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High-valent Pd complexes are potent agents for the oxidative functionalization of inert C-H bonds, and it was previously shown that rapid electrocatalytic methane monofunctionalization could be achieved by electro-oxidation of PdII to a critical dinuclear PdIII intermediate in concentrated or fuming sulfuric acid. However, the structure of this highly reactive, unisolable intermediate, as well as the structural basis for its mechanism of electrochemical formation, remained elusive. Herein, we use X-ray absorption and Raman spectroscopies to assemble a structural model of the potent methane-activating intermediate as a PdIII dimer with a Pd-Pd bond and a 5-fold O atom coordination by HxSO4(x-2) ligands at each Pd center. We further use EPR spectroscopy to identify a mixed-valent M-M bonded Pd2II,III species as a key intermediate during the PdII-to-PdIII2 oxidation. Combining EPR and electrochemical data, we quantify the free energy of Pd dimerization as

Electrophilic high-valent metal ions are potent intermediates for the catalytic functionalization of methane, but in many cases, their high redox potentials make these intermediates difficult or impossible to access using mild stoichiometric oxidants derived from O2. Herein, we establish electrochemical oxidation as a versatile new strategy for accessing high-valent methane monofunctionalization catalysts. We provide evidence for the electrochemical oxidation of simple PdSO4 in concentrated sulfuric acid electrolytes to generate a putative Pd2III,III species in an all-oxidic ligand field. This electrogenerated high-valent Pd complex rapidly activates methane with a low barrier of 25.9 (±2.6) kcal/mol, generating methanol precursors methyl bisulfate (CH3OSO3H) and methanesulfonic acid (CH3SO3H) via concurrent faradaic and nonfaradaic reaction pathways. This work enables new electrochemical approaches for promoting rapid methane monofunctionalization.

Electrophilic high-valent metal ions are potent intermediates for the catalytic functionalization of methane, but in many cases, their high redox potentials make these intermediates difficult or impossible to access using mild stoichiometric oxidants derived from O2. Herein, we establish electrochemical oxidation as a versatile new strategy for accessing high-valent methane monofunctionalization catalysts. We provide evidence for the electrochemical oxidation of simple PdSO4 in concentrated sulfuric acid electrolytes to generate a putative Pd2III,III species in an all-oxidic ligand field. This electrogenerated high-valent Pd complex rapidly activates methane with a low barrier of 25.9 (±2.6) kcal/mol, generating methanol precursors methyl bisulfate (CH3OSO3H) and methanesulfonic acid (CH3SO3H) via concurrent faradaic and nonfaradaic reaction pathways. This work enables new electrochemical approaches for promoting rapid methane monofunctionalization.

Transition-metal-mediated nucleophilic aromatic substitution (SNAr) reactions prefer that a suitably strong nucleophile be in an aprotic medium. Usually, using protic nucleophile/medium requires high reaction temperatures (>180 °C) to overcome the attenuated nucleophilicity for attack on the arene π system. Surprisingly, we demonstrate herein a RhIII-mediated SNAr reaction of a fluoroarene moiety with RCO2H (R = CH3, CF3) in acid media that proceeds at moderate temperatures (<100 °C). We show both by experimental and with DFT calculations that the mechanism proceeds through an internal nucleophilic aromatic substitution (I-SNAr), where the nucleophile coordinates to the metal ion prior to substitution, thereby mitigating the acid influence. [ABSTRACT FROM AUTHOR]

A series of rhodium(III) bis(quinolinyl)benzene (bisqx) complexes was studied as candidates for the homogeneous partial oxidation of methane. Density functional theory (DFT) (M06 with Poisson continuum solvation) was used to investigate a variety of (bisqx) ligand candidates involving different functional groups to determine the impact on RhIII(bisqx)-catalyzed methane functionalization. The free energy activation barriers for methane CH activation and Rh-methyl functionalization at 298 K and 498 K were determined. DFT studies predict that the best candidate for catalytic methane functionalization is RhIII coordinated to unsubstituted bis(quinolinyl)benzene (bisq). Support is also found for the prediction that the η2-benzene coordination mode of (bisqx) ligands on Rh encourages methyl group functionalization by serving as an effective leaving group for SN2 and SR2 attack. [ABSTRACT FROM AUTHOR]

The reductive functionalization ofRhIII–Me bondsin acids is a key step for RhI/III-based catalysts formethane functionalization. Heating electronic-rich [tBu3terpy]Rh(Me)(Cl)I (1; tBu3terpy = 4,4′,4″-tri-tert-butylterpyridine) in H2O, AcOD, or trifluoroaceticacid (HTFA) released a stoichiometric amount of methane. With useof a less donating ligand, [(NO2)3terpy]Rh(Me)(Cl)I(2; (NO2)3terpy = 4,4′,4″-trinitroterpyridine),a mixture of CH4and CH3X (X = Cl, TFA, OAc,OH) was obtained. The selectivity between MeX and CH4wasfound to be dependent on the halide present (I–orCl–). In a key experiment with the removal of onehalide from 2, {[(NO2)3terpy]Rh(Me)Cl}{BF4} (3) was completely protonated in acidic solvent.DFT calculations were employed to investigate the mechanism of reductivefunctionalization and protonation. [ABSTRACT FROM AUTHOR]

Traditional C-H bond activation by a concerted metalation-deprotonation (CMD) mechanism involves precoordination of the C-H bond followed by deprotonation from an internal base. Reported herein is a “through-arene” activation of an uncoordinated benzylic C-H bond that is 6 bonds away from a RhIII ion. The mechanism, which was investigated by experimental and DFT studies, proceeds through a dearomatized xylene intermediate. This intermediate was observed spectroscopically upon addition of a pyridine base to provide a thermodynamic trap. [ABSTRACT FROM AUTHOR]

Synthetic protocols for a pyrrolide-centeredONO3–trianionic pincer-type ligand are presented.Treating (tBuO)3WCtBu with the proligand [pyr-ONO]H3(2) results in the formation of the trianionic pinceralkylidene complex [pyr-ONO]WCHtBu(OtBu) (3). Addition ofa mild base to complex 3provides the trianionic pinceralkylidyne complex {MePPh3}{[pyr-ONO]WCtBu(OtBu)} (4). All new compounds were characterized by NMR spectroscopy, combustionanalysis, and, in the case of complex 4, single-crystalX-ray crystallography. DFT calculations performed on 4provide insight into its electronic structure and indicate thatthe HOMO is ligand-based and localized on the pyrrolide π orbitals. [ABSTRACT FROM AUTHOR]

Appending an amine to a C=C double bond drastically increases the nucleophilicity of the β-carbon atom of the alkene to form an enamine. In this report, we present the synthesis and characterization of a novel CF3-ONO3- trianionic pincer-type ligand, rationally designed to mimic enamines within a metal coordination sphere. Presented is a synthetic strategy to create enhanced nucleophilic tungsten-alkylidene and -alkylidyne complexes. Specifically, we present the synthesis and characterization of the new CF3-ONO3- trianionic pincer tungsten-alkylidene [CF3-ONO]W=CH(Et)(OtBu) (2) and -alkylidyne {MePPh3}{[CF3-ONO]W≡C(Et)(OtBu)} (3) complexes. Characterization involves a combination of multinuclear NMR spectroscopy, combustion analysis, DFT computations, and single crystal X-ray analysis for complexes 2 and 3. Exhibiting unique nucleophilic reactivity, 3 reacts with MeOTf to yield [CF3-ONO]W=C(Me)(Et)(OtBu) (4), but the bulkier Me3SiOTf silylates the tert-butoxide, which subsequently undergoes isobutylene expulsion to form [CF3-ONO]WCH(Et)(OSiMe3) (5). A DFT calculation performed on a model complex of 3, namely, [CF3-ONO]W≡C(Et)(OtBu) (3'), reveals the amide participates in an enamine-type bonding combination. For complex 2, the Lewis acids MeOTf, Me3SiOTf, and B(C6F5)3 catalyze isobutylene expulsion to yield the tungsten-oxo complex [CF3-ONO]W(O)(IIPr) (6). [ABSTRACT FROM AUTHOR]

Synthetic and kinetic experiments designed to probe the mechanism of O2 activation bT the trianionic pincer chromium(III) complex [tBuOCO]Cr' `(THF)3 (1) (where tBuOCO = [2,6-(tBuC6H3O)2C6H3]3-, THF = tetrahydrofuran) are described. Whereas analogous porphyrin and corrole oxidation catalysts can become inactive toward 02 activation upon dimerization (forming au-oxo species) or product inhibition, complex 1 becomes more active toward O2 activation when dimerized. The product from O2 activation, [tB OCO]CV(0)(THF) (2), catalyzes the oxidation of 1 via formation of the u-0 dimer {[tBUOCO]CrlV(THF)2}(μ-O) (3). Complex 3existsinequilibrium with 1 and 2 and thus could not be isolated in pure form. However, single crystals of 3 and 1 co-deposit, and the molecular stucture of 3 was determined using single-crystal X-ray crystallography methods. Variable (9.5,35, and 240 GHz) frequency electron paramagnetic resonance spectroscopy supports the assignment of complex 3 as a Cr"-O-Cr" dimer, with a high (S = 2) spin ground state, based on detailed computer simulations. Complex 3 is the first condusively assigned example of a complex containing a Cr(1V) dimer; its spin Hamiltonian parameters are g = 1.976, D = 2400 G, and E = 750 G. The reaction of 1 with O2 was monitored by UV-visible spectrophotometry, and the kinetic orders of the reagents were determined. The reaction does not exhibit first-order behavior with respect to the concentrations of complex 1 and O2. Altering the THF concentration reveals an inverse order behavior in THF. A proposed autocatalytic mechanism, with 3 as the key intermediate, was employed in numerical simulations of concentration versus time decay plots, and the individual rate constants were calculated. The simulations agree well with the experimental observations. The acceleration is not unique to 2; for example, the presence of OPPh3 accelerates O2 activation by forming the five-coordinate complex trans-[tBuOCO]CrIII(0PPh3)2 (4). [ABSTRACT FROM AUTHOR]

This review describes organometallic compounds and materials that are capable of mediating a rarely encountered but fundamentally important reaction: β-alkyl elimination at the metal-Cα-Cβ-R moiety, in which an alkyl group attached to the Cβ atom is transferred to the metal or to a coordinated substrate. The objectives of this review are to provide a cohesive fundamental understanding of β-alkyl-elimination reactions and to highlight its applications in olefin polymerization, alkane hydrogenolysis, depolymerization of branched polymers, ring-opening polymerization of cycloalkanes, and other useful organic reactions. To provide a coherent understanding of the β-alkyl elimination reaction, special attention is given to conditions and strategies used to facilitate β-alkyl-elimination/transfer events in metal-catalyzed olefin polymerization, which provide the well-studied examples.

Traditional C-H bond activation by a concerted metalation-deprotonation (CMD) mechanism involves precoordination of the C-H bond followed by deprotonation from an internal base. Reported herein is a "through-arene" activation of an uncoordinated benzylic C-H bond that is 6 bonds away from a Rh(III) ion. The mechanism, which was investigated by experimental and DFT studies, proceeds through a dearomatized xylene intermediate. This intermediate was observed spectroscopically upon addition of a pyridine base to provide a thermodynamic trap.

Appending an amine to a C═C double bond drastically increases the nucleophilicity of the β-carbon atom of the alkene to form an enamine. In this report, we present the synthesis and characterization of a novel CF(3)-ONO(3-) trianionic pincer-type ligand, rationally designed to mimic enamines within a metal coordination sphere. Presented is a synthetic strategy to create enhanced nucleophilic tungsten-alkylidene and -alkylidyne complexes. Specifically, we present the synthesis and characterization of the new CF(3)-ONO(3-) trianionic pincer tungsten-alkylidene [CF(3)-ONO]W═CH(Et)(O(t)Bu) (2) and -alkylidyne {MePPh(3)}{[CF(3)-ONO]W≡C(Et)(O(t)Bu)} (3) complexes. Characterization involves a combination of multinuclear NMR spectroscopy, combustion analysis, DFT computations, and single crystal X-ray analysis for complexes 2 and 3. Exhibiting unique nucleophilic reactivity, 3 reacts with MeOTf to yield [CF(3)-ONO]W═C(Me)(Et)(O(t)Bu) (4), but the bulkier Me(3)SiOTf silylates the tert-butoxide, which subsequently undergoes isobutylene expulsion to form [CF(3)-ONO]W═CH(Et)(OSiMe(3)) (5). A DFT calculation performed on a model complex of 3, namely, [CF(3)-ONO]W≡C(Et)(O(t)Bu) (3'), reveals the amide participates in an enamine-type bonding combination. For complex 2, the Lewis acids MeOTf, Me(3)SiOTf, and B(C(6)F(5))(3) catalyze isobutylene expulsion to yield the tungsten-oxo complex [CF(3)-ONO]W(O)((n)Pr) (6).

The oxygen-atom-transfer (OAT) from [(t)BuOCO]Cr(V)(O)(THF) (2) (where (t)BuOCO = [2,6-C(6)H(3)(6-(t)BuC(6)H(3)O)(2)](3-), THF = tetrahydrofuran) to triphenylphosphine (PPh(3)) in THF produces [(t)BuOCO]Cr(III)(THF)(3) (1) and triphenylphosphine oxide (OPPh(3)) at a rate of 69.5(±1.9) M(-1) s(-1) (22 °C). Identical rate constants were attained when acetonitrile (MeCN) and dichloromethane/THF (CH(2)Cl(2)/THF) were used as solvents. Electron paramagnetic resonance (EPR) data shows that the six-coordinate complex, [(t)BuOCO]Cr(V)(O)(THF)(2) (2a) forms upon addition of THF to 2, suggesting 2a as the active OAT species in THF. Similarly, addition of OPPh(3) has no influence on the rate of OAT, but the addition of triphenylphosphorus ylide (CH(2)PPh(3)) to form [(t)BuOCO]Cr(V)(O)(CH(2)PPh(3)) (4) prevents OAT to PPh(3). In CH(2)Cl(2), a [Cr(IV)](2)(μ-O) intermediate forms during the OAT from 2 to PPh(3). The OAT from {[(t)BuOCO]Cr(IV)(THF)}(2)(μ-O) (3) to PPh(3) reveals a zero-order dependence in PPh(3) indicating the dimer must first dissociate prior to OAT. The decay of 3versus time does not follow first-order kinetics due to the formation of a [(t)BuOCO]Cr(III)(THF) species (5) that inhibits the dissociation of 3. The change in concentration of 3versus time during OAT was simulated to obtain approximate rate constants.

Synthetic and kinetic experiments designed to probe the mechanism of O(2) activation by the trianionic pincer chromium(III) complex [(t)BuOCO]Cr(III)(THF)(3) (1) (where (t)BuOCO = [2,6-((t)BuC(6)H(3)O)(2)C(6)H(3)](3-), THF = tetrahydrofuran) are described. Whereas analogous porphyrin and corrole oxidation catalysts can become inactive toward O(2) activation upon dimerization (forming a μ-oxo species) or product inhibition, complex 1 becomes more active toward O(2) activation when dimerized. The product from O(2) activation, [(t)BuOCO]Cr(V)(O)(THF) (2), catalyzes the oxidation of 1 via formation of the μ-O dimer {[(t)BuOCO]Cr(IV)(THF)}(2)(μ-O) (3). Complex 3 exists in equilibrium with 1 and 2 and thus could not be isolated in pure form. However, single crystals of 3 and 1 co-deposit, and the molecular stucture of 3 was determined using single-crystal X-ray crystallography methods. Variable (9.5, 35, and 240 GHz) frequency electron paramagnetic resonance spectroscopy supports the assignment of complex 3 as a Cr(IV)-O-Cr(IV) dimer, with a high (S = 2) spin ground state, based on detailed computer simulations. Complex 3 is the first conclusively assigned example of a complex containing a Cr(IV) dimer; its spin Hamiltonian parameters are g(iso) = 1.976, D = 2400 G, and E = 750 G. The reaction of 1 with O(2) was monitored by UV-visible spectrophotometry, and the kinetic orders of the reagents were determined. The reaction does not exhibit first-order behavior with respect to the concentrations of complex 1 and O(2). Altering the THF concentration reveals an inverse order behavior in THF. A proposed autocatalytic mechanism, with 3 as the key intermediate, was employed in numerical simulations of concentration versus time decay plots, and the individual rate constants were calculated. The simulations agree well with the experimental observations. The acceleration is not unique to 2; for example, the presence of OPPh(3) accelerates O(2) activation by forming the five-coordinate complex trans-[(t)BuOCO]Cr(III)(OPPh(3))(2) (4).