Reversing Conventional Reactivity of Mixed Oxo/Alkyl Rare‐Earth Complexes: Non‐Redox Oxygen Atom Transfer

The preferential substitution of oxo ligands over alkyl ones of rare‐earth complexes is commonly considered as “impossible” due to the high oxophilicity of metal centers. Now, it has been shown that simply assembling mixed methyl/oxo rare‐earth complexes to a rigid trinuclear cluster framework cannot only enhance the activity of the Ln‐oxo bond, but also protect the highly reactive Ln‐alkyl bond, thus providing a previously unrecognized opportunity to selectively manipulate the oxo ligand in the presence of numerous reactive functionalities. Such trimetallic cluster has proved to be a suitable platform for developing the unprecedented non‐redox rare‐earth‐mediated oxygen atom transfer from ketones to CS₂ and PhNCS. Controlled experiments and computational studies shed light on the driving force for these reactions, emphasizing the importance of the sterical accessibility and multimetallic effect of the cluster framework in promoting reversal of reactivity of rare‐earth oxo complexes.     

Cerium masquerading as a group 4 element: synthesis,structure and computational characterisation of [CeCl(N(SiMe3)2)3

Oxidation of the three-coordinate cerium amide [Ce(N-(SiMe3)2)3] with TeCl4 in toluene solution yields purple, diamagnetic [CeCl(N(SiMe3)2)3], whose structure has been examined by X-ray crystallographic and computational methods.     

Hydrogen for fluorine exchange in CH4-xFx by monomeric [1,2,4-(Me3C)3C5H2]2CeH: experimental and computational studies

The monomeric metallocenecerium hydride, Cp'(2)CeH (Cp' = 1,2,4-tri-tert-butylcyclopentadienyl), reacts instantaneously with CH(3)F, but slower with CH(2)F(2), to give Cp'(2)CeF and CH(4) in each case, a net H for F exchange reaction. The hydride reacts very slowly with CHF(3), and not at all with CF(4), to give Cp'(2)CeF, H(2), and 1,2,4- and 1,3,5-tri-tert-butylbenzene. The substituted benzenes are postulated to result from trapping of a fluorocarbene fragment derived by alpha-fluoride abstraction from Cp'(2)CeCF(3). The fluoroalkyl, Cp'(2)CeCF(3), is generated by reaction of Cp'(2)CeH and Me(3)SiCF(3) or by reaction of the metallacycle, [(Cp')(Me(3)C)(2)C(5)H(2)C(Me(2))CH(2)]Ce, with CHF(3), and its existence is inferred from the products of decomposition, which are Cp'(2)CeF, the isomeric tri-tert-butylbenzenes and in the case of Me(3)SiCF(3), Me(3)SiH. The fluoroalkyls, Cp'(2)CeCH(2)F and Cp'(2)CeCHF(2), generated from the metallacycle and CH(3)F and CH(2)F(2), respectively, are also inferred by their decomposition products, which are Cp'(2)CeF, CH(2), and CHF, respectively, which are trapped. DFT(B3PW91) calculations have been carried out to examine several reaction paths that involve CH and CF bond activation. The calculations show that the CH activation by Cp(2)CeH proceeds with a low barrier. The carbene ejection and trapping by H(2) is the rate-determining step, and the barrier parallels that found for reaction of H(2) with CH(2), CHF, and CF(2). The barrier of the rate-determining step is raised as the number of fluorines increases, while that of the CH activation path is lowered as the number of fluorines increases, which parallels the acidity.     

6-Hydroxyaurone aminomethyl derivatives in the inverse electron-demand Diels–Alder reaction

Chemistry of Heterocyclic Compounds - The transformations of 7-dimethylaminomethyl-6-hydroxyaurone and 5-dimethylaminomethyl-6-hydroxy-7-methylurone in the inversed electron-demand...     

Grafting of Lanthanum Complexes on a Functionalized Graphene Surface: Theoretical Investigation on Ethylene and 1,3-Butadiene Homo- and Co-Polymerization

In this contribution, we describe the use of graphene as an efficient catalyst support and the role it plays in increasing the Lewis acidity of the supported metal complexes. By a density functional theory study, we show that the [La(N(SiMe₃)₂)₃] complex can be easily grafted on graphene-OH and -COOH functionalized surfaces. Two stable mono-grafted compounds, (gO)-[La(N(SiMe₃)₂)₂] and (gOO)-[La(N(SiMe₃)₂)₂], are formed, behaving as stronger Lewis acids than the previously reported silica grafted analogues. To study the role of the graphene support in catalysis, we also computed the catalytic activity of the alkylated (gO)-[La(CH₃)₂] and (gOO)-[La(CH₃)₂] complexes in the ethylene and 1,3-butadiene homo- and co-polymerization reactions. Both compounds are efficient catalysts for the homo-polymerization of the ethylene and 1,3-butadiene. For the 1,3-butadiene homo-polymerization, the stereoselectivity outcome of the reaction differs according to the grafting site. The results computed for the co-polymerization reaction, finally, show that the high stability of the allylic products leads to the formation of block copolymers.     

Probing the stereo‐electronic properties of cationic rhodium complexes bearing chiral diphosphine ligands by 103Rh NMR

¹⁰³Rh NMR represents a powerful tool to assess the global electronic and steric contribution of diphosphine ligands on [Rh(COD)(diphosphine)]⁺ complexes. In the case of DIOP, BINAP and MeDUPHOS, this approach proved to be more informative than classical CO‐stretching frequency measurements. After validation, this method has been extended to a set of seven diphosphines. ¹⁰³Rh NMR measurements on [Rh(COD)(diphosphine)]PF₆ lead to the following order of donor properties: dppe > MeBPE > MeDUPHOS > dppb > DIOP > BINAP > Tol‐BINAP. This trend has been validated by DFT in the case of DIOP, BINAP and MeDUPHOS. In conjunction, ³¹P NMR chemical shift has been shown to reflect the ring constraints of the Rh‐diphosphine scaffold. This contribution is a step towards a mechanistic investigation of the catalytic hydrogenation of unsaturated substrates by ¹⁰³Rh NMR and DFT. Copyright © 2010 John Wiley & Sons, Ltd.     

Mixing of mutually soluble liquids in turbulent flow in a pipe

The problem concerned with mixing of mutually soluble liquids in turbulent flow in a pipe [1–11] is considered. To describe the distribution of concentration in the region of mixture, taken average across the section of the pipe, we use a model based on a one-dimensional model of the type of heat-conduction equation with an effective coefficient which, as tests show, is different from the coefficients of molecular and turbulent transfer. The dimensionless value of this coefficient depends on a number of parameters, such as the Reynolds number calculated for one of the liquids, roughness, ratio of the densities and viscosities of the liquids, as well as on the concentration, gradients of concentration, etc. These relationships can be established either by means of tests or on the basis of theoretical consideration of the mixing phenomenon. In this paper we theoretically derive a dispersion model with an effective diffusion coefficient which depends on Reynolds and Schmidt numbers, as well as on roughness.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 96–102, September–October, 1971.     

Early Main Group Metal Catalysis: How Important is the Metal?

Organocalcium compounds have been reported as efficient catalysts for various alkene transformations. In contrast to transition metal catalysis, the alkenes are not activated by metal–alkene orbital interactions. Instead it is proposed that alkene activation proceeds through an electrostatic interaction with a Lewis acidic Ca²⁺. The role of the metal was evaluated by a study using the metal‐free catalysts: [Ph₂N⁻][Me₄N⁺] and [Ph₃C⁻][Me₄N⁺]. These “naked” amides and carbanions can act as catalysts in the conversion of activated double bonds (CO and CN) in the hydroamination of Ar NCO and R NCN R (R=alkyl) by Ph₂NH. For the intramolecular hydroamination of unactivated CC bonds in H₂CCHCH₂CPh₂CH₂NH₂ the presence of a metal cation is crucial. A new type of hybrid catalyst consisting of a strong organic Schwesinger base and a simple metal salt can act as catalyst for the intramolecular alkene hydroamination. The influence of the cation in catalysis is further evaluated by a DFT study.