Mechanism of formation of chelated tricarbonyliron complexes of α,β-unsaturated Schiff bases from the corresponding tetracarbonyliron complexes: factors affecting the stability of the trigonal-bipyramidal complexes

Abstract

The chelation reaction of the complexes [Fe(CO)₄{N(C₆H₄X)CH–CHCHPh}](1) to form [Fe(η-Ph–CHCH–CHN–C₆H₄X)(CO)₃](2)(X = 4-Br, 4-OMe, and H) in the presence of excess Schiff base has been studied. The kinetics are first order in the complex concentration and the inverse of the rate constantvaries linearly with the concentration of the Schiff base. The effect of CO on both the rate constants and the final reaction products has been studied. The results support a reaction mechanism similar to that proposed for the corresponding complexes of α,β-unsaturated aldehydes and ketones. The substitution reaction of (1) with PPh₃has also been investigated. It proceeds via a dissociative mechanism. The observed effect of the substituents X on the rate constants allows the conclusion that in derivatives of tetracarbonyliron having a trigonal-bipyramidal structure the bond energy at the apical position depends mainly on the σ-bond strength. while that at the equatorial position depends on the amount of π-back bonding present.