Synthesen und Strukturen von Übergangsmetall‐Komplexen mit Dithiophosphinato‐ und Trithiophosphonato‐Liganden

Syntheses and Structures of Transition Metal Complexes with Dithiophosphinato and Trithiophosphinato Ligands The reactions of MnCl₂ with Ph₂P(S)(SSiMe₃) produced [Mn(S₂PPh₂)₂(thf)₂] ( 1 ) and [Mn(S₂PPh₂)₂(dme)] ( 2 ) (DME = 1,2‐Dimethoxyethane). The compounds [Co₆(S₃PPh)₂(μ⁴‐S)₂(μ³‐S)₂(PPh₃)₄] ( 3 ), [Co₂(S₃PPh)₂(PPh₃)₂] ( 4 ), [Ni(S₂PPh)(PPhEt₂)₂] ( 5 ), [Ni(S₃PPh)(PPhEt₂)₂] ( 6 ) and [Cu₄(S₃PPh)₂(dppp)₂] ( 8 ) [dppp = 1,3‐Bis(diphenylphosphanyl)propane] were obtained from reactions of first‐row transition metal halides with PhP(S)(SSiMe₃)₂ in the presence of tertiary phosphines. In a reaction of PhP(S)(SSiMe₃)₂ with PhPEt₂ PhPEt₂PS₂Ph ( 7 ) was isolated. All compounds were characterized by X‐ray crystallography.     

Facile Hydrolysis and Alcoholysis of Palladium Acetate

Palladium(II) acetate is readily converted into [Pd₃(μ²‐OH)(OAc)₅] ( 1 ) in the presence of water in a range of organic solvents and is also slowly converted in the solid state. Complex 1 can also be formed in nominally anhydrous solvents. Similarly, the analogous alkoxide complexes [Pd₃(μ²‐OR)(OAc)₅] ( 3 ) are easily formed in solutions of palladium(II) acetate containing a range of alcohols. An examination of a representative Wacker‐type oxidation shows that the Pd‐OH complex 1 and a related Pd‐oxo complex 4 can be excluded as potential catalytic intermediates in the absence of exogenous water.     

Sterically‐Directed Consecutive and Size‐Selective Self‐Assembly of Palladium Diphosphane Complexes with an Ar‐BIAN Ligand: Unexpected Formation of Pentameric and Hexameric Aggregates

The coordination properties of N,N′‐bis[4‐(4‐pyridyl)phenyl]acenaphthenequinonediimine (L¹) and N,N′‐bis[4‐(2‐pyridyl)phenyl]acenaphthenequinonediimine (L²) were investigated in self‐assembly with palladium diphosphane complexes [Pd(P^P)(H₂O)₂](OTf)₂ (OTf=triflate) by using various analytical techniques, including multinuclear (¹H, ¹⁵N, and ³¹P) NMR spectroscopy and mass spectrometry (P^P=dppp, dppf, dppe; dppp=bis(diphenylphosphanyl)propane, dppf= bis(diphenylphosphanyl)ferrocene, and dppe=bis(diphenylphosphanyl)ethane). Beside the expected trimeric and tetrameric species, the interaction of an equimolar mixture of [Pd(dppp)]²⁺ ions and L¹ also generates pentameric aggregates. Due to the E/Z isomerism of L¹, a dimeric product was also observed. In all of these species, which correspond to the general formula [Pd(dppp)L¹]_n(OTf)_2n (n=2–5), the L¹ ligand is coordinated to the Pd center only through the terminal pyridyl groups. Introduction of a second equivalent of the [Pd(dppp)]²⁺ tecton results in coordination to the internal, sterically more encumbered chelating site and induces enhancement of the higher nuclearity components. The presence of higher‐order aggregates (n=5, 6), which were unexpected for the interaction of cis‐protected palladium corners with linear ditopic bridging ligands, has been demonstrated both by mass‐spectrometric and DOSY NMR spectroscopic analysis. The sequential coordination of the [Pd(dppp)]²⁺ ion is attributed to the dissimilar steric properties of the two coordination sites. In the self‐assembled species formed in a 1:1:1 mixture of [Pd(dppp)]²⁺/[Pd(dppe)]²⁺/L¹, the sterically more demanding [Pd(dppp)]²⁺ tectons are attached selectively to the pyridyl groups, whereas the more hindered imino nitrogen atoms coordinate the less bulky dppe complexes, thus resulting in a sterically directed, size‐selective sorting of the metal tectons. The propensity of the new ligands to incorporate hydrogen‐bonded solvent molecules at the chelating site was confirmed by X‐ray diffraction studies.     

Synthesis and characterization of supramolecular complexes of a tetranuclear metallocycle with platinum(II) bis-ethynylphenylethynylpyridine organometallic complexes

A novel Pt(II) organometallic complex with 4-(4-ethynyl-phenylethynyl)-pyridine (DEBPy-H), namely [Pt(dppp)(DEBPy)₂] (dppp = 1,3-bis-(diphenylphosphino)-propane), has been prepared by two synthetic routes. The DEBPy-H ligand was prepared by a shorter synthetic pathway and in higher yield than the method of previous reports. New Pt(II) organometallic complexes with DEBPy-H and four bidentate ligands [Pt(L–L)(DEBPy)₂] (L–L = 1,2-bis-(diphenylphosphino)-benzene, bis-(dicyclohexyl)-ethane, 4,4′-dimethyl-2,2′-bipyridine, or 5,5′-dimethyl-2,2′-bipyridine) have also been prepared. These five Pt(II) complexes have two pyridyl units as an available coordination site that can operate as a metalloligand in nanoscale tectonics. A supramolecular complex of a tetranuclear metallocycle, [Pt(dppp)(DEBPy)₂]₂[Pt(dppp)]₂(OTf)₄, was synthesized from [Pt(dppp)(DEBPy)₂] and [Pt(dppp)(OTf)₂] as a corner connection through coordination-driven self-assembly. This supramolecular Pt(II) complex exists as a tetranuclear structure of the square type according to the interpretation of the ESI-mass and NMR spectra in solution. Five Pt(II) organometallics demonstrated the formation of similar tetranuclear metallocycles with [Pt(dppp)(OTf)₂], as indicated by their ESI-mass and UV–vis spectra in solution.     

A New Class of Remote N‐Heterocyclic Carbenes with Exceptionally Strong σ‐Donor Properties: Introducing Benzo[c]quinolin‐6‐ylidene

We make the case for benzo[c]quinolin‐6‐ylidene ( 1 ) as a strongly electron‐donating carbene ligand. The facile synthesis of 6‐trifluoromethanesulfonylbenzo[c]quinolizinium trifluoromethanesulfonate ( 2 ) gives straightforward access to a useful precursor for oxidative addition to low‐valent metals, to yield the desired carbene complexes. This concept has been achieved in the case of [Mn(benzo[c]quinolin‐6‐ylidene)(CO)₅]⁺ ( 15 ) and [Pd(benzo[c]quinolin‐6‐ylidene)(PPh₃)₂(L)]²⁺ L=THF ( 21 ), OTf ( 22 ) or pyridine ( 23 ). Attempts to coordinate to nickel result in coupling products from two carbene precursor fragments. The CO IR‐stretching‐frequency data for the manganese compound suggests benzo[c]quinolin‐6‐ylidene is at least as strong a donor as any heteroatom‐stabilised carbene ligand reported.     

Organo‐Palladium(II)‐Verbindungen mit PdC4‐Koordination: Phenylethinyl‐ und Methylphenylethinylkomplexe

Tetrakis(p‐tolyl)oxalamidinato‐bis[acetylacetonatopalladium(II)] ([Pd₂(acac)₂(oxam)]) reacted with Li–C≡C–C₆H₅ in THF with formation of [Pd(C≡C–C₆H₅)₄Li₂(thf)₄] ( 1a ). Reaction of [Pd₂(acac)₂(oxam)] with a mixture of 6 equiv. Li–C≡C–C₆H₅ and 2 equiv. LiCH₃ resulted in the formation of [Pd(CH₃)(C≡C–C₆H₅)₃Li₂(thf)₄] ( 2 ), and the dimeric complex [Pd₂(CH₃)₄(C≡C–C₆H₅)₄Li₄(thf)₆] ( 3 ) was isolated upon reaction of [Pd₂(acac)₂(oxam)] with a mixture of 4 equiv. Li–C≡C–C₆H₅ and 4 equiv. LiCH₃. 1 – 3 are extremely reactive compounds, which were isolated as white needles in good yields (60–90%). They were fully characterized by IR, ¹H‐, ¹³C‐, ⁷Li‐NMR spectroscopy, and by X‐ray crystallography of single crystals. In these compounds Li ions are bonded to the two carbon atoms of the alkinyl ligand. 1a reacted with Pd(PPh₃)₄ in the presence of oxygen to form the already known complexes trans‐[Pd(C≡C–C₆H₅)₂(PPh₃)₂] and [Pd(η²‐O₂)(PPh₃)₂]. In addition, 1a is an active catalyst for the Heck coupling reaction, but less active in the catalytic Sonogashira reaction.     

(S)P(SSiMe3)3 als Ausgangsverbindung für die Synthese neuer Palladium-Clusterkomplexe

(S)P(SSiMe₃)₃ as Starting Material for the Synthesis of New Palladium Cluster Complexes The silylated derivative of thiophosphoric acid (S)P(SSiMe₃)₃ is used as a convenient starting compound for the synthesis of multinuclear Pd cluster complexes. (S)P(SSiMe₃)₃ reacts with Pd(acac)₂/PPh₃ and [PdCl₂(PPh₃)₂] to give the following crystalline compounds: [Pd₂(PS₄)Cl(PEt₃)₃] ( 1 ), [Pd₂(PS₄)(PEt₃)₄]Cl ( 2 ), [Pd₃(PS₄)(PS₃)(PEt₃)₄] ( 3 ), [Pd₆(PS₄)₄(PPh₃)₆] ( 4 ) and [Pd₆(PS₂(OEt)₂)₂(μ₃-S)₄(μ₂-SH)₂(PPh₃)₄] ( 5 ). The above mentioned compounds have been characterised by X-ray structure determination.     

Tripropylarsane Complexes of Palladium(II) and Platinum(II) — Syntheses,Spectroscopy, and Structures

Several palladium(II) and platinum(II) complexes of tripropylarsanes (AsR₃; R = Pr, iPr) with the formulae, [MCl₂(AsR₃)₂], [M₂Cl₂(μ‐Cl)₂(AsR₃)₂], [Pd₂Me₂(μ‐Cl)₂(AsR₃)₂], [Pd₂X₂(μ‐Pz)₂(AsR₃)₂] (X = Cl or Me, Pz = pyrazolate), [Pd₂Cl₂(μ‐Y)₂(AsR₃)₂] (Y = OAc or SPh), [MCl(S₂CNEt₂)(AsR₃)] and [PdCp(Cl)(AsiPr₃)] (M = Pd or Pt) have been prepared. All the complexes have been characterised by elemental analyses, IR and ¹H NMR spectroscopy. The stereochemistry of the complexes has been deduced from the spectroscopic data. The structures of [Pd₂Me₂(μ‐X)₂(AsiPr₃)₂] (X = Cl or Pz) have been established by single crystal X‐ray diffraction analyses. Both of the complexes have sym‐trans configuration. Strong trans influence of the methyl group is reflected on the Pd—X bond distances.     

Pd(II) complexes of N(4)‐substituted phenylaminoacetohydrazone and biacetylmonooxime: synthesis,characterization, structures and catalytic behaviour towards Suzuki–Miyaura coupling reactions

The reaction of [PdCl₂(CH₃CN)₂] and N(4)‐substituted phenylaminoacetohydrazone ligands (LH) in methanol at room temperature afforded air‐ and moisture‐stable palladium(II) complexes of two types with general formulae [Pd(LH)Cl] and [Pd₂(LH)(L)]Cl. An unusual coordination mode of ligand LH is observed, in which the ligand coordinates through N(4)H nitrogen and without enolization of the carbonyl group of the hydrazone moiety in both mono‐ and bimetallic complexes. The crystal structure of the complexes reveals that the oxime LH reacts with [PdCl₂(CH₃CN)₂] presumably via the elimination of HCl from hydrazine NH. All the synthesized Pd(II) complexes were evaluated as catalysts in the Suzuki cross‐coupling reaction of aryl halides, activated 4‐bromoacetophenone and non‐activated bromobenzene, with phenylboronic acid in aqueous medium. In both cases, i.e. with activated and non‐activated aryl halides, all the complexes show moderate conversion leading to biaryls with yields in the range 50–65%. Copyright © 2016 John Wiley & Sons, Ltd.     

Tetrapyrroles as π Donors: A Pd22+ Unit Sandwiched between Two Helical Bilindione–Palladium Moieties

Metalation of octaethylbilindione (H₃OEB) with palladium(II ) acetate produces the novel tetranuclear complex [Pd₄(OEB)₂] in which a (Pd^I₂)²⁺ unit is sandwiched between two helical Pd(OEB) units (see picture). Treatment of [Pd₄(OEB)₂] with pyridine/ethanol gives the odd-electron helical complex [Pd(OEB)].     

Pd‐Catalyzed Carbonylative α‐Arylation of Aryl Bromides: Scope and Mechanistic Studies

Reaction conditions for the three‐component synthesis of aryl 1,3‐diketones are reported applying the palladium‐catalyzed carbonylative α‐arylation of ketones with aryl bromides. The optimal conditions were found by using a catalytic system derived from [Pd(dba)₂] (dba=dibenzylideneacetone) as the palladium source and 1,3‐bis(diphenylphosphino)propane (DPPP) as the bidentate ligand. These transformations were run in the two‐chamber reactor, COware, applying only 1.5 equivalents of carbon monoxide generated from the CO‐releasing compound, 9‐methylfluorene‐9‐carbonyl chloride (COgen). The methodology proved adaptable to a wide variety of aryl and heteroaryl bromides leading to a diverse range of aryl 1,3‐diketones. A mechanistic investigation of this transformation relying on ³¹P and ¹³C NMR spectroscopy was undertaken to determine the possible catalytic pathway. Our results revealed that the combination of [Pd(dba)₂] and DPPP was only reactive towards 4‐bromoanisole in the presence of the sodium enolate of propiophenone suggesting that a [Pd(dppp)(enolate)] anion was initially generated before the oxidative‐addition step. Subsequent CO insertion into an [Pd(Ar)(dppp)(enolate)] species provided the 1,3‐diketone. These results indicate that a catalytic cycle, different from the classical carbonylation mechanism proposed by Heck, is operating. To investigate the effect of the dba ligand, the Pd⁰ precursor, [Pd(η³‐1‐PhC₃H₄)(η⁵‐C₅H₅)], was examined. In the presence of DPPP, and in contrast to [Pd(dba)₂], its oxidative addition with 4‐bromoanisole occurred smoothly providing the [PdBr(Ar)(dppp)] complex. After treatment with CO, the acyl complex [Pd(CO)Br(Ar)(dppp)] was generated, however, its treatment with the sodium enolate led exclusively to the acylated enol in high yield. Nevertheless, the carbonylative α‐arylation of 4‐bromoanisole with either catalytic or stoichiometric [Pd(η³‐1‐PhC₃H₄)(η⁵‐C₅H₅)] over a short reaction time, led to the 1,3‐diketone product. Because none of the acylated enol was detected, this implied that a similar mechanistic pathway is operating as that observed for the same transformation with [Pd(dba)₂] as the Pd source.     

Cavitand‐Based Pd‐Pyridyl Coordination Capsules: Guest‐Induced Homo‐ or Heterocapsule Selection and Applications of Homocapsules to the Protection of a Photosensitive Guest and Chiral Capsule Formation

A 2 : 4 mixture of tetrakis[4‐(4‐pyridyl)phenyl]cavitand ( 1 ) or tetrakis[4‐(4‐pyridyl)phenylethynyl]cavitand ( 2 ) and Pd(dppp)(OTf)₂ self‐assembles into a homocapsule { 1 ₂ ? [Pd(dppp)]₄}⁸⁺ ? (TfO^?)₈ ( C1 ) or { 2 ₂ ? [Pd(dppp)]₄}⁸⁺ ? (TfO^?)₈ ( C2 ), respectively, through Pd?Npy coordination bonds. A 1 : 1 : 4 mixture of 1 , 2 , and Pd(dppp)(OTf)₂ produced a mixture of homocapsules C1 , C2 , and a heterocapsule { 1 ? 2 ? [Pd(dppp)]₄}⁸⁺ ? (TfO^?)₈ ( C3 ) in a 1 : 1 : 0.98 mole ratio. Selective formation (self‐sorting) of homocapsules C1 and C2 or heterocapsule C3 was controlled by guest‐induced encapsulation under thermodynamic control. Applications of Pd?Npy coordination capsules with the use of 1 were demonstrated. Capsule C1 serves as a guard nanocontainer for trans‐4,4′‐diacetoxyazobenzene to protect against the trans‐to‐cis photoisomerization by encapsulation. A chiral capsule { 1 ₂ ? [Pd((R)‐BINAP)]₄}⁸⁺ ? (TfO^?)₈ ( C5 ) was also constructed. Capsule C5 induces supramolecular chirality with respect to prochiral 2,2′‐bis(alkoxycarbonyl)‐4,4′‐bis(1‐propynyl)biphenyls by diastereomeric encapsulation through the asymmetric suppression of rotation around the axis of the prochiral biphenyl moiety.     

Syntheses,Structures and Properties of Four Manganese Coordination Polymers Derived from 4-Amino-3,5-bis(imidazol-1-ylmethyl)-1,2,4-triazole

The reaction of 4‐amino‐3,5‐bis(imidazol‐1‐ylmethyl)‐1,2,4‐triazole (abit) and manganese(II) salts yields four coordination polymers {[Mn(abit)₃](ClO₄)₂}_n ( 1 ), {[Mn(abit)₃](PF₆)₂}_n ( 2 ), [Mn(abit)(dca)₂(H₂O)₂]_n ( 3a ) and [Mn(abit)(dca)₂(H₂O)₂]_n ( 3b ). Compounds 1 and 2 are one‐dimensional triple‐stranded chain. Compounds 3a and 3b are polymorphous and construct one‐dimensional single chain. The conformational analysis is performed. The thermal properties have been investigated.     

A trinuclear palladium(II) complex containing N,S‐coordinating 2‐(benzylsulfanyl)anilinide and 1,3‐benzothiazole‐2‐thiolate ligands with a central square‐planar PdN4 motif

The reaction of dichlorido(cod)palladium(II) (cod = 1,5‐cyclooctadiene) with 2‐(benzylsulfanyl)aniline followed by heating in N,N‐dimethylformamide (DMF) produces the linear trinuclear Pd₃ complex bis(μ₂‐1,3‐benzothiazole‐2‐thiolato)bis[μ₂‐2‐(benzylsulfanyl)anilinido]dichloridotripalladium(II) N,N‐dimethylformamide disolvate, [Pd₃(C₇H₄NS₂)₂(C₁₃H₁₂NS)₂Cl₂]·2C₃H₇NO. The molecule has symmetry and a Pd...Pd separation of 3.2012 (4) Å. The outer Pd^II atoms have a square‐planar geometry formed by an N,S‐chelating 2‐(benzylsulfanyl)anilinide ligand, a chloride ligand and the thiolate S atom of a bridging 1,3‐benzothiazole‐2‐thiolate ligand, while the central Pd^II core shows an all N‐coordinated square‐planar geometry. The geometry is perfectly planar within the PdN₄ core and the N—Pd—N bond angles differ significantly [84.72 (15)° for the N atoms of ligands coordinated to the same outer Pd atom and 95.28 (15)° for the N atoms of ligands coordinated to different outer Pd atoms]. This trinuclear Pd₃ complex is the first example of one in which 1,3‐benzothiazole‐2‐thiolate ligands are only N‐coordinated to one Pd centre. The 1,3‐benzothiazole‐2‐thiolate ligands were formed in situ from 2‐(benzylsulfanyl)aniline.     

Conformational change induced by hydration: trans‐dichloro­bis­[(1R,2R,3R,5S)‐(–)‐isopinocampheylamine]palladium(II) and trans‐dichloro­bis[(1S,2S,3S,5R)‐(+)‐isopinocampheylamine]palladium(II) hemihydrate

The title compounds, trans‐dichloro­bis[(1R,2R,3R,5S)‐(−)‐2,6,6‐trimethyl­bicyclo­[3.1.1]heptan‐3‐amine]palladium(II), [PdCl₂(C₁₀H₁₉N)₂], and trans‐dichloro­bis[(1S,2S,3S,5R)‐(+)‐2,6,6‐trimethyl­bicyclo­[3.1.1]heptan‐3‐amine]palladium(II) hemihydrate, [PdCl₂(C₁₀H₁₉N)₂]·0.5H₂O, present different arrangements of the amine ligands coordinated to Pd^II, viz. antiperiplanar in the former case and (−)anticlinal in the latter. The hemihydrate is an inclusion compound, with a Pd coordination complex and disordered water mol­ecules residing on crystallographic twofold axes. The crystal structure for the hemihydrate includes a short Pd⋯Pd separation of 3.4133 (13) Å.     

New palladium complexes with phosphino‐ and phosphinitopyridine ligands

Three new palladium complexes containing a difunctional P,N‐chelate, namely tris­(chloro­{[1‐methyl‐1‐(6‐methyl‐2‐pyridyl)ethoxy]diphenylphospine‐κ²N,P}methyl­palladium(II)chloro­form solvate, 3[Pd(CH₃)Cl(C₂₁H₂₂NOP)]·CHCl₃, (III), dichloro­[2‐(2,6‐dimethyl­phen­yl)‐6‐(diphenyl­phosphinometh­yl)­pyridine‐κ²N,P]palladium(II), [PdCl₂(C₂₆H₂₄NP)], (IV), and chloro­[2‐(2,6‐dimethyl­phen­yl)‐6‐(diphenyl­phos­phino­meth­yl)pyridine‐κ²N,P]methyl­palladium(II), [Pd(CH₃)Cl(C₂₆H₂₄NP)], (V), are reported. Geometric data and the conformations of the ligands around the metal centers, as well as slight distortions of the Pd coordination environments from idealized square‐planar geometry, are discussed and compared with the situations in related compounds. Non‐conventional hydrogen‐bond inter­actions (C—H⋯Cl) have been found in all three complexes. Compound (III) is the first six‐membered chloro–meth­yl–phosphinite P,N‐type Pd^II complex to be structurally characterized.     

Two isomers of Pd2(S2NC7H4)4

The dichloromethane solvates of the isomers tetrakis(μ‐1,3‐benzothiazole‐2‐thiolato)‐κ⁴N:S;κ⁴S:N‐dipalladium(II)(Pd—Pd), (I), and tetrakis(μ‐1,3‐benzothiazole‐2‐thiolato)‐κ⁶N:S;κ²S:N‐dipalladium(II)(Pd—Pd), (II), both [Pd₂(C₇H₄NS₂)₄]·CH₂Cl₂, have been synthesized in the presence of (o‐isopropylphenyl)diphenylphosphane and (o‐methylphenyl)diphenylphosphane. Both isomers form a lantern‐type structure, where isomer (I) displays a regular and symmetric coordination and isomer (II) an asymmetric and distorted structure. In (I), sitting on an centre of inversion, two 1,3‐benzothiazole‐2‐thiolate units are bonded by a Pd—N bond to one Pd atom and by a Pd—S bond to the other Pd atom, and the other two benzothiazolethiolate units are bonded to the same Pd atoms by, respectively, a Pd—S and a Pd—N bond. In (II), three benzothiazolethiolate units are bonded by a Pd—N bond to one Pd atom and by a Pd—S bond to the other Pd atom, and the fourth benzothiazolethiolate unit is bonded to the same Pd atoms by, respectively, a Pd—S bond and a Pd—N bond.     

Pincer‐Type Heck Catalysts and Mechanisms Based on PdIV Intermediates: A Computational Study

Pincer‐type palladium complexes are among the most active Heck catalysts. Due to their exceptionally high thermal stability and the fact that they contain Pd^II centers, controversial Pd^II/Pd^IV cycles have been often proposed as potential catalytic mechanisms. However, pincer‐type Pd^IV intermediates have never been experimentally observed, and computational studies to support the proposed Pd^II/Pd^IV mechanisms with pincer‐type catalysts have never been carried out. In this computational study the feasibility of potential catalytic cycles involving Pd^IV intermediates was explored. Density functional calculations were performed on experimentally applied aminophosphine‐, phosphine‐, and phosphite‐based pincer‐type Heck catalysts with styrene and phenyl bromide as substrates and (E)‐stilbene as coupling product. The potential‐energy surfaces were calculated in dimethylformamide (DMF) as solvent and demonstrate that Pd^II/Pd^IV mechanisms are thermally accessible and thus a true alternative to formation of palladium nanoparticles. Initial reaction steps of the lowest energy path of the catalytic cycle of the Heck reaction include dissociation of the chloride ligands from the neutral pincer complexes [{2,6‐C₆H₃(XPR₂)₂}Pd(Cl)] [X=NH, R=piperidinyl ( 1 a ); X=O, R=piperidinyl ( 1 b ); X=O, R=iPr ( 1 c ); X=CH₂, R=iPr ( 1 d )] to yield cationic, three‐coordinate, T‐shaped 14e^? palladium intermediates of type [{2,6‐C₆H₃(XPR₂)₂}Pd]⁺ ( 2 ). An alternative reaction path to generate complexes of type 2 (relevant for electron‐poor pincer complexes) includes initial coordination of styrene to 1 to yield styrene adducts [{2,6‐C₆H₃(XPR₂)₂}Pd(Cl)(CH₂?CHPh)] ( 4 ) and consecutive dissociation of the chloride ligand to yield cationic square‐planar styrene complexes [{2,6‐C₆H₃(XPR₂)₂}Pd(CH₂?CHPh)]⁺ ( 6 ) and styrene. Cationic styrene adducts of type 6 were additionally found to be the resting states of the catalytic reaction. However, oxidative addition of phenyl bromide to 2 result in pentacoordinate Pd^IV complexes of type [{2,6‐C₆H₃(XPR₂)₂}Pd(Br)(C₆H₅)]⁺ ( 11 ), which subsequently coordinate styrene (in trans position relative to the phenyl unit of the pincer cores) to yield hexacoordinate phenyl styrene complexes [{2,6‐C₆H₃(XPR₂)₂}Pd(Br)(C₆H₅)(CH₂?CHPh)]⁺ ( 12 ). Migration of the phenyl ligand to the olefinic bond gives cationic, pentacoordinate phenylethenyl complexes [{2,6‐C₆H₃(XPR₂)₂}Pd(Br)(CHPhCH₂Ph)]⁺ ( 13 ). Subsequent β‐hydride elimination induces direct HBr liberation to yield cationic, square‐planar (E)‐stilbene complexes with general formula [{2,6‐C₆H₃(XPR₂)₂}Pd(CHPh?CHPh)]⁺ ( 14 ). Subsequent liberation of (E)‐stilbene closes the catalytic cycle.     

Synthesis of dinuclear palladium complexes having two parallel isocyanide ligands,and their application as catalysts to pyrrole formation from tert-butylisocyanide and alkynes

Novel dinuclear palladium complexes having two isocyanide ligands were synthesized by using a binucleating ligand, N,N′-bis[2-(diphenylphsphino)phenyl]formamidinate (dpfam). The structure of [Pd₂(μ-dpfam)(tert-BuNC)₂]Cl was confirmed by X-ray analysis, showing that the Pd–Pd bond length of 2.5824(3) Å falls well within the range of those for known dipalladium complexes having the edge-sharing structure and two isocyanides coordinate to palladium in almost parallel and in close proximity. The dinuclear complex [Pd₂(μ-dpfam)(tert-BuNC)₂]PF₆ served as catalyst for pyrrole formation from tert-butylisocyanide and various alkynes.     

Trinuclear Pd3O2 Intermediate in Aerobic Oxidation Catalysis

The activation of O₂ is a key step in selective catalytic aerobic oxidation reactions mediated by transition metals. The bridging trinuclear palladium species, [(LPd^II)₃(μ³‐O)₂]²⁺ (L=2,9‐dimethylphenanthroline), was identified during the [LPd(OAc)]₂(OTf)₂‐catalyzed aerobic oxidation of 1,2‐propanediol. Independent synthesis, structural characterization, and catalytic studies of the trinuclear compound show that it is a product of oxygen activation by reduced palladium species and is a competent intermediate in the catalytic aerobic oxidation of alcohols. The formation and catalytic activity of the trinuclear Pd₃O₂ species illuminates a multinuclear pathway for aerobic oxidation reactions catalyzed by Pd complexes.