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The controlled oxidation of methane to methanol has been an area of intense research over the past decades. Despite the efforts, the identification of an efficient catalyst with high selectivity is still elusive. Here we propose a thoroughly different strategy employing catalysts containing a metal methoxy unit. This family of catalysts has been used for the activation of C–H bonds but this is the first systematic investigation for the conversion of methane to methanol highlighting the advantages over the typically used metal oxides. Specifically, we start our investigation with an Fe(III) center coordinated by four ammonia ligands, (NH3)4FeOCH32+. Structures and energetics are reported for two mechanisms ([2+2] and proton coupled electrons transfer) and for different spin multiplicities via density functional theory, multi-reference, and coupled cluster quantum chemical calculations. The excited low-spin doublet state of this model system exhibits the best performance in terms of activation barriers and selectivity. Therefore, we then switched to the corresponding Ru(III) complex, which has a doublet ground state and manifests better performance than the doublet state of Fe(III). For both systems the activation barrier for methanol is larger than that of methane due to the interaction of the OH group of methanol with the coordinated NH3 ligands (hydrogen bonding) and/or the metal center. This observation suggests that the activation of methanol is slower, hindering its oxidation. In addition, we show that the metal–methoxy family of catalysts offers a potential mechanism that can prevent the oxidation of an activated methanol molecule (self-correcting chemical loop). This work aspires to induce experimental interest and pave the road for the development of high-performance high-selectivity methane to methanol direct conversion routes under mild conditions. [ABSTRACT FROM AUTHOR]

The dissimilatory sulfite reductase enzyme has very characteristic active site where the substrate binds to an iron site, ligated by a siroheme macrocycle and a thiol directly connected to a [Fe4S4] cluster. This arrangement gives the enzyme remarkable efficiency in reducing sulfite and nitrite all the way to hydrogen sulfide and ammonia. For the first time we present a theoretical study where substrate binding modalities and activation are elucidated using active site models containing proton supply side chains and the [Fe4S4] cluster. Density functional theory (DFT) was deployed in conjunction with the energy decomposition scheme (as implemented in AMS), the quantum theory of atoms in molecules (QTAIM), and conceptual DFT (cDFT) descriptors. We quantified the role of the electrostatic interactions inside the active site created by the side chains as well as the influence of the [Fe4S4] cluster on the substrate binding. Furthermore, using conceptual DFT results we shed light of the activation process, thus, laying foundation for further mechanistic studies. We found that the bonding of the ligands to the iron complex is dominated by electrostatic interactions, but the presence of the [Fe4S4] cubane leads to substantial changes in electronic interaction. The spin state of the cubane, however, affects the binding energy only marginally. The conceptual DFT results show that the presence of the [Fe4S4] cubane affects the reactivity of the active site as it is involved in electron transfer. This is corroborated by an increase in the electrophilicity index, thus making the active site more prone to react with the ligands. The interaction energies between the ligand and the siroheme group are also increased upon the presence of the cubane group, thus, suggesting that the siroheme group is not an innocent spectator but plays an active role in the reactivity of the dSIR active site. [ABSTRACT FROM AUTHOR]

The dissimilatory sulfite reductase enzyme has very characteristic active site where the substrate binds to an iron site, ligated by a siroheme macrocycle and a thiol directly connected to a [Fe 4 S 4 ] cluster. This arrangement gives the enzyme remarkable efficiency in reducing sulfite and nitrite all the way to hydrogen sulfide and ammonia. For the first time we present a theoretical study where substrate binding modalities and activation are elucidated using active site models containing proton supply side chains and the [Fe 4 S 4 ] cluster. Density functional theory (DFT) was deployed in conjunction with the energy decomposition scheme (as implemented in AMS), the quantum theory of atoms in molecules (QTAIM), and conceptual DFT (cDFT) descriptors. We quantified the role of the electrostatic interactions inside the active site created by the side chains as well as the influence of the [Fe 4 S 4 ] cluster on the substrate binding. Furthermore, using conceptual DFT results we shed light of the activation process, thus, laying foundation for further mechanistic studies. We found that the bonding of the ligands to the iron complex is dominated by electrostatic interactions, but the presence of the [Fe 4 S 4 ] cubane leads to substantial changes in electronic interaction. The spin state of the cubane, however, affects the binding energy only marginally. The conceptual DFT results show that the presence of the [Fe 4 S 4 ] cubane affects the reactivity of the active site as it is involved in electron transfer. This is corroborated by an increase in the electrophilicity index, thus making the active site more prone to react with the ligands. The interaction energies between the ligand and the siroheme group are also increased upon the presence of the cubane group, thus, suggesting that the siroheme group is not an innocent spectator but plays an active role in the reactivity of the dSIR active site.

High-level electronic structure calculations are initially performed to investigate the electronic structure of RhO 2+ . The construction of potential energy curves for the ground and low-lying excited states allowed the calculation of spectroscopic constants, including harmonic and anharmonic vibrational frequencies, bond lengths, spin-orbit constants, and excitation energies. The equilibrium electronic configurations were used for the interpretation of the chemical bonding. We further monitored how the Rh-O bonding scheme changes with the gradual addition of ammonia ligands. The nature of this bond remains unaffected up to four ammonia ligands but adopts a different electronic configuration in the pseudo-octahedral geometry of (NH 3 ) 5 RhO 2+ . This has consequences in the activation mechanism of the C-H bond of methane by these complexes, especially (NH 3 ) 4 RhO 2+ . We show that the [2 + 2] mechanism in the (NH 3 ) 4 RhO 2+ case has a very low energy barrier comparable to that of a radical mechanism. We also demonstrate that methane can coordinate to the metal in a similar fashion to ammonia and that knowledge of the electronic structure of the pure ammonia complexes provides qualitative insights into the optimal reaction mechanism.