[Pd(NHC)(μ-Cl)Cl]2: The Highly Reactive Air- and Moisture-Stable, Well-Defined Pd(II)-N-Heterocyclic Carbene (NHC) Complexes for Cross-Coupling Reactions

ConspectusPalladium-catalyzed cross-coupling reactions owing to their high specificity and superb chemoselectivity represent a powerful tool for the rapid construction of C-C and C-X bonds across various areas of chemical research, including pharmaceutical development, polymer and agrochemical industries, bioactive natural products, and advanced functional materials, rendering them indispensable for modern synthetic chemists. The major driving force for the advances in this critical field is the design of increasingly more reactive and more selective ligands and precatalysts that aim not only to address challenging cross-coupling processes but also to achieve optimal reactivity, selectivity, and functional group compatibility under mild, user-friendly, operationally simple, and broadly applicable conditions. In this context, Pd(II)-N-heterocyclic carbene complexes (NHC = N-heterocyclic carbene) have garnered prevalent attention among practitioners of organic synthesis due to their unique electronic and steric characteristics that are unmatched among other ligands. In particular, the superior σ-donating ability of NHC ligands in conjunction with conformational flexibility as well as the ease of steric and electronic modification and high stability to air and moisture enable highly effective fundamental elementary steps in catalytic cycles and facile formation of well-defined complexes.The key factor in the design of well-defined, air- and moisture-stable Pd(II) precatalysts involves the incorporation of supporting ligands, which are essential for ensuring the stability of Pd(II)-NHC complexes and facile activation of Pd(II)-NHC precatalysts to catalytically active monoligated Pd(0)-NHC species under the reaction conditions. Notably, [Pd(NHC)(μ-Cl)Cl]2 chloro dimers, which can be readily synthesized via a one-pot, atom-economic process, are the most reactive Pd(II)-NHC complexes synthesized to date. These well-defined, air- and moisture-stable dimers readily dissociate to monomers and are activated to Pd(0)-NHC catalysts under both mild and strong base conditions, showcasing enhanced reactivity and selectivity among their Pd(II)-NHC counterparts. This balance between high, operationally simple stability, which is characteristic of Pd(II) complexes together with the ease of activation to the strongly nucleophilic Pd(0)-NHC catalysts, renders [Pd(NHC)(μ-Cl)Cl]2 the most reactive Pd(II)-NHC precatalysts developed to date for a broad range of general cross-coupling processes, including C-X, C-O, C-N, and C-S activation and enabling the direct late-stage functionalization of complex compounds decorated with a wide range of sensitive functional groups.In this Account, we outline [Pd(NHC)(μ-Cl)Cl]2 as a highly reactive Pd(II)-NHC precatalyst that should be routinely used as the first choice Pd complexes for a wide range of challenging cross-coupling reactions. The advancements in this field over the past 20 years emphasize the critical role of catalyst design to achieve optimal reactivity. Consequently, [Pd(NHC)(μ-Cl)Cl]2 chloro dimers should be recommended as the go-to complexes in the powerful toolbox of Pd-catalyzed cross-coupling reactions. These now commercially available Pd(II)-NHC complexes see widespread use across the synthetic chemistry community and enable the accelerated application of challenging cross-couplings in the synthesis of new molecules.

Thermal desorption of PCBs contaminated soil with calcium hydroxide in a rotary kiln

In this study, thermal desorption was combined with the addition of calcium hydroxide to remediate polychlorinated biphenyls (PCBs) contaminated soil, collected from a storage point for PCB-contaminated capacitors and transformers. The thermal desorption test conditions were varied from 300 to 600 °C, both with blank soil and with 1% Ca(OH)₂ added. The results showed that the synergistic thermal desorption was effective to removal most of PCBs. At 600 °C, the removal efficiency (RE) reached 94.0% in presence of Ca(OH)₂, higher than that of 90.9% in blank soil. The dechlorination was significant when compared with blank soil. Ca(OH)₂ effectively decreased both the sum and the toxic equivalence quantity (TEQ) value of the 12 dioxin-like PCBs, with the RE based on TEQ of 90.0%. Ca(OH)₂ strengthened the removal, dechlorination and detoxication of PCBs. The synergistic effect factor proved the promotion did exist in the presence of Ca(OH)₂.

Highly Efficient Encapsulation and Phase Separation of Apolar Molecules by Magnetic Shell-by-Shell-Coated Nanocarriers in Water

We report on the development of a supramolecular nanocarrier concept that allows for the encapsulation and separation of small apolar molecules from water. The nanocarriers consist of shell-by-shell-coated nanoparticles such as TiO₂ and ferromagnetic Fe₃ O₄ . The first ligand shell is provided by covalently bound hexadecyl phosphonic acid (PAC₁₆ ) and the second shell by noncovalently assembled amphiphiles rendering the hybrid architecture soluble in water. Agitation of these constructs with water containing the hydrocarbons G1-G4, the fluorescent marker G5, the polychlorinated biphenyl PCB 77, or crude oil leads to a very efficient uptake (up to 411 %) of the apolar contaminant. In case of the hybrids containing a Fe₃ O₄ core, straightforward phase separation by the action of an external magnet is provided. The load can easily be released by a final treatment with an organic solvent.

Multiphase hydrodechlorination of polychlorinated aromatics - Towards scale-up

We describe a chemical technology for the reductive catalytic multiphase hydrodechlorination (HDC) of chlorinated aromatics to greatly reduce their toxicity and aid the disposal of such species. The system requires no solvent and the catalyst displays a high recycling efficiency. In the present case, 1,3-dichlorobenzene (1,3-DCB) was used as a model compound, and was quantitatively hydrodechlorinated to benzene with hydrogen, in a tri-phasic liquid system consisting of the chlorinated aromatic itself as the top organic phase, an aqueous sodium hydroxide bottom phase (that neutralises acids formed), and an Aliquat^®336 (A336) intermediate phase containing a Pd/C catalyst. Once the reaction was complete the top phase (now just benzene) and the bottom phase (now principally aqueous NaCl) were removed and the remaining catalytic A336/(Pd/C) phase recycled. This model study was conducted on a multi-gram scale with a view of demonstrating its applicability to the detoxification of PCBs. Comparison of the Mass Intensity (MI) and turnover frequency (TOF) of our model reaction with three examples of published procedures for the HDC of DCB, indicated that the MI for our system (MI = 6.33) was lower by an order of magnitude or more than that of the others (MI = 27.9, 64.6, 96016), and that TOFs were comparable. A preliminary cost analysis indicates approximately 2000 €/tonne to treat tonne-scale amounts of chlorinated aromatics, making the system in principle useful for industrial implementation.

Transfer hydrodehalogenation of aryl halides accelerated by a saturated sodium acetate aqueous solution

A saturated sodium acetate aqueous solution could be applied as an efficient and environmentally-friendly reaction medium to accelerate transfer hydrodehalogenation of various aryl halides.

Thermal desorption of PCB-contaminated soil with sodium hydroxide

The thermal desorption was combined with sodium hydroxide to remediate polychlorinated biphenyl (PCB)-contaminated soil. The experiments were conducted at different temperatures ranging from 300 to 600 °C with three NaOH contents of 0.1, 0.5, and 1 %. The results showed that thermal desorption was effective for PCB removal, destruction, and detoxication, and the presence of NaOH enhanced the process by significant dechlorination. After treatment with 0.1 % NaOH, the removal efficiency (RE) increased from 84.8 % at 300 °C to 98.0 % at 600 °C, corresponding to 72.7 and 91.7 % of destruction efficiency (DE). With 1 % NaOH content treated at 600 °C, the RE and DE were 99.0 and 93.6 %, respectively. The effect of NaOH content on PCB removal was significant, especially at lower temperature, yet it weakened under higher temperature. The interaction between NaOH content and temperature influenced the PCB composition. The higher temperature with the help of NaOH effectively increased the RE and DE of 12 dioxin-like PCBs (based on WHO-TEQ).

Ruthenium/base-catalyzed ortho-selective C–H arylation of acylarenes with halogenated arylboronates

Ruthenium-catalyzed ortho-selective C–H arylation of acylarenes with halogenated arylboronates was promoted by the catalytic amount of bases and provided desired halogenated biaryls in excellent yields under 0.2–1 mol% ruthenium catalyst loading.

Syntheses of sterically shielded stable carbenes of the 1,2,4-triazole series and their corresponding palladium complexes: efficient catalysts for chloroarene hydrodechlorination

Sterically shielded 1,2,4-triazol-5-ylidenes and their palladium complexes: catalysts for hydrodechlorination.

Exceptionally efficient catalytic hydrodechlorination of persistent organic pollutants: application of new sterically shielded palladium carbene complexes

Exceptionally efficient catalysis of haloarene hydrodehalogenation by palladium carbene complexes.

Vertically aligned Ag nanoplate-assembled film as a sensitive and reproducible SERS substrate for the detection of PCB-77

Vertically aligned Ag nanoplate-assembled film has been achieved by spin-coating Ag seeds on an ITO substrate and subsequent electrodeposition in a mixed aqueous solution of AgNO(3) and citric acid. As sufficient hot spots are located in the deep gaps between the neighboring nanoplates across the whole substrate, the Ag nanoplate-assembled film shows strong Surface enhanced Raman scattering (SERS) effect, together with good signal reproducibility. Therefore, the Ag nanoplate-assembled films were tried as robust, highly sensitive and reproducible SERS substrates for the rapid detection of 3,3',4,4'-tetrachlorobiphenyl (PCB-77) and a detection limit of about 10(-6)M was reached. For further reducing the detection limit, a layer of decanethiol was modified on the Ag nanoplate surface to capture the PCB-77 molecules efficiently, and a lower detection limit of 10(-7)M was achieved. A linear dependence was found between the logarithmic concentrations of PCB-77 and the intensities of the fingerprint peaks. Furthermore, the Ag nanoplate-assembled film can also be used as a SERS substrate to distinguish characteristic peaks of different polychlorinated biphenyls (PCBs) in their mixed solutions. Therefore the vertically aligned Ag nanoplate-assembled film has potentials as effective SERS substrates in rapid and direct detection of trace PCBs.