Spectroelectrochemical studies of some species in fused PbCl2 + KCl at 440°C

The electrochemical reduction of Pb(II), Ag(I). Ni(II), In(III)_and Rh(III), and the oxidation of O(?II), I(?I) and S(?II) have been investigated in a PbCl₂+KCl melt with 23% mol KCl at 440°C, by potentiometry, linear potential sweep and cyclic voltammetry, and potential step chronoamperometry. The potential of the Pb/Pb(II) electrode was found to change linearly with the melt composition. The Ag/Ag(I) system has a Nerstian behaviour and was used as a reference electrode. All species showed a one step oxidation or reduction process, leading to Ni(0), In(I), Rh(0), CO₂ and I₂, except that the sulphide oxidizes in two steps. Absorption of light by Co(II), Ni(II), and Rh(III) was measured with a new device using fibre optics, which allowed us to record the spectrum of Ni(II) and Rh(III) inside the electrolytic cell, during oxidation at controlled potential of a nickel wire, or reduction of Rh(III).     

A nickel precatalyst for efficient cross-coupling reactions of aryl tosylates with arylboronic acids: vital role of dppf

An air-stable and easy-to-handle nickel precatalyst, (9-phenanthrenyl)Ni(II)(PPh₃)₂Cl, was examined for the cross-coupling reactions of aryl tosylates with arylboronic acids. Under the optimized reaction conditions, the catalytic system tolerates a wide range of activated, neutral and deactivated substrates. The selectivity of this cross-coupling reaction towards aryl tosylates and arylboronic acids has been investigated. It is proposed that ligand 1,1′-bis(diphenylphosphino)ferrocene (dppf) plays a key role in the coupling by enforcing a cis geometry in key intermediates and the active Ni(0) species.     

Bis-diimidazolylidine complexes of nickel: investigations into nickel catalyzed coupling reactions

Air and moisture stable homoleptic bis(diimidazolylidine)nickel(II) complexes, ([(diNHC)(2)Ni](2+)) 3a,b and their corresponding silver(I) 4a,b and palladium(II) 5a,b complexes were synthesized and characterized by NMR and single crystal X-ray analysis. The catalytic potential of complex 3a was assessed in Mizoroki-Heck and Suzuki-Miyaura coupling reactions. In the Suzuki-Miyaura coupling reaction, nickel precatalyst 3a was active for the coupling of aryl chlorides as well as aryl fluorides. The analogously synthesized Pd(II) complexes resulted in formation of (diNHC)PdCl(2) species which were not active for the coupling of aryl fluorides. For the Mizoroki-Heck reaction, it was found that aryl iodides could be activated in the absence of nickel or palladium precatalysts when using Na(2)CO(3) or NEt(3) as base while aryl iodides and aryl bromides could be activated in the Suzuki-Miyaura reaction sans precatalyst when K(3)PO(4) was used as base.     

Role of Pd(0) particles in oxidation of lower olefins in the catalytic system Fe(III)_Pd/ZrO<Subscript>2</Subscript>

Oxidation of metallic Pd(0) particles applied onto an oxide support with Fe(III) ions in a concentration not exceeding 0.06 M at 70°C was studied. In contrast to palladium black, with the supported catalyst Pd/ZrO₂ Pd(II) is formed in the solution in the concentration corresponding to the thermodynamic equilibrium. With an increase in the initial Fe(III) concentration, the equilibrium yield of Pd(II) increases. The initial reaction rate grows with an increase in the weight of the initial Pd-containing catalyst and in the initial Fe(III) concentration. The revealed kinetic relationships of the dissolution of Pd(0) in the reaction with Fe(III) aqua ions allow a conclusion that, in oxidation of lower olefins C₂-C₄ in the catalytic system Fe(III)_Pd/ZrO₂ in aqueous solution, Pd(II) is regenerated in the catalytic cycle by oxidation of Pd(I) species, rather than of Pd(0), with Fe(III) aqua ions.     

Discovery and mechanistic study of Al(III)-catalyzed transamidation of tertiary amides

Cleavage of the C-N bond of carboxamides generally requires harsh conditions. This study reveals that tris(amido)Al(III) catalysts, such as Al2(NMe2)6, promote facile equilibrium-controlled transamidation of tertiary carboxamides with secondary amines. The mechanism of these reactions was investigated by kinetic, spectroscopic, and density functional theory (DFT) computational methods. The catalyst resting state consists of an equilibrium mixture of a tris(amido)Al(III) dimer and a monomeric tris(amido)Al(III)-carboxamide adduct, and the turnover-limiting step involves intramolecular nucleophilic attack of an amido ligand on the coordinated carboxamide or subsequent rearrangement (intramolecular ligand substitution) of the tetrahedral intermediate. Fundamental mechanistic differences between these tertiary transamidation reactions and previously characterized transamidations involving secondary amides and primary amines suggest that tertiary amide/secondary amine systems are particularly promising for future development of metal-catalyzed amide metathesis reactions that proceed via transamidation.     

Me4N](Ni(II)(BEAAM)): a synthetic model for nickel superoxide dismutase that contains Ni in a mixed amine/amide coordination environment

Nickel superoxide dismutase (NiSOD) is a metalloenzyme that converts O2*- into H2O2 and O2 by cycling between Ni(II) and Ni(III) oxidation states. Reduced NiSOD contains Ni(II) in a square-planar N2S2 coordination environment formed by two cysteinate S atoms, an amide N, and an amine N to Ni(II). [Me4N](Ni(II)(BEAAM)) represents the first NiN2S2 complex containing Ni in a mixed amine/amide environment. [Me4](Ni(II)(BEAAM)) contains Ni-S bonds at 2.177(2) and 2.137(2) A and Ni-N bonds at 1.989(7) and 1.858(6) A, which compare well with the metalloenzyme. Orange solutions of [Me(4)N](Ni(II)(BEAAM)) in MeCN are diamagnetic and stable toward O2 for weeks. A quasireversible Ni(II/III) redox couple is observed for [Ni(II)(BEAAM)](NMe4) at 0.12(1) V vs Ag/AgCl. These data suggest that NiSOD utilizes the mixed amine/amide ligands to modulate the Ni(II/III) redox couple to best match the O2*- reduction/oxidation couples while maintaining O2 stability.     

Well-Defined Ni(0) and Ni(II) Complexes of Bicyclic (Alkyl)(Amino)Carbene (MeBICAAC): Catalytic Activity and Mechanistic Insights in Negishi Cross-Coupling Reaction

Negishi cross-coupling reaction of organozinc compounds as nucleophiles with aryl halides has drawn immense focus for C−C bond formation reactions. In comparison to the well-established library of Pd complexes, the C−C cross-coupling of this particular approach is largely primitive with nickel-complexes. Herein, we describe the syntheses of Ni(II) complexes, [(^MeBICAAC)₂NiX₂] (X=Cl ( 1 ), Br ( 2 ), and I ( 3 )) by employing the bicyclic (alkyl)(amino)carbene (^MeBICAAC) ligand. The reduction of complexes 1 – 3 using KC₈ afforded the two coordinate low valent, Ni(0) complex, [(^MeBICAAC)₂Ni(0)] ( 4 ). Complexes 1 – 4 have been characterized by spectroscopic techniques and their solid-state structures were also confirmed by X-ray crystallography. Furthermore, complexes 1 – 4 have been applied in a direct and convenient method to catalyze the Negishi cross-coupling reaction of various aryl halides with 2,6-difluorophenylzinc bromide or phenylzinc bromide as the coupling partner in the presence of 3 mol % catalyst. Comparatively, among all-pristine complexes, 1 exhibit high catalytic potential to afford value-added C−C coupled products without the use of any additive. The UV-vis studies and HRMS measurements of controlled stochiometric reactions vindicate the involvement of Ni(I)−NI(III) cycle featured with a penta-coordinated Ni(III)-aryl species as the key intermediate for 1 whereas Ni(0)/Ni(II) species are potentially involved in the catalytic cycle of 4 .     

Ni-catalyzed cross-coupling reaction of aryl chlorides with arylboronic acids in IPA without using a reducing reagent

The coupling reaction of aryl chlorides with arylboronic acids was successfully performed in isopropanol (IPA) by using [NiCl(Ph₂PCH₂CH₂OH)₂(H₂O)]Cl (5), a cationic Ni(II)-complex, as a precatalyst in the absence of a reducing agent. The coupling reaction proceeded smoothly under mild conditions to provide biaryls in satisfactory to excellent yields, and formation of the undesired dechlorination products of aryl chlorides was completely prevented.     

Zur Oxydation von Gemischtligand-Nickel(0)-Komplexen mit organischen Halogeniden

Oxidation of Mixed Ligand Nickel(0) Complexes by Organic Halides The oxidation of (dipy)Ni(PPh₃)₂ by alkyl and aryl iodides or bromides affords the nickel(I) complexes (dipy)Ni(PPh₃)X (X = Br, I). No normal products of oxidative addition are obtained. But in the case of methyl and ethyl halides complexes of the type (dipy)NiR₂ are formed as intermediates. Basing on the identified final products and on the correalation between the reactivity of the organic halides and their polarographic half wave potentials a mechanism of the reaction is proposed. The first step is a charge transfer from nickel(0) to the organic halide. Further synthesis, reactions, and the ESR-spectra of the complexes (dipy)Ni(PPh₃)X and a synthesis of (dipy)Ni(CH₂Ph)₂ are described. Experiments to prepare pure (dipy)Ni(PPh₃)Cl had no success.     

Shuttling of nickel oxidation states in N4S2 coordination geometry versus donor strength of tridentate N2S donor ligands

Seven bis-Ni(II) complexes of a N(2)S donor set ligand have been synthesized and examined for their ability to stabilize Ni(0), Ni(I), Ni(II) and Ni(III) oxidation states. Compounds 1-5 consist of modifications of the pyridine ring of the tridentate Schiff base ligand, 2-pyridyl-N-(2'-methylthiophenyl)methyleneimine ((X)L1), where X = 6-H, 6-Me, 6-p-ClPh, 6-Br, 5-Br; compound 6 is the reduced amine form (L2); compound 7 is the amide analog (L3). The compounds are perchlorate salts except for 7, which is neutral. Complexes 1 and 3-7 have been structurally characterized. Their coordination geometry is distorted octahedral. In the case of 6, the tridentate ligand coordinates in a facial manner, whereas the remaining complexes display meridional coordination. Due to substitution of the pyridine ring of (X)L1, the Ni-N(py) distances for 1~5 < 3 < 4 increase and UV-vis λ(max) values corresponding to the (3)A(2g)(F)→(3)T(2g)(F) transition show an increasing trend 1~5 < 2 < 3 < 4. Cyclic voltammetry of 1-5 reveals two quasi-reversible reduction waves that correspond to Ni(II)→Ni(I) and Ni(I)→Ni(0) reduction. The E(1/2) for the Ni(II)/Ni(I) couple decreases as 1 > 2 > 3 > 4. Replacement of the central imine N donor in 1 by amine 6 or amide 7 N donors reveals that complex 6 in CH(3)CN exhibits an irreversible reductive response at E(pc) = -1.28 V, E(pa) = +0.25 V vs saturated calomel electrode (SCE). In contrast, complex 7 shows a reversible oxidation wave at E(1/2) = +0.84 V (ΔE(p) = 60 mV) that corresponds to Ni(II)→Ni(III). The electrochemically generated Ni(III) species, [(L3)(2)Ni(III)](+) is stable, showing a new UV-vis band at 470 nm. EPR measurements have also been carried out.     

Oxidative Coupling of Aryl Boron Reagents with sp3‐Carbon Nucleophiles: The Enolate Chan–Evans–Lam Reaction

Reported is a versatile new oxidative method for the arylation of activated methylene species. Under mild reaction conditions (RT to 40 °C), Cu(OTf)₂ mediates the selective coupling of functionalized aryl boron species with a variety of stabilized sp³‐nucleophiles. Tertiary malonates and amido esters can be employed as substrates to generate quaternary centers. Complementing either traditional cross‐coupling or S_NAr protocols, the transformation is chemoselective in the presence of halogen electrophiles, including aryl bromides and iodides. Substrates bearing amide, sulfonyl, and phosphonyl groups, which are not amenable to coupling under mild Hurtley‐type conditions, are suitable reaction partners.     

Preparation of iron amido complexes via putative Fe(IV) imido intermediates

The isolation and characterization of monomeric Fe(III) amido complexes with hybrid ureate/amidate ligands is described. An aryl azide serves as the source of the amido ligand in preparing the complexes from trigonal monopyramidal Fe(II) precursors. Aryl azides more commonly react with transition metal complexes by a two-electron oxidation process to yield imido complexes, suggesting that the Fe(III) amido complexes may be formed from high valent species by hydrogen atom abstraction from an external species. The mechanistic basis for formation of the amido complexes is investigated using substrates that readily donate hydrogen atoms. Results from these experiments suggest that the Fe(III) amido complexes are generated from Fe(IV) imido intermediates that can facilitate homolytic X-H bond cleavage. The Fe(III) amido complexes are high spin (S = 5/2) with a strong absorbance band at lambdamax approximately 600 nm and extinction coefficients between 2000 and 3000 M-1 cm-1. These complexes are hygroscopic, reacting with 1 equiv of water to produce the corresponding Fe(III)-OH complexes and p-toluidine.     

Comparison of dppf‐Supported Nickel Precatalysts for the Suzuki–Miyaura Reaction: The Observation and Activity of Nickel(I)

Ni‐based precatalysts for the Suzuki–Miyaura reaction have potential chemical and economic advantages compared to commonly used Pd systems. Here, we compare Ni precatalysts for the Suzuki–Miyaura reaction supported by the dppf ligand in 3 oxidation states, 0, I and II. Surprisingly, at 80 °C they give similar catalytic activity, with all systems generating significant amounts of Ni^I during the reaction. At room temperature a readily accessible bench‐stable Ni^II precatalyst is highly active and can couple synthetically important heterocyclic substrates. Our work conclusively establishes that Ni^I species are relevant in reactions typically proposed to involve exclusively Ni⁰ and Ni^II complexes.     

A STUDY ON THE SELECTIVE REDUCTION OF AROMATIC AMIDE WITH LiAlH_4

In the synthesis of l-phenyl-5-substituted amino-4-pyrazole N-alkyl amide,it was found for the first time that one of the two aromatic amido groups in the moleculeof 1-phenyl-5-benzoyl amino-4-pyrazole N-alkyl amide was reduced selectively by LiAlH_4.new conclusion was drawn after several experiments have been done that ortho-amino(orsubstituted amino)aryl amide or the aryl amide with its ortho substituent which canbe reduced into an amino group(or substituted amino group)can not be reduced by LiAlH_4.It was further rationalized by quantum chemical calculation.     

Pyrazolidine-3,5-dione angiotensin-II receptor antagonists

The crystal structures of three angiotensin-II receptor antagonists involving different spacer groups (CO, CONH and NHCO) between the aryl rings are presented, namely 2-{4-[(3-butyl-1,4-dioxo-2,3-di­aza­spiro­[4.4]­non-2-yl)­methyl]­benzoyl}benzoic acid, C₂₆H₂₈N₂O₅, (I), 2-{4-[(3-butyl-1,4-dioxo-2,3-di­aza­spiro­[4.4]­non-2-yl)­methyl]­benz­amido}benzoic acid, C₂₆H₂₉N₃O₅, (II), and 2-{4-[(3-butyl-1,4-dioxo-2,3-di­azaspiro­[4.4]­non-2-yl)­methyl]­anilino­carbonyl}benzoic acid monohydrate, C₂₆H₂₉N₃O₅·H₂O, (III). The aryl rings of (II) are almost coplanar, in contrast with compounds (I) and (III). The conformation of (II) is induced by an intramolecular N—H⋯O hydrogen bond between the amide and carboxyl­ic acid groups.     

Electron transfer reactions of tris(polypyridine)ruthenium(III) complexes with organic sulfides: importance of hydrophobic interaction

Ruthenium(III)-polypyridyl complexes, generated from the photochemical oxidation of Ru(II) complexes with molecular oxygen, undergo facile electron transfer reaction with dialkyl and aryl methyl sulfides. The rate controlling electron transfer process is confirmed from the absorption spectrum of the transient sulfide radical cation. The spectrophotometric kinetic study shows that the reaction is of total second order, first order in Ru(III) complex and in the organic sulfide. The reaction rate is susceptible to the change of ligand in [Ru(NN)₃]³⁺ and the structure of organic sulfide.     

N,N′-Bis(aryl)pyridine-2,6-dicarboxamide complexes of ruthenium: Synthesis,structure and redox properties

Reaction of five N,N′-bis(aryl)pyridine-2,6-dicarboxamides (H₂L-R, where H₂ denotes the two acidic protons and R (R = OCH₃, CH₃, H, Cl and NO₂) the para substituent in the aryl fragment) with [Ru(trpy)Cl₃](trpy = 2,2′,2″-terpyridine) in refluxing ethanol in the presence of a base (NEt₃) affords a group of complexes of the type [Ru^II(trpy)(L-R)], each of which contains an amide ligand coordinated to the metal center as a dianionic tridentate N,N,N-donor along with a terpyridine ligand. Structure of the [Ru^II(trpy)(L-Cl)] complex has been determined by X-ray crystallography. All the Ru(II) complexes are diamagnetic, and show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ru^II(trpy)(L-R)] complexes shows a Ru(II)–Ru(III) oxidation within 0.16–0.33 V versus SCE. An oxidation of the coordinated amide ligand is also observed within 0.94–1.33 V versus SCE and a reduction of coordinated terpyridine ligand within −1.10 to −1.15 V versus SCE. Constant potential coulometric oxidation of the [Ru^II(trpy)(L-R)] complexes produces the corresponding [Ru^III(trpy)(L-R)]⁺ complexes, which have been isolated as the perchlorate salts. Structure of the [Ru^III(trpy)(L-CH₃)]ClO₄ complex has been determined by X-ray crystallography. All the Ru(III) complexes are one-electron paramagnetic, and show anisotropic ESR spectra at 77 K and intense LMCT transitions in the visible region. A weak ligand-field band has also been shown by all the [Ru^III(trpy)(L-R)]ClO₄ complexes near 1600 nm.     

The mechanism of the modified Ullmann reaction

The copper-mediated aromatic nucleophilic substitution reactions developed by Fritz Ullmann and Irma Goldberg required stoichiometric amounts of copper and very high reaction temperatures. Recently, it was found that addition of relatively cheap ligands (diamines, aminoalcohols, diketones, diols) made these reactions truly catalytic, with catalyst amounts as low as 1 mol% or even lower. Since these catalysts are homogeneous, it has opened up the possibility to investigate the mechanism of these modified Ullmann reactions. Most authors agree that Cu(I) is the true catalyst even though Cu(0) and Cu(II) catalysts have also shown to be active. It should be noted however that Cu(I) is capable of reversible disproportionation into Cu(0) and Cu(II). In the first step, the nucleophile displaces the halide in the LnCu(I)X complex forming LnCu(I)ZR (Z = O, NR′, S). Quite a number of mechanisms have been proposed for the actual reaction of this complex with the aryl halide: 1. Oxidative addition of ArX forming a Cu(III) intermediate followed by reductive elimination; 2. Sigma bond metathesis; in this mechanism copper remains in the Cu(II) oxidation state; 3. Single electron transfer (SET) in which a radical anion of the aryl halide is formed (Cu(I)/Cu(II)); 4. Iodine atom transfer (IAT) to give the aryl radical (Cu(I)/Cu(II)); 5. π-complexation of the aryl halide with the Cu(I) complex, which is thought to enable the nucleophilic substitution reaction. Initially, the radical type mechanisms 3 and 4 where discounted based on the fact that radical clock-type experiments with ortho-allyl aryl halides failed to give the cyclised products. However, a recent DFT study by Houk, Buchwald and co-workers shows that the modified Ullmann reaction between aryl iodide and amines or primary alcohols proceeds either via an SET or an IAT mechanism. Van Koten has shown that stalled aminations can be rejuvenated by the addition of Cu(0), which serves to reduce the formed Cu(II) to Cu(I); this also corroborates a Cu(I)/Cu(II) mechanism. Thus the use of radical clock type experiments in these metal catalysed reactions is not reliable. DFT calculations from Hartwig seem to confirm a Cu(I)/Cu(III) type mechanism for the amidation (Goldberg) reaction, although not all possible mechanisms were calculated.     

Pd(OAc)2/M(NO3)n (M = Cu(II), Fe(III); n = 2, 3): Kinetic investigations of an alternative Wacker system for the oxidation of natural olefins

Pd-catalyzed oxidative coupling of camphene by dioxygen afforded mainly a diene, which subsequently underwent oxidation to a ring-expanded β,γ-unsaturated ketone with LiNO₃ as reoxidant. However, the instability of LiNO₃ results to the decomposition of NO₃⁻ ions which subsequently deactivates the catalyst. The present investigation describes the oxidation of terpenes catalyzed by Pd(OAc)₂/M(NO₃)_n (M = Cu(II), Fe(III); n = 2 or 3), using dioxygen as final oxidant. Fe(III) and Cu(II) effectively stabilize the nitrate reoxidant as determined by the significant increase of both catalytic activity and stability of the system. Turnover frequency suggests that Fe(III) is the most efficient co-catalyst. Moreover, it is established that the co-catalysts NO₃⁻, Cu(II) and especially Fe(III) ions, change the product distribution (diene/ketone) remarkably. Their involvement in the rate-determining step was investigated and the results of the kinetic investigations clarified important aspects of Pd(II)-catalyzed oxidation reactions. The described protocol offers an alternative to the traditional Wacker system which uses CuCl₂ as co-catalyst and is not effective in promoting the oxidation of bicycle olefins.     

A Theoretical DFT‐Based and Experimental Study of the Transmetalation Step in Au/Pd‐Mediated Cross‐Coupling Reactions

In this work a combined theoretical and experimental investigation of the cross‐coupling reaction involving two metallic reaction centers, namely gold and palladium, is described. One metal center (Au) hereby is rather inert towards change in its oxidation state, whereas Pd undergoes oxidative insertion and reductive elimination steps. Detailed mechanistic and energetic studies of each individual step, with the focus on the key transmetalation step are presented and compared for different substrates and ligands on the catalytic Pd center. Different aryl halides (Cl, Br, I) and aryl triflates were investigated. Hereby the nature of the counteranion X turned out to be crucial. In the case of X=Cl and L=PMe₃ the oxidative addition is rate‐determining, whereas in the case of X=I the transmetalation step becomes rate‐determining in the Au/Pd‐cross‐coupling mechanism. A variety of Au–Pd transmetalation reaction scenarios are discussed in detail, favoring a transition state with short intermetallic Au–Pd contacts. Furthermore, without a halide counteranion the transmetalation from gold(I) to palladium(II) is highly endothermic, which confirms our experimental findings that the coupling does not occur with aryl triflates and similar weakly coordinating counteranions—a conclusion that is essential in designing new Au–Pd catalytic cycles. In combination with experimental work, this corrects a previous report in the literature claiming a successful coupling potentially catalytic in both metals with weakly coordinating counteranions.