Redox reactions in palladium catalysis: on the accelerating and/or inhibiting effects of copper and silver salt additives in cross-coupling chemistry involving electron-rich phosphine ligands

Challenging a catalytic cycle: Pd(0) catalysts are readily oxidized by Cu and Ag salts to give dinuclear Pd(I) complexes and Cu(I) or Ag(I) cubanes (see scheme). The reactivities of the resulting Pd(I) dimers are consistent with several observations of additive effects in cross-coupling chemistry. The results indicate the possibility for alternative catalytic cycles involving dinuclear Pd(I) complexes over the currently accepted synergistic cycles involving Pd(0)/Pd(II) intermediates and Cu or Ag.

Autocatalytic oxidative addition of PhBr to Pd(PtBu3)2 via Pd(PtBu3)2(H)(Br)

We report that oxidative addition of bromobenzene to Pd(PtBu3)2 occurs by an unusual autocatalytic mechanism. Studies on the effect of various additives showed that the degree of rate acceleration followed the trend: (PtBu3)Pd(Ph)(Br) approximately (HPtBu3)Br < [(PtBu3)Pd(mu-Br)]2 < (PtBu3)2Pd(H)(Br). Studies on the reactions of Pd(PtBu3)2 in the presence of (PtBu3)2Pd(H)(Br) showed that the concentration of (PtBu3)2Pd(H)(Br) decreased only after the Pd(0) complex had been consumed. These data indicated that the catalyst in this process is (PtBu3)2Pd(H)(Br). Thermal decomposition of the three-coordinate oxidative addition product (PtBu3)Pd(Ar)(Br) during the reaction of Pd(PtBu3)2 and bromoarenes ultimately leads to formation of (PtBu3)2Pd(H)(Br). Parallel reactions of bromobenzene with (PtBu3)2Pd(H)(Br) and Pd(PtBu3)2 showed that the bromoarenes reacted considerably faster with the Pd(II) species than with the Pd(0) species. We therefore propose a catalytic cycle for oxidative addition in which PBut3.HBr reacts with the Pd(0) species to form (PtBu3)2Pd(H)(Br), and (PtBu3)2Pd(H)(Br) reacts with the bromoarene, possibly though the anionic species [HPtBu3+][(PtBu3)Pd(Br)-], to form [Pd(PtBu3)(Ar)(Br)].

Comparative Bonding Behavior of Functional Cyclopentadienyl Ligands and Boron-Containing Analogues in Heterometallic Complexes and Clusters

The reactivity of isolobal molybdenum carbonylmetalates containing a 2-boratanaphthalene, [Mo(eta5-2,4-MeC9H6BMe)(CO)3]- (5a) and [Mo(eta5-2,4-MeC9H6BNi-Pr2)(CO)3]- (5b), a 1-boratabenzene, [Mo(eta5-3,5-Me2C5H3BNi-Pr2)(CO)3]- (8), or a functionalized cyclopentadienyl ligand, the new metalate [Mo(eta5-C5H4Ph)(CO)3]- (7) and [Mo(eta5-C5H4NMe2)(CO)3]- (9), toward palladium (I and II) or platinum (I and II) complexes, such as trans-[PdCl2(NCPh)2], [Pd2(NCMe)6](BF4)2, trans-[PtCl2(PEt3)2], and [N(n-Bu)4]2 [Pt2Cl4(CO)2], has been investigated, and this has allowed an evaluation of the influence of the pi-bonded ligands on the structures and unprecedented coordination modes observed in the resulting metal-metal-bonded heterometallic clusters. The new 58 CVE planar-triangulated centrosymmetric clusters, [Mo2Pd2(eta5-C5H4Ph)2(CO)6(PEt3)2] (11), [Mo2Pd2(eta5-2,4-MeC9H6BNi-Pr2)2(CO)6] (12), [Mo(2)Pd(2)(eta5-3,5-Me2C5H3BNi-Pr2)2(CO)6] (13), [Mo2Pd2(eta5-C5H4NMe2)2(CO)6(PEt3)2] (15), [Mo2Pt2(eta5-C5H4NMe2)2(CO)6(PEt3)2] (16), and [Mo2Pt2(eta5-C5H4NMe2)2(CO)8] (20), have been characterized by single-crystal X-ray diffraction. Their structural features were compared with those of the 54 CVE cluster [Re2Pd2(eta5-C4H4BPh)2(CO)6)] (4), previously obtained from the borole-containing metalate [Re(eta5-C4H4BPh)(CO)3]- (2), in which a 2e-3c B-C(ipso)-Pd interaction involving the pi-ring was observed. As an extension of what has been observed in 4, clusters 12 and 13 present a direct interaction of the boratanaphthalene (12) and the boratabenzene (13) ligands with palladium. In clusters 11, 15, 16, and 20, the pi-ring does not interact with the palladium (11 and 15) or platinum centers (16 and 20), which confers to these clusters a geometry very similar to that of [Mo2Pd2(eta5-C5H5)2(CO)6(PEt3)2] (3b). The carbonylmetalates [Mo(pi-ring)(CO)3]- are thus best viewed as formal four electron donors which bridge a dinuclear d9-d9 unit. The orientation of this building block in the clusters influences the shape of their metal cores and the bonding mode of the bridging carbonyl ligands. The crystal structure of new centrosymmetric complex [Mo(eta5-C5H4Ph)(CO)3]2 (10) was determined, and it revealed intramolecular contacts of 2.773(4) A between the carbon atoms of carbonyl groups across the metal-metal bond and intermolecular bifurcated interactions between the carbonyl oxygen atoms (2.938(4) and 3.029(4) A), as well as intermolecular C-H...pi(Ar)(C=C) interactions (2.334(3) and 2.786(4) A) involving the phenyl substituents.

Monoligated palladium species as catalysts in cross-coupling reactions

Palladium-mediated cross-coupling reactions are attractive organometallic transformations for the generation of C--C, C--N, C--O, and C--S bonds. Despite being widely employed in small-scale syntheses, cross-coupling reactions have not found important industrial applications because until recently, only reactive aryl bromides and iodides could be used as substrates. These substrates are generally more expensive and less widely available than their chloride counterparts. Over the past few years, new catalytic systems with the ability to activate unreactive and sterically hindered aryl chlorides have been developed. The new catalysts are based on palladium complexes that contain electron-rich and bulky phosphine or carbene ligands. The enhanced reactivity observed with these new systems has been attributed to the formation of unsaturated and reactive [PdL] species which can readily undergo oxidative addition reactions with ArX to yield [Pd(Ar)X(L)].

[(Pd−Pd)@Ge18]4-: A Palladium Dimer Inside the Largest Single-Cage Deltahedron

The largest single-cage deltahedral cluster is made of 18 germanium atoms and is centered by a pair of palladium atoms. Its shape is very close to the ideal D3d symmetry expected and predicted for a cluster of this size.

Scope and mechanism of palladium-catalyzed amination of five-membered heterocyclic halides

Detailed studies have been conducted to determine the activity of palladium catalysts for the amination of five-membered heterocyclic halides and to determine the factors that control the scope of this reaction. Palladium-catalyzed aminations of the electron-rich furanyl, thiophenyl, and indolyl halides and of the related 2-halogenated thiazoles, benzimidazole, and benzoxazole have been shown to occur with a subset of amines. Various combinations of palladium precursors and P(t)Bu(3) were tested as catalysts for reaction of 3-bromothiophene with N-methylaniline, and the fastest reactions occurred with the Pd(I) dimer, [PdBr(P(t)Bu(3))](2). The fastest aminations of thiazoles, benzimidazoles, and benzoxazoles occurred with the combination of palladium trifluoroacetate and P(t)Bu(3) as catalyst.

Preparation, structure and catalytic properties of a binuclear Pd(0) complex with bridging silylene ligands

In contrast to the N-heterocyclic carbene (NHC) 1, the homologous N-heterocyclic silylene (NHS) 4 acts as a bridging ligand to Pd(0), giving rise to the dinuclear complex 5 which is catalytically active in Suzuki reactions.