Steric effect of phosphorus ligands on (P donor)iron tetracarbonyl (Fe(CO)4(P donor)) and tris(P donor)triruthenium nonacarbonyl (Ru3(CO)9(P donor)3) photocatalyzed 1-pentene isomerization

Abstract

Photoexcitation at 355 nm of [Fe(CO),L] and [Ru,(CO)&,] (L = CO, PPh,, P(OMe)3, and P(O-o-tolyl),)can be used to generate catalysts for the isomerization of 1-pentene to cis-and trans-2-pentene. Each complex gives a different initial ratio of trans-to cis-2-pentene ranging from approximately 6 for [RU~(CO),~] and [RU~(CO)~(P(OM~)J~] to approximately 1 for [Fe(CO)4P(O-o-tolyl)3]. Comparisons of the initial ratios of the isomeric products shows that steric effects, not electronic effects, of the P-donor ligands are responsible for the variation in isomer ratio. The more sterically demanding complexes give the smallest ratio of transto cis-2-pentene. Thus, sterically crowded complexes give enrichment of the less thermodynamically stable alkene. Similarly, beginning with cis-2-pentene, [Fe(C0)3(PPh3)2] gives a much smaller ratio of trans-2-pentene to 1-pentene at low conversion than does [Fe(C0)6] under 355-nm irradiation. Initial quantum yields for isomerization generally exceed unity, and the ultimate distribution of alkenes is the thermodynamic ratio, evidencing the photochemical formation of a thermally active catalyst. For [Fe(CO)4L] the only detected primary photoprocess is diesocitive loas of CO with a quantum yield of -0.4, while for [Ru,(CO)&] the primary photoprocess is proposed to be rupture of a metal-metal bond.

FTIR Studies of Iron–Carbonyl Intermediates in Allylic Alcohol Photoisomerization

“…Hexane and toluene were distilled before use and [Fe(CO) 4 PPh 3 ] was prepared according to literature methods. [12] [Fe 3 (CO) 12 ] and [Ru 3 (CO) 12 ] were obtained from Strem Chemicals, while allyl alcohol, methylallyl alcohol, and phenylallyl alcohol were from Sigma-Aldrich and used without further purification. A typical preparation for each kinetic run would be to mix the allyl alcohol (~0.01 mol) and the metal-carbonyl complex (~10 À5 mol, loading = 0.1-1.0 %) in hexane or toluene (30 mL).…”

“…In this work, we have largely focused on triiron dodecacarbonyl [Fe 3 (CO) 12 ] as the catalytic precursor, because of its much milder toxicity as well as the ease of handling the solid. Since the toxic properties of volatile [Fe(CO) 5 ] could limit its use as a versatile reagent in most organic syntheses, photoisomerization in the presence of [Fe 3 (CO) 12 ] at room temperature may offer a better alternative.…”

“…Since the toxic properties of volatile [Fe(CO) 5 ] could limit its use as a versatile reagent in most organic syntheses, photoisomerization in the presence of [Fe 3 (CO) 12 ] at room temperature may offer a better alternative. [4] [Fe 3 (CO) 12 ] is also a much less studied catalytic precursor in terms of its mechanism, as it is often assumed to behave in a similar manner to [Fe(CO) 5 ]. Another catalytic percursor, [Fe-A C H T U N G T R E N N U N G (CO) 4 PPh 3 ] has also been used in this work for comparison studies.…”

“…Another catalytic percursor, [Fe-A C H T U N G T R E N N U N G (CO) 4 PPh 3 ] has also been used in this work for comparison studies. [12] We have used pulsed laser FTIR absorption spectroscopy to monitor directly the spectral evolution of the precursor, intermediates, and final product of the photoisomerization of allyl alcohols to aldehydes during isomerization. Compared to a broadband source, the intense laser light, either in the visible (532 nm) or UV region (355 nm) could enhance the probability of detecting low concentrations of intermediate species.…”

“…Usually the area under the curve for only one band of each species (the one of least spectral interference from other species) was used for the concentration determination as well as monitoring the progress of the species. 12 ] and allyl alcohol in hexane (35 mL) was first carried out to verify that catalysis was indeed ocurring and to characterize the main transition-metal-carbonyl products formed. Figure 3 (top) shows the concentration profiles of propionaldehyde and allyl alcohol during 532 nm photoisom-A C H T U N G T R E N N U N G erization.…”