Microsolvation of cobalt, nickel, and copper atoms with ammonia: a theoretical study of the solvated electron precursors

CONTEXT

The s-block metals dissolved in ammonia form metal-ammonia complexes with diffuse electrons which could be used for redox catalysis. In this theoretical paper, we investigated the possibility of the d-bloc transition metals (Mn, Fe, Co, Ni, and Cu) solvated by ammonia. It has been demonstrated that both Mn and Fe atoms undergo into an oxidative reaction with NH3 forming an inserted species, HMNH2. On the contrary, the Co, Ni, and Cu atoms can accommodate four NH3, via the coordination bond, to form the first solvation sphere within C2v, D2d, and Td point groups, respectively. Addition of a fifth NH3 constitute the second solvation shell by forming hydrogen bond with the other NH3s. Interestingly, M(NH3)4 (M = Co, Ni, and Cu) is a so-called solvated electron precursor and should be considered as a monocation M(NH3)4+ kernel in tight contact with one electron distributed over its periphery. This nearly free electron could be used to capture a CO2 molecule and engages in a reduction reaction.

METHODS

Geometry optimization of the stationary points on the potential energy surface was performed using density functional theory - CAM-B3LYP functional including the GD3BJ dispersion contribution - in combination with the 6-311 +  + G(2d, 2p) basis set for all the atoms. All first-principles calculations were performed using the Gaussian 09 quantum chemical packages. The natural electron configuration of transition atom engaged in the compounds has been found using the natural bond orbital (NBO) method. We used the EDR (electron delocalization range) approach to analyze the structure of solvated electrons in real space. We also used the electron localization function (ELF) to measure the degree of electronic localization within a chemical compound. The EDR and ELF analyses are done using the TopMod and Multiwfn packages, respectively.

Electronic structure analysis and DFT benchmarking of Rydberg-type alkali-metal-crown ether, -cryptand, and -adamanzane complexes

Density functional theory (DFT) and electron propagator theory (EPT) calculations were performed to study ground and excited electronic structures of alkali-metal (M) coordinated 9-crown-3, 24-crown-8, [2.1.1]cryptand, o-Me2-1.1.1, and 36Adamanzane complexes. Each complex bears an expanded electron in the periphery and occupies diffuse 1p-, 1d-, 1f-type molecular orbitals (or superatomic 1P, 1D, 1F orbitals) in excited electronic states. The calculated superatomic shell model of the M(9-crown-3)2 is 1S, 1P, 1D, 1F, 2S, 2P, 2D, 1G and it is held by all other complexes up to the studied 1F level. Due to the highly diffuse nature of the electron, the ionization energies of these complexes are significantly lower (1.6-2.0 eV) and hence these complexes belong to the superalkali category. The ab initio EPT ionization energy and the excitation energies of the Li(9-crown-3)2 were used to evaluate DFT errors associated with a series of exchange correlation functionals that span multiple rungs of Jacob's ladder (i.e., GGA, meta-GGA, global GGA hybrid, meta-GGA hybrid, range-separated hybrid, double-hybrid). Among these, the best performing functional is the range-separated hybrid CAM-B3LYP and the errors are within 6% of high-level ab initio EPT results. The accuracy of CAM-B3LYP is indeed transferable to similar complexes and hence the findings are expected to accelerate the progression of studies of Rydberg-type systems.

Ground and excited electronic structures of electride and alkalide units: The cases of Metal-Tren, -Azacryptand, and -TriPip222 complexes

A systematic electronic structure analysis was conducted for M(L)n molecular electrides and their corresponding alkalide units M(L)n @M' (M/M' = Na, K; L = Tren, Azacryptand, TriPip222; n = 1, 2). All complexes belong to the "superalkali" category due to their low ionization potentials. The saturated molecular electrides display M+ (L)n - form with a greatly diffuse quasispherical electron cloud. They were identified as "superatoms" considering the contours of populating atomic-type molecular orbitals. The observed superatomic Aufbau order of M(Tren)2 is 1S, 1P, 1D, 1F, 2S, 2P, and 1G and it is consistent with those of M(Azacryptand) and M(TriPip222) up to the analyzed 1F level. Their excitation energies decrease gradually moving from M(Tren)2 to M(Azacryptand) and to M(TriPip222). The studied alkalide complexes carry [M(L)n ]+ @M'- ionic structure and their dissociation energies vary in the sequence of K(L)n @Na > Na(L)n @Na > K(L)n @K > Na(L)n @K. Similar to molecular electrides, the anions of alkalide units occupy electrons in diffuse Rydberg-like orbitals. In this work, excited states of [M(L)n @M']0/+/- and their trends are also analyzed.

A two-electron reducing reaction of CO2 to an oxalate anion: a theoretical study of delocalized (presolvated) electrons in Al(CH3)n(NH3)m, n = 0-2 and m = 1-6, clusters

Presolvated electron possibility in three oxidation states of aluminum - Al(0), Al(I), and Al(II) - has been theoretically investigated for the Al + 6NH3, Al(CH3) + 5NH3, and Al(CH3)2 + 4NH3 reactions. It has been shown that the metal center adopts a tetrahedral shape for its most stable geometric structure, irrespective of the degree of Al oxidation states. Using different analysis techniques (highest occupied molecular orbital shapes, spin density distributions, and electron delocalization ranges), we showed that presolvated (delocalized) electrons are only formed in the Al(CH3)2(NH3)p coordination complexes when 2 ≤ p ≤ 4. It has also been evidenced that these delocalized electrons being powerful reducing agents allowed two CO2 molecules to be captured and form an oxalate ion in close contact with the [Al2(CH3)2(CH2)2(NH3)4]2+ dication core.

A fresh perspective on metal ammonia molecular complexes and expanded metals: opportunities in catalysis and quantum information

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.

Electron Presolvation in Tetrahydrofuran-Incorporated Supramolecular Sodium Entities

Alkali metal atoms can repopulate their valence electrons toward solvation due to impact from solvents or microsurroundings and provide the remaining alkali metal cations for coordinating with a variety of specific solvents, forming various electron-expanded complexes or solvated ionic pairs with special interactions. Such special solute-solvent interactions not only affect their electronic structures but also enable the formation of entirely new species. Taking Na(THF)_n (n = 1-6, THF = tetrahydrofuran) and Na₂@THF complexes as typical representatives, density functional theory calculations are carried out to explore the solvation of a sodium atom and its dimer in THF and characterize their complexes as solvent-incorporated supramolecular entities and particularly valence electron presolvation due to their interaction with solvent THF. Electron presolvation is caused by the Pauli repulsion between THF containing a coordinating O atom with a lone pair of electrons and the alkali metal Na or Na₂ containing valence electrons, and THF coordination to them forces their valence electrons to redistribute, which can be easily realized in such solvents. Compared with strongly bound valance electrons of alkali metal atoms, THF coordination enables Na or Na₂ electrons to exhibit much more active states (i.e., the presolvated states) featuring small vertical detachment energies of electrons and distorted diffuse distributions in the frames of the generally structured metal cation complexes, acting as the electron-expanded chemical entities. Furthermore, the degree of electron diffusion and the polarity of the Na-Na bond are proportional to the coordination number (n) and the coordination number difference (Δn) between two Na centers in Na₂@THF. The unique properties of such entities are also discussed. This work offers a theoretical support to the supramolecular entities formed by alkali-metal atoms or their dimers with ligands containing O or N and uncovers the unique electron presolvation phenomena and also enriches our understanding of the novel metal atom complexes.

Transition metal oxide complexes as molecular catalysts for selective methane to methanol transformation: any prospects or time to retire?

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.

The nature of supermolecular bonds: Investigating hydrocarbon linked beryllium solvated electron precursors

Beryllium ammonia complexes Be(NH₃)₄ are known to bear two diffuse electrons in the periphery of a Be(NH₃)₄ ²⁺ skeleton. The replacement of one ammonia with a methyl group forms CH₃Be(NH₃)₃ with one peripheral electron, which is shown to maintain the hydrogenic-type shell model observed for Li(NH₃)₄. Two CH₃Be(NH₃)₃ monomers are together linked by aliphatic chains to form strongly bound beryllium ammonia complexes, (NH₃)₃Be(CH₂)_nBe(NH₃)₃, n = 1-6, with one electron around each beryllium ammonia center. In the case of a linear carbon chain, this system can be seen as the analog of two hydrogen atoms approaching each other at specific distances (determined by n). We show that the two electrons occupy diffuse s-type orbitals and can couple exactly as in H₂ in either a triplet or singlet state. For long hydrocarbon chains, the singlet is an open-shell singlet nearly degenerate with the triplet spin state, which transforms to a closed-shell singlet for n = 1 imitating the σ-covalent bond of H₂. The biradical character of the system is analyzed, and the singlet-triplet splitting is estimated as a function of n based on multi-reference calculations. Finally, we consider the case of bent hydrocarbon chains, which allows the closer proximity of the two diffuse electrons for larger chains and the formation of a direct covalent bond between the two diffuse electrons, which happens for two Li(NH₃)₄ complexes converting the open-shell to closed-shell singlets. The energy cost for bending the hydrocarbon chain is nearly compensated by the formation of the weak covalent bond rendering bent and linear structures nearly isoenergetic.

Ground and Electronically Excited States of Main-Group-Metal-Doped B20 Double Rings

Ab initio coupled-cluster, electron propagator, and Møller-Plesset second-order perturbation theory calculations are utilized to analyze the low-lying electronic states of several metal-doped B₂₀. In the ground state, the presently focused AB₂₀/EB₂₀ (A = Li, Na, and K; E = Mg and Ca) consist of charge-separated A⁺B₂₀^-/E²⁺B₂₀^2- frameworks. The excited electronic states of AB₂₀ and EB₂₀⁺ were analyzed by computing the vertical electron attachment energies (VEAEs) of AB₂₀⁺ and EB₂₀²⁺. In several excited states, the radical electron is predominantly localized on the B₂₀ frames, which are counterparts of the low-lying states of bare B₂₀^-. A variety of basis sets were tested on obtaining VEAEs, and the aug-cc-pVDZ/_A,E d-aug-cc-pVDZ/_B combination provided the best accuracy-efficiency compromise on them. Furthermore, this work analyzes the Rydberg-like excited states of AB₂₀ and EB₂₀⁺ and will serve as a guide for future studies on similar metal-doped boron systems.

Superatomic Chelates: The Cases of Metal Aza-Crown Ethers and Cryptands

Li, Na, and Mg⁺-coordinated hexaaza-18-crown-6 ([18]aneN₆) and 1,4,7-triazacyclononane ([9]aneN₃), Li[1.1.1]cryptand, and Na[2.2.2]cryptand species possess a diffuse electron in a quasispherical s-type orbital. They populate expanded p-, d-, f-, and g-shape orbitals in low-lying excited states and hence are identified as "superatoms". By means of quantum calculations, their superatomic shell models are revealed. The observed orbital series of M([9]aneN₃)₂ and M[18]aneN₆ (M = Li, Na, Mg⁺) are identical to the 1s, 1p, 1d, 1f, 2s, and 2p. The electronic spectra of Li[1.1.1]cryptand and Na[2.2.2]cryptand were analyzed up to the 1f¹ configuration, and their transitions were found to occur at lower energies compared to their aza-crown ethers. The introduced superatomic shell models in this work closely resemble the Aufbau principle of "solvated electrons precursors". All reported alkali metal complexes bear lower ionization potentials than any atom in the periodic table; thus, they can also be recognized as "superalkalis".

Ground and excited states analysis of alkali metal ethylenediamine and crown ether complexes

High-level electronic structure calculations are carried out to obtain optimized geometries and excitation energies of neutral lithium, sodium, and potassium complexes with two ethylenediamine and one or two crown ether molecules. Three different sizes of crowns are employed (12-crown-4, 15-crown-5, 18-crown-6). The ground state of all complexes contains an electron in an s-type orbital. For the mono-crown ether complexes, this orbital is the polarized valence s-orbital of the metal, but for the other systems this orbital is a peripheral diffuse orbital. The nature of the low-lying electronic states is found to be different for each of these species. Specifically, the metal ethylenediamine complexes follow the previously discovered shell model of metal ammonia complexes (1s, 1p, 1d, 2s, 1f), but both mono- and sandwich di-crown ether complexes bear a different shell model partially due to their lower (cylindrical) symmetry and the stabilization of the 2s-type orbital. Li(15-crown-5) is the only complex with the metal in the middle of the crown ether and adopts closely the shell model of metal ammonia complexes. Our findings suggest that the electronic band structure of electrides (metal crown ether sandwich aggregates) and expanded metals (metal ammonia aggregates) should be different despite the similar nature of these systems (bearing diffuse electrons around a metal complex).

Ground and excited electronic structures of metal encapsulated nanocages: the cases of endohedral M@C20H20 (M = K, Rb, Ca, Sr) and M@C36H36 (M = Na, K, Rb)

High-level electronic structure calculations were performed to analyze ground and excited states of neutral and cationic endohedral M@C₂₀H₂₀ (M = K, Rb, Ca, Sr) and M@C₃₆H₃₆ (M = Na, K, Rb). In their ground states, one or two electrons occupy a diffuse atomic s-type orbital, thus 1s¹ and 1s² superatomic electronic configurations are assigned for M = Na, K, Rb and M = Ca, Sr cases, respectively. These species populate 1p-, 1d-, 1f-superatomic orbitals in electronically excited states. The specific superatomic Aufbau model introduced for M@C₂₀H₂₀ (M = K, Rb) is 1s, 1p, 1d, 2s, 1f, 2p, 2d, 1g, 2f. On the other hand, excited electronic spectra of M@C₂₀H₂₀ (M = Ca, Sr) are rich in multireference characters. Excited states of bigger M@C₃₆H₃₆ molecules were investigated up to the 1d level and the transitions were found to require slightly higher energies compared to M@C₂₀H₂₀. These superatoms possess lower ionization potentials, hence can also be categorized as superalkalis.

Electronic and geometric structure of cationic and neutral chromium and molybdenum ammonia complexes

High level quantum chemical approaches are used to study the geometric and electronic structures of M(NH₃)_n and M(NH₃)_n ⁺ (M = Cr, Mo for n = 1-6). These complexes possess a dual shell electronic structure of the inner metal (3d or 4d) orbitals and the outer diffuse orbitals surrounding the periphery of the complex. Electronic excitations reveal these two shells to be virtually independent of the other. Molybdenum and chromium ammonia complexes are found to differ significantly in geometry with the former adopting an octahedral geometry and the latter a Jahn-Teller distorted octahedral structure where only the axial distortion is stable. The hexa-coordinated complexes and the tetra-coordinated complexes with two ammonia molecules in the second solvation shell are found to be energetically competitive. Electronic excitation energies and computed IR spectra are provided to allow the two isomers to be experimentally distinguished. This work is a component of an ongoing effort to study the periodic trends of transition metal solvated electron precursors.

Spontaneous bond dissociation cascades induced by Ben clusters (n = 2,4)

High-level single and multireference ab initio calculations show that the Be₄ cluster behaves as a very efficient Lewis acid when interacting with conventional Lewis bases such as ammonia, water or hydrogen fluoride, to the point that the corresponding acid-base interaction triggers a sequential dissociation of all the bonds of the Lewis base. Notably, this behavior is already found for the simplest beryllium cluster, the Be₂ dimer. However, whereas for Be₂ the first dissociation process involves a low activation barrier which is above the reactants, for Be₄ all the bond dissociation processes involve barriers below the entrance channel leading to a cascade of successive exothermic processes, which end up spontaneously in a global minimum in which the bonding patterns of both the base and the Lewis acid are completely destroyed. Indeed, the global minimum, in all cases, is stabilized by three-center Be-H-Be bonds and covalent interactions between the Be atoms and the basic center of the base, which replace the initial metallic bond stabilizing the Be₄ cluster. As a consequence, in the global minimum the basic atoms (N, O and F) behave as hyper-coordinated centers. Also importantly, the Be₄ cluster and its complexes present RHF-UHF instabilities (not reported before for Be₄), which require the use of multireference methods to correctly describe them.

Ground and excited electronic structure analysis of XM4 (X = N, P and M = Li, Na) and their anions

High-level coupled-cluster, electron propagator, and multi-reference ab initio methods are employed to study the ground and excited electronic states of the XM4 (X = N, P and M = Li, Na) series. All XM4 species bear lower ionization potentials and can be classified as superalkalis. In the ground state each possesses a diffuse electron in the periphery. This expanded electron cloud of tetrahedral NLi4, NNa4, and PNa4 molecules is spherical (similar to an s-orbital) and evenly distributed around the XM4+ core. The outer electron is promoted to higher-angular momentum p-, d-, 2s-type orbitals in excited states. Singly occupied molecular orbitals of excited PLi4 are deformed due to its lower C1 symmetry. The aug-cc-pVQZ basis set was found to describe the excited states of XM4 accurately and efficiently. The bound singlet and triplet electronic states of XM4- that possess two peripheral electrons are also analyzed.

Geometric and electronic structure analysis of calcium water complexes with one and two solvation shells

The stability of calcium water complexes is investigated quantum mechanically. Ground and excited electronic states are studied for hexa-, octa-, and octakaideca-coordinated complexes, where calcium valence electrons move to outer diffuse orbitals.