HOMOGENEOUS AND CHARCOAL-CATALYZED ISOMERIZATIONS OF (DIETHYLENETRIAMINE)(METHYLDIETHYLENETRIAMINE)COBALT(III) AND BIS(DIETHYLENETRIAMINE)COBALT(III) IONS

Each of the s-fac and u-fac isomers of [Co(dien)(medien)](C104)3 isomerizes in homogeneous aqueous solution to the same equilibrium mixture of the three geometric isomers, with no other products detectable. Proportions after 2 days at 90 °C were s-fac:u-fac:mer=81:10:9. This reaction is the basis for obtaining on a preparative scale the new u-fac and mer isomers which have been characterized by 13C NMR spectra. Over catalytic charcoal disproportionation occurs to a mixture of seven complexes which can be mostly separated on SP-Sephadex C-25 cation-exchange resin. Equilibrium molar proportions after 4 h at 80 °C were Co(dien)23+s-fac:u-fac:mer:Co(dien)(medien)3+s-fac:u-fac:Co(medien)23+s-fac=5:~23:19 [which includes a little nonseparable mer-Co(dien)(medien)3+]:~23:4:26. The charcoal reactions must involve complete dissociations of ligands, and by comparison the homogeneous reactions must involve intramolecular processes. In the homogeneous isomerizations at least two rearrangment processes must be involved, and probably three rearrangements occur. Similar experimental results have been obtained for the catalyzed and uncatalyzed reactions of the three isomers of Co(dien)23+. All the homogneous reactions are base-catalyzed and probably involve conjugate-base intermediates (deprotonated at the secondary amine nitrogens of coordinated dien). All the experimental data for the homogeneous reactions are most satisfactorily accommodated by the proposal of transient five-coordinated intermediates, formed by bond rupture, rearranging. The possibility of twisting mechanisms occurring through trigonal-prismatic transition states is also considered. Racemizations of Co(en)33+ under both charcoal-catalyzed and homogeneous conditions are also base catalyzed. © 1978, American Chemical Society. All rights reserved.