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-diphenylsilene and
1,1-diphenylgermene have been determined in hexane and isooctane solution at 23 °C 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 2 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 stepwise dimerization
mechanism. The quantum yield for photocycloreversion of the digermacyclobutane 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 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.