An investigation into oxo analogues of molybdenum olefin metathesis complexes as epoxidation catalysts for alkenes

The oxo-imido molybdenum complex 2a is an effective catalyst at low catalyst loadings (0.5 mol % or below) for the epoxidation of a range of alkenes with ^tBuOOH in PhMe at 90 °C. Reactions are complete in less than 4 h and the products are isolated in high yields. The catalytic system is chemoselective for the epoxidation of electron-rich alkenes and allylic alcohols.     

Iron(II) complexes with bio-inspired N,N,O ligands as oxidation catalysts: olefin epoxidation and cis-dihydroxylation

The Rieske dioxygenases are a group of non-heme iron enzymes, which catalyze the stereospecific cis-dihydroxylation of its substrates. Herein, we report the iron(II) coordination chemistry of the ligands 3,3-bis(1-methylimidazol-2-yl)propionate (L1) and its neutral propyl ester analogue propyl 3,3-bis(1-methylimidazol-2-yl)propionate (PrL1). The molecular structures of two iron(II) complexes with PrL1 were determined and two different coordination modes of the ligand were observed. In [Fe(II)(PrL1)(2)](BPh(4))(2) (3) the ligand is facially coordinated to the metal with an N,N,O donor set, whereas in [Fe(II)(PrL1)(2)(MeOH)(2)](OTf)(2) (4) a bidentate N,N binding mode is found. In 4, the solvent molecules are in a cis arrangement with respect to each other. Complex 4 is a close structural mimic of the crystallographically characterized non-heme iron(II) enzyme apocarotenoid-15-15'-oxygenase (APO). The mechanistic features of APO are thought to be similar to those of the Rieske oxygenases, the original inspiration for this work. The non-heme iron complexes [Fe(II)(PrL1)(2)](OTf)(2) (2) and [Fe(II)(PrL1)(2)](BPh(4))(2) (3) were tested in olefin oxidation reactions with H(2)O(2) as the terminal oxidant. Whereas 2 was an active catalyst and both epoxide and cis-dihydroxylation products were observed, 3 showed negligible activity under the same conditions, illustrating the importance of the anion in the reaction.     

DFT-UX3LYP studies on the coordination chemistry of Ni2+. Part 1: Six coordinate [Ni(NH3)n(H2O)(6-n)]2+ complexes

DFT calculations with the UX3LYP hybrid functional and a medium-sized 6-311++G(d,p) basis set were performed to examine the gas-phase structure of paramagnetic (S = 1) six-coordinate complexes [Ni(NH3)n(H2O)(6-n)](2+), 0 < or = n < or = 6. Significant interligand hydrogen bonding was found in [Ni(H2O)6](2+), but this becomes much less significant as NH3 replaces H2O in the coordination sphere of the metal. Bond angles and bond lengths obtained from these calculations compare reasonably well with available crystallographic data. The mean calculated Ni-O bond length in [Ni(H2O)6](2+) is 2.093 A, which is 0.038 A longer than the mean of the crystallographically observed values (2.056(22) A, 108 structures) but within 2sigma of the experimental values. The mean calculated Ni-N bond length in [Ni(NH3)6](2+) is 2.205(3) A, also longer (by 0.070 A) than the crystallographically observed mean (2.135(18) A, 7 structures). Valence bond angles are reproduced within 1 degree. The successive replacement of H2O by NH3 as ligands results in an increase in the stabilization energy by 7 +/- 2 kcal mol(-1) per additional NH3 ligand. The experimentally observed increase in the lability of H2O in Ni(II) as NH3 replaces H2O in the coordination sphere is explained by an increase in the Ni-OH2 bond length. It was found from a natural population analysis that complexes with the highest stabilization energies are associated with the greatest extent of ligand-to-metal charge transfer, and the transferred electron density is largely accommodated in the metal 4s and 3d orbitals. An examination of the charge density rho bcp and the Laplacian of the charge density nabla(2)rho(bcp) at the metal-ligand bond critical points (bcp) in the series show a linear correlation with the charge transferred to the metal. Values of nabla(2)rho(bcp) are positive, indicative of a predominantly closed-shell interaction. The charge transferred to the metal increases as n, the number of NH3 ligands in the complex, increases. This lowers the polarizing ability of the metal on the ligand donors and the average metal-ligand bond length increases, resulting in a direct correlation between the dissociation energy of the complexes and the reciprocal of the average metal-ligand bond length. There is a strong correlation between the charge transferred to the metal and experimental DeltaH values for successive replacement of H2O by NH3, but a correlation with stability constants (log beta values) breaks when n = 5 and 6, probably because of entropic effects in solution. Nevertheless, DFT calculations may be a useful way of estimating the stability constants of metal-ligand systems.     

Spin surface crossing in chromium-mediated olefin epoxidation with O(2)

[CpCr(mu-Cl)Cl](2) reacted with dioxygen (O(2)) to produce CpCr(O)Cl(2) (1), which has been structurally characterized. Although 1 oxidized PPh(3) and 1,4-cyclohexadiene catalytically, it did not epoxidize olefins. DFT calculations have been performed on the system to characterize the potential energy surface for the epoxidation of ethylene and, in particular, the consequences of the crossing from the doublet surface of the starting materials to the quartet surface of the product (i.e. a chromium(III) epoxide adduct). These calculations suggested that "spin-blocking" was not a significant problem and that the reaction of CpCr(O)Cl(2) (3) with ethylene should have a lower activation barrier. On the basis of this computational prediction, 3 was prepared; it was found to epoxidize olefins stoichiometrically.     

A highly efficient dioxo(mu-oxo)molybdenum(VI) dimer catalyst for olefin epoxidation

The oxo-bridged dimer [Mo(2)O(4)(mu(2)-O)Cl(2)(pzH)(4)] (1; pzH = pyrazole) exhibits unusually high activity in the liquid-phase catalytic epoxidation of the cyclic olefins cyclooctene and (R)-(+)-limonene under mild conditions and in the absence of additional organic solvents, using tert-butyl hydroperoxide as the oxidant. The complex is stable under the reaction conditions and can be used in further catalytic runs without significant loss of performance. An X-ray crystallographic investigation reveals that 1 has an unprecedented and extremely rare all-cis configuration at each of the MoO(2)-(mu(2)-O)Cl(pzH)(2) cores, which can be understood by considering supramolecular contacts and geometric factors.     

Chiral monomeric organorhenium(VII) and organomolybdenum(VI) compounds as catalysts for chiral olefin epoxidation reactions

Several attempts have been made to transform the organometallic Re(VII) compound MTO and the (MoO₂)²⁺ moiety to chiral epoxidation catalysts by addition of chiral organic ligands. Being very efficient epoxidation catalysts in achiral reactions, it was hoped that these compounds could be transformed into chiral epoxidation catalysts by adding chiral Lewis base ligands. The major flaw of most of these attempts, however, was the weak coordination of the chiral Lewis base ligands to the metal center, which led either to high ees only at the very beginning of the catalytic reaction (low conversion) or to generally low enantiomeric excesses. The heterogenisation of the Mo(VI) complexes was, at least in some cases, successfully achieved but with the same drawbacks with respect to the ees as in the homogeneous phase. Currently, attempts are being made to synthesize organometallic Re(VII) and Mo(VI) complexes with stronger interactions between the metal containing moiety and the chiral ligand(s).     

MgCl2/R'nAl(OR)3-n: an excellent activator/support for transition-metal complexes for olefin polymerization

A new and effective method for the activation, and simultaneously, immobilization of bis(phenoxyimine) early-transition-metal complexes for olefin polymerization (known as FI catalysts), which makes use of MgCl(2)/R'(n)Al(OR)(3-n) as an activator/support, has been developed. Ti-, Zr-, and V-FI catalysts combined with this MgCl(2)-based compound can form highly active MgCl(2)-supported single-site catalysts capable of demonstrating superior catalytic properties, compared to the corresponding homogeneous methylaluminoxane- (Ti- and Zr-FI catalysts) or alkylaluminum-activation systems (V-FI catalysts), in terms of their catalytic activity, molecular weight, stereoselectivity, and comonomer incorporation. Additionally, these new catalysts can produce polymers of significant morphology with high efficiency. Notably, the MgCl(2)-based compounds can also effectively activate and immobilize the early-to-late transition-metal complexes that have emerged recently. Thus, the application of MgCl(2)-based compounds as activators/supports for transition-metal complexes for olefin polymerization provides a conceptually new strategy for the development of methylaluminoxane- and borate-free, high-performance, single-site catalysts capable of controlling polymer morphology.     

Dinuclear peroxo complexes of molybdenum(VI) containing Mannich base ligands

Dinuclear molybdenum(VI) peroxo complexes containing Mannich base ligands having formulae [Mo₂O₄(O₂)₂L-L(H₂O)₂] · H₂O [where L-L = N-[1-morpholinobenzyl] acetamide (MBA), N-[1-piperidinobenzyl] acetamide (PBA), N-[1-morpholino(-4-nitrobenzyl)] benzamide (MPNBB), N-[1-piperidino(-3-nitrobenzyl)] benzamide (PMNBB), N-[1-morpholino(-2-nitrobenzyl)] acetamide (MONBA), and N-[1-morpholino(-3-nitrobenzyl)] acetamide (MMNBA)] have been synthesized by stirring ammonium heptamolybdate with excess 30% aqueous hydrogen peroxide followed by treatment with ethanolic solution of corresponding ligands. The complexes have been characterized by elemental analysis, molar conductance, magnetic measurements, infrared (IR), electronic, TGA/DTA, mass spectral, and ¹H NMR studies. The complexes are non-electrolytes and diamagnetic. The IR spectral studies suggest that the ligands are bidentate to metal through carbonyl oxygen and ring nitrogen. Thermal analyses provide conclusive evidence for the presence of coordinated, as well as lattice water in the complexes. Dinuclear complexes preserve the individuality of the molybdenum oxo peroxo core. The complexes exhibit higher antibacterial activity against bacterium Ralastonia solanacearum (Pseudomonas solanacearum) than the free ligands.     

Homonuclear molybdenum(VI) and heteronuclear molybdenum(VI) copper(II) peroxo complexes containing amino acids

Summary MoO₃·H₂O reacts with two equivalents of glycylglycine (H₂Gg) or aspartic acid (H₂Asp) in an excess of aqueous H₂O₂ at ambient temperature to give [MoO(O₂)₂-(H₂Gg)₂(H₂O)] (1) and [MoO(O₂)₂(H₂Asp)(H₂O)] (2), respectively. An equimolar mixture of Cu powder and MoO₃·H₂O, on interaction with H₂Gg or H₂Asp in the presence or absence of H₂O₂, results in formation of [CuMoO(O₂)(Gg)₂(H₂O)₂] (3), [CuMoO₂(Gg)₂-(H₂O)₄] (4) and [CuMoO₂(Asp)₂(H₂O)₄] (5), respectively. These novel complexes have been characterized by elemental and thermogravimetric analysis, and e.s.r., electronic and i.r. spectral data.     

Bis(β‐enaminoketonato) vanadium (III or IV) complexes as catalysts for olefin polymerization

Bis(β‐enaminoketonato) vanadium(III) complexes ( 2a–c ) [O(R₁)C?C(H)_xC(R₂)?NC₆H₅]₂VCl(THF) and the corresponding vanadium(IV) complexes ( 3a–c ) [O(R₁)C?C(H)_xC(R₂)? NC₆H₅]₂VO (R₁ = ? (CH₂)₄? , R₂ = H, x = 0, a ; R₁ = ? C₆H₅, R₂ = H, x = 1, b ; R₁ = ? C₆H₅, R₂ = ? C₆H₅, x = 1, c ) have been synthesized from VCl₃(THF)₃ and VOCl₂(THF)₂, respectively, by treating with 2.0 equivalent β‐enaminoketonato ligands in tetrahydrofuran. Structures of 2b and 3a–c were further confirmed by X‐ray crystallographic analysis. The complexes were investigated as the catalysts for ethylene polymerization in the presence of Et₂AlCl. Complexes 2a–c and 3a–c exhibited high catalytic activities (up to 23.76 kg of PE/mmol_V h bar), and afforded polymers with unimodal molecular weight distributions at 70 °C indicating the good thermal stability. The catalytic behaviors were influenced not only by the oxidation state of the catalyst precursors but also by the ligand structures. Complexes 2a–c and 3a–c were also effective catalyst precursors for ethylene/1‐hexene copolymerization. The influence of polymerization parameters such as reaction temperature, Al/V molar ratio and hexene feed concentration on the ethylene/hexene copolymerization behaviors have bee also investigated in detail. In addition, the agents such as AlMe₃, AlⁱBu₃, MeMgBr, MgCl₂, and ZnEt₂ were applied to control the molecular weight and molecular weight distribution modal. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3062–3072, 2010     

New molybdenum catalysts for alkyl olefin epoxidation. Their implications for the mechanism of oxygen atom transfer.

We report here the design, synthesis, and characterization of new (dioxo)Mo(VI) epoxidation catalysts based on monoanionic tridentate ligands. Two important features distinguish these catalysts from those previously reported. First, their coordination environment remains well-defined during the epoxidation reaction. Second, the ligand design does not permit simultaneous coordination of olefin and alkyl hydroperoxide. Based on the study of these new catalysts, we conclude that direct oxygen atom transfer from coordinated alkyl peroxide to olefin remains the simplest mechanism consistent with the available data. We discuss literature discrepancies in this regard.     

胺功能化的拟薄水铝石纳米颗粒负载的Mn-V复合物：高效的烯烃环氧化催化剂

将具有高比表面积和表面高度羟基化的拟薄水铝石纳米颗粒与3-(3甲氧基硅烷)-正丙胺进行共价结合而官能团化,再用于负载硫酸氧钒和六羰基钼络合物。所得样品采用红外光谱、粉末X射线衍射、热重-差热分析、X射线光电子能谱、元素分析、电感耦合等离子体和透射电镜等技术进行了表征,并用于顺-环辛烯的环氧化反应中,优化了诸如溶剂和氧化剂等反应条件。反应过程采用气-液色谱进行监测。重复使用实验表明,该纳米催化剂可重复使用多次,并保持顺-环辛烯接近完全环氧化。所得到的优化反应条件也成功用于其它的取代烯烃的环氧化反应中。     

PPh4]3[W(CN)7(O2)].4H2O as the representative of the [M(L)7(LL)] class for nine-coordinate complexes

The photoinduced dissociation of a W-CN bond in [W(CN)8]4- in an aqueous solution under ambient conditions, in conjunction with the uptake of molecular oxygen, affords the W(VI) mixed-ligand complex anion [W(CN)(7)(eta2-O2)]3-, conveniently isolable as its [PPh4+] salt. Although research into the chemistry of cyanomolybdates and cyanotungstates has been pursued with great interest and vigor over several decades, there is a paucity of structurally characterized cyano-peroxo complexes of Mo and W. The side-on coordination mode of the peroxo moiety in [W(CN)7(eta2-O2)]3- has been ascertained with X-ray crystal structure determination [d(O-O) = 1.41 A; peroxo bite angle: 41.0 degrees ] and corroborated with vibrational spectroscopy [nu(O-O) = 915 cm(-1)]. The complex ion exhibits trapezoidal tridecahedral geometry and represents the new class of nine-coordinate complexes with one bidentate and seven monodentate ligands. Cyclic voltammetry shows a reversible redox behavior of [W(CN)7(eta2-O2)]3- in CH3CN with its standard reduction potential equal to 1.130 V. Generally, interest in atmospheric oxygen derives from the versatility of this molecule as a ligand and oxidant and extends to the physicochemical features it imparts to transition metals such as copper and iron in biological oxygen carriers.     

Ligand topology effects on olefin oxidations by bio-inspired [FeII(N2Py2)] catalysts

Linear tetradentate N2Py2 ligands can coordinate to an octahedral FeII center in three possible topologies (cis-alpha, cis-beta, and trans). While for the N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane (bpmen) complex, only the cis-alpha topology has been observed, for N,N'-bis(2-pyridylmethyl)-1,2-diaminocyclohexane (bpmcn) both cis-alpha and cis-beta isomers have been reported. To date, no facile interconversion between cis-alpha and cis-beta topologies has been observed for ironII complexes even at high temperatures. However, this work provides evidence for facile interconversion in solution of cis-alpha, cis-beta, and trans topologies for [Fe(bpmpn)X2] (bpmpn=N,N'-bis(2-pyridylmethyl)-1,3-diaminopropane; X=triflate, CH3CN) complexes. As reported previously, the catalytic behavior of cis-alpha and cis-beta isomers of [Fe(bpmcn)(OTf)2] with respect to olefin oxidation depends dramatically on the geometry adopted by the iron complex. To establish a general pattern of the catalysis/topology dependence, this work presents an extended comparison of the catalytic behavior for oxidation of olefins of a family of [Fe(N2py2)] complexes that present different topologies. 18O labeling experiments provide evidence for a complex mechanistic landscape in which several pathways should be considered. Complexes with a trans topology catalyze only non-water-assisted epoxidation. In contrast, complexes with a cis-alpha topology, such as [Fe(bpmen)X2] and [Fe(alpha-bpmcn)(OTf)2], can catalyze both epoxidation and cis-dihydroxylation through a water-assisted mechanism. Surprisingly, [Fe(bpmpn)X2] and [Fe(beta-bpmcn)(OTf)2] catalyze epoxidation via a water-assisted pathway and cis-dihydroxylation via a non-water-assisted mechanism, a result that requires two independent and distinct oxidants.     

Heterotrinuclear manganese(II) and vanadium(IV) Schiff base complexes as epoxidation catalysts

The catalytic epoxidation of styrene using urea-hydrogen peroxide and heterotrinuclear Cu(II) complexes with general formula (ML ⁿ )₂Cu(acac)₂, where n = 1–3 and M = VO²⁺ or Mn²⁺ is reported. Schiff base complexes ML ⁿ involving a 3,4-diaminopyridine bridge with free coordination site were used as the ligand, where (Lⁿ)²⁻ is [(5-x-Sal)₂Py]² and x = H, Br or NO₂. The complexes were characterized by physico-chemical and spectroscopic methods. The electrochemical properties of M were modified upon trinuclear complex formation. The trinuclear complexes show high catalytic activity, with up to 86% conversion and 93% selectivity, while no catalytic properties were observed for the monomeric complexes. The catalyst could be reused with some loss of activity.     

Aminopropyl group-modified SBA-15 covalent attachment Mn(salen) complexes as catalysts for styrene epoxidation

A series of aminopropyl group-modified ordered mesoporous silica materials impregnated with Mn(salen) were prepared using successive grafting procedures. The prepared composite catalysts were well characterized by inductively coupled plasma atomic emission spectroscopy, Fourier transform-infrared, UV–Vis diffuse reflectance spectroscopy, X-ray diffraction analysis, and transmission electron microscopy in order to confirm the structure integrities of the Mn(salen) units after the incorporation, to evidence the formation of a covalent bond between the starting Mn(salen) units and the aminopropyl group-modified SBA-15 matrix in the presence of NaOH by abstraction of an HCl molecule. These heterogeneous catalysts exhibited comparable catalytic activity and selectivity to those of the homogeneous counterpart in the epoxidation of styrene by using NaClO as oxidant. In addition, the effects of key reaction parameters, including the loadings of the neat Mn(salen), molar ratios of NaClO to styrene, and PPNO amount on the reactivity and selectivity, were also studied. Finally, the reusability of the prepared heterogeneous catalyst was evaluated.     

简单烯烃不对称环氧化的手性金属配合物催化剂

介绍了多类简单烯烃不对称环氧化反应的手性过渡金属配合物催化剂系统的催化效果。论述了配体结构与配合物作为环氧化反应催化剂的活性，稳定性，催化效率的关系。从配合物的电子和立体因素，几何构型与过渡态的构象和非对映异构体的能量差别探讨对不对称诱导效果的影响。同时陈述机理的研究情况。     

Syntheses and structures of nine-coordinate K3[DyIII(nta)2(H2O)]·5H2O and eight-coordinate (NH4)3[DyIII(nta)2] complexes

K₃[Dy^III(nta)₂(H₂O)]·5H₂O and (NH₄)₃[Dy^III(nta)₂] have been synthesized in aqueous solution and characterized by IR, elemental analysis and single-crystal X-ray diffraction techniques. In K₃[Dy^III(nta)₂(H₂O)]·5H₂O the Dy^III ion is nine coordinated yielding a tricapped trigonal prismatic conformation, and its crystal belongs to monoclinic system and C2/c space group. The crystal data are as follows: a = 15.373(5) Å, b = 12.896(4) Å, c = 26.202(9) Å; β = 96.122(5)°, V = 5165(3) ų, Z = 8, D _c = 1.965 g·cm^?3, μ = 3.458 mm^?1, F(000) = 3016, R ₁ = 0.0452 and wR ₂ = 0.1025 for 4550 observed reflections with I ≥ 2σ(I). In (NH₄)₃[Dy^III(nta)₂] the Dy^III ion is eight coordinated yielding a usual dicapped trigonal anti-prismatic conformation, and its crystal belongs to monoclinic system and C2/c space group. The crystal data are as follows: a = 13.736(3) Å, b = 7.9389(16) Å, c = 18.781(4) Å; β = 104.099(3)°, V = 1986.3(7) ų, Z = 2, D _c = 1.983 g·cm^?3, μ = 3.834 mm^?1, F(000) = 1172, R ₁ = 0.0208 and wR ₂ = 0.0500 for 2022 observed reflections with I ≥ 2σ(I). The results indicate that the difference in counter ion also influences coordination numbers and structures of rare earth metal complexes with aminopolycarboxylic acid ligands.     

Kristallstruktur von (NMe4)2[Re3Br11(H2O)] [Re3Br9(H2O)3](H2O)2

Crystal Structure of (NMe₄)₂[Re₃Br₁₁(H₂O)] [Re₃Br₉(H₂O)₃](H₂O)₂ . (NMe₄)₂[Re₃Br₁₁(H₂O)] [Re₃Br₉(H₂O)₃](H₂O)₂ crystallizes from hydrobromic acid solution of Re₃Br₉ · 2 H₂O and NMe₄Br at 0 – 5°C. The crystal structure (monoclinic; P2₁/m (Nr. 11); a = 967.9(3); b = 1 529.7(4); c = 1 710.9(4) pm; β = 91.66(2)°; Z = 2; R = 0.113; R_w = 0.068) has been determined from four-circle diffractometer data. The structure contains two different cluster units of trivalent rhenium, isolated anionic [Re₃Br₁₁(H₂O)]^2? units and neutral cluster units that are connected through crystal water molecules to chains{[Re₃Br₉(H₂O)₃](H₂O)₂}.