Reversible complexation of ethylene by a silylene under ambient conditions

Treatment of toluene solutions of the silylenes Si(SAr(Me6))2 (Ar(Me6) = C6H3-2,6(C6H2-2,4,6-Me3)2, 1) or Si(SAr(Pr(i)4))2 (Ar(Pr(i)4) = C6H3-2,6(C6H3-2,6-Pr(i)2)2, 2) with excess ethylene gas affords the siliranes (Ar(Me6)S)2SiCH2CH2 (3) or (Ar(Pr(i)4)S)2SiCH2CH2 (4). Silirane 4 evolves ethylene spontaneously at room temperature in toluene solution. A Van't Hoff analysis by variable-temperature (1)H NMR spectroscopy showed that ΔG(assn) = -24.9(2.5) kJ mol(-1) for 4. A computational study of the reaction mechanism using a model silylene Si(SPh)2 (Ph = C6H5) was in harmony with the Van't Hoff analysis, yielding ΔG(assn) = -24 kJ mol(-1) and an activation energy ΔG(‡) = 54 kJ mol(-1).