Synthesis and characterisation of zinc(ii) complexes of tripodal N7 ligands involving pyridine and amine or amide nitrogen donors. Crystal structure of a zinc(ii) complex

Abstract

Two tripodal heptadentate ligands tris[2-(2-pyridylmethyl)aminoethyl]amine (tpaa), and tris[2-picolinamidoethyl]amine (trenpicam) were synthesized as their hydrochloride salts and their protonation constants determined by potentiometry; variable pH 1H NMR spectroscopy allows the determination of the order of protonation of their different basic nitrogen atoms. The stability constants of the zinc(II) chelates with these ligands, determined by potentiometry and UV spectrometry, show the formation of [Zn(LH2)]4+, [Zn(LH)]3+ and [ZnL]2+ species with tpaa, whereas amide group deprotonation of trenpicam permits the detection of the variable stoichiometry species [ZnL]2+, [ZnL2]2+, [Zn(L2H−1)]+, [Zn(L2H−3)]−, [Zn2(LH−2)2] and [Zn2(L2H−5)]−, where LH−x is an amide-deprotonated ligand. The zinc(II) complexes [ZnL](ClO4)2 (L = tpaa or trenpicam), and the dinuclear complex [Zn2(LH−2)Cl2] (L = trenpicam) have been prepared and characterized by IR and mass spectroscopy. The crystal structure of the protonated ligand [Htrenpicam](ClO4) and the complex [Zn(tpaa)](ClO4)2 are reported. [Htrenpicam]+ contains one intra-molecular H-bond and in the cation [Zn(tpaa)]2+ the geometry around the Zn2+ ion is distorted octahedral with one pyridyl nitrogen uncoordinated.