Electronic Structure of the [Cu₃(μ-O)₃]²⁺ Cluster in Mordenite Zeolite and Its Effects on the Methane to Methanol Oxidation

Identifying Cu-exchanged zeolites able to activate C–H bonds and selectively convert methane to methanol is a challenge in the field of biomimetic heterogeneous catalysis. Recent experiments point to the importance of trinuclear [Cu₃(μ-O)₃]²⁺ complexes inside the micropores of mordenite (MOR) zeolite for selective oxo-functionalization of methane. The electronic structures of these species, namely, the oxidation state of Cu ions and the reactive character of the oxygen centers, are not yet fully understood. In this study, we performed a detailed analysis of the electronic structure of the [Cu₃(μ-O)₃]²⁺ site using multiconfigurational wave-function-based methods and density functional theory. The calculations reveal that all Cu sites in the cluster are predominantly present in the Cu­(II) formal oxidation state with a minor contribution from Cu­(III), whereas two out of three oxygen anions possess a radical character. These electronic properties, along with the high accessibility of the out-of-plane oxygen center, make this oxygen the preferred site for the homolytic C–H activation of methane by [Cu₃(μ-O)₃]²⁺. These new insights aid in the construction of a theoretical framework for the design of novel catalysts for oxyfunctionalization of natural gas and suggest further spectroscopic examination

<i>Ab Initio</i> Study of the Adsorption of Small Molecules on Metal–Organic Frameworks with Oxo-centered Trimetallic Building Units: The Role of the Undercoordinated Metal Ion

The interactions of H₂, CO, CO₂, and H₂O with the undercoordinated metal centers of the trimetallic oxo-centered M₃^III(μ₃-O)­(X) (COO)₆ moiety are studied by means of wave function and density functional theory. This trimetallic oxo-centered cluster is a common building unit in several metal–organic frameworks (MOFs) such as MIL-100, MIL-101, and MIL-127 (also referred to as soc-MOF). A combinatorial computational screening is performed for a large variety of trimetallic oxo-centered units M₃^IIIO (M = Al³⁺, Sc³⁺, V³⁺, Cr³⁺, Fe³⁺, Ga³⁺, Rh³⁺, In³⁺, Ir³⁺) interacting with H₂O, H₂, CO, and CO₂. The screening addresses interaction energies, adsorption enthalpies, and vibrational properties. The results show that the Rh and Ir analogues are very promising materials for gas storage and separations

<i>Ab Initio</i> Study of the Adsorption of Small Molecules on Metal–Organic Frameworks with Oxo-centered Trimetallic Building Units: The Role of the Undercoordinated Metal Ion

The interactions of H₂, CO, CO₂, and H₂O with the undercoordinated metal centers of the trimetallic oxo-centered M₃^III(μ₃-O)­(X) (COO)₆ moiety are studied by means of wave function and density functional theory. This trimetallic oxo-centered cluster is a common building unit in several metal–organic frameworks (MOFs) such as MIL-100, MIL-101, and MIL-127 (also referred to as soc-MOF). A combinatorial computational screening is performed for a large variety of trimetallic oxo-centered units M₃^IIIO (M = Al³⁺, Sc³⁺, V³⁺, Cr³⁺, Fe³⁺, Ga³⁺, Rh³⁺, In³⁺, Ir³⁺) interacting with H₂O, H₂, CO, and CO₂. The screening addresses interaction energies, adsorption enthalpies, and vibrational properties. The results show that the Rh and Ir analogues are very promising materials for gas storage and separations

Catechol-Ligated Transition Metals: A Quantum Chemical Study on a Promising System for Gas Separation

Metal–organic frameworks (MOFs) have received a great deal of attention for their potential in atmospheric filtering, and recent work has shown that catecholate linkers can bind metals, creating MOFs with monocatecholate metal centers and abundant open coordination sites. In this study, M–catecholate systems (with M = Mg²⁺, Sc²⁺, Ti²⁺, V²⁺, Cr²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺) were used as computational models of metalated catecholate linkers in MOFs. Nitric oxide (NO) is a radical molecule that is considered an environmental pollutant and is toxic if inhaled in large quantities. Binding NO is of interest in creating atmospheric filters, at both the industrial and personal scale. The binding energies of NO to the metal–catecholate systems were calculated using density functional theory (DFT) and complete active space self-consistent field (CASSCF) followed by second-order perturbation theory (CASPT2). Selectivity was studied by calculating the binding energies of additional guests (CO, NH₃, H₂O, N₂, and CO₂). The toxic guests have stronger binding than the benign guests for all metals studied, and NO has significantly stronger binding than other guests for most of the metals studied, suggesting that metal–catecholates are worthy of further study for NO filtration. Certain metal–catecholates also show potential for separation of N₂ and CO₂ via N₂ activation, which could be relevant for carbon capture or ammonia synthesis

Selective, Tunable O₂ Binding in Cobalt(II)–Triazolate/Pyrazolate Metal–Organic Frameworks

The air-free reaction of CoCl₂ with 1,3,5-tri­(1<i>H</i>-1,2,3-triazol-5-yl)­benzene (H₃BTTri) in <i>N</i>,<i>N</i>-dimethylformamide (DMF) and methanol leads to the formation of Co-BTTri (Co₃[(Co₄Cl)₃(BTTri)₈]₂·DMF), a sodalite-type metal–organic framework. Desolvation of this material generates coordinatively unsaturated low-spin cobalt­(II) centers that exhibit a strong preference for binding O₂ over N₂, with isosteric heats of adsorption (<i>Q</i>_st) of −34(1) and −12(1) kJ/mol, respectively. The low-spin (<i>S</i> = 1/2) electronic configuration of the metal centers in the desolvated framework is supported by structural, magnetic susceptibility, and computational studies. A single-crystal X-ray structure determination reveals that O₂ binds end-on to each framework cobalt center in a 1:1 ratio with a Co–O₂ bond distance of 1.973(6) Å. Replacement of one of the triazolate linkers with a more electron-donating pyrazolate group leads to the isostructural framework Co-BDTriP (Co₃[(Co₄Cl)₃(BDTriP)₈]₂·DMF; H₃BDTriP = 5,5′-(5-(1<i>H</i>-pyrazol-4-yl)-1,3-phenylene)­bis­(1<i>H</i>-1,2,3-triazole)), which demonstrates markedly higher yet still fully reversible O₂ affinities (<i>Q</i>_st = −47(1) kJ/mol at low loadings). Electronic structure calculations suggest that the O₂ adducts in Co-BTTri are best described as cobalt­(II)–dioxygen species with partial electron transfer, while the stronger binding sites in Co-BDTriP form cobalt­(III)–superoxo moieties. The stability, selectivity, and high O₂ adsorption capacity of these materials render them promising new adsorbents for air separation processes

Catalytic Silylation of Dinitrogen with a Dicobalt Complex

A dicobalt complex catalyzes N₂ silylation with Me₃SiCl and KC₈ under 1 atm N₂ at ambient temperature. Tris­(trimethylsilyl)­amine is formed with an initial turnover rate of 1 N­(TMS)₃/min, ultimately reaching a turnover number of ∼200. The dicobalt species features a metal–metal interaction, which we postulate is important to its function. Although N₂ functionalization occurs at a single cobalt site, the second cobalt center modifies the electronics at the active site. Density functional calculations reveal that the Co–Co interaction evolves during the catalytic cycle: weakening upon N₂ binding, breaking with silylation of the metal-bound N₂ and reforming with expulsion of [N₂(SiMe₃)₃]⁻

Mechanism of Oxidation of Ethane to Ethanol at Iron(IV)–Oxo Sites in Magnesium-Diluted Fe₂(dobdc)

The catalytic properties of the metal–organic framework Fe₂(dobdc), containing open Fe­(II) sites, include hydroxylation of phenol by pure Fe₂(dobdc) and hydroxylation of ethane by its magnesium-diluted analogue, Fe_0.1Mg_1.9(dobdc). In earlier work, the latter reaction was proposed to occur through a redox mechanism involving the generation of an iron­(IV)–oxo species, which is an intermediate that is also observed or postulated (depending on the case) in some heme and nonheme enzymes and their model complexes. In the present work, we present a detailed mechanism by which the catalytic material, Fe_0.1Mg_1.9(dobdc), activates the strong C–H bonds of ethane. Kohn–Sham density functional and multireference wave function calculations have been performed to characterize the electronic structure of key species. We show that the catalytic nonheme-Fe hydroxylation of the strong C–H bond of ethane proceeds by a quintet single-state σ-attack pathway after the formation of highly reactive iron–oxo intermediate. The mechanistic pathway involves three key transition states, with the highest activation barrier for the transfer of oxygen from N₂O to the Fe­(II) center. The uncatalyzed reaction, where nitrous oxide directly oxidizes ethane to ethanol is found to have an activation barrier of 280 kJ/mol, in contrast to 82 kJ/mol for the slowest step in the iron­(IV)–oxo catalytic mechanism. The energetics of the C–H bond activation steps of ethane and methane are also compared. Dehydrogenation and dissociation pathways that can compete with the formation of ethanol were shown to involve higher barriers than the hydroxylation pathway

Catalytic Silylation of Dinitrogen with a Dicobalt Complex

A dicobalt complex catalyzes N₂ silylation with Me₃SiCl and KC₈ under 1 atm N₂ at ambient temperature. Tris­(trimethylsilyl)­amine is formed with an initial turnover rate of 1 N­(TMS)₃/min, ultimately reaching a turnover number of ∼200. The dicobalt species features a metal–metal interaction, which we postulate is important to its function. Although N₂ functionalization occurs at a single cobalt site, the second cobalt center modifies the electronics at the active site. Density functional calculations reveal that the Co–Co interaction evolves during the catalytic cycle: weakening upon N₂ binding, breaking with silylation of the metal-bound N₂ and reforming with expulsion of [N₂(SiMe₃)₃]⁻

Gas-Phase Ion Chemistry of Metalloporphyrin Anions with Molecular Oxygen: Probing the Influence of the Oxidation and Spin State of the Central Transition Metal by Experiment and Theory

We performed a comprehensive gas-phase experimental and quantum-chemical study of the binding properties of molecular oxygen to iron and manganese porphyrin anions. Temperature-dependent ion–molecule reaction kinetics as probed in a Fourier-transform ion-cyclotron resonance mass spectrometer reveal that molecular oxygen is bound by, respectively, 40.8 ± 1.4 and 67.4 ± 2.2 kJ mol^–1 to the Fe^II or Mn^II centers of isolated tetra­(4-sulfonatophenyl)­metalloporphyrin tetraanions. In contrast, Fe^III and Mn^III trianion homologues were found to be much less reactiveindicating an upper bound to their dioxygen binding energies of 34 kJ mol^–1. We modeled the corresponding O₂ adsorbates at the density functional theory and CASPT2 levels. These quantum-chemical calculations verified the stronger O₂ binding on the Fe^II or Mn^II centers and suggested that O₂ binds as a superoxide anion

Pushing Single-Oxygen-Atom-Bridged Bimetallic Systems to the Right: A Cryptand-Encapsulated Co–O–Co Unit

A dicobalt­(II) complex, [Co₂­(<i>m</i>BDCA-5t)]^2– (<b>1</b>), demonstrates a cofacial arrangement of trigonal mono­pyramidal Co­(II) ions with an inter-metal separation of 6.2710(6) Å. Reaction of <b>1</b> with potassium superoxide generates an encapsulated Co–O–Co core in the dianionic complex, [Co₂O­(<i>m</i>BDCA-5t)]^2– (<b>2</b>); to form the linear Co–O–Co core, the inter-metal distance has diminished to 3.994(3) Å. Co K-edge X-ray absorption spectroscopy data are consistent with a +2 oxidation state assignment for Co in both <b>1</b> and <b>2</b>. Multi­reference complete active space calculations followed by second-order perturbation theory support this assignment, with hole equivalents residing on the bridging O-atom and on the cryptand ligand for the case of <b>2</b>. Complex <b>2</b> acts as a 2-e^– oxidant toward substrates including CO and H₂, in both cases efficiently regenerating <b>1</b> in what represent net oxygen-atom-transfer reactions. This dicobalt system also functions as a catalase upon treatment with H₂O₂