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.     

Pd(II)-catalyzed deprotection of acetals and ketals containing acid sensitive functional groups

The pincer complex [Pd(C¹,O¹,N¹-L)(NCMe)]ClO₄ (L = monoanionic ligand resulting from deprotonation of the acetyl group of the dimethyl monoketal of 2,6-diacetylpyridine) is used for the high-yield and selective catalytic hydrolysis of aliphatic, aromatic, cyclic, and acyclic dimethyl-acetals, -ketals, and dioxolanes, even in the presence of large substituents. Other protecting groups, such as THP or TBDMS, or very acid-sensitive alcohols were not affected. The catalyst is easily prepared in high yield from Pd(AcO)₂ and 2,6-diacetylpyridinium perchlorate stable to air and moisture, easily and fully recoverable and reusable.     

Mono‐ and dinuclear oxime palladacycles bearing diphosphine ligands: An unusual coordination mode for dppe

Three new oxime‐based palladacycles, namely [Pd{C,N‐C₆H₄{C(Me)?NOH}‐2}(dppm)]ClO₄ ( 1 ), [Pd₂{C,N‐C₆H₄{C(Me)?NOH}‐2}₂(dppe)₂(μ‐dppe)](ClO₄)₂ ( 2 ) and [Pd{C,N‐C₆H₄{C(Me)?NOH}‐2}(dppmS₂)]ClO₄ ( 3 ), were synthesized by the reaction of dinuclear oxime complex [Pd{C,N‐C₆H₄{C(Me)?NOH}‐2}(μ‐Cl)]₂ with different diphosphine ligands (dppm, dppe and dppmS₂). The synthesized complexes were characterized using Fourier transform infrared, ³¹P NMR, ¹H NMR and ¹³C NMR spectroscopic methods and elemental analyses, and their molecular structures were elucidated using X‐ray crystallography. The structure of 2 is worthy of note as it is the first oxime palladacycle where there are both bridging (P–) and chelating (P^P) dppe ligands, giving rise to a dinuclear complex. The palladium atom is in a five‐coordinate, square pyramidal P₃NC environment, while in 3 the palladium atom is in a distorted square planar environment, coordinated by the oxime ligand and a chelating (S^S) dppmS₂ ligand. These complexes were employed as efficient catalysts for the Suzuki–Miyaura cross‐coupling reaction of several aryl bromides with phenylboronic acid. The in vitro cytotoxicity of the compounds was also evaluated against human tumour cell lines (HT29, A549 and HeLa) using the MTT assay method. The results indicate that the dinuclear complex 2 has greater catalytic and anticancer activity in comparison with the mononuclear complexes 1 and 3 .     

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.     

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.     

Palladium(II) Acetylide Complexes with Pincer‐Type Ligands: Photophysical Properties,Intermolecular Interactions,and Photo‐cytotoxicity

Two classes of cationic palladium(II) acetylide complexes containing pincer‐type ligands, 2,2′:6′,2′′‐terpyridine (terpy) and 2,6‐bis(1‐butylimidazol‐2‐ylidenyl)pyridine (C^N^C), were prepared and structurally characterized. Replacing terpy with the strongly σ‐donating C^N^C ligand with two N‐heterocyclic carbene (NHC) units results in the Pd^II acetylide complexes displaying phosphorescence at room temperature and stronger intermolecular interactions in the solid state. X‐ray crystal structures of [Pd(terpy)(C≡CPh)]PF₆ ( 1 ) and [Pd(C^N^C)(C≡CPh)]PF₆ ( 7 ) reveal that the complex cations are arranged in a one‐dimensional stacking structure with pair‐like Pd^II⋅⋅⋅Pd^II contacts of 3.349 Å for 1 and 3.292 Å for 7 . Density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) calculations were used to examine the electronic properties. Comparative studies of the [Pt(L)(C≡CPh)]⁺ analogs by ¹H NMR spectroscopy shed insight on the intermolecular interactions of these Pd^II acetylide complexes. The strong Pd−C_carbene bonds render 7 and its derivative sufficiently stable for investigation of photo‐cytotoxicity under cellular conditions.     

Neutral and cationic orthopalladated ( ) complexes ( = phenylazophenyl, dimethylbenzylamine, 8-methylquinoline, 2-methoxy-3-N,N-dimetrylaminopropyl)

By reacting [Pd( )(μ-Cl)]₂ with AgClO₄ in NCMe, the corresponding cationic complexes [Pd( )(NCMe)₂]ClO₄ ( = phenylazophenyl-C²,N¹; dimethylbenzylamine-C²,N; 8-methylquinoline-C⁸,N) can be obtained. Solutions containing the cations [Pd( )(S)₂]⁺ are obtained when the reaction is carried out in tetrahydrofuran or acetone (S). The treatment of these solutions with bidentate ligands (L—L) (Ph₂PCH₂PPh₂,Ph₂PNHPPh₂ or Ph₂PCH₂PPh₂CHC(O)Ph) gives the mononuclear [Pd( )(L3l)]ClO₄ complexes, with L3l acting as a chelate ligand. On the other hand [Pd( (μ-Cl)]₂ reacts with L3l (Ph₂PCH₂PPh₂, Ph₂PNHPPh₂) yielding [Pd( )Cl(L3l)] with L3l acting as monodentate. The reactions between [Pd( )(NCMe)₂]ClO₄ and 2,2′-bipyrimidyl give rise to the formation of the mononuclear [Pd( ) (bipym)]ClO₄ or binuclear [Pd₂( )₂(μ-bipym)](ClO₄)₂, [( )Pd(μ-bipym)Pd( )](ClO₄)₂ derivatives. Finally [Pd( )Cldppm] (dppm = Ph₂PCH₂PPh₂) react with NaH producing the neutral complexes [Pd( )(ddppm)] (ddppm = Ph₂PCHPPh₂) which by reaction with HCl lead again to the starting materials [Pd( )Cl(dppm)].     

A barium perchlorate complex with a lateral macrobicycle derived from 1,10‐di­aza‐15‐crown‐5 containing a phenol Schiff base spacer

In the crystal structure of (acetonitrile‐κN)[13‐methyl‐39‐oxido‐1,17,25‐tri­aza‐9‐azonia‐28,31,36‐trioxapentacyclo[23.8.5.1^11,15.0^3,8.0^18,23]nonatriaconta‐3,5,7,9,11,13,15(39),16,18,20,22‐un­decaene‐κ⁷N¹,N¹⁷,N²⁵,O²⁸,O³¹,O³⁶,O³⁹](perchlorato‐κ²O,O′)barium(II) perchlorate acetonitrile hemisolvate, [Ba(ClO₄)(C₂H₃N)(C₃₃H₄₀N₄O₄)]ClO₄·0.5CH₃CN, the barium(II) cation is asymmetrically situated in the macrobicyclic cavity and is bound to seven of the eight heteroatoms of the macrobicyclic ligand, to the N atom of an aceto­nitrile mol­ecule and to two O atoms of one perchlorate group. The azonia N atom is not coordinated to the barium(II) cation and is involved in an intramolecular hydrogen‐bonding interaction with the oxido O atom.     

An acetatopalladium(II) complex with 1‐benzyl‐N‐(3,5‐di‐tert‐butylsalicylidene)piperidin‐4‐amine: Synthesis,structure and catalytic applications in Suzuki–Miyaura coupling of arylboronic acids with hydroxyaryl halides

The Schiff base 1‐benzyl‐N ‐(3,5‐di‐tert ‐butylsalicylidene)piperidin‐4‐amine (HL) and its acetatopalladium(II) complex having the formula [Pd(L)(OAc)] were synthesized. Both HL and [Pd(L)(OAc)] were characterized using elemental analysis and various spectroscopic (infrared, UV–visible, ¹H NMR and ¹³C NMR) and mass spectrometric measurements. The molecular structure of the complex was determined using X‐ray crystallographic analysis. In the complex, the pincer‐like NNO‐donor L⁻ and the monodenate OAc⁻ provide a distorted square‐planar N₂O₂ coordination environment around the metal centre. The physicochemical properties and the spectroscopic features of [Pd(L)(OAc)] are consistent with its molecular structure. The complex was found to be an effective catalyst for the Suzuki–Miyaura cross‐coupling reactions of hydroxyaryl halides with arylboronic acids in predominantly aqueous media. The reactions afforded hydroxybiaryl products in good to excellent yields with a wide substrate scope.     

Exploring the Oxidative‐Addition Pathways of Phenyl Chloride in the Presence of PdII Abnormal N‐Heterocyclic Carbene Complexes: A DFT Study

DFT calculations were performed to elucidate the oxidative addition mechanism of the dimeric palladium(II) abnormal N‐heterocyclic carbene complex 2 in the presence of phenyl chloride and NaOMe base under the framework of a Suzuki–Miyaura cross‐coupling reaction. Pre‐catalyst 2 undergoes facile, NaOMe‐assisted dissociation, which led to monomeric palladium(II) species 5 , 6 , and 7 , each of them independently capable of initiating oxidative addition reactions with PhCl. Thereafter, three different mechanistic routes, path a, path b, and path c, which originate from the catalytic species 5 , 7 , and 6 , were calculated at M06‐L ‐D3(SMD)/LANL2TZ(f)(Pd)/6–311++G**//M06‐L/LANL2DZ(Pd)/6–31+G* level of theory. All studied routes suggested the rather uncommon Pd^II/Pd^IV oxidative addition mechanism to be favourable under the ambient reaction conditions. Although the Pd⁰/Pd^II routes are generally facile, the final reductive elimination step from the catalytic complexes were energetically formidable. The Pd^II/Pd^IV activation barriers were calculated to be 11.3, 9.0, 26.7 kcal mol^?1 (ΔΔ^≠G_L^S‐D3) more favourable than the Pd^II/Pd⁰ reductive elimination routes for path a, path b, and path c, respectively. Out of all the studied pathways, path a was the most feasible as it comprised of a Pd^II/Pd^IV activation barrier of 24.5 kcal mol^?1 (ΔG_L^S‐D3). To further elucidate the origin of transition‐state barriers, EDA calculations were performed for some key saddle points populating the energy profiles.     

Cyclooctenyl complexes of palladium(II) with multidentate N-heterocycles

[Pd(cod)(cotl)]ClO₄ (cod = 1,5-cyclooctadiene, cotl = cyclooctenyl, C₈H₁₃ ^–) undergoes substitutions with multidentate N-heterocycles: 1,3-bis(benzimidazolyl)benzene (L¹), 1,3-bis(1-methylbenzimidazol-2-yl)benzene (L²), 2,6-bis(benzimidazolyl)pyridine (L³) and 2,6-bis(1-methylbenzimidazol-2-yl)pyridine (L⁴) to yield mono/binuclear complexes: [Pd(cotl)(L¹)(OClO₃)], [Pd(cotl)(L)]ClO₄ (L = L² or L³) and [Pd(cotl)₂(L⁴)](ClO₄)₂. Dihalobridged binuclear complexes [PdX(cotl)]₂ (X = Cl or Br) undergo halogen bridge cleavages with the multidentate N-heterocycles to form binuclear complexes of the type [PdX(cotl)₂L] (X = Cl or Br; L = L¹, L², L³ or L⁴). The complexes were characterized by elemental analyses, ¹H-, ¹³C-n.m.r., i.r., far-i.r. and FAB-mass spectral studies.     

Osmium(III) and Osmium(V) Complexes Bearing a Macrocyclic Ligand: A Simple and Efficient Catalytic System for cis‐Dihydroxylation of Alkenes with Hydrogen Peroxide

A simple protocol that uses [Os^III(OH)(H₂O)(L ‐N₄Me₂)](PF₆)₂ ( 1 ; L ‐N₄Me₂=N,N′‐dimethyl‐2,11‐diaza[3.3](2,6)pyridinophane) as a catalyst and H₂O₂ as a terminal oxidant for efficient cis‐1,2‐dihydroxylation of alkenes is presented. Unfunctionalized (or aliphatic) alkenes and alkenes/styrenes containing electron‐withdrawing groups are selectively oxidized to the corresponding vicinal diols in good to excellent yields (46–99 %). In the catalytic reactions, the stoichiometry of alkene:H₂O₂ is 1:1, and thus the oxidant efficiency is very high. For the dihydroxylation of cyclohexene, the catalytic amount of 1 can be reduced to 0.01 mol % to achieve a very high turnover number of 5500. The active oxidant is identified as the Os^V(O)(OH) species ( 2 ), which is formed via the hydroperoxide adduct, an Os^III(OOH) species. The active oxidant 2 is successfully isolated and crystallographically characterized.     

Synthesis and Self‐Assembly of NCN‐Pincer Pd‐Complex‐Bound Norvalines

The NCN‐pincer Pd‐complex‐bound norvalines Boc‐D /L ‐[PdCl(dpb)]Nva‐OMe ( 1 ) were synthesized in multigram quantities. The molecular structure and absolute configuration of 1 were unequivocally determined by single‐crystal X‐ray structure analysis. The robustness of 1 under acidic/basic conditions provides a wide range of N‐/C‐terminus convertibility based on the related synthetic transformations. Installation of a variety of functional groups into the N‐/C‐terminus of 1 was readily carried out through N‐Boc‐ or C‐methyl ester deprotection and subsequent condensations with carboxylic acids, R¹COOH, or amines, R²NH₂, to give the corresponding N‐/C‐functionalized norvalines R¹‐D /L ‐[PdCl(dpb)]Nva‐R² 2 – 9 . The dipeptide bearing two Pd units 10 was successfully synthesized through the condensation of C‐free 1 with N‐free 1 . The robustness of these Pd‐bound norvalines was adequately demonstrated by the preservation of the optical purity and Pd unit during the synthetic transformations. The lipophilic Pd‐bound norvalines L ‐ 2 , Boc‐L ‐[PdCl(dpb)]Nva‐NH‐n‐C₁₁H₂₃, and L ‐ 4 , n‐C₄H₉CO‐L ‐[PdCl(dpb)]Nva‐NH‐n‐C₁₁H₂₃, self‐assembled in aromatic solvents to afford supramolecular gels. The assembled structures in a thermodynamically stable single crystal of L ‐ 2 and kinetically stable supramolecular aggregates of L ‐ 2 were precisely elucidated by cryo‐TEM, WAX, SAXS, UV/Vis, IR analyses, and single‐crystal X‐ray crystallography. An antiparallel β‐sheet‐type aggregate consisting of an infinite one‐dimensional hydrogen‐bonding network of amide groups and π‐stacking of PdCl(dpb) moieties was observed in the supramolecular gel fiber of L ‐ 2 , even though discrete dimers are assembled through hydrogen bonding in the thermodynamically stable single crystal of L ‐ 2 . The disparate DSC profiles of the single crystal and xerogel of L ‐ 2 indicate different thermodynamics of the molecular assembly process.     

Electrospray Mass Spectrometry Study on Polynuclear Pd(II) and Ni (II) Complexes of 3, 6, 9, 16, 19, 22‐Hexaazatricyclo [22. 2.2.211,14] triaconta‐11, 13, 24, 26 (1), 27, 29‐Hexaene

Polynuclear Pd(II) and Ni(II) complexes of macrocyclic polyamine 3,6,9,16,19,22‐hexaazatricyclo[22.2.2.2^11,14]‐triaconta 11,13,24,26(l),27,29‐hexaene (L) in solution were investigated by electrospray ionization mass spectrometry (ESIMS). For methanol solution of complexes M₂LX₄ (M = Pd(II) and Ni(II), X= Cl and I), two main clusters of peaks were observed which can be assigned to [M₂LX₃]⁺ and [M₂LX₂]²⁺. When Pd₂LCl₄ was treated with 2 or 4 mol of AgNO₃, it gave rise formation of Pd₂LCl₂ (NO₃)₂ · H₂O and [Pd₂L(H₂O)_m(NO₃)_n]^(4‐n)+, respectively. ESMS spectra show that the dissociation of the former in the ionization process gave peaks of [Pd₂LCl₂]²⁺ and [(Pd₂LCl₂)NO₃]⁺, while dissociation of the later gave the peaks of [Pd₂L(CH₃CO₂)₂]²⁺ and [Pd₂L(CH₃CO₂)₂](NO₃) ⁺ in the presence of acetic acid. Similar species were observed for Pd₂LI₄ when treated with 4 mol of AgNO₃. When [Pd₂L · (H₂O)_m(NO₃)_n]^(4‐n)+ reacted with 2 mol of oxalate anions at 40°C, [Pd₄L₂(C₂O₄)₂(NO₃)₂]²⁺ and [Pd₄L₂(C₂O₄)₂ (NO₃)]³⁺ were detected. This implies the formation of square‐planar molecular box Pd₄L₂(C₂O₄)₂(NO₃)₄ in which C₂O₄^? may act as bridging ligands as confirmed by crystal structure analysis. The dissociation form and the stability of complex cations in gaseous state are also discussed. This work provides an excellent example of the usefulness of ESIMS in the identification of metal complexes in solution.     

Bis[S‐benzyl‐3‐(4‐n‐octyloxybenzylidene)dithiocarbazato‐κ2N3,S′]palladium(II)

In the title compound, [Pd(C₂₃H₂₉N₂OS₂)₂], the Pd^II atom displays the expected square‐planar coordination geometry. However, the trans configuration, which allows the Pd^II atom to be located on a crystallographic inversion centre, is unusual with respect to the cis arrangement found in analogous Pd complexes comprising similar N,S‐chelating ligands.     

Triple ring contraction of a [2+2] macrocyclic ligand

Schiff base condensation of 2,6‐diformylpyridine and 1,3‐diaminopropan‐2‐ol in the presence of a Ba^II template ion yields a complex containing a [2+2] macrocycle, [Ba₂(μ_1,2‐ClO₄)₂(H₂L1)₂], where H₂L1 is 3,7,15,19,25,26‐hexaazatricyclo[19.3.1.1^9,13]hexacosa‐1(25),2,7,9(26),10,12,14,19,21,23‐decaene‐5,17‐diol. On transmetallation with Cu^II cations, the macrocycle undergoes three successive ring contractions, yielding crystals of (acetato‐κO)[26,28‐dioxa‐3,7,15,19,25,27‐hexaazahexacyclo[19.3.1.1^2,5.1^9,13.1^17,10.0^3,8]octacosa‐1(25),9(27),10,12,14,21,23‐heptaene‐κ⁵N]copper(II) perchlorate, [Cu(CH₃COO)(C₂₀H₂₂N₆O₂)]ClO₄ or [Cu(CH₃COO)(L2)]ClO₄, in which the macrocycle ring size has been reduced from 20 members in H₂L1 to 16 in L2.     

Cationic palladium(II) complexes involving unprecedented O-coordination of acetylmethylenetriphenylphosphorane

Cationic pentafluorophenyl palladium(II) complexes of the type [Pd(C₆F₅)L₂(APPY)]ClO₄ (L = PPh₃, PBu₃ⁿ; L₂ = bipy and A acetylmethylenetriphenylphosphorane) have been prepared by addition of APPY to the perchlorato complexes [Pd(OClO₃)(C₆F₅)L₂]; the APPY ligand is O-coordinated, which is unprecedented in keto-stabilized ylide complexes of palladium.The neutral complex Pd(C₆F₅)(Cl)(tht)(APPY) has been made by addition of APPY to the binuclear complex Pd₂(μ-Cl)₂(C₆F₅)₂(tht)₂ (tht = tetrahydrothiophene); in which the APPY ligand shows the normal C-coordination.     

Unraveling the Mechanism of Water Oxidation Catalyzed by Nonheme Iron Complexes

Density functional theory (DFT) is employed to: 1) propose a viable catalytic cycle consistent with our experimental results for the mechanism of chemically driven (Ce^IV) O₂ generation from water, mediated by nonheme iron complexes; and 2) to unravel the role of the ligand on the nonheme iron catalyst in the water oxidation reaction activity. To this end, the key features of the water oxidation catalytic cycle for the highly active complexes [Fe(OTf)₂(Pytacn)] (Pytacn: 1‐(2′‐pyridylmethyl)‐4,7‐dimethyl‐1,4,7‐triazacyclononane; OTf: CF₃SO₃^?) ( 1 ) and [Fe(OTf)₂(mep)] (mep: N,N′‐bis(2‐pyridylmethyl)‐N,N′‐dimethyl ethane‐1,2‐diamine) ( 2 ) as well as for the catalytically inactive [Fe(OTf)₂(tmc)] (tmc: N,N′,N′′,N′′′‐tetramethylcyclam) ( 3 ) and [Fe(NCCH₃)(^MePy₂CH‐tacn)](OTf)₂ (^MePy₂CH‐tacn: N‐(dipyridin‐2‐yl)methyl)‐N′,N′′‐dimethyl‐1,4,7‐triazacyclononane) ( 4 ) were analyzed. The DFT computed catalytic cycle establishes that the resting state under catalytic conditions is a [Fe^IV(O)(OH₂)(LN₄)]²⁺ species (in which LN₄=Pytacn or mep) and the rate‐determining step is the O?O bond‐formation event. This is nicely supported by the remarkable agreement between the experimental (ΔG^≠=17.6±1.6 kcal mol^?1) and theoretical (ΔG^≠=18.9 kcal mol^?1) activation parameters obtained for complex 1 . The O?O bond formation is performed by an iron(V) intermediate [Fe^V(O)(OH)(LN₄)]²⁺ containing a cis‐Fe^V(O)(OH) unit. Under catalytic conditions (Ce^IV, pH 0.8) the high oxidation state Fe^V is only thermodynamically accessible through a proton‐coupled electron‐transfer (PCET) process from the cis‐[Fe^IV(O)(OH₂)(LN₄)]²⁺ resting state. Formation of the [Fe^V(O)(LN₄)]³⁺ species is thermodynamically inaccessible for complexes 3 and 4 . Our results also show that the cis‐labile coordinative sites in iron complexes have a beneficial key role in the O?O bond‐formation process. This is due to the cis‐OH ligand in the cis‐Fe^V(O)(OH) intermediate that can act as internal base, accepting a proton concomitant to the O?O bond‐formation reaction. Interplay between redox potentials to achieve the high oxidation state (Fe^V?O) and the activation energy barrier for the following O?O bond formation appears to be feasible through manipulation of the coordination environment of the iron site. This control may have a crucial role in the future development of water oxidation catalysts based on iron.     

The crystal and molecular structures of three copper‐containing complexes and their activities in mimicking galactose oxidase

The structures of three copper‐containing complexes, namely (benzoato‐κ²O,O′)[(E)‐2‐({[2‐(diethylamino)ethyl]imino}methyl)phenolato‐κ³N,N′,O]copper(II) dihydrate, [Cu(C₇H₅O₂)(C₁₃H₁₉N₂O)]·2H₂O, 1 , [(E)‐2‐({[2‐(diethylamino)ethyl]imino}methyl)phenolato‐κ³N,N′,O](2‐phenylacetato‐κ²O,O′)copper(II), [Cu(C₈H₇O₂)(C₁₃H₁₉N₂O)], 2 , and bis[μ‐(E)‐2‐({[3‐(diethylamino)propyl]imino}methyl)phenolato]‐κ⁴N,N′,O:O;κ⁴O:N,N′,O‐(μ‐2‐methylbenzoato‐κ²O:O′)copper(II) perchlorate, [Cu₂(C₈H₇O₂)(C₁₂H₁₇N₂O)₂]ClO₄, 3 , have been reported and all have been tested for their activity in the oxidation of d ‐galactose. The results suggest that, unlike the enzyme galactose oxidase, due to the precipitation of Cu₂O, this reaction is not catalytic as would have been expected. The structures of 1 and 2 are monomeric, while 3 consists of a dimeric cation and a perchlorate anion [which is disordered over two orientations, with occupancies of 0.64 (4) and 0.36 (4)]. In all three structures, the central Cu atom is five‐coordinated in a distorted square‐pyramidal arrangment (τ parameter of 0.0932 for 1 , 0.0888 for 2 , and 0.142 and 0.248 for the two Cu centers in 3 ). In each species, the environment about the Cu atom is such that the vacant sixth position is open, with very little steric crowding.     

Complexes of Crown Ether‐Containing N‐Phosphorylated Thioamides and Thioureas with CoII,ZnII and PdII Cations

The reaction of the potassium salts of N‐phosphorylated thioureas [4′‐benzo‐15‐crown‐5]NHC(S)NHP(Y)(OiPr)₂ (Y = S, HL^I ; Y = O, HL^II ) with Zn^II and Co^II cations in aqueous EtOH leads to complexes of formulae Zn(L^I,II‐S,Y)₂ (Y = S, 1 ; Y = O, 2 ) and Co(L^I‐S,S′)₂ ( 3 ), while interaction of the potassium salt of N‐phosphorylated thioamide [4′‐benzo‐15‐crown‐5]C(S)NHP(O)(OiPr)₂ ( HL^III ) with Zn^II in the same conditions leads to the complex Zn(HL^III)(L^III‐S,O)₂ ( 4 ). The reaction of the potassium salt of crown ether‐containing N‐phosphorylated bis‐thiourea N,N′‐[C(S)NHP(O)(OiPr)₂]₂‐1,10‐diaza‐18‐crown‐6 ( H₂L ) with Co^II, Zn^II and Pd^II cations in anhydrous CH₃OH leads to complexes M₂(L‐O,S)₂ (M = Co, 5 ; Zn, 6 ; M = Pd, 7 ). Thioamide HL^III was investigated by single‐crystal X‐ray diffraction.