Dual Reactivity of a Stable Zwitterionic N-Heterocyclic Silylene and Its Carbene Complex Probed with Muonium

Abstract

The reactivity of the multifunctional cyclic silylene <b>4</b> and its carbene complex <b>5</b> have been investigated by a combination of muon spin spectroscopy and computation. The free radicals formed by muonium (Mu) addition to <b>4</b> were identified, showing that there are two dominant sites of free radical attack: on the Si atom and on the exocyclic methylene carbon. Reaction of muonium with <b>5</b> also produced two radicals, but with markedly different hyperfine constants. For both compounds avoided level-crossing resonance spectra and calculation of hyperfine constants show that one of the radicals results from Mu addition to the methylene group, yielding radicals <b>4a</b> and <b>5a</b>. Each contains a muoniated methyl group, −CH₂Mu, which undergoes restricted rotation with respect to the plane of the ring. For <b>4</b> the second product is readily assigned as the muoniated silyl radical <b>4b</b>, on the grounds of its high muon hyperfine constant (716 MHz). The second product from <b>5</b> shows instead a very small coupling constant, 19 MHz, assignable to the muoniated complex <b>5b</b>, in which the spin density has been transferred from the silicon to the carbenic carbon