Photochemistry of alkyltricarbonyl(η5-cyclopentadienyl)tungsten (alkyl = Et, Prn, Pri, Bun, or CH2Ph), tricarbonyl(η5-cyclopentadienyl)(phenyl)tungsten, tricarbonyl(η5-pentamethylcyclopentadienyl)(n-propyl)tungsten, and tricarbonyl(η5-cyclopentadienyl)(ethyl)-molybdenum in gas matrices at 12 K and in solutions at 243 K

Abstract

The photoreactions of [M(CO)3R(η5-C5R′5)] complexes (M = Mo or W; R = Et, Prn, Pri, Bun, Ph, or CH2Ph; R′= H or Me) have been studied in solution (–30 to 20 °C) and in gas matrices (12–30 K). In alkane solutions in the absence of ligands the alkyl complexes that contain β-hydrogens initially undergo β-photoelimination at –30 °C to give [MH(CO)2(olefin)(η5-C5R′5)] complexes of which only the trans isomers could be detected, isolated, and characterised (i.r., n.m.r., and mass spectra). Intramolecular rotation of the olefin ligands about the tungsten–olefin bond axis was observed by low-temperature (–80 °C) n.m.r. spectroscopy; asymmetric olefins gave rotamers in different proportions. Prolonged photolysis of [M(CO)3(alkyl)(η5-C5R′5)] complexes in alkane solutions gave [MH(CO)3(η5-C5R′5)] complexes and ultimately [{M(CO)3(η5-C5R′5)}2]. The dimer [{W(CO)3(η5-C5H5)}2] was the only metal-containing photoproduct when [W(CO)3Ph(η5-C5H5)] was photolysed alone in pentane at –30 °C, while for [W(CO)3(σ-CH2Ph)(η5-C5H5)] the major photoproduct was [W(CO)2(η3-CH2Ph)(η5-C5H5)]. In the presence of C2H4, the phenyl and benzyl complexes gave the new monosubstitution products [W(CO)2(C2H4)R(η5-C5H5)](R = Ph or CH2Ph) whereas the alkyl complexes all gave [MH(CO)2(C2H4)(η5-C5R′5)] as the main metal-containing product. In CH4 and CO gas matrices at 12 K the primary photolysis step was shown to be photo-ejection of a CO ligand and the formation of the 16-electron species [M(CO)2R(η5-C5R′5)](R = alkyl or aryl). The identity of the co-ordinatively unsaturated species was confirmed by 13CO-labelling in [W(CO)3Ph(η5-C5H5)] and fitting the terminal CO stretching bands using an energy-factored force-field program. For the alkyl complexes with β-hydrogens, thermal and photochemical reactions led to the conversion of [M(CO)2(alkyl)(η5-C5R′5)] species into the olefin–hydride complexes [MH(CO)2(olefin)(η5-C5R′5)]. Gas matrix studies for the W complexes at 12 K showed the presence of both cis and trans isomers together with intramolecular cis⇌trans isomerisation whereas in a previous paraffin-wax disc study of [M(CO)3R(η5-C5R′5)] complexes (M = Mo or W; R = Et or n-C5H11; R′= H or Me) at 77 K only trans isomers were observed. Photolysis of the benzyl complex, [W(CO)3(σ-CH2Ph)(η5-C5H5)], led to the formation of the η3-bonded benzyl complex, [W(CO)2(η3-CH2Ph)(η5-C5H5)] in CH4, Ar, and CO matrices. The olefin–hydride species with asymmetric olefins, [MH(CO)2(olefin)(η5-C5H5)], were found to be formed and to exist as rotamers in gas matrices at 12 K. Prolonged photolysis of the alkyl complexes resulted in the formation of [MH(CO)3(η5-C5R′5)] complexes. In the presence of ligands L (L = C2H4 or N2) the 16-electron intermediate [W(CO)2Ph(η5-C5H5)] gave addition products [W(CO)2(L)Ph(η5-C5H5)] at 12 K but no such products were observed for [W(CO)2(CH2Ph)(η5-C5H5)]. The combination of solution and matrix isolation studies established that the primary photolysis step for [M(CO)3R(η5-C5R′5)] complexes is photo-ejection of a CO ligand and that this can be followed by β-hydrogen transfer to give cis and trans isomers of [MH(CO)2(olefin)(η5-C5R′5)]. The very low temperature used in this study enabled the cis isomer to be observed for the first time.