Energetics of the Reactions of (η6-C6H6)Cr(CO)3 with n-Heptane, N2, and H2 Studied by High-Pressure Photoacoustic Calorimetry

The enthalpy changes associated with the photosubstitution of (η6-C6H6)Cr(CO)3 by n-heptane, H2, and N2 have been measured by high-pressure photoacoustic calorimetry (PAC). The kinetics of the reaction of (η6-C6H6)Cr(CO)3 to form (η6-C6H6)Cr(CO)2L (L = H2 and N2) under high pressures of H2 and N2 have been monitored by time-resolved infrared spectroscopy (TRIR), showing that the increased concentration of ligand at high pressure greatly accelerates the reaction, making it possible to use the PAC technique to study these reactions. PAC experiments in n-heptane show the enthalpy change for the reaction of (η6-C6H6)Cr(CO)3 + H2 → (η6-C6H6)Cr(CO)2(η2-H2) + CO to be +91 ± 4 kJ mol-1. The PAC data indicate that the strength of the Cr−L bond in (η6-C6H6)Cr(CO)2L (L = H2 and N2) is similar to that in the corresponding Cr(CO)5L complexes. We estimate the Cr−H2 bond dissociation enthalpy (BDE) in (η6-C6H6)Cr(CO)2(η2-H2) to be 60 ± 4 kJ mol-1 and the Cr−N2 BDE to be 6 kJ mol-1 stronger (66 ± 4 kJ mol-1). The thermal equilibrium between (η6-C6H6)Cr(CO)2(η2-H2) and (η6-C6H6)Cr(CO)2(N2) was studied by FTIR in a polyethylene matrix pressurized with a mixture of H2 and N2 (130 bar). Conversion of (η6-C6H6)Cr(CO)2(N2) to (η6-C6H6)Cr(CO)2(η2-H2) was achieved by using a high pressure of H2 at 22 °C.