Anionic Pd Research Articles

Salt-Enhanced Oxidative Addition of Iodobenzene to Pd: An Interplay Between Cation, Anion, and Pd–Pd Cooperative Effects

Halide salts facilitate the oxidative addition of organic halides to Pd(0). This phenomenon originates from a combination of anionic, cationic, and Pd-Pd cooperative effects. Exhaustive computational exploration at the density functional theory level of the complexes obtained from [Pd0(PPh3)2] and a salt (NMe4Cl or LiCl) showed that chlorides promote phosphine release, leading to a mixture of mononuclear and dinuclear Pd(0) complexes. Anionic Pd(0) dinuclear complexes exhibit a cooperativity between Pd(0) centers, which favors the oxidative addition of iodobenzene. The higher activity of Pd(0) dimers toward oxidative addition rationalizes the previously reported kinetic laws. In the presence of Li+, the oxidative addition to mononuclear [Pd0L(Li2Cl2)] is estimated barrierless. LiCl coordination polarizes Pd(0), enlarging both the electrophilicity and the nucleophilicity of the complex, which promotes both coordination of the substrate and the subsequent insertion into the C-I bond. These conclusions are paving the way to the rational use of the salt effects in catalysis for the activation of more challenging bonds.

One-pot Sonogashira–Hydroarylation reaction catalyzed by anionic palladium complexes in an aqueous medium

It was found that anionic Pd(II) complexes of type [CA]2[PdCl4] and [CA]2[Pd2Cl6] (CA = imidazolium or pyridinium cation) are effective catalysts for copper-free Sonogashira coupling in an aqueous medium without additional ligands or co-catalysts. Both types of substrates, containing electron donor or electron acceptor groups, formed the corresponding products in good to excellent yield. The most active complex, [bmpy]2[Pd2Cl6] (bmpy = 1-butyl-4-methylpyridinium cation), was employed to the hydroarylation of the Sonogashira product in a one-pot process, using microwave radiation, NaBPh4 as a phenyl source and water as a proton donor. In these reactions the presence of electron donor substituents facilitated a higher conversion of diarylacetylenes to triarylethenes than electron acceptor groups.

Experimental evidence for the direct involvement of Pd(0) and Pd(II) anionic phosphine complexes in the Mizoroki–Heck coupling reaction

The role of ionic Pd species in the catalytic systems used for cross-coupling reactions has been revised in recent years, mainly utilizing in situ analytical techniques to detect catalyst species in the reaction mixture. The possible inclusion of phosphine molecules in the active Pd complexes is still debated. We have carried out a comparative study on the differential selectivity of the Mizoroki–Heck reaction with competition between two aryl halides or two alkenes to elucidate the active Pd species in different types of catalytic systems, including those favoring the formation of ionic complexes. The sensitivity of the reaction differential selectivity of two competing aryl halides to the additives of tertiary phosphine and halide salts points to the catalytic activity of anionic Pd(0) complexes containing at least one phosphine ligand when the phosphine-containing catalytic systems are used. The changes of the reaction differential selectivity of two competing alkenes to phosphine and halide salt allow to propose the entrance of both phosphine and halide ions into the coordination sphere of active Pd(II) complexes possessing anionic character. In addition, evidence for the effect of phosphine ligand and salt additives on the composition of the active Pd complexes that determine the regioselectivity of the reaction is presented. The data obtained indicate the decisive role of anionic Pd species in the Mizoroki–Heck reaction using both phosphine-free and phosphine-containing catalytic systems.

Three-Coordinate Pd(0) with Rare-Earth Metalloligands: Synergetic CO Activation and Double P-C Bond Cleavage-Formation Reactions.

Metalation of β-diketiminato rare-earth metal complexes LnacnacLn(PhNCH2PPh2)2 (Ln = Y, Yb, Lu) with (COD)Pd(CH2SiMe3)2 afforded three-coordinate Pd(0) complexes supported by two sterically less bulky phosphines and a Pd → Ln dative interaction. The Pd(0) center is prone to ligation with isonitrile and CO; in the latter case, the insertion of a second CO with the Y-N bond was assisted via a precoordination of CO on the Pd(0) center, which led to the formation of an anionic Pd(0) carbamoyl. The reaction of the Pd-Y complex with iodobenzene showed a remarkable double P-C bond cleavage-formation pathway within the heterobimetallic Pd-Y core to afford (Ph3P)2PdI(Ph), imine PhNCH2, and a β-diketiminato yttrium diiodide. In the related reaction of LnacnacY(PhNCH2PPh2)2 with (Ph3P)2PdI(Ph), the P-C bond cleavage following with a N-C bond formation was observed. Computational studies revealed a synergetic bimetallic mechanism for these reactions.

Ionic Pd/NHC Catalytic System Enables Recoverable Homogeneous Catalysis: Mechanistic Study and Application in the Mizoroki-Heck Reaction.

N-Heterocyclic carbene (NHC) ligands are ubiquitously utilized in catalysis. A common catalyst design model assumes strong M-NHC binding in this metal-ligand framework. In contrast to this common assumption, we demonstrate here that lability and controlled cleavage of the M-NHC bond (rather than its stabilization) could be more important for high-performance catalysis at low catalyst concentrations. The present study reveals a dynamic stabilization mechanism with labile metal-NHC binding and [PdX3 ]- [NHC-R]+ ion pair formation. Access to reactive anionic palladium intermediates formed by dissociation of the NHC ligands and plausible stabilization of the molecular catalyst in solution by interaction with the [NHC-R]+ azolium ion is of particular importance for an efficient and recyclable catalyst. These ionic Pd/NHC complexes allowed for the first time the recycling of the complex in a well-defined form with isolation at each cycle. Computational investigation of the reaction mechanism confirms a facile formation of NHC-free anionic Pd in polar media through either Ph-NHC coupling or reversible H-NHC coupling. The present study formulates novel ideas for M/NHC catalyst design.

Energy Decomposition Analyses Reveal the Origins of Catalyst and Nucleophile Effects on Regioselectivity in Nucleopalladation of Alkenes.

Nucleopalladation is one of the most common mechanisms for Pd-catalyzed hydro- and oxidative functionalization of alkenes. Due to the electronic bias of the π-alkene-palladium complexes, nucleopalladations with terminal aliphatic alkenes typically deliver the nucleophile to the more substituted sp2 carbon to form the Markovnikov-selective products. The selective formation of the anti-Markovnikov nucleopalladation products requires the inherent electronic effects to be overridden, which is still a significant challenge for reactions with simple aliphatic alkenes. Because the interactions between the nucleophile and the alkene substrate are influenced by a complex combination of multiple types of steric and electronic effects, a thorough understanding of the interplay of these underlying interactions is needed to rationalize and predict the regioselectivity. Here, we employ an energy decomposition approach to quantitatively separate the different types of nucleophile-substrate interactions, including steric, electrostatic, orbital interactions, and dispersion effects, and to predict the impacts of each factor on regioselectivity. We demonstrate the use of this approach on the origins of catalyst-controlled anti-Markovnikov-selectivity in Hull's Pd-catalyzed oxidative amination reactions. In addition, we evaluated the regioselectivity in a series of nucleopalladation reactions with different neutral and anionic Pd catalysts and N- and O-nucleophiles with different steric and electronic properties. On the basis of these computational analyses, a generalized scheme is established to identify the dominant nucleophile-substrate interaction affecting the regioselectivity of nucleopalladations with different Pd catalysts and nucleophiles.

Production of highly catalytic, archaeal Pd(0) bionanoparticles using Sulfolobus tokodaii.

The thermo-acidophilic archaeon, Sulfolobus tokodaii, was utilized for the production of Pd(0) bionanoparticles from acidic Pd(II) solution. Use of active cells was essential to form well-dispersed Pd(0) nanoparticles located on the cell surface. The particle size could be manipulated by modifying the concentration of formate (as electron donor; e-donor) and by addition of enzymatic inhibitor (Cu2+) in the range of 14-63nm mean size. Since robust Pd(II) reduction progressed in pre-grown S. tokodaii cells even in the presence of up to 500mM Cl-, it was possible to conversely utilize the effect of Cl- to produce even finer and denser particles in the range of 8.7-15nm mean size. This effect likely resulted from the increasing stability of anionic Pd(II)-chloride complex at elevated Cl- concentrations, eventually allowing involvement of greater number of initial Pd(0) crystal nucleation sites (enzymatic sites). The catalytic activity [evaluated based on Cr(VI) reduction reaction] of Pd(0) bionanoparticles of varying particle size formed under different conditions were compared. The finest Pd(0) bionanoparticles obtained at 50mM Cl- (mean 8.7nm; median 5.6nm) exhibited the greatest specific Cr(VI) reduction rate, with four times higher catalytic activity compared to commercial Pd/C. The potential applicability of S. tokodaii cells in the recovery of highly catalytic Pd(0) nanoparticles from actual acidic chloride leachate was, thus, suggested.

Cationic—Anionic Pd(II) Complexes with Adamantylimidazolium Cation: Synthesis, Structural Study, and MAO-Inhibiting Activity

New cationic-anionic palladium(II) complexes have been prepared from adamantyl-substituted imidazolium salts and effect of structure of the adamantyl-substituted salts and synthesis conditions on the structure of these complexes with \({\rm{Pd}}({\rm{DMSO}}){\rm{Hal}}_3^ - \) (1–4), \({\rm{P}}{{\rm{d}}_2}{\rm{Br}}_6^{2 - }\) (5, 6), or \({\rm{PdCl}}_4^{2 - }\) (7) anions has been studied. A number of palladium(II) complexes active against monoamine oxidase B has been prepared, effect of composition and structure on their biological activity have been revealed. The structure of the complexes has been confirmed by X-ray diffraction analysis, a conductometric study of complex 1 has been performed. MAO-inhibiting activity of the obtained complexes has been found to be on the level of reference compounds: 17.6% of residual enzyme activity upon inhibition by complex 3 as compared with 16.9% for reference compound (selegiline). Complexes with bromine ligand show higher activity than those with chlorine ligand. The results of this study can be used in organometallic and bioinorganic chemistry.

A tailored polymeric cationic tag–anionic Pd(ii) complex as a catalyst for the low-leaching Heck–Mizoroki coupling in flow and in biomass-derived GVL

A new polystyrene-type resin loaded with pincer-type imidazolium ionic tag has been very effective in the immobilization of [PdCl4]2− palladium complex leading to a very low leaching of the metal during its use in flow.

Mechanistic investigation into Et3N C[sbnd]H activation and chemoselectivity by Pd-Catalyzed intramolecular heck reaction of N-Vinylacetamides

Density functional theory (DFT) calculations have been conducted to elucidate the detailed mechanism of Pd-Catalyzed intramolecular Heck reaction of N-vinylacetamides. Pd(PPh3)2 and (PPh3)PdOAc−, generated in situ from Pd(OAc)2/PPh3, are predicted to be the active Pd(0) catalyst. The reaction undergoes C(aryl)Br oxidative addition, alkene insertion, β-N group elimination, and nitrogen protonation. The CH activation of Et3N, which is requisite for nitrogen protonation, was explored emphatically. An ion-pair intermediate containing both an anionic Pd(II) complex (a 16e 4-coordinate square planar structure) and a cationic moiety Et2NCHCH3+ was found to be involved in the CH activation. The nitrogen atom of Et3N plays a crucial role in stabilizing the cationic moiety by means of its lone pair to afford a NC double bond, and thus ruling out the possibility of the β-CH bond activation. Additionally, the high chemoselectivity for P1 rather than P2 and P3 were rationalized theoretically.

Computation and Experiment: A Powerful Combination to Understand and Predict Reactivities.

Computational chemistry has become an established tool for the study of the origins of chemical phenomena and examination of molecular properties. Because of major advances in theory, hardware and software, calculations of molecular processes can nowadays be done with reasonable accuracy on a time-scale that is competitive or even faster than experiments. This overview will highlight broad applications of computational chemistry in the study of organic and organometallic reactivities, including catalytic (NHC-, Cu-, Pd-, Ni-catalyzed) and noncatalytic examples of relevance to organic synthesis. The selected examples showcase the ability of computational chemistry to rationalize and also predict reactivities of broad significance. A particular emphasis is placed on the synergistic interplay of computations and experiments. It is discussed how this approach allows one to (i) gain greater insight than the isolated techniques, (ii) inspire novel chemistry avenues, and (iii) assist in reaction development. Examples of successful rationalizations of reactivities are discussed, including the elucidation of mechanistic features (radical versus polar) and origins of stereoselectivity in NHC-catalyzed reactions as well as the rationalization of ligand effects on ligation states and selectivity in Pd- and Ni-catalyzed transformations. Beyond explaining, the synergistic interplay of computation and experiments is then discussed, showcasing the identification of the likely catalytically active species as a function of ligand, additive, and solvent in Pd-catalyzed cross-coupling reactions. These may vary between mono- or bisphosphine-bound or even anionic Pd complexes in polar media in the presence of coordinating additives. These fundamental studies also inspired avenues in catalysis via dinuclear Pd(I) cycles. Detailed mechanistic studies supporting the direct reactivity of Pd(I)-Pd(I) with aryl halides as well as applications of air-stable dinuclear Pd(I) catalysts are discussed. Additional combined experimental and computational studies are described for alternative metals, these include the discussion of the factors that control C-H versus C-C activation in the aerobic Cu-catalyzed oxidation of ketones, and ligand and additive effects on the nature and favored oxidation state of the active catalyst in Ni-catalyzed trifluoromethylthiolations of aryl chlorides. Examples of successful computational reactivity predictions along with experimental verifications are then presented. This includes the design of a fluorinated ligand [(CF3)2P(CH2)2P(CF3)2] for the challenging reductive elimination of ArCF3 from Pd(II) as well as the guidance of substrate scope (functional group tolerance and suitable leaving group) in the Ni-catalyzed trifluoromethylthiolation of C(sp(2))-O bonds. In summary, this account aims to convey the benefits of integrating computational studies in experimental research to increase understanding of observed phenomena and guide future experiments.

Oxidative addition transition states of Pd(0) complexes in polar solvent—a DFT study involving implicit and explicit solvation

Previous reports in the literature have suggested that concerted transition states for oxidative addition would not exist in solution. By contrast, this report describes the successful location of transition states for the oxidative addition of monoligated Pd(PtBu3), bisligated Pd(PtBu3)(MeCN), and anionic Pd(PtBu3)F− complexes to C–Cl and C–OTf of an aryl chloro triflate in polar solvent (MeCN). Optimizations were done with implicit solvation and a combined implicit/explicit solvation approach in which six acetonitrile molecules were additionally involved in the optimization. The geometries obtained from optimizations employing an implicit solvation model are structurally largely analogous to those found using a combination of implicit and explicit solvation.

ChemInform Abstract: Orthogonal Selectivities under Pd(0) Catalysis with Solvent Polarity: An Interplay of Computational and Experimental Studies

AbstractReview: 65 refs.

Dipotassium tetrakis(thiocyanato-κS)palladate(II)–(2,2′-bipyrimidine-κ2N1,N1′)bis(thiocyanato-κS)palladium(II) (1/2)

The asymmetric unit of the title compound, K2[Pd(NCS)4]·2[Pd(NCS)2(C8H6N4)], contains two crystallographically independent half-mol­ecules of the anionic PdII complex, two K+ cations and two independent neutral PdII complexes; an inversion centre is located at the centroid of each anionic complex. In the anionic complexes, each PdII ion is four-coordinated in an almost regular square-planar environment by four S atoms from four SCN− anions, and the PdS4 unit is exactly planar. In the neutral complexes, the PdII ion has a slightly distorted square-planar coordination environment defined by two pyrimidine N atoms derived from a chelating 2,2′-bipyrimidine ligand and two mutually cis S atoms from two SCN− anions. Both 2,2′-bipyrimidine ligands are almost planar [dihedral angle between the rings = 3.98 (16) and 4.57 (17)°] and also chelate to a potassium ion from their other two N atoms. In the crystal, the K+ ions inter­act with various S and N atoms of the ligands, forming a three-dimensional polymeric network, in which the shortest K⋯K contacts between the KN7S polyhedra are 4.4389 (17) and 4.4966 (18) Å. Intra- and inter­molecular C—H⋯S and C—H⋯N hydrogen bonds are also observed.

Structural changes of Pd13 upon charging and oxidation/reduction

First-principle generalized gradient corrected density functional calculations have been performed to study the stability of cationic and anionic Pd(13) (+∕-), and neutral Pd(13)O(2) clusters. It is found that while cationic Pd(13) (+) favors a C(s) geometry similar to the neutral Pd(13), both anionic Pd(13)(-) and neutral Pd(13)O(2) favor a compact ~I(h) structure. A detailed analysis of the electronic structure shows that the stabilization of the delocalized 1P and 2P cluster orbitals, and the hybridization of the 1D orbitals with the oxygen atomic p orbitals play an important role in the energetic ordering of C(s) and ~I(h) isomers. A structural oscillation is predicted during an oxidation/reduction cycle of Pd(13) in which small energy barriers between 0.3 and 0.4 eV are involved.

Orthogonal Selectivities under Pd(0) Catalysis with Solvent Polarity: An Interplay of Computational and Experimental Studies

We recently communicated a combined computational and experimental study of our discovery of selectivity reversal in Pd cross-coupling in polar solvent for a competition reaction of C-Cl vs. C-OTf functionalization. Our data suggested that the reactivity in polar solvent is consistent with anionic Pd(0) and in nonpolar solvent with neutral Pd(0) catalysis. This article will discuss the power and limits of current computational approaches to explore mechanisms in the context of this study and will highlight the use of experiments in parallel to computational approaches to test theoretical conclusions and aid theoretical analyses.

The First Carbodiphosphorane Complex with Two Palladium Centers Attached to the CDP Carbon: Assembly of a Single-Stranded di-Pd Helicate by the PCP Pincer ligand C(dppm)2

The reaction of the protonated PCP pincer carbodiphosphorane (CDP) complex [PdCl(CH(dppm)2)]Cl2 (1) with [PdCl2(MeCN)2] results in the opening of one ring of the pincer structure and affords the betaine [Pd2Cl5(CH(dppm)2)] (2). The [CH(dppm)2]+ ligand forms a five-membered chelate ring with the Pd(1)Cl2 moiety via the protonated CDP carbon (positively charged) and one phosphine functionality and bridges to the anionic Pd(2)Cl3 fragment via the second phosphine phosphorus. The complex is chiral due to a stereocenter at the protonated CDP carbon atom. Treatment of 2 with water effects deprotonation of the CDP carbon, C–H activation at one methylene group, and the coordination of the second Pd to the CDP carbon. The resulting tricyclic complex [Pd2Cl3(C(dppm)(Ph2CHPh2))] (3) is the first compound with two Pd centers coordinated to a CDP carbon. Compound 3 forms a single-stranded helicate structure and exhibits two interdependent carbon stereocenters, resulting in only two possible enantiomers. The complexes 2·2CH2Cl2 and 3·3CH2Cl2 crystallize in the chiral space group P21, whereby a spontaneous resolution of the enantiomers occurs in both cases.

Palladium‐Mediated Annulation of Vinyl Aziridines with Michael Acceptors: Stereocontrolled Synthesis of Substituted Pyrrolidines and Its Application in a Formal Synthesis of (−)‐α‐Kainic Acid

route to vinyl epoxides and vinyl aziridines via chiral sulfur ylides, we were keen to develop their potential in synthesis further. It had been reported that palladium-catalyzed reactions of vinyl epoxides and aziridines with doubly activated Michael acceptors gave tetrahydrofurans and pyrrolidines in good yield but usually with poor stereocontrol. However, related reactions with singly activated Michael acceptors were not effective. Nevertheless, we recognized that if we could find a way of coercing vinyl aziridines to react with singly activated enones/acrylates, and we were able to control stereochemistry in the annulation process, then we could potentially utilize this methodology in synthesis. Herein, we describe our success in simultaneously meeting these two significant challenges and also describe its application in a formal synthesis of ( )-a-kainic acid 1. Our initial efforts at promoting reaction between vinyl aziridine 2a and methyl vinyl ketone (MVK, A) using [Pd2(dba)3·CHCl3], (p-FC6H5)3P in THF (conditions employed by Yamamoto), however, were fruitless—we only observed decomposition. Using trimethylsilyl-substituted trans vinyl aziridine 2e, we now observed isomerization to a mixture of trans/cis aziridines (1:20). With this substrate, clearly the Pd was performing its role in generating the p-allyl palladium complex as this resulted in isomerization of the vinyl aziridine. However, the amide anion that was generated did not react with the enone. This result reinforced the observations by Yamamoto and Knight that doubly activated Michael acceptors were required to capture the relatively unreactive amide anion. We reasoned that ion pairing of the amide anion with the cationic Pd complex might compromise its reactivity, and that increased nucleophilicity should therefore be possible with an anionic Pd complex instead (Scheme 2).

An anionic d-valine–palladium(ii) complex supported on a hydroxy double salt with a Brønsted basic phosphate anion: application for a heterogeneous catalyst toward aerobic alcohol oxidation

An anionic D-valine–Pd(II) complex intercalated into an anion exchangeable Ni–Zn mixed basic salt (NiZn), which is classified as a layered hydroxy double salt, was synthesised in situ via a simple anion exchange procedure. A Bronsted basic PO43− anion was also intercalated into the NiZn interlayer along with the anionic D-valine–Pd(II) complex. The Bronsted basic clay–Pd(II) nanocomposite catalysed the aerobic oxidation of a wide variety of alcohols to the corresponding aldehydes and ketones. In the aerobic oxidation of 1-phenylethanol, for example, a turnover number (TON) of 2000 based on Pd was achieved. During the aerobic alcohol oxidation, the D-valine–Pd(II) complex in the NiZn interlayer maintained its original monomeric Pd(II) structure due to the strong electrostatic interaction between the NiZn host and the anionic Pd(II) complex. This catalyst could be reused without loss of either catalytic activity or selectivity for the aerobic oxidation of alcohols.

Inorganic−Organic Hybrid Materials: Layered Zinc Hydroxide Salts with Intercalated Porphyrin Sensitizers

Layered materials provide a two-dimensional interlayer space suitable for accommodating molecules with a designed functionality. In this study, inorganic−organic hybrids were prepared by intercalation of anionic porphyrin sensitizers into the host structure of layered zinc hydroxide salts. The inorganic host offers stabilization and protection, whereas the guest species provide the photofunction. The properties and arrangement of porphyrin molecules in the interlayer space were studied by a combination of experimental techniques and molecular simulations. Intercalation of porphyrins led to a gallery height that is comparable with the size of porphyrin molecules. Molecular simulations showed that the interlayer space is filled with disordered porphyrin units. The porphyrin sulfonate groups interact with the brucite-like layers via dominant electrostatic interactions similarly to layered double hydroxides. The photophysical experiments proved that intercalated anionic Pd porphyrins produce singlet oxygen, O2(1Δg), with long effective lifetimes, suggesting that layered zinc hydroxide salts are good carriers of porphyrin sensitizers.