The photochemical behaviour of matrix-isolated W(CO) 6 in the presence of alkynes

Abstract

The outcome of UV photolysis of W(CO) 6 molecules isolated in a solid CH 4 or Ar matrix in the presence of an alkyne RCCR′ (R,R′H,H; Me,H; or Me, Me) at ca. 20 K has been monitored by recording the IR and UV-vis absorption spectra of the deposit. The results indicate that irradiation with light having wavelengths near 313 nm results in the formation of the ( η 2-alkyne)tungsten pentacarbonyl complex, [( η 2-RCCR′)W(CO) 5], in addition to [X … W(CO) 5] (X  CH 4 or Ar), and free CO. In the case of a terminal alkyne, e.g. HCCH or MeCCH, however, photolysis under these conditions gives, in addition to the η 2-alkyne complex and in significantly greater yield, what appears to be the vinylidene derivative [R(H)CCW(CO) 5] (RH or Me). Prolonged irradiation with UV light ( λ = ca. 313 or 369 nm) causes the various pentacarbonyl complexes to lose CO with the formation of tetracarbonyl species; prominent among the products issuing from the W(CO) 5/C 2H 2 system are [( η 2-HCCH)( η 1-H 2CC)W(CO) 4] (in C 2H 2-rich matrices) and [( η 2-HCCH)W(CO) 4] in which the alkyne is acting formally as a 4e ligand. The results are significant in relation to the likely intermediate stages in the photocatalytic polymerization of terminal alkynes induced by tungsten hexacarbonyl.