Homo- and cross-[2+2]-cycloaddition of 1,1-diphenylsilene and 1,1-diphenylgermene. Absolute rate constants for dimerization and the molecular structures and photochemistry of the resulting 1,3-dimetallacyclobutanes

Absolute rate constants for the head-to-tail [2+2]-dimerization of 1,1-diph enylsilene and 1,1-diphenylgermene have been determined in hexane and isooc tane solution at 23 degrees C by laser flash photolysis, using the correspo nding 1,1-diphenylmetallacyclobutanes as precursors. This requires knowledg e of the molar extinction coefficients at the UV absorption maxima of the t wo transient species, which have been determined by a transient actinometri c procedure. The rate constants for dimerization of the two compounds are s imilar and within a factor of about 2 of the diffusional rate constant in b oth cases. The 1,3-dimetallacyclobutane dimerization products have been pre pared by direct photolysis of the corresponding 1,1-diphenylmetallacyclobut anes in dry isooctane. In addition, 1,1,3,3- tetraphenyl-1-germa-3-silacycl obutane has been prepared by photolysis of a 1:1 mixture of the two metalla cyclobutanes. The solid-state structures of the three 1,3-dimetallacyclobut anes have been determined by single-crystal X-ray crystallography, and thei r photochemistry has been studied. Laser flash and/or steady-state photolys is experiments indicate that all three compounds cyclorevert to the corresp onding 1,1-diphenylmetallaenes upon photolysis in hydrocarbon solvents, mos t likely via the same (M-.-C-M'-C-.) 1,4-biradical intermediates which link the metallaenes with their corresponding dimers via stepwise dimerization mechanism. The quantum yield for photocycloreversion of the digermacyclobut ane is roughly one-fourth that of 1,1-diphenylgermacyclobutane. However, it is about 3 times higher than that for the silagermacyclobutane and roughly 15 times higher than that for the disilacyclobutane. Singlet lifetimes hav e been determined for the metallacyclobutanes and the disila- and digermacy clobutanes using single photon counting techniques. The implications of the se results for the mechanisms of the head-to-tail [2+2]-dimerization of sil enes and germenes are discussed.