Methane Activation with Rhenium Catalysts. 1. Bidentate Oxygenated Ligands

Trends in methane activation have been explored for rhenium-based catalysts in conjunction with bidentate oxygenated ligands of the form (L_1)(L_2)Re(OH)(OH_2) [L_1, L_2 = acac, catechol, glycol]. When placed in acidic media, the equilibrium for this reference catalyst shifts to the protonated forms (L_1)(L_2)Re(OH_2)(OH_2) in almost all cases. In all cases the activation of the reference complex proceeds through a concerted metathesis type transition state, and only one of the 13 reference complexes proceeds with methane activation through a barrier of less than 35 kcal mol-1. Study of the identity complexes (L_1 = L_2) revealed that protonation of the ligand oxygens is unfavorable for acac and catechol, but favorable for glycol; however in only one case is the barrier for methane activation improved by this route. Electron density on the central rhenium is the best predictor for the magnitude of the methane activation barrier; namely, increased electron density (obtained by considering lower oxidation states) on the metal leads to lower barriers. Lower oxidation states form weaker Re−O bonds, which increase lability of the leaving groups and decrease the barrier to proton transfer from methane

Rhodium complexes bearing tetradentate diamine-bis(phenolate) ligands

Using tetradentate, dianionic ligands, several new rhodium complexes have been prepared. Some of these diamine-bis(phenolate) compounds, are active for C–H activation of benzene. These complexes are air and thermally stable. All four complexes were characterized by X-ray diffraction

Methylrhenium Trioxide Revisited: Mechanisms for Nonredox Oxygen Insertion in an M−CH_3 Bond

Methylrhenium trioxide (MTO) has the rare ability to stoichiometrically generate methanol at room temperature with an external oxidant (H_2O_2) under basic conditions. In order to use this transformation as a model for nonredox oxidative C−O coupling, the mechanisms have been elucidated using density functional theory (DFT). Our studies show several possible reaction pathways to form methanol, with the lowest net barrier (ΔH‡) being 23.3 kcal mol^(-1). The rate-determining step is a direct “Baeyer−Villiger” type concerted oxygen insertion into MTO, forming methoxyrhenium trioxide. The key to the low-energy transition state is the donation of electron density, first, from HOO(−) to the –CH_3 group (making –CH_3 more nucleophilic and HOO− more electrophilic) and, second, from the Re−C bond to both the forming Re−O and breaking O−O bonds, simultaneously (thus forming the Re−O bond as the Re−C bond is broken). In turn, the ability of MTO to undergo these transfers can be traced to the electrophilic nature of the metal center and to the absence of accessible d-orbitals. If accessible d-orbitals are present, they would most likely donate the required electron density instead of the M−CH_3 moiety, and this bond would thus not be broken. It is possible that other metal centers with similar qualities, such as Pt^(IV) or Ir^V, could be competent for the same type of chemistry

Heterolytic CH Activation with a Cyclometalated Platinum(II) 6-Phenyl-4,4‘-di-tert-butyl-2,2-Bipyridine Complex

The more electron-rich, thermally, air, and protic stable, cyclometalated Pt(II)(NNC) trifluoroacetate complex (3) (NNC = κ^3-6-phenyl-4,4‘-di-tert-butyl-2,2‘-bipyridine) was synthesized with the expectation that it would be less susceptible to H_2O inhibition than the Pt(bpym)(TFA)_2 system (bpym = κ^2-2,2‘-bipyrimidine) for the catalytic oxidation of hydrocarbons. Complex 3 was found to catalyze the H/D exchange between benzene and trifluoroacetic acid via CH activation but at a rate slower than the Pt(bpym) complex. Experimental and theoretical studies show that while the use of the more electron-rich NNC ligand motif lowered the ΔH for substrate coordination relative to the Pt(bpym) system, a larger increase in the barrier for CH cleavage led to an increase in the overall barrier for CH activation

Examining the Rotation Period Distribution of the 40 Myr Tucana-Horologium Association with TESS

The Tucana-Horologium Association (Tuc-Hor) is a 40 Myr old moving group in the southern sky. In this work, we measure the rotation periods of 313 Tuc-Hor objects with TESS light curves derived from TESS full frame images and membership lists driven by Gaia EDR3 kinematics and known youth indicators. We recover a period for 81.4% of the sample and report 255 rotaion periods for Tuc-Hor objects. From these objects we identify 11 candidate binaries based on multiple periodic signals or outlier Gaia DR2 and EDR3 re-normalised unit weight error (RUWE) values. We also identify three new complex rotators (rapidly rotating M dwarf objects with intricate light curve morphology) within our sample. Along with the six previously known complex rotators that belong to Tuc-Hor, we compare their light curve morphology between TESS Cycle 1 and Cycle 3 and find they change substantially. Furthermore, we provide context for the entire Tuc-Hor rotation sample by describing the rotation period distributions alongside other youth indicators such as H{\alpha} and Li equivalent width, as well as near ultra-violet and X ray flux. We find that measuring rotation periods with TESS to be a fast and effective means to confirm members in young moving groups.Comment: 27 pages, 12 figure

Catalytic Mechanism and Efficiency of Methane Oxidation by Hg(II) in Sulfuric Acid and Comparison to Radical Initiated Conditions

Methane conversion to methyl bisulfate by Hg^(II)(SO_4) in sulfuric acid is an example of fast and selective alkane oxidation catalysis. Dichotomous mechanisms involving C–H activation and electron transfer have been proposed based on experiments. Radical oxidation pathways have also been proposed for some reaction conditions. Hg^(II) is also of significant interest because as a d^(10) transition metal it is similar to d^(10) main-group metals that also oxidize alkanes. Density-functional calculations are presented that use both implicit and a mixture of implicit/explicit solvent models for the complete Hg_(II) catalytic cycle of methane oxidation to methyl bisulfate. These calculations are consistent with experiment and reveal that methane is functionalized to methyl bisulfate by a C–H activation and reductive metal alkyl functionalization mechanism. This reaction pathway is lower in energy than both electron transfer and proton-coupled electron transfer pathways. After methane C–H functionalization, catalysis is completed by conversion of the proposed resting state, [Hg^I(HSO_4)]_2, into Hg^0 followed by Hg^0 to Hg^(II) oxidation induced by SO_3 from dehydration of sulfuric acid. This catalytic cycle is efficient because in sulfuric acid the Hg^(II)/Hg^0 potential results in a moderate free energy barrier for oxidation (∼40 kcal/mol) and Hg^(II) is electrophilic enough to induce barriers of <40 kcal/mol for C–H activation and reductive metal alkyl functionalization. Comparison of Hg^(II) to Tl^(III) shows that while C–H activation and reductive metal alkyl functionalization have reasonable barriers for Tl^(III), the oxidation of Tl^I to Tl^(III) has a significantly larger barrier than Hg^0 to Hg^(II) oxidation and therefore Tl^(III) is not catalytic in sulfuric acid. Comparison of Hg^(II) to Cd^(II) and Zn^(II) reveals that while M^0 to M^(II) oxidation and C–H activation are feasible for these first-row and second-row transition metals, reductive metal alkyl functionalization barriers are very large and catalysis is not feasible. Calculations are also presented that outline the mechanism and energy landscape for radical-initiated (K_2S_2O_8) methane oxidation to methanesulfonic acid in sulfuric acid

Facile Functionalization of a Metal Carbon Bond by O-Atom Transfer

The facile conversion of M−R to M−OR that could be useful for the functionalization of electron-rich metal alkyl intermediates is shown to proceed via a Baeyer−Villiger-type pathway involving a nonredox, electrophilic, O-atom insertion in reactions with non-peroxo O-donors

Facile oxy-functionalization of a nucleophilic metal alkyl with a cis-dioxo metal species via a (2+3) transition state

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