A novel nontransient acyclic iminosilylene (1), bearing a bulky super silyl group (-SitBu3) and N-heterocyclic imine ligand with a methylated backbone, was prepared and isolated. The methylated backbone is the feature of 1 that distinguishes it from the previously reported nonisolable iminosilylenes, as it prevents the intramolecular silylene center insertion into an aromatic C-C bond of an aryl substituent. Instead, 1 exhibits an intermolecular Büchner-ring-expansion-type reactivity; the silylene is capable of dearomatization of benzene and its derivatives, giving the corresponding silicon analogs of cycloheptatrienes, i.e. silepins, featuring seven-membered SiC6 rings with nearly planar geometry. The ring expansion reactions of 1 with benzene and 1,4-bis(trifluoromethyl)benzene are reversible. Similar reactions of 1 with N-heteroarenes (pyridine and DMAP) proceed more rapidly and irreversibly forming the corresponding azasilepins, also with nearly planar seven-membered SiNC5 rings. DFT calculations reveal an ambiphilic nature of 1 that allows the intermolecular aromatic C-C bond insertion to occur. Additional computational studies, which elucidate the inherent reactivity of 1, the role of the substituent effect, and reaction mechanisms behind the ring expansion transformations, are presented.