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1. DFT Analysis of Methane C−H Activation and Over‐Oxidation by [Cu 2 O] 2+ and [Cu 2 O 2 ] 2+ Sites in Zeolite Mordenite: Intra‐ versus Inter‐site Over‐Oxidation

   https://doi.org/10.1002/cphc.202100580  

   Panthi, Dipak ; Adeyiga, Olajumoke ; Odoh, Samuel O. ( October 2021 , ChemPhysChem)

   Methane over‐oxidation by copper‐exchanged zeolites prevents realization of high‐yield catalytic conversion. However, there has been little description of the mechanism for methane over‐oxidation at the copper active sites of these zeolites. Using density functional theory (DFT) computations, we reported that tricopper [Cu₃O₃]²⁺active sites can over‐oxidize methane. However, the role of [Cu₃O₃]²⁺sites in methane‐to‐methanol conversion remains under debate. Here, we examine methane over‐oxidation by dicopper [Cu₂O]²⁺and [Cu₂O₂]²⁺sites using DFT in zeolite mordenite (MOR). For [Cu₂O₂]²⁺, we considered the μ‐(η²:η²) peroxo‐, and bis(μ‐oxo) motifs. These sites were considered in the eight‐membered (8MR) ring of MOR. μ‐(η²:η²) peroxo sites are unstable relative to the bis(μ‐oxo) motif with a small interconversion barrier. Unlike [Cu₂O]²⁺which is active for methane C−H activation, [Cu₂O₂]²⁺has a very large methane C−H activation barrier in the 8MR. Stabilization of methanol and methyl at unreacted dicopper sites however leads to over‐oxidation via sequential hydrogen atom abstraction steps. For methanol, these are initiated by abstraction of the CH₃group, followed by OH and can proceed near 200 °C. Thus, for [Cu₂O]²⁺and [Cu₂O₂]²⁺species, over‐oxidation is an inter‐site process. We discuss the implications of these findings for methanol selectivity, especially in comparison to the intra‐site process for [Cu₃O₃]²⁺sites and the role of Brønsted acid sites.

    

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2. Performance of density functional theory for describing hetero‐metallic active‐site motifs for methane‐to‐methanol conversion in metal‐exchanged zeolites

   https://doi.org/10.1002/jcc.25714  

   Dandu, Naveen K. ; Adeyiga, Olajumoke ; Panthi, Dipak ; Bird, Shaina A. ; Odoh, Samuel O. ( October 2018 , Journal of Computational Chemistry)

   Methane‐to‐methanol conversion (MMC) can be facilitated with high methanol selectivities by copper‐exchanged zeolites. There are however two open questions regarding the use of these zeolites to facilitate the MMC process. The first concerns the possibility of operating the three cycles in the stepwise MMC process by these zeolites in an isothermal fashion. The second concerns the possibility of improving the methanol yields by systematic substitution of some copper centers in these active sites with other earth‐abundant transition metals. Quantum‐mechanical computations can be used to compare methane activation by copper oxide species and analogous mixed‐metal systems. To carry out such screening, it is important that we use theoretical methods that are accurate and computationally affordable for describing the properties of the hetero‐metallic catalytic species. We have examined the performance of 47 exchange‐correlation density functionals for predicting the relative spin‐state energies and chemical reactivities of six hetero‐metallic [M‐O‐Cu]²⁺and [M‐O₂‐Cu]²⁺, (where MCo, Fe, and Ni), species by comparison with coupled cluster theory including iterative single, double excitations as well as perturbative treatment of triple excitations, CCSD(T). We also performed multireference calculations on some of these systems. We considered two types of reactions (hydrogen addition and oxygen addition) that are relevant to MMC. We recommend the use of τ‐HCTH and OLYP to determine the spin‐state energy splittings in the hetero‐metallic motifs. ωB97, ωB97X, ωB97X‐D3, and MN15 performed best for predicting the energies of the hydrogen and oxygen addition reactions. In contrast, local, and semilocal functionals do poorly for chemical reactivity. Using [Fe‐O‐Cu]²⁺as a test, we see that the nonlocal functionals perform well for the methane CH activation barrier. In contrast, the semilocal functionals perform rather poorly. © 2018 Wiley Periodicals, Inc.

    

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3. Encapsulation of tricopper cluster in a synthetic cryptand enables facile redox processes from Cu I Cu I Cu I to Cu II Cu II Cu II states

   https://doi.org/10.1039/D0SC05441K  

   Zhang, Weiyao ; Moore, Curtis E. ; Zhang, Shiyu ( March 2021 , Chemical Science)

   null (Ed.)

   One-pot reaction of tris(2-aminoethyl)amine (TREN), [Cu I (MeCN) 4 ]PF 6 , and paraformaldehyde affords a mixed-valent [ TREN4 Cu II Cu I Cu I (μ 3 -OH)](PF 6 ) 3 complex. The macrocyclic azacryptand TREN4 contains four TREN motifs, three of which provide a bowl-shape binding pocket for the [Cu 3 (μ 3 -OH)] 3+ core. The fourth TREN caps on top of the tricopper cluster to form a cryptand, imposing conformational constraints and preventing solvent interaction. Contrasting the limited redox capability of synthetic tricopper complexes reported so far, [ TREN4 Cu II Cu I Cu I (μ 3 -OH)](PF 6 ) 3 exhibits several reversible single-electron redox events. The distinct electrochemical behaviors of [ TREN4 Cu II Cu I Cu I (μ 3 -OH)](PF 6 ) 3 and its solvent-exposed analog [ TREN3 Cu II Cu II Cu II (μ 3 -O)](PF 6 ) 4 suggest that isolation of tricopper core in a cryptand enables facile electron transfer, allowing potential application of synthetic tricopper complexes as redox catalysts. Indeed, the fully reduced [ TREN4 Cu I Cu I Cu I (μ 3 -OH)](PF 6 ) 2 can reduce O 2 under acidic conditions. The geometric constraints provided by the cryptand are reminiscent of Nature's multicopper oxidases (MCOs). For the first time, a synthetic tricopper cluster was isolated and fully characterized at Cu I Cu I Cu I ( 4a ), Cu II Cu I Cu I ( 4b ), and Cu II Cu II Cu I ( 4c ) states, providing structural and spectroscopic models for many intermediates in MCOs. Fast electron transfer rates (10 5 to 10 6 M −1 s −1 ) were observed for both Cu I Cu I Cu I /Cu II Cu I Cu I and Cu II Cu I Cu I /Cu II Cu II Cu I redox couples, approaching the rapid electron transfer rates of copper sites in MCO. 

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4. Methane Over‐Oxidation by Extra‐Framework Copper‐Oxo Active Sites of Copper‐Exchanged Zeolites: Crucial Role of Traps for the Separated Methyl Group

   https://doi.org/10.1002/cphc.202100103  

   Adeyiga, Olajumoke ; Odoh, Samuel O. ( April 2021 , ChemPhysChem)

   Copper‐exchanged zeolites are useful for stepwise conversion of methane to methanol at moderate temperatures. This process also generates some over‐oxidation products like CO and CO₂. However, mechanistic pathways for methane over‐oxidation by copper‐oxo active sites in these zeolites have not been previously described. Adequate understanding of methane over‐oxidation is useful for developing systems with higher methanol yields and selectivities. Here, we use density functional theory (DFT) to examine methane over‐oxidation by [Cu₃O₃]²⁺active sites in zeolite mordenite MOR. The methyl group formed after activation of a methane C−H bond can be stabilized at a μ‐oxo atom of the active site. This μ‐(O−CH₃) intermediate can undergo sequential hydrogen atom abstractions till eventual formation of a copper‐monocarbonyl species. Adsorbed formaldehyde, water and formates are also formed during this process. The overall mechanistic path is exothermic, and all intermediate steps are facile at 200 °C. Release of CO from the copper‐monocarbonyl costs only 3.4 kcal/mol. Thus, for high methanol selectivities, the methyl group from the first hydrogen atom abstraction stepmust bestabilizedawayfrom copper‐oxo active sites. Indeed, it must be quickly trapped at an unreactive site (short diffusion lengths) while avoiding copper‐oxo species (large paths between active sites). This stabilization of the methyl group away from the active sites is central to the high methanol selectivities obtained with stepwise methane‐to‐methanol conversion.

    

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5. Expansion of the (BB)Ru metallacycle with coinage metal cations: formation of B–M–Ru–B (M = Cu, Ag, Au) dimetalacyclodiboryls

   https://doi.org/10.1039/C8SC00190A  

   Eleazer, Bennett J. ; Smith, Mark D. ; Popov, Alexey A. ; Peryshkov, Dmitry V. ( January 2018 , Chemical Science)

   In this work, we introduce a novel approach for the selective assembly of heterometallic complexes by unprecedented coordination of coinage metal cations to strained single ruthenium–boron bonds on a surface of icosahedral boron clusters. M( i ) cations (M = Cu, Ag, and Au) insert into B–Ru bonds of the (BB)–carboryne complex of ruthenium with the formation of four-membered B–M–Ru–B metalacycles. Results of theoretical calculations suggest that bonding within these metalacycles can be best described as unusual three-center-two-electron B–M⋯Ru interactions that are isolobal to B–H⋯Ru borane coordination for M = Cu and Ag, or the pairs of two-center-two electron B–Au and Au–Ru interactions for M = Au. These transformations comprise the first synthetic route to exohedral coinage metal boryl complexes of icosahedral closo -{C 2 B 10 } clusters, which feature short Cu–B (2.029(2) Å) and Ag–B (2.182(3) Å) bonds and the shortest Au–B bond (2.027(2) Å) reported to date. The reported heterometallic complexes contain Cu( i ) and Au( i ) centers in uncharacteristic square-planar coordination environments. These findings pave the way to rational construction of a broader class of multimetallic architectures featuring M–B bonds. 

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