Aromaticity of 1-Heterocyclopropenes Containing an Atom of Group 14 or 4

Abstract

The aromaticity of three-membered monoheterocycles containing a group 14 or 4 element was investigated by using modern aromaticity criteria: structural, electronic (FBO), optical (vibrational spectroscopy), energetic (ISE), and magnetic (NICS-scan, GIMIC). The results of all methods used show that the group 14 compounds are aromatic, with cyclopropene exhibiting σ aromaticity, whereas sila-and germacyclopropenes X 2 EC 2 R 2 exhibit pseudo-π aromaticity due to the interaction of the CC π bond orbital with the low-lying vacant antibonding σ* orbital of the X 2 E fragment (E = Si, Ge). At the same time, the results of MO and magnetic descriptor calculations testify against the aromaticity of group 4 cyclopropenes Cp 2 MC 2 R 2 (M = Ti, Zr, Hf). Thus, a fundamental dissimilarity in bonding schemes and electronic structure was disclosed between 1-heterocyclopropenes containing E = C, Si, Ge atoms and those containing M = Ti, Zr, Hf atoms, the latter molecules being in essence π complexes of acetylene.

Latest News: Reactions of Group 4 Bis(trimethylsilyl)acetylene Metallocene Complexes and Applications of the Obtained Products

“…Very recently, further calculations of such compounds were presented in a paper by Aysin, Leites and Bukalov for a better general understanding of the bonding in 1-heterocyclopropenes regarding the "Aromaticity of 1 Heterocyclopropenes Containing an Atom of Group 14 or 4". [3] In addition to the earlier published results of calculations from the authors for the group 4 metallocene alkyne complexes, structural, electronic (FBO), optical (vibrational spectroscopy), energetic (ISE), and magnetic (NICS scan, GIMIC) criteria for aromaticity were discussed. The obtained results characterize the hetero-cyclopropenes of Si and Ge with aromatic properties as σ* or pseudo-π, whereas all-C-cyclopropenes are σ-aromatic.…”

Update for Reactions of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes: A Never-Ending Story?

“…The low-valent metal could reduce the alkyne ligand, or the metallocyclopropene could serve as the origin of electrons. Details for both views of bonding in these complexes are given present in refs and .…”

“…Very recently another study was presented by Leites and coworkers, investigating the aromaticity of three-membered monoheterocycles of group 14 and 4 elements by using modern aromaticity criteria such as structural, electronic (FBO), optical (vibrational spectroscopy), energetic (ISE), and magnetic properties (NICS-scan, GIMIC). 8 The results of all these methods show that the group 14 compounds are aromatic with pseudo-π aromaticity due to the interaction of the CC π bond orbital with the low-lying vacant antibonding σ* orbital of the X 2 E fragment for the element E Si, Ge. The results of MO and magnetic descriptor calculations describe no aromaticity of group 4 cyclopropenes Cp 2 M(η 2 -RC 2 R) of the metals M = Ti, Zr, Hf.…”

Aromaticity of 5- and 7-Membered Metallacyclocumulenes of Group 4

“…The structures 1 and 2 and related compounds 3 and 4 were studied earlier using various computational − and experimental methods. − Based on the bond distances, electron density distributions, vibrational spectra, and reactivity, cyclocumulenes 1 and 2 are usually considered by means of two resonance forms. − ,, For 1 , it is M-cyclocumulene and diacetylene complex (Scheme ), and for 2 , it is M-cyclocumulene and M-cyclopentadiene with a coordinated lateral alkyne branch (Scheme ). It is well known that frontier canonical molecular orbitals (CMOs) of the Cp 2 M moiety, , which are essentially d-orbitals of the metal, are located in the bisecting plane.…”

“…It is well known that frontier canonical molecular orbitals (CMOs) of the Cp 2 M moiety, , which are essentially d-orbitals of the metal, are located in the bisecting plane. For this reason, no additional π-interaction occurs in cyclopropenes 3 . In the case of 1 and 2 cumulenes, the bonding pattern is quite opposite since some π-orbitals of the ligand (π || ) are located in the plane of the metallacycles, and thus the overlapping between π || –orbitals and the boundary CMO of Cp 2 M can be symmetrically allowed.…”