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

Abstract

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.

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Identifiers

ORCID for Julie M. Herniman: orcid.org/0000-0003-4834-1093

ORCID for Rhys P. King: orcid.org/0000-0002-6793-1075

ORCID for Mark E. Light: orcid.org/0000-0002-0585-0843

ORCID for Gillian Reid: orcid.org/0000-0001-5349-3468

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