Abstract:Both ~, ( d~~r n ) ,~O~+ clusters (M = Pd, Pt) photoreact with chlorocarbons (Cl-R; R = CCl,, CHCl,, CH2C1, C6H5, ClOHl5 (adamant~l)) and chloride ion (slowly) to produce the oxidized mononuclear species M(dppm)Cl, as a sole isolated Mcoordinated inorganic product. Such reactions do not proceed in the dark, except for R = CH2C6H5. Among the organic products, the coupling compound R-R (R = C6H5) is observed along with many phosphine compounds such as P(C6H5),. In an attempt to elucidate the photoinduced mechani… Show more
“…These distances are well located within the sum of the van der Waals radii ( r vdw = 1.60 Å (Pd), 1.90 Å (Br)), and therefore, the presence of reactivity is not surprising. These findings corroborate recent studies on the photochemical oxidation of Pd 3 2+ in the presence of chlorocarbons, where rates of reactivity (i.e., quantum yields) were functions of the substrate dimension 7 Example of a computed ball-and-stick model of the Pd 3 2+ ···β-Xyl−Br host−guest system.…”
The title cluster (Pd(3)(2+)) exhibits a pronounced affinity for Br(-) ions to form the very stable Pd(3)(Br)(+) adduct. Upon a 2-electron reduction, a dissociative process occurs generating Pd(3)(0) and eliminating Br(-) according to an ECE mechanism (electrochemical, chemical, electrochemical). At a lower temperature (i.e. -20 degrees C), both ECE and EEC processes operate. This cluster also activates the C-Br bond, and this work deals with the reactivity of Pd(3)(2+) with 2,3,4-tri-O-acetyl-5-thioxylopyranosyl bromide (Xyl-Br), both alpha- and beta-isomers. The observed inorganic product is Pd(3)(Br)(+) again, and it is formed according to an associative mechanism involving Pd(3)(2+).Xyl-Br host-guest assemblies. In an attempt to render the C-Br bond activation catalytic, these species are investigated under reduction conditions at two potentials (-0.9 and -1.25 V vs SCE). In the former case, the major product is Xyl-H, issued from a radical intermediate Xyl(*) abstracting an H atom from the solvent. Evidence for Xyl(*) is provided by the trapping with TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) and DMPO (5,5'-dimethylpyrroline-N-oxyde). In the second case, only one product is observed, 3,4-di-O-acetyl-5-thioxylal, which is issued from the Xyl(-)() intermediate anion.
“…These distances are well located within the sum of the van der Waals radii ( r vdw = 1.60 Å (Pd), 1.90 Å (Br)), and therefore, the presence of reactivity is not surprising. These findings corroborate recent studies on the photochemical oxidation of Pd 3 2+ in the presence of chlorocarbons, where rates of reactivity (i.e., quantum yields) were functions of the substrate dimension 7 Example of a computed ball-and-stick model of the Pd 3 2+ ···β-Xyl−Br host−guest system.…”
The title cluster (Pd(3)(2+)) exhibits a pronounced affinity for Br(-) ions to form the very stable Pd(3)(Br)(+) adduct. Upon a 2-electron reduction, a dissociative process occurs generating Pd(3)(0) and eliminating Br(-) according to an ECE mechanism (electrochemical, chemical, electrochemical). At a lower temperature (i.e. -20 degrees C), both ECE and EEC processes operate. This cluster also activates the C-Br bond, and this work deals with the reactivity of Pd(3)(2+) with 2,3,4-tri-O-acetyl-5-thioxylopyranosyl bromide (Xyl-Br), both alpha- and beta-isomers. The observed inorganic product is Pd(3)(Br)(+) again, and it is formed according to an associative mechanism involving Pd(3)(2+).Xyl-Br host-guest assemblies. In an attempt to render the C-Br bond activation catalytic, these species are investigated under reduction conditions at two potentials (-0.9 and -1.25 V vs SCE). In the former case, the major product is Xyl-H, issued from a radical intermediate Xyl(*) abstracting an H atom from the solvent. Evidence for Xyl(*) is provided by the trapping with TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) and DMPO (5,5'-dimethylpyrroline-N-oxyde). In the second case, only one product is observed, 3,4-di-O-acetyl-5-thioxylal, which is issued from the Xyl(-)() intermediate anion.
“…This compound completes the series for Pd 3 (X) + (Figure 5 and Table 2) where X = Cl [49,50] and I. [21] A comparison of the skeleton (Table 3) indicates a fairly robust core in which the nature of the halide does not perturb the bond distances within the experimental uncertainties.…”
Section: X-ray Structure Of Pd 3 (Br) +mentioning
confidence: 72%
“…The stoichiometric reactivity toward halocarbons of the Pd 3 + and Pd 3 0 species has been investigated for the first Average X = Br X = Cl [49] X = Cl [50] X = I [21] distance [a] (this work) [a] The uncertainties are based on the maximum difference between the average value and experimental data.…”
“…Since the first report on the synthesis and characterization of the Pd 3 (dppm) 3 (CO) 2+ cluster (dppm = bis(diphenylphosphino)methane) in the 1980s, by Puddephatt et al, , and its Pt analogue, numerous and exhaustive works on its reactivity and properties have appeared in the literature. − This cluster exhibits a triangular Pd 3 frame with three dppm-supported M−M single bonds. While one M 3 face is capped by a CO group, the other one is unsaturated, giving rise to rich coordination chemistry. − The dppm-phenyl groups form a cavity above this M 3 plane, limiting access to smaller substrates only. , Of particular interest in this work, thebinding of halide ions onto this Pd 3 2+ species leads to very stable architectures, , and the approximated binding constants spectroscopically evaluated by UV−visible methods indicate that the stability varies as I ≫ Br ≫ Cl. − …”
The reduction mechanism of the title cluster has been investigated by means of cyclic voltammetry (CV), rotating disk electrode (RDE) voltammetry, and coulometry. The 2-electron reduction proceeds via two routes simultaneously. The first one involves two 1-electron reduction steps, followed by an iodide elimination to form the neutral Pd(3)(dppm)(3)(CO)(0) cluster (EEC mechanism). The second one is a 1-electron reduction process, followed by an iodide elimination, then by a second 1-electron step (ECE mechanism) to generate the same final product. Control over these two competitive mechanisms can be achieved by changing temperature, solvent polarity, iodide concentration, or sweep rate. The reoxidation of the Pd(3)(dppm)(3)(CO)(0) cluster in the presence of iodide proceeds via a pure ECE pathway. The overall results were interpreted with a six-member square scheme, and the cyclic and RDE voltammograms were simulated, in order to extract the reaction rate and equilibrium constants for iodide exchange for all three Pd(3)(dppm)(3)(CO)(I)(n)() (n = +1, 0, -1) adducts.
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