Steric Effect of Phosphorus Ligands on Fe(CO)₄(P donor) and Ru₃(CO)₉(P donor)₃ Photocatalyzed 1-Pentene Isomerization

Photoexcitation at 355 nm of [Fe(CO)₄L] and [Ru₃(CO)₉] (L = CO, PPh₃, P(OMe)₃, and P(O-o-tolyl)₃) 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 [Ru₃(CO)₁₂] and [Ru₃(CO)₉(P(OMe)₃)₃] 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 trans- to cis-2-pentene. Thus, sterically crowded complexes give enrichment of the less thermodynamically stable alkene. Similarly, beginning with cis-2-pentene, [Fe(CO)₃(PPh₃)₂] gives a much smaller ratio of trans-2-pentene to 1-pentene at low conversion than does [Fe(CO)₅] 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 dissociative loss of CO with a quantum yield of −0.4, while for [Ru₃(CO)₉L₃] the primary photoprocess is proposed to be rupture of a metal-metal bond.