1,721,020 research outputs found
Measuring the Olefin-to-Pd(0) Bond Strength: A Kinetic Study Involving Olefin Exchange Reactions on Palladium(0) Complexes Bearing Isocyanide Ligands
No full textIn this article, the synthesis and characterization of new palladium(η2-olefin) complexes bearing isocyanides as spectator ligands are described. These derivatives have been tested on olefin exchange reactions, allowing to define a scale of strength of the palladium(0)-olefin bond. In this way we have proved that dimethyl fumarate can be released by the metal center much more easily than the other olefins and that (Z)-1,2-ditosylethene is by far the most firmly anchored to palladium. When the sterically encumbered tetramethylethene-1,1,2,2-tetracarboxylate (tmetc) is involved in the olefin exchange, the reaction kinetics can be monitored by UV-Vis spectrophotometry, showing that the process follows a second-order rate law. This response indicates that the reaction mechanism is associative, as is also confirmed by a DFT study
The unexpected case of reactions of halogens and interhalogens with halide substituted Pd(II) σ-butadienyl complexes
No full textWe have experimentally studied and theoretically interpreted the addition under stoichiometric conditions of halogens or interhalogens to σ-butadienyl palladium complexes bearing the heteroditopic thioquinolines as spectator ligands. The observed reactions do not involve the expected extrusion of the butadienyl fragment but rather the unpredictable substitution of the halide coordinated to palladium and in some cases also of that bound to the terminal butadienyl carbon. We have explained this peculiar reactivity with a mechanistic hypothesis based on a sequence of selective processes of oxidative addition and reductive elimination involving Pd(iv) intermediates
Synthesis of novel olefin complexes of palladium(0) bearing monodentate NHC, phosphine and isocyanide spectator ligands
No full textWe have synthesized and characterized seventeen new bis-NHC, mixed NHC–phosphines or NHC–isocyanides Pd(0) olefin complexes that can potentially act as catalysts. The complexes were characterized by standard spectroscopic methods and elemental analysis and in two cases by SC-XRD technique. We have analyzed with particular care the thermodynamic and kinetic conditions governing the one-pot synthesis of the mixed complexes. In this respect we tried to validate our results by a dedicated computational study on the mutual distribution of the isomers that could be potentially formed. However, the computational result is not clear-cut owing to the not significant value of the calculated ΔG0. Finally, in one case we have measured the rate of the exchange reaction between not particularly encumbered olefins
Chemoselective oxidative addition of vinyl sulfones mediated by palladium complexes bearing picolyl-N-heterocyclic carbene ligands
No full textThe manifold interactions of (E)- or (Z)-1,2-ditosylethene with a palladium(0) centre bearing picolyl-NHC carbene ligands have been studied thoroughly. (E)-1,2-Ditosylethene produces the expected and stable η2-olefin palladium complexes, whereas the coordination of theZderivative alternatively promotes the isomerization of the olefin itself or an oxidative addition process depending on the steric bulkiness of carbene substituents and/or the adopted synthetic procedure. Remarkably, the oxidative addition pathway involves a selectiveS-vinyl (notS-aryl) breaking and produces selectively the S- rather than O-coordinated sulfinate. A mechanistic study has clarified the reasons of the chemoselectivity of the process, which was proved to be kinetically controlled. All the involved species have been isolated and exhaustively characterized. In particular, we report the first example of the X-ray crystal structure of a complex bearing one vinyl and one S-sulfinate fragment coordinated to palladium
Reactivity of N-heterocyclic carbene-pyridine palladacyclopentadiene complexes toward halogen addition. the unpredictable course of the reaction
No full textAs an extension of a previously published work we have reacted some palladacyclopentadiene complexes stabilized by bidentate N-heterocyclic carbene-pyridine or monodentate N-heterocyclic carbene-pyridine and isocyanide ligands with the halogens I2 and Br2. All the bidentate and monodentate complexes react with halogens to give at first the expected σ-coordinated butadienyl fragment. However, two of the less hindered NHC carbene-pyridine bidentate butadienyl iodo derivatives undergo a further rearrangement and novel Pd(ii) complexes characterized by a ten term coordinative ring were isolated and characterized. In the most favorable case we were able to carry out the kinetics of rearrangement and measure its reaction rate. Moreover, we have surmised a plausible mechanism on the basis of a dedicated computational approach and in one case the surprising structure characterized by the ten term coordinative ring was resolved by X-ray diffraction
The addition of halogens and interhalogens on palladacyclopentadienyl complexes bearing quinolyl-thioether as spectator ligands. A kinetic and computational study
No full textWe have studied the oxidative addition of halogens (I2 and Br2) and interhalogens (ICl and IBr) on complexes of the type [Pd(thioquinoline)C4(COOMe)4], (thioquinoline = 8-(methylthio)quinoline, 8-(t-butylthioquinoline, 2-methyl-8-(methylthio)quinoline, 2-methyl-8-(t-butylthio)quinoline). The expected palladium(thioquinoline)-σ-butadienyl derivatives have been obtained by the stoichiometric addition of I2 and Br2 to a solution of the starting palladacyclopentadienyl complexes. The bromine in excess induces the extrusion of the di-bromo-(E, E)-σ-butadiene and the formation of the thioquinoline palladium(II) di-bromide species. The kinetics and mechanism of these reactions have been determined. Except for one case which was analyzed in detail by a computational study, the oxidative addition of the interhalogens ICl and IBr yields the species that is less predictable from the thermodynamic point of view. In general the computational approach justifies the reaction progress and allows an interpretative clue suggesting a kinetically governed path to the reaction products. Finally, the solid state structures of two reaction products were resolved and reported
Substitution reactions between bis-chelate ligands in palladium(II) alkenyl complexes: an unusual way to form unstable trans-P complexes. A study on the isomerization mechanism
No full textThe substitution reactions between asymmetric bis-chelate ligands and alkenyl chloro derivatives of palladium(II) of the type [Pd(L-L')(Rx)Cl] (L-L' = 2-phenylsulfanylmethyl-pyridine (HN-SPh), 2-methyl-6-phenylsulfanylmethyl-pyridine (MeN-SPh), 2,2'-bipyridinyl (BiPy), Rx = -CCOOMe=CMeCOOMe (Ra), -CCOOEt=CMeCOOEt (Rb), -CCOOt-Bu=CMeCOOt-Bu (Rc), -(CCOOMe=CCOOMe)(2)Me (Rd)) with phosphoquinoline moieties (8-diphenylphosphanyl-quinoline (DPPQ), 8-diphenylphosphanyl-2-methyl-quinoline (DPPQ-Me)) usually leads to the formation of the stable geometrical isomer bearing these groups in the cis position thanks to the mutual trans influence of the alkenyl and phosphine groups. However, when the leaving group MeN-SPh and the entering ligand DPPQ are involved, the fast and quantitative substitution reaction leads to the formation of a couple of geometrical isomers [Pd(DPPQ)(Rx)Cl]-trans P and [Pd(DPPQ)(Rx)Cl]-cis P (Rx = Ra, Rb, Rc, Rd) in which the alkenyl and the phosphine groups are in mutual trans or cis position. The substrate [Pd(DPPQ)(Rx)Cl]-trans P (Rx = Ra, Rb, Rc) slowly interconverts into its thermodynamically stable -cis P counterpart while the bulky [Pd(DPPQ)(Rd)Cl]-trans P displays no tendency to isomerize, thereby allowing separation of the two geometrical forms. Also, the ligand DPPQ-Me induces the formation of the -trans P geometrical isomer which is only detectable at low temperature since it rapidly interconverts into the -cis P derivative at RT. The kinetics of the interconversion process, a reasonable explanation of the observed phenomenon based on theoretical calculations, and eventually an unequivocal structure determination of the stable [Pd(DPPQ)(Rx)Cl]-cis P substrate are reported in the present paper
Synthesis and characterization of palladacyclopentadiene complexes with N-heterocyclic carbene ligands
No full textNew palladacyclopentadiene compounds containing different chelate NHC-thioether and NHC-pyridine ligands have been prepared by transfer of the functionalized carbenes from the respective silver complexes to the polymeric precursors [PdC-COOR)4]n (R = Me, t-Bu). Their dynamic behaviour in solution was discussed and the solid-structure of 2c was determined by X-ray crystallography. The treatment of [Pd(C-COOCH3)4]n with two equivalents of the carbene silver complexes led to the (NHC)2Pd(C4-COOCH3)4 derivatives (3c-i), a new class of compounds with only Pd-C bonds. A serious limitation to this synthetic procedure is an excessive steric crowding around the metal centre. The complexes 3 are present in solution as a mixture of two atropoisomers, due to restricted rotation around the Carbene-Pd bond. The kinetics of equilibration between the two configurational isomers was studied for complex 3c, which was also structurally defined by X-ray crystallography (anti isomer). Finally a synthetic protocol was set up for the synthesis of mixed NHC-Phosphine and NHC-Isocyanide palladacyclopentadiene complexes. In this procedure the order of addition of the reactants is of great importance
Synthesis and characterization of novel olefin complexes of palladium(0) with chelating bis(N-heterocyclic carbenes) as spectator ligands
Full text linkWe have synthesized several novel palladium(0) olefin complexes stabilized by strong σ-donating bis-chelating carbene ligands characterized by one or two CH2spacers and electron-withdrawing olefins. Although it appears obvious that the σ-donating carbenes and electron-withdrawing olefins should cooperate in the stabilization of the ensuing complexes, the limit of their coexistence was not hitherto clear. On the basis of previously measured stabilizing capability of the olefins toward Pd(0) complexes we were able to synthesize ten complexes (nine new and one synthesized by a different protocol from that of the literature). The less electron-withdrawing olefin capable of stabilizing the complex was dimethylfumarate. However, the most interesting results were obtained with the (Z)-1,2-bis(p-tolylsulfonyl)ethene (cis-sulf) which instantly isomerizes upon coordination and in the case of the derivative bearing the olefin tetramethyl ethene-1,1,2,2-tetracarboxylate (tmetc) whose decomposition in CD2Cl2yields the saturated tetramethyl ethane-1,1,2,2-tetracarboxylate (D2). The solid-state structure of the complex 4d bearing the bis-carbene 1,1′-dibenzyl-3,3′methylenediimidazol-2,2′-diylidene and the olefin (E)-1,2-bis(p-tolylsulfonyl)ethene (trans-sulf) was also determined
Insertion of isocyanides across the Pd-C bond in alkyl or aryl palladium(II) complexes bearing mixed nitrogen-sulfur and nitrogen-phosphorus ancillary ligands. The mechanism of reaction
No full textAn exhaustive study dealing with the kinetic and mechanistic behavior of alkyl- and arylpalladium complexes bearing pyridyl-thioethers (NS-R) and quinoline-phosphines (NP) as ancillary ligands when reacting with 2,6-dimethyl isocyanide (DIC) and tosylmethyl isocyanide (TosMIC) was undertaken. In these reactions some differently substituted isocyanides insert into the palladium -carbon bond of alkyl and aryl complexes bearing mixed (NS or NP) ligands. The reactions were carried out under equimolecular conditions since such a restrictive approach allows the determination of the rate constants related to the isocyanide insertion attack. Reactions carried out under nonstoichiometric conditions were also taken into account and the reaction products characterized. Usually the formation of an inserted bis-substituted isocyanide halide derivative of palladium(II) was observed. In a particular case the formation of an imidoyl dimer was detected. The structures of the monoinserted [Pd(NSt-Bu)((C(Tol)=NR2)I] (NSt-Bu = 2-(tertbutylthiomethyl)pyridine) and of the dimer [Pd(CNR 2) (C(=NR2)Me)C( (R-2 = 2,6-Me2C6H3) were reported
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