Mechanistic Insights of a Concerted Metalation–Deprotonation Reaction with [Cp*RhCl₂]₂

The effect of the carboxylate used in a concerted metalation–deprotonation reaction is probed and shows a direct correlation of p<i>K</i>_a to observed rate up to a p<i>K</i>_a of 4.3, where the rate drops off at higher p<i>K</i>_a. The rate of the C–H activation of 2-(4-methoxyphenyl)­pyridine with [Cp*RhCl₂]₂ and carboxylate follows first-order kinetics in the active metal species, Cp*RhCl­(κ²-OAc), and zero-order kinetics in substrate when in a 1:1 ratio. There is a first-order dependence on substrate observed when excess substrate is present. The evaluation of the mechanism using kinetic studies allowed for a mechanistic proposal in which a second Ph′Py coordinates prior to the rate-determining C–H activation

Effect of Carboxylate Ligands on Alkane Dehydrogenation with (^<i>dm</i>Phebox)Ir Complexes

A series of carboxylate-ligated iridium complexes (^<i>dm</i>Phebox)­Ir­(O₂CR)₂(H₂O) (R = −CH₃, −CH₂CH₃, −CMe₃, −CH₂C₆H₅, and −CHCMe₂) were designed and synthesized to understand the carboxylate ligand effects on the reactivity of the complex for alkane dehydrogenation. Kinetic studies showed that the different R groups of the carboxylate iridium complexes can affect the reactivity with octane in the β-H elimination step. The rate constants for octene formation with different carboxylate ligands follow the order R = −CHCMe₂ > −CMe₃ > −CH₂CH₃ > −CH₃ > −CH₂C₆H₅. In contrast, there is no significant effect of carboxylate ligand on the rate of the C–H activation step at 160 °C. These experimental results support the findings in the previously reported density functional theory study of the (^<i>dm</i>Phebox)Ir complex in alkane C–H activation

Effect of Carboxylate Ligands on Alkane Dehydrogenation with (^<i>dm</i>Phebox)Ir Complexes

A series of carboxylate-ligated iridium complexes (^<i>dm</i>Phebox)­Ir­(O₂CR)₂(H₂O) (R = −CH₃, −CH₂CH₃, −CMe₃, −CH₂C₆H₅, and −CHCMe₂) were designed and synthesized to understand the carboxylate ligand effects on the reactivity of the complex for alkane dehydrogenation. Kinetic studies showed that the different R groups of the carboxylate iridium complexes can affect the reactivity with octane in the β-H elimination step. The rate constants for octene formation with different carboxylate ligands follow the order R = −CHCMe₂ > −CMe₃ > −CH₂CH₃ > −CH₃ > −CH₂C₆H₅. In contrast, there is no significant effect of carboxylate ligand on the rate of the C–H activation step at 160 °C. These experimental results support the findings in the previously reported density functional theory study of the (^<i>dm</i>Phebox)Ir complex in alkane C–H activation

Room-Temperature Carbon–Sulfur Bond Activation by a Reactive (dippe)Pd Fragment

The reactivity of [Pd­(dippe)­(μ-H)]₂ (<b>1</b>) and [(μ-dippe)­Pd]₂ (<b>2</b>) (dippe = 1,2-bis­(diisopropylphosphino)­ethane) toward C–S bonds in thiophene derivatives and thioethers was investigated, which led to C–S bond activation products. The thiapalladacycles derived from thiophenic substrates were fully characterized by ¹H, ³¹P, and ¹³C NMR spectroscopy, elemental analysis, and X-ray diffraction. The stability of the C–S insertion products was probed by performing competition experiments which follow the thermodynamic stability order (dippe)­Pd­(κ²<i>C</i>,<i>S</i>-benzothiophene) (<b>6</b>) > (dippe)­Pd­(κ²<i>C</i>,<i>S</i>-dibenzothiophene) (<b>8</b>) > (dippe)­Pd­(κ²<i>C</i>,<i>S</i>-thiophene) (<b>3</b>). The reactivity of the thiapalladacycles with small molecules such as H₂, CO, and alkynes was investigated

Mechanistic Insights in the Exchange of Arylthiolate Groups in Aryl(arylthiolato)palladium Complexes Supported by a Dippe Ligand

Carbon–sulfur activation of phenyl <i>p</i>-tolyl sulfide by a mixture of [Pd­(dippe)­(μ-H)]₂ (<b>1a</b>) and dinuclear Pd(0), [(μ-dippe)­Pd]₂ (<b>1b</b>) (dippe = 1,2-bis­(diisopropylphosphino)­ethane), to yield four carbon–sulfur activation products, (dippe)­Pd­(<i>p-</i>tolyl)­(SPh) (<b>3a</b>), (dippe)­Pd­(Ph)­(S-<i>p</i>-tolyl) (<b>3b</b>), (dippe)­Pd­(SPh)­(Ph)­(<b>3c</b>), and (dippe)­Pd­(<i>p-</i>tolyl)­(S-<i>p</i>-tolyl) (<b>3d</b>), was investigated. The carbon–sulfur complexes <b>3a</b>–<b>3d</b> were completely characterized by ¹H, ³¹P, and ¹³C NMR spectroscopy, elemental analysis, and X-ray diffraction. Exchange interactions between arylthiolate groups in (dippe)­Pd­(Ar)­(SAr′) (<b>3a</b>–<b>3d</b>) were investigated, leading to understanding the mechanism of interconversions among the complexes

Oxidative Addition of Chlorohydrocarbons to a Rhodium Tris(pyrazolyl)borate Complex

The reactive fragment [Tp′Rh­(PMe₃)], generated from the thermal precursor Tp′Rh­(PMe₃)­(Me)­H, is found to cleave the C–Cl bonds of chlorohydrocarbons under mild conditions. Reaction with chloromethane gives clean formation of an initial C–H activation product, which rearranges to form the C–Cl activation product at 30 °C. Reaction with dichloromethane or benzyl chloride gives a mixture of C–Cl activation products as well as products from chlorination. Reaction with chlorocyclohexane gives a mixture of intermediates from C–H activation, which react further upon heating to give a C–Cl cleavage product as well as the β-chloride elimination product Tp′Rh­(PMe₃)­(Cl)H plus cyclohexene. Complete conversion from a C–H activation product to a C–Cl activation product was observed in the reaction with 1,2-dichloroethylene, where β-elimination is circumvented. Activation of 1-chlorobutane, 1,2-dichloroethane, or 1,4-dichlorobutane gives a mixture of C–Cl activation products as well as Tp′Rh­(PMe₃)­(Cl)­H plus olefin. Similar to the case for activation of methylene chloride, C–Cl activation and hydride/chloride exchange was observed in the reaction with benzyl chloride, where C–H activation was not seen. The reaction with chlorobenzene gives isomeric species resulting from C–H activation, which react further to give the corresponding chloride derivatives upon heating. Reaction with pentachlorobenzene gives a cyclometalated product from C–H bond cleavage in the phosphine ligand. These reactions are compared and contrasted with related photoreactions with the [Tp′Rh­(CNneopentyl)] analogue, where C–H activation is solely observed in most cases. Mechanistic studies suggest the spectator ligand dependent reactivity relies greatly on the dissociation energy of the Tp′Rh–L bond

A Molecular Iron Catalyst for the Acceptorless Dehydrogenation and Hydrogenation of N‑Heterocycles

A well-defined iron complex (<b>3</b>) supported by a bis­(phosphino)­amine pincer ligand efficiently catalyzes both acceptorless dehydrogenation and hydrogenation of N-heterocycles. The products from these reactions are isolated in good yields. Complex <b>3</b>, the active catalytic species in the dehydrogenation reaction, is independently synthesized and characterized, and its structure is confirmed by X-ray crystallography. A <i>trans</i>-dihydride intermediate (<b>4</b>) is proposed to be involved in the hydrogenation reaction, and its existence is verified by NMR and trapping experiments

Investigation of C–C Bond Activation of sp–sp² C–C Bonds of Acetylene Derivatives via Photolysis of Pt Complexes

Carbon–carbon bond activation reactions of acetylene derivatives featuring sp–sp³ C–C bonds or both sp–sp² and sp–sp single C–C bonds were studied via photolysis of platinum compounds. Novel Pt⁰–acetylene complexes with η² coordination of the alkynes were synthesized and characterized. Irradiation of (dtbpe)­Pt­(η²-H₃CCCCH₃) (<b>1</b>), (dtbpe)­Pt­[η²-(H₃C)₃CCCC­(CH₃)₃] (<b>3</b>), and [(dtbpe)­Pt]₂(μ₂-η²:η²-H₃CCCCCCH₃) (<b>6</b>) with UV light (λ >300 nm) produced the activation product (dtbpe)­Pt­(D)­(C₆D₅) (<b>2</b>) as a result of C–D bond activation of the solvent (C₆D₆), whereas (dtbpe)­Pt­(η²-F₃CCCCF₃) (<b>4</b>) and (dippe)­Pt­(η²-F₃CCCCF₃) (<b>5</b>) remained unchanged upon irradiation for 22 h. Photolysis of [(dtbpe)­Pt]₂(μ₂-η²:η²-PhCCCCPh) (<b>7</b>) and (dippe)­Pt­(η²-PhCCCCPh) (<b>9</b>) resulted in [(dtbpe)­(Ph)­Pt]₂(μ-CCCC−) (<b>8</b>) and (dippe)­Pt­(Ph)­(CCCCPh) (<b>10</b>), respectively, showing exclusive C–C bond activation through sp–sp² type C–C bonds. Both of the products stayed unchanged upon heating to 150 °C overnight

Oxidative Addition of Chlorohydrocarbons to a Rhodium Tris(pyrazolyl)borate Complex

The reactive fragment [Tp′Rh­(PMe₃)], generated from the thermal precursor Tp′Rh­(PMe₃)­(Me)­H, is found to cleave the C–Cl bonds of chlorohydrocarbons under mild conditions. Reaction with chloromethane gives clean formation of an initial C–H activation product, which rearranges to form the C–Cl activation product at 30 °C. Reaction with dichloromethane or benzyl chloride gives a mixture of C–Cl activation products as well as products from chlorination. Reaction with chlorocyclohexane gives a mixture of intermediates from C–H activation, which react further upon heating to give a C–Cl cleavage product as well as the β-chloride elimination product Tp′Rh­(PMe₃)­(Cl)H plus cyclohexene. Complete conversion from a C–H activation product to a C–Cl activation product was observed in the reaction with 1,2-dichloroethylene, where β-elimination is circumvented. Activation of 1-chlorobutane, 1,2-dichloroethane, or 1,4-dichlorobutane gives a mixture of C–Cl activation products as well as Tp′Rh­(PMe₃)­(Cl)­H plus olefin. Similar to the case for activation of methylene chloride, C–Cl activation and hydride/chloride exchange was observed in the reaction with benzyl chloride, where C–H activation was not seen. The reaction with chlorobenzene gives isomeric species resulting from C–H activation, which react further to give the corresponding chloride derivatives upon heating. Reaction with pentachlorobenzene gives a cyclometalated product from C–H bond cleavage in the phosphine ligand. These reactions are compared and contrasted with related photoreactions with the [Tp′Rh­(CNneopentyl)] analogue, where C–H activation is solely observed in most cases. Mechanistic studies suggest the spectator ligand dependent reactivity relies greatly on the dissociation energy of the Tp′Rh–L bond

Catalytic Upgrading of Ethanol to <i>n</i>‑Butanol via Manganese-Mediated Guerbet Reaction

Replacement of precious metal catalysts in the Guerbet upgrade of ethanol to <i>n</i>-butanol with first-row metal complex catalysts is highly appreciated due to their economic and environmental friendliness. The manganese pincer complexes of the type [(^RPNP)­MnBr­(CO)₂] (R = ^<i>i</i>Pr, Cy, ^<i>t</i>Bu, Ph or Ad) are found to be excellent catalysts for upgrading ethanol to <i>n</i>-butanol. Under suitable reaction conditions and with an appropriate base, about 34% yield of <i>n</i>-butanol can be obtained in high selectivity. A detailed account on the effect of the temperature, solvent, nature, and proportion of base used and the stereoelectronic effects of the ligand substituents on the catalytic activity of the catalysts as well as the plausible deactivation pathways is presented