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

he 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-triazacyclo-nonane ([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 ZnI 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 CuII complexes increase on going from L1 to L2 and then decrease on going from L2 to L3 and L4, those for ZnII complexes increase from L 1 to L3 and then decrease for L4. The X-ray crystal structures of the complexes [Cu(L1)(Br)]Br, [Zn(L 1)(NO3)]NO3, [Cu(L2)](ClO 4)2, [Ni(L2)(MeCN)](BF4)2, [Zn(L 4)](BF4)2·MeCN and [Mn(L4)](NO 3)2-·1/2H2O have been determined. In both [CuCL1)(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)](BF 4)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 NiII in the latter. In both [Zn(L4)](BF4)2-·MeCN and [Mn(L4)] (NO3)2-·1/2H2O 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 CuII 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 CuII complex [Cu(L 6)](BF4)2 where one of the three propylamino pendant arms of the starting ligand has been cleaved to give L6.