Steric changes at labelled NH2 sites during the base-catalysed ammoniation of the trans-azido(dimethyl sulphoxide)bis(ethylenediamine)cobalt(III) ion

Abstract

A quantitative evaluation of the acid dependence of the kinetics of ammoniation of trans-[CoN₃(en)₂(dmso)][ClO₄]₂(en = ethylenediamine, dmso = dimethyl sulphoxide) to give trans(70%)-and cis(30%)-[CoN₃(en)₂(NH₃)][ClO₄]₂ in liquid ammonia is presented. The reaction follows the normal conjugate-base mechanism, consisting of a fast pre-equilibrium, followed by the rate-determining elimination of dmso. Independent methods are presented for the assignment of the ¹H n.m.r. resonances of co-ordinated ethylenediamine in the trans complexes. The large difference in exchange rates of the protons at either side (near dmso or N₃) of the Co(en)₂‘plane’ make it possible to study the ammoniation reaction in liquid N²H₃ of a complex selectively deuteriated at the N₃-side. In this complex the remaining hydrogens function as ¹H n.m.r. labels. In the trans-[CoN₃(en)₂(NH₃)]²⁺ reaction product the position of the labels indicates inversion at the active amido-site and rotation of the chelate ring not involved in acid dissociation during the ammoniation. The results lead to a proposal for the stereochemistry of the five-co-ordinate intermediate involving N-donor π bonding.