Stereospecific Formation of Pentaamine Complexes of Co(III) with Terdentate 2,6‐Bis(aminomethyl)pyridine

Abstract

SummarySome pentaamine complexes of Co (111) with 2,6-bis (aminomethy1)pyridine (bamp), a diamine ligand (or two ammonia ligands) and one unidentate ligand have been prepared (Table 1). In all these species, bamp remains coordinated meridionally under a variety of conditions as shown by 'H-and 13C-NMR. spectroscopy and correlations by stereoretentive reaction cycles. The rates of amine proton exchange and of spontaneous aquation, Hg2+-induced aquation and base hydrolysis of some chloropentaamine complexes have been determined. They essentially follow the patterns observed for complexes with purely aliphatic amine ligands; the presence of a pyridine donor in these complexes does not suggest deviations from the mechanistic schemes usually proposed for the solvolytic reactions investigated. Introduction.For stereochemical work in progress we require a terdentate nitrogen donor which exclusively coordinates in a meridional arrangement at an octahedral Co(II1) centre. Such a ligand must have a rigid backbone to prevent it from occupying facial positions. We further require that the chain of atoms containing the nitrogen donor atoms be as closely as possible in one plane and that further atoms of this ligand be at least arranged pairwise in symmetrical positions relative to that plane. For our purpose, the donor atom set should be as closely related as possible to that of a purely aliphatic linear-chain triamine, viz. diethylenetriamine. We therefore did not consider ligands like 2,2', 6', 2"-terpyridine [ 11 or any of the numerous terdentate Schiff s base-type ligands known.However, our structural requirements appear not to be met in any of the reasonably accessible purely aliphatic triamines. As a compromise, we have chosen to use 2,6-bis (aminomethy1)pyridine (I,= bamp) in a study of Co (111) pentaamine complexes.