Synthesis and Properties of NHC-Supported Palladium(I) Dimers with Bridging Allyl, Cyclopentadienyl, and Indenyl Ligands

The anticancer activity of an air-stable Pd(I)-NHC (NHC = N-heterocyclic carbene) dimer

A new dinuclear Pd(i) complex coordinating two bis(NHC) ligands revealed an unsuspected stability despite the unsaturation of the two metal centres. Even more surprisingly, the compound showed high and selective antiproliferative activity against different cancer cell lines and ovarian cancer tumoroids, and the mechanism of action was different from that of cisplatin.

P-Alkynyl functionalized benzazaphospholes as transmetalating agents

Exposure of 10π-electron benzazaphosphole 1 to HCl, followed by nucleophilic substitution with the Grignard reagent BrMgCCPh afforded alkynyl functionalized 3 featuring an exocyclic -C[triple bond, length as m-dash]C-Ph group with an elongated P-C bond (1.7932(19) Å). Stoichiometric experiments revealed that treatment of trans-Pd(PEt3)2(Ar)(i) (Ar = p-Me (C) or p-F (D)) with 3 generated trans-Pd(PEt3)2(Ar)(CCPh) (Ar = p-Me (E) or p-F (F)), 5, which is the result of ligand exchange between P-I byproduct 4 and C/D, and the reductively eliminated product (Ar-C[triple bond, length as m-dash]C-Ph). Cyclic voltammetry studies showed and independent investigations confirmed 4 is also susceptible to redox processes including bimetallic oxidative addition to Pd(0) to give Pd(i) dimer 6-Pd2-(P(t-Bu)3)2 and reduction to diphosphine 7. During catalysis, we hypothesized that this unwanted reactivity could be circumvented by employing a source of fluoride as an additive. This was demonstrated by conducting a Sonogashira-type reaction between 1-iodotoluene and 3 in the presence of 10 mol% Na2PdCl4, 20 mol% P(t-Bu)Cy2, and 5 equiv. of tetramethylammonium fluoride (TMAF), resulting in turnover and the isolation of Ph-C[triple bond, length as m-dash]C-(o-Tol) as the major product.

NHC Pd(II) and Ag(I) Complexes: Synthesis, Structure, and Catalytic Activity in Three Types of C-C Coupling Reactions

Four bis-benzimidazolium salts, 1,4-bis[1'-(N-R-benzimidazoliumyl)methyl]-2,3,5,6-tetramethylbenzene 2X- (L 1 H 2 ·(PF 6 ) 2 : R = ethyl, X = PF6; L 2 H 2 ·Br 2 : R = picolyl, X = Br; L 3 H 2 ·Br 2 : R = benzyl, X = Br; and L 4 H 2 ·Br 2 : R = allyl, X = Br), and their three N-heterocyclic carbene (NHC) Pd(II) and Ag(I) complexes, L 1 Pd 2 Cl 4 (1), L 2 Ag 2 Br 2 (2), and L 4 (AgBr) 2 (3), as well as one anionic complex L 3 H 2 ·(Ag 4 Br 8 ) 0.5 (4), have been synthesized and characterized. Complex 1 adopts a funnel-like type of structure, complex 2 adopts a cyclic structure, and complex 3 is an open structure. In the crystal packing of 1-4, one-dimensional polymeric chains and two-dimensional supramolecular layers are formed via intermolecular weak interactions, including hydrogen bonds, π-π interactions, and C-H···π contacts. The catalytic activities of NHC Pd(II) complex 1 in three types of C-C coupling reactions (Suzuki-Miyaura, Heck-Mizoroki, and Sonogashira reactions) were studied. The results show that this catalytic system is efficient for these C-C coupling reactions.

Dinuclear oxidative addition reactions using an isostructural series of Ni2, Co2, and Fe2 complexes

Dinuclear oxidative additions at metal–metal bonds are facilitated by redox-active supporting ligands.

Dinuclear Pd(I) complexes with bridging allyl and related ligands

There are many important synthetic methods that utilize palladium catalysts. In most of these reactions, the palladium species are proposed to exist exclusively in either the Pd(0) or Pd(II) oxidation states. However, in the last decade, dinuclear Pd(I) complexes have repeatedly been isolated from reaction mixtures previously suggested to involve only species in the Pd(0) and Pd(II) oxidation states. As a consequence, in order to design improved catalysts there is considerable interest in understanding the chemistry of dinuclear Pd(I) complexes. A significant proportion of the known dinuclear Pd(I) complexes are supported by bridging allyl or related ligands such as cyclopentadienyl or indenyl ligands. This review provides a detailed account of the synthesis, electronic structure and stoichiometric reactivity of dinuclear Pd(I) complexes with bridging allyl and related ligands. Additionally, it describes recent work where dinuclear Pd(I) complexes with bridging allyl ligands have been detected in catalytic reactions, such as cross-coupling, and discusses the potential implications for catalysis.

Comparison of the catalytic activity for the Suzuki-Miyaura reaction of (η(5)-Cp)Pd(IPr)Cl with (η(3)-cinnamyl)Pd(IPr)(Cl) and (η(3)-1-t-Bu-indenyl)Pd(IPr)(Cl)

Complexes of the type (η(3)-allyl)Pd(L)(Cl) and (η(3)-indenyl)Pd(L)(Cl) are highly active precatalysts for the Suzuki-Miyaura reaction. Even though allyl and indenyl ligands are similar to cyclopentadienyl (Cp) ligands, there have been no detailed comparative studies exploring the activity of precatalysts of the type (η(5)-Cp)Pd(L)(Cl) for Suzuki-Miyaura reactions. Here, we compare the catalytic activity of (η(5)-Cp)Pd(IPr)(Cl) (IPr = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene, Cp) with two commercially available catalysts (η(3)-cinnamyl)Pd(IPr)(Cl) (Cin) and (η(3)-1-t-Bu-indenyl)Pd(IPr)(Cl) ( (tBu) Ind). We show that Cp gives slightly better catalytic activity than Cin, but significantly inferior activity than (tBu) Ind. This order of activity is rationalized by comparing the rates at which the precatalysts are activated to the monoligated Pd(0) active species along with the tendency of the starting precatalysts to comproportionate with monoligated Pd(0) to form inactive Pd(I) dimers. As part of this work the Cp supported Pd(I) dimer (μ-Cp)(μ-Cl)Pd2(IPr)2 (Cp (Dim) ) was synthesized and crystallographically characterized. It does not readily disproportionate to form monoligated Pd(0) and consequently Cp (Dim) is a poor catalyst for the Suzuki-Miyaura reaction.

Electrocatalytic reduction of CO2 with palladium bis-N-heterocyclic carbene pincer complexes

A series of pyridine- and lutidine-linked bis-N-heterocyclic carbene (NHC) palladium pincer complexes were electrochemically characterized and screened for CO2 reduction capability with 2,2,2-trifluoroethanol, acetic acid, or 2,2,2-trifluoroacetic acid (TFA) as proton sources. The lutidine-linked pincer complexes electrocatalytically reduce CO2 to CO at potentials as low as -1.6 V versus Ag/AgNO3 in the presence of TFA. The one-electron reduction of these complexes is shown to be chemically reversible, yielding a monometallic species, with density functional theory studies indicating charge storage on the redox-active ligand, thus addressing a major source of deactivation in earlier triphosphine electrocatalysts.