Direct detection of dimethylstannylene and tetramethyldistannene in solution and the gas phase by laser flash photolysis of 1,1- dimethylstannacyclopent-3-enes

The photochemistry of 1,1-dimethyl- and 1,1,3,4-tetramethylstannacyclopent- 3-ene (4a and 4b, respectively) has been studied in the gas phase and in hexane solution by steady-state and 193-nm laser flash photolysis methods. Photolysis of the two compounds results in the formation of 1,3-butadiene (from 4a) and 2,3-dimethyl-1,3-butadiene (from 4b) as the major products, suggesting that cycloreversion to yield dimethylstannylene (SnMe₂) is the main photodecomposition pathway of these molecules. Indeed, the stannylene has been trapped as the Sn-H insertion product upon photolysis of 4a in hexane containing trimethylstannane. Flash photolysis of 4a in the gas phase affords a transient absorbing in the 450-520-nm range that is assigned to SnMe₂ by comparison of its spectrum and reactivity to those previously reported from other precursors. Flash photolysis of 4b in hexane solution affords results consistent with the initial formation of SnMe₂ (λ _max ≈ 500 nm), which decays over ∼10 μs to form tetramethyldistannene (5b; λ_max ≈ 470 nm). The distannene decays over the next ca. 50 μs to form at least two other longer-lived species, which are assigned to higher SnMe₂ oligomers. Time-dependent DFT calculations support the spectral assignments for SnMe₂ and Sn₂Me₄, and calculations examining the variation in bond dissociation energy with substituent (H, Me, and Ph) in disilenes, digermenes, and distannenes rule out the possibility that dimerization of SnMe₂ proceeds reversibly. Addition of methanol leads to reversible reaction with SnMe₂ to form a transient absorbing at λ_max ≈ 360 nm, which is assigned to the Lewis acid-base complex between SnMe ₂ and the alcohol.