Silaiminyl-Silylene-Derived Unsymmetrical Diradicaloid Silacycles Isolated in Three Different Charge and Spin States

Organosilicon diradicals with singlet or triplet spin states of the two unpaired electrons are one of the foci in main group chemistry. However, to date, the reported isolable ones are still scarce owing to their high reactivity and lack of suitable synthetic methods. In this article, a series of diradical silacycles and their corresponding isomers 1,3-disila-pyrroles were facilely obtained via the reactions of a silaiminyl-silylene (1) serving as a precursor of "masked" bis-silylene, with various diarylacetylenes. The computed reaction mechanisms indicated a concomitant and energetically favored formation of both isomers through distinct paths. Theoretical studies demonstrated that the singlet diradicaloid silacycles (2) bear two unpaired electrons asymmetrically distributed in the molecular backbone and have a remarkably small singlet-triplet energy gap that results in the observable NMR and electron paramagnetic resonance signals. Moreover, the reactivity studies of the diradical silacycle were also conducted, giving rise to multiple hydrochlorinated products under the treatment of HCl and a monoradical cation (6) and a dication (7) via stepwise oxidation of AgOTf. Compounds 6 and 7 were isolated, characterized, and studied by density functional theory calculations. Interestingly, the heterocyclic compound 4 containing a CNPSi₂ ring isolated from the reaction of 1 and Ad-C≡P showed Möbius-type aromaticity with the NICS(0) value of -6.8.