, when the external pressure is zero. For the SmW10O36—LDH system, the uniaxial stress varies significantly from 75 at 28 K to 140 kbar at 290 K (Park et al., 2002).

The only complexes of lanthanum or cerium to be described are [La(terpy)3][C104]3 175) and Ce(terpy)Cl3 H20 411). The lanthanum compound is a 1 3 electrolyte in MeCN or MeN02, and is almost certainly a nine-coordinate mononuclear species the structure of the cerium compound is not known with any certainty. A number of workers have reported hydrated 1 1 complexes of terpy with praseodymium chloride 376,411,438), and the complex PrCl3(terpy)-8H20 has been structurally characterized 376). The metal is in nine-coordinate monocapped square-antiprismatic [Pr(terpy)Cl(H20)5] cations (Fig. 24). Complexes with a 1 1 stoichiometry have also been described for neodymium 33, 409, 411, 413, 417), samarium 33, 411, 412), europium 33, 316, 411, 414, 417), gadolinium 33, 411), terbium 316, 410, 414), dysprosium 33, 410, 412), holmium 33, 410), erbium 33, 410, 417), thulium 410, 412), and ytterbium 410). The 1 2 stoichiometry has only been observed with the later lanthanides, europium 33, 411, 414), gadolinium, dysprosium, and erbium 33). 

Hart and co-workers have demonstrated that the nine-coordinate cations [M(terpy)3] may be prepared in the absence of coordinating counterions in the cases of europium, samarium, lanthanum, and lutetium 175, 201, 202). The most widely investigated compound in this series is [Eu(terpy)3][C104]3, which has been structurally characterized. The metal is in a nine-coordinate tricapped trigonal-prismatic arrangement (Fig. 25) 201). The distortion from Dj symmetry to is explained by the nonplanarity of the terpy ligands, and is predicted from spectroscopic observations. It is not clear how the above observations may be correlated with a report that... 

According to the structure determination results, it is presumed that the reversion from the carbinolamine form to the tetraimine one exists probably because of the optimal cation-cavity criteria and the fact that the samarium ion can be accommodated by either form of the two macrocycles. Carbinolamine, acting as the intermediate of the tetraimine Schiff base 27, is the kinetically favored product. In contrast, the latter species is the thermodynamically favored product. On dissolution and recrystallization in water, a higher temperature is reached than in the original reaction in alcohol, which facilitates completion of the reaction. Furthermore,... 

Fundamental studies have been reported using the cationic liquid ion exchanger di(2-ethylhexyl) phosphoric acid in the extraction of uranium from wet-process phosphoric acid (H34), yttrium from nitric acid solution (Hll), nickel and zinc from a waste phsophate solution (P9), samarium, neodymium, and cerium from their chloride solutions (12), aluminum, cobalt, chromium, copper, iron, nickel, molybdenum, selenium, thorium, titanium, yttrium, and zinc (Lll), and in the formation of iron and rare earth di(2-ethylhexyl) phosphoric acid polymers (H12). Other cationic liquid ion exchangers that have been used include naphthenic acid, an inexpensive carboxylic acid to separate copper from nickel (F4), di-alkyl phosphate to recover vanadium from carnotite type uranium ores (M42), and tributyl phosphate to separate rare earths (B24). 

Some substitution of strontium (up to 14 mol.%), of lead (2 mol.% reported) but no barium has been reported in aragonite, although investigations at elevated temperatures and pressures show almost complete miscibility of these elements in the structure (Gaines et al., 1997, p. 442), and SrCOs (strontionite), BaCOs (witherite), and PbCOs (cerussite) are common minerals. A calculated plot (Figure 3(b)) for cations in ninefold coordination shows that this coordination theoretically allows trivalent rare earth elements and quadravalent and many other elements to be substituents in the structure. Ytterbium, europium, samarium, and radium carbonates with aragonite structure have been synthesized (Spear, 1983). 

Cationic lanthanide complexes of samarium and ytterbium, [Cp"Sm([18]-crown-6)][SmCp"3] 0.5C6H6 and [Gp"Yb([18]-crown-6)][Cp"]-3C6H6 [Cp" = 7]S-CsH3(SiMe3)2-l,3], have been prepared by the reaction between bis (cyclopentadienyl) lanthanide and [18]-crown-6 in benzene (Scheme 32).190... 

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