A ¹H NMR study of the complex of cobalt(II) with 2,5,8,11-tetramethyl-2,5,8,11-tetraazadodecane in aerated aqueous solutions

The fully methylated open chain polyamine ligand 2,5,8,11-tetramethyl-2,5,8,11-tetraazadodecane forms a stable 1:1 complex with cobalt(II) over the pH range 8-9.5. The paramagnetic ¹H NMR spectrum of the complex exhibits a number of relatively narrow and nicely resolved isotropically shifted signals spread between 200 and -50 ppm. Saturation transfer experiments and the analysis of the temperature dependence of the ¹H NMR spectra provide information on the peculiar dynamic properties of this complex in solution; an assignment of all the paramagnetically shifted signals to methylene and methyl protons is proposed. From the analysis of the spectroscopic results it is argued that over the investigated pH range the geometry of the complex is pseudooctahedral with all the four nitrogen atoms coordinated equatorially to the metal. This complex is not oxidized by dioxygen in contrast to its unmethyled form and other cobalt(II) complexes with amine ligands.