Sibo Lin

Nickel-Mediated Hydrogenolysis of C–O Bonds of Aryl Ethers: What Is the Source of the Hydrogen?

Mechanistic studies of the hydrogenolysis of aryl ethers by nickel were undertaken with (diphosphine)aryl methyl ethers. A Ni(0) complex containing Ni-arene interactions adjacent to the aryl-O bond was isolated. Heating led to aryl-O bond activation and generation of a nickel aryl methoxide complex. Formal β-H elimination from this species produced a nickel aryl hydride which can undergo reductive elimination in the presence of formaldehyde to generate a carbon monoxide adduct of Ni(0). The reported complexes map out a plausible mechanism of aryl ether hydrogenolysis catalyzed by nickel. Investigations of a previously reported catalytic system using isotopically labeled substrates are consistent with the mechanism proposed in the stoichiometric system, involving β-H elimination from a nickel alkoxide rather than cleavage of the Ni-O bond by H(2).

Nickel Hydrides Supported by a Non-Innocent Diphosphine Arene Pincer: Mechanistic Studies of Nickel−Arene H-Migration and Partial Arene Hydrogenation

Nickel hydrides supported by a terphenyl diphosphine were synthesized and found to undergo nickel-to-arene H-transfers. Some of the resulting complexes also undergo the reverse (C-to-Ni) H-migration, indicating the potential for storing H-equivalents in this type of pincer ligand. NMR spectroscopy, single crystal X-ray diffraction, and isotopic labeling studies investigating the mechanism of these processes are discussed.

Reversible Halide‐Modulated Nickel–Nickel Bond Cleavage: Metal–Metal Bonds as Design Elements for Molecular Devices

The dinickel chloride affair: In dinuclear nickel(I) complexes supported by a tris(phosphinoaryl)benzene and stabilized by metal–arene interactions, chloride addition causes reversible Ni-Ni bond cleavage that induces 180° rotation around an aryl–aryl bond (see scheme). A dinickel–chloride moiety was found to rotate around the bridging arene by a mechanism involving breaking and forming Ni-P bonds.

Dipalladium(I) terphenyl diphosphine complexes as models for two-site adsorption and activation of organic molecules.

A para-terphenyl diphosphine was employed to support a dipalladium(I) moiety. Unlike previously reported dipalladium(I) species, the present system provides a single molecular hemisphere for binding of ligands across two metal centers, enabling the characterization and comparison of the binding of a wide variety of saturated and unsaturated organic molecules. The dipalladium(I) terphenyl diphosphine toluene-capped complex was synthesized from a dipalladium(I) hexaacetonitrile precursor in the presence of toluene. The palladium centers display interactions with the π-systems of the central ring of the terphenyl unit and that of the toluene. Exchange of toluene for anisole, 1,3-butadiene, 1,3-cyclohexadiene, thiophenes, pyrroles, or furans resulted in well-defined π-bound complexes which were studied by crystallography, nuclear magnetic resonance (NMR) spectroscopy, and density functional theory. Structural characterization shows that the interactions of the dipalladium unit with the central arene of the diphosphine does not vary significantly in this series allowing for a systematic comparison of the binding of the incoming ligands to the dipalladium moiety. Several of the complexes exhibit rare μ-η(2):η(2) or μ-η(2):η(1)(O or S) bridging motifs. Hydrogenation of the thiophene and benzothiophene adducts was demonstrated to proceed at room temperature. The relative binding strength of the neutral ligands was determined by competition experiments monitored by NMR spectroscopy. The relative equilibrium constants for ligand substitution span over 13 orders of magnitude. This represents the most comprehensive analysis to date of the relative binding of heterocycles and unsaturated ligands to bimetallic sites. Binding interactions were computationally studied with electrostatic potentials and molecular orbital analysis. Anionic ligands were also demonstrated to form π-bound complexes.

Combination of Redox-Active Ligand and Lewis Acid for Dioxygen Reduction with π-Bound Molybdenum−Quinonoid Complexes

A series of π-bound Mo-quinonoid complexes supported by pendant phosphines have been synthesized. Structural characterization revealed strong metal-arene interactions between Mo and the π system of the quinonoid fragment. The Mo-catechol complex (2a) was found to react within minutes with 0.5 equiv of O(2) to yield a Mo-quinone complex (3), H(2)O, and CO. Si- and B-protected Mo-catecholate complexes also react with O(2) to yield 3 along with (R(2)SiO)n and (ArBO)(3) byproducts, respectively. Formally, the Mo-catecholate fragment provides two electrons, while the elements bound to the catecholate moiety act as acceptors for the O(2) oxygens. Unreactive by itself, the Mo-dimethyl catecholate analogue reduces O(2) in the presence of added Lewis acid, B(C(6)F(5))(3), to generate a Mo(I) species and a bis(borane)-supported peroxide dianion, [[(F(5)C(6))(3)B](2)O(2)(2-)], demonstrating single-electron-transfer chemistry from Mo to the O(2) moiety. The intramolecular combination of a molybdenum center, redox-active ligand, and Lewis acid reduces O(2) with pendant acids weaker than B(C(6)F(5))(3). Overall, the π-bound catecholate moiety acts as a two-electron donor. A mechanism is proposed in which O(2) is reduced through an initial one-electron transfer, coupled with transfer of the Lewis acidic moiety bound to the quinonoid oxygen atoms to the reduced O(2) species.

Lewis Acid Accelerated Aryl Ether Bond Cleavage with Nickel: Orders of Magnitude Rate Enhancement Using AlMe3.

Study of the kinetics of intramolecular aryl ether C-O bond cleavage by Ni was facilitated by access to a family of metal complexes supported by diphosphines with pendant aryl-methyl ethers. The nature of the aryl substituents was found to have little effect on the rate of cleavage. In contrast, soluble Lewis acidic additives accelerate the aryl ether cleavage dramatically. The effect of AlMe3 was studied in detail, and showed an increase in rate by several orders of magnitude. Low temperature NMR spectroscopy studies demonstrate quantitative coordination of ether to Al. From the Lewis acid-bound precursor, the activation parameters for ether cleavage are significantly lower. These findings provide a mechanistic basis for milder catalyst design for the activation of strong bonds.

Carbon Nanotube Formic Acid Sensors Using a Nickel Bis(ortho-diiminosemiquinonate) Selector

Formic acid is corrosive, and a sensitive and selective sensor could be useful in industrial, medical, and environmental settings. We present a chemiresistor for detection of formic acid composed of single-walled carbon nanotubes (CNTs) and nickel bis( ortho-diiminosemiquinonate) (1), a planar metal complex that can act as a ditopic hydrogen-bonding selector. Formic acid is detected in concentrations as low as 83 ppb. The resistance of the material decreases on exposure to formic acid, but slightly increases on exposure to acetic acid. We propose that 1 assists in partial protonation of the CNT by formic acid, but the response toward acetic acid is dominated by inter-CNT swelling. This technology establishes CNT-based chemiresistive discrimination between formic and acetic acid vapors.

Bio‐Inspired Carbon Monoxide Sensors with Voltage‐Activated Sensitivity

Carbon monoxide (CO) outcompetes oxygen when binding to the iron center of hemeproteins, leading to a reduction in blood oxygen level and acute poisoning. Harvesting the strong specific interaction between CO and the iron porphyrin provides a highly selective and customizable sensor. We report the development of chemiresistive sensors with voltage-activated sensitivity for the detection of CO comprising iron porphyrin and functionalized single-walled carbon nanotubes (F-SWCNTs). Modulation of the gate voltage offers a predicted extra dimension for sensing. Specifically, the sensors show a significant increase in sensitivity toward CO when negative gate voltage is applied. The dosimetric sensors are selective to ppm levels of CO and functional in air. UV/Vis spectroscopy, differential pulse voltammetry, and density functional theory reveal that the in situ reduction of FeIII to FeII enhances the interaction between the F-SWCNTs and CO. Our results illustrate a new mode of sensors wherein redox active recognition units are voltage-activated to give enhanced and highly specific responses.

A Semiconducting Conjugated Radical Polymer: Ambipolar Redox Activity and Faraday Effect

Investigations of magnetism in electronically coupled polyradicals have largely focused on applications in photonic and magnetic devices, wherein radical polymers were found to possess molecularly tunable and cooperative magnetic properties. Radical polymers with nonconjugated insulating backbones have been intensively investigated previously; however the integration of radical species into conducting polymer backbones is at an early stage. We report herein 1,3-bisdiphenylene-2-phenylallyl (BDPA)-based conjugated radical polymers that display ambipolar redox activities and conductivities. Moreover, these radical polymers were demonstrated to be promising magneto-optic (MO) materials with Faraday rotations wherein the sign is modulated by the radical character and display absolute Verdet constants up to (2.80 ± 0.84) × 104 deg T-1 m-1 at 532 nm. These values rival the performance of the present-day commercial inorganic MO materials (e.g., terbium gallium garnet, V = -1.0 × 104 deg T-1 m-1 at 532 nm). The structure property studies detailed herein reveal the promise of multifunctional conjugated radical polymers as responsive MO materials.

Carbon Nanotube Chemical Sensors

Carbon nanotubes (CNTs) promise to advance a number of real-world technologies. Of these applications, they are particularly attractive for uses in chemical sensors for environmental and health monitoring. However, chemical sensors based on CNTs are often lacking in selectivity, and the elucidation of their sensing mechanisms remains challenging. This review is a comprehensive description of the parameters that give rise to the sensing capabilities of CNT-based sensors and the application of CNT-based devices in chemical sensing. This review begins with the discussion of the sensing mechanisms in CNT-based devices, the chemical methods of CNT functionalization, architectures of sensors, performance parameters, and theoretical models used to describe CNT sensors. It then discusses the expansive applications of CNT-based sensors to multiple areas including environmental monitoring, food and agriculture applications, biological sensors, and national security. The discussion of each analyte focuses on the strategies used to impart selectivity and the molecular interactions between the selector and the analyte. Finally, the review concludes with a brief outlook over future developments in the field of chemical sensors and their prospects for commercialization.