Macrocyclic complexes of Fe(III) with mixed hydroxypropyl and phenolate or amide pendants as T₁ MRI probes

High-spin Fe(III) complexes of 1,4,7-triazacyclononane (TACN) with mixed oxygen donor pendants including hydroxypropyl, phenolate or amide groups are prepared for study as T₁ MRI probes. Complexes with two hydroxypropyl pendants and either amide (Fe(TOAB)) or phenolate (Fe(PTOB)) groups are compared to an analog with three hydroxypropyl groups (Fe(NOHP)), in order to study the effect of the third pendant on the coordination sphere as probed by solution chemistry, relaxivity and structural studies. Solution studies show that Fe(PTOB) has two ionizations with the phenol pendant deprotonating with a pK_a of 1.7 and a hydroxypropyl pendent with pK_a of 6.3. The X-ray crystal structure of [Fe(PTOB)]Br₂ features a six-coordinate complex with two bound hydroxypropyl groups, and a phenolate in a distorted octahedral geometry. The Fe(TOAB) complex has a single deprotonation, assigned to a hydroxypropyl group with a pK_a value of 7.0. Both complexes are stabilized as high-spin Fe(III) in solution as shown by their effective magnetic moments and Fe(III)/Fe(II) redox potentials of -390 mV and -780 mV versus NHE at pH 7 and 25 °C for Fe(TOAB) and Fe(PTOB) respectively. Both Fe(PTOB) and Fe(TOAB) are kinetically inert to dissociation under a variety of challenges including phosphate/carbonate buffer, one equivalent of ZnCl₂, two equivalents of transferrin or 100 mM HCl, or at basic pH values over 24 h at 37 °C. The r₁ relaxivity of Fe(TOAB) at 1.4 T, pH 7.4 and 33 °C is relatively low at 0.6 mM^-1 s^-1 whereas the r₁ relaxivity of Fe(PTOB) is more substantial and shows an increase of 2.5 fold to 2.5 mM^-1 s^-1 at acidic pH. The increase in relaxivity at acidic pH is attributed to protonation of the phenolate group to provide an additional pathway for proton relaxation.