Characterization of the Fe(III)-Tiron System in Solution through an Integrated Approach Combining NMR Relaxometric, Thermodynamic, Kinetic, and Computational Data

Abstract

The Fe(III)-Tiron system (Tiron = 4,5-dihydroxy-1,3-benzenedisulfonate) was investigated using a combination of 1 H and 17 O NMR relaxometric studies at variable field and temperature and theoretical calculations at the DFT and NEVPT2 levels. These studies require a detailed knowledge of the speciation in aqueous solution at different pH values. This was achieved using potentiometric and spectrophotometric titrations, which afforded the thermodynamic equilibrium constants characterizing the Fe(III)-Tiron system. A careful control of the pH of the solution and the metal-to-ligand stoichiometric ratio allowed the relaxometric characterization of [Fe(Tiron) 3 ] 9− , [Fe(Tiron) 2 (H 2 O) 2 ] 5− , and [Fe(Tiron)(H 2 O) 4 ] − complexes. The 1 H nuclear magnetic relaxation dispersion (NMRD) profiles of [Fe(Tiron) 3 ] 9− and [Fe(Tiron) 2 (H 2 O) 2 ] 5− complexes evidence a significant second-sphere contribution to relaxivity. A complementary 17 O NMR study provided access to the exchange rates of the coordinated water molecules in [Fe(Tiron) 2 (H 2 O) 2 ] 5− and [Fe(Tiron)(H 2 O) 4 ] − complexes. Analyses of the NMRD profiles and NEVPT2 calculations indicate that electronic relaxation is significantly affected by the geometry of the Fe 3+ coordination environment. Dissociation kinetic studies indicated that the [Fe(Tiron) 3 ] 9− complex is relatively inert due to the slow release of one of the Tiron ligands, while the [Fe(Tiron) 2 (H 2 O) 2 ] 5− complex is considerably more labile.

Coordination-driven self-assembled Mn(ii)-metallostar with high relaxivity and synergistic photothermal and photodynamic effects

“…As reported by Nucera et al 30 and Charkoudian et al , 37 Fe( iii )-catechol (Cat) type complexes demonstrated pH-dependent coordination equilibria, existing as mono-, bis-, and tris-Cat coordination forms with different Fe( iii )-hydrated states ([Fe(Cat) x (H 2 O) 6−2 x ], x = 1, 2, 3) when the pH varied from acidic to alkaline values. In this work, as shown in Fig.…”

“…As shown in Chart 2, the ditopic ligand (L) derived from L-Dopa contains an EDTA moiety and a catechol group, serving as the complexing unit for the Mn(II) ion and transition metal ions (M = Ti 4+ or Fe 3+ ), respectively. Additionally, as demonstrated in precedent studies like in the Fe(III)-Tiron system, 30 the Fe(III)-catechol core can provide a relatively intense and broad absorption band in the visible to near-infrared (NIR) region due to ligand-to-metal charge transfer (LMCT) and ligand-localized p-p* electronic transitions, providing this Mn(II)-metallostar with potential for NIRresponsive phototherapy properties.…”

“…Additionally, as demonstrated in precedent studies like in the Fe( iii )-Tiron system, 30 the Fe( iii )-catechol core can provide a relatively intense and broad absorption band in the visible to near-infrared (NIR) region due to ligand-to-metal charge transfer (LMCT) and ligand-localized π–π* electronic transitions, providing this Mn( ii )-metallostar with potential for NIR-responsive phototherapy properties.…”

Structural, stability and relaxation features of lanthanide‐complexes designed for multimodal imaging detection of enzyme activities

“…The rotational correlation time, τ R 298 , has a value which corresponds to the size of the complex. We note that the scalar coupling constant has a negative sign in the formalism of the Solomon‐Bloembergen‐Morgan equations used (supporting information), although it should be theoretically positive as the consequence of a negative spin density at the oxygen nucleus and the negative magnetic moment of the 17 O nucleus, as it was demonstrated by different authors [45–46] . The electron spin relaxation is notably faster for GdL2 than for GdDOTA , as expressed by its seven times higher Δ 2 value (Table 6).…”

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

“…2,3 However, the development of Fe(III) MRI probes has received more attention over the past few years. [4][5][6][7][8][9][10][11] A motivation for the development of iron MRI probes is to take advantage of the biochemical mechanisms in animals as well as humans to sequester and store excess iron. [12][13][14] However, ligands designed for Fe(III) chelation need to take into account the unique chemistry of the highly Lewis acidic and small-sized Fe(III) ion.…”