DIRECT DETECTION OF DIPHENYLSTANNYLENE (SnPh2) AND STUDIES OF ITS REACTIVITY IN SOLUTION BY LASER FLASH PHOTOLYSIS METHODS

While the reactivities of several transient silylenes and germylenes (the heavy homologues of singlet carbenes) have been characterized in detail in the gas phase and in solution, the chemistry of the corresponding tin derivatives (“stannylenes”) has remained virtually unexplored. To this end, the photochemistry of 3,4-dimethyl-1,1-diphenyl-1-stannacyclopent-3-ene (2) has been studied through steady state and laser flash photolysis techniques with the goal of detecting and studying diphenylstannylene (SnPh2, 1). Laser flash photolysis of 2 affords a promptly-formed transient exhibiting λmax = 290 and 500 nm, which can be assigned to 1. The species decays with clean second order decay kinetics (2k/500 = (4.7 ± 0.4) × 10 cm s), forming a much longer-lived transient exhibiting a broad absorption band centered at λmax ≈ 270 nm. The latter is assigned to the corresponding cyclotristannane, (SnPh2)3; the expected intermediate in this process, tetraphenyldistannene (Sn2Ph4), cannot be detected. The reactions of 1 with representative oxygen-, sulphur-, and nitrogencontaining substrates proceed reversibly, exhibiting forward rate and equilibrium constants similar to those of the germanium homologue, GePh2. These reactions afford new transient species absorbing at shorter wavelengths (λmax = 340 370 nm), which are assigned to the corresponding Lewis acid-base complexes. The reactions of 1 with C-C unsaturated compounds also proceed reversibly, affording transient products exhibiting absorption maxima in the 380-410 nm range which are assigned to the corresponding π-complexes. Similarly, equilibrium type behaviour is observed with n-Bu3SnH, affording a new species (λmax = 430 nm) that is assigned to the hydrido-bridged complex. The reaction of 1 with tin halides proceeds with characteristics consistent with irreversible scavenging. Steady state trapping experiments utilizing Me2SnCl2 results in the formation of 2,3-dimethylbutadiene, Ph2SnCl2, MePhSnCl2 and other products consistent with trapping of free SnPh2. AN ASSESSMENT OF STERIC AND ELECTRONIC EFFECTS ON THE RATE AND EQUILIBRIUM CONSTANTS FOR LEWIS ACID-BASE COMPLEXATION OF SILYLENES AND GERMYLENES WITH CHALCOGEN AND PNICTOGEN DONORS.