Heterogeneous zirconocene catalyst on magnesium support MgCl2(THF)2 modified by AlEt2Cl for ethylene polymerisation

The heterogeneous bis(cyclopentadienyl)zirconium(IV) dichloride catalyst of the composition MgCl₂(THF)/(AlEt₂Cl)_0.34/(Cp₂ZrCl₂)_0.01 as determined by FTIR, XRD, and AAS analyses was synthesised and, after activation by MAO, applied for ethylene polymerisation. The catalyst turned out to be stable and more active than those magnesium supported catalysts already known from the literature. The polyethylene produced has a relatively high molecular weight (M_w > 200,000 g/mol), a narrow and monomodal molecular weight distribution (MWD = 2.4), a bulk density of about 180 g/dm³, and monomodal particle size distribution. Application of a ternary Al(i-Bu)₃/MAO/B(C₆F₅)₃ activator decreased the amount of MAO needed and increased catalyst activity, but did not change the reaction mechanism.     

Structural and catalytic aspects of copper(II) complexes containing 2,6-bis(imino)pyridyl ligands

The synthesis, structure, and ligand substitution mechanism of a new five-coordinate trigonal-bipyramidal copper(II) complex, [Cu^II(py ^tBuMe₂N₃)Cl₂] (1), with a sterically constrained py ^tBuMe₂N₃ chelate ligand, py ^tBuMe₂N₃?=?2,6-bis-(ketimino)pyridyl, are reported. The kinetics and mechanism of chloride substitution by thiourea, as a function of nucleophile concentration, temperature, and pressure, were studied in detail and compared with an earlier study reported for the analogous complex [Cu^II(py ^tBuN₃)Cl₂] (2) [py ^tBuN₃?=?2,6-bis-(aldimino)pyridyl]. Catalysis of the oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone by 1 and 2 was studied. Correlations between the reactivity, chloride substitution behavior, and reduction potentials of both complexes were made. These show that the rate of oxidation is independent of the rate of chloride substitution, indicating that the substitution of chloride by catechol as substrate occurs in a fast step. Spectral data show a non-linear relationship between the ability of the complexes to oxidize 3,5-DTBC and the Lewis acidity of their copper(II) centers. Electrochemical data demonstrate that the most effective complex 1 has a E ⁰ value that approaches the E ⁰ value of the natural tyrosinase enzyme.     

Polyethylene with bimodal molecular weight distribution synthesized by 2,6-bis(imino)pyridyl complexes of Fe(II) activated with various activators

Polyethylenes with bimodal molecular weight distribution (MWD) were synthesized by 2,6-bis(imino)pyridyl complexes of Fe(II) combined with different activators, which were prepared from alkylaluminium. It is found that the molecular weight (MW) and MWD was influenced by not only iron complexes but activators as well. The activator plays key important role in determination of the MW and MWD of final polymer and the MWD shape could be regulated by selection of various activators and polymerization conditions. The study on the variation of the MWD with the polymerization time and fitting of bimodal MWD with two Flory distributions suggests that bimodal MWD is caused by chain transfer reaction to activator or two active sites.     

Synthesis and characterization of para-nitro substituted 2,6-bis(phenylimino)pyridyl Fe(II) and Co(II) complexes and their ethylene polymerization properties

Four iron(II) and cobalt(II) complexes ligated by 2,6-bis(4-nitro-2,6-R₂-phenylimino)pyridines, LMCl₂ (1: R = Me, M = Fe; 2: R = iPr, M = Fe; 3: R = Me, M = Co; 4: R = iPr, M = Co) have been synthesized and fully characterized, and their catalytic ethylene polymerization properties have been investigated. Among these complexes, the iron(II) pre-catalyst bearing the ortho-isopropyl groups (complex 2) exhibited higher activities and produced higher molecular weight polymers than the other complexes in the presence of methylaluminoxane (MAO). A comparison of 2 with the reference non-nitro-substituted catalyst (2,6-bis(2,6-diisopropylphenylimino)pyridyl)FeCl₂ (FeCat 5) revealed a modest increase of the catalytic activity and longer lifetime upon substitution of the para-positions with nitro groups (activity up to 6.0 × 10³ kg mol⁻¹ h⁻¹ bar⁻¹ for 2 and 4.8 × 10³ kg mol⁻¹ h⁻¹ bar⁻¹ for 5), converting ethylene to highly linear polyethylenes with a unimodal molecular weight distribution around 456.4 kg mol⁻¹. However, the iron(II) pre-catalyst 1 on changing from ortho-isopropyl to methyl groups displayed much lower activities (over an order of magnitude) than 2 under mild conditions. As expected, the cobalt analogues showed relatively low polymerization activities.     

Mg(BO2)2在MgCl2水溶液中的相平衡与化学平衡

借助拉曼光谱和X射线衍射(XRD)检测手段,对Mg(BO_2)_2在MgCl_2水溶液中水解的固液相平衡与物种化学平衡规律进行了研究。结果表明,MgCl_2对Mg(BO_2)_2的溶解转化、多硼氧配阴离子的物种分布有很大影响:(1)随着MgCl_2浓度从0达到饱和,Mg(BO_2)_2的表观饱和浓度从0.79%增加到1.96%,pH值从9.96降到6.27;(2)Mg(BO_2)_2在纯水中水解形成固相Mg_2B_6O_(11)·15H_2O和Mg(OH)_2,在MgCl_2溶液中形成固相Mg_2B_6O_(11)·15H_2O和Mg_3Cl_2(OH)_4·4H_2O;(3)Mg(BO_2)_2在纯水中水解,硼的物种主要为B_4O_5(OH)_4~(2-)和B_3O_3(OH)_4~-,分别占液相总硼含量的49.81%和19.54%。在MgCl_2饱和溶液中,主要为B_3O_3(OH)_4~-和B_5O_6OH)_4~-,分别占液相总硼含量的44.57%和40.00%。     

过渡金属铁配合物Fe(CO)5-x(PR3)x的结构与性质的理论研究

采用密度泛函理论（DFT）对一系列低价铁化合物Fe（CO）_5-x（PR₃）_x（x=1~3,R=H,F,Me）的几何结构、电子结构、成键特点以及热力学性质进行了理论研究。结果表明引入膦配体后不会造成Fe（CO）_x（PR₃）_5-x的几何结构畸变,为略扭曲的三角双锥形。自然键轨道（NBO）分析显示,膦配体与羰基铁基团间存在电荷转移,有效增强Fe-CO之间的共价作用。多数稳定结构Fe（CO）_x（PR₃）_5-x的第一膦配体解离能要比第一羰基解离能低,预示Fe（CO）_5-x（PR₃）_x的反应活性比Fe（CO）₅有明显提高。     

过渡金属铁配合物Fe(CO)_(5-x)(PR_3)_x的结构与性质的理论研究

采用密度泛函理论(DFT)对一系列低价铁化合物Fe(CO)_(5-x)(PR_3)_x(x=1~3,R=H,F,Me)的几何结构、电子结构、成键特点以及热力学性质进行了理论研究。结果表明引入膦配体后不会造成Fe(CO)x(PR_3)_(5-x)的几何结构畸变,为略扭曲的三角双锥形。自然键轨道(NBO)分析显示,膦配体与羰基铁基团间存在电荷转移,有效增强Fe-CO之间的共价作用。多数稳定结构Fe(CO)x(PR_3)_(5-x)的第一膦配体解离能要比第一羰基解离能低,预示Fe(CO)_(5-x)(PR_3)_x的反应活性比Fe(CO)5有明显提高。     

Selective and efficient oxidation of ethylene to ethylene glycol acetates with H2O2 catalyzed by Pd(OAc)2-di(2-pyridyl)ketone-di(2-pyridyl)methanesulfonate system

Novel systems for palladium-catalyzed selective oxidation of ethylene to a mixture of ethylene glycol mono- and di-acetates as the major reaction products (90-95% selectivity) with H₂O₂ in acetic acid solution at ambient pressure and 20 °C were developed. The catalytic reaction is very efficient with up to 90% combined yield of glycol acetates with H₂O₂ as a limiting reagent and 1 mol% catalyst loading. The catalytic systems developed are comprised of a mixture of Pd(OAc)₂, and 6-methyl substituted (2-pyridyl)methanesulfonate and/or di(6-pyridyl)ketone ligands. Compositions of the binary, Pd(OAc)₂-dpk, Pd(OAc)₂-Me-dpms, and ternary, Pd(OAc)₂-dpk-Me-dpms, systems have been studied by means of ¹H NMR spectroscopy and ESI mass spectrometry. Kinetics studies were performed as well and plausible reaction mechanism was suggested, which features facially chelating ligand-enabled facile oxidation of Pd^IIC₂H₄OAc intermediates with H₂O₂ to form Pd^IVC₂H₄OAc transients.     

Structure and stability of closo-BnHn−2(CO)2 (n = 5–12)

Closo-B_nH_n−2(CO)₂ (n = 5–12), isolobal analogues of closo-C₂B_n−2H_n, have been investigated at the B3LYP/6-311+G^**density functional level of theory. The most stable isomers of closo-B_nH_n−2(CO)₂ are similar to those of closo-C₂B_n−2H_n in geometric patterns apart from closo-B₆H₄(CO)₂, and closo-B_nH_n−2(CO)₂ is much less strained than closo-C₂B_n−2H_n. Energetic analysis identifies closo-B₆H₄(CO)₂, closo-B₁₂H₁₀(CO)₂ and closo-B₁₀H₈(CO)₂ to be most stable, of which the latter two cages have been prepared experimentally. On the basis of the negative and rather large nucleus independent chemical shifts (NICS), closo-B_nH_n−2(CO)₂ are aromatic. To aid further experimental study, the CO stretching frequencies have been computed.     

Synthesis and X-ray crystal structures of [Ru4H4(CO) 11(PPh3)] and [Ru4−x IrxH2−x(CO) 12(PPh3) ] (x = 0 or 1)

Three tetranuclear clusters [Ru₄H₄(CO)₁₁(PPh₃)] (1), [Ru₄H₂(CO)₁₂(PPh₃)] (2) and [Ru₃IrH(CO)₁₂(PPh₃)] (3) were formed in the reaction of [Ir(CO)Cl(PPh₃)₂] and Na[Ru₃H(CO)₁₁] in tetrahydrofuran. Complexes 1–3 were characterized by IR and ¹H and ³¹P NMR, and the structure of the clusters was confirmed by single crystal X-ray analysis. In 2 and 3 one of the carbonyls bridges between two ruthenium atoms; otherwise the compounds contain only terminal carbonyls.     

Efficient UV-emitting X-ray phosphors: octahedral Zr(PO4)6 luminescence centers in potassium hafnium–zirconium phosphates K2Hf1−xZrx(PO4)2 and KHf2(1−x)Zr2x(PO4)3

Potassium hafnium–zirconium phosphates, K₂Hf_1−xZr_x(PO₄)₂ and KHf_2(1−x)Zr_2x(PO₄)₃, are broad-band UV-emitting phosphors. At room temperature, they have emission peak maxima at approximately 322 and 305 nm, respectively, under 30 kV peak molybdenum X-ray excitation. Both phosphors demonstrate luminescence efficiencies that make them up to 60% as bright as commercially available CaWO₄ Hi-Plus. The solid-state and flux synthesis conditions, and X-ray excited UV luminescence of these two phosphors are discussed. Even though the two compounds have different atomic structures, they contain zirconium in the same active luminescence environment as that found in highly efficient UV-emitting BaHf_1−xZr_x(PO₄)₂. All the three materials have hafnium and zirconium in octahedral coordination via oxygen-atom corner sharing with six separate PO₄ tetrahedra. This octahedral Zr(PO₄)₆ moiety appears to be an important structural element for efficient X-ray excited luminescence, as are the edge-sharing octahedral TaO₆ chains for tantalate emission.     

Mössbauer studies of thiospinels. V. The systems Cu1−xFexMe2S4 (Me = Cr, Rh) and Cu1−xFexCr2(S0.7Se0.3)4

With the exception of FeRh₂S₄, powder samples of all systems studied have been obtained as spinel phase without essential impurities. The lattice constants follow Vegard's law. From the Seebeck coefficients and the Mössbauer spectra the valence distribution Cu¹⁺_1−xFe²⁺_2x−1Fe³⁺_1−x[Me³⁺₂]X²⁻₄ is derived for 0.5 x 1, while there is only Fe³⁺ present for 0 < x 0.5. Samples with the overall composition FeRh₂S₄ contain mostly Rh₂S₃ and iron sulfide phases, but less than 20% of a spinel phase.     

Bis(di-t-butylphosphino)methane complexes of rhodium: homogeneous alkyne hydrosilylation by catalyst-dependent alkyne insertion into Rh---Si or Rh---H bonds. Molecular structures of the dimer [(dtbpm)RhCl]2 and of the silyl complex (dtbpm) Rh[Si(OEt)3](PMe3)

The homogeneous, Rh-catalysed hydrosilylation of but-2-yne with triethoxysilane has been studied. All rhodium complexes employed as catalyst precursors contain ^tBu₂PCH₂P^tBu₂ (“dtbpm”) as a chelating ligand. The crystal and molecular structure of the dimer [(dtbpm)RhCl]₂ (10) has been determined by X-ray diffraction. Complex 10 is shown to be a sluggish catalyst in hydrosilylation reactions of hex-1-ene, whereas but-2-yne is hydrosilylated more rapidly. A much more efficient and highly selective catalyst is 10 with added PPh₃, equivalent to the use of monomeric (dtbpm)RhCl(PPh₃). (E)-2-Triethoxysilylbut-2-ene is formed exclusively and with high turnover numbers in this case. For both 10 and its PPh₃ derivative, the 14-electron fragment [(dtbpm)RhCl], formed by dissociation processes, is the most likely active intermediate in a Harrod-Chalk-type catalytic cycle. The PPh₃ dissociation equilibrium has been studied in detail for (dtbpm)RhCl(PPh₃) and its thermodynamic parameters have been determined. With rhodium alkyl complexes as catalyst precursors, a different type of alkyne hydrosilylation catalysis, involving direct alkyne insertion into the Rh---Si bond of an intermediate rhodium silyl complex, (dtbpm)Rh[Si(OEt)₃](PMe₃) (14), has been found. Complex 14 was synthesized independently from (dtbpm)RhMe(PMe₃) and characterized by X-ray diffraction. It is an equally active catalyst itself, yielding (E)-2-triethoxysilylbut-2-ene as the major product (90%) from but-2-yne and HSi(OEt)₃ (turnover number 1000 per 30 min). The insertion step of the alkyne into the Rh---Si bond of 14 and the formation of two stereoisomeric rhodium vinyl complexes were established independently for MeO₂CCCCO₂Me as a more reactive alkyne substrate. A catalytic cycle is proposed for this unprecedented hydrosilylation reaction. The synthesis of the ν³-benzyl complex (dtbpm)Rh(η³-CH₂C₆H₅) (23) is described. This compound allows an alternative, more efficient access to the new silyl complex (dtbpm)Rh[Si(OEt)₃](PMe₃).     

Experimental evidence of structural transition from fcc to bulk bcc in nanophase metal clusters of (Fe)n, (Cr)n, (Mo)n and (W)n produced by CO2 laser multiphoton decomposition of metal carbonyls

We have produced nanophase metal clusters, (Fe)_n, (Cr)_n, (Mo)_n and (W)_n, by multiphoton decomposition of the corresponding metal carbonyls with a 10.6 μm CO₂ laser in the presence of Ar and SF₆. The size distribution was narrow and the average diameter was 6, 3.5, 2 and 1 nm for Fe, Cr, Mo and W clusters, respectively. The structure was found to be bcc for both Fe and Cr clusters, fcc for Mo clusters, and amorphous for W clusters (note that all the bulk metals have bcc structure). Considering the cluster sizes (9630, 1870, 230 and 30 for Fe, Cr, Mo and W clusters, respectively) estimated from their average diameters, it is likely that there exists a structural transition from fcc to bulk bcc with increasing cluster size in these metal clusters.     

Formation of [CoCl4(H2O)2] complex in CoCl2·MgCl2·8H2O double salt: structural and energetic properties of [MCl4(H2O)2] and [M(H2O)6] (M=Mg, Co)

The crystal structure of the double salt CoCl₂·MgCl₂·8H₂O has been determined by the X-ray diffraction method. It crystallizes in the space group with a=6.0976(9), b=6.308(1), c=8.579(3) Å, α=81.99(2)°, β=88.40°, γ=84.61(1)°, Z=1, and R=0.027. The crystal consists of two kinds of well separated octahedra, [CoCl₄(H₂O)₂]²⁻ and [Mg(H₂O)₆]²⁺. The former is unique as aquachloro complexes of Co²⁺. In order to elucidate the reason prepared as such unique complexes in the double salts, formation energies for [MCl₄(H₂O)₂]²⁻ and [M(H₂O)₆]²⁺ (M=Co, Mg) have been calculated by using the density functional methods, and it has been revealed that the formation energies of the first coordination sphere for the metal ions and the Cl⁻?H₂O hydrogen bond networks around [CoCl₄(H₂O)₂]²⁻ play a decisive role in forming [CoCl₄(H₂O)₂]²⁻ with the regular octahedral geometry in the double salt.     

M(CO)4 (M = Mo or W) complexes of 2,6-bis(methylthiomethyl)pyridine (L) and 2,6-bis(p-tolylthiomethyl)pyridine (L): a dynamic NMR study

The hybrid S/N/S donor ligands 2,6-bis(methylthiomethyl)pyridine (L¹) and 2,6-bis(p-tolylthiomethyl)pyridine (L²) react with the [M(CO)₅(THF)] (M = Mo or W) compounds to form complexes of general formula [M(CO)₄L] (M = Mo, L = L²; M = W, L = L¹ or L²), where both L¹ and L² act in a S/N bidentate chelate fashion. In solution, these complexes undergo three fluxional processes, viz. inversion at the coordinated S atom, S¹–S² switching, and combined inversion and S¹–S² switching, leading to an interconversion of the four possible permutational isomers. Energy barriers for all three processes have been evaluated by standard one-dimensional band-shape analysis techniques. The mechanism of the S¹–S² switch is discussed.     

Polarized absorption spectra and polarized Raman spectra of single crystals of trans(Cl2)-[CoCl2(NH3)n(H2O)4−n]Cl complexes (n = 2, 3, 4)

The title cobalt(III) complexes have been investigated by polarized absorption and Raman spectroscopies of the single crystals. The symmetry properties of the d-electron orbitals and of the vibrational modes attributable to the Raman bands of trans(Cl₂)-[CoCl₂(NH₃)_n(H₂O)_4−n]Cl complexes (n = 2, 3, or 4) were examined to elucidated the peculiar observation that ligand substitution causes no splitting of the 15 200-cm⁻¹ absorption band and the 250-cm⁻¹ Raman band. Effects of replacing the NH₃ ligand with H₂O on the electronic structure, atom–atom force constants and vibrational modes of these complex ions are briefly described.     

配合物Fe(pda)2(H2O)4和[FeCo(pda)4(H2O)4]n的合成与晶体结构

采用水热法合成了2个3-(3-吡啶基)丙烯酸的配合物:Fe(pda)2(H2O)4(1)和[FeCo(pda)4(H2O)4]n(2)(pda=3-(3-吡啶基)丙烯酸),用红外光谱、元素分析、热重-差热以及X-射线衍射单晶结构分析进行了表征.2个配合物都属于单斜晶系,配合物1的空间群为P21/n,配合物2的为P21/c.配合物1是一个pda配体中仅吡啶基氮原子参与配位、而羧基上的氧原子未参与配位的单核结构,通过大量的氢键作用形成三维超分子体系.2是pda配体桥联Fe和Co的异核二维层状配位聚合物;配体吡啶基上的氮原子和羧基上的氧原子都参与了配位,其中羧基采用单齿配位模式.     

Polymorphism and Pressure Driven Thermal Spin Crossover Phenomenon in [Fe( abpt )2(NC X )2] ( X =?S, and Se): Synthesis, Structure and Magnetic Properties

Summary.  The monomeric compounds [Fe(abpt)₂(NCX)₂] (X = S (1), Se (2) and abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) have been synthesized and characterized. They crystallize in the monoclinic P2₁/n space group with a = 11.637(2) ?, b = 9.8021(14) ?, c = 12.9838(12) ?, β = 101.126(14)°, and Z = 2 for 1, and a = 11.601(2) ?, b = 9.6666(14) ?, c = 12.883(2) ?, β = 101.449(10)°, and Z = 2 for 2. The unit cell contains a pair mononuclear [Fe(abpt)₂(NCX)₂] units related by a center of symmetry. Each iron atom, located at a molecular inversion center, is in a distorted octahedral environment. Four of the six nitrogen atoms coordinated to the Fe(II) ion belong to the pyridine-N(1) and triazole-N(2) rings of two abpt ligands. The remaining trans positions are occupied by two nitrogen atoms, N(3), belonging to the two pseudo-halide ligands. The magnetic susceptibility measurements at ambient pressure have revealed that they are in the high-spin range in the 2 K–300 K temperature range. The pressure study has revealed that compound 1 remains in high-spin as pressure is increased up to 4.4 kbar, where an incomplete thermal spin crossover appears at around T _1/2 = 65 K. Quenching experiments at 4.4 kbar have shown that the incomplete character of the conversion is a consequence of slow kinetics. Relatively sharp spin transition takes place at T _1/2 = 106, 152 and 179 K, as pressure attains 5.6, 8.6 and 10.5 kbar, respectively. Corresponding author. E-mail: jose.a.real@uv.es Received June 12, 2002; accepted July 1, 2002     

The metastable Ni7±xS6, and mixed Ni6±x(S1−ySey)5, phases

The closely related, narrowly non-stoichiometric, metastable as well as thermodynamically stable “phases” in the metal-rich part of the Ni–S phase diagram near the nominal composition Ni_7±xS₆ have been carefully re-investigated via electron diffraction and transmission electron microscope imaging. Two quite distinct polymorphs have been identified, a minority incommensurate interface-modulated polymorph and a (heavily twinned) majority I1a1, a=2a_p, b=2b_p, c=−a_p+c_p superstructure (of an underlying Bmmb, a_p3.3, b_p16.4, c_p11.3 Å parent structure) polymorph. The incommensurate polymorph is shown to be very closely related to the only known polymorph of Ni_6±xSe₅ and is rapidly stabilized to room temperature upon doping of the sulfide compounds with selenium.