Overcoming the chelate effect: hypodentate coordination of ethylenediamine, diethylenetriamine and tris(2-aminoethyl)amine in Co(III) complexes

The syntheses and characterization of the hypodentate complexes cis-[Co(en)2(enH)(imH)]Br4, [Co(NH3)5(enH)]Cl4, [Co(NH3)5(dienH2)]Cl5, [Co(NH3)5(trenH3)]Cl6·3H2O and [Co(tren)(NH3)(trenH3)]Cl6·2H2O containing protonated monodentate ethylenediamine (en), diethylenetriamine (dien) and tris(2-aminoethyl)amine (tren) are described. The complexes were prepared by reaction of a stoichiometric or less than stoichiometric amount of the appropriate polyamine with [Co(NH3)5OSMe2]3+, cis-[Co(en)2(imH)Br]2+ or p-[Co(tren)(NH3)(OH2)]3+ under mild conditions. The crystal structures of the complexes cis-[Co(en)2(enH)(imH)]Br4 (enH=NH2CH2CH2NH3 +, imH=imidazole) and [Co(NH3)5(enH)]Cl4 are also reported. These confirm the presence of protonated monodentate ethylenediamine in both complexes, and the structure of the former shows the monodentate protonated ethylenediamine ligand in a cis relationship to the imidazole ligand. The protonated ethylenediamine ligands adopt a gauche conformation in both structurally characterized complexes. pKa values for the monodentate ethylenediamine and imidazole ligands in cis-[Co(en)2(enH)(imH)]4+ are 7.86±0.02 and 9.8±0.1, respectively.