Syntheses, structures and hydrolytic properties of copper(II) complexes of asymmetrically N-functionalised 1,4,7-triazacyclononane ligands

Matthew James Belousoff, Andrew Raymond Battle, Bimbil Graham, Leone Spiccia

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Abstract

A series of new asymmetrically N-substituted derivatives of the 1,4,7-triazacyclononane (tacn) macrocycle have been prepared from the common precursor 1,4,7-triazatricyclo[5.2. 1.0(4,10)]decane: 1-ethyl-4-isopropyl-1,4,7-triazacyclononane (L1), 1-isopropyl-4-propyl-1,4,7-triazacyclononane (L2),]1-(3-aminopropyl)-4-benzyl-7-isopropyl-1,4,7-triazacyclononane (L3), 1-benzyl-4-isopropyl-1,4,7-triazacyclononane (L4) and 1,4-bis(3-aminopropyl)-7-isopropyl-1,4,7-triazacyclononane (L5). The corresponding monomeric copper(11) complexes were synthesised and were found to be of composition: [Cu(L1)Cl-2] . 1/2H(2)O (0), [Cu(L4)Cl-2] . 4H(2)O (C2), [Cu(L3)(MeCN)I(ClO4)(2) (C3), [Cu(L5)](ClO4)2 . MeCN . NaClO4 (C4) and [Cu(L2)Cl-2] . 1/2H(2)O (C5). The X-ray crystal structures of each complex revealed a distorted square-pyramidal copper(II) geometry, with the nitrogen donors on the ligands occupying 3 (C1 and C2), 4 (0) or 5 (C4) coordination sites on the Cu(II) centre. The metal complexes were tested for the ability to hydrolytically cleave phosphate esters at near physiological conditions, using the model phosphodiester, bis(p-nitrophenyl)phosphate (BNPP). The observed rate constants for BNPP cleavage followed the order k(C1), P: k(C2) > k(C5) > k(C3) > k(C4), confirming that tacn-type Cu(II) complexes efficiently accelerate phosphate ester hydrolysis by being able to bind phosphate esters and also form the nucleophile necessary to carry out intramolecular cleavage. Complexes C1 and C2, featuring asymmetrically disubstituted ligands, exhibited rate constants of the same order of magnitude as those reported for the Cu(11) complexes of symmetrically tri-N-alkylated tacn ligands (k similar to 1.5 x 10(-5) s(-1)). (c) 2006 Elsevier Ltd. All rights reserved.
Original languageEnglish
Pages (from-to)344 - 355
Number of pages12
JournalPolyhedron
Volume26
Issue number2
Publication statusPublished - 2007

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