Synthesis and adsorption properties of p-sulfonated calix[4 and 6]arene-intercalated layered double hydroxides

The intercalation of water-soluble p-sulfonated calix[4 and 6]arene (CS4 and CS6) in the interlayer of the Mg-Al and Zn-Al layered double hydroxide (LDH) by the coprecipitation method has been investigated, as well as the adsorption properties of the resulting CS/LDHs for benzyl alcohol (BA) and p-nitrophenol (NP) to prepare new microporous organic-inorganic hybrid adsorbents. The amount and arrangement of CS intercalated was different by the kind of the host metal ions. CS4 cavity axis was perpendicular for the Mg-Al LDH basal layer and parallel for the Zn-Al LDH basal layer, while CS6 cavity axis was perpendicular for both the LDH basal layers. In the BET surface area measurement, the surface area of the Zn-Al/CS4/LDH was four times than that of the Mg-Al/CS4/LDH, expecting that the former has higher adsorption capacity than the latter. In fact, the adsorption ability of the CS/LDHs for BA and NP in aqueous solution was found to be larger in the Zn-Al/CS4/LDH than in the Mg-Al/CS4/LDH. In addition, the adsorption ability of both the LDHs was larger in the CS6/LDHs than in the CS4/LDHs. These results were explained by the difference in the amount and arrangement of CS intercalated in the LDH interlayer space.     

Synthesis and characterization of molybdenum oxo complexes of two tripodal ligands: reactivity studies of a functional model for molybdenum oxotransferases

Reaction of the tetradentate ligand N-(2-hydroxybenzyl)-N,N-bis(2-pyridylmethyl)amine (L-OH) with MoO2Cl2 in methanol in the presence of NaOMe and PF6- results in the formation of [MoO2(L-O)]PF6. Similarly, the reaction of N-(2-mercaptobenzyl)-N,N-bis(2-pyridylmethyl)amine (L-SH) with MoO2(acac)2 leads to the formation of [MoO2(L-S)]+. The dioxo-molybdenum complex [MoO2(L-O)]+ reacts with phosphines in methanol to afford phosphine oxides and an air-sensitive molybdenum complex, tentatively identified as [Mo(IV)O(L-O)(OCH3)]. The latter complex is capable of reducing biological oxygen donors such as DMSO or nitrate, thereby mimicking the activity of DMSO reductase and nitrate reductase. Reaction of [MoO2(L-O)]PF6 with PPh3 in other solvents than methanol leads to the formation of the Mo(V) dimer [(L-O)OMo(micro-O)MoO(L-O)](PF6)2. The crystal structures of [MoO2(L-O)]PF6 and the micro-oxo bridged dimer are presented.     

Hydrothermal Synthesis and Structure of a Novel Binuclear Molybdenum Complex with Oxalate Ligand,(enH2){NH4[Co(en)3][Mo2O7(C2O4)]}2·2H2O

The title complex (enH2){NH4[Co(en)3][Mo2O7(C2O4)]}2·2H2O (C18H70Co2Mo4- N16O24, Mr = 1396.52) was obtained under hydrothermal conditions and its crystal structure has been determined by single-crystal X-ray diffraction. It crystallizes in the monoclinic system, space group P21/c with a = 17.8023(8), b = 7.7527(4), c = 16.9781(4)A,β= 103.878(7)°, V = 2274.8(2) A3, Dc = 2.039 g/cm3, Z = 2,μ(MoKα) = 1.878 mm-1 and F(000) = 1408. The final R = 0.0410 and Wr = 0.1070 for 4065 observed reflections with I≥2σ(I). The crystal structure is composed of bi- nuclear [Mo2O7(C2O4)]4- anions, complex [Co(en)3]2+ cations, protonated ethylenediamine cations, ammonium cations and crystal water molecules, which are held together into a three-dimensional network via hydrogen-bonding interactions. The binuclear structure of [Mo2O7(C2O4)]4- consist of one MoO4 and one MoO6 octahedra through sharing a bridging oxygen atom, where the oxalate ligand acts as a bidentate ligand coordinating to the octahedral molybdenum atom though two deprotonated corboxylate groups.     

Molybdenum(V) on an oxide string. Formation and solution stability of the linear complex (mu-trans-dioxo(tetraphenylporphyrinato)molybdenato(V))- bis(oxo(tetraphenylporphyrinato)molybdenum(V)) perchlorate

The unusual linear trinuclear complex [Mo3O4(TPP)3]+ is formed in solution upon the reaction of [MoO(TPP)-(OClO3)] with [[MoO(TPP)]2O], and an equilibrium between [Mo3O4(TPP)3]+ and its constituent species is rapidly established. Spectrophotometric experiments suggest that [Mo3O4(TPP)3]+ is the predominant species found in solutions resulting from the mixture of [MoO(TPP)(OClO3)] and [[MoO(TPP)]2O], and its formation is strongly favored (log K = 5.5 +/- 0.5 M-1). No evidence of higher oligomers has been observed. A mechanism for the formation of [Mo3O4(TPP)3]+ by the controlled hydrolysis of [MoO(TPP)(OClO3)] is proposed.     

An octanuclear molybdenum(VI) complex containing coordinatively bound 4,4'-di-tert-butyl-2,2'-bipyridine, [Mo8O22(OH)4(di-tBu-bipy)4]: synthesis, structure, and catalytic epoxidation of bio-derived olefins

The reaction of [MoO(2)Cl(2)(di-tBu-bipy)] (1) (di-tBu-bipy = 4,4'-di-tert-butyl-2,2'-bipyridine) with water at 100-120 °C in a Teflon-lined stainless steel autoclave, in an open reflux system, or in a microwave synthesis system gave the octanuclear complex [Mo(8)O(22)(OH)(4)(di-tBu-bipy)(4)] (2) as a microcrystalline powder in good yields. Single crystals of 2 suitable for X-ray diffraction were obtained by the reaction of MoO(3) and di-tBu-bipy in water at 160 °C for 3 days. The molecular structure of 2 comprises a purely inorganic core, Mo(4)O(8)(μ(3)-OH)(2)(μ(2)-O)(2), attached to two peripheral oxo-bridged binuclear units, Mo(2)O(4)(μ(2)-O)(2)(OH)(di-tBu-bipy)(2). The inorganic core is composed of a unique assembly of four {MoO(5)} distorted square pyramids connected to each other via edge-sharing. Overall, the octanuclear complex adopts a highly distorted form strongly resembling an "S"-shaped molecular unit. Complex 2 was applied in the catalytic epoxidation of the biorenewable olefins DL-limonene (Lim) and methyl oleate (Ole), using tert-butylhydroperoxide (TBHP) as an oxygen donor, under mild reaction conditions (55 °C, air). The reactions of Lim and Ole gave the respective epoxide monomers in fairly high selectivities at high conversions (89% 1,2-epoxy-p-menth-8-ene selectivity at 96% Lim conversion; 99% methyl 9,10-epoxystearate selectivity at 94% Ole conversion, reached within 24 h reaction). Iodometric titrations revealed no measurable "non-productive" decomposition of TBHP.     

Sugar-anionic clay composite materials: intercalation of pentoses in layered double hydroxide

The intercalation of non-ionized guest pentoses (ribose and 2-deoxyribose) into the Mg-Al and Zn-Al layered double hydroxides (LDHs) was carried out at 298 K by the calcination-rehydration reaction using the Mg-Al and Zn-Al oxide precursors calcined at 773 K. The resulting solid products reconstructed the LDH structure with incorporating pentoses, and the maximum amount of ribose intercalated by the Mg-Al oxide precursor was approximately 20 times that by the Zn-Al oxide precursor. The ribose/Mg-Al LDH was observed to have the expanded LDH structure with a broad (003) spacing of 0.85 nm. As the thickness of the LDH hydroxide basal layer is 0.48 nm, the interlayer distance of the ribose/Mg-Al LDH is 0.37 nm. This value corresponds to molecular size of ribose in thickness (0.36 nm), supporting that ribose is horizontally oriented in the interlayer space of LDH. The maximum amount of ribose intercalated by the Mg-Al oxide precursor was approximately 5 times that of 2-deoxyribose. Ribose is substituted only by the hydroxyl group at C-2 position for 2-deoxyribose. Therefore, the number of hydroxyl group of sugar is essentially important for the intercalation of sugar molecule into the LDH, suggesting that the intercalation behavior of sugar for the LDH was greatly influenced by hydrogen bond between hydroxyl group of the intercalated pentose and the LDH hydroxide basal layers.     

Molecular structure-activity relationships for the oxidation of organic compounds using mesoporous silica catalysts derivatised with bis(halogeno)dioxomolybdenum(VI) complexes

Mo K-edge XAFS spectra have been measured for ordered mesoporous silica MCM-41 grafted with the complexes [MoO2X2(thf)2] (X=Cl, Br). For grafting reactions in the absence of triethylamine, materials with 1 wt. % Mo are obtained; the Mo K-edge EXAFS results indicate the co-existence of isolated surface-fixed monomeric species [MoO2[(-O)3SiO]2(thf)(n)] and [MoO2[(-O)3SiO]X(thf)(n)]. When Et3N is used in the grafting reactions, materials with 4 wt. % Mo are obtained. The EXAFS data for the material prepared using [MoO2Cl2(thf)2] and Et3N indicate the presence of dinuclear species with two Mo(VI) centres, each with two Mo=O groups and each linked by one or two oxo bridges (Mo...Mo 3.27 A). The molybdenum centres in the material prepared using the dibromo complex comprise mainly isolated four-coordinate dioxomolybdenum(VI) and trioxomolybdenum(VI) monomeric species, with a small contribution from dimeric species. All materials were further characterised in the solid state by powder X-ray diffraction, N2 adsorption analysis, MAS NMR (13C, 29Si) and FTIR spectroscopy. The derivatised MCMs perform differently as catalysts in the liquid-phase oxidation of various olefins and alcohols with tert-butyl hydroperoxide. The highest alkene epoxidation activity was recorded for the catalysts with low metal loading, whereas the material containing oxo-bridged dimers had the highest activity for oxidation of alcohols. The recyclability of all the catalysts was tested: the catalytic activity of the derivatised materials tended to stabilize with ageing.     

钼酸根阴离子在增强聚苯胺包覆的镍钴层状氢氧化物的电容和析氧行为中的关键作用

理论容量大且过电位低的层状氢氧化物(LDHs)是极有前景的超级电容电池和析氧反应的电极材料;然而,体相LDHs的低电导率和活性位点不足增加了电极的内阻,降低了电极容量和产氧效率.本文采用两步法制备了聚苯胺包覆的MoO42?插层的镍钴层状双金属氢氧化物复合电极(M-LDH@PANI).随着LDH中MoO42?含量的增加,针状的LDH微球逐渐演化为具有较高比表面积的片状M-LDH微球,这为整个电极提供了更多的电化学位点.此外,非晶态的聚苯胺包覆提高了复合电极的电导率.在引入适量MoO42?插层离子时,M-LDH@PANI表现出显著强化的储能和催化性能.所获得的M-LDH@PANI-0.5在析氧反应中表现出优越的电催化活性(10 mA cm?2时的过电位为266 mV),作为超级电容电池电极则具有864.8 C g?1的高容量.采用M-LDH@PANI-0.5作为正极及以活性炭作为负极组装的超级电容电池在功率密度为8,300.0 W kg?1时能量密度为44.6 Wh kg?1,且具有优异的循环稳定性(10000次循环后保留83.9％的初始容量).本文为LDH基材料的阴离子插层改性增强材料性能的机理提供了一个非传统的解释.在上述研究基础上,采用射线衍射(XRD)、X射线光电子能谱(XPS)、扫描电子显微镜(SEM)、高分辨透射电镜(HRTEM)和比表面积测试(BET)等手段对样品进行了深入表征.XRD结果表明,MoO42?插层的LDH材料的层间晶面(003)的峰随着MoO42?含量的增加而逐渐消失,这是由于晶面间距越大越容易受到晶粒细化的影响,间距大的晶格更容易受到破坏,导致晶格的展宽和弱化,从而间接证明MoO42?的成功插层.SEM、HRTEM和BET测试结果表明,MoO42?的含量对材料的形貌和比表面积具有重大影响.利用XPS对样品的价态进行了研究,发现随着MoO42?含量的增加,Co和Ni的价态没有明显变化.电化学测试结果表明,电极的储能和催化性能随MoO42?含量的增加而先增加后减小.利用理论计算分析了MoO42?在LDH中的插层行为,发现少量的MoO42?有利于扩大LDH的层间间距,而过量的MoO42?则会与LDH的H原子结合,从而与电解液中的OH?竞争,导致复合电极的电化学性能下降.此外,MoO42?插层的片状微球能有效调节材料的去质子化能,大大加速电极表面的氧化还原反应.因此,MoO42?插层能够显著强化LDH基材料的超级电容电池电极和OER催化剂电化学性能.     

钼酸根阴离子在增强聚苯胺包覆的镍钴层状氢氧化物的电容和析氧行为中的关键作用

理论容量大且过电位低的层状氢氧化物(LDHs)是极有前景的超级电容电池和析氧反应的电极材料;然而,体相LDHs的低电导率和活性位点不足增加了电极的内阻,降低了电极容量和产氧效率.本文采用两步法制备了聚苯胺包覆的MoO42?插层的镍钴层状双金属氢氧化物复合电极(M-LDH@PANI).随着LDH中MoO42?含量的增加,针状的LDH微球逐渐演化为具有较高比表面积的片状M-LDH微球,这为整个电极提供了更多的电化学位点.此外,非晶态的聚苯胺包覆提高了复合电极的电导率.在引入适量MoO42?插层离子时,M-LDH@PANI表现出显著强化的储能和催化性能.所获得的M-LDH@PANI-0.5在析氧反应中表现出优越的电催化活性(10 mA cm?2时的过电位为266 mV),作为超级电容电池电极则具有864.8 C g?1的高容量.采用M-LDH@PANI-0.5作为正极及以活性炭作为负极组装的超级电容电池在功率密度为8,300.0 W kg?1时能量密度为44.6 Wh kg?1,且具有优异的循环稳定性(10000次循环后保留83.9％的初始容量).本文为LDH基材料的阴离子插层改性增强材料性能的机理提供了一个非传统的解释.在上述研究基础上,采用射线衍射(XRD)、X射线光电子能谱(XPS)、扫描电子显微镜(SEM)、高分辨透射电镜(HRTEM)和比表面积测试(BET)等手段对样品进行了深入表征.XRD结果表明,MoO42?插层的LDH材料的层间晶面(003)的峰随着MoO42?含量的增加而逐渐消失,这是由于晶面间距越大越容易受到晶粒细化的影响,间距大的晶格更容易受到破坏,导致晶格的展宽和弱化,从而间接证明MoO42?的成功插层.SEM、HRTEM和BET测试结果表明,MoO42?的含量对材料的形貌和比表面积具有重大影响.利用XPS对样品的价态进行了研究,发现随着MoO42?含量的增加,Co和Ni的价态没有明显变化.电化学测试结果表明,电极的储能和催化性能随MoO42?含量的增加而先增加后减小.利用理论计算分析了MoO42?在LDH中的插层行为,发现少量的MoO42?有利于扩大LDH的层间间距,而过量的MoO42?则会与LDH的H原子结合,从而与电解液中的OH?竞争,导致复合电极的电化学性能下降.此外,MoO42?插层的片状微球能有效调节材料的去质子化能,大大加速电极表面的氧化还原反应.因此,MoO42?插层能够显著强化LDH基材料的超级电容电池电极和OER催化剂电化学性能.     

Coordination complexes of iron(III) with 3-hydroxy-2(1H)-pyridinone, 2,3-dihydroxybenzoic acid and 3,4-dihydroxybenzoic acid: preparation and characterization in the solid state

Summary The synthesis and study of a number of new iron(III) complexes of the ligands 3-hydroxy-2(1H)-pyridinone (3,2-opoH), 2,3-dihydroxybenzoic acid (2,3-dhbH₃) and 3,4-dihydroxybenzoic acid (3,4-dhbH₃) are described. These complexes have the formulae [Fe(3,2-opo)₂Cl]·PrⁿOH, K[Fe(2,3-dhbH)₂(H₂O)₂], [Fe(2,3-dhb)(H₂O)₂], K[Fe-(3,4-dhbH)₂(H₂O)₂], [Fe(3,4-dhb)(H₂O)₂] and K₆[Fe(3,4-dhb)₃]·3H₂O. The complexes were characterized by elemental analyses. X-ray powder patterns, t.g.a./d.t.g. techniques, magnetic susceptibilities and spectroscopic (u.v.-vis., i.r. and variable-temperature ⁵⁷Fe-M?ssbauer) studies. Monomeric octahedral structures are assigned for the 1∶2 2,3-dhbH²⁻ complex and the 1:3 3,4-dhb³⁻ compound. Dinuclear and/or oligonuclear structures are tentatively proposed for the remaining complexes in the solid state. In [FeL(H₂O)₂] (L³⁻ = 2,3-dhb³⁻ or 3,4-dhb³⁻), iron(III) appears to be 5-coordinate. Both oxygens of 3,2-opo⁻ participate in coordination, while the dihydroxybenzoato ligands exhibit various coordination modes, depending mainly on the positions of the hydroxy groups, their anionic charge and the ligand∶metal molar ratio used.     

Synthesis and Characterization of Poly(ethylene terephthalate)/Modified Lithium‐Aluminum‐Layered Double Hydroxide Nanocomposites Prepared Using In‐situ Polymerization

Layered double hydroxides are a type of layered stacked compound, which can be intercalated with organic‐molecule modifiers. An ion‐exchange process for layered double hydroxide (LDH) was used to intercalate water‐soluble sulfanilic acid salt (SAS) and dimethyl 5‐sulfoisopthalate (DMSI) into lithium aluminum layered double hydroxides (LiAl LDHs). In this work, a hydrothermal process was used to modify LiAl LDHs, and the modified LiAl LDHs were treated with either SAS or DMSI through an ion‐exchange process and were then intercalated using bis‐hydroxyethylene terephthalate (BHET). The results indicate that the modified LiAl LDHs improved the interlayer compatibility between the PET and LiAl LDH layers; thus, enabling the oligomer molecules to more easily enter the gallery of the LiAl LDH layers so that polymer chains could be included between the LDH layers during polymerization of the matrix. The better barrier, mechanical properties, and thermal stability of these new types of PET nanocomposites are discussed.     

Facile and large-scale production of ZnO/Zn-Al layered double hydroxide hierarchical heterostructures

ZnO/Zn-Al layered double hydroxide (ZnO/Zn-Al LDH) hierarchical architecture, a new type of ZnO-based heterostructure, has been synthesized directly on an Al substrate via a facile solution phase process. The firecracker-like heterostructures consist of uniform ZnO nanorods orderly standing at the edges of two-dimensional (2D) surfaces of Zn-Al LDH nanoplatelets. Experimental result obtained from the early growth stage indicates that the underlying Zn-Al LDH nanoplatelet arrays are well constructed with their (00l) planes perpendicular to the surface of Al substrate. We propose that the "edge effect" of Zn-Al LDH and the "lattice match" between ZnO and Zn-Al LDH are vital to the growth of such heterostructures. The effects of total solution volume and NH3.H2O concentration on the formation of heterostructures are investigated. It is found that other LDH-based complex structures can also be achieved controllably by varying the mentioned experimental factors. Our work is the first demonstration of fabricating intricate ZnO/Zn-Al LDH heterostructures as well as well-defined Zn-Al LDH arrays on an Al substrate, for which several promising applications such as optoelectronics, biosensors, and catalysis can be envisioned.     

Molybdocalixarene structure control via rim deprotonation. synthesis, characterization, and crystal structures of calix[4]arene Mo(VI) monooxo complexes and calix[4]arene alkali metal/Mo(VI) dioxo complexes

We report a series of calix[4]arene Mo(VI) dioxo complexes M2RC4MoO2 (M = alkali metal, R = H or Bu(t)) that were fully characterized by NMR, X-ray, IR, UV/vis, and elemental analysis. Molybdocalix[4]arene structures can be controlled via lower rim deprotonation, groups at para positions of calix[4]arene, and alkali metal counterions. Mono deprotonation at the lower rim leads to calix[4]arene Mo(VI) monooxo complexes RC4MoO (R = H, Bu(t), or allyl), and full deprotonation gives rise to calix[4]arene Mo(VI) dioxo complexes. Structural studies indicate that HC4 Mo(VI) dioxo complexes easily form polymeric structures via cation-pi interaction and coordination between different calixarene units. However, Bu(t)C4 Mo(VI) dioxo complexes tend to form dimers or tetramers due to steric hindrance of the tert-butyl groups at para positions in calixarene. The structures of the reduced side products A and C were determined by X-ray diffraction studies. The mechanism of RC4MoO formation from the reaction of calixarene monoanions with MoO2Cl2 appears to include the addition of a calixarene -OH group across a Mo=O bond.     

Syntheses of hydrated molybdenum bronzes by reduction of MoO3 with NaBH4

Hydrated molybdenum bronzes have been prepared by reduction reaction of MoO3 with NaBH4 in ethanol and DMSO. The reduction reactions in both solvents occur smoothly; thus, the layered structure of MoO3 is maintained in the product. Divalent cation Ca2+ has been intercalated between the MoO3 layers, which leads to highly reduced molybdenum bronze (Mo5.26+). Solvated molybdenum bronze catalyzes the reduction reaction of DMSO by NaBH4, producing CH3SCH3. The structure model of hydrated sodium molybdenum bronze has also been reinvestigated by using the Rietveld analysis. The hydrated molybdenum bronze crystallizes in an orthorhombic structure, in which the structure of Mo octahedron layers is closely related to that in MoO3. However, the structure refinement reveals that the Mo octahedron in the MoO3 layers is axially distorted, which is different from that in MoO3 but similar to an isoelectron compound H0.33MoO3.     

The effect of substituents on the phenyl portion of the imido ligand on the structure and properties of molybdenum(VI) imido complexes

Anilines with alkyl substituents on the phenyl ring (ArNH2 = 2,4,6-trimethylaniline; 2,3-, 2,4-, 2,6-, and 3,4-dimethylaniline; and 2,6-diisopropylaniline) react with MoO(X)2(dtc)2 (X = Cl or Br; dtc = diethyldithiocarbamate) in methanol in the presence of 2 equiv of triethylamine to form ionic imido complexes of the type [MoNAr(dtc)3]2[Mo6O19] or MoNAr(dtc)3]4[Mo8O26]. The same reaction in THF with butyllithium as base yields imido complexes of the type MoNAr(X)2(dtc)2. The structures of three ionic, five chloro, and two bromo complexes have been determined by X-ray crystallography. In all complexes, the molybenum center is a distorted pentagonal bipyramid. While the structures are similar, the angles of the imido linkages differ. The effect of the substituents on the phenyl ring of the imido ligand on the 95Mo NMR chemical shifts was determined. The Mo nucleus becomes more deshielded with the substituents in the following order: 3,4-Me2 < 2,3-Me2 < 2,4-Me2 < 2,6-Me2 < 2,4,6-Me3 < 2,6 isopropyl. Complexes with more deshielded 95Mo centers tend to have angles of the imido linkage that are closer to 180 degrees.     

Synthesis and Crystal Structure of [H-amp]4[Mo8O26（DMF）2].2H2O（amp = 2-Amino-6-methyl-pyridine）

1INTRODUCTIONMetal-oxoclusterchemistryhasbeenactivelypurchasedduetotheinterestinchemistryitselfanditsvariousapplicationsinfieldssuchascatalysis,electronicconductivity,magnetism,nonlinearopti-calpropertiesandmedicine[1～3].Recently,anim-portantadvanceinmet…     

Catalytic gas phase oxidation of methanol to formaldehyde

Two gas-phase catalytic cycles for the two-electron oxidation of primary and secondary alcohols were detected by multistage mass spectrometry experiments. A binuclear dimolybdate center [Mo(2)O(6)(OCHR(2))](-) acts as the catalyst in both these cycles. The first cycle proceeds via three steps: (1) reaction of [Mo(2)O(6)(OH)](-) with alcohol R(2)HCOH and elimination of water to form [Mo(2)O(6)(OCHR(2))](-); (2) oxidation of the alkoxo ligand and its elimination as aldehyde or ketone in the rate-determining step; and (3) regeneration of the catalyst via oxidation by nitromethane. Step 2 does not occur at room temperature and requires the use of collisional activation to proceed. The second cycle is similar but differs in the order of reaction with alcohol and nitromethane. The nature of each of these reactions was probed by kinetic measurements and by variation of the substrate alcohols (structure and isotope labeling). The role of the binuclear molybdenum center was assessed by examination of the relative reactivities of the mononuclear [MO(3)(OH)](-) and binuclear [M(2)O(6)(OH)](-) ions (M = Cr, Mo, W). The molybdenum and tungsten binuclear centers [M(2)O(6)(OH)](-) (M = Mo, W) were reactive toward alcohol but the chromium center [Cr(2)O(6)(OH)](-) was not. This is consistent with the expected order of basicity of the hydroxo ligand in these species. The chromium and molybdenum centers [M(2)O(6)(OCHR(2))](-) (M = Cr, Mo) oxidized the alkoxo ligand to aldehyde, while the tungsten center [W(2)O(6)(OCHR(2))](-) did not, instead preferring the non-redox elimination of alkene. This is consistent with the expected order of oxidizing power of the anions. Each of the mononuclear anions [MO(3)(OH)](-) (M = Cr, Mo, W) was inert to reaction with methanol, highlighting the importance of the second MoO(3) unit in these catalytic cycles. Only the dimolybdate center has the mix of properties that allow it to participate in each of the three steps of the two catalytic cycles. The three reactions of these cycles are equivalent to the three essential steps proposed to occur in the industrial oxidation of gaseous methanol to formaldehyde at 300-400 degrees C over solid-state catalysts based upon molybdenum(VI)-trioxide. The new gas-phase catalytic data is compared with those for the heterogeneous process.     

Molybdenum oxo and imido complexes of beta-diketiminate ligands: synthesis and structural aspects

Treatment of [MoO2(eta2-Pz)2] (Pz = 3,5-di-tert-butylpyrazolate) with the diketiminate ligand NacNacH (NacNac = CH[C(Me)NAr]2-, Ar = 2,6-Me2C6H3) at 55 degrees C leads under reduction of the metal to the formation of the dimeric molybdenum(V) compound [{MoO2(NacNac)}2] (1). The compound was characterized by spectroscopic means and by X-ray crystal structure analysis. The dimer consists of a [Mo2O4]2+ core with a short Mo-Mo bond (2.5591(5) A) and one coordinated diketiminate ligand on each metal atom. The reaction of [MoO2(eta2-Pz)2] with NacNacH in benzene at room temperature leads to a mixture of 1 and the monomeric molybdenum(VI) compound [MoO2(NacNac)(eta2-Pz)] (2). From such solutions, yellow crystals of 2 suitable for X-ray structural analysis were obtained revealing the coordination of one bidentate NacNac and one eta2-coordinate Pz ligand. This renders the two oxo groups inequivalent. Further high oxidation state molybdenum compounds containing the NacNac ligand were obtained by the reaction of [Mo(NAr)2Cl2(dme)] (Ar = 2,6-Me2C6H3) and [Mo(N-t-Bu)2Cl2(dme)] (dme = dimethoxyethane) with 1 equiv of the potassium salt NacNacK forming [Mo(NAr)2Cl(NacNac)] (3) and [Mo(N-t-Bu)2Cl(NacNac)] (4), respectively, in good yields. The X-ray structure analysis of 3 revealed a penta-coordinate compound where the geometry is best described as trigonal-bipyramidal.     

Dioxomolybdenum(VI) and dioxotungsten(VI) complexes supported by an amido ligand

Stepwise addition of one equivalent of n-butyllithium and trimethylsilyl chloride to 2-tert-butylmercaptoaniline affords the new ligand 1-(Me3SiNH)-2-(t-BuS)C6H4 (LH), that reacts with one equivalent of butyllithium to its lithium salt LLi. Dioxodichloromolybdenum [MoO2Cl2] and dioxodichlorotungsten dimethoxyethane [WO2Cl2(dme)] react in tetrahydrofuran solution at low temperature with two equivalents LLi to monomeric dioxomolybdenum(VI) [MoO2L2] (1) and dioxotungsten(VI) complex [WO2L2] (2) employing two bidentate amido thioether ligands. The crystallographic determination of the molecular structures of 1 and 2 show evidence for M...S contacts. The reaction of [MoO2Cl2] with LLi in tetrahydrofuran solution at room temperature leads next to 1 to two compounds where silyl group migration from nitrogen to oxygen atoms occurs forming [Mo(=NL')2(OSiMe)2] (3) and [Mo(=NL')2(OSiMe3)L] (4, L' = N-2-t-BuSC6H4) as determined by NMR spectroscopy. Compound 4 was isolated in low yield and its molecular structure determined by X-ray crystallography. Higher yields of a bisimido complex can be obtained by the direct reaction of one equivalent of LLi with [Mo(NAr)2Cl2(dme)] (Ar = 2,6-Me2C6H4) forming [Mo(NAr)2LCl] (5).     

Molybdenum(VI) dioxo and oxo-imido complexes of fluorinated β-ketiminato ligands and their use in OAT reactions

Substitution of a methyl by a trifluoromethyl moiety in well-known β-ketimines afforded the ligands (Ar)NC(Me)CH(2)CO(CF(3)) (HL(H), Ar = C(6)H(5); HL(Me), A r= 2,6-Me(2)C(6)H(3); HL(iPr), Ar = 2,6-(i)Pr(2)C(6)H(3)). Subsequent complexation to the [MoO(2)](2+) core leads to the formation of novel complexes of general formula [MoO(2)(L(R))(2)] (R = H, 1; R = Me, 2; R = iPr, 3). For reasons of comparison the oxo-imido complex [MoO(N(t)Bu)(L(Me))(2)] (4) has also been synthesized. Complexes 1-4 were investigated in oxygen atom transfer (OAT) reactions using the substrate trimethylphosphine. The respective products after OAT, the reduced Mo(IV) complexes [MoO(PMe(3))(L(R))(2)] (R = H, 5; R = Me, 6; R = iPr, 7) and [Mo(N(t)Bu)(PMe(3))(L(Me))(2)] (8), were isolated. All complexes have been characterized by NMR spectroscopy, and 1-4 also by cyclic voltammetry. A positive shift of the Mo(VI)-Mo(V) reduction wave upon fluorination was observed. Furthermore, molecular structures of complexes 2, 4, 5, and 8 have been determined via single crystal X-ray diffraction analysis. Complex 8 represents a rare example of a Mo(IV) phosphino-imido complex. Kinetic measurements by UV-vis spectroscopy of the OAT reactions from complexes 1-4 to PMe(3) showed them to be more efficient than previously reported nonfluorinated ones, with ligand L' = (Ar)NC(Me)CH(2)CO(CH(3)) [MoO(2)(L')(2)] (9) and [MoO(N(t)Bu)(L')(2)] (10), respectively. Thermodynamic activation parameters ΔH(?) and ΔS(?) of the OAT reactions for complexes 2 and 4 have been determined. The activation enthalpy for the reaction employing 2 is significantly smaller (12.3 kJ/mol) compared to the reaction with the nonfluorinated complex 9 (60.8 kJ/mol). The change of the entropic term ΔS(?) is small. The reaction of the oxo-imido complex 4 to 8 revealed a significant electron-donating contribution of the imido substituent.