Revisiting the Structure and Bonding in Li5H6- and the Exploration of Reactivity: Planar Pentacoordinate Hydrogen.

Recently, Guha and co-workers (Sarmah, K.; Kalita, A.; Purkayastha, S.; Guha, A. K. Pushing The Extreme of Multicentre Bonding: Planar Pentacoordinate Hydride. Angew. Chem. Int. Ed. 2024, e202318741) reported a highly intriguing bonding motif: planar pentacoordinate hydrogen (ppH) in Li5H6-, featuring C2v symmetry in the singlet state with two distinct H-Li (center-ring) bond distances. We herein revisited the potential energy surface of Li5H6- by using a target-oriented genetic algorithm. Our investigation revealed that the lowest-energy structure of Li5H6- exhibits a ppH configuration with very high D5h symmetry and a 1A1' electronic state. We did not find any electronic effect like Jahn-Teller distortion that could be responsible for lowering its symmetry. Moreover, our calculations demonstrated significant differences in the relative energies of other low-lying isomers. An energetically very competitive planar tetracoordinate hydrogen (ptH) isomer is also located, but it corresponds to a very shallow minimum on the potential energy surface depending on the used level of theory. Chemical bonding analyses, including AdNDP and EDA-NOCV, uncover that the optimal Lewis structure for Li5H6- involves H- ions stabilized by the Li5H5 crown. Surprisingly, despite the dominance of electrostatic interactions, the contribution from covalent bonding is also significant between ppH and the Li5H5 moiety, derived from H-(1s) → Li5H5 σ donation. Magnetically induced current density analysis revealed that due to minimal orbital overlap and the highly polar nature of the H-Li covalent interaction, the ppH exhibits local diatropic ring currents around the H centers, which fails to result in a global aromatic ring current. The coordination of Li5H6- with Lewis acids, BH3 and BMe3, instantly converts the ppH configuration to (quasi) ptH. These Lewis acid-bound ptH complexes show high electronic stability and high thermochemical stability against dissociation and, therefore, will be ideal candidates for the experimental realization.

Exploring the Use of "Honorary Transition Metals" To Push the Boundaries of Planar Hypercoordinate Alkaline-Earth Metals.

The quest for planar hypercoordinate atoms (phA) beyond six has predominantly focused on transition metals, with dodecacoordination being the highest reported thus far. Extending this bonding scenario to main-group elements, which typically lack d orbitals despite their larger atomic radius, has posed significant challenges. Intrigued by the potentiality of covalent bonding formation using the d orbitals of the heavier alkaline-earth metals (Ae = Ca, Sr, Ba), the so-called "honorary transition metals", we aim to push the boundaries of planar hypercoordination. By including rings formed by 12-15 atoms of boron-carbon and Ae centers, we propose a design scheme of 180 candidates with a phA. Further systematic screening, structural examination, and stability assessments identified 10 potential clusters with a planar hypercoordinate alkaline-earth metal (phAe) as the lowest-energy form. These unconventional structures embody planar dodeca-, trideca-, tetradeca-, and pentadecacoordinate atoms. Chemical bonding analyses reveal the important role of Ae d orbitals in facilitating covalent interactions between the central Ae atom and the surrounding boron-carbon rings, thereby establishing a new record for coordination numbers in the two-dimensional realm.

New Perspectives on Delocalization Pathways in Aromatic Molecular Chameleons.

This study comprehensively analyzes the magnetically induced current density of polycyclic compounds labeled as "aromatic chameleons" since they can arrange their π-electrons to exhibit aromaticity in both the ground and the lowest triplet state. These compounds comprise benzenoid moieties fused to a central skeleton with 4n π-electrons and traditional magnetic descriptors are biased due to the superposition of local magnetic responses. In the S0 state, our analysis reveals that the molecular constituent fragments preserve their (anti)aromatic features in agreement with two types of resonant structures: one associated with aromatic benzenoids and the other with a central antiaromatic ring. Regarding the T1 state, a global and diatropic ring current is revealed. Our aromaticity study is complemented with advanced electronic and geometric descriptors to consider different aspects of aromaticity, particularly important in the evaluation of excited state aromaticity. Remarkably, these descriptors consistently align with the general features on the main delocalization pathways in polycyclic hydrocarbons consisting of fused 4n π-electron rings. Moreover, our study demonstrates an inverse correlation between the singlet-triplet energy difference and the antiaromatic character of the central ring in S0.

Update for Isomerization Stabilization Energies: The Fulvenization Approach.

An alternative approach for calculating aromatic stabilization energies is proposed based on transforming an (anti)aromatic ring into a fulvene isomer. This fulvenization process gives a value of 34.05 kcal·mol-1 for benzene in the singlet state and a value of -17.85 kcal·mol-1 in the triplet state. Additionally, it is possible to use experimental values (as long as they exist) for the calculation as the gas-phase formation enthalpies of benzene and fulvene, whose difference is 33.72 kcal·mol-1. On the other hand, this same approach has been evaluated on several six-membered rings, including those persubstituted, biradicals, azines, and inorganic analogues, giving results in agreement with those reported in the literature using different criteria. Additionally, it is possible to differentiate the aromaticity of the rings in polycyclic aromatic hydrocarbons according to Clar's rules. Assigning the (anti)aromatic character in various nonbenzenoid rings (neutral and charged), except for five- and seven-membered rings, is also possible. The construction of the fulvene isomers in PAHs is set such that nonaromaticity-related effects are not considered. The results show that the fulvenization approach is an effective and efficient approach that can serve as an alternative or complement to existing tools.

Li6 E5 Li6 : Tetrel Sandwich Complexes with 10-π-Electrons.

When (4n +2) π-electrons are located in single planar ring, it conventionally qualifies as aromatic. According Hückel's rule, systems possessing ten π-electrons should be aromatic. Herein we report a series of D5h  Li6 E5 Li6 sandwich structures, representing the first global minima featuring ten π-electrons E5 10- ring (E=Si-Pb). However, these π-electrons localize as five π-lone-pairs rather than delocalized orbitals. The high symmetry structure achieved is a direct consequence of σ-aromaticity, particularly favored in elements from Si to Pb, resulting in a pronounced diatropic ring current flow that contributes to the enhanced stability of these systems.

Aromaticity and Magnetic Behavior in Benzenoids: Unraveling Ring Current Combinations.

Nowadays, an active research topic is the connection between Clar's rule, aromaticity, and magnetic properties of polycyclic benzenoid hydrocarbons. In the present work, we employ a meticulous magnetically induced current density analysis to define the net current flowing through any cyclic circuit, connecting it to aromaticity based on the ring current concept. Our investigation reveals that the analyzed polycyclic systems display a prominent global ring current, contrasting with subdued semi-local and local ring currents. These patterns align with Clar's aromatic π-sextets only in cases where migrating π-sextet structures are invoked. The results of this study will enrich our comprehension of aromaticity and magnetic behavior in such systems, offering significant insights into coexisting ring current circuits in these systems.

Revisiting the potential-energy surface of CnBe3n+2H2n+22+ (n = 2-4) clusters: are planar pentacoordinate carbon structures the global minima?

Using various exploration strategies, in this study, we investigated the potential energy surfaces (PES) of CBe5H5+ and CnBe3n+2H2n+22+ (n = 2-4) clusters. Previous studies proposed that the planar pentacoordinate carbons (ppCs) were the global minima of these clusters. However, our study identified new putative global minima and competitive isomers, refuting some previous assignments. We employed several methods, including evolutive-inspired stochastic approaches guided by "chemical criteria", and ab initio molecular dynamics simulations at elevated temperatures. Our results showed that the size of the scanned population significantly affected the evolutive method and that constrained or guided procedures showed an advantage in identifying better minima for larger systems. This study demonstrated that using multiple complementary strategies can result in a wider variety of minima in a given energy range. Our findings provide valuable insights into exploring the potential energy surfaces of clusters, mainly medium-sized clusters, which could be the connections between small clusters and nanomaterials.

B7 Be6 B7 : A Boron-Beryllium Sandwich Complex.

Planar boron clusters have often been regarded as "π-analogous" to aromatic arenes because of their similar delocalized π-bonding. However, unlike arenes such as C5 H5 - and C6 H6 , boron clusters have not previously shown the ability to form sandwich complexes. In this study, we present the first sandwich complex involving beryllium and boron, B7 Be6 B7 . The global minimum of this combination adopts a unique architecture having a D6h geometry, featuring an unprecedented monocyclic Be6 ring sandwiched between two quasi-planar B7 motifs. The thermochemical and kinetic stability of B7 Be6 B7 can be attributed to strong electrostatic and covalent interactions between the fragments. Chemical bonding analysis shows that B7 Be6 B7 can be considered as a [B7 ]3- [Be6 ]6+ [B7 ]3- complex. Moreover, there is a significant electron delocalization within this cluster, supported by the local diatropic contributions of the B7 and Be6 fragments.

E6C15 (E = Si-Pb): polycyclic aromatic compounds with three planar tetracoordinate carbons.

A systematic exploration of the potential energy surface reveals two global minima with three planar tetra coordinate carbons (ptCs) and two global minima with three quasi-ptCs for E6C15 (E = Si-Pb) combinations. These consist of aromatic polycyclic templates suitable for further design of different materials without hindering the ptC texture.

Bowl-shaped Cluster CuB12 - : A Viable Global Minimum with Twofold Aromaticity.

A low-lying structure is revealed for the CuB12 - cluster, which is bowl-shaped. It consists of a triangular CuB2 base and a B10 rim. Molecular dynamics simulations indicates its structural robustness; at an elevated temperature (600 K), the base rotates reversibly within the B10 perimeter. Chemical bonding analysis detects 2σ- and 3π-delocalized bonds, suggesting double aromaticity. This is also confirmed by two diatropic and concentric ring currents under an external magnetic field.

Planar Elongated B12 Structure in M3B12 Clusters (M = Cu-Au).

Here, it is shown that the M3B12 (M = Cu-Au) clusters' global minima consist of an elongated planar B12 fragment connected by an in-plane linear M3 fragment. This result is striking since this B12 planar structure is not favored in the bare cluster, nor when one or two metals are added. The minimum energy structures were revealed by screening the potential energy surface using genetic algorithms and density functional theory calculations. Chemical bonding analysis shows that the strong electrostatic interactions with the metal compensate for the high energy spent in the M3 and B12 fragment distortion. Furthermore, metals participate in the delocalized π-bonds, which infers an aromatic character to these species.

Planar Hypercoordinate Carbons in Alkali Metal Decorated CE3 2- and CE2 2- Dianions.

After exploring the potential energy surfaces of Mm CE2 p (E=S-Te, M=Li-Cs, m=2, 3 and p=m-2) and Mn CE3 q (E=S-Te, M=Li-Cs, n=1, 2, q=n-2) combinations, we introduce 38 new global minima containing a planar hypercoordinate carbon atom (24 with a planar tetracoordinate carbon and 14 with a planar pentacoordinate carbon). These exotic clusters result from the decoration of V-shaped CE2 2- and Y-shaped CE3 2- dianions, respectively, with alkali counterions. All these 38 systems fulfill the geometrical and electronic criteria to be considered as true planar hypercoordinate carbon systems. Chemical bonding analyses indicate that carbon is covalently bonded to chalcogens and ionically connected to alkali metals.

Planar Tetracoordinate Carbons in Allene-Type Structures.

The exhaustive exploration of the potential energy surfaces of CE2M2 (E = Si-Pb; M = Li and Na) revealed seven global minima containing a planar tetracoordinate carbon (ptC). The design, based on a π-localization strategy, resulted in a ptC with two double bonds forming a linear or a bent allene-type E═C═E motif. The magnetic response of the bent E═C═E fragments support a σ-aromaticity. The bonding analysis indicated that the ptCs form C-E covalent bonds and C-M electrostatic interactions.

Li8 Si8 , Li10 Si9 , and Li12 Si10 : Assemblies of Lithium-Silicon Aromatic Units.

We report the global minima structures of Li8 Si8 , Li10 Si9 , and Li12 Si10 systems, in which silicon moieties maintain structural and chemical bonding characteristics similar to those of their building blocks: the aromatic clusters Td -Li4 Si4 and C2v -Li6 Si5 . Electron counting rules, chemical bonding analysis, and magnetic response properties verify the silicon unit's aromaticity persistence. This study demonstrates the feasibility of assembling silicon-based nanostructures from aromatics clusters as building blocks.

Planar Hexacoordinate Carbons: Half Covalent, Half Ionic.

Herein, the first global minima containing a planar hexacoordinate carbon (phC) atom are reported. The fifteen structures belong to the CE3 M3 + (E=S-Te and M=Li-Cs) series and satisfy both geometric and electronic criteria to be considered as a true phC. The design strategy consisted of replacing oxygen in the D3h  CO3 Li3 + structure with heavy and less electronegative chalcogens, inducing a negative charge on the C atom and an attractive electrostatic interaction between C and the alkali-metal cations. The chemical bonding analyses indicate that carbon is covalently bonded to three chalcogens and ionically connected to the three alkali metals.