Direct Detection of Diphenylsilylene and Tetraphenyldisilene in Solution

Photolysis of 1,1,3,3-tetramethyl-2,2-diphenyl-1,2,3-trisilacyclohexane (9) in hydrocarbon solvents affords diphenylsilylene (SiPh2) and 1,1,2,2-tetramethyl-1,2-disilacyclopentane in nearly quantitative yields, as shown by the results of steady-state trapping experiments with methanol and acetone. SiPh2 (λmax = 290, 515 nm) can be detected directly by laser flash photolysis of 9 in hexane, where it decays over 1−2 μs with the concomitant growth of a second transient species that is assigned to the dimer, tetraphenyldisilene (Si2Ph4; λmax = 290, 370, 460 nm), on the basis of its spectrum and kinetic behavior. The transient spectroscopic behavior is similar to that observed in laser photolysis experiments with the dimethylsilylene (SiMe2) and dimesitylsilylene (SiMes2) precursors dodecamethylcyclohexasilane (1) and 2,2-dimesityl-1,1,1,3,3,3-hexamethyltrisilane (2), respectively, in the same solvent. Absolute rate constants for the reactions of the three silylenes with methanol and acetone have been measured under common conditions, providing the first quantitative glimpse of the electronic and steric effects of substituents on silylene reactivity in solution. The time-resolved spectroscopic behavior observed in laser photolysis experiments with 9 is also compared to those observed with the related, previously reported SiPh2 precursors, 1,1,3,3-tetramethyl-2,2-diphenyl-1,2,3-trisilacycloheptane (6) and 1,1,1,3,3,3-hexamethyl-2,2-diphenyltrisilane (4), which are also briefly reinvestigated. The results show that the relatively weak absorptions due to SiPh2 can in fact be detected from the latter compound, superimposed on the much more intense, long-lived transient absorptions due to the conjugated silene derivative that is known to be formed as a coproduct in the photolysis of this compound in solution at room temperature.