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Barfod R, Eriksen J, Golding BT, Hammershoi A, Jacobsen TA, Langkilde A, Larsen S, Monsted O, Sargeson AM, Sorensen HO: Facile cobalt (III) template synthesis of novel branched hexadentate polyamine monocarboxylates. Dalton Trans. 2005 Feb 7;(3):491-500. Epub 2004 Dec 21.
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-([small omega]-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)] ZnCl (4)(s-6), s-[Co (tren-tngly)] ZnCl4.H2O (s-7) and s-[Co (tren-bngly) ZnCl3] 2ZnCl4 (s-8) were structurally characterized by X-ray crystallography. Charcoal-catalyzed equilibration of s-[Co (tren-engly)] Cl (2).2H (2) O dissolved in water produced the s- (s-6), p- (p-6) and t-[Co (tren-engly)] 2+ (t-6) isomers in comparable amounts. p-6 and t-6 were also structurally characterized as their tetrachlorozincate and chloride salts, respectively. In base-catalyzed reactions, s-6 and t-6 each also formed p-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. |
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