Organometallics, 18, 5643-5652 (1999).
© 1999 American Chemical Society
Nicholas P. Toltl, Mark J. Stradiotto, Tracy L. Morkin and William J. Leigh*
Contribution from the Department of Chemistry, McMaster University, Hamilton, ON Canada L8S 4M1
Abstract: Absolute rate constants for the head-to-tail [2+2]-dimerization of 1,1-diphenylsilene and 1,1-diphenylgermene have been determined in hexane and isooctane solution at 23 oC by laser flash photolysis, using the corresponding 1,1-diphenylmetallacyclobutanes as precursors. This requires knowledge of the molar extinction coefficients at the UV absorption maxima of the two transient species, which have been determined by a transient actinometric procedure. The rate constants for dimerization of the two compounds are similar, and within a factor of about two of the diffusional rate constant in both cases. The 1,3-dimetallacyclobutane dimerization products have been prepared by direct photolysis of the corresponding 1,1-diphenylmetallacyclobutanes in dry isooctane. In addition, 1,1,3,3-tetraphenyl-1-germa-3-silacyclobutane has been prepared by photolysis of a 1:1 mixture of the two metallacyclobutanes. The solid state structures of the three 1,3-dimetallacyclobutanes have been determined by single crystal X-ray crystallography, and their photochemistry has been studied. Laser flash and/or steady state photolysis experiments indicate that all three compounds cyclorevert to the corresponding 1,1-diphenylmetallaenes upon photolysis in hydrocarbon solvents, most likely via the same (·M-C-M'-C·) 1,4-biradical intermediates which link the metallaenes with their corresponding dimers via the stepwise dimerization mechanism. The quantum yield for photocycloreversion of the digermacyclobutane is roughly one-fourth that of 1,1-diphenylgermacyclobutane. However, it is about three times higher than that for the silagermacyclobutane and roughly fifteen times higher than that for the disilacyclobutane. Singlet lifetimes have been determined for the metallacyclobutanes and the disila- and digermacyclobutanes using single photon counting techniques. The implications of these results for the mechanisms of the head-to-tail [2+2]-dimerization of silenes and germenes are discussed.
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