Formation of Spherical Aromatic Endohedral Metallic Fullerenes. Evaluation of Magnetic Properties of M@C28 (M = Ti, Zr, and Hf) from DFT calculations

Cuboctahedral Pd13 as a spherical aromatic noble metal core: insights from a ligand-protected [Pd13(Tr)6]2+ cluster

Low-valent palladium nanoparticles are efficient species promoting catalytic activity and selectivity in a number of chemical reactions. Recently, an atom-centered cuboctahedral Pd13 motif has been characterized as a ligand-protected [Pd13(Tr)6]2+ cluster featuring a 1s2 superatomic shell structure. In this report, we describe the ligand-cluster of and endohedral-cage interaction in [Pd13(Tr)6]2+, which accounts for a favorable situation in the overall cluster. In addition, the spherical aromatic properties of the cluster were evaluated to understand the behavior of the ligand-protected Pd13 cluster core. Our results indicate a sizable interaction towards carbon-based ligands in an overall spherical aromatic cluster featuring a long-range shielding cone. Thus, [Pd13(Tr)6]2+ is rationalized as the first ligand-protected palladium cluster to date exhibiting spherical aromatic properties, serving as a stable building block for molecule-based materials or as a dopant in porous carbon materials.

Smallest Endohedral Metallofullerenes [Mg@C20]n (n = 4, 2, 0, -2, and -4): Endo-Ionic Interaction in Superatoms

This work reports a series of endohedral metallofullerene superatoms [Mg@C₂₀]ⁿ, where n = 4, 2, 0, -2, and -4. It was found that Mg transfers virtually all of its 3s electrons to the C₂₀ shell, resulting in the ionic states of Mg²⁺@[C₂₀]^n-2. Detailed calculations revealed that the superatomic electronic configuration of these clusters is 1S²1P⁶1D¹⁰1F^4-n. The first nine superatomic molecular orbitals (SAMOs), 1S²1P⁶1D¹⁰, housed with 18 electrons, are largely based on [C₂₀]^n-2 with small contributions from magnesium, while the outmost SAMOs, 1F^4-n, with 4 - n extra electrons, reside solely on [C₂₀]^n-2. The interaction between the Mg²⁺ ion and [C₂₀]^n-2 was found to be predominately ionic in character. Furthermore, ultraviolet-visible spectra provide a theoretical basis for fingerprinting these clusters. It is hoped that this work will encourage the synthetic pursuit of these smallest superatomic systems.

Investigation of the substituted-titanium nanocages using computational chemistry

We are investigated substitution effects of titanium heteroatoms on band gap, charge and local reactivity of C_20-nTi_n heterofullerenes (n = 1-5), at different levels and basis sets. The C₁₈Ti_2-2 nanocage is considered as the most kinetically stable species with the widest band gap of 2.86 eV, in which two carbon atoms are substituted by two Ti atoms in equatorial position, individually. The charges on carbon atoms of C₂₀ are roughly zero, while high positive charge (1.256) on the surface of C₁₉Ti₁ prompts this heteofullerene for hydrogen storage. The positive atomic charge on Ti atoms and negative atomic charge on their adjacent C atoms implies that these sites can be influenced more readily by nucleophilic and electrophilic regents, respectively. We examined the usefulness of local reactivity descriptors to predict the reactivity of Ti-C atomic sites on the external surface of the heterofullerenes. The properties determined include Fukui function (F.F.); f (k) and local softness s (k) on the surfaces of the investigated hollow cages. Geometry optimization results reveal that titanium atoms can be comfortably incorporated into the CC network of fullerene. It is most likely associated with the triple-coordination characteristic of titanium atoms, which can well match with the sp²-hybridized carbon bonding structure. According to the values of f (k) and s (k) for the C₁₅Ti₅ heterofullerene; the carbon atoms in the cap regions exhibit a different reactivity pattern than those in the equatorial portion of the heterofullerene. The titanium impurity can significantly improve the fullerene's surface reactivity and it allows controlling their surface properties. The band gap of C_20-nTi_n …..(H₂)_n structures is decreased with increasing n. Hence, C₁₅Ti₅ is found as the best hydrogen adsorbent.

Super-Excimer: Anomalous Bonding in a Metastable Excited-State Dimer of Superatomic Dimers

A new type of excimer formation was reported, which stems from an unexpected discovery of a short-lived excited-state dimer of superatomic dimers. In theoretical investigation of the dimer formation, it was found that the physical adsorption states maintain the closed-shell properties of the dimeric units via van der Waals interaction, while the chemical adsorption excited state is a broken-symmetry (BS) state, having a higher energy of about 0.5 eV. Potential energy surface calculations indicate that the short-lived metastable chemical bonding state can transform into energetically lower physical adsorption states by crossing a shallow energy barrier and eventually disintegrate into two ground-state dimers. Since the basic unit is a superatomic cluster, the chemical adsorption state discovered may be called "super-excimer", which opens up a new avenue for the discovery of tailorable excimer materials.

Antiaromaticity-aromaticity transition of cyclo[16]carbon upon metal encapsulation

In contrast to aromatic compounds with particular stability, antiaromatic compounds are usually less stable due to their high reactivity and unfavorable formation energies. Cyclo[16]carbon (C₁₆) is a carbon ring molecule with a dual antiaromatic character. In this study, we demonstrate that C₁₆ can be transformed into highly aromatic molecules upon metal encapsulation. The geometrical characteristics, electronic properties and thermodynamic stability of MC₁₆ compounds (M = Ca, Sc, Ti, V, Ce, U) are fully investigated from a theoretical perspective. Based on natural population analysis, atom-in-molecules theory and localized molecular orbital analysis, the nature of the metal-carbon interaction in the MC₁₆ compounds is investigated. It has been proved that the bonding between Ca and C₁₆ corresponds to a typical ionic interaction, while other metal atoms form polar covalent bonds with C₁₆. By analyzing the frontier molecular orbitals and magnetic response of MC₁₆, we have found that all the encapsulated metal atoms donate two electrons to the in-plane π orbitals via either electron transfer or orbital hybridization, which makes the in-plane π orbitals completely satisfy the 4n + 2 (n = 4) Hückel aromaticity rule. The U atom formally transfers four electrons to the carbon ring in total, two to the in-plane π orbitals and two to the out-of-plane π orbitals, which results in the remarkable dual aromaticity feature of UC₁₆. The transformation of aromaticity can be utilized to develop new strategies for the synthesis of novel carbon ring molecules.

Open-shell nature of non-IPR fullerene С40: isomers 29 (C2) and 40 (Td)

It is well-known that the small non-IPR fullerenes C_n (n < 60) are highly unstable and that is why they cannot be obtained as empty cages. However, they become stable as exohedral or endohedral derivatives. In this report, the molecular structures of non-IPR isomers 29 (C₂) and 40 (T_d) of fullerene C₄₀ are investigated using a semiempirical approach developed earlier for higher fullerenes. Quantum-chemical calculations (DFT) show that isomers 29 (C₂) and 40 (T_d) have open-shell structures. The distributions of single, double, and delocalized π-bonds in the isomer molecules in question are presented for the first time as well as their molecular formulas. It is found unusual for higher fullerenes chain of π-bonds passing through some cycles. Identified features in the structures of small fullerene molecules can be predictive of the ability to their synthesis as derivatives and will assist in their structure determination.

Interaction potential energy surface between superatoms

In this work, we report the potential energy surface between superatoms (SPES) based on first-principles theory. The calculations show that superatoms have SPES crossing behavior between different electronic states similar to an atomic potential energy surface (PES). However, unlike atoms, the relative rotation between superatoms results in new dimensions for the SPES. The rotation is divided into two types, around the direction of the line between two superatoms and perpendicular to it. At the equilibrium distance, the former rotation results in maximum energy and charge changes of 0.03 eV and 10^-2 e respectively. However, the latter rotation yields changes that are 17 times and 7 times those of the former. These findings help promote the understanding of intermolecular interactions, and will contribute to the bottom-up superatomic-based assembly of novel materials.

On the formation of spherical aromatic endohedral buckminsterfullerene. Evaluation of M@C60 (M = Cr, Mo, W) from relativistic DFT calculations

Endohedral metallofullerenes are key species for expanding the range of viable fullerenes, their versatility, and applications. Here we report our computational evaluation on the formation of spherical aromatic counterparts of the C60 fullerene from relativistic DFT calculations, based on the inclusion of Cr, Mo and W endohedral atoms. The resulting M@C60 endohedral fullerenes are 66-π electron neutral species exhibiting bonding properties and electronic structure mimicking the aromaticity and diamagnetic insulator behavior of alkali-C606- phases. The resulting structures are interesting candidates for further experimental realization as novel neutral building blocks for more flexible nanostructured organic materials, highlighting truly spherical aromatic neutral species retaining the truncated icosahedral structure of the seminal Buckminster fullerene.

Shielding cone behavior in the spherical aromatic He@C606-: origin of the record for the most shielded encapsulated 3He nucleus and comparison to He@C706

The textbook concept of shielding cone is one of the characteristic properties for planar aromatic species, which explains the nuclear shielding of neighbor molecules located above and below the molecular plane. Here, we explore its resemblance in spherical aromatic fullerenes, where particularly for C₆₀^6-, it explains the record for the highest shielding observed via ³He-NMR for the endohedral ³He@C₆₀^6- species. Our results compare the behavior for He@C₆₀ and He@C₆₀^6- denoting relevant changes in their magnetic response behavior, where an induced shielding cone was observed for the latter with its long-range shielding character. In contrast to planar aromatics which give rise to a shielding cone property only for a perpendicularly oriented field, it was found that the shielding cone in spherical aromatic fullerenes occurs to any orientation. Thus, the orientation dependence behavior of the shielding cone is accounted, unraveling a characteristic shielding cone pattern in He@C₆₀^6- which further rationalizes its record on the most shielded He nucleus, upon rotation or tumbling from the aromatic ring about the applied field. For the C₇₀ case, the opposite situation has been characterized via ³He-NMR experiments, which is explained in terms of the deshielding/shielding region inside the C₇₀ cage. Graphical abstract.

Aromatic character of [Au13]5+ and [MAu12]4+/6+ (M = Pd, Pt) cores in ligand protected gold nanoclusters – interplay between spherical and planar σ-aromatics

Ligand-protected superatoms are able to behave as both spherical and planar aromatic species, providing a strong link between spherical and planar σ-aromatics, which can be controlled selectively by tuning their redox charge states.