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A series of mononuclear nickel­(II) thiolate complexes (Et₄N)­Ni­(X-pyS)₃ (Et₄N = tetraethylammonium; X = 5-H (<b>1a</b>), 5-Cl (<b>1b</b>), 5-CF₃ (<b>1c</b>), 6-CH₃ (<b>1d</b>); pyS = pyridine-2-thiolate), Ni­(pySH)₄(NO₃)₂ (<b>2</b>), (Et₄N)­Ni­(4,6-Y₂-pymS)₃ (Y = H (<b>3a</b>), CH₃ (<b>3b</b>); pymS = pyrimidine-2-thiolate), and Ni­(4,4′-Z-2,2′-bpy)­(pyS)₂ (Z = H (<b>4a</b>), CH₃ (<b>4b</b>), OCH₃ (<b>4c</b>); bpy = bipyridine) have been synthesized in high yield and characterized. X-ray diffraction studies show that <b>2</b> is square planar, while the other complexes possess tris-chelated distorted-octahedral geometries. All of the complexes are active catalysts for both the photocatalytic and electrocatalytic production of hydrogen in 1/1 EtOH/H₂O. When coupled with fluorescein (Fl) as the photosensitizer (PS) and triethylamine (TEA) as the sacrificial electron donor, these complexes exhibit activity for light-driven hydrogen generation that correlates with ligand electron donor ability. Complex <b>4c</b> achieves over 7300 turnovers of H₂ in 30 h, which is among the highest reported for a molecular noble metal-free system. The initial photochemical step is reductive quenching of Fl* by TEA because of the latter’s greater concentration. When system concentrations are modified so that oxidative quenching of Fl* by catalyst becomes more dominant, system durability increases, with a system lifetime of over 60 h. System variations and cyclic voltammetry experiments are consistent with a CECE mechanism that is common to electrocatalytic and photocatalytic hydrogen production. This mechanism involves initial protonation of the catalyst followed by reduction and then additional protonation and reduction steps to give a key Ni–H^–/N–H⁺ intermediate that forms the H–H bond in the turnover-limiting step of the catalytic cycle. A key to the activity of these catalysts is the reversible dechelation and protonation of the pyridine N atoms, which enable an internal heterocoupling of a metal hydride and an N-bound proton to produce H₂

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

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

Light-Driven Hydrogen Production from Aqueous Protons using Molybdenum Catalysts

Homogeneous light-driven systems employing molecular molybdenum catalysts for hydrogen production are described. The specific Mo complexes studied are six-coordinate bis­(benzenedithiolate) derivatives having two additional isocyanide or phosphine ligands to complete the coordination sphere. Each of the complexes possesses a trigonal prismatic coordination geometry. The complexes were investigated as proton reduction catalysts in the presence of [Ru­(bpy)₃]²⁺, ascorbic acid, and visible light. Over 500 TON are obtained over 24 h. Electrocatalysis occurs between the Mo^IV/Mo^III and Mo^III/Mo^II redox couples, around 1.0 V vs SCE. Mechanistic studies by ¹H NMR spectroscopy show that upon two-electron reduction the Mo­(CNR)₂(bdt)₂ complex dissociates the isocyanide ligands, followed by addition of acid to result in the formation of molecular hydrogen and the Mo­(bdt)₂ complex

Light-Driven Hydrogen Production from Aqueous Protons using Molybdenum Catalysts

Homogeneous light-driven systems employing molecular molybdenum catalysts for hydrogen production are described. The specific Mo complexes studied are six-coordinate bis­(benzenedithiolate) derivatives having two additional isocyanide or phosphine ligands to complete the coordination sphere. Each of the complexes possesses a trigonal prismatic coordination geometry. The complexes were investigated as proton reduction catalysts in the presence of [Ru­(bpy)₃]²⁺, ascorbic acid, and visible light. Over 500 TON are obtained over 24 h. Electrocatalysis occurs between the Mo^IV/Mo^III and Mo^III/Mo^II redox couples, around 1.0 V vs SCE. Mechanistic studies by ¹H NMR spectroscopy show that upon two-electron reduction the Mo­(CNR)₂(bdt)₂ complex dissociates the isocyanide ligands, followed by addition of acid to result in the formation of molecular hydrogen and the Mo­(bdt)₂ complex

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

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