Iron(III) complexes are under investigation as MRI contrast agents, and new challenges have delineated the way they are being developed. This review article describes the enhanced features that have been develolped in the last 15 years of research. The consequences of relaxation theory impose kinetic, thermodynamic, and structural requirements on the potential contrast agent. Iron, as an endogenous metal, might fulfil these expectations, so its specificity is discussed here. The

Iron-filled multiwall carbon nanotubes (Fe@MWCNTs) were functionalized toward a variety of potential magnetic resonance imaging contrast agents. Oxidized Fe@MWNCTs were covered with PEG5000 via direct esterification or using acyl chloride derivatives. Alternatively, the latter were functionalized with an aminophenol ligand (Fe@O-MWCNT-L). Moreover, pristine Fe@MWCNTs were functionalized with N -phenylaziridine groups (Fe@f-MWCNT) via [2+1] cycloaddition of nitrene. All of these chemically modified nanotubes served as a vehicle for anchoring Fe 3+ ions. The new hybrids – Fe(III)/Fe@(f-/O-)MWCNTs – containing 6%–14% of the “tethered” Fe 3+ ions were studied in terms of the acceleration of relaxation of water protons in nuclear magnetic resonance. The highest transverse relaxivity r 2 =63.9±0.9 mL mg −1 s −1 was recorded for Fe(III)/Fe@O-MWCNT-L, while for Fe(III)/Fe@f-MWCNT, with r 2 =57.9±2.9 mL mg −1 s −1 , the highest impact of the anchored Fe(III) ions was observed. The T 1 / T 2 ratio of 30–100 found for all of the nanotube hybrids presented in this work is a very important factor for their potential application as T 2 contrast agents. Increased stability of the hybrids was confirmed by ultraviolet–visible spectrophotometry.

Iron(III) complexes with N,O-ligands are compounds of high interest because they can be applied in catalysis and play an important role in living organisms, e.g., as models of catechol dioxygenase. Several N,O-ligands were studied: their synthesis, iron(III) complexation and the potential of the latter as T 1 -MRI contrast agents. A route to the tetrapodal N 3 O 2 -naphthyl ligand was investigated. The resulting iron complex was obtained in 26% total yield and its relaxivity value was moderate (r 1 = 1.03 in water and 2.54 s À1 mM À1 in serum). Thus, phenyl isomeric salan complexes were obtained. These complexes differed in charge (positive and neutral) and in the presence of polar hydrogen-bonding substituents. The highest relaxivities (r 1 = 2.39 in water and 5.37 s À1 mM À1 in serum) were obtained for the Fe(III) cationic complex with MeO groups in the ligand. EPR studies confirmed a high spin configuration of rhombically distorted Fe(III) complexes. ª 2014 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).