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

The reactivity of the multifunctional cyclic silylene 4 and its carbene complex 5 have been investigated by a combination of muon spin spectroscopy and computation. The free radicals formed by muonium (Mu) addition to 4 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 5 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 4a and 5a. Each contains a muoniated methyl group, -CH(2)Mu, which undergoes restricted rotation with respect to the plane of the ring. For 4 the second product is readily assigned as the muoniated silyl radical 4b, on the grounds of its high muon hyperfine constant (716 MHz). The second product from 5 shows instead a very small coupling constant, 19 MHz, assignable to the muoniated complex 5b, in which the spin density has been transferred from the silicon to the carbenic carbon.