Remarkable effect of alkali metal on polymerization of cyclic esters catalyzed by samarium-alkali metal multinuclear alkoxide clusters

The remarkable effect of alkali metal on catalytic reactivity of samarium-alkali metal multinuclear alkoxide clusters is systematically studied. Three samarium-alkali metal multinuclear alkoxide clusters are synthesized in high yield by the reaction of anhydrous SmCl(3) with different molar ratios of alkali metal alkoxide and MOH (M = Na or K) in tetrahydrofuran (THF). These clusters were fully characterized by elemental analysis, IR, (1)H NMR and single-crystal structural analysis. These clusters exhibited good catalytic activity for the ring-opening polymerization of ε-caprolactone (ε-CL), L-lactide (L-LA) and trimethylene carbonate (TMC). It is interesting to note that the catalytic activity is much influenced by the alkali metals of the clusters. For the polymerization of these cyclic esters, the catalytic activities all increase with the increase of the molar ratio of alkali metal to samarium metal.     

Structure of alkali metal ionic clusters

Quantum mechanical calculations show that alkali metal ionic clusters of the type M³⁺₄ have the tetrahedral structure, the stability varying as Na³⁺₄ > K³⁺₄ > Rb³⁺₄. Raman spectroscopy has been employed to establish the tetrahedral structure of Na³⁺₄.     

Synthesis of barium titanate/polymer composites from metal alkoxide

Barium titanate (BaTiO₃)/polymer composite was successfully synthesized by methacryltriisopropoxytitanium (MTPT) and barium alkoxide. MTPT undergoes radical polymerization using azobisisobutyronitrile at temperatures from 90 to 150°C. ¹H NMR spectra showed that MTPT reacted with barium alkoxides yielding a complex alkoxide. BaTiO₃ particles/polymer was formed after the polymerization and hydrolysis of the complex alkoxide. The transmission electron microscopic observation revealed that crystalline BaTiO₃ particles of around 3 nm in size were dispersed in the polymer matrix.     

Hydrotalcite-derived Co-containing mixed metal oxide catalysts for methanol incineration

The Co–Mg–Al mixed metal oxides were prepared by calcination of co-precipitated hydrotalcite-like precursors at various temperatures (600–800 °C), characterised with respect to chemical (AAS) and phase (XRD) composition, textural parameters (BET), form and aggregation of cobalt species (UV–vis-DRS) and their redox properties (H₂-TPR, cyclic voltammetry). Moreover, the process of thermal decomposition of hydrotalcite-like materials to mixed metal oxide systems was studied by thermogravimetric method combined with the analysis of gaseous decomposition products by mass spectrometry. Calcined hydrotalcite-like materials were tested as catalysts for methanol incineration. Catalytic performance of the oxides depended on cobalt content, Mg/Al ratio and calcination temperature. The catalysts with lower cobalt content, higher Mg/Al ratio and calcined at lower temperatures (600 or 700 °C) were less effective in the process of methanol incineration. In a series of the studied catalysts, the best results, with respect to high catalytic activity and selectivity to CO₂, were obtained for the mixed oxide with Co:Mg:Al molar ratio of 10:57:33 calcined at 800 °C. High activity of this catalyst was likely connected with the presence of a Co–Mg–Al spinel-type phases, containing easy reducible Co³⁺ cations, formed during high-temperature treatment of the hydrotalcite-like precursor.     

Anionic polymerization. III. Polymerization of butadiene with alkali metal alkoxide-modified alkali metal system

A high molecular weight polybutadiene was prepared in hexane solvent by using alkali metal (Li, Na, K) and metal tert-butoxide (Li, Na, K) as a polymerization initiator. The microstructure of polybutadiene varies, depending on the type of modifiers and polymerization and temperatures. The results and mechanistic implications of this study are discussed.     

Structure prediction of high-pressure phases for alkali metal sulfides

For a given chemical system we present a systematic approach to predict structures, which may exist at high pressure, by investigating the global enthalpy landscape. We combine global optimizations, based on empirical potential energy functions, and local optimizations (volume, cell shape, and atomic positions) on both Hartree-Fock and density functional theory level. We predict the existence of high-pressure phases for the alkali metal sulfides of the composition M2S (M = Li, Na, K, Rb, Cs), together with the transition pressures among these phases.     

Structure of mixed group IB metal trimers

The structures of all mixed group IB trimers have been calculated in the diatomics-in-molecules (DIM) approximation. The optimum trimer geometries correspond to bent structures in C₂ or C_2v symmetry. All trimers are bound with respect to dissociation to an atom plus a diatomic. The structures can all be characterized as having an “obtuse” structure with a bond angle greater than 60°. The structure of the trimer is controlled by the atom which forms the strongest diatomic bonds. This is confirmed by calculations on the possible linear structures. The most stable linear structure has the atom which forms the strongest diatomic bonds in the central position.     

Synthesis and physicochemical study of hexafluorocomplexes of tin with mixed alkali metal cations

Conclusions Hexafluorocomplexes of tin with mixed cations of alkali metals with the composition NaMSnF₆ (M=K, Rb, Cs) and crystallizing in the orthorhombic system have been synthesized for the first time from aqueous solutions. The parameters of the unit cells of these substances have been determined.The compounds synthesized are optically positive biaxial crystals; their optical and x-ray diffraction constants are given.In the IR spectra of all the fluorocomplexes synthesized a splitting is observed in the frequency of the stretching vibration (₃) for the octahedral anion SnF₆ ^2–.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 1927–1931, September, 1987.     

Structure and electronic properties of the surface of alkali metal peroxides

The CRYSTAL09 program with the implemented B3PW hybrid density functional in a localized basis of atomic orbitals is used to determine the atomic and electronic structure of the surface of lithium, sodium, and potassium peroxides. Geometric parameters, surface energies, partial densities of states, electron density distributions, overlap populations, and atomic charges are calculated. It is found that the geometry relaxation has a characteristic depth up to ～10 Å, while the surface states are located in the upper layers at a depth up to ～2.5 Å. Structural displacements of atoms do not exceed ～ 0.2 Å; the charge of the upper surface layers is positive, whereas the energy state shifts relative to the bulk ones can reach ～1 eV. The surface energy of peroxides decreases with an increase in the atomic number of the cation.     

Molybdenum oxo-imido aryloxide complexes: oxo analogues of olefin metathesis catalysts

The novel 16-electron molybdenum oxo-imido bis(aryloxide) complexes [Mo(NtBu)(O)(2,6-Me2C6H3O)2(py)] (1) and [Mo(NtBu)(O)(2,6-iPr2C6H3O)2(py)] (2) have been prepared by the salt elimination reactions of [Mo(NtBu)(O)Cl2(DME)] with the appropriate lithium aryloxide and from the cycloaddition reactions of tert-butyl isocyanate with the appropriate molybdenum dioxo bis(aryloxide) complex [Mo(O)2(OAr)2(py)n]. Complexes 1 and 2 are the first isolable and crystallographically characterized molybdenum oxo-imido aryloxide complexes. The geometry around the metal in complexes 1 and 2 is best described as a distorted trigonal bipyramid, with the imido and pyridine ligands occupying the axial positions and the oxo and aryloxide ligands in the equatorial plane. X-ray and IR data have confirmed that the imido ligand is the dominant pi donor in the complexes, resulting in an Mo-O bond order of less than 2.5. Reaction of [Mo(NtBu)(O)Cl2(DME)] with Li(OCH2tBu) instead gave the novel complex [Mo(NtBu)(OCH2tBu)3Cl(py)] (3).     

Preparation of site-differentiated mixed ligand and mixed ligand/mixed metal metallacrowns

Assembly reactions that can prepare reliably regioselective metallamacrocyclic complexes have been a target in the development of metallacrowns. To this end, a series of mixed ligand and mixed ligand/mixed metal metallacrowns have been synthesized in high yield and structurally characterized. Two distinct connectivities have been observed in these types of metallacrowns. The monomeric, vacant metallacrown with mixed ligand composition [12-MC(Ni(II)N(Hshi)2(pko)2-4)] (1a) shows the connectivity pattern [-O-Ni-O-N-Ni-N-]2 while the other Ni metallacrowns, [12-MC(Ni(II)N(shi)2(pko)2-4)] (2a) and the coupled [12-MC(Ni(II)N(shi))2(pko)2-4)][12-MC(Ni(II)N(shi))3(pko)-4)] (3a) fused metallacrowns as well as the mixed metal Mn-Ni metallacrown [12-MC(Ni(II)Mn(III)N(shi)2(pko)2-4)] (4a), follow the pattern [-Ni-O-N-]4. Also, three distinct arrangements of the chelate rings around the metal ions have been observed. The syntheses are completely general, allowing for the substitution of different ligands into the metallacrown core. Compounds 1 and 4 show the 6-5-6-5-6-5-6-5 arrangement, compounds 2 and 3(1) the 6-6-5-5-6-6-5-5, and the 3(2) component the 6-6-5-5-6-5-6-5. The obtained structures can be rationalized by balancing the charge at each metal site in the metallacrown. Variable temperature magnetic susceptibility measurements show that exchange interactions for all the compounds are weak and dominantly antiferromagnetic (e.g., 2a gives an exchange coupling of J = -1.2 cm(-1) with g = 2.2). In solution, the metallacrowns are shown to be stable both to decomposition and ligand exchange.     

Synthesis and physicochemical investigation of hexafluoro complexes of germanium with mixed alkali metal cations

Hexafluoro complexes of germanium with mixed alkali-metal cations having the composition formula MM'GeF₆, which crystallize in a rhombic syngony were synthesized. The unit cell parameters of the compounds have been determined. It was found that MM'GeF₆ are biaxial optical crystals, and their optical and refractometric constants are given. It was shown that the substitution of the Na⁺ cation in Na₂GeF₆ by K⁺, Rb⁺, Cs⁺ leads to a varied degree of distortion of the hexafluoro ion. It was found (NMR data) that a change in the dynamic state of the octahedral ions is due to a transition from a restrained rotation to an isotropic one.Institute of Chemistry of the Far-Eastern Scientific Center, Russian Academy of Sciences, 690022 Vladivostok. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 11, pp. 2473–2482, November, 1992.