Synthesis and structural features of α-acyloxy-(η-allyl)palladium complexes

α-Acetoxy (η³-allyl)palladium complexes were prepared from acyloxy functionalized allylsilanes under mild conditions and in good isolated yields. The substituent and ligand effects of the acetoxy group on the palladium-allyl bonding were studied by X-ray diffraction. These studies show that the acetoxy group generates a strongly deformed bonding between the metal atom and the allyl moiety. This unsymmetrical bonding is modulated by the σ-donor/π-acceptor properties of the ligands. The ¹³C NMR studies indicated that the shift values correlate with the carbon-palladium bond lengths and the inductive effects of the acetoxy group.     

In vitro assessment of cytotoxicity, anti-inflammatory, antifungal properties and crystal structures of metallacyclic palladium(II) complexes

Metallacyclic palladium(II) complexes [Pd(L)(R₃P)Cl], L = TIQDTC (1,2,3,4-tetrahydroisoquinolinedithiocarbamate), 4MpipDTC (4-methylpipradinedithiocarbamate), MPizDTC (N-methylpiperazinedithiocarbamate), R₃P = Ph₃P, (o-tolyl)₃P, Ph₂ClP, were synthesized in a 1:1 molar metal-ligand ratio. These complexes were characterized by elemental analyses, FT-IR, multinuclear (¹H, ¹³C and ³¹P) NMR. The X-ray crystal structures of [Pd(TIQDTC)(Ph₃P)Cl] and [Pd(TIQDTC)((o-tolyl)₃P)Cl] show a slightly distorted square planar environment around the Pd(II) ion with S-Pd-S and P-Pd-Cl average bond angles of 74.51 and 92.41, respectively. These complexes were screened for cytotoxic, antifungal, anti-inflammatory and antibacterial activity. Some complexes exhibit a significant activity against fungi.     

Synthesis, characterization, antibacterial and cytotoxic activity of new palladium(II) complexes with dithiocarbamate ligands: X-ray structure of bis(dibenzyl-1-S:S′-dithiocarbamato)Pd(II)

Six palladium(II) dithiocarbamates of general formula Pd(AmDTC)₂, where HAmDTC = aminedithiocarbamic acid, [Pd(II) piperidinedithiocarbamate (1), Pd(II) 4-methylpiperidinedithiocarbamate (2), Pd(II) N-methylbenzyldithiocarbamate (3), Pd(II) dibenzyldithiocarbamate (4), Pd(II) dicyclohexyldithiocarbamate (5), Pd(II) N-cyclohexyl-N-methyldithiocarbamate (6)] have been synthesized and characterized by elemental analyses, FT-IR, ¹H and ¹³C NMR. The X-ray structure of Pd(II), compounds 3 and 4, showed that the ligands are chelated by both sulfur atoms with bond angles S1-Pd-S4 = 179.24(2)° and S2-Pd-S3 = 179.09(5)°, with a distorted square planar geometry around Pd. All these complexes were screened for cytotoxic and antibacterial effects and showed significant antibacterial activity and no substantial in vitro cytotoxicity indicating specificity of the compounds.     

Synthesis and characterization of water-soluble palladium(II)-functionalized diphosphine complexes

Water-soluble functionalized bis(phosphine) ligands L (a–h) of the general formula CH₂(CH₂PR₂)₂, where for a: R = (CH₂)₆OH; b–g: R = (CH₂)_nP(O)(OEt)₂, n = 2–6 and n = 8; h: R = (CH₂)₃NH₂ ( Scheme 1), have been prepared photochemically by hydrophosphination of the corresponding 1-alkenes with H₂P(CH₂)₃PH₂. Water-soluble palladium complexes cis-[Pd(L)(OAc)₂] (1–8) were obtained by the reaction of Pd(OAc)₂ with the ligands a–h in a 1:1 mixture of dichloromethane:acetonitrile. The water-soluble phosphine ligands and their palladium complexes were characterized by IR, ¹H and ³¹P NMR. A crystallographic study of complex 1 shows that the Pd(II) ion has a square planar coordination sphere in which the acetate ligands and the diphosphine ligand deviate by less than 0.12 Å from ideal planar.     

Bicyclic N-heterocyclic carbene (NHC) ligand precursors and their palladium complexes

Eight bicyclic amidinium precursors (3), prepared from R,S-tmcp (R,S-tmcp: (1R,3S)-diamino-1,2,2-trimethylcyclopentane) were described. Only five of the precursors (3a–e) could be converted to palladium complexes, (PdX₂(6,7-NHC)PEPPSI) (4) by treatment with PdCl₂, K₂CO₃, and pyridine (additional KBr was used for (PdBr₂(6,7-NHC)PEPPSI)). The salts and complexes were fully characterized by spectroscopic methods and X-ray crystallography.     

Synthesis and structure of new palladium complexes with the ligand 2-(diphenylphosphino)-1-methylimidazole: Evidence of hemilability

Several Pd(II) complexes containing the potentially bidentate ligand 2-(diphenylphosphino)-1-methylimidazole, dpim, have been synthesized and characterized: [PdCl₂(dpim)]_n (1), [PdCl₂(H₂O)(dpim-κP)] (2), [PdClMe(μ-dpim-κP,κN)]₂ (3) (previously described), [PdClMe(dpim-κP)₂] (4), [Pd(C₆F₅)₂(dpim-κP)₂] (5) and [Pd(η³-2-Me-C₃H₄)(μ-dpim-κP,κN)]₂[PF₆]₂ (6). The highly insoluble complex 1 dissolves in wet DMSO-d₆ to give the water adduct 2 in which a hydrogen bond is established between one of the water hydrogens and the imidazolyl nitrogen. Two types of coordination mode have been found for the dpim ligand in these derivatives, with the ligand behaving as P monodentate and also as a P,N bridge. The transformations between 3 and 4 demonstrate the hemilability of the dpim ligand. Complex 6 was obtained as a mixture of two pairs of enantiomers (R,S)/(S,R) and (R,R)/(S,S). Analysis of the fluxional behaviour of 6, in which the allyl group acts as a “reporter ligand”, indicates that Pd-N bond rupture takes place - again providing evidence of the hemilabile character of the dpim ligand.     

A mechanistic study on modern palladium catalyst precursors as new gateways to Pd(0) in cationic Heck reactions

Electrospray ionization mass spectrometry (ESI-MS) was used as a means to directly identify catalytic cationic organopalladium species in ligand-controlled Heck reactions involving electron-rich olefins and different Pd-sources. In these high-temperature Heck arylations, the oxidative addition intermediates were observed as bidentate ligand chelated cationic aryl palladium species, suggesting that the used ligand attaches to the metal center at the very beginning of the catalytic cycle. This was also in agreement with the obtained regioisomeric profile of the isolated products. The investigation supports the standard Pd(0)/Pd(II) Heck mechanism and provides further insight regarding the conceivable composition of fundamental Pd(II) intermediates in an ongoing Heck reaction.     

Grafting and anchoring of molecular complexes as metal precursors for alumina-supported Pd catalysts

Abstract

The surface coordination chemistry of Pd complexes on alumina has been studied in the framework of synthesizing Pd/γ-Al₂O₃ catalytic materials. Two methodologies were explored: the direct grafting of Pd complexes on hydroxyl functions present at the alumina surface and the anchoring of the precursors via amine-bearing silanes previously grafted on the support. Suitable conditions to graft and anchor Pd complexes on alumina surface were found and experimental proofs of grafting and anchoring processes are provided. The results show that covalent grafting indeed took place for samples prepared in acetonitrile with [Pd(CF₃CO₂)₂(bipy)] and [PdCl₂(PhCN)₂] complexes or with [Pd(OAc)₂] and [Pd(CF₃CO₂)₂] in acetone. The anchoring was successful for catalysts prepared in acetone with 1 wt.% of [Pd(CF₃CO₂)₂] loading. Grafting and anchoring were found to stabilize palladium in its Pd(II) oxidation state. This has an adverse effect on the activation step that should lead to reduction of the complex to give the metallic catalytic supported active phase.     

New studies on the apparent allyl rotation in scorpion-like palladium complexes. The influence of non-directly bonded groups. X-ray molecular structures of [Pd(η-2-Me-C3H4)L]TfO, L=bpzmArOMe and bpz*mCy

New cationic 2-Me-allylpalladium complexes were prepared with the N,N-donor chelate ligands bis(pyrazol-1-yl)(R)methane (R=anisol-2-yl, bpzmArOMe; 2-hydroxyphenyl, bpzmArOH) and bis(3,5-dimethylpyrazol-1-yl)(R)methane (R=anisol-2-yl, bpz*mArOMe; cyclohexyl, bpz*mCy and ferrocenyl, bpz*mFc). The bpz′mR ligands adopt a rigid boat conformation after coordination to the Pd center and the R group is in the axial position of the metallacycle. The new complexes exhibit two isomeric forms in solution that differ in the relative orientation of the 2-Me-allyl group with respect to the bpz′mRPd fragment. The fluxional behavior of the new complexes, mainly in the context of the isomerization process, has been analyzed. Conclusions concerning the influence on this isomerization of the R group and the pyrazole substituents in positions 3 and 5 are discussed. The isomerization process was found to be affected by the presence of coordinating anions (Cl⁻) or by a change in the complex concentration. The molecular structures of the complexes [Pd(η³-2-Me-C₃H₄)(bpzmArOMe)]TfO and [Pd(η³-2-Me-C₃H₄)(bpz*mCy)]TfO have been determined by X-ray diffraction studies.     

Phosphite ligands derived from distally and proximally substituted dipropyloxy calix[4]arenes and their palladium complexes: Solution dynamics, solid-state structures and catalysis

Two bisphosphite ligands, 25,27-bis-(2,2′-biphenyldioxyphosphinoxy)-26,28-dipropyloxy-p-tert-butyl calix[4]arene (3) and 25,26-bis-(2,2′-biphenyldioxyphosphinoxy)-27,28-dipropyloxy-p-tert-butyl calix[4]arene (4) and two monophosphite ligands, 25-hydroxy-27-(2,2′-biphenyldioxyphosphinoxy)-26,28-dipropyloxy-p-tert-butyl calix[4]arene (5) and 25-hydroxy-26-(2,2′-biphenyldioxyphosphinoxy)-27,28-dipropyloxy- p-tert-butyl calix[4]arene (6) have been synthesized. Treatment of (allyl) palladium precursors [(η³-1,3-R,R′-C₃H₄)Pd(Cl)]₂ with ligand 3 in the presence of NH₄PF₆ gives a series of cationic allyl palladium complexes (3a-3d). Neutral allyl complexes (3e-3g) are obtained by the treatment of the allyl palladium precursors with ligand 3 in the absence of NH₄PF₆. The cationic allyl complexes [(η³-C₃H₅)Pd(4)]PF₆ (4a) and [(η³-Ph₂C₃H₃)Pd(4)]PF₆ (4b) have been synthesized from the proximally (1,2-) substituted bisphosphite ligand 4. Treatment of ligand 4 with [Pd(COD)Cl₂] gives the palladium dichloride complex, [PdCl₂(4)] (4c). The solid-state structures of [{(η³-1-CH₃-C₃H₄)Pd(Cl)}₂(3)] (3f) and [PdCl₂(4)] (4c) have been determined by X-ray crystallography; the calixarene framework in 3f adopts the pinched cone conformation whereas in 4c, the conformation is in between that of cone and pinched cone. Solution dynamics of 3f has been studied in detail with the help of two-dimensional NMR spectroscopy.The solid-state structures of the monophosphite ligands 5 and 6 have also been determined; the calix[4]arene framework in both molecules adopts the cone conformation. Reaction of the monophosphite ligands (5, 6) with (allyl) palladium precursors, in the absence of NH₄PF₆, yield a series of neutral allyl palladium complexes (5a-5c; 6a-6d). Allyl palladium complexes of proximally substituted ligand 6 showed two diastereomers in solution owing to the inherently chiral calix[4]arene framework. Ligands 3, 6 and the allyl palladium complex 3f have been tested for catalytic activity in allylic alkylation reactions.     

A simple approach to azirines containing an aldehyde functionality and their stabilization as palladium(II) complexes

A simple and useful method for the synthesis of azirines containing an aldehyde functionality, from open chain bromo/chloro-aldehydes at room temperature and their stabilization as palladium(II) complexes are reported.     

Substituted (pyridinyl)benzoazole palladium complexes: Synthesis and application as Heck coupling catalysts

The synthesis of 2-(4-tert-butylpyridin-2-yl)-benzooxazole (L3), 2-(4-tert-butyl-pyridin-2-yl)-benzothiazole (L4) and 6-tert-Butyl-2-(4-tert-butyl-pyridin-2-yl)-benzothiazole (L5) by intramolecular cyclization under basic conditions is described. Reactions of 2-pyridin-2-yl-1H-benzoimidazole (L1), 2-pyridin-2-yl-benzothiazole (L2) and L3–L5 with either [Pd(NCMe)₂Cl₂] or [Pd(COD)MeCl] afforded the corresponding mononuclear palladium complexes [Pd(L1)MeCl] (1), [Pd(L2)MeCl] (2), [Pd(L3)Cl₂] (3), [Pd(L3)MeCl] (4), [Pd(L4)Cl₂] (5), [Pd(L4)MeCl] (6) and [Pd(L4)MeCl] (7) as confirmed by mass spectrometry and elemental analyses. The palladium complexes are efficient Heck coupling catalysts for the reaction of iodobenzene with butylacrylate under mild conditions. Benzothiazole and benzooxazole containing complexes show faster induction periods compared to the benzoimidazole analogues.     

Synthesis and characterization of methylpalladium(II) dithiolate complexes of the type [PdMe(SS)(ER3)] (SS = S2CNR 2, S2COEt, S2P(OR)2, S2 PPh2; ER3 = PMePh2, PPh3, AsPh3)

Methylpalladium(II) dithiolate complexes of the type [PdMe(SS)(ER₃] (SS = S₂ CNR₂ (R = Me or Et), S₂COEt, S₂P(OR)₂ (R = Et, ⁿPr, ⁱPr), S₂PPh₂; ER₃ = PMePh₂, PPh₃, AsPh₃) have been synthesized by the reaction of [Pd₂Me₂(μ-Cl)₂(PMePh₂)₂] with sodium/potassium/ammonium salts of the dithio acid or by treatment of [PdMeCl(cod)] with ER₃ followed by sodium/potassium/ammonium salts of the dithio ligand. All the complexes were characterized by elemental analysis, IR and nuclear magnetic resonance (¹H, ³¹P) data.     

Synthesis,characterization, and structures of nickel(II) and palladium(II) complexes of aromatic thiohydrazides

Nickel(II) and palladium(II) form neutral 1?:?2 chelates with aromatic thiohydrazides, for example. thiobenzhydrazide, o-hydroxythiobenzhydrazide, furan-2-thiohydrazide, and thiophen-2-thiohydrazide. All the compounds are diamagnetic and have been characterized by elemental analysis and spectroscopic methods. o-Hydroxythiobenzhydrazido complexes of nickel(II) and palladium(II) were crystallized from DMSO and their structures were solved by X-ray diffraction. The complexes are isostructural with planar structures. Metal ion is linked to two identical deprotonated ligands through trans hydrazinic nitrogen and sulfur. Hydrogen of OH is involved in intramolecular hydrogen-bonding.     

Impregnated palladium on magnetite as catalyst for multicomponent reductive amination reactions and other related reducing processes

The impregnated palladium on magnetite catalyst is a versatile system for different reduction processes using inexpensive polymehtylhydrosiloxane, including multicomponent reductive amination reactions, and aldehyde, imine, sulfinimide and sulfoxide reductions. This catalyst avoids the use of any type of expensive and quite expensive organic ligand, showing excellent yields, under mild reaction conditions. The catalyst is easily removed from the reaction medium, just by using a magnet. The catalytic system is very selective permitting the discrimination between ketones and aldehydes in the reductive amination process.     

Steric and electronic effects in stabilizing allyl-palladium complexes of “P-N-P” ligands, X2PN(Me)PX2 (X = OC6H5 or OC6H3Me2-2,6)

The chemistry of η³-allyl palladium complexes of the diphosphazane ligands, X₂PN(Me)PX₂ [X = OC₆H₅ (1) or OC₆H₃Me₂-2,6 (2)] has been investigated.The reactions of the phenoxy derivative, (PhO)₂PN(Me)P(OPh)₂ with [Pd(η³-1,3-R′,R″-C₃H₃)(μ-Cl)]₂ (R′ = R″ = H or Me; R′ = H, R″ = Me) give exclusively the palladium dimer, [Pd₂{μ-(PhO)₂PN(Me)P(OPh)₂}₂Cl₂] (3); however, the analogous reaction with [Pd(η³-1,3-R′,R″-C₃H₃)(μ-Cl)]₂ (R′ = R″ = Ph) gives the palladium dimer and the allyl palladium complex [Pd(η³-1,3-R′,R″-C₃H₃)(1)](PF₆) (R′ = R″ = Ph) (4). On the other hand, the 2,6-dimethylphenoxy substituted derivative 2 reacts with (allyl) palladium chloro dimers to give stable allyl palladium complexes, [Pd(η³-1,3-R′,R″-C₃H₃)(2)](PF₆) [R′ = R″ = H (5), Me (7) or Ph (8); R′ = H, R″ = Me (6)].Detailed NMR studies reveal that the complexes 6 and 7 exist as a mixture of isomers in solution; the relatively less favourable isomer, anti-[Pd(η³-1-Me-C₃H₄)(2)](PF₆) (6b) and syn/anti-[Pd(η³-1,3-Me₂-C₃H₃)(2)](PF₆) (7b) are present to the extent of 25% and 40%, respectively. This result can be explained on the basis of the steric congestion around the donor phosphorus atoms in 2. The structures of four complexes (4, 5, 7a and 8) have been determined by X-ray crystallography; only one isomer is observed in the solid state in each case.     

Dimethylaminoalkyl chalcogenolate palladium(II) complexes as an efficient copper- and phosphine-free catalyst for Sonogashira reaction

N,N-Dimethylaminoalkyl chalcogenolate Pd(II) complexes [PdCl(E^∩NMe₂)]_n has been investigated as a moisture/air-stable and robust catalyst for Sonogashira cross-coupling reaction in the absence of copper and phosphine ligand. The dimeric palladium(II) complex of selenium containing ligand shows the best catalytic activity as compared with monomeric and trimeric complexes. The variety of functional groups are tolerated under optimized catalytic systems and provide excellent yields of the products.     

Synthesis, X-ray structures, and catalytic applications of palladium(II) complexes bearing N-heterocyclic iminocarbene ligands

Two new Pd(II) N-heterocyclic iminocarbene complexes (C-N)PdCl₂ that contain 5-membered chelate rings have been prepared by carbene transfer from a silver iminocarbene precursor to (COD)PdCl₂. The new Pd imonocarbene complexes, as well as two that have been previously reported (altogether three 5-membered and one 6-membered chelate ring complexes) have been evaluated as catalysts for the Suzuki-Miyaura coupling reaction. The complexes were found to be active in the reaction, but without exceptional catalytic performances. The 5-membered chelate ring complexes appeared to be more robust and remained active for a longer time than the 6-membered ring congener. The catalytic performance of the 5-membered chelate ring complexes appeared to be rather insensitive to the steric demands of the imine-N-aryl group. The X-ray structure of one of the Ag iminocarbene complexes reveals the κ¹(C) bonding of the iminocarbene moiety in a nearly linear Ag(I) complex; two monomeric units are associated through a weak Ag-Ag interaction. The X-ray structures of two new Pd iminocarbene complexes (C-N)PdCl₂ confirm the chelating κ²(C,N) nature of the iminocarbene moiety; in both complexes, the Pd-Cl distances trans to carbene-C are slightly longer than those trans to imine-N.     

Efficient palladium catalysts for the amination of aryl chlorides: a comparative study on the use of phosphium salts as precursors to bulky, electron-rich phosphines

Alkyl-di-(1-adamantyl)phosphonium salts are practical ligand precursors for the palladium-catalyzed amination of aryl chlorides. In the presence of typically 0.5 mol% Pd(OAc)₂ and 1 mol% of ligand precursor a variety of activated and deactivated aryl chlorides can be aminated in good to excellent yield (73-99%). Applying optimized conditions catalyst turnover numbers up to 10,000 have been achieved.     

Synthesis, spectroscopy and structures of palladium(II) and platinum(II) complexes containing mercapto-o-carborane

The reactions of [M₂Cl₂(μ-Cl)₂(PMe₂Ph)₂] with mercapto-o-carboranes in the presence of pyridine afforded mono-nuclear complexes of composition, [MCl(SCb°R)(py)(PMe₂Ph)] (M = Pd or Pt; Cb° = o-C₂B₁₀H₁₀; R = H or Ph). The treatment of [PdCl₂(PEt₃)₂] with PhCb°SH yielded trans-[Pd(SCb°Ph)₂(PEt₃)₂] (4) which when left in solution in the presence of pyridine gave another substitution product, [Pd(SCb°Ph)₂(py)(PEt₃)] (5). The structures of [PdCl(SCb°Ph)(py)(PMe₂Ph)] (1), [Pd(SCb°Ph)₂(PEt₃)₂] (4) and [Pd(SCb^oPh)₂(py)(PEt₃)] (5) were established unambiguously by X-ray crystallography. The palladium atom in these complexes adopts a distorted square-planar configuration with neutral donor atoms occupying the trans positions. Thermolysis of [PdCl(SCb°)(py)(PMe₂Ph)] (2) in TOPO (trioctylphosphine oxide) at 200 °C gave nanocrystals of TOPO capped Pd₄S which were characterized by XRD pattern and SEM.