Samarium vapor

The title complex was first prepared from the reaction between zerovalent samarium vapor and pentamethylcyclopentadiene at -120°C [37a]. Later it was found that Sml2(THF)2 was a more convenient entry to the samarocene(II) complex as treated with K(C5Me5). 

Recently, a novel rf-laser double resonance method for optical heterodyne detection of sublevel coherence phenomena was introduced. This so-called Raman heterodyne technique relies on a coherent Raman process being stimulated by a resonant rf field and a laser field (see Fig.l(a)). The method has been applied to impurity ion solids for studying nuclear magnetic resonances at low temperature3 5 and to rf resonances in an atomic vapor /, jn this section we briefly review our results on Raman heterodyne detection of rf-induced resonances in the gas phase. As a specific example, we report studies on Zeeman resonances in a J=1 - J =0 transition in atomic samarium vapor in the presence of foreign gas perturbers. 

The reaction kinetics for the dehydrogenation of ethanol are also weU documented (309—312). The vapor-phase dehydrogenation of ethanol ia the presence of a chromium-activated copper catalyst at 280—340°C produces acetaldehyde ia a yield of 89% and a conversion of 75% per pass (313). Other catalysts used iaclude neodymium oxide and samarium hydroxide (314). 

The metal vapor reaction products differed from traditional organolanthanide complexes in many ways. First, the observed stoichiometries had low ligand to metal ratios. For example, the ytterbium and samarium 3-hexyne products (Reaction 4) had formal ligand to metal ratios of one, whereas most organolanthanides are commonly nine or ten coordinate (6-10). Second, the stoichiometries varied in an... 

The (C Me ) Sm(THF) metal vapor product provided the first opportunity ta see if Smdl) complexes (y =3.5—3.8 Ufi) could be characterized by H NMR spectroscopy (24). Fortunately, the paramagnetism doesn t cause large shifting and broadening of the resonances and hence samarium provides the only Ln(III)/Ln(II) couple in which both partners are NMR accessible. Once the existence and identity of (C Mej- SmdHF) was known, a solution synthesis was developed from KC Me and Sml THF) (44). This system is the preferred preparative route and also provides another soluble organosamarium(II) complex, [(C Me )Sm(THF)2(u-I)]2, under appropriate conditions. This is another xample of how solution studies subsequently catch up to the research targets often identified first in metal vapor reactions. 

Metallic samarium is obtained by heating the oxide, Sm203 with lanthanum turnings or cerium in slight excess amounts in a tantalum crucible under high vacuum. The metal is recovered by condensation of its vapors at 300 to 400°C. The metal cannot be obtained by reduction of its halides, SmFs or SmCls, or by heating with calcium or barium. In such reduction, trihalides are reduced to dihalides, but not to the metal. 

Metals. Kruglikh, et al. (104) measured saturated vapor pressures of erbium, samarium, and ytterbium by the Knudsen effusion method, and standard (average) sublimation entropies of 18.4, 20.7, and 25.6 cal./(gram atom °K.) were derived. Nesmeyanov, et al. (146) studied the vapor pressure of yttrium by an integral variant of the effusion technique. Similar studies at higher temperatures by Herrick (70) on samarium metal have been interpreted in good accord by both first and second law methods. Ideal gas thermodynamic functions have been derived from 100 °K. to 6000°K. at 100° intervals for both actinide and lanthanide elements by Feber and Herrick (45). 

In the specific case of scandium, mono- and mixed-valence species can also be isolated together with divalent complexes for instance, a Sc organometallic compoimd could be obtained imder relatively mild conditions. Finally, the author describes the few knovm zerovalent bis(arene) rare-earth complexes which have been obtained by co-condensation of arenes or heteroarenes with metal vapors. In his conclusion, F. Nief notes that the low-valence molecular chemistry of rare earths, which was once thought to be restricted to divalent samarium, europium, and ytterbium, has been extended to several other rare earths, as well as to lower valence oxidation states. It is the opinion of the author that this research area is likely to find fascinating developments in a near future. 

Rare-earth monochalcogenides are trivalent in the ground state and have metal-type conduction. These compounds, particularly the monosulfides, are highly stable in the thermal sense [8j. They melt without decomposing. For example, the vapor of lanthanum monosulfide consists mainly of LaS molecules [9]. Europium, ytterbium, and samarium monochalcogenides are semiconductors. All the monochalcogenides have the NaCl-type structure. 

Acetylide hydride complexes of samarium, erbium and ytterbium have been made by the cocondensation reactions of Sm, Er, and Yb metal vapor with 1-hexyne at 77K. Polymeric compounds containing [(BuC C)2SmH], [(BuC C)2ErH] and [(BuC=C)3Yb2H] units are isolated and shown to be active catalysts for hydrogenation reactions (W.J. Evans et al., 1981c). 

A bis(pentamethylcyclopentadienyl) samarium(II) complex containing two tetra-hydrofuranes was made by W.J. Evans et al. (1981a) by vaporization of samarium metal into a mixture of pentamethylcyclopentadiene in hexane at — 120°C. From the reaction mixture a purple crystalline compound could be isolated and characterized by an X-ray structural analysis (fig. 45, table 41), as well as by H and C NMR spectra. 

Complexes between some lanthanides in a low oxidation state and olefins were isolated by W.J. Evans et al. (1978a, 1981b). Cocondensation of lanthanum, neodymium, samarium or erbium metal with butadiene or 2,3-dimethylbutadiene at — 196°C in a metal vaporization reactor produces a brown solid, which can be extracted by toluene and tetrahydrofuran yielding soluble brown products with the empirical formulas R(C4H4)j for R = Nd, Sm, Er, and R[(CH3)2C4H4]2 for R = La, Er. For these complexes the following three formulas have been suggested ... 

Samarium, erbium and ytterbium metal vapor also react with ethene, propene, and 1,2-propadiene at — 196°C. The colored matrices, orange to black, contain up to 80% of the appropriate metal as shown by the infrared absorptions of the coordinated olefinic double bond. The reaction product of erbium metal and propene was characterized by elemental analysis as Er(CH2=CHCH,)3. The predominant volatile products after hydrolysis of the complexes are CH4, C2H, C3Hft/C3Hg and C4Hg/C4H,o for the ethene complexes, CjH /CjHg, H2C=C=CH2, and CH3propene complexes. Since ethene is only a minor reaction product in the hydrolysis of... 

The major breakthrough occurred in 1953 when the Ames Laboratory team (Daane et al. 1953) reported the preparation of samarium, europium and ytterbium in high purity and high yields by the reduction of their oxides with lanthanum metal in a vacuum. With the preparation of samarium metal, finally, 126 years after the first rare earth element was reduced to its metallic state, all of the naturally occurring rare earths were now available in their elemental state in sufficient quantity and purity to measure their physical and chemical properties. The success of this reaction is due to the low vapor pressure of lanthanum and the extremely high vapor pressures of samarium, europium and ytterbium (Daane 1951, 1961, Habermann and Daane 1961). It is interesting to note that this same technique has been the method of choice for the preparation of some transplutonium metals (Cunningham 1964). 

Small particules (15-20 nm) of samarium or ytterbium metal are obtained by vaporization on a THF matrix (14). Samarium readily catalyzes reduction of olefins but not of triple bonds. The corresponding ytterbium catalyst is less active. Methylacetylene is rapidly converted by the Sm catalyst into allene at 0°C. 

The final step, vaporization of calcium, does not work for metals with high vapor pressure, as the metal itself would vaporize with the calcium. Instead the oxide is reduced with metallic lanthanum, which has a very low vapor pressure. Lanthanum oxide and a melt of the actual RE metal are formed. The reduction reaction occurs in a tantalum container. The reduced RE metals e.g. samarium, are vaporized and deposit on the walls of the tantalum container. This method is used for [boiling point (°C) in parenthesis] Sm (1794), Eu (1529), Tm (1950), Yb (1196). 

Fig. 3. Comparison between the 3d spectra of Sm vapor, SmS solid and Sm in clusters for two different concentrations R (argon to samarium ratio). Notice that at very low concentrations multiplets are present. (By courtesy of Drs. R.C. Kamatak and J.M. Esteva).

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