Real Time Infrared Spectroscopic Probe of the Reactions of Fe(CO)₃ and Fe(CO)₄ with N₂ in the Gas Phase

Abstract

Time-resolved infrared absorption spectroscopy has been used to study the gas-phase reactions of Fe(CO)3, Fe(CO)3N2, and Fe(CO)4 with N2, where Fe(CO)3 is generated by 308 nm laser photolysis of Fe(CO)5. The heretofore unknown complex Fe(CO)3(N2)2 forms by addition of N2 to Fe(CO)3N2 with a rate constant of (5.4 ± 1.8) × 10-16 cc molecule-1 s-1. This rate constant is much smaller than is typical for the addition of small ligands to coordinately unsaturated metal carbonyls, and data are consistent with this reaction being activated. The bond dissociation energy (BDE) for the loss of a N2 ligand from Fe(CO)4N2 is 17.6 ± 1.8 kcal mol-1. The activation energy for the loss of N2 from Fe(CO)3(N2)2 is 14.1 ± 5.2 kcal mol-1. The kinetics of the system are consistent with a model that involves equilibria between Fe(CO)3, Fe(CO)3N2, and Fe(CO)3(N2)2 as well as reactions of coordinatively unsaturated species with Fe(CO)5. Using this kinetic model, an upper limit for the BDE for the Fe−N2 bond in Fe(CO)3N2 has been estimated and the BDE for the Fe−N2 bond in Fe(CO)3(N2)2 has been determined under the assumption that one of the relevant reactions has a minimal activation energy.