Matrix-isolation studies of alkyne complexes in the photochemical reactions of Mo(CO) 6 with alkynes

Abstract

Photolysis of a low-temperature matrix X (X  Ar or CH 4) doped with molybdenum hexacarbonyl and an alkyne RCCR′ (HCCH, MeCCH, or MeCCMe) leads to the formation of the corresponding alkyne complex of molybdenum pentacarbonyl, [( η 2-RCCR′)Mo(CO) 5], in addition to [X … Mo(CO) 5] and CO; the products have been detected and characterised by their IR and UV-vis absorption spectra. The species [( η 2-RCCR′)Mo(CO) 5], unlike [X … Mo(CO) 5], is stable with respect to irradiation in the green region of the visible spectrum, and a scheme has been devised for the photochemical interconversion of the three species [Mo(CO) 6], [( η 2-RCCR′)Mo(CO) 5] and [X … Mo(CO) 5] in a mixed matrix composed of RCCR′ and X molecules. An η 2-alkyne complex of the type [(RCCH)Mo(CO) 5] (R  H or Me) undergoes rearrangement to give what is believed to be a vinylidene complex [R(H)CCMo(CO) 5]. Continued irradiation at λ = 313 nm causes the various pentacarbonyl complexes to lose CO with the formation of tetracarbonyl species. More significantly, irradiation at λ = 369 nm also results in photoejection of CO; this favours the formation of a product believed, on the evidence of its IR spectrum and of experiments with C 2D 2 and 12C 13CH 2, to be [( η 2-RCCR′)Mo(CO) 4] (R,R′ = H,H or Me,Me) in which the alkyne is acting formally as a 4e ligand. The IR spectra serve notice of further changes on prolonged photolysis under these conditions. The results are discussed in relation to intermediates in the catalytic polymerization of 1-alkynes.