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.     

Enantiospecific C?H Activation Using Ruthenium Nanocatalysts

The activation of C H bonds has revolutionized modern synthetic chemistry. However, no general strategy for enantiospecific C H activation has been developed to date. We herein report an enantiospecific C H activation reaction followed by deuterium incorporation at stereogenic centers. Mechanistic studies suggest that the selectivity for the α‐position of the directing heteroatom results from a four‐membered dimetallacycle as the key intermediate. This work paves the way to novel molecular chemistry on nanoparticles.     

Small Changes Have Consequences: Lessons from Tetrabenzyltitanium and ‐zirconium Surface Organometallic Chemistry

Homoleptic benzyl derivatives of titanium and zirconium have been grafted onto silica that was dehydroxylated at 200 and 700 °C, thereby affording bi‐grafted and mono‐grafted single‐site species, respectively, as shown by a combination of experimental techniques (IR, MAS NMR, EXAFS, and elemental analysis) and theoretical calculations. Marked differences between these compounds and their neopentyl analogues are discussed and rationalized by using DFT. These differences were assigned to the selectivity of the grafting process, which, depending on the structure of the molecular precursors, led to different outcomes in terms of the mono‐ versus bi‐grafted species for the same surface concentration of silanol species. The benzylzirconium derivatives were active towards ethylene polymerization in the absence of an activator and the bi‐grafted species displayed higher activity than their mono‐grafted analogues. In contrast, the benzyltitanium and neopentylzirconium counterparts were not active under similar reaction conditions.     

Al2O3基Ni-Cu双金属催化剂上乙酰丙酸氢解（英）

Inexpensive γ-alumina-based nickel-copper bimetallic catalysts were studied for the hydrogenolysis of levulinic acid,a key platform molecule for biomass conversion to biofuels and other valued chemicals,into γ-valerolactone as a first step towards the production of 2-methyltetrahydrofurane.The activities of both monometallic and bimetallic catalysts were tested.Their textural and chemical characteristics were determined by nitrogen physisorption,elemental analysis,temperature-programmed ammonia desorption,and temperature-programmed reduction.The monometallic nickel catalyst showed high activity but the highest bγ-product production and significant amounts of carbon deposited on the catalyst surface.The copper monometallic catalyst showed the lowest activity but the lowest carbon deposition.The incorporation of the two metals generated a bimetallic catalyst that displayed a similar activity to that of the Ni monometallic catalyst and significantly low bγ-product and carbon contents,indicating the occurrence of important synergetic effects.The influence of the preparation method was also examined by studying impregnated- and sol-gel-derived bimetallic catalysts.A strong dependency on the preparation procedure and calcination temperature was observed.The highest activity per metal atom was achieved using the sol-gel-derived catalyst that was calcined at 450 ℃.High reaction rates were achieved;the total levulinic acid conversion was obtained in less than 2 h of reaction time,yielding up to 96%γ-valerolactone,at operating temperature and pressure of 250 ℃ and 6.5 MPa hydrogen,respectively.     

An Experimental and Computational Investigation Rules Out Direct Nucleophilic Addition on the N2 Ligand in Manganese Dinitrogen Complex [Cp(CO)2Mn(N2)]

We have re-examined the reactivity of the manganese dinitrogen complex [Cp(CO)₂Mn(N₂)] ( 1 , Cp=η⁵-cyclopentadienyl, C₅H₅) with phenylithium (PhLi). By combining experiment and density functional theory (DFT), we have found that, unlike previously reported, the direct nucleophilic attack of the carbanion onto coordinated dinitrogen does not occur. Instead, PhLi reacts with one of the CO ligands to provide an anionic acylcarbonyl dinitrogen metallate [Cp(CO)(N₂)MnCOPh]Li ( 3 ) that is stable only below −40 °C. Full characterization of 3 (including single crystal X-ray diffraction) was performed. This complex decomposes quickly above −20 °C with N₂ loss to give a phenylate complex [Cp(CO)₂MnPh]Li ( 2 ). The latter compound was erroneously formulated as an anionic diazenido compound [Cp(CO)₂MnN(Ph)=N]Li in earlier reports, ruling out the claimed and so-far unique behavior of the N₂ ligand in 1 . DFT calculations were run to explore both the hypothesized and the experimentally verified reactivity of 1 with PhLi and are fully consistent with our results. Direct attack of a nucleophile on metal-coordinated N₂ remains to be demonstrated.     

Magnesium(I) Reduction of Aluminum(III) Hydride Complexes: Generation of Mixed Valence Aluminum (AlI/Al0) Hydride Cluster Compounds, [Al6H8(NR3)2{Mg(β-diketiminate)}4]**

Reduction of a range of amido- and aryloxy-aluminum dihydride complexes, e.g. [AlH₂(NR₃){N(SiMe₃)₂}] (NR₃=NMe₃ or N-methylpiperidine (NMP)), with β-diketiminato dimagnesium(I) reagents, [{(^ArNacnac)Mg}₂] (^ArNacnac=[HC(MeCNAr)₂]⁻, Ar=mesityl (Mes) or 2,6-xylyl (Xyl)), have afforded deep red mixed valence aluminum hydride cluster compounds, [Al₆H₈(NR₃)₂{Mg(^ArNacnac)}₄], which have an average Al oxidation state of +0.66, the lowest for any well-defined aluminum hydride compound. In the solid-state, the clusters are shown to have distorted octahedral Al₆ cores, having zero-valent Al axial sites and mono-valent AlH₂⁻ equatorial units. Several novel by-products were isolated from the reactions that gave the clusters, including the Mg−Al bonded magnesio-aluminate complexes, [(^ArNacnac)(Me₃N)Mg−Al(μ-H)₃[{Mg(^ArNacnac)}₂(μ-H)]]. Computational analyses of one aluminum hydride cluster revealed its Al₆ core to be electronically delocalized, and to possess one unoccupied, and six occupied, skeletal molecular orbitals.     

(Mini)emulsion Polymerization: Effect of the Segregation Degree on Polymer Architecture

A continuous loop reactor was used for the production of 2‐ethylhexyl acrylate (2‐EHA), methyl methacrylate (MMA) and acrylic acid (AA) pressure sensitive adhesive by both emulsion and miniemulsion polymerization. Similar high monomer conversions were achieved in both processes, but striking differences in polymer architecture were found. A mathematical model was used to analyze these differences concluding that because the costabilizer suppressed monomer diffusion from miniemulsion droplets, the average polymer concentration in the polymerization loci was lower in the miniemulsion process. This resulted in less chain transfer to polymer, and hence in lower sol molecular weight and gel content.

     

Highly Stable trans‐Cyclooctene Amino Acids for Live‐Cell Labeling

trans‐Cyclooctene groups incorporated into proteins via non‐canonical amino acids (ncAAs) are emerging as specific handles for bioorthogonal chemistry. Here, we present a highly improved synthetic access to the axially and the equatorially linked trans‐cyclooct‐2‐ene isomers ( 1 a , b ). We further show that the axially connected isomer has a half‐life about 10 times higher than the equatorial isomer and reacts with tetrazines much faster, as determined by stopped‐flow experiments. The improved properties resulted in different labeling performance of the insulin receptor on the surface of intact cells.     

DFT study of the ring opening polymerization of ε-caprolactone by grafted lanthanide complexes: 2--Effect of the initiator ligand

The influence of the initiator ligand on the Ring Opening Polymerization (ROP) of ε-caprolactone by lanthanide complexes grafted on silica have been investigated by means of density functional theory (DFT) calculations. Three different initiator ligands (alkyl, dialkylamido and borohydride) and three grating modes (mono-grafted, bi-grafted or bi-grafted after breaking of a Si-O-Si bridge) have been considered. This study highlights that lanthanum grafted complexes (alkyl, amide or borohydride) are active in lactone polymerization. In any case the reaction process is demonstrated to be similar to the one found for homogeneous catalysts. However, even if the different grafting modes are energetically equivalent for the ε-caprolactone ROP initiation reaction, some differences are observed according to the ligand involved in the initiation reaction. In agreement with experimental data, grafted lanthanide amides rapidly polymerise the ε-caprolactone. The grafted alkyl lanthanum complexes are also predicted to be very efficient catalysts. The borohydride is thus predicted to be the least efficient due to the difficulties in the ring opening. Indeed, the rate-determining step is the nucleophilic attack for the methyl and dialkylamido ligands (occuring with a low barrier) whereas it is the ring opening for the borohydride ligands (highest barrier) and the formation of -CH(2)C(=O)(X) (X = CH(3) or NMe(2)) terminal group is more favorable than that of a -CH(2)OBH(2) end group.