A coordinatively flexible hexadentate ligand gives structurally isomeric complexes M2(L)X3 (M = Cu, Zn; X = Br, Cl)

Polypyridyl multidentate ligands based on ethylenediamine backbones are important metal-binding agents with applications in biomimetics and homogeneous catalysis. The seemingly hexadentate tpena ligand [systematic name: N,N,N'-tris(pyridin-2-ylmethyl)ethylenediamine-N'-acetate] reacts with zinc chloride and zinc bromide to form trichlorido[μ-N,N,N'-tris(pyridin-2-ylmethyl)ethylenediamine-N'-acetato]dizinc(II), [Zn ₂(C ₂₂H ₂₄N ₅O ₂)Cl ₃], and tribromido[μ-N,N,N'-tris(pyridin-2-ylmethyl)ethylenediamine-N'-acetato]dizinc(II), [Zn ₂Br ₃(C ₂₂H ₂₄N ₅O ₂)]. One Zn ^II ion shows the anticipated N5O coordination in an irregular six-coordinate site and is linked by an anti carboxylate bridge to a tetrahedral ZnX 3 (X = Cl or Br) unit. In contrast, the Cu ^II ions in aquatribromido[μ-N,N,N'-tris(pyridin-2-ylmethyl)ethylenediamine-N'-acetato]dicopper(II)-tribromido[μ-N,N,N'-tris(pyridin-2-ylmethyl)ethylenediamine-N'-acetato]dicopper(II)-water (1/1/6.5) [Cu ₂Br ₃(C ₂₂H ₂₄N ₅O ₂)][Cu ₂Br ₃(C ₂₂H ₂₄N ₅O ₂)(H ₂O)]·6.5H2O, occupy two tpena-chelated sites, one a trigonal bipyramidal N ₃Cl ₂ site and the other a square-planar N2OCl site. In all three cases, electrospray ionization mass spectra were dominated by a misleading ion assignable to [M(tpena)] ⁺ (M = Zn ²⁺ and Cu ²⁺).