Co-ordination chemistry of amino pendant arm derivatives of 1,4,7-triazacyclononane

Abstract

The binding properties of 1,4,7-triazacyclononane ([9]aneN₃) to metal cations can be adapted through sequential functionalisation of the secondary amines with aminoethyl or aminopropyl pendant arms to generate ligands with increasing numbers of donor atoms. The new amino functionalised pendant arm derivative of 1,4,7-triazacyclononane ([9]aneN₃), L¹, has been synthesised and its salt [H₂L¹]Cl₂ characterised by X-ray diffraction. The protonation constants of the ligands L¹–L⁴ having one, two or three aminoethyl or three aminopropyl pendant arms, respectively, on the [9]aneN₃ framework, and the thermodynamic stabilities of their mononuclear complexes with Cu^II and Zn^II have been investigated by potentiometric measurements in aqueous solutions. In order to discern the protonation sites of ligands L¹–L⁴, ¹H NMR spectroscopic studies were performed in D₂O as a function of pH. While the stability constants of the Cu^II complexes increase on going from L¹ to L² and then decrease on going from L² to L³ and L⁴, those for Zn^II complexes increase from L¹ to L³ and then decrease for L⁴. The X-ray crystal structures of the complexes [Cu(L¹)(Br)]Br, [Zn(L¹)(NO₃)]NO₃, [Cu(L²)](ClO₄)₂, [Ni(L²)(MeCN)](BF₄)₂, [Zn(L⁴)](BF₄)₂·MeCN and [Mn(L⁴)](NO₃)₂·½H₂O have been determined. In both [Cu(L¹)(Br)]Br and [Zn(L¹)(NO₃)]NO₃ the metal ion is five co-ordinate and bound by four N-donors of the macrocyclic ligand and by one of the two counter-anions. The crystal structures of [Cu(L²)](ClO₄)₂ and [Ni(L²)(MeCN)](BF₄)₂ show the metal centre in slightly distorted square-based pyramidal and octahedral geometry, respectively, with a MeCN molecule completing the co-ordination sphere around Ni^II in the latter. In both [Zn(L⁴)](BF₄)₂·MeCN and [Mn(L⁴)](NO₃)₂·½H₂O the metal ion is bound by all six N-donors of the macrocyclic ligand in a distorted octahedral geometry. Interestingly, and in agreement with the solution studies and with the marked preference of Cu^II to assume a square-based pyramidal geometry with these types of ligands, the reaction of L⁴ with one equivalent of Cu(BF₄)₂·4H₂O in MeOH at room temperature yields a square-based pyramidal five co-ordinate Cu^II complex [Cu(L⁶)](BF₄)₂ where one of the three propylamino pendant arms of the starting ligand has been cleaved to give L⁶.