Octadentate Zirconium(IV)-Loaded Macrocycles with Varied Stoichiometry Assembled From Hydroxamic Acid Monomers using Metal-Templated Synthesis

The reaction between Zr(IV) and the forward endo-hydroxamic acid monomer 4-[(5-aminopentyl)(hydroxy)amino]-4-oxobutanoic acid (for-PBH) in a 1:4 stoichiometry in the presence of diphenylphosphoryl azide and triethylamine gave the octadentate Zr(IV)-loaded tetrameric hydroxamic acid macrocycle for-[Zr(DFOT₁)] ([M + H]⁺ calc 887.3, obs 887.2). In this metal-templated synthesis (MTS) approach, the coordination preferences of Zr(IV) directed the preorganization of four oxygen-rich bidentate for-PBH ligands about the metal ion prior to ring closure under peptide coupling conditions. The replacement of for-PBH with 5-[(5-aminopentyl) (hydroxy)amino]-5-oxopentanoic acid (for-PPH), which contained an additional methylene group in the dicarboxylic acid region of the monomer, gave the analogous Zr(IV)-loaded macrocycle for-[Zr(PPDFOT₁)] ([M + H]⁺ calc 943.4, obs 943.1). A second, well-resolved peak in the liquid chromatogram from the for-PPH MTS system also characterized as a species with [M + H]⁺ 943.3, and was identified as the octadentate complex between Zr(IV) and two dimeric tetradentate hydroxamic acid macrocycles for-[Zr(PPDFOT_1D)₂]. Treatment of for-[Zr(PPDFOT₁)] or for-[Zr(PPDFOT_1D)₂] with EDTA at pH 4.0 gave the respective hydroxamic acid macrocycles as free ligands: octadentate PPDFOT₁ or two equivalents of tetradentate PPDFOT_1D (homobisucaberin, HBC). At pH values closer to physiological, EDTA treatment of for-[Zr(DFOT₁)], for-[Zr(PPDFOT₁)], or Zr(IV) complexes with related linear tri- or tetrameric hydroxamic acid ligands showed the macrocycles were more resistant to the release of Zr(IV), which has implications for the design of ligands optimized for the use of Zr(IV)-89 in positron emission tomography (PET) imaging of cancer.