Transition metal complexes with phosphine oxide appended aza-macrocycles - effects of ring size and denticity

Two new phosphine oxide appended aza-macrocyclic ligands, L1 and L3, have been prepared in good yield from the reaction of Bn-tacn and Bn-cyclen (tacn = 1,4,7-triazacyclononane; cyclen = 1,4,7,10-tetraazacyclododecane) with paraformaldehyde and diphenylphosphine oxide. They are characterised spectroscopically and via a crystal structure determination for L3·MeCN. The coordination chemistry of these ligands and the known NOTP-Ph (L2) with divalent late 3d metal ions, Co, Ni, Cu and Zn, has been investigated, showing that L1 and L2 bind in a pentadentate and hexadentate manner, respectively, in all cases. The potentially heptadentate L3 is found to coordinate to Co(II) and Zn(II) in a hexadentate manner (N₄O₂ donor set) giving distorted trigonal prismatic cations, and to Ni(II) through the same N₄O₂ donor set, but in a distorted octahedral geometry. These assignments follow from a combination of elemental analyses, mass spectrometry, IR, UV-visible, ¹H and ³¹P{¹H} NMR spectroscopic data, as appropriate, and X-ray crystal structures of several representative examples incorporating L1-L3. The properties of the new complexes are also compared to the literature data from the corresponding complexes of the cyclen macrocycle bearing four diphenylphosphine oxide pendant groups, DOTP-Ph, which are distinctly different. Overall, these studies across this series of ligands and metals demonstrate that the macrocyclic ring size, metal ion type and the number of pendant phosphine oxide arms all play an important role in determining the resulting coordination numbers and geometries.