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

Dalton Transactions Pub Date: 2004-12-21 DOI: 10.1039/B416994H

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.

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