Abstract
Investigating the relaxation of water 1H nuclei induced by paramagnetic Mn(II) complexes is important to understand the mechanisms that control the efficiency of contrast agents used in diagnostic magnetic resonance imaging (MRI). Herein, a series of potentially hexadentate triazacyclononane (TACN) derivatives containing different pendant arms were designed to explore the relaxation of the electron spin in the corresponding Mn(II) complexes by using a combination of 1H NMR relaxometry and theoretical calculations. These ligands include 1,4,7-triazacyclononane-1,4,7-triacetic acid (H3NOTA) and three derivatives in which an acetate group is replaced by sulfonamide (H3NO2ASAm), amide (H2NO2AM), or pyridyl (H2NO2APy) pendants. The analogue of H3NOTA containing three propionate pendant arms (H3NOTPrA) was also investigated. The X-ray structure of the derivative containing two acetate groups and a sulfonamide pendant arm [Mn(NO2ASAm)]− evidenced six-coordination of the ligand to the metal ion, with the coordination polyhedron being close to a trigonal prism. The relaxivities of all complexes at 20 MHz and 25 °C (1.1–1.3 mM–1 s–1) are typical of systems that lack water molecules coordinated to the metal ion. The nuclear magnetic relaxation profiles evidence significant differences in the relaxivities of the complexes at low fields (<1 MHz), which are associated with different spin relaxation rates. The zero field splitting (ZFS) parameters calculated by using DFT and CASSCF methods show that electronic relaxation is relatively insensitive to the nature of the donor atoms. However, the twist angle of the two tripodal faces that delineate the coordination polyhedron, defined by the N atoms of the TACN unit (lower face) and the donor atoms of the pendant arms (upper face), has an important effect in the ZFS parameters. A twist angle close to the ideal value for an octahedral coordination (60°), such as that in [Mn(NOTPrA)]−, leads to a small ZFS energy, whereas this value increases as the coordination polyhedron approaches to a trigonal prism.
Highlights
Mn(II) complexes stable in aqueous media, in terms of dissociation and redox state, are currently the subject of intense research focused to find candidates as contrast agents for application in magnetic resonance imaging (MRI).[1−4] MRI is an imaging technique used by radiologists to aid clinical diagnosis, as it provides high-resolution three-dimensional anatomical images.[5]
The [Mn(NOTPrA)]− complex is characterized by a pseudooctahedral coordination environment evidenced by a twist angle of the N3 and O3 planes close to 60°. This is in nice agreement with the splitting of the metal 3d orbitals obtained with ab initio ligand field theory (AILFT) analysis based on complete active space self-consistent field (CASSCF)/NEVPT2 calculations (Figure 6)
Nuclear magnetic relaxation dispersion (NMRD) studies and theoretical calculations revealed that the rationalization of electron relaxation using simple rules remains difficult
Summary
Mn(II) complexes stable in aqueous media, in terms of dissociation and redox state, are currently the subject of intense research focused to find candidates as contrast agents for application in magnetic resonance imaging (MRI).[1−4] MRI is an imaging technique used by radiologists to aid clinical diagnosis, as it provides high-resolution three-dimensional anatomical images.[5]. The [Mn(NOTPrA)]− complex is characterized by a pseudooctahedral coordination environment evidenced by a twist angle of the N3 and O3 planes close to 60° This is in nice agreement with the splitting of the metal 3d orbitals obtained with ab initio ligand field theory (AILFT) analysis based on CASSCF/NEVPT2 calculations (Figure 6). The energies calculated for the lowest-energy quartet state with NEVPT2 (before SOC) in the structurally related [Mn(NOTA)]−, [Mn(NO2ASAm)]−, [Mn(NO2AM)], and [Mn(NO2APy)] complexes are 22474, 22188, 21986, and 21610 cm−1 These values correlate with the oxidation potentials measured in aqueous solution of 728, 1011, 1138, and >1150 mV.
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