Catalyst Section Pulsed-Laser-Initiated Photocatalysis in the Liquid Phase

We illustrate the use of pulsed-laser irradiation to promote homogeneous organometallic catalysis under mild conditions in the liquid phase. We concentrate on the Iron carbonyl catalyzed isomerization of olefins. Kinetics data on the Fe(CO)₅/1-pentene system establish the sequence of photolytic events leading from Fe(CO)₅ starting material through Fe(CO)₄(pentene) catalyst precursor to the true photocatalyst, Fe(CO)₃(pentene). Measurement of the average lifetime of the photocatalyst with respect to reversion to its inactive, precursor form, combined with a knowledge of the photocatalytic quantum yield establishes the turnover rate under our conditions, which is a fast 10⁴ s^-1. We also find that Fe(CO)₅ photocatalytic quantum yields are temperature dependent with an apparent activation energy of 2.8 kcal mol^-1. We present a mechanistic analysis of the isomerization cycle, which compares and contrasts the reactivity of group 8 coordinatively unsaturated organometallic intermediates with those of group 6.