Pincer‐Type Heck Catalysts and Mechanisms Based on Pd^IV Intermediates: A Computational Study

Abstract

Pincer-type palladium complexes are among the most active Heck catalysts. Due to their exceptionally high thermal stability and the fact that they contain Pd(II) centers, controversial Pd(II)/Pd(IV) cycles have been often proposed as potential catalytic mechanisms. However, pincer-type Pd(IV) intermediates have never been experimentally observed, and computational studies to support the proposed Pd(II)/Pd(IV) mechanisms with pincer-type catalysts have never been carried out. In this computational study the feasibility of potential catalytic cycles involving Pd(IV) intermediates was explored. Density functional calculations were performed on experimentally applied aminophosphine-, phosphine-, and phosphite-based pincer-type Heck catalysts with styrene and phenyl bromide as substrates and (E)-stilbene as coupling product. The potential-energy surfaces were calculated in dimethylformamide (DMF) as solvent and demonstrate that Pd(II)/Pd(IV) mechanisms are thermally accessible and thus a true alternative to formation of palladium nanoparticles. Initial reaction steps of the lowest energy path of the catalytic cycle of the Heck reaction include dissociation of the chloride ligands from the neutral pincer complexes [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Cl)] [X=NH, R=piperidinyl (1 a); X=O, R=piperidinyl (1 b); X=O, R=iPr (1 c); X=CH(2), R=iPr (1 d)] to yield cationic, three-coordinate, T-shaped 14e(-) palladium intermediates of type [{2,6-C(6)H(3)(XPR(2))(2)}Pd](+) (2). An alternative reaction path to generate complexes of type 2 (relevant for electron-poor pincer complexes) includes initial coordination of styrene to 1 to yield styrene adducts [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Cl)(CH(2)=CHPh)] (4) and consecutive dissociation of the chloride ligand to yield cationic square-planar styrene complexes [{2,6-C(6)H(3)(XPR(2))(2)}Pd(CH(2)=CHPh)](+) (6) and styrene. Cationic styrene adducts of type 6 were additionally found to be the resting states of the catalytic reaction. However, oxidative addition of phenyl bromide to 2 result in pentacoordinate Pd(IV) complexes of type [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Br)(C(6)H(5))](+) (11), which subsequently coordinate styrene (in trans position relative to the phenyl unit of the pincer cores) to yield hexacoordinate phenyl styrene complexes [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Br)(C(6)H(5))(CH(2)=CHPh)](+) (12). Migration of the phenyl ligand to the olefinic bond gives cationic, pentacoordinate phenylethenyl complexes [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Br)(CHPhCH(2)Ph)](+) (13). Subsequent beta-hydride elimination induces direct HBr liberation to yield cationic, square-planar (E)-stilbene complexes with general formula [{2,6-C(6)H(3)(XPR(2))(2)}Pd(CHPh=CHPh)](+) (14). Subsequent liberation of (E)-stilbene closes the catalytic cycle.

Spin-labelled cyclometallated palladium complexes. EPR study of dynamic processes in coordination sphere

“…Most probably, initial organohalogenides in polar media are partly dissociated [4], and formed chloride anion readily reacts with thallium o-semiquinonate forming uncoordinated anion-radical (Scheme 6). Nonpolar solvents suppress the dissociation, so thallium semiquinonate reacts directly with palladium organohalogenide forming desired product (Scheme 6).…”

“…The formation of four-coordinated complexes with bidentate bonded pincer ligand is proposed as initial step for one of the pathways of Heck reaction including Pd II /Pd IV mechanism [4]. The fact of formation of similar compounds 3a-b means that such pathway is really possible.…”

“…From the other point of view, the unusual Pd II /Pd IV mechanism is often proposed for Heck reactions [2,3] and even DFT calculations of possible catalytic pathways have been made recently [4]. Moreover the oxidative addition of aryl iodides to pincer-like complexes forming stable Pd IV species was recently confirmed experimentally [5].…”

Dinuclear macrocyclic palladium complexes having pincer coordinating groups and their catalytic properties in Mizoroki–Heck reactions

“…Some papers pointed out that colloidal palladium(0) particles released from the pincer complexes at elevated temperature act as real catalyst [6,11,12]. Meanwhile, DFT calculations show that Pd(II)ePd(IV) cycle without decomposing the complexes is a viable mechanistic alternative to a palladium nanoparticle catalyzed reaction [13]. We have no information on the reaction mechanism of the catalysis mediated by our pincer dinuclear complexes; however it should be pointed out that decomposition of pincer complexes under the applied reaction conditions starts with the coordination of amine to palladium by opening one of the side arms [14].…”

Mizoroki–Heck Reactions Catalyzed by Dichloro{bis[1‐(dicyclohexylphosphanyl)piperidine]}palladium: Palladium Nanoparticle Formation Promoted by (Water‐Induced) Ligand Degradation

“…In the case of pincer‐type complexes, which have been introduced in the late 1990s as Heck catalysts,5–7 nanoparticles are generally considered to be their catalytically active form, even though the often proposed controversial Pd(II)/Pd(IV) cycles still cannot be excluded completely as being operative 8. In contrast, recent experimental and computational investigations indicated that pincer‐type Pd(IV) intermediates are thermally accessible and hence, are to be considered as reactive intermediates in reactions performed with aryl halides in polar, non‐protic solvents and at elevated temperatures 9,10…”