Interaction of lead(II) with highly-dentate linear and cyclic polyamines

Abstract

The interaction of Pb²⁺ with the macrocyclic amines 1,4,7,10,13,16-hexaazacyclooctadecane (L¹), 1,4,7,10,13,16,19-heptaazacyclohenicosane (L²), 1,4,7,10,13,16,19,22-octaazacyclotetracosane (L³), 1,4,7,10,13,16,19,22,25-nonaazacycloheptacosane (L⁴), 1,4,7,10,13,16,19,22,25,28-decaazacyclotriacontane (L⁵), 1,4,7,10,13,16,19,22,25,28,31-undecaazacyclotritriacontane (L⁶) and 1,4,7,10,13,16,19,22,25,28,31,34-dodecaazacyclohexatriacontane (L⁷) belonging to the [3k]aneN_g series and with their related open-chain, terminally methylated polyamines 1,14-bis(methylamino)-3,6,9,12-tetraazatetra-decane (L⁸), 1,17-bis(methylamino)-3,6,9,12,15-pentaazaheptadecane (L⁹), 1,20-bis(methylamino)-3,6,9,12,15,18-hexaazaicosane (L¹⁰) and 1,23-bis(methylamino)-3,6,9,12,15,18,21-heptaazatricosane (L¹¹) has been studied by potentiometry at 298.1 K in 0.15 mol dm^–3 NaClO₄. The cyclic amines L¹ and L² form only mononuclear complexes, while L³–L⁵ can form both mono- and bi-nuclear species. The largest amines of this series L⁶ and L⁷ form bi- and tri-nuclear lead(II) complexes. Of the open-chain ligands, L⁸ forms only mononuclear complexes, while L⁹–L¹¹ form both mono- and bi-nuclear species. The crystal structure of the solid compound [Pb₂L₅][ClO₄]₄ has been solved by X-ray diffraction analysis [space group P2₁/n, Z= 2, a= 9.052(9), b= 11.400(9), c= 18.689(8)Å, β= 94.42(6)°, R= 0.082 and R′= 0.068]. It shows that both Pb²⁺ ions are tightly co-ordinated to four nitrogens of the macrocycle (Pb–N ranging from 2.27 to 2.71 Å). A fifth nitrogen and three oxygens of two perchlorate anions are weakly interacting at much larger distances (Pb–N 3.04; Pb–O 2.98, 3.38, 3.41 Å). The complex cation is situated around a crystallographic symmetry centre. The arrangement of the eight atoms around the lead ion is rather asymmetric leaving a zone free from co-ordinated donor atoms which is believed to be occupied by a lone pair of electrons.