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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]

Recent advances in our comprehension of the electronic structure of metal ammonia complexes have opened avenues for novel materials with diffuse electrons. These complexes in their ground state can host peripheral "Rydberg" electrons which populate a hydrogenic-type shell model imitating atoms. Aggregates of such complexes form the so-called expanded or liquid metals. Expanded metals composed of d- and f-block metal ammonia complexes offer properties, such as magnetic moments and larger numbers of diffuse electrons, not present for alkali and alkaline earth (s-block) metals. In addition, tethering metal ammonia complexes via hydrocarbon chains (replacement of ammonia ligands with diamines) yields materials that can be used for redox catalysis and quantum computing, sensing, and optics. This perspective summarizes the recent findings for gas-phase isolated metal ammonia complexes and projects the obtained knowledge to the condensed phase regime. Possible applications for the newly introduced expanded metals and linked solvated electrons precursors are discussed and future directions are proposed. [ABSTRACT FROM AUTHOR]

Transition metal oxides have been extensively used in the literature for the conversion of methane to methanol. Despite the progress made over the past decades, no method with satisfactory performance or economic viability has been detected. The main bottleneck is that the produced methanol oxidizes further due to its weaker C–H bond than that of methane. Every improvement in the efficiency of a catalyst to activate methane leads to reduction of the selectivity towards methanol. Is it therefore prudent to keep studying (both theoretically and experimentally) metal oxides as catalysts for the quantitative conversion of methane to methanol? This perspective focuses on molecular metal oxide complexes and suggests strategies to bypass the current bottlenecks with higher weight on the computational chemistry side. We first discuss the electronic structure of metal oxides, followed by assessing the role of the ligands in the reactivity of the catalysts. For better selectivity, we propose that metal oxide anionic complexes should be explored further, while hydrophylic cavities in the vicinity of the metal oxide can perturb the transition-state structure for methanol increasing appreciably the activation barrier for methanol. We also emphasize that computational studies should target the activation reaction of methanol (and not only methane), the study of complete catalytic cycles (including the recombination and oxidation steps), and the use of molecular oxygen as an oxidant. The titled chemical conversion is an excellent challenge for theory and we believe that computational studies should lead the field in the future. It is finally shown that bottom-up approaches offer a systematic way for exploration of the chemical space and should still be applied in parallel with the recently popular machine learning techniques. To answer the question of the title, we believe that metal oxides should still be considered provided that we change our focus and perform more systematic investigations on the activation of methanol. [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]

Graphical abstract Highlights • The sketch of the French sculptor Auguste Rodin (1840–1917) Le Penseur (The Thinker) reflects directly to René Descartes famous saying "cogito ergo sum" (I Think therefore I exist) Abstract Bonding in chemistry refers to the stability of molecules, therefore it is of paramount importance to natural sciences and certainly to all aspects of life. Yet bonding is a chameleon–like fuzzy concept, leaving enough space to the sprouting of a variety of "theories" supposedly explaining the structure and bonding of molecular systems. Admittedly some of these "theoretical ideas" played a constructive and useful role to the development of chemistry in the last one hundred years, however they are no longer needed. Intertwined with binding the hypervalency definition introduced half a century ago, started to be used not only for communication purposes as intended, but as an "explanation" of the structure and bonding. In the present communication and within the spirit of nonproliferation of redundant concepts, we have tried to prove that bonding between atoms can be understood and clarified based solely on the tenets of quantum mechanics as applied to molecules. More specifically the indisputable role of the excited state of atoms in the bonding process is emphatically projected. Clearly chemistry is an excited state rather than a ground state field. [ABSTRACT FROM AUTHOR]

High-valent Fe(iv)-oxo species have been found to be key oxidizing intermediates in the mechanisms of mononuclear iron heme and non-heme enzymes that can functionalize strong C–H bonds. Biomimetic Fe(iv)-oxo molecular complexes have been successfully synthesized and characterized, but their catalytic reactivity is typically lower than that of the enzymatic analogues. The C–H activation step proceeds through two competitive mechanisms, named σ- and π-channels. We have performed high-level wave function theory calculations on bare FeO2+ and a series of non-heme Fe(iv)-oxo model complexes in order to elucidate the electronic properties and the ligand field effects on those channels. Our results suggest that a coordination environment formed by a weak field gives access to both competitive channels, yielding more reactive Fe(iv)-oxo sites. In contrast, a strong ligand environment stabilizes only the σ-channel. Our concluding remarks will aid the derivation of new structure–reactivity descriptors that can contribute to the development of the next generation of functional catalysts. [ABSTRACT FROM AUTHOR]

The role of the first excited state of oxygen (1D) is proven essential for the description of terminal iodine–oxygen chemical bonds. The description of the I–O bond as a dative one from iodine to O(1D) provides a simple and accurate picture which explains the oxidation properties of iodosobenzene and similar in nature molecules. [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.

High-level quantum chemical approaches are performed to study the stability and electronic structure of tri-, di-, monocationic, and neutral scandium ammonia complexes. The calculated binding energies of all Sc(NH 3 ) 1-8 3+,2+,+,0 complexes reveal the higher stability of hexa- and octacoordinated systems. The ground states of Sc(NH 3 ) 6,8 2+ and Sc(NH 3 ) 6,8 have a Sc 2+ (3d 1 ) center, while there are two competitive electronic states for Sc(NH 3 ) 6,8 + with a Sc 2+ (3d 1 ) or a Sc 3+ center. The remaining electrons occupy an outer diffuse s-type orbital (1s). The lower lying states involve 3d-3d transitions for Sc(NH 3 ) 6,8 2+ reduces the binding energies but shortens the Sc-N bond lengths. The comparison with the vanadium and yttrium ammonia complexes (studied earlier) reveal the unique identity of scandium balancing between a d- and s-block element.3 ) 6,8 +,0 . The addition of one electron to Sc(NH 3 ) 6,8 3+,2+,+ reduces the binding energies but shortens the Sc-N bond lengths. The comparison with the vanadium and yttrium ammonia complexes (studied earlier) reveal the unique identity of scandium balancing between a d- and s-block element.

The conversion of methane to methanol (MTM) catalyzed by FeOCH 3 + and FeCH 3 + is investigated by means of multireference configuration interaction (MRCI), single-reference coupled clusters (CC), and density functional theory (DFT) approaches. Our dual purpose is the assessment of the applied methodologies and the performance of the proposed catalytic cycle, which involves both of the titled units. The investigated cycle aims to bypass the limitations of metal-oxide catalysts and offers an alternative promising method for efficient MTM transformation. From the technical viewpoint, we found that generally accurate electron correlation treatment is more important than accurately calculated geometries. The combination of optimal DFT geometries with MRCI and CC energetics provides a good compromise between accuracy and efficiency, although there are cases where multireference calculations must be used to obtain correct structures.

High-valent Fe(iv)-oxo species have been found to be key oxidizing intermediates in the mechanisms of mononuclear iron heme and non-heme enzymes that can functionalize strong C-H bonds. Biomimetic Fe(iv)-oxo molecular complexes have been successfully synthesized and characterized, but their catalytic reactivity is typically lower than that of the enzymatic analogues. The C-H activation step proceeds through two competitive mechanisms, named σ- and π-channels. We have performed high-level wave function theory calculations on bare FeO2+ and a series of non-heme Fe(iv)-oxo model complexes in order to elucidate the electronic properties and the ligand field effects on those channels. Our results suggest that a coordination environment formed by a weak field gives access to both competitive channels, yielding more reactive Fe(iv)-oxo sites. In contrast, a strong ligand environment stabilizes only the σ-channel. Our concluding remarks will aid the derivation of new structure-reactivity descriptors that can contribute to the development of the next generation of functional catalysts.

Tetra-amino beryllium complexes and ions, Be(NH 3 ) 4 0,± , have a tetrahedral Be(NH 3 ) 4 2+ core with one, two, or three outer electrons orbiting its periphery. Our calculations reveal a new class of molecular entities, solvated electron precursors, with Aufbau rules (1s, 1p, 1d, 2s, 1f, 2p, 2d) that differ from their familiar hydrogenic counterparts and resemble those of jellium or nuclear-shell models. The core's radial electrostatic potential suffices to reproduce the chief features of the ab initio results. Wave function and electron-propagator methods combined with diffuse basis sets are employed to calculate accurate geometries, ionization energies, electron affinities, and excitation energies.

The role of the first excited state of oxygen ( 1 D) is proven essential for the description of terminal iodine-oxygen chemical bonds. The description of the I-O bond as a dative one from iodine to O( 1 D) provides a simple and accurate picture which explains the oxidation properties of iodosobenzene and similar in nature molecules.