Divalent Samarium Research Articles

Optical properties of divalent samarium-doped fluorochlorozirconate glasses and glass ceramics

Glass ceramics comprising Sm 2+-doped barium chloride in both hexagonal and orthorhombic phases embedded in a fluorochlorozirconate glass matrix have been prepared. Divalent samarium doping was effected with the aid of NaBH 4 to partly reduce Sm 3+ in the starting materials to Sm 2+. The optical absorption spectrum in unannealed samples shows a strong, broad 4f → 5d band centred at around 340 nm and attributed to Sm 2+, but no photoluminescence is observed. Following annealing to induce crystallisation of Sm 2+-doped barium chloride crystals in the glass, photoluminescence is observed in the form of sharp line 4f → 4f transitions, and two broad 5d → 4f bands, the latter centred at around 680 and 900 nm. The 680 nm band disappears below about 250 K whilst the 900 nm band intensity increases with decreasing temperature. The 4f → 4f transitions for Sm 2+ ions in the orthorhombic phase of barium chloride show an abrupt change in the relative intensity of transitions originating from the 5D 1 and 5D 0 levels at around 90 K. The effect is attributed to the role of the 5d level in thermally assisted indirect transitions between the 5D 1 and 5D 0 levels. In contrast, transitions from only the 5D 0 level are observed for the hexagonal phase. The temperature dependence of the photoluminescence intensity, and the observation of a different crystal field splitting pattern for the 7F 1 level, can be used to distinguish between the two phases. The splitting pattern for the 7F 1 level for the hexagonal phase of barium chloride suggests that both possible crystallographic barium sites are occupied by Sm 2+ ions.

Divalent Samarium Triflate Mediated Stereoselective Pinacol Coupling of Planar Chiral Phosphanyl and Phosphoryl Ferrocenecarbaldehyde

AbstractThe pinacol coupling reaction of (Rp)‐2‐diphenylphosphanyl ferrocenecarbaldehyde (1) was smoothly mediated by divalent samarium triflate to give (R,R)‐diol 2a predominantly, whereas the use of samarium(II) iodide resulted in low selectivity as described in the previous literature. In contrast, the coupling reaction of (Rp)‐2‐diphenylphosphoryl ferrocenecarbaldehyde (3) with Sm(OTf)2 gave the (S,S)‐diol as the major isomer, which was the opposite stereochemistry of that obtained in the reaction with 1. The rhodium complexes of diphosphanes 2a were good catalysts for the asymmetric hydrogenation of α‐acetamidocinnamic acid, and the product was obtained quantitatively with up to 92 % ee. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Bis(phosphinimino)methanides as Ligands in Divalent Samarium Chemistry: Synthesis, Structures and Catalysis

AbstractThe reaction of K{CH(PPh2NSiMe3)2} with samarium diiodide in a 1:1 molar ratio in thf affords the corresponding divalent samarium complex [{(Me3SiNPPh2)2CH}Sm(μ‐I)(thf)]2 (1), whereas treatment of K{CH(PPh2NSiMe3)2} with samarium diiodide in a 2:1 molar ratio in thf gives the homoleptic complex [{(Me3SiNPPh2)2CH}2Sm] (2). When 1 is treated with KNPh2 in toluene in a 2:1 molar ratio the mixed dimeric compound [({(Me3SiNPPh2)CH}2Sm)2(μ‐I)(μ‐NPh2)] (3) is obtained. The single‐crystal X‐ray structures of all these complexes have been determined. [{(Me3SiNPPh2)2CH}Sm(μ‐I)(thf)]2 has also been successfully used as a catalyst for the polymerization of ϵ‐caprolactone. Good activities and molecular masses are observed with this catalyst.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Synthesis of star-shaped poly(ε-caprolactone) by samarium-based tetrafunctional initiator and its dilute-solution properties

An in situ–generated tetrafunctional samarium enolate from the reduction of 1,1,1,1-tetra(2-bromoisobutyryloxymethyl)methane with divalent samarium complexes [Sm(PPh2)2 and SmI2] in tetrahydrofuran has proven to initiate the ring-opening polymerization of ϵ-caprolactone (CL) giving star-shaped aliphatic polyesters. The polymerization proceeded with quantitative conversions at room temperature in 2 h and exhibited good controllability of the molecular weight of polymer. The resulting four-armed poly(ϵ-caprolactone) (PCL) was fractionated, and the dilute-solution properties of the fractions were studied in tetrahydrofuran and toluene at 30°C. The Mark–Houwink relations for these solvents were [η] = 2.73 × 10−2Mw0.74 and [η] = 1.97 × 10−2Mw0.75, respectively. In addition, the unperturbed dimensions of the star-shaped PCL systems were also evaluated, and a significant solvent effect was observed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 175–182, 2006

Stereoselective Pinacol Coupling of Chiral Formylferrocene Using Divalent Samarium Triflate: Preparation of a New Chiral Bisferrocenyl Oxazoline Ligand and Its Application to Asymmetric Diels−Alder Reactions

[reaction: see text] The pinacol coupling reaction of planar chiral ortho-oxazoline-substituted formylferrocene was smoothly mediated by SmI2 or Sm(OTf)3 to give the (R,R) isomer selectivity (up to 76% diastereomeric excess). The combination of Yb(OTf)3 and the (R,R)-ferrocenyl diol was revealed to be a good catalyst for the asymmetric Diels-Alder reaction of 3-acyloxazolidinone with cyclopentadiene, and the endo adduct was produced in up to 80% enantiomeric excess.

Comparative Reductive Reactivity of SmI2 with TmI2 in the Synthesis of Lanthanide Arene Complexes

AbstractArene complexes of divalent samarium, [(C6R6)Sm(AlCl3I)2]n (C6R6 = C6H5Me (1); C6Me6 (2)), were obtained by reaction of unsolvated SmI2 with AlCl3 in toluene and hexamethylbenzene. Crystallization of the tetrahalo‐aluminate lanthanide arene complexes gave single crystals that showed disorder in the Cl and I positions by X‐ray crystallography, but provided detailed information on η6‐arene coordination to SmII. The more strongly reducing lanthanide diiodide TmI2 did not form analogous products under the same conditions, but the reaction of TmI2 with AlCl3 in toluene in the presence of naphthalene formed the trivalent arene complex (C6H5Me)Tm(AlCl4‐xIx)3 (3).

Ring‐opening polymerization of ε‐caprolactone with a divalent samarium bis(phosphido) complex

AbstractThe ring‐opening polymerization of ε‐caprolactone initiated with a divalent samarium bis(phosphido) complex [Sm(PPh2)2] is reported. The polymerization proceeded under mild reaction conditions and resulted in polyesters with number‐average molecular weights of 8.2 × 103 to 12.5 × 103. The yield and molecular weight of poly(ε‐caprolactone)s were dependent on the experimental parameters, such as the monomer/initiator molar ratio, the monomer concentration, the reaction temperature, and the polymerization time. The obtained polymers were characterized with Fourier transform infrared, NMR, gel permeation chromatography, and differential scanning calorimetry. On the basis of an end‐group analysis of low‐molecular‐weight polymers by NMR spectroscopy, a coordination–insertion mechanism is proposed for the polymerization. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1558–1564, 2005

Homo‐ and copolymerization of 2,2‐dimethyltrimethylene carbonate promoted by samarium thiolate derivatives: Novel and versatile initiators

The tri- and divalent samarium thiolate derivatives were first found to be highly active initiators for the ring-opening polymerization of 2,2-dimethyltrimethylene carbonate (DTC) and the copolymerization of DTC and ε-caprolactone (ε-CL).

The synthesis of a sterically hindered samarium(ii) bis(amidinate) and conversion to its homoleptic trivalent congener

The first divalent samarium bis(amidinate) has been prepared and aspects of its novel chemistry, including the preparation of a sterically hindered homoleptic Sm(III) tris(amidinate), explored.

The mechanism of photobleaching in Sm 2+-doped alkaline-earth fluorohalides

The fluorescence intensity of divalent samarium (Sm 2+) ions doped in alkaline-earth fluorohalides were found to decrease under the optical excitation from a blue laser, a green laser or even a red laser. This photobleaching effect has a mysterious and unknown mechanism. After the bleaching effect happened, no Sm 3+ ions generation, no relative spectral change in Sm 2+ emissions and no lifetime decrease for the levels of Sm 2+ ions could be observed. Under blue- or green-laser irradiation, the decrease of Sm 2+ fluorescence showed a perfect biexponential function; however, when the exciting photon energy was lower than that of 5D 0→ 7F 0 transition, the photobleaching effect could not be observed anymore. An identical mechanism in the photobleaching with that in the spectral hole burning was proposed.

Samarium(III) carbenoid as a competing reactive species in samarium-promoted cyclopropanation reactions.

The trivalent samarium carbenoid I2SmCH2I-promoted cyclopropanation reactions with ethylene have been investigated and are predicted to be highly reactive, similarly to the divalent samarium carbenoid ISmCH2I. The methylene transfer and carbometalation pathways were explored and compared with and without coordination of THF solvent molecules to the carbenoid. The methylene transfer was found to be favored, with the barrier to reaction going from 12.9 to 9.2 kcal/mol compared to barriers of 15.4-17.5 kcal/mol for the carbometalation pathway upon the addition of one THF molecule.

Interaction between Sm and GaN––a photoemission study

We report on a comprehensive study of the Sm/GaN(0 0 0 1 ) interface using synchrotron radiation photoemission. Spectra were taken after stepwise Sm deposition onto a clean GaN(0 0 0 1 ) surface at room temperature and after annealing. Based on the analysis of resonant photoemission spectra taken for hν=136 eV (divalent samarium resonance) and hν=141 eV (trivalent samarium resonance) we find that both divalent and trivalent samarium states are present in the GaN–Sm interface layer after each step of deposition. The average valence of the system increases from 2.57 for a coverage of 0.6 Å to 2.73 for 3.2 Å. Above this coverage the average valence drops and reaches a final value of 2.63 for a coverage of 10 Å. The Ga3d core level evolution confirms that two valence Sm states are present in the interface region. A large chemical shift of the Ga3d core level after samarium deposition and a moderate decrease in the Ga3d attenuation curve indicate that a reactive interface is created when Sm atoms are deposited on the GaN(0 0 0 1 ) surface. Annealing of the Sm/GaN system to 500 °C induces a valence change from Sm 2+ to Sm 3+ and promotes diffusion of Sm from the interface into the bulk.

Two complexes of Sm(II) with crown ethers-electrochemical synthesis, structure and spectroscopy.

Two complexes of divalent samarium have been synthesized by electrochemical reduction in methanol-tetrahydrofuran solutions: [Sm(18-crown-6)(ClO4)2] and [Sm(15-crown-5)2](ClO4)2. In [Sm(18-crown-6)(ClO4)2] the metal cation is ten-coordinate and its coordination sphere comprises six oxygen atoms of the crown ligand and four oxygen atoms from two perchlorate anions. [Sm(15-crown-5)2](ClO4)2 shows a sandwich structure with decacoordinate samarium located between two 15-crown-5 molecules. At 77 K both compounds show f-f luminescence originating from the 5D0 level, and also the 15-crown-5 complex shows a weak luminescence in the range 20000-25000 cm(-1), which has been tentatively interpreted as originating from 3P0 and 5H3 levels. At room temperature the emission of [Sm(15-crown-5)2](ClO4)2 is dominated by broad f-d bands. In the excitation spectra some Fano resonances have been observed. The 18-crown-6 compound is unstable, but the 15-crown-5 compound is fairly stable in air.

Theoretical study of samarium (II) carbenoid (ISmCH2I) promoted cyclopropanation reactions with ethylene and the effect of THF solvent on the reaction pathways.

A computational study of the cyclopropanation reactions of divalent samarium carbenoid ISmCH(2)I with ethylene is presented. The reaction proceeds through two competing pathways: methylene transfer and carbometalation. The ISmCH(2)I species was found to have a "samarium carbene complex" character with properties similar to previously investigated lithium carbenoids (LiCH(2)X where X = Cl, Br, I). The ISmCH(2)I carbenoid was found to be noticeably different in structure with more electrophilic character and higher chemical reactivity than the closely related classical Simmons-Smith (IZnCH(2)I) carbenoid. The effect of THF solvent was investigated by explicit coordination of the solvent THF molecules to the Sm (II) center in the carbenoid. The ISmCH(2)I/(THF)(n)() (where n = 0, 1, 2) carbenoid methylene transfer pathway barriers to reaction become systematically lower as more THF solvent is added (from 12.9 to 14.5 kcal/mol for no THF molecules to 8.8 to 10.7 kcal/mol for two THF molecules). In contrast, the reaction barriers for cyclopropanation via the carbometalation pathway remain high (>15 kcal/mol). The computational results are briefly compared to other carbenoid reactions and related species.

Reactivity and structural characterization of divalent samarium aryloxide with diethylaluminum chloride

Reaction of (ArO)2Sm(THF)4 (ArO = 2,6-di-tert-butyl-4-methylphenolate) with Et2AlCl in THF gives SmCl2 (1) and (ArO)AlEt2(THF) (2) via ligand exchange. Complex 1 reacts with cyclooctatetraene (C8H8) to give [(C8H8)SmCl(THF)2]2 (3). Crystal structural determinations show that complex 2 has a monomeric structure, the aluminum atom is coordinated by two carbon atoms and two oxygen atoms, to form a distorted tetrahedron. Complex 3 has a dimeric structure, the samarium atom is coordinated by a cyclooctatetraenyl ring, two chorine atoms and two oxygen atoms, the coordination number of the central metal is 9.

Serendipitous Isolation of the First Example of a Mixed-Valence Samarium Tripyrrole Complex

The transamination reaction of Sm{N(SiMe3)2}2(THF)2 with two dipyrrole ligands of formula RR‘C(α-C4H3NH)2 (R = R‘ = Et; R = Me, R‘ = Ph) has been examined. The reactions in THF and under N2 gave two similar tetranuclear dinitrogen complexes, {[Et2C(α-C4H3N)2Sm}4(THF)2](μ-N2)·2THF and [{[(C6H5)(CH3)C(α-C4H3N)2]Sm}4(DME)2](μ-N2), where the dinitrogen unit has undergone a four-electron reduction via cooperative attack of four divalent samarium atoms and remained coordinated both side-on and end-on between the four coplanar metal centers. The same reaction carried out with diethyldipyrrolylmethane under an argon atmosphere afforded the divalent samarium macrocyclic cluster {[Et2C(α-C4H3N)2]Sm}8(THF)4·4THF. In the case of the reaction with the methylphenyldipyrrolylmethane ligand under N2, the unprecedented hexanuclear mixed-valence SmII/SmIII cluster [([(C6H5)(CH3)C(α-C4H3N)2][(C6H5)(CH3)C(α-C4H3N)(β-C4H3N)]{[(C6H5)(CH3)C]2(α-C4H3N)2(α,α‘-C4H2N)}Sm3)(THF)3]2, containing tripyrrolide, “N-confused”, and regular dipyrrolide ligands was serendipitously formed by reaction with the impurities contained in the nonpurified ligand.

Polymerization of Methyl Methacrylate Initiated by a Divalent Samarium Phenoxide Complex with an Alkyl Aluminium Compound

AbstractA divalent samarium complex that bears two bulky phenoxide ligands promotes the living polymerization of methyl methacrylate in the absence of cocatalyst. The initiation involves a tail‐to‐tail coupling reaction of the samarium‐enolate radical followed by a one‐electron reduction of the monomer with the divalent samarium initiator. The polymerization takes place stereospecifically and the samarium initiator affords highly syndiotactic poly(methyl methacrylate). The stereoregularity of the polymer is variable with the ratio of the samarium initiator to aluminium compound (2,6‐di‐tert‐butyl‐4‐methylphenoxy)2AlMe. The polymer obtained when the [Al]/[Sm] ratio is 4.0 has a unique tacticity, [mm]/[mr]/[rr] = 42:11:47, and the polymer has a melting point at 161 °C, which suggests the formation of a stereocomplex between syndiotactic and isotactic poly(methyl methacrylate).Part of the possible initiation mechanism and equimolar reaction of BHT‐SmII with MMA.magnified imagePart of the possible initiation mechanism and equimolar reaction of BHT‐SmII with MMA.

Reactivity and structural characterization of divalent samarium aryloxide with diethylaluminum chloride

Reactivity and structural characterization of divalent samarium aryloxide with diethylaluminum chloride

Novel Sm(PPh 2 ) 2 initiator for the synthesis of poly(ε-caprolactone) with linear and star-shaped structures§

Divalent samarium bis(phosphido) complex Sm(PPh2)2 was first applied as an initiator for the ring-opening polymerization of e-caprolactone (e-CL). In comparison to samarium diiodide (SmI2), this Sm(II) complex exhibited much higher activity and yielded poly(e-CL) with higher molecular weight. At the same time, an in-situ generated tetrafunctional samarium enolate from the reduction of 1,1,1,1-tetra(2-bromoisobutyryloxymethyl)methane with Sm(PPh2)2 or SmI2 has proven to initiate the polymerization of e-CL under mild conditions. The resulting linear and star-shaped poly(e-CL)s have been characterized by 1H NMR, GPC, and SLS.

Effect of the Alkali-Metal Cation on the Bonding Mode of 2,5-Dimethylpyrrole in Divalent Samarium and Ytterbium Complexes

The reactions of SmI2(THF)2 and YbI2(THF)2 with the alkali-metal salts of 2,5-dimethylpyrrole, or the reaction of SmCl3(THF)3 and YbCl3(THF)3 with the same ligands followed by reduction with the ap...