Experimental and Computational Investigation of the Aerobic Oxidation of a Late Transition Metal-Hydride.

The rational development of homogeneous catalytic systems for selective aerobic oxidations of organics has been hampered by the limited available knowledge of how oxygen reacts with important organometallic intermediates. Recently, several mechanisms for oxygen insertion into late transition metal-hydride bonds have been described. Contributing to this nascent understanding of how oxygen reacts with metal-hydrides, a detailed mechanistic study of the reaction of oxygen with the IrIII hydride complex (dmPhebox)Ir(OAc)(H) (1) in the presence of acetic acid, which proceeds to form the IrIII complex (dmPhebox)Ir(OAc)2(OH2) (2), is described. The evidence supports a multifaceted mechanism wherein a small amount of an initially formed metal hydroperoxide proceeds to generate a metal-oxyl species that then initiates a radical chain reaction to rapidly convert the remaining IrIII-H. Insight into the initiation step was gained through kinetic and mechanistic studies of the radical chain inhibition by BHT (butylated hydroxytoluene). Computational studies were employed to contribute to a further understanding of initiation and propagation in this system.

Multiple Bonds in Uranium-Transition Metal Complexes.

Novel heterobimetallic complexes featuring a uranium atom paired with a first-row transition metal have been computationally predicted and analyzed using density functional theory and multireference wave function based methods. The synthetically inspired metalloligands U{(iPr2PCH2NAr)3tacn} (1) and U(iPr2PCH2NPh)3 (2) are explored in this study. We report the presence of multiple bonds between uranium and chromium, uranium and manganese, and uranium and iron. The calculations predict a 5-fold bonding between uranium and manganese in the UMn(iPr2PCH2NPh)3 complex, which is unprecedented in the literature.

Computational Screening of Bimetal-Functionalized Zr6O8 MOF Nodes for Methane C-H Bond Activation.

Zr-based metal-organic frameworks (MOFs) are promising supports for copper-based catalysts able to activate methane. Homo- and heterobimetal-functionalized NU-1000 MOF nodes were selected to computationally screen the effect of ancillary metals for C-H bond activation, allowing us to correlate activation free energies with chemical descriptors.

Tuning the properties of metal-organic framework nodes as supports of single-site iridium catalysts: node modification by atomic layer deposition of aluminium.

The metal-organic framework NU-1000, with Zr6-oxo, hydroxo, and aqua nodes, was modified by incorporation of hydroxylated Al(iii) ions by ALD-like chemistry with [Al(CH3)2(iso-propoxide)]2 followed by steam (ALD = atomic layer deposition). Al ions were installed to the extent of approximately 7 per node. Single-site iridium diethylene complexes were anchored to the nodes of the modified and unmodified MOFs by reaction with Ir(C2H4)2(acac) (acac = acetylacetonate) and converted to Ir(CO)2 complexes by treatment with CO. Infrared spectra of these supported complexes show that incorporation of Al weakened the electron donor tendency of the MOF. Correspondingly, the catalytic activity of the initial supported iridium complexes for ethylene hydrogenation increased, as did the selectivity for ethylene dimerization. The results of density functional theory calculations with a simplified model of the nodes incorporating Al(iii) ions are in qualitative agreement with some catalyst performance data.

Methane Oxidation to Methanol Catalyzed by Cu-Oxo Clusters Stabilized in NU-1000 Metal-Organic Framework.

Copper oxide clusters synthesized via atomic layer deposition on the nodes of the metal-organic framework (MOF) NU-1000 are active for oxidation of methane to methanol under mild reaction conditions. Analysis of chemical reactivity, in situ X-ray absorption spectroscopy, and density functional theory calculations are used to determine structure/activity relations in the Cu-NU-1000 catalytic system. The Cu-loaded MOF contained Cu-oxo clusters of a few Cu atoms. The Cu was present under ambient conditions as a mixture of ∼15% Cu+ and ∼85% Cu2+. The oxidation of methane on Cu-NU-1000 was accompanied by the reduction of 9% of the Cu in the catalyst from Cu2+ to Cu+. The products, methanol, dimethyl ether, and CO2, were desorbed with the passage of 10% water/He at 135 °C, giving a carbon selectivity for methane to methanol of 45-60%. Cu oxo clusters stabilized in NU-1000 provide an active, first generation MOF-based, selective methane oxidation catalyst.

An Exceptionally Stable Metal-Organic Framework Supported Molybdenum(VI) Oxide Catalyst for Cyclohexene Epoxidation.

Molybdenum(VI) oxide was deposited on the Zr6 node of the mesoporous metal-organic framework NU-1000 via condensed-phase deposition where the MOF is simply submerged in the precursor solution, a process named solvothermal deposition in MOFs (SIM). Exposure to oxygen leads to a monodisperse, porous heterogeneous catalyst, named Mo-SIM, and its structure on the node was elucidated both computationally and spectroscopically. The catalytic activity of Mo-SIM was tested for the epoxidation of cyclohexene. Near-quantitative yields of cyclohexene oxide and the ring-opened 1,2-cyclohexanediol were observed, indicating activity significantly higher than that of molybdenum(VI) oxide powder and comparable to that of a zirconia-supported analogue (Mo-ZrO2) prepared in a similar fashion. Despite the well-known leaching problem of supported molybdenum catalysts (i.e., loss of Mo species thus causes deactivation), Mo-SIM demonstrated no loss in the metal loading before and after catalysis, and no molybdenum was detected in the reaction mixture. In contrast, Mo-ZrO2 led to significant leaching and close to 80 wt % loss of the active species. The stability of Mo-SIM was further confirmed computationally, with density functional theory calculations indicating that the dissociation of the molybdenum(VI) species from the node of NU-1000 is endergonic, corroborating the experimental data for the Mo-SIM material.

Competing amination and C-H arylation pathways in Pd/xantphos-catalyzed transformations of binaphthyl triflates: switchable routes to chiral amines and helicene derivatives.

A Pd(OAc)2/xantphos catalyst system was found effective for benzylaminations of binaphthyl 2-triflates bearing a variety of alkyl, benzyl, and substituted phenyl substituents at the 2'-position. With 2'-aryl substituents, an intramolecular Pd-catalyzed C-H arylation was observed as a competing side reaction under some conditions. By adjusting the solvent and quantity of the amine, the reaction was optimized to favor either the amination or the C-H arylation pathway, affording two distinct and potentially useful sets of products. The amines represent tunable chiral ligand precursors, while the C-H arylation pathway affords a series of benzofused [5]helicene derivatives. Kinetic studies and activation parameters for the C-H arylation pathway, supported by DFT calculations, are consistent with a concerted metalation-deprotonation (CMD) mechanism involving a Pd-bound carbonate as the base. Xantphos is proposed to facilitate the turnover-limiting inner-sphere CMD step by acting as a hemilabile ligand, while its wide bite angle engenders a low reductive elimination barrier.

Oxyfunctionalization with Cp*Ir(III)(NHC)(Me)(Cl) with O2: identification of a rare bimetallic Ir(IV) µ-oxo intermediate.

Methanol formation from [Cp*Ir(III)(NHC)Me(CD2Cl2)](+) occurs quantitatively at room temperature with air (O2) as the oxidant and ethanol as a proton source. A rare example of a diiridium bimetallic complex, [(Cp*Ir(NHC)Me)2(µ-O)][(BAr(F)4)2], 3, was isolated and shown to be an intermediate in this reaction. The electronic absorption spectrum of 3 features a broad observation at ∼660 nm, which is primarily responsible for its blue color. In addition, 3 is diamagnetic and can be characterized by NMR spectroscopy. Complex 3 was also characterized by X-ray crystallography and contains an Ir(IV)-O-Ir(IV) core in which two d(5) Ir(IV) centers are bridged by an oxo ligand. DFT and MCSCF calculations reveal several important features of the electronic structure of 3, most notably, that the µ-oxo bridge facilitates communication between the two Ir centers, and σ/π mixing yields a nonlinear arrangement of the µ-oxo core (Ir-O-Ir ∼ 150°) to facilitate oxygen atom transfer. The formation of 3 results from an Ir oxo/oxyl intermediate that may be described by two competing bonding models, which are close in energy and have formal Ir-O bond orders of 2 but differ markedly in their electronic structures. The radical traps TEMPO and 1,4-cyclohexadiene do not inhibit the formation of 3; however, methanol formation from 3 is inhibited by TEMPO. Isotope labeling studies confirmed the origin of the methyl group in the methanol product is the iridium-methyl bond in the [Cp*Ir(NHC)Me(CD2Cl2)][BAr(F)4] starting material. Isolation of the diiridium-containing product [(Cp*Ir(NHC)Cl)2][(BAr(F)4)2], 4, in high yields at the end of the reaction suggests that the Cp* and NHC ligands remain bound to the iridium and are not significantly degraded under reaction conditions.

Reductive functionalization of a rhodium(III)-methyl bond by electronic modification of the supporting ligand.

Net reductive elimination (RE) of MeX (X = halide or pseudo-halide: Cl(-), CF3CO2(-), HSO4(-), OH(-)) is an important step during Pt-catalyzed hydrocarbon functionalization. Developing Rh(I/III)-based catalysts for alkane functionalization is an attractive alternative to Pt-based systems, but very few examples of RE of alkyl halides and/or pseudo-halides from Rh(III) complexes have been reported. Here, we compare the influence of the ligand donor strength on the thermodynamic potentials for oxidative addition and reductive functionalization using [(t)Bu3terpy]RhCl (1) {(t)Bu3terpy = 4,4',4''-tri-tert-butylpyridine} and [(NO2)3terpy]RhCl (2) {(NO2)3terpy = 4,4',4''-trinitroterpyridine}. Complex 1 oxidatively adds MeX {X = I(-), Cl(-), CF3CO2(-) (TFA(-))} to afford [(t)Bu3terpy]RhMe(Cl)(X) {X = I(-) (3), Cl(-) (4), TFA(-) (5)}. By having three electron-withdrawing NO2 groups, complex 2 does not react with MeCl or MeTFA, but reacts with MeI to yield [(NO2)3terpy]RhMe(Cl)(I) (6). Heating 6 expels MeCl along with a small quantity of MeI. Repeating this experiment but with excess [Bu4N]Cl exclusively yields MeCl, while adding [Bu4N]TFA yields a mixture of MeTFA and MeCl. In contrast, 3 does not reductively eliminate MeX under similar conditions. DFT calculations successfully predict the reaction outcome by complexes 1 and 2. Calorimetric measurements of [(t)Bu3terpy]RhI (7) and [(t)Bu3terpy]RhMe(I)2 (8) were used to corroborate computational models. Finally, the mechanism of MeCl RE from 6 was investigated via DFT calculations, which supports a nucleophilic attack by either I(-) or Cl(-) on the Rh-CH3 bond of a five-coordinate Rh complex.

Mechanism of hydrogenolysis of an iridium-methyl bond: evidence for a methane complex intermediate.

Evidence for key σ-complex intermediates in the hydrogenolysis of the iridium-methyl bond of (PONOP)Ir(H)(Me)(+) (1) [PONOP = 2,6-bis(di-tert-butylphosphinito)pyridine] has been obtained. The initially formed η(2)-H(2) complex, 2, was directly observed upon treatment of 1 with H(2), and evidence for reversible formation of a σ-methane complex, 5, was obtained through deuterium scrambling from η(2)-D(2) in 2-d(2) into the methyl group of 2 prior to methane loss. This sequence of reactions was modeled by density functional theory calculations. The transition state for formation of 5 from 2 showed significant shortening of the Ir-H bond for the hydrogen being transferred; no true Ir(V) trihydride intermediate could be located. Barriers to methane loss from 2 were compared to those of 1 and the six-coordinate species (PONOP)Ir(H)(Me)(CO)(+) and (PONOP)Ir(H)(Me)(Cl).

C-H functionalization reactivity of a nickel-imide.

We report bifunctional reactivity of the ß-diketiminato Ni(III)-imide [Me(3)NN]Ni═NAd (1), which undergoes H-atom abstraction (HAA) reactions with benzylic substrates R-H (indane, ethylbenzene, toluene). Nickel-imide 1 competes with the nickel-amide HAA product [Me(3)NN]Ni-NHAd (2) for the resulting hydrocarbyl radical R(•) to give the nickel-amide [Me(3)NN]Ni-N(CHMePh)Ad (3) (R-H = ethylbenzene) or aminoalkyl tautomer [Me(3)NN]Ni(η(2)-CH(Ph)NHAd) (4) (R-H = toluene). A significant amount of functionalized amine R-NHAd is observed in the reaction of 1 with indane along with the dinickel imide {[Me(3)NN]Ni}(2)(µ-NAd) (5). Kinetic and DFT analyses point to rate-limiting HAA from R-H by 1 to give R(•), which may add to either imide 1 or amide 2, each featuring significant N-based radical character. Thus, these studies illustrate a fundamental competition possible in C-H amination systems that proceed via a HAA/radical rebound mechanism.

Pronounced effects of substituents on the iridium-catalyzed borylation of aryl C-H bonds.

Iridium trisboryl complexes containing bisphosphine and bipyridine ligands and pinacolate and catecholate substituents on boron are reported. A large difference in reactivity towards the borylation of C-H bonds is observed for this series of trisboryl complexes, and this difference is attributed to the electron-donating properties of the pinacolate vs. catecholate groups, and the steric and electronic properties of bipyridine vs. bisphosphine ligands.