James L. Graff

Photoactivation of cluster catalysis: a comparison of 1-pentene isomerization by tetracarbonyl(triphenylphosphine)ruthenium and 1,1,1,2,2,2,3,3,3-nonacarbonyl-1,2,3-tris(triphenylphosphine)-triangulo-triruthenium

Abstract : The photocatalyzed isomerization of 1-pentene to trans- and cis-2-pentene by Ru(CO)4PPh3 and Ru3(CO)9(PPh3)3 is reported along with data for Ru3(CO)12, Fe3(CO)12, and Fe(CO)5. The primary photoprocess in Ru(CO)4PPh3 is dissociative loss of CO giving a coordinatively unsaturated species having the same empirical formula Ru3(CO)9(PPh3)3; the trinuclear species undergoes Ru-Ru bond rupture and ultimate declusterification subsequent to photo-excitation giving a quantitative yield of Ru(CO)4PPh3 under CO or Ru(CO)3(PPh3)2 in the presence of PPh3. The crucial result is that the cluster yields a different catalytically active species compared to Ru(CO)4PPh3, since the initial ratio of trans- and cis-2-pentene is different for the two photocatalysts. The photocatalysis and primary photoprocesses suggest that the isomerization from the Ru3(CO)9(PPh3)3 results from an active form of the cluster. By way of contrast, Fe(CO)5 and Fe3(CO)12 yields the same initial ratio of photocatalytic products, implicating a common, mononuclear catalytic species. Since the clusters are good visible absorbers compared to the mononuclear species, photoactivation of cluster catalysis can be effected with low energy visible light.

Photogeneration of reactive organometallic species

Abstract : A large number of thermally inert organometallic complexes can be photoactivated for stoichiometric and catalytic reactions using visible or ultraviolet light. In favorable cases, low temperature irradiation leads to the generation of spectroscopically detectable intermediates, e.g. (eta 5-C5H5)W(CO)2C2H5 or Fe(CO)eta(alkene)5-eta, that likely participate in catalytic cycles at higher temperatures where the steady state concentration is too low to detect. The dissociative process of CO or H2 loss and metal-metal bond rupture can lead to the coordinatively unsaturated intermediates that pervade homogeneous and heterogeneous catalytic systems. In some cases, the same catalyst as generated thermally at high temperature can be prepared photochemically at low temperature using the same precursor, as in the case of H4Ru4(CO)12 where loss of CO thermally or photochemically produces an active species. In other situations different catalysis results, as reflected in product ratios, when photo-excitation is used compared to thermal activation as in hydrogenation vs. isomerization using H2Os3(CO10. New spectroscopic techniques such as Fourier transform infrared photoacoustic spectroscopy will become important in the in situ analysis of heterogenizea catalyst precursors, where gas/solid interfacial photoreactions can be monitored.

Photochemistry and photocatalytic activity of H4Ru4(CO)12 and H2Ru4(CO)13 for isomerization and hydrogenation of alkenes

Abstract The tetranuclear complex H4Ru4(CO)12 undergoes dissociative loss of CO upon near-uv, 355 nm, irradiation to give substitution products H4Ru4(CO)11L (L = P(OMe)3, PPh3) in the presence of L with a quantum yield of ∼5 X 10−3. In the presence of an olefin H2Ru4(CO)13 is the principal Ru containing photoproduct and stoichiometric reduction of the olefin is observed, e.g. cyclopentene gives cyclopentane. Presumably the intermediate H4Ru4(CO)11 (olefin) gives alkane and the unsaturated H2Ru4(CO)11 that rapidly reacts with CO to form H4Ru4(CO)13. The H2Ru4(CO)13 reacts slowly with H2 to give H4Ru4(CO)12 at 25 °C. Irradiation of H2Ru4(CO)13 in the presence of an olefin such as 1-pentene accelerates the catalyzed isomerization and effects stoichiometric reduction of the olefin, e.g. n-pentane forms from 1-pentene at 25 °C under irradiation whereas no reduction occurs thermally at 25 °C.

PHOTOACTIVATION OF ORGANOMETALLIC CATALYSTS

Abstract : Photochemistry offers a technique to synthesize unique catalysts, control catalytic reactions, and perturb and better understand conventional catalytic cycles. Recent studies in the authors laboratory concerning photoinduced catalysis using mono-, di-, and trinuclear organometallic catalyst precursors are summarized. Specific systems considered here are (M(CO)nL5-n) (M = Fe, Ru; L = PPh; n = 5,4,3), (M3(CO)12) (M = Fe, Ru, Os), (Ru3(CO)g(PPh3)3), and (Co2(CO)6L2) (L = P(n-Bu)3, P(OPh)3) used to effect catalytic chemistry of 1-pentene including isomerization, hydrogenation, and hydrosilation. (Author)