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

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
X2EC2R2 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 X2E fragment (E = Si, Ge). At the same time, the results of MO and
magnetic descriptor calculations testify against the aromaticity of
group 4 cyclopropenes Cp2MC2R2 (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.