Zinc(II) and copper(II) complexes of edampda(2-): octahedral trigonal bipyramidal and in between [edampda(2-) = N,N '-bis-(pyridylmethyl)ethylenediamine-N,N '-diacetate

Zn-II and Cu-II complexes of the ligand edampda(2-) [N, N'-bis(pyridylmethy l)-ethylenediamine-N,N'-diacetate] have been studied in solution by H-1 n.m .r, and e.p.r. spectroscopies, respectively. [Zn-II(edampda)] exists in sol ution in a major octahedral isomer (ca. 83%) in which the two carboxylate d onors and two pyridylmethyl donors remain stereochemically rigid up to 333 K at pD = 6.0. The major octahedral complex has equivalent glycinato and py ridyl donors as shown by equivalent AB quartets for each type of chelate. B y contrast, the [Zn-II(edta)](2-) analogue complex is known to have process es which rapidly equilibrate the coordinated carboxylates leading to coales ced, broad singlets instead of AB quartets down to 273 K (freezing point of the sample). The minor [Zn-II(edampda)] species has one pendant pyridylmet hyl arm. The complex does not increase in abundance up to 333 K via dissoci ation of the major species, suggesting that it possesses a different five-c oordinate geometry (approximate trigonal bipyramid). The [Cu-II(edampda)] c omplex exhibits an e.p.r. spectrum that is intermediate between rhombic or tetragonal Cu-II complexes (near D-4h) and the "reversed-e.p.r," type of tr igonal bipyramidal Cu-II complexes (ca. D-3h) The single g value of 2.079 f or g(parallel to) congruent to g(perpendicular to) > 2.03 identifies the [C u (edampda)] complex as distorted toward trigonal bipyramidal whereas its [ Cu-II(edta-H-2)(H2O)] analogue is known to be distorted toward square pyram idal. A binuclear CUT complex of edampda(2-) is formed only as a transient, and it rapidly disproportionates into [Cu-II(edampda)] and Cu metal. A mon onuclear [Cu-I(edampda)](-) complex persists for up to 8 h, but is oxidized within 3 min by O-2 to the Cu-II complex. [Cu-II(edampda)] oxidizes to Cu- III with a highly irreversible wave on glassy-carbon at +1.09 V compared to the [Ni-II/III(edampda)] wave at +1.32 V.