Suzuki Cross-Coupling Reactions Catalyzed by an Aliphatic Phosphine-Based Pincer Complex of Palladium: Evidence for a Molecular Mechanism

Accessing Ni(0) to Ni(IV) via nickel-carbon-phosphorus bond reorganization

Two-electron oxidation of a NiIIPh(PCP) pincer complex initiates phosphine ligand insertion, generating an η6-arylphosphonium moiety coordinated to NiII. The reaction is fully reversible under reducing conditions. X-ray crystallography, NMR/EPR spectroscopy, electrochemistry, and DFT calculations support the proposed Ni-C-P bond reorganization mechanisms, which access oxidation states from Ni0 to NiIV.

Aqueous Suzuki couplings mediated by a hydrophobic catalyst

The catalytic activity of [(Ph₂P-o-C₆H₄)₂N]PdCl in aerobic aqueous Suzuki couplings is described. Though hydrophobic, this molecular catalyst is competent in cross-coupling reactions of arylboronic acids with a variety of electronically activated, unactivated, and deactivated aryl iodides, bromides, and chlorides upon heating in aqueous solutions under aerobic conditions to give biphenyl derivatives without the necessity of amphiphiles even in the presence of an excess amount of mercury.

Coordination-Induced Weakening of a C(sp3)-H Bond: Homolytic and Heterolytic Bond Strength of a CH-Ni Agostic Interaction

The scission of a C(sp³)-H bond to form a new metal-alkyl bond is a fundamental step in coordination chemistry and catalysis. However, the extent of C-H bond weakening when this moiety interacts with a transition metal is poorly understood and quantifying this phenomenon could provide insights into designing more efficient C-H functionalization catalysts. We present a nickel complex with a robust adamantyl reporter ligand that enables the measurement of C-H acidity (pK_a) and bond dissociation free energy (BDFE) for a C(sp³)-H agostic interaction, showing a decrease in pK_a by dozens of orders of magnitude and BDFE decrease of about 30 kcal/mol upon coordination. X-ray crystallographic data is provided for all molecules, including a distorted square planar Ni^III metalloradical and "doubly agostic" Ni^II(κ²-CH₂) complex.

Facile synthesis of Pd(ii) and Ni(ii) pincer carbene complexes by the double C–H bond activation of a new hexahydropyrimidine-based bis(phosphine): catalysis of C–N couplings

A new hexahydropyrimidine-based NHC proligand undergoes facile double C–H bond activation to give Pd(ii) and Ni(ii) NHC pincer cationic complexes. The Pd complex catalyzes C–N cross couplings very efficiently.

Adamantyl metal complexes: new routes to adamantyl anions and new transmetallations

New routes to 1- and 2-adamantyl anion equivalents are described, starting from commercially available 1- and 2-adamantylzinc bromides and employing reducing metals (Mg; Li). Adamantylmagnesium bromides (both 1-AdMgBr and 2-AdMgBr) can reliably be produced via reaction of the corresponding adamantylzinc bromides with excess magnesium metal. Reactions of adamantylzinc bromides with stoichiometic lithium biphenylide or lithium 2,2'-bipyridylide afford the new diadamantylzinc species, 1-Ad₂Zn and 2-Ad₂Zn, isolable free of solvent and salt impurities. Addition of 2,2'-bipyridine (bipy) leads to the crystalline adducts 1-Ad₂Zn(bipy) and 2-Ad₂Zn(bipy), which were structurally characterized. The resulting adamantyl anions were used in order to generate the first adamantyl complexes of mercury (1- and 2-Ad₂Hg), gold (1- and 2-AdAu(PPh₃), 1- and 2-AdAu(PCy₃)) and bismuth (2-Ad₂BiBr), of which 1- and 2-Ad₂Hg, 2-AdAu(PPh₃), 2-AdAu(PCy₃), and 2-Ad₂BiBr were isolated. These include the first structurally characterized unsupported 2-adamantyl metal complexes.

Development of (quinolinyl)amido-based pincer palladium complexes: a robust and phosphine-free catalyst system for C–H arylation of benzothiazoles

Well-defined (quinolinyl)amido-pincer palladium complexes are developed and employed for the catalytic C–H bond arylation of benzothiazoles with aryl iodides, which can be recycled and reused for several cycles.

The synthesis and efficient one-pot catalytic “self-breeding” of asymmetrical NC(sp3)E-hybridised pincer complexes

Asymmetric NC(sp³)E (E = S, O) pincer complexes were examined as catalysts in asymmetric hydrophosphination.

A new type of oxidative addition of an iodoarene to a Pd(II) complex

Oxidative addition of N-(2-iodophenyl)formamide to Pd(dba)2 (dba = dibenzylideneacetone) in the presence of 4,4'-ditertbutyl-2,2'-bipyridine ((t)Bubpy) produces [Pd(C6H4NHCHO-2)I((t)Bubpy)] (1) which inserts 2-iodophenyl isocyanide to give [Pd{C(=NC6H4I-2)C6H4NHCHO-2}I((t)Bubpy)] (2). Dehydroiodination of 2 with Tl(acac) (acacH = acetylacetone) gives the stable Pd(IV) complex OC-6-35-[Pd{C,N,N-C(=NC6H4-2)C6H4NCHO-2}I((t)Bubpy)] (4) likely resulting from the spontaneous oxidative addition of the I-Ar moiety present in the unstable intermediate Pd(II) complex [Pd{C,N-C(=NC6H4I-2)C6H4NCHO-2}((t)Bubpy)] (3). The crystal structure of 4 shows various C-H···O hydrogen bonds resulting in chains of dimers stacked along the a axis.

Mechanistic Aspects of Aryl-Halide Oxidative Addition, Coordination Chemistry, and Ring-Walking by Palladium

This contribution describes the reactivity of a zero-valent palladium phosphine complex with substrates that contain both an aryl halide moiety and an unsaturated carbon-carbon bond. Although η(2) -coordination of the metal center to a C=C or C≡C unit is kinetically favored, aryl halide bond activation is favored thermodynamically. These quantitative transformations proceed under mild reaction conditions in solution or in the solid state. Kinetic measurements indicate that formation of η(2) -coordination complexes are not nonproductive side-equilibria, but observable (and in several cases even isolated) intermediates en route to aryl halide bond cleavage. At the same time, DFT calculations show that the reaction with palladium may proceed through a dissociation-oxidative addition mechanism rather than through a haptotropic intramolecular process (i.e., ring walking). Furthermore, the transition state involves coordination of a third phosphine to the palladium center, which is lost during the oxidative addition as the C-halide bond is being broken. Interestingly, selective activation of aryl halides has been demonstrated by adding reactive aryl halides to the η(2) -coordination complexes. The product distribution can be controlled by the concentration of the reactants and/or the presence of excess phosphine.

Design and development of POCN-pincer palladium catalysts for C-H bond arylation of azoles with aryl iodides

Well-defined and efficient POCN-ligated palladium complexes have been developed for the direct C-H bond arylation of azoles with aryl iodides. The phosphinite-amine pincer ligands 1-(R2PO)-C6H4-3-(CH2N(i)Pr2) [(R2)POCN(iPr2)-H; R = (i)Pr (), R = (t)Bu ()] and corresponding palladium complexes {2-(R2PO)-C6H3-6-(CH2N(i)Pr2)}PdCl [((R2)POCN(iPr2))PdCl; R = (i)Pr (), R = (t)Bu ()] were synthesized in good yields. Treatment of palladium complex with KI and AgOAc afforded the complexes ((iPr2)POCN(iPr2))PdI () and ((iPr2)POCN(iPr2))Pd(OAc) (), respectively. Similarly, the reaction of with benzothiazolyl-lithium produces the ((iPr2)POCN(iPr2))Pd(benzothiazolyl) () complex in a quantitative yield. The pincer palladium complex efficiently catalyzes the C-H bond arylation of benzothiazole, substituted-benzoxazoles and 5-aryl oxazoles with diverse aryl iodides in the presence of CuI as a co-catalyst under mild reaction conditions. This represents the first example of a pincer palladium complex being applied for the direct C-H bond arylation of any heterocycle with low catalyst loading. A preliminary mechanistic investigation reveals that palladium nanoparticles are presumably not the catalytically active form of and supports the direct involvement of the catalyst , with complex being a probable key intermediate in the catalytic reaction.

Mono- and binuclear palladacycles via regioselective C–H bond activation: syntheses, mechanistic insights and catalytic activity in direct arylation of azoles

Hybrid “POCN”-ligated mono- and binuclear palladacycles have been synthesized via the base-assisted regioselective C–H bond activation, and their mechanistic aspects and catalytic application for the arylation of azoles have been described.

Synthesis and characterization of a family of POCOP pincer complexes with nickel: reactivity towards CO2 and phenylacetylene

A cyclohexyl-based POC sp 3OP pincer ligand (POC sp 3OP=cis-1,3-bis(di-tert-butylphosphinito)cyclohexyl) cyclometalates with nickel to generate a series of new POC sp 3OP-supported Ni(II) complexes, including the halide, hydride, methyl, and phenyl species. trans-[NiCl{cis-1,3-bis(di-tert-butylphosphinito)cyclohexane}], [(POC sp 3OP)NiCl] (1 a) and the analogous bromide complex (1 b) were synthesized and fully characterized by NMR spectroscopy and X-ray crystallography. Cyclic voltammetry measurements of 1 a and 1 b alongside their bis(phosphine) analogues [(PC sp 3P)NiCl] (2 a) and [(PC sp 3P)NiCl] (2 a) (PC sp 3P=cis-1,3-bis(di-tert-butylphosphino)cyclohexyl) indicate a reduced electron density at the metal center upon introducing electron-withdrawing oxygen atoms in the pincer arms. The methyl [(POC sp 3OP)NiMe] (3) and phenyl [(POC sp 3OP)NiPh] (4) complexes were formed from 1 a by reaction with the corresponding organolithium reagents. 1 a also reacts with LiAlH4 to give the hydride complex [(POC sp 3OP)NiH] (5). The methyl complex 3 reacts with phenyl acetylene to give the acetylide complex [(POC sp 3OP)NiCCPh] (6). The reactivity of compounds 3-5 towards CO2 was studied. The hydride complex 5 and the methyl complex 3 both underwent CO2 insertion to form the formate species [(POC sp 3OP)NiOCOH] (7) and acetate species [(POC sp 3OP)NiOCOCH3 ] (8), respectively, although with a higher barrier of insertion in the latter case. Compound 4 was unreactive towards CO2 even at elevated temperatures. Complexes 3-8 were all characterized by NMR spectroscopy and X-ray crystallography.

Stepwise cyclopalladation of 2-phenacylthiopyridine to give C,C,N-pincer complexes

The synthesis of a family of palladium C,C,N-pincer complexes derived from 2-phenacylthiopyridine, obtained by an unprecedented stepwise double cyclopalladation process, is reported. The pincer moiety behaves as a metallaligand.

Novel access to carbodiphosphoranes in the coordination sphere of group 10 metals: template synthesis and protonation of PCP pincer carbodiphosphorane complexes of C(dppm)2

In a novel template synthesis of carbodiphosphoranes (CDPs), the phosphine functionalized CDP ligand C(dppm)(2) (dppm = Ph(2)PCH(2)PPh(2)) is formed in the coordination sphere of group 10 metals from CS(2) and 4 equivalents of dppm. The products are the PCP pincer complexes [M(Cl)(C(dppm)(2)-κ3P,C,P)]Cl (M = Ni, Pd, Pt) and 2 equivalents of dppmS. The compound C(dppm)(2), which is composed of a divalent carbon atom and two dppm subunits, represents a new PCP-type pincer ligand with the formally neutral carbon Lewis base of the CDP functionality as the central carbon. Treatment of [M(Cl)(C(dppm)(2)-κ3P,C,P)]Cl (M = Pd, Pt) with hydrochloric acid results in protonation at the CDP carbon atom and the formation of the PCP pincer complexes [M(Cl)(CH(dppm)(2)-κ3P,C,P)]Cl(2) (M = Pd, Pt). The PCP pincer ligand [CH(dppm)(2)](+) involves a formally cationic central carbon donor. The reaction of [Ni(Cl)(C(dppm)(2)-κ3P,C,P)]Cl with HCl leads to the extrusion of NiCl(2) and formation of the diprotonated CDP compound [CH(2)(dppm)(2)]Cl(2), from which the monoprotonated conjugate base [CH(dppm)(2)]Cl is obtained upon addition of bases, such as NH(3). The crystal structures of [M(Cl)(C(dppm)(2)-κ3P,C,P)]Cl (M = Ni, Pd, Pt), [Ni(Cl)(C(dppm)(2)-κ3P,C,P)](2)[NiCl(4)], [M(Cl)(CH(dppm)(2)-κ3P,C,P)]Cl(2) (M = Pd, Pt) as well as [CH(2)(dppm)(2)]Cl(2) and [CH(dppm)(2)]Cl are presented. A comparison of the solid state structures reveals interesting features, e.g. infinite supramolecular networks mediated by C-H···Cl hydrogen bond interactions and an unexpected loss of molecular symmetry upon protonation in the complexes [M(CH(dppm)(2)-κ3P,C,P)(Cl)]Cl(2) (M = Pd, Pt) as a result of the flexible ligand backbone. Additionally the new compounds were characterized comprehensively in solution by multinuclear (31)P, (13)C and (1)H NMR spectroscopy: Several spectroscopic parameters show a striking variability in particular regarding the carbodiphosphorane functionality. Furthermore the compound [Ni(Cl)(C(dppm)(2)-κ3P,C,P)]Cl was examined by cyclic voltammetry (CV) and could be shown to display quasi-reversible oxidative as well as reductive behaviour.