The anions of PdCl(2)L(2) and Pd(OAc)(2), precursors of palladium(0) used in cross-coupling and Heck reactions, play a crucial role in these reactions. Tricoordinated anionic complexes Pd(0)L(2)Cl(-) and Pd(0)L(2)(OAc)(-) are the effective catalysts instead of the usually postulated Pd(0)L(2) complex. The anion ligated to the palladium(0) affects the kinetics of the oxidative addition to ArI as well as the structure and reactivity of the arylpalladium(II) complexes produced in this reaction. Thus, pentacoordinated anionic complexes are formed, ArPdI(Cl)L(2)(-) or ArPdI(OAc)L(2)(-), the precursor of neutral trans-ArPd(OAc)L(2), instead of the usually postulated trans-ArPdIL(2) complex (L = PPh(3)).
Fast Scan Cyclic Voltammetry (FSCV) at high aspect ratio carbon microelectrodes shows adequate high temporal and spatial resolution for in vivo analysis of catecholamines. Though the presence of their surface heterogeneities has been recognized since their earliest introduction for in vivo measurements in the brain, the kinetic consequences on the measurements have not been investigated and FSCV measurements are treated based on pre- and post-calibrations. We establish here that surface heterogeneities play a consequent dynamic role on the oxidation of dopamine taken as an example of catecholamines. Hence, the FSCV current peak intensities do not scale with the scan rate v or its square root. This is rationalized with a simple model involving a co-existence of at least two types of surface nanodomains with different electrochemical reactivities and different time responses. At low scan rates (<100 V s−1) dopamine molecules that initially adsorbed onto non-electroactive nanodomains have enough time to migrate toward highly electroactive ones so all molecules initially adsorbed on the whole electrode surface may be oxidized during one FSCV cycle. Current peak intensities then increase proportionally to the scan rate. However, above 100 V s−1, dopamine migration between sites starts to be kinetically limited so that FSCV current peak intensities do not increase any more proportionally to the scan rate. Ultimately, i.e., above 1000 V s−1, the dopamine exchange between sites is almost totally blocked so only dopamine molecules initially adsorbed on the electroactive surface nanodomains may be oxidized; the current peak intensities then increase again proportionally with the scan rate though with a smaller slope than that observed at small scan rates. Since carbon fibers with large aspect ratios are frequently used in brain investigations, this effect should be a concern when extracting quantitative results even when each carbon fiber response is properly pre- or post-calibrated using the exact CV waveform and scan rate used during the in vivo measurements.
Abstract The mechanism of the reaction of trans ‐ArPdBrL 2 (Ar= p ‐Z‐C 6 H 4 , Z=CN, H; L=PPh 3 ) with Ar′B(OH) 2 (Ar′= p ‐Z′‐C 6 H 4 , Z′=H, CN, MeO), which is a key step in the Suzuki–Miyaura process, has been established in N , N ‐dimethylformamide (DMF) with two bases, acetate ( n Bu 4 NOAc) or carbonate (Cs 2 CO 3 ) and compared with that of hydroxide ( n Bu 4 NOH), reported in our previous work. As anionic bases are inevitably introduced with a countercation M + (e.g., M + OH − ), the role of cations in the transmetalation/reductive elimination has been first investigated. Cations M + (Na + , Cs + , K + ) are not innocent since they induce an unexpected decelerating effect in the transmetalation via their complexation to the OH ligand in the reactive ArPd(OH)L 2 , partly inhibiting its transmetalation with Ar′B(OH) 2 . A decreasing reactivity order is observed when M + is associated with OH − : n Bu 4 N + > K + > Cs + > Na + . Acetates lead to the formation of trans ‐ArPd(OAc)L 2 , which does not undergo transmetalation with Ar′B(OH) 2 . This explains why acetates are not used as bases in Suzuki–Miyaura reactions that involve Ar′B(OH) 2 . Carbonates (Cs 2 CO 3 ) give rise to slower reactions than those performed from n Bu 4 NOH at the same concentration, even if the reactions are accelerated in the presence of water due to the generation of OH − . The mechanism of the reaction with carbonates is then similar to that established for n Bu 4 NOH, involving ArPd(OH)L 2 in the transmetalation with Ar′B(OH) 2 . Due to the low concentration of OH − generated from CO 3 2− in water, both transmetalation and reductive elimination result slower than those performed from n Bu 4 NOH at equal concentrations as Cs 2 CO 3 . Therefore, the overall reactivity is finely tuned by the concentration of the common base OH − and the ratio [OH − ]/[Ar′B(OH) 2 ]. Hence, the anionic base (pure OH − or OH − generated from CO 3 2− ) associated with its countercation (Na + , Cs + , K + ) plays four antagonist kinetic roles: acceleration of the transmetalation by formation of the reactive ArPd(OH)L 2 , acceleration of the reductive elimination, deceleration of the transmetalation by formation of unreactive Ar′B(OH) 3 − and by complexation of ArPd(OH)L 2 by M + .
Carbon-fluorine bonds of Teflon (polytetrafluoroethylene, PTFE) can be reduced electrochemically with the purpose of modifying its adhesive and wetting surface properties by micrometrically controlled surface carbonization of the material. This can be performed adequately by redox catalysis provided that the redox mediator couple has a sufficiently negative reduction potential. The process is investigated kinetically with benzonitrile as the mediator and a gold-band ultramicroelectrode mounted adjacent to a PTFE block, though separated from it by an insulating micrometric mylar gap. For moderate fluxes of reduced mediator, the whole device behaves as a generator-collector double-band assembly with a constant current amplification factor. This is maintained over long periods of time, during which the carbonized PTFE zones extends over distances that are much wider than the slowly expanding cylindrical diffusion layer generated at the gold-microband electrode. This establishes that the overall redox catalysis proceeds through electronic conduction in the n-doped carbonized material. Thus, carbonization progresses at the external edge of the freshly carbonized surface in a diffusion-like fashion (dependence on the square root of time), while the redox-mediator oxidized form is regenerated at the carbonized PTFE edge facing to the gold ultramicroelectrode, so that the overall rate of carbonization is controlled by solution diffusion only. For larger fluxes of mediator, the heterogeneous rate of reduction and doping of PTFE becomes limiting, and the situation is more complex. A conceptually simple model is developed which predicts and explains all the main dynamic features of the system under these circumstances and allows the determination of the heterogeneous rate constant of carbon-fluorine bonds at the interface between the carbonized zone and the fresh PTFE. This model can be further refined to account for the effect of ohmic drop inside the carbonized zone on the heterogeneous reduction rate constants and henceforth gives an extremely satisfactory quantitative agreement with the experimental data.
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