Facile cobalt(iii) template synthesis of novel branched hexadentate polyamine monocarboxylates

Abstract

New hexadentate polyamine monocarboxylate ligands, 11-amino-9-(2-aminoethyl)-3,6,9-triazaundecanoate (tren-engly⁻), 12-amino-10-(2-aminoethyl)-3,7,10-triazadodecanoate (tren-tngly⁻) and 13-amino-11-(2-aminoethyl)-3,8,11-triazatridecanoate (tren-bngly⁻), were synthesized by intramolecular coupling of tetradentate tris(2-aminoethyl)amine (tren) and didentate N-(ω-formylalkyl)glycinates, OCH(CH₂)_nNHCH₂CO₂⁻, in easily and stereoselectively assembled cobalt(III) templates, p-[Co(tren){(RO)₂CH(CH₂)_nNHCH₂CO₂}](O₃SCF₃)₂, n = 1–3 (R = Me or Et). The reaction sequences comprised assembly of the template from [Co(tren)(O₃SCF₃)₂]O₃SCF₃ (1) and (RO)₂CH(CH₂)_nNHCH₂CO₂Et, deprotection of the pendant acetal in acid, intramolecular condensation of the resulting aldehyde with a coordinated primary amine at intermediate pH to form the imine and reduction of this by NaBH₄. For n = 1, imine formation occurred exclusively at the primary amine trans to the carboxylate producing the hexadentate 11-amino-9-(2-aminoethyl)-3,6,9-triazaundeca-5-enoato (tren-enimgly⁻) complex, i-[Co(tren-enimgly)]Cl₂·3.5H₂O. In all instances, subsequent imine reduction gave the s isomer complex, exclusively. Complexes p-[Co(tren){(MeO)₂CHCH₂gly}](O₃SCF₃)₂ (3), i-[Co(tren-enimgly)]ZnCl₄·H₂O (5), s-[Co(tren-engly)]ZnCl₄ (sss-6), s-[Co(tren-tngly)]ZnCl₄·H₂O (ss-7) and s-[Co(tren-bngly)ZnCl₃]₂ZnCl₄ (ss-8) were structurally characterized by X-ray crystallography. Charcoal-catalyzed equilibration of s-[Co(tren-engly)]Cl₂·2H₂O dissolved in water produced the s- (sss-6), p- (pp-6) and t-[Co(tren-engly)]²⁺ (tt-6) isomers in comparable amounts. pp-6 and tt-6 were also structurally characterized as their tetrachlorozincate and chloride salts, respectively. In base-catalyzed reactions, sss-6 and tt-6 each also formed pp-6. Reduction of s-[Co(tren-engly)]Cl₂·2H₂O with (NH₄)₂S and acidification liberated the pentaamino carboxylic acid ligand which was isolated as the hydrochloride salt.