The presence of a delocalized π-bond is often considered an essential criterion for achieving planar hypercoordination. Herein, we show that σ-delocalization could be sufficient to make the planar configuration the most stable isomer in a series of planar pentacoordinate s-block metals. High-level computations reveal that the global minimum of a series of interalkali and interalkali-alkaline earth clusters (LiNa, LiMg, NaMg, KCa, CaRb, RbSr, and SrCs) adopts a singlet structure with a planar pentacoordinate lithium or alkaline earth metal (AE = Mg, Ca, Sr). These clusters are unusual combinations to stabilize a planar pentacoordinate atom, as all their constituents are electropositive. Despite the absence of π-electrons, Hückel's rule is fulfilled by the six σ-electrons. Furthermore, the systems exhibit a diatropic ring current in response to an external magnetic field and a strong magnetic shielding, so they might be classified as σ-aromatic. Therefore, multicenter σ-bonds and the resulting σ-delocalization stabilize these clusters, even though they lack π-aromaticity.
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http://dx.doi.org/10.1039/d2sc05939h | DOI Listing |
Chem Commun (Camb)
December 2024
Centro de Química Teórica & Computacional (CQT&C), Facultad de Ciencias Exactas, Departamento de Ciencias Químicas, Universidad Andrés Bello, Avenida República 275, Santiago 837014, Chile.
Ten planar pentacoordinate boron (ppB) systems are reported, each featuring a pentagonal ring composed of tetrels, pnictogens, or their combination around boron. These structures exhibit double aromaticity (σ and π), consistent with Hückel's 4+2 rule, as confirmed by magnetically induced current density analysis.
View Article and Find Full Text PDFChem Commun (Camb)
December 2024
Key Laboratory of Materials for Energy Conversion and Storage of Shanxi Province, Institute of Molecular Science, Shanxi University, Taiyuan 030006, China.
A hydrocopper H©CuH star with planar pentacoordinate hydrogen (ppH) was designed. This structure is the global minimum, exhibiting dynamic stability. Its planar geometry is supported by five peripheral Cu-H-Cu 3c-2e σ-bonds and a central 6c-2e σ-bond.
View Article and Find Full Text PDFAngew Chem Int Ed Engl
November 2024
Institute of Atomic and Molecular Physics, Jilin University, Changchun, 130023, China.
Planar hypercoordinate motifs represent an intriguing frontier in chemistry, challenging traditional bonding norms. As electronegativity of the central atom increases, achieving planar hypercoordination becomes more difficult due to restricted delocalization, making the design of planar hypercoordinate halogens particularly puzzling. Here, we conduct an extensive computational survey of LiX (n=4, 5, 6; X=F, Cl, Br, I) clusters, revealing a starlike D-symmetry global minimum in LiX (X=F, Cl, Br) with a planar pentacoordinate halogen (ppX), where X is located at the center of LiX crown.
View Article and Find Full Text PDFChemphyschem
October 2024
Department of Chemistry and Biochemistry, Utah State University, Logan, UT, 84322-0300, USA.
The triel bond (TrB) formed between Be(CH)/Mg(CH) and TrX (Tr=B, Al, and Ga; X=H, F, Cl, Br, and I) is investigated via the MP2/aug-cc-pVTZ(PP) quantum chemical protocol. The C atoms of the methyl groups in M(CH) are characterized by a negative electrostatic potential and act as an electron donor in a triel bond with the π-hole above the Tr atom of planar TrX. The interaction energy spans a wide range between -2 and -69 kcal/mol.
View Article and Find Full Text PDFMolecules
August 2024
Key Laboratory of Materials for Energy Conversion and Storage of Shanxi Province, Institute of Molecular Science, Shanxi University, Taiyuan 030006, China.
Among the known planar pentacoordinate atoms, chlorine is missing due to its large radius and high electronegativity. Herein, we report the first star-like superhalogen anion Cl©LiCl (), which contains a planar pentacoordinate chlorine (ppCl) at the center. Computer structural searches and high-level calculations reveal that is a true global minimum (GM) on the potential energy surfaces.
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