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)]Br-4, [Co(NH3)(5)(enH)]Cl-4, [Co(NH3)(5)(dienH(2))]Cl-5, [Co( NH3)(5)(trenH(3))]Cl-6. 3H(2)O and [Co(tren)(NH3)(trenH(3))]Cl-6. 2H(2)O co ntaining protonated monodentate ethylenediamine (en), diethylenetriamine (d ien) and tris(2-aminoethyl)amine (tren) are described. The complexes were p repared by reaction of a stoichiometric or less than stoichiometric amount of the appropriate polyamine with [Co(NH3)(5)OSMe2](3+), cis-[Co(en)(2)(imH )Br](2+) or p-[Co(tren)(NH3)(OH2)](3+) under mild conditions. The crystal s tructures of the complexes cis-[Co(en)(2)(enH)(imH)]Br-4 (enH = NH2CH2CH2NH 3+, imH = imidazole) and [Co(NH3)(5)(enH)]Cl-4 are also reported. These con firm the presence of protonated monodentate ethylenediamine in both complex es, and the structure of the former shows the monodentate protonated ethyle nediamine ligand in a cis relationship to the imidazole ligand. The protona ted ethylenediamine ligands adopt a gauche conformation in both structurall y characterized complexes. pK(a) values for the monodentate ethylenediamine and imidazole ligands in cis-[Co(en)(2)(enH)(imH)](4+) are 7.86 +/- 0.02 a nd 9.8 +/- 0.1, respectively. (C) 2000 Elsevier Science S.A. All rights res erved.