Photochemistry of >. Formation of Alkyne-Carbene Complexes and Studies of Their Decomposition Reactions

Article abstract of DOI:10.1021/ja00348a020

The photochemistry of > has been examined and been shown to involve CO loss as the only detectable photoreaction.Irradiation in acetonitrile solution leads smoothly to the formation of > in quantitative yield.The 366 and 313-nm quantum yields for disappearance of > in CH3CN solution are 0.009 and 0.011, respectively, but the quantum yield drops to -4 at 436 nm, even though the complex absorbs strongly at the latter wavelength.The lowest intense absorption band (λ_max 402 nm) (ε_max 10560 L mol^-1 cm^-1) has been assigned as a W -> carbene(?^*) charge-transfer transition with ligand field bands lying at higher energy.The low-lying W -> carbene charge-transfer state is inactive with respect to CO loss, with the latter occuring from ligand field excited states.Low-temperature photolysis in the presence of PhC=*CPh, MeC=*CPh, MeC=*CMe, PhC=*CH, and n-BuC=*CH leads to spectroscopically observable alkyne-carbene adducts, with the diphenylacetylene complex > isolated as a crystalline solid.None of the alkyne adducts are stable at 25 deg C in solution, and they decompose to give products that depend markedly upon the nature of the alkyne.With terminal alkynes, only polyacetylenes form, whereas the PhC=*CPh and PhC=*CMe adducts decay exclusively to form 1-methoxy-2,3-diphenylindene and 1-methoxy-2-methyl-3-phenylindene, respectively.The MeC=*CMe adduct leads to both poly-2-butyne and 1-methoxy-2,3-dimethylindene.