Bis- and tris-(methylphosphonic) acid derivatives of a 14-membered tetraazamacrocycle containing pyridine: synthesis, protonation and complexation studies
authors Guerra, KP; Delgado, R; Lima, LMP; Drew, MGB; Felix, VT
nationality International
journal DALTON TRANSACTIONS
keywords CAMBRIDGE STRUCTURAL DATABASE; TRANSITION-METAL IONS; MRI CONTRAST AGENTS; MACROCYCLIC LIGANDS; X-RAY; TETRAAZA MACROCYCLES; COPPER(II) COMPLEXES; STABILITY-CONSTANTS; CRYSTAL-STRUCTURES; PHOSPHONIC-ACIDS
abstract Two N-methylphosphonic acid derivatives of a 14-membered tetraazamacrocycle containing pyridine have been synthesized, H4L1 and H6L2. The protonation constants of these compounds and the stability constants of complexes of both ligands with Ni2+, Cu2+ and Zn2+ were determined by potentiometric methods at 298 K and ionic strength 0.10 mol dm(-3) in NMe4NO3. The high overall basicity of both compounds is ascribed to the presence of the phosphonate arms. H-1 and P-31 NMR spectroscopic titrations were performed to elucidate the sequence of protonation, which were complemented by conformational analysis studies. The complexes of these ligands have stability constants of the order of or higher than those formed with ligands having the same macrocyclic backbone but acetate arms. At pH = 7 the highest pM values were found for solutions containing the compound with three acetate groups, followed immediately by those of H6L2, however, as expected, the increasing pH favours the complexes of ligands containing phosphonate groups. The single-crystal structure of Na-2[Cu(HL1)]NO3.8H(2)O has shown that the coordination geometry around the copper atom is a distorted square pyramid. Three nitrogen atoms of the macrocyclic backbone and one oxygen atom from one methylphosphonate arm define the basal plane, and the apical coordination is accomplished via the nitrogen atom trans to the pyridine ring of the macrocycle. To achieve this geometric arrangement, the macrocycle adopts a folded conformation. This structure seems consistent with Uv-vis-NIR spectroscopy for the Ni2+ and the Cu2+ complexes and with the EPR for the latter.
publisher ROYAL SOC CHEMISTRY
issn 1477-9226
year published 2004
issue 12
beginning page 1812
ending page 1822
digital object identifier (doi) 10.1039/b403977g
web of science category Chemistry, Inorganic & Nuclear
subject category Chemistry
unique article identifier WOS:000221908400008
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