Kinetics of nucleophilic attack upon coordinated organic moieties.: XXIII. Attack by cyclohexylamine upon the diene ligands in [MX2(1,5-cyclooctadiene)] (M = Pd,Pt; X = Cl,Br) complexes

Kinetic and spectroscopic studies of the reactions of cyclohexylamine with the complexes [MX₂(1,5-cyclooctadiene)] (I) (M = Pd; X = Cl, Br; M = Pt, X = Br) in acetone reveal the rate law, k_obs = K₁k₂[amine]², for the rapid sequence

For X = Br, the palladium(II) complex is ca. 70 times more reactive than its platinum(II) analogue. This is the first quantitative comparison reported to date for nucleophilic attack upon olefins coordinated to Pd^II and Pt^II centres. The reactivity order Pd^II ⪢ Pt^II may arise from the higher ionization potential of Pd²⁺ compared to Pt²⁺, which makes Pd^II a less effective back-π-bonder. Replacing the bromo ligands in [PdBr₂(1,5-COD)] by chloro ligands lowers the rate of formation of III by a factor of 8.     

PdII Complexes of [44]‐ and [46]Decaphyrins: The Largest Hückel Aromatic and Antiaromatic,and Möbius Aromatic Macrocycles

Reductive metalation of [44]decaphyrin with [Pd₂(dba)₃] provided a Hückel aromatic [46]decaphyrin Pd^II complex, which was readily oxidized upon treatment with DDQ to produce a Hückel antiaromatic [44]decaphyrin Pd^II complex. In CH₂Cl₂ solution the latter complex underwent slow tautomerization to a Möbius aromatic [44]decaphyrin Pd^II complex which exists as a mixture of conformers in dynamic equilibrium. To the best of our knowledge, these three Pd^II complexes represent the largest Hückel aromatic, Hückel antiaromatic, and Möbius aromatic complexes to date.     

PdII Complexes of [44]‐ and [46]Decaphyrins: The Largest Hückel Aromatic and Antiaromatic,and Möbius Aromatic Macrocycles

Reductive metalation of [44]decaphyrin with [Pd₂(dba)₃] provided a Hückel aromatic [46]decaphyrin Pd^II complex, which was readily oxidized upon treatment with DDQ to produce a Hückel antiaromatic [44]decaphyrin Pd^II complex. In CH₂Cl₂ solution the latter complex underwent slow tautomerization to a Möbius aromatic [44]decaphyrin Pd^II complex which exists as a mixture of conformers in dynamic equilibrium. To the best of our knowledge, these three Pd^II complexes represent the largest Hückel aromatic, Hückel antiaromatic, and Möbius aromatic complexes to date.     

Insertion of CO2 Mediated by a (Xantphos)NiI–Alkyl Species

The incorporation of CO₂ into organometallic and organic molecules represents a sustainable way to prepare carboxylates. The mechanism of reductive carboxylation of alkyl halides has been proposed to proceed through the reduction of Ni^II to Ni^I by either Zn or Mn, followed by CO₂ insertion into Ni^I‐alkyl species. No experimental evidence has been previously established to support the two proposed steps. Demonstrated herein is that the direct reduction of (tBu‐Xantphos)Ni^IIBr₂ by Zn affords Ni^I species. (tBu‐Xantphos)Ni^I‐Me and (tBu‐Xantphos)Ni^I‐Et complexes undergo fast insertion of CO₂ at 22 °C. The substantially faster rate, relative to that of Ni^II complexes, serves as the long‐sought‐after experimental support for the proposed mechanisms of Ni‐catalyzed carboxylation reactions.     

Studies on the stereochemistry of diphenyl diketone monothio-semicarbazone and its transition metal complexes

Summary The stereochemistry and complexation behaviour of diphenyl diketone monothiosemicarbazone (DKTS) with Cu^II, Co^II, Ni^II, Cd^II, Zn^II, Pd^II, Pt^II, Ru^III, Rh^III and Ir^III have been investigated by means of chemical, magnetic and spectral (i.r., Raman, ¹H- and ¹³C-n.m.r. and electronic) studies. The ligand forms complexes of the M(DKTS)₂ type with Ni^II, Cu^II and Co^II having a distorted octahedral geometry. The absence of a v(M—X) band in the i.r. spectra, coupled with their 1:1 electrolytic conductances, suggests that Ru^III, Rh^III and Ir^III form octahedral complexes of the [M(DKTS)₂]Cl type. A four-coordinate structure involving bridging halides is proposed for the Zn^II, Cd^II, Pd^II and Pt^II complexes, which have relatively low v(M—X) vibration modes.     

Room‐Temperature Decarboxylative Couplings of α‐Oxocarboxylates with Aryl Halides by Merging Photoredox with Palladium Catalysis

Enabled by merging iridium photoredox catalysis and palladium catalysis, α‐oxocarboxylate salts can be decarboxylatively coupled with aryl halides to generate aromatic ketones and amides at room temperature. DFT calculations suggest that this reaction proceeds through a Pd⁰–Pd^II–Pd^III pathway, in which the Pd^III intermediate is responsible for reoxidizing Ir^II to complete the Ir^III–*Ir^III–Ir^II photoredox cycle.     

Providing Support in Favor of the Existence of a PdII/PdIV Catalytic Cycle in a Heck‐Type Reaction

The complex [Pd(O,N,C‐L)(OAc)], in which L is a monoanionic pincer ligand derived from 2,6‐diacetylpyridine, reacts with 2‐iodobenzoic acid at room temperature to afford the very stable pair of Pd^IV complexes (OC‐6‐54)‐ and (OC‐6‐26)‐[Pd(O,N,C‐L)(O,C‐C₆H₄CO₂‐2)I] (1.5:1 molar ratio, at ?55 °C). These complexes and the Pd^II species [Pd(O,N,C‐L)(OX)] and [Pd(O,N,C‐L′)(NCMe)]ClO₄, (X=MeC(O) or ClO₃, L′=another monoanionic pincer ligand derived from 2,6‐diacetylpyridine), are precatalysts for the arylation of CH₂?CHR (R?CO₂Me, CO₂Et, Ph) using IC₆H₄CO₂H‐2 and AgClO₄. These catalytic reactions have been studied and a tentative mechanism is proposed. The presence of two Pd^IV complexes was detected by ESI(+)‐MS during the catalytic process. All the data obtained strongly support a Pd^II/Pd^IV catalytic cycle.     

On the nature of the chemical bond in heterobimetallic palladium(II) complexes with divalent 3<Emphasis Type="Italic">d</Emphasis> metals

The complex Pd(μ-OOCMe)₄Cu(OH₂) · 2Pd₃(μ-OOCMe)₆ was synthesized and characterized by X-ray crystallography. In the heterometallic moiety of this complex, the Pd^II and Cu^II atoms are at an extraordinary short distance (2.521(3) Å). DFT quantum-chemical calculations of the geometric and electronic structure of a series of heterobinuclear paddlewheel complexes Pd^IIM^II(μ-OOCMe)₄L (M = Zn^II, Ni^II, Cu^II, Co^II, Fe^II; L = OH₂ and NCH) and their formate analogues Pd^IIM^II(μ-OOCH)₄L (M = Zn^II, Ni^II, Fe^II) showed that the extraordinary short Pd?M distance in all these complexes is caused only by the tightening effect of carboxylate bridges rather than by the metal-metal bond. The direct Pd-M interaction becomes possible only after removal of electrons from the antibonding orbitals and formation of oxidized complexes of the [Pd^III(μ-OOCMe)₄Ni^III]²⁺ type.     

Alkylative Carboxylation of Ynamides and Allenamides with Functionalized Alkylzinc Halides and Carbon Dioxide by a Copper Catalyst

The alkylative carboxylation of ynamides and allenamides with CO₂ and alkylzinc halides catalyzed by a copper catalyst was developed. A variety of alkylzinc halides bearing functional groups were used for this transformation to afford α,β-unsaturated carboxylic acids, which contain the α,β-dehydroamino acid skeleton, introducing the corresponding alkyl group and CO₂ across the carbon–carbon triple or double bond. This alkylative carboxylation formally consists of Cu-catalyzed carbozincation of ynamides or allenamides with alkylzinc halides and the subsequent nucleophilic carboxylation of the resulting alkenylzinc species with CO₂. This protocol would be a useful method for the synthesis of α,β-dehydroamino acid derivatives possessing a functionalized alkyl group due to the high regio- and stereoselectivity, simple one-pot procedure as well as the use of CO₂ as a starting material.     

Chelatbildung mit 1,3-Diamino-2-methylenpropanen

Chelate Formation with 1,3-Diamino-2-methylene Propane1 1,3-Diamino-2-methylene propane and its N, N′ alkylated derivatives form crystalline chelates with Co^II (1:3), Ni^II (1:1, 1:2 and 1:3), Pd^II (1:1, 1:2), Rh^III (1:1) and Cu^II (1:2). Experiments for preparation of olefin complexes were unsuccessful. By potentiometric measurements the base strengths of the ligands as well as the stability constants of the Co^II, Ni^II, Pd^II, Cu^II, Zn^II, Cd^II chelates were evaluated and the kinetics of the formation of the 1:1 Pd^II complex is investigated. The magnetic behaviour of the Co^II?, Pd^II? and Cu^II? chelates is normal, whereas[Ni(dia)₂(H₂O)₂] (ClO₄)₂ shows anormal behaviour due to configurational isomerism between square planar and octahedral ligand geometry in solid state in type of a LIFSCHITZ -isomerism. The ESR-spectra of the Cu^II?compounds are discussed and the bonding parameters of the Cu? N?bonds were calculated.     

Mechanistic Insights into the Ni‐Catalyzed Reductive Carboxylation of C−O Bonds in Aromatic Esters with CO2: Understanding Remarkable Ligand and Traceless‐Directing‐Group Effects

The mechanism of the Ni⁰‐catalyzed reductive carboxylation reaction of C(sp²)?O and C(sp³)?O bonds in aromatic esters with CO₂ to access valuable carboxylic acids was comprehensively studied by using DFT calculations. Computational results revealed that this transformation was composed of several key steps: C?O bond cleavage, reductive elimination, and/or CO₂ insertion. Of these steps, C?O bond cleavage was found to be rate‐determining, and it occurred through either oxidative addition to form a Ni^II intermediate, or a radical pathway that involved a bimetallic species to generate two Ni^I species through homolytic dissociation of the C?O bond. DFT calculations revealed that the oxidative addition step was preferred in the reductive carboxylation reactions of C(sp²)?O and C(sp³)?O bonds in substrates with extended π systems. In contrast, oxidative addition was highly disfavored when traceless directing groups were involved in the reductive coupling of substrates without extended π systems. In such cases, the presence of traceless directing groups allowed for docking of a second Ni⁰ catalyst, and the reactions proceed through a bimetallic radical pathway, rather than through concerted oxidative addition, to afford two Ni^I species both kinetically and thermodynamically. These theoretical mechanistic insights into the reductive carboxylation reactions of C?O bonds were also employed to investigate several experimentally observed phenomena, including ligand‐dependent reactivity and site‐selectivity.     

Formation of XPhos-Ligated Palladium(0) Complexes and Reactivity in Oxidative Additions

Understanding the nature of the intermediate species operating within a palladium catalytic cycle is crucial for developing efficient cross-coupling reactions. Even though the XPhos/Pd(OAc)₂ catalytic system has found numerous applications, the nature of the active catalytic species remains elusive. A Pd⁰ complex ligated to XPhos has been detected and characterized in situ for the first time using cyclic voltammetry and NMR techniques. In the presence of XPhos, Pd(OAc)₂ initially associates with the ligand to form a complex in solution, which has been characterized as Pd^II(OAc)₂(XPhos). This Pd^II center is then reduced to the Pd⁰(XPhos)₂ species by an intramolecular process. This study also sheds light on the formation of Pd^I–Pd^I dimers. Finally, a kinetic study probes a dissociative mechanism for the oxidative addition with aryl halides involving Pd⁰(XPhos) as the reactive species in equilibrium with the unreactive Pd⁰(XPhos)₂. Remarkably, the reportedly poorly reactive PhCl reacts at room temperature in the oxidative addition, which confirms the crucial role of the XPhos ligand in the activation of aryl chlorides.     

Metal chelates of thiazole-2-ylthiourea

Summary The formation constants of 1-phenyl-3-thiazole-2-ylthiourea complexes with some bivalent metal ions (Cu^II, Ni^II, Zn^II and Mn^II) have been determined in 75% EtOH–H₂O. Complexes of Cu^II, Ni^II, Zn^II, Hg^II and Pd^II have been isolated and characterized by conductance, i.r., electronic spectra and magnetic measurements. The ligand forms ML complexes with Cu^II and Hg^II and ML₂ with Ni^II, Zn^II and Pd^II, where L is the uninegatively charged bidentate ligand and binds through the ring nitrogen and thiocarbonyl sulphur atoms.     

Ligational behaviour of a tetradentate NONS donor ligand towards some transition metal ions

Summary The synthesis and characterization of Cr^II, Mn^II, Fe^II, Co^II, Ni^II, Pd^II, Cu^II, Zn^II, Cd^II and UO ₂ ²⁺ complexes of 1-meotinoyl-4-phenyl-3-thiosemicarbazide (H₂NTS) are reported. I.r. spectral data show that the ligand behaves in a bidentate and/or tetradentate manner. An octahedral structure is proposed for the Cr^II, Fe^II and Ni^II complexes; a tetrahedral structure for the Mn^II, Co^II and Cu(NTS)·2H₂O complexes; and a square planar structure for the Pd^II and Cu(HNTS)Cl·H₂O complexes. The i.r. data suggest that the Fe^II complex contains a hydroxo bridge.     

Activation of CH bonds by transition metals : V. A study of the mechanism of metallation reactions of benzyl- and meta-fluorobenzylphosphines with RhI,IrI, PdII and PtII compounds

Reaction of R₂PCH₂C₆H₅ (R = cyclohexyl or t-butyl) with [(COT)₂RhCl]₂, [(COT)₂IrCl]₂, PdCl₂ or PtCl₂(benzonitrile)₂ yields cyclometallated compounds. The reactivity appears to decrease in the order Ir^I ρ Rh^I ρ ρ Pd^II ≈ Pt^II, suggesting a different reaction mechanism for univalent and bivalent d⁸ metal atoms. Reaction of meta-FC₆H₄CH₂PR₂ with the same metal chlorides shows that for Rh^I and Ir^I a nucleophilic mechanism operates and for Pd^II an electrophilic one. For Pt^II no decision could be made on the basis of these experiments. Steric effects have a large influence on the rates of the reactions.     

Catalytic hydroallylation of norbornadiene with allyl formate

Norbornadiene (NBD) reacts with allyl esters All—OC(O)R (R = Me, Bu^t, Ph, CCl₃, CF₃) in acetonitrile solutions of palladium(0) complexes to give a mixture of four isomeric nontraditional allylation products and the corresponding carboxylic acids. Under similar conditions, the reaction of NBD with allyl formate in solutions of Pd⁰ and Pd^II complexes occurs selectively, resulting in the product of addition of the allyl fragment and the H atom to an NBD double bond, 5-allylbicyclo[2.2.1]hept-2-ene, and CO₂. The hydroallylation of NBD is accompanied by catalytic addition of formic and acetic acids to one double bond of the diene to give bicyclo[2.2.1]hept-2-en-5-ol and nortricyclan-3-ol acetates and formates. Unlike most known palladium-based catalyst systems, these complexes exhibit catalytic activity also in the absence of phosphines. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 309–313, February, 2007.     

Synthesis,Spectroscopic Characterization and Properties of New Metal Complexes

A new series of the polydentate Schiff base Cu^II, Co^II, Ni^II, Pd^II and Zn^II complexes derived from ethylenediamine (eda), diethylenetriamine (dea) and tris(2-aminoethyl)amine (taa) have been prepared by template condensation in MeOH solution, and characterized by i.r., electronic spectral data, elemental analyses, conductivity and magnetic measurements. The ¹H- and ¹³C-n.m.r. and mass spectral data of the Ni^II, Pd^II and Zn^II complexes have been recorded. In all complexes, some of the chloride ions coordinate to the metal ions. From conductivity measurements, it is shown that the complexes are electrolytes. The Ni^II, Pd^II and Zn^II complexes have diamagnetic character. In this study, the Schiff base Cu^II and Co^II complexes have sub-normal magnetic moments commensurate with their binuclear or tetranuclear nature. Some show antimicrobial activity against bacteria and yeast.     

Solvolysis of palladium(II) and platinum(II) complexes of asymmetric ligands: Synthesis and structural characterization of [Pd(AMP)(dmso)Cl]BF4 and [Pt(AMP)(dmso)Cl]ClO4

Treatment of [M(AMP)Cl₂] (M = Pt^II, Pd^II; AMP = 2-aminomethylpyridine) with 1 mole of AgX (X = ClO₄, BF₄, PF₆) in dmso yields [M(AMP)(dmso)Cl]X. Single crystal X-ray structure determinations of the Pd^II and Pt^II complexes indicate that dmso is S-bondedtrans to the pyridyl ring in both complexes. (2-Aminomethylpyridine)chloro(dimethylsulphoxide-S) palladium(II) tetrafluoroborate.     

[(IPent)PdCl2(morpholine)]: A Readily Activated Precatalyst for Room‐Temperature,Additive‐Free Carbon–Sulfur Coupling

A series of new, easily activated NHC–Pd^II precatalysts featuring a trans‐oriented morpholine ligand were prepared and evaluated for activity in carbon‐sulfur cross‐coupling chemistry. [(IPent)PdCl₂(morpholine)] (IPent=1,3‐bis(2,6‐di(3‐pentyl)phenyl)imidazol‐2‐ylidene) was identified as the most active precatalyst and was shown to effectively couple a wide variety of deactivated aryl halides with both aryl and alkyl thiols at or near ambient temperature, without the need for additives, external activators, or pre‐activation steps. Mechanistic studies revealed that, in contrast to other common NHC–Pd^II precatalysts, these complexes are rapidly reduced to the active NHC–Pd⁰ species at ambient temperature in the presence of KOtBu, thus avoiding the formation of deleterious off‐cycle Pd^II–thiolate resting states.     

Activation of C–H and H–H bonds by dinuclear iridium complexes. Oxidative addition to highly active unsaturated 32e− diiridium species

Reactions of [(Cp¹Ir)₂(μ-dmpm)(μ-H)₂][OTf]₂ (1) with NaO^tBu in aromatic solvent at room temperature give [(Cp¹Ir)(H)(μ-dmpm)(μ-H)(Cp¹Ir)(Ar)][OTf] [Ar = Ph (3), p-Tol (4a), m-Tol (4b), 2-furyl (5a), 3-furyl (5b)] via intermolecular aromatic C–H activation. Treatment of [(Cp¹Ir)₂(μ-dppm)(μ-H)₂][OTf]₂ (2) with weak base (Et₂NH) results in intramolecular C–H activation of a phenyl group in the dppm ligand to give [(Cp¹Ir)(H){μ-PPh(C₆H₄)CH₂PPh₂}(μ-H)(Cp¹Ir)][OTf] (6). Reaction of 1 with NaO^tBu in tetrahydrofuran under H₂ (1 atm) results in activation of the H–H bond to give [{(Cp¹Ir)(H)}₂(μ-dmpm)(μ-H)][OTf] (7). Reaction of 1 with NaO^tBu in dichloromethane under carbon monoxide (1 atm) gives a carbonyl-bridged Ir^II–Ir^II complex, [(Cp¹Ir)₂(μ-dmpm)(μ-H)(μ-CO)][OTf] (8-OTf). These results strongly suggest that the active species in C–H and H–H bond activation starting with 1 and 2 would be unsaturated 32e^? diiridium species. The structures of 3, 5a, 6, 7, and 8-BPh₄ have been determined by X-ray diffraction methods.