Photochemistry of group 14 1,1,1-trimethyl-2,2,2-triphenyldimetallanes (Ph3MM ' Me-3; M, M ' = Si, Ge). Direct detection and characterization of silene and germene reactive intermediates

The photochemistry of trimethylsilyltriphenylgermane (Ph3GeSiMe3), tripheny lsilyltrimethylgermane (Ph3SiGeMe3), and 1,1,1-trimethyl-2,2,2-triphenyldig ermane (Ph3GeGeMe3) has been studied in hydrocarbon solution by steady stat e and laser flash photolysis methods and is compared to previously reported data for the homologous disilane Ph3SiSiMe3. A variety of products are for med upon photolysis of the three compounds in the presence of 2,3-dimethyl- 1,3-butadiene or chloroform, but in each case the major ones are derived fr om M-M' bond homolysis and dimethyl- or diphenylgermylene extrusion. The tr apping products of the 1,3,5-(1-metalla)hexatriene derivatives formed by [1 ,3]-MMe3 migration into the ortho-position of one of the phenyl rings are f ormed as well, in yields of 9-30%. While these experiments indicate that ge rmylenes are formed in at least twice the yield of the 1,3,5-(1-metalla)hex atrienes, only the tatter and triphenylsilyl or triphenylgermyl radicals ca n be detected by laser flash photolysis techniques. The metallaenes have be en identified on the basis of their time-resolved UV absorption spectra and absolute rate constants for reaction T-Vith 2,3-dimethylbutadiene, methano l, acetone, acetic acid, oxygen, and carbon tetrachloride and can be distin guished from germylenes by their lack of reactivity toward triethylsilane a nd chloroform. Radical formation is shown to result from reaction of the tr iplet states of these compounds, and a triplet lifetime is estimated for Ph 3GeSiMe3 and compared to that of the disilane homologue. The results of tim e-resolved experiments on other, related compounds are discussed in light o f these results.