Bis-<i>N</i>-heterocyclic Carbene Palladium(IV) Tetrachloride Complexes: Synthesis, Reactivity, and Mechanisms of Direct Chlorinations and Oxidations of Organic Substrates

This Article describes the preparation and isolation of novel octahedral CH2-bridged bis-(N-heterocyclic carbene)palladium(IV) tetrachlorides of the general formula LPdIVCl4 [L = (NHC)CH2(NHC)] from LPdIICl2 and Cl2. In intermolecular, nonchelation-controlled transformations LPdIVCl4 reacted with alkenes and alkynes to 1,2-dichlorination adducts. Aromatic, benzylic, and aliphatic C−H bonds were converted into C−Cl bonds. Detailed mechanistic investigations in the dichlorinations of alkenes were conducted on the 18VE PdIV complex. Positive solvent effects as well as kinetic measurements probing the impact of cyclohexene and chloride concentrations on the rate of alkene chlorination support a PdIV−Cl ionization in the first step. Product stereochemistry and product distributions from various alkenes also support Cl+-transfer from the pentacoordinated PdIV-intermediate LPdIVCl3+ to olefins. 1-Hexene/3-hexene competition experiments rule out both the formation of π-complexes along the reaction coordinate as well as in situ generated Cl2 from a reductive elimination process. Instead, a ligand-mediated direct Cl+-transfer from LPdIVCl3+ to the π-system is likely to occur. Similarly, C−H bond chlorinations proceed via an electrophilic process with in situ formed LPdIVCl3+. The presence of a large excess of added Cl− slows cyclohexene chlorination while the presence of stoichiometric amounts of chloride accelerates both PdIV−Cl ionization and Cl+-transfer from LPdIVCl3+. 1H NMR titrations, T1 relaxation time measurements, binding isotherms, and Job plot analysis point to the formation of a trifurcated Cl−···H−C bond in the NHC-ligand periphery as a supramolecular cause for the accelerated chemical events involving the metal center

Bis-<i>N</i>-heterocyclic Carbene Palladium(IV) Tetrachloride Complexes: Synthesis, Reactivity, and Mechanisms of Direct Chlorinations and Oxidations of Organic Substrates

This Article describes the preparation and isolation of novel octahedral CH2-bridged bis-(N-heterocyclic carbene)palladium(IV) tetrachlorides of the general formula LPdIVCl4 [L = (NHC)CH2(NHC)] from LPdIICl2 and Cl2. In intermolecular, nonchelation-controlled transformations LPdIVCl4 reacted with alkenes and alkynes to 1,2-dichlorination adducts. Aromatic, benzylic, and aliphatic C−H bonds were converted into C−Cl bonds. Detailed mechanistic investigations in the dichlorinations of alkenes were conducted on the 18VE PdIV complex. Positive solvent effects as well as kinetic measurements probing the impact of cyclohexene and chloride concentrations on the rate of alkene chlorination support a PdIV−Cl ionization in the first step. Product stereochemistry and product distributions from various alkenes also support Cl+-transfer from the pentacoordinated PdIV-intermediate LPdIVCl3+ to olefins. 1-Hexene/3-hexene competition experiments rule out both the formation of π-complexes along the reaction coordinate as well as in situ generated Cl2 from a reductive elimination process. Instead, a ligand-mediated direct Cl+-transfer from LPdIVCl3+ to the π-system is likely to occur. Similarly, C−H bond chlorinations proceed via an electrophilic process with in situ formed LPdIVCl3+. The presence of a large excess of added Cl− slows cyclohexene chlorination while the presence of stoichiometric amounts of chloride accelerates both PdIV−Cl ionization and Cl+-transfer from LPdIVCl3+. 1H NMR titrations, T1 relaxation time measurements, binding isotherms, and Job plot analysis point to the formation of a trifurcated Cl−···H−C bond in the NHC-ligand periphery as a supramolecular cause for the accelerated chemical events involving the metal center

Competitive Nucleophilic Attack Chemistry Based on Undecenoic Acid: A New Chemical Route for Plant-Oil-Based Epoxies

Plant oil is one of the world’s most abundant renewable resources; however, its derived epoxies are low in thermal resistance and mechanical strength. In this work, a new chemical route referred to “competitive nucleophilic attack (CNA)” was discovered to achieve plant-oil-based epoxy with high thermal resistance and mechanical strength as well as many other unique properties comparable to those of diglycidyl ether of bisphenol A (DGEBA), one of the most popular petroleum-based epoxies. The CNA route was realized by using 10-undecenoic acid (UA), a plant-derived monomer, as a building block reacting with alicyclic oxirane chemicals, such as 4-ethenyl-7-oxabicyclo[4.1.0]­heptanes (ECP), to achieve epoxy monomers with ether-bridged cycloaliphatic ring structure. A newly formed hydroxyl (NFH) is involved in the nucleophilic attack upon oxonium to compete with UA anion during the UA–ECP reaction. The resultant epoxy is UV-curable in a few seconds, possessing high tensile strength (∼48 MPa), high glass transition temperature (∼142 °C), high transparency (∼90%), as well as low viscosity (∼1.9 Pa s). These properties are superior to the plant-oil-based epoxies published and comparable to or better than commercial DGEBA. Structure analysis revealed that the ether-bridged cycloaliphatic ring structure via the CNA route played a key role in maximizing the network performance. With the CNA feature, chain structure can be further regulated via introducing a methyl group to hinder the NFH nucleophilic attack, achieving a conversion of epoxy resin from rigid to semiductile. This finding suggests that CNA strategy could be a new direction for the design of biobased epoxies using all possible chemicals with acid–alkene structures from various renewable resources rather than plant oils only

Channel Blocking of MspA Revisited

Porin A from Mycobacterium smegmatis (MspA) is a highly stable, octameric channel protein, which acts as the main transporter of electrolytes across the cell membrane. MspA features a narrow, negatively charged constriction zone, allowing stable binding of various analytes thereby blocking the channel. Investigation of channel blocking of mycobacterial porins is of significance in developing alternate treatment methods for tuberculosis. The concept that ruthenium­(II)­quaterpyridinium complexes have the capability to act as efficient channel blockers for MspA and related porins, emerged after very high binding constants were measured by high-performance liquid chromatography and steady-state luminescence studies. Consequently, the interactions between the ruthenium­(II) complex RuC2 molecules and MspA, leading to RuC2@MspA assemblies, have been studied utilizing time-resolved absorption/emission, atomic force microscopy, dynamic light scattering, ζ potential measurements, and isothermal titration calorimetry. The results obtained provide evidence for the formation of clusters/large aggregates of RuC2 and MspA. The results are of interest with respect to utilizing prospective channel blockers in porins. The combination of results from conceptually different techniques shed some light onto the chemical nature of MspA–channel blocker interactions thus contributing to the development of a paradigm for channel blocking

Attenuation of Mouse Melanoma by A/C Magnetic Field after Delivery of Bi-Magnetic Nanoparticles by Neural Progenitor Cells

Localized magnetic hyperthermia as a treatment modality for cancer has generated renewed interest, particularly if it can be targeted to the tumor site. We examined whether tumor-tropic neural progenitor cells (NPCs) could be utilized as cell delivery vehicles for achieving preferential accumulation of core/shell iron/iron oxide magnetic nanoparticles (MNPs) within a mouse model of melanoma. We developed aminosiloxane−porphyrin functionalized MNPs, evaluated cell viability and loading efficiency, and transplanted neural progenitor cells loaded with this cargo into mice with melanoma. NPCs were efficiently loaded with core/shell Fe/Fe3O4 MNPs with minimal cytotoxicity; the MNPs accumulated as aggregates in the cytosol. The NPCs loaded with MNPs could travel to subcutaneous melanomas, and after A/C (alternating current) magnetic field (AMF) exposure, the targeted delivery of MNPs by the cells resulted in a measurable regression of the tumors. The tumor attenuation was significant (p < 0.05) a short time (24 h) after the last of three AMF exposures