Kinetics of substitution reactions of trans-[Mo(N2)2(PPh2Me)4] with tripodal phosphines

Abstract

The kinetics of substitution reactions of trans-[Mo(N2)2(PPh2Me)4]1 with the tripodal ligands tris(2-diphenylphosphinoethyl)phosphine and tris(2-diphenylphosphinoethyl)amine has been studied in benzene solution at different temperatures. The absorbance vs. time profiles obtained at 25.0 °C are very complicated because of the existence of four consecutive steps. Although the curves can be fitted by consecutive exponentials, the values of the rate constants are very similar, which hinders a detailed interpretation of the data. The reactions have also been monitored by 31P-{1H} NMR spectroscopy to determine the order of occurrence of the different steps and to propose reasonable structures for the reaction intermediates. The proposed mechanism involves initial dissociation of PPh2Me to form an intermediate which is also detected upon decomposition of 1. This then reacts through two parallel pathways leading to complete decomposition of 1 and to the formation of reaction products with the tripodal phosphines. The second pathway goes through the formation of several intermediates containing one or two molecules of tripodal phosphine acting as mono-, bi-, tri- or tetra-dentate ligands.