Persistent Planar Tetracoordinate Carbon in Global Minima Structures of Silicon-Carbon Clusters

Abstract

Recently, we reported a series of global minima whose structures consist of carbon rings decorated with heavier group 14 elements. Interestingly, these structures feature planar tetracoordinate carbons (ptCs) and result from the replacement of five or six protons (H+) from the cyclopentadienyl anion (C5H5−) or the pentalene dianion (C8H62−) by three or four E2+ dications (E = Si–Pb), respectively. The silicon derivatives of these series are the Si3C5 and Si4C8 clusters. Here we show that ptC persists in some clusters with an equivalent number of C and Si atoms, i.e., Si5C5, Si8C8, and Si9C9. In all these species, the ptC is embedded in a pentagonal C5 ring and participates in a three-center, two-electron (3c-2e) Si-ptC-Si σ-bond. Furthermore, these clusters are π-aromatic species according to chemical bonding analysis and magnetic criteria.

Si6C18: A bispentalene derivative with two planar tetracoordinate carbons

“…The design rule to be challenged in this work was tested in aromatic hydrocarbons with pentagonal rings [4,5,22,29,71]. The C 18 H 10 polycycle system is globally non-aromatic and locally antiaromatic.…”

“…On the left is a representation of how ptCs are formed by replacing 3 H + with 2 Si 2+ on a C 5 ring [4,22,29]. On the right is the bispentalene derivative used as a molecular template in this work.…”

“…A striking feature of these clusters is that they are globally π‐aromatic and possess three‐center, two‐electron (3c‐2e) E‐ptC‐E σ‐bonds (local σ‐aromaticity). More recently, we have shown that whenever there is the possibility of forming a C 5 ring bridged with at least two Si in small CSi clusters (Si n C n ; n = 5, 8, and 9), aromaticity and ptCs in the GM structures are evidenced (see Scheme 1) [29]. Besides, there are other aromatic hydrocarbon derivatives with ptCs; however, all stem from small cyclic hydrocarbons [4, 22, 30, 31].…”

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

“…It is important to note that AUTOMATON is a program that uses a probabilistic cellular automated method to generate the initial population, which then evolves through genetic operations. This method has been designed to identify the global minimum, assessing its efficiency in different systems, 41 with recent application to design global minima with planar hypercoordinate carbons, [49][50][51][52] organic dications, 53 deltahedral Zintl clusters, 54 among others. 55,56 Our initial search yielded 1450 different geometries.…”

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

“…C 15 H 9 3− was designed from 1 ([6]radialenes), see Scheme 1 (left), searching for a template with three pentagonal aromatic rings suitable to stabilize ptCs. 19,25–27 Note that even in very small systems, the group 14 elements favors ptCs. 28–30…”