Theoretical Study on the Stability and Aromaticity of Metallasilapentalynes

Antiaromatic compounds and small cyclic alkynes or carbynes are
both challenging for synthetic chemists because of the destabilization
caused by their antiaromaticity and highly distorted triple bonds,
respectively. These dual destabilizations could be the reason why
pentalyne (I), a highly antiaromatic and extremely strained
cyclic alkyne, has never been synthesized. Recently, we have successfully
synthesized the first metallapentalyne (II), benefiting
from the stabilization of a metal fragment by reducing the ring strain
and switching the antiaromaticity in pentalyne to the aromaticity
in metallapentalyne. An interesting question is raised: can the aromaticity
in metallasilapentalyne (III) be retained, considering
the fact that the silicon atom is reluctant to participate in π
bonding? Here we report a thorough theoretical study on the stability
and aromaticity of metallasilapentalynes. The computed energies and
magnetic properties reveal the reduced aromatic character of osmasilapentalyne
in comparison with osmapentalyne. The effect of the ligands, substituents,
and base on the aromaticity and stability of osmasilapentalyne is
also discussed, thus providing an important guide to the synthesis
of osmasilapentalyne.