Hsueh-Ju Liu

Silica-Supported Cu Nanoparticle Catalysts for Alkyne Semihydrogenation: Effect of Ligands on Rates and Selectivity

Narrowly dispersed, silica-supported Cu nanoparticles (ca. 2 nm) prepared via surface organometallic chemistry from a mesityl complex [Cu5Mes5] are highly active for the hydrogenation of a broad range of alkynes. High-throughput experimentation allows for identifying the optimal ligand and reaction conditions to turn these supported Cu nanoparticles into highly chemo- and stereoselective catalysts for the preparation of Z-olefins (overall, 23 examples). For instance, PCy3-modified Cu nanoparticles semihydrogenate 1-phenyl-1-propyne to cis-β-methylstyrene (20 bar H2, 40 °C) with turnover number and turnover frequency of ca. 540 and 1.9 min-1, respectively, and with 94% selectivity at full conversion.

Cyclometalated N-Heterocyclic Carbene Complexes of Ruthenium for Access to Electron-Rich Silylene Complexes That Bind the Lewis Acids CuOTf and AgOTf

The synthesis of the cyclometalated complexes Cp*Ru(IXy-H) (2) [IXy = 1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene; IXy-H = 1-(2-CH2C6H3-6-methyl)-3-(2,6-dimethylphenyl)imidazol-2-ylidene-1-yl (the deprotonated form of IXy); Cp* = η(5)-C5Me5] and Cp*Ru(IXy-H)(N2) (3) was achieved by dehydrochlorination of Cp*Ru(IXy)Cl (1) with KCH2Ph. Complexes 2 and 3 activate primary silanes (RSiH3) to afford the silyl complexes Cp*(IXy-H)(H)RuSiH2R [R = p-Tol (4), Mes (5), Trip (6)]. Density functional theory studies indicated that these complexes are close in energy to the corresponding isomeric silylene species Cp*(IXy)(H)Ru═SiHR. Indeed, reactivity studies indicated that various reagents trap the silylene isomer of 6, Cp*(IXy)(H)Ru═SiHTrip (6a). Thus, benzaldehyde reacts with 6 to give the [2 + 2] cycloaddition product 7, while 4-bromoacetophenone reacts via C-H bond cleavage and formation of the enolate Cp*(IXy)(H)2RuSiH[OC(═CH2)C6H4Br]Trip (8). Addition of the O-H bond of 2,6-dimethylphenol across the Ru═Si bond of 6a gives Cp*(IXy)(H)2RuSiH(2,6-Me2C6H3O)Trip (9). Interestingly, CuOTf and AgOTf also react with 6 to provide unusual Lewis acid-stabilized silylene complexes in which MOTf bridges the Ru-Si bond. The AgOTf complex, which was crystallographically characterized, exhibits a structure similar to that of [Cp*((i)Pr3P)Ru(μ-H)2SiHMes](+), with a three-center, two-electron Ru-Ag-Si interaction. Natural bond orbital analysis of the MOTf complexes supported this type of bonding and characterized the donor interaction with Ag (or Cu) as involving a delocalized interaction with contributions from the carbene, silylene, and hydride ligands of Ru.

1,2-hydrogen migration to a saturated ruthenium complex via reversal of electronic properties for tin in a stannylene-to-metallostannylene conversion.

An intramolecular 1,2(α)-H migration in a saturated ruthenium stannylene complex, to form a ruthenostannylene complex, involves a reversal of the role for a coordinated stannylene ligand, from that of an electron donor to an acceptor in the transition state. This change in the bonding properties for a stannylene group, with a simple molecular motion, lifts the usual requirement for generation of an unsaturated metal center in migration chemistry.

Multishelled CaO Microspheres Stabilized by Atomic Layer Deposition of Al2O3 for Enhanced CO2 Capture Performance

CO2 capture and storage is a promising concept to reduce anthropogenic CO2 emissions. The most established technology for capturing CO2 relies on amine scrubbing that is, however, associated with high costs. Technoeconomic studies show that using CaO as a high-temperature CO2 sorbent can significantly reduce the costs of CO2 capture. A serious disadvantage of CaO derived from earth-abundant precursors, e.g., limestone, is the rapid, sintering-induced decay of its cyclic CO2 uptake. Here, a template-assisted hydrothermal approach to develop CaO-based sorbents exhibiting a very high and cyclically stable CO2 uptake is exploited. The morphological characteristics of these sorbents, i.e., a porous shell comprised of CaO nanoparticles coated by a thin layer of Al2 O3 (<3 nm) containing a central void, ensure (i) minimal diffusion limitations, (ii) space to accompany the substantial volumetric changes during CO2 capture and release, and (iii) a minimal quantity of Al2 O3 for structural stabilization, thus maximizing the fraction of CO2 -capture-active CaO.

The Ruthenostannylene Complex [Cp*(IXy)H2Ru-Sn-Trip]: Providing Access to Unusual Ru-Sn Bonded Stanna-imine, Stannene, and Ketenylstannyl Complexes†

Reactivity studies of the thermally stable ruthenostannylene complex [Cp*(IXy)(H)2 Ru-Sn-Trip] (1; IXy=1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene; Cp*=η(5) -C5 Me5 ; Trip=2,4,6-iPr3 C6 H2 ) with a variety of organic substrates are described. Complex 1 reacts with benzoin and an α,β-unsaturated ketone to undergo [1+4] cycloaddition reactions and afford [Cp*(IXy)(H)2 RuSn(κ(2) -O,O-OCPhCPhO)Trip] (2) and [Cp*(IXy)(H)2 RuSn(κ(2) -O,C-OCPhCHCHPh)Trip] (3), respectively. The reaction of 1 with ethyl diazoacetate resulted in a tin-substituted ketene complex [Cp*(IXy)(H)2 RuSn(OC2 H5 )(CHCO)Trip] (4), which is most likely a decomposition product from the putative ruthenium-substituted stannene complex. The isolation of a ruthenium-substituted stannene [Cp*(IXy)(H)2 RuSn(=Flu)Trip] (5) and stanna-imine [Cp*(IXy)(H)2 RuSn(κ(2) -N,O-NSO2 C6 H4 Me)Trip] (6) complexes was achieved by treatment of 1 with 9-diazofluorene and tosyl azide, respectively.