Organopalladium Complexes Research Articles

Reactivity of a Palladacyclic Complex: A Monodentate Carbonate Complex and the Remarkable Selectivity and Mechanism of a Neophyl Rearrangement

The ligand N(CH2-2-C5H4N)2(CH2CH2CH2OH), L1, reacted with [Pd(CH2CMe2C6H4)(COD)] to give a new fluxional “cycloneophyl” organopalladium complex [Pd(CH2CMe2C6H4)(κ2-L1)], 1, which on attempted recrystallization from THF gave the monodentate carbonate complex [Pd(CO3)(κ3-L1)], 2. Complex 2 was prepared in designed syntheses by reaction of [PdCl(κ3-L1)]+ with silver carbonate or by reaction of [Pd(OH)(κ3-L1)]+ with CO2. Complex 1 reacted with aqueous CO2 to give the cationic neophylpalladium complex [Pd(CH2CMe2C6H5)(κ3-L1)]+(HCO3)−, 6. Complex 6 reacts with hydrogen peroxide to give complex 2 with release of a mixture of organic products, the major one being 2-phenyl-2-butanol, PB. The formation of PB involves a neophyl rearrangement with the unprecedented preference for methyl over phenyl migration. A mechanistic basis for this unexpected reaction is proposed, involving β-carbon elimination at a palladium(IV) center.

Kinetics, Mechanism and Theoretical Studies of Norbornene-Ethylene Alternating Copolymerization Catalyzed by Organopalladium(II) Complexes Bearing Hemilabile α-Amino-pyridine.

Cationic methylpalladium complexes bearing hemilabile bidentate α-amino–pyridines can serve as effective precursors for catalytic alternating copolymerization of norbornene (N) and ethylene (E), under mild conditions. The norbornyl palladium complexes in the formula of {[RHNCH2(o-C6H4N)]Pd(C7H10Me)(NCMe)}(BF4) (R = iPr (2a), tBu (2b), Ph (2c), 2,6-Me2C6H3 (2d), 2,6-iPr2C6H3 (2e)) were synthesized via single insertion of norbornene into the corresponding methylpalladium complexes 1a–1e, respectively. Both square planar methyl and norbornyl palladium complexes exhibit facile equilibria of geometrical isomerization, via sterically-controlled amino decoordination–recoordination of amino–pyridine. Kinetic studies of E-insertion, N-insertion of complexes 1 and 2, and the geometric isomerization reactions have been examined by means of VT-NMR, and found in excellent agreement with the results estimated by DFT calculations. The more facile N-insertion in the cis-isomers, and ready geometric isomerization, cooperatively lead to a new mechanism that accounts for the novel catalytic formation of alternating COC.

Palladium-Catalyzed C–H Activation: Mass Spectrometric Approach to Reaction Kinetics in Solution

We report a new method for determination of rate constants of processes in solution using electrospray ionization mass spectrometry (ESI-MS). The investigated reaction is C–H activation of acetanilides by palladium(II)trifluoroacetate leading to stable organopalladium complexes. The rate constants can be determined from an experiment with a couple of differently substituted acetanilides being in competition for being activated by the palladium salt. The formed organopalladium complexes can be detected by ESI-MS. The time dependence is achieved by adding one of the acetanilides to the reaction mixture with a time delay. The kinetics can be then evaluated from the evolution of the ratio of the ESI-MS signals of differently substituted complexes as a function of the time delay. The Hammett analysis of the rate constants obtained for a series of meta- and para-substituted acetanilides provides a ρ value of −1.5, which is in agreement with values reported for similar C–H activations. We have also investigated ...

Reversible Gas-Solid Ammonia N-H Bond Activation Mediated by an Organopalladium Complex.

N-H bond activation of gaseous ammonia is achieved at room temperature in a reversible solvent-free reaction using a solid dicyclopalladated azobenzene complex. Monitoring of the gas-solid reaction in real-time by in situ solid-state Raman spectroscopy enabled a detailed insight into the stepwise activation pathway proceeding to the final amido complex via a stable diammine intermediate. Gas-solid synthesis allowed for isolation and subsequent structural characterization of the intermediate and the final amido product, which presents the first dipalladated complex with the PdII-(μ-NH2)-PdII bridge. Gas-solid reaction is readily followed via color changes associated with conformational switching of the palladated azobenzene backbone. The reaction proceeds analogously in solution and was characterized by UV-vis and NMR spectroscopies showing the same stepwise route to the amido complex. Combining the experimental data with density functional theory calculations we propose a stepwise mechanism of this heterolytic N-H bond activation assisted by exogenous ammonia.

Palladium–cadmium sulfide nanopowder at oil–water interface as an effective catalyst for Suzuki–Miyaura reactions

A simple and effective strategy is described for the synthesis of Pd–CdS nanopowder by the reduction of an organopalladium(II) complex, [PdCl2(cod)] (cod = cis,cis‐1,5‐cyclooctadiene), in the presence of CdS quantum dots (QDs) at a toluene–water interface. We investigated the impact of addition of CdS QDs on catalytic activity of Pd nanoparticles (NPs). The Pd–CdS nanopowder functions as an efficient catalyst for Suzuki–Miyaura reactions for the formation of carbon–carbon bonds. There is a high electron density on Pd NPs and due to their high electron affinity they behave as an electron scavenger from CdS increasing the rate of oxidative addition, which is the rate‐determining step of the catalytic cycle, and, just as we expect, the C─C coupling reaction with the Pd–CdS nanopowder is faster and occurs in less time than that with Pd nanocatalysts. Compared to classical reactions, this method consistently has the advantages of short reaction times, high yields in a green solvent, reusability of the catalyst without considerable loss of catalytic activity and low cost, and is a facile method for the preparation of the catalyst.

ONO pincer type binuclear Pd(II) complex: Synthesis, crystal structure and catalytic utilization of the resulting organopalladium complex in catalytic copper-free Sonogashira coupling reaction

New binuclear palladium (II) complex featuring ONO pincer type hydrazone ligand was synthesized and characterized by spectroscopic and single-crystal XRD analysis. This complex showed excellent catalytic activity towards the copper-free Sonogashira cross coupling reaction. The main advantages over previous methodologies include low catalyst loading, less problematic reaction media (H2O–DMSO) and reusability in homogenous systems for optimal performance.

Asymmetric Diels–Alder reaction between 3,4-dimethyl-1-phenylphosphole and (Z/E)-diphenyl-1-styrylphosphine

The organopalladium complex containing ortho-metalated (S)-[1-(dimethylamino)ethyl]naphthalene as the chiral auxiliary has been successfully employed to promote the asymmetric Diels–Alder reaction between 3,4-dimethyl-1-phenylphosphole and (Z)-diphenyl-1-styrylphosphine or (E)-diphenyl-1-styrylphosphine. In the intramolecular cycloaddition reaction between (E)-diphenyl-1-styrylphosphine and 3,4-dimethyl-1-phenylphosphole, a pair of diastereomeric endo-cycloadducts were obtained in the ratio of 1:1. However, the asymmetric reaction between (Z)-diphenyl-1-styrylphosphine and 3,4-dimethyl-1-phenylphosphole also produced the same endo-product in the same stereoselectivity unexpectedly. The chiral naphthylamine auxiliary could be removed chemoselectively from the template by treatment with concentrated hydrochloric acid to generate the neutral stable dichloro complex [(P–P)PdCl2]. Treatment of the dichloro complex with KCN gave the pure bis-phosphine ligand in quantitative yield.

Palladacyclo-promoted asymmetric hydrophosphination reaction between diphenylphosphine and 2-ethynylpyridine

The organopalladium complex containing ortho-metalated (R)-[1-(dimethylamino) ethyl]naphthalene as the chiral auxiliary has been used as the chiral template to promote the asymmetric hydrophosphination reaction between diphenylphosphine and 2-ethynylpyridine in high selectivity under mild conditions. The double hydrophosphination reaction produces four stereoisomeric products in the ratio of 1:19.4:2.4:1.3. The naphthylamine auxiliary could be removed chemoselectively from the template products by treatment with concentrated hydrochloric acid to form the corresponding chiral dichloro complexes [(P–P)PdCl2]. The molecular structure and absolute configuration of the dichloro complex have been resolved by single-crystal X-ray crystallography. It confirms that the asymmetric diphosphines as P–P bidentate chelate on the chiral naphthylamine palladium templates. Finally, the enantiomerically pure diphosphine ligand with chirality on the carbon backbone can be readily liberated from the dichloro palladium complex as an air-sensitive solid by treatment of the complex with aqueous potassium cyanide in high yield.

Individual steps of the Mizoroki-Heck reaction and intrinsic reactivity of intermediate organopalladium complexes studied in the gas phase.

The mechanism of the Mizoroki-Heck reaction (MHR) was analyzed by collision-induced dissociation (CID) tandem-mass spectrometry and gas-phase ion/molecule reactions (IMRs) as well as by DFT computational analysis. The MHR was performed in the gas phase and the intrinsic reactivity of important intermediates was examined individually. Kinetics and substituent effects of cationic palladium- Pcy3-aryl complexes (Cy = cyclohexyl) with 2,3-dimethylbutadiene in the MHR were analyzed via IMRs and CID. The kinetics and ion structures of the species involved in the olefin insertion, i.e., the carbopalladation, were investigated. Moreover, linear free-energy correlations were applied and a concerted mechanism proceeding via a four-membered transition state for the carbopalladation step that exhibited only a minor charge separation was deduced.

Organo-palladium(II) complexes bearing unsymmetrical N,N,N-pincer ligands: synthesis, structures and oxidatively induced coupling reactions.

The 2-(2′-aniline)-6-imine-pyridines, 2-(C6H4-2′-NH2)-6-(CMe=NAr)C5H3N (Ar = 4-i-PrC6H4 (HL1a), 2,6-i-Pr2C6H3 (HL1b)), have been synthesised via sequential Stille cross-coupling, deprotection and condensation steps from 6-tributylstannyl-2-(2-methyl-1,3-dioxolan-2-yl)pyridine and 2-bromonitrobenzene. The palladium(II) acetate N,N,N-pincer complexes, [{2-(C6H4-2′-NH)-6-(CMe=NAr)C5H3N}Pd(OAc)] (Ar = 4-i-PrC6H4 (1a), 2,6-i-Pr2C6H3 (1b)), can be prepared by reacting HL1 with Pd(OAc)2 or, in the case of 1a, more conveniently by the template reaction of ketone 2-(C6H4-2′-NH2)-6-(CMe=O)C5H3N, Pd(OAc)2 and 4-isopropylaniline; ready conversion of 1 to their chloride analogues, [{2-(C6H4-2′-NH)-6-(CMe=NAr)C5H3N}PdCl] (Ar = 4-i-PrC6H4 (2a), 2,6-i-Pr2C6H3 (2b)), has been demonstrated. The phenyl-containing complexes, [{2-(C6H4-2′-NH)-6-(CMe=NAr)C5H3N}PdPh] (Ar = 4-i-PrC6H4 (3a), 2,6-i-Pr2C6H3 (3b)), can be obtained by treating HL1 with (PPh3)2PdPh(Br) in the presence of NaH or with regard to 3a, by the salt elimination reaction of 2a with phenyllithium. Reaction of 2a with silver tetrafluoroborate or triflate in the presence of acetonitrile allows access to cationic [{2-(C6H4-2′-NH)-6-(CMe=N(4-i-PrC6H4)C5H3N}Pd(NCMe)][X] (X = BF4 (4), X = O3SCF3 (5)), respectively; the pyridine analogue of 5, [{2-(C6H4-2′-NH)-6-(CMe=N(4-i-PrC6H4)C5H3N}Pd(NC5H5)][O3SCF3] (5′), is also reported. Oxidation of phenyl-containing 3a with one equivalent of 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor™) in acetonitrile at low temperature leads to a new palladium species that slowly decomposes to give 4 and biphenyl; biphenyl formation is also observed upon reaction of 3a with XeF2. However, no such oxidatively induced coupling occurs when using 3b. Single crystal X-ray diffraction studies have been performed on HL1b, 1a, 1b, 2a, 2b, 3a, 3b and 5′.

Palladium(0)-mediated C-H bond activation of N-(naphthyl)salicylaldimine and related ligands: utilization of the resulting organopalladium complexes in catalytic C-C and C-N coupling reactions.

N-(Naphthyl)-4-R-salicylaldimines (R = OCH(3), H and Cl; H(2L)(1)-H(2)L(3)) and 2-hydroxy-N-(naphthyl)naphthaldimine (H(2)L(4)) readily undergo, upon reaction with Na(2)[PdCl(4)] in the presence of triphenylphosphine, cyclopalladation via C-H bond activation at the peri-position to afford complexes of type [Pd(L)(PPh(3))] (L = L(1)-L(4)). The C-H bond activation has been found to be mediated by palladium(0) formed in situ. A similar reaction of H(2)L(1) with Na(2)[PdCl(4)] in the presence of 1,2-bis(diphenylphosphino)ethane (dppe), in a 2 : 2 : 1 mole ratio, yields a dinuclear complex of type [{Pd(L(1))}(2)(dppe)]. Reaction of H(2)L(1) with Na(2)[PdCl(4)] in the presence of 4-picoline (pic) yields [Pd(L(1))(pic)]. The molecular structures of the six complexes have been determined by X-ray crystallography. The aldiminate ligand in each compound is coordinated to the metal center as a di-anionic tridentate ONC-donor, with the fourth coordination site occupied by a phosphine or picoline ligand. The new complexes show intense absorptions in the visible and ultraviolet regions, and the nature of the optical transitions has been analyzed by TDDFT calculations. The palladium complexes display notable efficiency in catalyzing C-C and C-N bond coupling reactions. The thermodynamics for the formation of the cyclometalated catalyst precursor [Pd(L(2))(PPh(3))] has been evaluated by DFT calculations.

Synthetic studies toward polytwistane hydrocarbon nanorods.

A synthetic strategy toward the intriguing hydrocarbon nanorod polytwistane is outlined. Our approach aims toward the polymerization of acetylene starting from precursors that would provide a helical bias for the formation of polytwistane. Both transition-metal-catalyzed and radical polymerizations were investigated. Two potential initiator molecules were synthesized that could be used for either approach. Although the intended regioselectivities were not observed, unusual organopalladium complexes and numerous compounds with novel carbon skeletons were obtained.

Strongly coupled Pd nanotetrahedron/tungsten oxide nanosheet hybrids with enhanced catalytic activity and stability as oxygen reduction electrocatalysts.

The design and synthesis of highly active oxygen reduction reaction (ORR) catalysts with strong durability at low cost is extremely desirable but still remains a significant challenge. Here we develop an efficient strategy that utilizes organopalladium(I) complexes containing palladium-palladium bonds as precursors for the synthesis of strongly coupled Pd tetrahedron-tungsten oxide nanosheet hybrids (Pd/W18O49) to improve the electrocatalytic activity and stability of Pd nanocrystals. The hybrid materials are synthesized by direct nucleation, growth, and anchoring of Pd tetrahedral nanocrystals on the in situ-synthesized W18O49 nanosheets. Compared to supportless Pd nanocrystals and W18O49, their hybrids exhibited not only surprisingly high activity but also superior stability to Pt for the ORR in alkaline solutions. X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and electrochemical analyses indicated that the enhanced electrocatalytic activity and durability are associated with the increased number and improved catalytic activity of active sites, which is induced by the strong interaction between the Pd tetrahedrons and W18O49 nanosheet supports. The present study provides a novel strategy for synthesizing hybrid catalysts with strong chemical attachment and electrical coupling between nanocatalysts and supports. The strategy is expected to open up exciting opportunities for developing a novel class of metal-support hybrid nanoelectrocatalysts with improved ORR activity and durability for both fuel cells and metal-air batteries.

Asymmetric synthesis of a chiral diarsine ligand via a cycloaddition reaction between 3,4-dimethyl-1-phenylarsole and diphenylvinylarsine

The organopalladium complex containing ortho-metallated (S)-[1-(dimethylamino)ethyl]naphthalene as a chiral auxiliary has been successfully employed to promote the asymmetric cycloaddition reaction between 3,4-dimethyl-1-phenylarsole and diphenylvinylarsine. In the intramolecular cycloaddition reaction, a pair of separable diastereomeric palladium complexes was obtained in the ratio of 6:1. The chiral naphthylamine auxiliary could be removed chemoselectively from the template by treatment with HCl and subsequently NaI to generate the neutral diiodo complex [(As–As)PdI2]. Treatment of the diiodo complex with KCN gave the enantiomerically pure As–As bidentate ligand in quantitative yield. In contrast to the reported similar P–P and As–P analogues, both arsenic donors in the diiodo complex could be readily eliminated to produce a structurally novel dimetallic complex.

Thin film formation of Pd/reduced-graphene oxide and Pd nanoparticles at oil–water interface, suitable as effective catalyst for Suzuki–Miyaura reaction in water

A facile and efficient route was used to synthesise Pd nanoparticles by reduction of an organopalladium(II) complex, [PdCl2(cod)] (cod = cis,cis-1,5-cyclooctadiene), at the oil–water interface. Pd/reduced-graphene oxide and Pd free stabilizer nanoparticle thin films were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction and transmission electron microscopy (TEM). Our study shows that both of these nanoparticles were suitable for the Suzuki–Miyaura carbon–carbon coupling reaction. Compared to the classical reactions, this method consistently has the advantages of a green solvent, short reaction times, low catalyst loading, high yields and reusability of the catalyst. Furthermore, Pd nanoparticles show a high catalytic activity toward aryl chloride derivatives.

Controlling the Ambiphilic Nature of σ-Arylpalladium Intermediates in Intramolecular Cyclization Reactions

The reactivity of main group organometallics, such as organolithium compounds (RLi) and Grignard reagents (RMgX), is quite straightforward. In these species the R group usually exhibits nucleophilic reactivity without any possibility of inducing electrophilic character. In contrast, in organopalladium complexes, researchers can switch the reactivity from electrophilic to nucleophilic relatively simply. Although σ-aryl and σ-vinylpalladium complexes are commonly used as electrophiles in C-C bond-forming reactions, recent research has demonstrated that they can also react with carbon-heteroatom multiple bonds in a nucleophilic manner. Nevertheless, researchers have completely ignored the issue of controlling the ambiphilic nature of such species. This Account describes our efforts toward selectively promoting the same starting materials toward either electrophilic α-arylation or nucleophilic addition reactions to different carbonyl groups. We could tune the properties of the σ-arylpalladium intermediates derived from amino-tethered aryl halides and carbonyl compounds to achieve chemoselective transformations. Therefore, chemists can control the ambiphilic nature of such intermediates and, consequently, the competition between the alternative reaction pathways by the adequate selection of the reaction conditions and additives (base, presence/absence of phenol, bidentate phosphines). The nature of the carbonyl group (aldehydes, ketones, esters, and amides) and the length of the tether connecting it to the aniline moiety also play an important role in the outcome of these processes. Our joint computational and experimental efforts to elucidate the reaction mechanism of these palladium-catalyzed transformations suggest that beyond the formation of the four-membered azapalladacycle, two major factors help to control the dual character of the palladium(II) intermediates derived from 2-haloanilines. First, their high nucleophilicity strongly modifies the interaction of the metal center with the carbonyl group. Second, the additive phenol exchanges the iodide ligand to give an arylpalladium(II) phenoxide complex, which has a beneficial effect on the arylation. The formation of this transient intermediate not only stabilizes the arylpalladium moiety, thus preventing the nucleophilic attack at the carbonyl group, but also assists the enolization reaction, which takes place in a more favorable intramolecular manner. The azapalladacycle intermediate is, in the words of J. R. R. Tolkien, "the one ring to bring them all and in the darkness to bind them." With this intermediate, we can easily achieve the synthesis of a variety of heterocyclic systems by selectively promoting electrophilic α-arylation or nucleophilic addition reactions from the same precursors.

ChemInform Abstract: Palladium Mediated C—H Bond Activation of Thiosemicarbazones: Catalytic Application of Organopalladium Complexes in C—C and C—N Coupling Reactions.

AbstractA novel palladium thiosemicarbazone complex is prepared and efficiently applied as a catalyst for the arylation of amines (IV) and (VII) with halobenzenes (V).

Formation of organopalladium complexes via C–Br and C–C bond activation. Application in C–C and C–N coupling reactions

Reaction of [Pd2(dba)3] (dba = dibenzylideneacetone) with two Schiff base ligands (L1 and L2), derived from the condensation of 8-aminoquinoline with 2-bromobenzaldehyde or 2-bromoacetophenone, in refluxing tert-butanol afforded two organopalladium complexes 1 and 2. Crystal structure of complex 1 has been determined by X-ray diffraction studies. Structure of complex 2 has been optimized by DFT method. In both the complexes the imine ligands are coordinated, via C–Br bond activation, as tridentate CNN-donor and the fourth coordination position is occupied by an acetylide ion provided by an outgoing dba ligand via C–C bond cleavage. Both the complexes display intense absorptions in the visible and ultraviolet regions. Both the complexes catalyze C–C and C–N coupling reactions efficiently.

The Clicked Pyridyl‐Triazole Ligand: From Homogeneous to Robust, Recyclable Heterogeneous Mono‐ and Polymetallic Palladium Catalysts for Efficient Suzuki–Miyaura, Sonogashira, and Heck Reactions

AbstractVarious mono‐ and polymetallic palladium complexes containing a 2‐pyridyl‐1,2,3‐triazole (pyta) ligand or a nonabranch‐derived (nonapyta) ligand have been synthesized by reaction of palladium acetate with these ligands according to a 1:1 metal‐ligand stoichiometry and used as catalysts for carbon‐carbon cross‐coupling including the Suzuki–Miyaura, Sonogashira and Heck reactions. The unsubstituted monopalladium and nonapalladium complexes were insoluble in all the reaction media, whereas tri‐ and tetranuclar palladium complexes were soluble, which allowed conducting catalysis under either homogeneous or heterogeneous conditions. The organopalladium complexes were characterized by standard analytical and spectroscopic methods and by thermogravimetry showing decomposition above 110 °C. Both types of catalysts showed excellent activity for these cross carbon‐carbon bond formations involving aryl halides including activated aryl chlorides or acyl chloride. Besides the comparison between homogeneous and heterogeneous catalysis, the key feature of these catalysts is their remarkable robustness that allowed recycling at least ten times in the example of the Heck reaction with excellent yields and without significant reduction of the conversion.

Controlled interaction of benzaldehyde thiosemicarbazones with palladium: formation of bis-complexes with cis-geometry and organopalladium complexes, and their catalytic application in C–C and C–N coupling

Reaction of a 1 : 1 mixture of 4-R-benzaldehyde thiosemicarbazone (R = OCH3, CH3, H, Cl and NO2) and 2-picolinic acid with an equivalent quantity of Na2[PdCl4] in ethanolic medium, under relatively mild reflux, affords a group of complexes of type [Pd(NS–R)2] as the major product and another family of complexes of type [{Pd(CNS–R)}n] as the minor product (where NS–R and CNS–R denote the N,S- and C,N,S-coordinated thiosemicarbazone, respectively). The [{Pd(CNS–R)}n] complexes are conveniently obtained in good yields from similar reactions carried out under prolonged reflux. The sulfur-bridge in the [{Pd(CNS–R)}n] complexes undergoes facile cleavage upon their reaction with triphenylphosphine to yield complexes of type [Pd(CNS–R)(PPh3)]. Crystal structures of [Pd(NS–OCH3)2], [Pd(NS–CH3)2], [Pd(NS–Cl)2], [Pd(CNS–CH3)(PPh3)], [Pd(CNS–H)(PPh3)] and [Pd(CNS–Cl)(PPh3)] have been determined. In the [Pd(NS–R)2] complexes the thiosemicarbazones are coordinated to the metal center, via dissociation of the acidic proton, as monoanionic bidentate N,S-donors forming five-membered chelate rings, while in the [Pd(CNS–R)(PPh3)] complexes they are coordinated to palladium, via dissociation of the acidic proton and activation of an ortho C–H bond, as dianionic tridentate C,N,S-donors forming two adjacent five-membered chelate rings. The [Pd(NS–R)2] and [Pd(CNS–R)(PPh3)] complexes show characteristic 1H NMR signals, and in dichloromethane solution they all display intense absorptions in the visible and ultraviolet regions. The [Pd(NS–R)2] and [Pd(CNS–R)(PPh3)] complexes have shown notable efficiency in catalyzing C–C and C–N coupling reactions.