Planar pentacoordinate carbon in a sulphur-surrounded boron wheel: the global minimum of CB5S5. Chem Commun (Camb) 2022;58:2552-2555. [PMID: 35103735 DOI: 10.1039/d1cc07313c]

We report a σ + π double aromatic CB₅S₅⁺ cluster, the first global minimum unusually having a planar hypercoordinate carbon inside a boron wheel. Five peripheral sulfur atoms stabilize the carbon-centered boron wheel by weakening the electron deficiency of the boron atoms through strong S → B π back-bonding.

Sulphur-Bridged BAl5S5+ with 17 Counting Electrons: A Regular Planar Pentacoordinate Boron System

At present, most of the reported planar pentacoordinate clusters are similar to the isoelectronic substitution of CAl₅⁺, with 18 counting electrons. Meanwhile, the regular planar pentacoordinate boron systems are rarely reported. Hereby, a sulphur-bridged BAl₅S₅⁺ system with a five-pointed star configuration and 17 counting electrons is identified at the global energy minimum through the particle-swarm optimization method, based on the previous recognition on bridged sulphur as the peripheral tactics to the stable planar tetracoordinate carbon and boron. Its outstanding stability has been demonstrated by thermodynamic analysis at 900 K, electronic properties and chemical bonding analysis. This study provides adequately theoretical basis and referable data for its experimental capture and testing.

Planar pentacoordinate silicon and germanium atoms

The global minimum of XMg₄Y^- (X = Si, Ge; Y = In, Tl) and SiMg₃In₂ contains a planar pentacoordinate atom of group 14 other than carbon. Its design is based on the "localization" approach, replacing one or two peripheral atoms in XMg₅^2- by more electronegative ones. This change diminishes the repulsion and leads to stronger covalent X-Y bonds, stabilizing the planar pentacoordinate atom species.

Avoided spin coupling: an unexpected σ-σ diradical in global planar pentacoordinate carbon

We present a global planar pentacoordinate carbon (ppC) featuring a hitherto unreported σ-σ diradical characteristic. Using the multi-reference approach combined with the CCSD(T)/aug-cc-pVTZ method, the ppC C₃Li₃^- was found to be an intriguing triplet ground state, in which the unpaired density is mostly located at three Li ligands. Chemical bonding analysis reveals that the 2p_zπ electrons of C₃Li₃^- are fully located at the C₃ ring formed by C-C multiple bonds, in contrast to the perfect 2p_zπ-delocalization found in the well-known ppCs.

A sixteen-valence-electron carbon-group 13 family with global penta-atomic planar tetracoordinate carbon: an ionic strategy

The design of planar tetracoordinate carbon (ptC) has always been a challenge due to its unique bonding mode that necessitates the perfect balance between the carbon center and surrounding ligands both electronically and mechanically. A unique type of 18-valence-electron (18ve) template, i.e., CAl42-, has been found to be very effective in designing various novel 18ve-species upon skeletal substitution. In this work, we showed that though ptC is not the global structure for the parent 16ve-CAl4, suitable skeletal substitution can allow for a series of global minimum ptC species. Theoretical calculations at the level of CCSD(T)/def2-QZVP//B3LYP/def2-QZVP for 35 carbon-group 13 systems with 16-ve, i.e., CXaYbZcKd (X, Y, Z, K = Al/Ga/In/Tl; 0 ≤ a, b, c, d ≤ 4, a + b + c + d = 4), showed that 9 systems (CAl3Tl, CGa3Tl, CGa2Tl2, CAl2GaTl, CAl2InTl, CGa2InTl, CAlGa2Tl, CGa2InTl and CAlGaInTl) possess global minimum ptC and 2 systems (CAl3In and CAl2Tl2) have quasi-GM ptC. Except for CAl3Tl and CAl3In, all the ptCs were predicted for the first time. All these stable ptC structures have the same skeleton and can be described as the same ionic sub-structure, i.e., [A-]B+. This study not only enriches 16ve-ptC, but also directly demonstrates that utilizing an ionic strategy, non-ptC CAl4 also can be used as a template to extend the ptC family.

Embedding a Planar Hypercoordinate Carbon Atom into a [4n+2] π-System

Through delicate tuning of the electronic structure, we report herein a rational design of seventeen new putative global minimum energy structures containing a planar tetra- or pentacoordinate carbon atom embedded in an aromatic hydrocarbon. These structures are the result of replacing three consecutive hydrogen atoms of an aromatic hydrocarbon by less electronegative groups, forming a multicenter σ-bond with the planar hypercoordinate carbon atom and participating in the π-electron delocalization. This strategy that maximizes both mechanical and electronic effects through aromatic architectures can be extended to several molecular combinations to achieve new and diverse compounds containing planar hypercoordinate carbon centers.

Mono-silicon isoelectronic replacement in CAl4 : van't hoff/le bel carbon or not?

In cluster studies, the isoelectronic replacement strategy has been successfully used to introduce new elements into a known structure while maintaining the desired topology. The well-known penta-atomic 18 valence electron (ve) species C Al 4 2 - and its Al^- /Si or Al/Si⁺ isoelectronically replaced clusters CAl₃ Si^- , CAl₂ Si₂ , C AlSi 3 - , and C Si 4 2 + , all possess the same anti-van't Hoff/Le Bel skeletons, that is, nontraditional planar tetracoordinate carbon (ptC) structure. In this article, however, we found that such isoelectronic replacement between Si and Al does not work for the 16ve-CAl₄ with the traditional van't Hoff/Le Bel tetrahedral carbon (thC) and its isoelectronic derivatives CAl₃ X (X = Ga/In/Tl). At the level of CCSD(T)/def2-QZVP//B3LYP/def2-QZVP, none of the global minima of the 16ve mono-Si-containing clusters CAl₂ SiX⁺ (X = Al/Ga/In/Tl) maintains thC as the parent CAl₄ does. Instead, X = Al/Ga globally favors an unusual ptC structure that has one long C─X distance yet with significant bond index value, and X = In/Tl prefers the planar tricoordinate carbon. The frustrated formation of thC in these clusters is ascribed to the CSi bonding that prefers a planar fashion. Inclusion of chloride ion would further stabilize the ptC of CAl₂ SiAl⁺ and CAl₂ SiGa⁺ . The unexpectedly disclosed CAl₂ SiAl⁺ and CAl₂ SiGa⁺ represent the first type of 16ve-cationic ptCs with multiple bonds. © 2019 Wiley Periodicals, Inc.

CB3E2q (q = ±1): a family of “hyparene” analogues with a planar pentacoordinate carbon

Highlighted elements are the feasible E atoms of CB₃E₂^q template ppC structures, in which CB₃Mg₂⁻ is suitable for photoelectron spectroscopy.

Planar pentacoordinate carbon atoms embedded in a metallocene framework

Viable planar pentacoordinate carbon (ppC) systems with a ppC bonded to a transition metal and embedded in a metallocene framework are reported. Our detailed global minima search shows that CAl₄MX₂ (M = Zr and Hf; X = F-I and C₅H₅) clusters with ppCs are appropriate candidates for experimental realization in the gas phase. The fulfillment of the 18 electron rule and electron delocalization is found to be crucial for the stabilization of these ppC arrangements.