Monometallic Pathway Research Articles

Mechanistic study of Selectfluor-mediated digold-catalyzed Csp3-Csp2 coupling reaction

To obtain preliminary information regarding the number of metal centers involved in redox transformations and address the role of metal centers in the redox steps, we initiated a detailed theoretical study of digold-catalyzed cross-coupling reaction. There would be a cross point of the free energy profiles for bimetallic and monometallic pathways, suggesting that the metal number involving in redox is alterable. Initially, oxidative addition of gold catalyst by Selectfluor takes place via bimetallic manner giving an [AuII aAuII b] complex, which then undergoes disproportionation to afford the mixed-valent [AuI aAuIII b] complex, followed by monometallic reduction.

Catalytic Ammonia Oxidation to Dinitrogen by Hydrogen Atom Abstraction

AbstractCatalysts for the oxidation of NH3 are critical for the utilization of NH3 as a large‐scale energy carrier. Molecular catalysts capable of oxidizing NH3 to N2 are rare. This report describes the use of [Cp*Ru(PtBu2NPh2)(15NH3)][BArF4], (PtBu2NPh2=1,5‐di(phenylaza)‐3,7‐di(tert‐butylphospha)cyclooctane; ArF=3,5‐(CF3)2C6H3), to catalytically oxidize NH3 to dinitrogen under ambient conditions. The cleavage of six N−H bonds and the formation of an N≡N bond was achieved by coupling H+ and e− transfers as net hydrogen atom abstraction (HAA) steps using the 2,4,6‐tri‐tert‐butylphenoxyl radical (tBu3ArO.) as the H atom acceptor. Employing an excess of tBu3ArO. under 1 atm of NH3 gas at 23 °C resulted in up to ten turnovers. Nitrogen isotopic (15N) labeling studies provide initial mechanistic information suggesting a monometallic pathway during the N⋅⋅⋅N bond‐forming step in the catalytic cycle.

Thermodynamic Properties of Hydrogen-Producing Cobaloxime Catalysts: A Density Functional Theory Analysis.

Density functional theory calculations were carried out to study the electrochemical properties including reduction potentials, pKa values, and thermodynamic hydricities of three prototypical cobaloxime complexes, Co(dmgBF2)2 (dmgBF2 = difluoroboryl-dimethylglyoxime), Co(dmgH)2 (dmgH = dimethylglyoxime), and Co(dmgH)2(py)(Cl) (py = pyridine) in the acetonitrile (AN)–water solvent mixture. The electrochemical properties of Co(dmgBF2)2 in pure AN and pure water were also considered for comparison to reveal the key roles of the solvent on the catalytic reaction. In agreement with previous studies, hydrogen production pathways starting from reduction of the resting state of CoII and involving formation of the CoIIIH and CoIIH intermediates are the favorable ones for both bimetallic and monometallic pathways. However, we found that in pure AN, both the CoIIIH and CoIIH intermediates can react with a proton to produce H2. In the presence of water in the solvent, the reduction of CoIIIH to CoIIH is necessary for the reaction with a proton to occur to form H2. This suggests that it is possible to design catalytic systems by suitably tuning the composition of the AN–water mixture. We also identified the key role of axial coordination of the solvent molecules in affecting the catalytic reaction, which allows further catalyst design strategy. The highest hydride donor ability of Co(dmgH)2(py)(Cl) indicates that this complex displays the best catalytic hydrogen-producing performance among the three cobaloximes studied in this work.

Single-Site Photocatalytic H2 Evolution from Covalent Organic Frameworks with Molecular Cobaloxime Co-Catalysts.

We demonstrate photocatalytic hydrogen evolution using COF photosensitizers with molecular proton reduction catalysts for the first time. With azine-linked N2-COF photosensitizer, chloro(pyridine)cobaloxime co-catalyst, and TEOA donor, H2 evolution rate of 782 μmol h–1 g–1 and TON of 54.4 has been obtained in a water/acetonitrile mixture. PXRD, solid-state spectroscopy, EM analysis, and quantum-chemical calculations suggest an outer sphere electron transfer from the COF to the co-catalyst which subsequently follows a monometallic pathway of H2 generation from the CoIII-hydride and/or CoII-hydride species.

Ring‐Opening Copolymerization of Epoxides with Cyclic Anhydrides Promoted by Bimetallic and Monometallic Phenoxy–Imine Aluminum complexes

AbstractTwo dinuclear aluminum alkyl complexes bearing a salen ligand with alkyl backbone of different length between the nitrogen atoms (1,3‐propylene and 1,5‐pentylene backbone) and the related hemisalen aluminum complex were tested as catalysts in the ring‐opening copolymerization of cyclohexene oxide and limonene oxide with succinic anhydride and phthalic anhydride. The effects of different reaction conditions (co‐catalyst and solvent) on the productivity and selectivity of the reactions were evaluated. The comparison of the catalytic behavior of the dinuclear complexes bearing the aluminum reactive centers at different distances as well as of their mononuclear counterpart suggested that the copolymerization follows a monometallic pathway differently from what was observed in the homopolymerization of CHO.

Nickel-catalyzed Negishi arylations of propargylic bromides: a mechanistic investigation.

Although nickel-catalyzed stereoconvergent couplings of racemic alkyl electrophiles are emerging as a powerful tool in organic chemistry, to date there have been no systematic mechanistic studies of such processes. Herein, we examine the pathway for enantioselective Negishi arylations of secondary propargylic bromides, and we provide evidence for an unanticipated radical chain pathway wherein oxidative addition of the C–Br bond occurs through a bimetallic mechanism. In particular, we have crystallographically characterized a diamagnetic arylnickel(II) complex, [(i-Pr-pybox)NiIIPh]BArF4, and furnished support for [(i-Pr-pybox)NiIIPh]+ being the predominant nickel-containing species formed under the catalyzed conditions as well as a key player in the cross-coupling mechanism. On the other hand, our observations do not require a role for an organonickel(I) intermediate (e.g., (i-Pr-pybox)NiIPh), which has previously been suggested to be an intermediate in nickel-catalyzed cross-couplings, oxidatively adding alkyl electrophiles through a monometallic pathway.

Uranium(III)-Mediated C–C-Coupling of Terminal Alkynes: Formation of Dinuclear Uranium(IV) Vinyl Complexes

The previously reported uranium(III) complex [(((Ad)ArO)(3)N)U(III)(DME)] (1; Ad = adamantane, DME = 1,2-dimethoxyethane) reacts with the terminal bis-alkynes 1,7-octadiyne or 1,6-heptadiyne in C-C-coupling reactions to form the uranium(IV) vinyl complexes [{(((Ad)ArO)(3)N)U(IV)}(2)(μ-η(2):η(1)-1,2-(CH)(2)-cyclohexane)] (2) and [{(((Ad)ArO)(3)N)U(IV)}(2)(μ-η(2):η(2)-1,2-(CH)(2)-cyclopentane)] (3). With the monoalkynes 1-hexyne or 4-(t)butyl-phenylacetylene, the complexes [{(((Ad)ArO)(3)N)U(IV)}(2)(μ-η(2)(C1):η(1)(C4)-2-(n)Bu-1,3-octadiene)] (4) and [{(((Ad)ArO)(3)N)U(IV)}(2)(μ-η(2)(C4):η(1)(C1)-1,3-di-(p-(t)Bu-phenyl)butadiene))] (5), are formed. These are the first four examples of uranium vinyl complexes that are reported and crystallographically characterized. In addition, detailed DFT calculations are presented to establish a possible mechanism for their formation and explain the differences found for the coordination of the hydrocarbon fragments. In contrast to a previously proposed monometallic pathway for catalytic hydroamination of alkynes and alkyne dimerization involving a uranium vinyl intermediate at uranium(III) complexes, the calculations clearly support a bimetallic mechanism, since its transition states are energetically the most favored.

DFT studies of cobalt hydride intermediate on cobaloxime‐catalyzed H2 evolution pathways

AbstractChemical, electrochemical, and photochemical methods all had been utilized to explore proton reduction catalysis by cobaloxime complexes. It was postulated in these studies that the initial step toward making H2 was protonation of CoI to form a CoIII hydride intermediate. However, in the following steps, different results from electrochemical studies had led to both monometallic and bimetallic pathways. In this article, theoretical computation method (BP86/6‐31G*) was firstly performed on possible cobalt hydride intermediates involved in the reactive pathway of cobaloxime‐catalyzed H2 evolution. The monometallic pathway B was excluded, both monometallic pathway A and bimetallic pathway were the possible process. However, the Gibbs free energy change for generation of H2 following monometallic pathway A was much more negative than that following bimetallic pathway. The calculation on monometallic pathway A indicated that the main driving force of the reaction (i) came from the step of the reduction of 11. The proton transfer steps were also studied in detail. The protonation of cobalt hydride intermediates could directly happen on the dimethylglyoximate part. All the results refer to gas‐phase calculations, not considering the solution. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Theoretical Analysis of Mechanistic Pathways for Hydrogen Evolution Catalyzed by Cobaloximes

The mechanistic pathways for hydrogen evolution catalyzed by cobalt complexes with supporting diglyoxime ligands are analyzed with computational methods. The cobaloximes studied are Co(dmgBF(2))(2) (dmg = dimethylglyoxime) and Co(dpgBF(2))(2) (dpg = diphenylglyoxime) in acetonitrile. The reduction potentials and pK(a) values are calculated with density functional theory in conjunction with isodesmic reactions, incorporating the possibility of axial solvent ligand loss during the reduction process. The solvent reorganization energies for electron transfer between the cobalt complex and a metal electrode and the inner-sphere reorganization energies accounting for intramolecular rearrangements and the possibility of ligand loss are also calculated. The relative reduction potentials agree quantitatively with the available experimental values. The pK(a)s and reorganization energies agree qualitatively with estimates based on experimental data. The calculations suggest that a peak measured at ca. -1.0 V vs SCE in cyclic voltammetry experiments for Co(dmgBF(2))(2) is more likely to correspond to the Co(II)H/Co(I)H reduction potential than the Co(III)H/Co(II)H reduction potential. The calculations also predict pK(a) values of Co-hydride complexes and reduction potentials for both cobaloximes that have not been determined experimentally. The results are consistent with a mechanism in which the Co(III) and Co(II) complexes have two axial solvent ligands and the Co(I) complex has a single axial ligand along the reaction pathway. Analysis of the free energy diagrams generated for six different monometallic and bimetallic hydrogen production pathways identified the most favorable pathways for Co(dmgBF(2))(2) and tosic acid. The thermodynamically favored monometallic pathway passes through a Co(III)H intermediate, and Co(II)H reacts with the acid to produce H(2). The thermodynamically favored bimetallic pathways also pass through the Co(III)H intermediate, but the pathways in which two Co(III)H or two Co(II)H complexes react to produce H(2) are not thermodynamically distinguishable with these methods. On the basis of the electrostatic work term associated with bringing the two cobalt complexes together in solution, the preferred bimetallic pathway involves the reaction of two Co(III)H complexes to produce H(2). This mechanistic insight is important for designing more effective catalysts for solar energy conversion.