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.

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.

Striking Borane Planarization in the Thermal Rearrangement (η5-C5H5)Fe(η3-B5H10)→(η5-C5H5)Fe(η5-B5H10).

In 1977 Weiss and Grimes, by means of mass spectrometry and 1H and 11B NMR spectroscopy, proposed two structures (I and II) for the ferraborane (η5-C5H5)Fe(B5H10), isoelectronic with ferrocene. In this work, by means of high-level quantum-chemical computations, we confirm the experimental structures of the two isomers with their corresponding energies, and assign the reported 1H and 11B NMR chemical shifts. A striking result from this study is the planarization (3D→2D) of the B5H10 - ligand - an unknown isolated anion, isoelectronic with aromatic cyclopentadienyl anion C5H5 - - when attached to the (η5-C5H5)Fe+ moiety, thus resulting in a more stable ferraborane isomer II.

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.

Breaking the plane: B5H5 is a three-dimensional structure.

In this study, we delved into the structure of B5H5 and questioned some of its accepted assumptions. By exploring the potential energy surface, we found a new three-dimensional structure as the global minimum. This finding is in contrast with the previously hypothesized planar and cage-like models. Our exploration extends to the kinetic stability of various B5H5 isomers, offering insights into the dynamic behavior of these molecules.

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.

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.

Quest for the Most Aromatic Pathway in Charged Expanded Porphyrins.

Despite the central role of aromaticity in the chemistry of expanded porphyrins, the evaluation of aromaticity remains difficult for these extended macrocycles. The presence of multiple conjugation pathways and different planar and nonplanar π-conjugation topologies makes the quantification of global and local aromaticity even more challenging. In neutral expanded porphyrins, the predominance of the aromatic conjugation pathway passing through the imine-type nitrogens and circumventing the amino NH groups is established. However, for charged macrocycles, the question about the main conjugation circuit remains open. Accordingly, different conjugation pathways in a set of neutral, anionic, and cationic expanded porphyrins were investigated by means of several aromaticity indices rooted in the structural, magnetic, and electronic criteria. Overall, our results reveal the predominance of the conjugation pathway that passes through all nitrogen atoms to describe the aromaticity of deprotonated expanded porphyrins, while the outer pathway through the perimeter carbon atoms becomes the most aromatic in protonated macrocycles. In nonplanar and charged macrocycles, a discrepancy between electronic and magnetic descriptors is observed. Nevertheless, our work demonstrates AVmin remains the best tool to determine the main conjugation pathway of expanded porphyrins.

BH4 Ng+ (Ar-Rn): Viable Compounds with a B-Ng Covalent Bond.

In this work, we explore, using high-level calculations, the ability of BH4 + to interact with noble gases. The He system is energetically unstable, while the Ne system could only be observed at cryogenic temperatures. In the case of the Ar, Kr and Xe systems, all are energetically stable, even at room temperature. The different chemical bond descriptors reveal a covalent character between B and the noble gas from Ar to Rn. However, this interaction gradually weakens the multicentric bond between the boron atom and the H2 fragment. Thus, although BH4 Rn+ exhibits a strong covalent bond, it tends to dissociate at room temperature into BH2 Rn+ +H2 .

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.

Hitting the Bull's Eye: Stable HeBeOH+ Complex.

It is now known that the heavier noble gases (Ng=Ar-Rn) show some varying degrees of reactivity with a gradual increase in reactivity along Ar-Rn. However, because of their very small size and very high ionization potential, helium and neon are the hardest targets to crack. Although few neon complexes are isolated at very low temperatures, helium needs very extreme situations like very high pressure. Here, we find that protonated BeO, BeOH+ can bind helium and neon spontaneously at room temperature. Therefore, extreme conditions like very low temperature and/or high pressure will not be required for their experimental isolation. The Ng-Be bond strength is very high for their heavier homologs and the bond strength shows a gradual increase from He to Rn. Moreover, the Ng-Be attractive energy is almost exclusively originated from the orbital interaction which is composed of one Ng(s/pσ )→BeOH+ σ-donation and two weaker Ng(pπ )→BeOH+ π-donations, except for helium. Helium uses its low-lying vacant 2p orbitals to accept π-electron density from BeOH+ . Previously, such electron-accepting ability of helium was used to explain a somewhat stronger helium bond than neon for neutral complexes. However, the present results indicate that such π-back donations are too weak in nature to decide any energetic trend between helium and neon.

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.

Lewis Superacidic Heavy Pnictaalkene Cations: Comparative Assessment of Carbodicarbene-Stibenium and Carbodicarbene-Bismuthenium Ions.

We report a comprehensive assessment of Lewis acidity for a series of carbone-stibenium and -bismuthenium ions using the Gutmann-Beckett (GB) method. These new antimony and bismuth cations have been synthesized by halide abstractions from (CDC)PnBr3 and [(pyCDC)PnBr2][Br] (CDC = carbodicarbene; Pn = Sb or Bi; py = pyridyl). The reaction of (CDC)SbBr3 (1) with one or two equivalents of AgNTf2 (NTf2 = bis(trifluoromethanesulfonyl)imide) or AgSbF6 gives stibaalkene mono- and dications of the form [(CDC)SbBr3-n][A]n (2-4; n = 1,2; A = NTf2 or SbF6). The stibaalkene trication [(CDC)2Sb][NTf2]3 (5) was also isolated and collectively these molecules fill the gap among the series of cationic pnictaalkenes. The Sb cations are compared to the related CDC-bismaalkene complexes 6-9. With the goal of preparing highly Lewis acidic compounds, a tridentate bis(pyridine)carbodicarbene (pyCDC) was used as a ligand to access [(pyCDC)PnBr2][Br] (10, 12) and trications [(pyCDC)Pn][NTf2]3 (Pn = Sb (11), Bi (13)), forgoing the need for a second CDC as used in the synthesis of 5. The bonding situation in these complexes is elucidated through electron density and energy decomposition analyses in combination with natural orbital for chemical valence theory. In each complex, there exists a CDC-Pn double bonding interaction, consisting of a strong σ-bond and a weaker π-bond, whereby the π-bond gradually strengthens with the increase in cationic charge in the complex. Notably, [(CDC)SbBr][NTf2]2 (4) has an acceptor number (AN) (84) that is comparable to quintessential Lewis acids such as BF3, and tricationic pnictaalkene complexes 11 and 13 exhibit strong Lewis acidity with ANs of 109 (Pn = Sb) and 84 (Pn = Bi), respectively, which are among the highest values reported for any antimony or bismuth cation. Moreover, the calculated fluoride ion affinities (FIAs) for 11 and 13 are 99.8 and 94.3 kcal/mol, respectively, which are larger than that of SbF5 (85.1 kcal/mol), which suggest that these cations are Lewis superacids.

A Multidimensional Approach to Carbodiphosphorane-Bismuth Coordination Chemistry: Cationization, Redox-Flexibility, and Stabilization of a Crystalline Bismuth Hydridoborate.

Bismuth complexes stabilized by carbon-based donor ligands are underserved by their instability, often due to facile ligand dissociation and deleterious protonolysis. Herein, we show that the ortho-bismuthination of hexaphenylcarbodiphosphorane enables a robust framework with geometrically constrained carbone-bismuth bonding interactions, which are highly tunable by cationization. The carbodiphosphorane bismuth halides (1 and 2) are remarkably air-stable and feature unprecedented trans carboneC-Bi-X ligation, resulting in highly elongated Bi-X bonds. In contrast to known carbone-bismuth complexes, hydrolytic activation of the carbone yields well-defined organobismuth complexes, and subsequent dehydrohalogenation is feasible using potassium bis(trimethylsilyl)amide or N-heterocyclic carbenes. The redox-flexibility of this framework was evaluated in the high catalytic activity of 1 and 2 for silylation of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) under mild conditions (50 °C, 24-96 h) and low catalyst loadings (5-10 mol %), which suggests the accessibility of short-lived hydridic and radical bismuth species. The reaction of 1, PhSiH3, and tris(pentafluorophenyl)borane (BCF) yields the first crystallographically characterized bismuth hydridoborate complex as an ionic species (9), presumably by BCF-mediated hydride abstraction from an unobserved [Bi]-H intermediate. All isolated compounds have been characterized by heteronuclear NMR spectroscopy and X-ray crystallography, and the bonding situation in representative complexes (1, 2, 5, and 9) were further evaluated using density functional theory.

Designing a Four-Ring Tubular Boron Motif through Metal Doping.

Tubular boron clusters represent a class of extremely unusual geometries that can be regarded as a key indicator for the 2D-to-3D boron structural evolution as well as the embryos for boron nanotubes. While a good number of pure boron or metal-doped boron tubular clusters have been reported so far, most of them are two-ring tubular structures, and their higher-ring analogues are very scarce. We report herein the first example of a four-ring tubular boron motif in the cagelike global minimum of Be2B24+. Global-minimum searches of MB24q and M2B24q (M = alkali/alkaline-earth metals; q = 1+, 0, 1-) reveal that the most stable structure of Be2B24+ is a C2v-symmetric cage having a four-ring tubular boron moiety, whereas it is a high-lying isomer for those having a two/three-ring tubular boron motif for all other systems. The B24 framework in Be2B24+ can be viewed as consisting of two two-ring B12 tubular structures linked together at one side of the B6 rings along the high-symmetry axis and two offside B6 rings capped by two Be atoms. The Be2-B24 bonding is best described as Be22+ in an excited triplet state, forming two highly polarized covalent bonds with B24- in a quartet spin state. The unique ability of beryllium to make strong covalent and electrostatic interactions makes the Be2B24+ cluster stable in such an unusual geometry.

Why an integrated approach between search algorithms and chemical intuition is necessary?

Though search algorithms are appropriate tools for identifying low-energy isomers, fixing several constraints seems to be a fundamental prerequisite to successfully running any structural search program. This causes some potential setbacks as far as identifying all possible isomers, close to the lowest-energy isomer, for any elemental composition. The number of explored candidates, the choice of method, basis set, and availability of CPU time needed to analyze the various initial test structures become necessary restrictions in resolving the issues of structural isomerism reasonably. While one could arrive at new structures through chemical intuition, reproducing or achieving those exact same structures requires increasing the number of variables in any given program, which causes further constraints in exploring the potential energy surface in a reasonable amount of time. Thus, it is emphasized here that an integrated approach between search algorithms and chemical intuition is necessary by taking the C12O2Mg2 system as an example. Our initial search through the AUTOMATON program yielded 1450 different geometries. However, through chemical intuition, we found eighteen new geometries within 40.0 kcal mol-1 at the PBE0-D3/def2-TZVP level. These results indirectly emphasize that an integrated approach between search algorithms and chemical intuition is necessary to further our knowledge in chemical space for any given elemental composition.

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.

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.