Use of a High-Pressure/Variable-Temperature Infrared Flow Cell To Examine the Reaction Kinetics of the Migratory Insertion Intermediate (η⁵-C₅H₅)Fe(CO)C(O)CH₃ by Time-Resolved Spectroscopy

Abstract

The reactive intermediate CpFe(CO)C(O)CH3 (I; Cp = η5-C5H5) was prepared by flash photolysis of CpFe(CO)2C(O)CH3 (A) in cyclohexane solutions and its reaction kinetics interrogated by time-resolved infrared spectroscopy in a flow system designed to allow equilibration of photolysis solutions at elevated temperatures and high carbon monoxide pressures. The high CO pressures (PCO ≤ 28 atm) allowed for the measurement of the previously undetermined second-order rate constant (kCO) of CO trapping of I to regenerate A, which under these conditions occurs in competition with migration of the methyl group from the carbonyl carbon to the iron (kM). These rates were also measured over the temperature range 25−45 °C, which allowed the determination of the activation parameters for methyl migration (ΔH⧧ = 44 ± 2 kJ mol-1, ΔS⧧ = −5 ± 8 J K-1 mol-1) and for reaction with CO (ΔH⧧ = 34 ± 2 kJ mol-1, ΔS⧧ = −28 ± 6 J K-1 mol-1). The mechanistic implications of these results are discussed. Details regarding the design of the high-pressure/variable-temperature TRIR flow cell are also discussed.