A trans-diacetate dysprosium complex with a flexible hexaazatetramine 18-membered macrocycle

Introduction. Hexadentate N₆ macrocycles could be adequate for obtaining lanthanoid complexes with coordination number 8 and axial hexagonal bipyramidal geometry. However, achieving this is not straightforward, as the geometry of these complexes is strongly influenced not only by the flexibility of the macrocycle but also by the features of the selected ancillary donors, as indicated by some prior findings using chloride and nitrate as auxiliary ligands in macrocyclic lanthanoid complexes.

Synthesis and methods. With the intention of delving deeper into this issue, we present here the result of the synthesis and crystallographic characterization of the dysprosium complex [DyL(OAc)₂]OAc·7H₂O, arisen from the interaction of dysprosium acetate tetrahydrate and a macrocyclic hexaazatetramine ligand, derived from diacetylpyridine (L = 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane) in a 1:1 molar ratio in chloroform. Its crystal structure was solved using standard methods of single-crystal X-ray diffraction.

Results. Despite the desired trans disposition of the acetate anions, with the macrocycle L in a rather plane disposition, the dysprosium atom is deca-coordinate, as both acetate complexes behave as bidentate chelating donors, forming a mutual angle of 72.6 °. The spatial arrangement of the ligand leads to a bicapped square antiprism calculated geometry, but with a significant distortion to a sphenocorona.

Conclusion. Despite achieving a desired trans disposition for the ancillary ligands, the substitution of acetate as an auxiliary donor, instead chloride or nitrate, has not allowed us to obtain an axial octacoordinate Dy³⁺ complex, but a decacoordinate one.