Well-defined silica-supported olefin metathesis catalysts

Two triethoxysilyl-functionalized N-heterocyclic carbene ligands have been synthesized and used to prepare the corresponding second-generation ruthenium olefin metathesis catalysts. These complexes were then grafted onto silica gel, and the resulting materials were efficient heterogeneous catalysts for a number of metathesis reactions. The solid-supported catalysts were shown to be recyclable over a number of reaction cycles, and no detectable levels of ruthenium were observed in reaction filtrates (ruthenium concentration of filtrate <5 ppb)

A Western Range Extension for Caprella scaura (Amphipoda: Caprellidae) in the Aransas Bay Ecosystem, Texas

During March 2009, the skeleton shrimp Caprella scaura and Paracaprella tenuis (Amphipoda: Caprellidae) were collected from several locations throughout the Aransas Bay, Texas ecosystem from Texas Parks and Wildlife fishery—independent trawl and oyster dredge samples. This is a western range expansion for C. scaura; P. tenuis has been reported from this area before. Both species were exclusively associated with a bryozoan, Bugula neritina. Densities of both species ranged between 0.1–3.4 individuals per gram of attached bryozoans. A reproductive population is likely established since several sizes, including adult males and gravid females, were observed. No caprellids were observed after early April, which coincided with a reduction in bryozoan occurrence in our routine monthly samples. These collections represent the first documented occurrence of C. scaura west of Florida

Synthesis of N-Heterocylic Carbene-Containing Metal Complexes from 2-(Pentafluorophenyl)imidazolidines

N-Heterocyclic carbene (NHC)-containing metal complexes are prepared through a simple, base-free method involving the decomposition of 2-(pentafluorophenyl)imidazolidines under mild thermolytic conditions. Ruthenium, iridium, and rhodium complexes containing NHC ligands with different electronic and steric parameters are reported

Preparation of heteronuclear bridging methylene complexes

The reactions of the titanacyclobutane complexes (η^5-C_5H_5)_2TiCH_2CRR'CH_2 (la, R = R' = Me; lb, R = Me, R' = Pr) with M-X compounds (M =Ti, Zr, Rh, Ir, Pd, Pt; X = Cl, OMe) have been investigated. The titanacyclobutanes have been found to react as sources of (η^5-C_5H_5)_2TiCH_2, giving (η^5-C_5H_5)_2Ti(μ –CH_2)(μ-X)ML_n (L_n = ancillary ligands) products. Several such complexes have been isolated as crystalline solids; however, only (η^5-C_5H_5_2Ti(μ-CH_2)(μ-X)Rh(η^4-l,5-cyclooctadiene) is stable in solution at 25°C. The complex η^5-C_5H_5)_2iTi(μ-CH_2)(μ-Cl)Ti(η^5-C_5Me_5)Cl was found to react with CO to form the bridging ketene complex (η^5-C_5H_5)_2Ti-μ, η^2(C,O)-(OCCH_2)Ti(η^5-C_5Me_5)Cl_2

Structure of a trinuclear dizirconium-aluminum (Zr_2Al) µ-ketone complex with a bridging trigonal-bipyramidal methyl group

The reaction of a zirconocene acyl chloride with 0.5 equiv of trimethylaluminum results in the formation of the trinuclear Zr_2Al complex [Cp_2Zr(η^2(C, O)-OC-(CH_3)_2]_2(µ-AIMe_2Χ-µMe), which contains a trigonal-bipyramidal bridging methyl group. The complex crystallizes in the monoclinic system, in space group P2_1ln (C^5_(2h) No. 14), with a = 10.574 (1) Å, b = 16.456 (2) Å, c =16.763 (2) Å, β = 103.71 (1)°, V = 2833.6 (9) Å^3, and Z = 4. This is one of the few structures that contains a bridging methyl group with a near-trigonal-bipyramidal structure in which the metals occupy the axial positions and the three hydrogen atoms are in the equatorial plane

Prevention of Undesirable Isomerization during Olefin Metathesis

1,4-Benzoquinones have been found to prevent olefin isomerization of a number of allylic ethers and long-chain aliphatic alkenes during ruthenium-catalyzed olefin metathesis reactions. Electron-deficient benzoquinones are the most effective additives for the prevention of olefin migration. This mild, inexpensive, and effective method to block olefin isomerization increases the synthetic utility of olefin metathesis via improvement of overall product yield and purity

Second-order nonlinearities and crystal structure of 2-methoxy-4'-nitro-(E)-stilbene

Second-order nonlinear optical (NLO) properties of crystalline materials depend both on the magnitude of the molecular hyperpolarizability (β) and on the orientation of the chromophores in the crystal lattice. If the molecular hyperpolarizability is to result in a nonzero macroscopic nonlinearity, the molecule must crystallize in a noncentrosymmetric space group. The design of new second-order NLO materials requires one both to address the structure-property relationships that govern β and to develop methodologies that favor noncentrosymmetric crystallization. Methods used to encourage noncentrosymmetric crystallization include the use of chiral molecules, the incorporation of functional groups that encourage asymmetric intermolecular hydrogen bonding, the synthesis of molecules with very small ground-state dipole moments but larger excited-state dipole moments, and the variation of counterion with ionic chromophore

ClaimChain: Improving the Security and Privacy of In-band Key Distribution for Messaging

The social demand for email end-to-end encryption is barely supported by mainstream service providers. Autocrypt is a new community-driven open specification for e-mail encryption that attempts to respond to this demand. In Autocrypt the encryption keys are attached directly to messages, and thus the encryption can be implemented by email clients without any collaboration of the providers. The decentralized nature of this in-band key distribution, however, makes it prone to man-in-the-middle attacks and can leak the social graph of users. To address this problem we introduce ClaimChain, a cryptographic construction for privacy-preserving authentication of public keys. Users store claims about their identities and keys, as well as their beliefs about others, in ClaimChains. These chains form authenticated decentralized repositories that enable users to prove the authenticity of both their keys and the keys of their contacts. ClaimChains are encrypted, and therefore protect the stored information, such as keys and contact identities, from prying eyes. At the same time, ClaimChain implements mechanisms to provide strong non-equivocation properties, discouraging malicious actors from distributing conflicting or inauthentic claims. We implemented ClaimChain and we show that it offers reasonable performance, low overhead, and authenticity guarantees.Comment: Appears in 2018 Workshop on Privacy in the Electronic Society (WPES'18

Can softer junctions lead to stiffer gels? Understanding the role of stereochemistry in associative polymer gels

The ability to create synthetic materials that mimic the structural and mechanical properties of soft biological tissues remains a significant challenge. In this presentation, we focus on creating stiff hydrogels and novel nanoscale and microscale structure by engineering crystalline domains into associative hydrogels of poly(lactic acid)-poly(ethylene oxide)-poly(lactic acid) (PLA-PEO-PLA) triblock copolymers. In aqueous media, these materials form associative gels of micelles with PLA cores that serve as network junctions. We extend previous studies from our group and others by varying the stereochemistry of the PLA block to create polymers with PLA blocks with ratios of L/D lactide units varying from 100/0 to 50/50. We had previously found that the 100/0 systems (triblocks with poly(L-lactide) blocks) formed gels with nanoscale crystalline domains, and these gels displayed a high value of the elastic modulus which was strongly dependent on PLA block length. Interestingly, our most recent results show that the storage modulus of these gels does not vary monotonically with L/D ratio. Rather, systems at intermediate L/D values are stiffer than the 100/0 systems, displaying higher storage moduli in spite of the fact that the PLA domains are expected to have a lower degree of crystallinity than in the 100/0 systems. Small-angle neutron scattering (SANS) results also indicate that the strongest interactions between micelles occurs for systems with intermediate L/D ratios, and ultra-small angle neutron scattering (USANS) shows evidence of larger structures in these gels, reminiscent of the hierarchical structures observed in biological gels. Collectively, our work shows that stereochemistry can be used in unexpected ways to access novel structures and properties in relatively simple synthetic polymers, giving insight into new routes for creating complex soft materials

Double quantum dot with tunable coupling in an enhancement-mode silicon metal-oxide semiconductor device with lateral geometry

We present transport measurements of a tunable silicon metal-oxide-semiconductor double quantum dot device with lateral geometry. Experimentally extracted gate-to-dot capacitances show that the device is largely symmetric under the gate voltages applied. Intriguingly, these gate voltages themselves are not symmetric. Comparison with numerical simulations indicates that the applied gate voltages serve to offset an intrinsic asymmetry in the physical device. We also show a transition from a large single dot to two well isolated coupled dots, where the central gate of the device is used to controllably tune the interdot coupling.Comment: 4 pages, 3 figures, to be published in Applied Physics Letter