Investigation of the Zero‐Field Splitting in Six‐ and Seven‐Coordinate Mononuclear Mn<sup>II</sup> Complexes with N/O‐Based Ligands by Combining EPR Spectroscopy and Quantum Chemistry

Investigating the relaxation of water 1 H 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 1 H NMR relaxometry and theoretical calculations. These ligands include 1,4,7-triazacyclononane-1,4,7-triacetic acid (H 3 NOTA) and three derivatives in which an acetate group is replaced by sulfonamide (H 3 NO2ASAm), amide (H 2 NO2AM), or pyridyl (H 2 NO2APy) pendants. The analogue of H 3 NOTA containing three propionate pendant arms (H 3 NOTPrA) 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.

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“…However, coordination of an increasing number of negatively charged donor ligands was found to turn the sign of D negative. 79 …”

Section: Resultsmentioning

confidence: 99%

Understanding the Effect of the Electron Spin Relaxation on the Relaxivities of Mn(II) Complexes with Triazacyclononane Derivatives

et al. 2021

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“…23,24 Moreover, the ZFS parameters could also be affected by the ratio between the N-and O-based ligands. 25 The reaction between Mn II carboxylate and bidentate ligands (NN), such as 2,2′-bipyridine (bpy) and 1,10-phenanthroline ( phen), leads to the formation of Mn II compounds with different nuclearity: mononuclear, dinuclear, trinuclear, or 1D systems. 26,27 The carboxylate groups display a wide variety of coordination modes, such as monodentate terminal, chelating, bidentate bridging, and monodentate bridging modes.…”

Section: Introductionmentioning

confidence: 99%

Determination of ZFS parameters from the EPR spectra of mono-, di- and trinuclear Mn^IIcomplexes: impact of magnetic coupling

et al. 2017

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A family of new Mn compounds, consisting of seven dinuclear, three mononuclear, and four trinuclear ones, were synthesised using benzoic acid derivatives n-RCHCOOH, where n-R = 2-MeO, 3-MeO, 4-MeO, or 4-Bu, and 2,2'-bipyridine (bpy) or 1,10-phenantroline (phen) as blocking ligands. The crystal structures of nine of these compounds and the magnetic studies of all of them are reported here. Each type of compound was formed depending on the presence or absence of ClO ions, the solvent used, and/or the presence of a small amount of water in the reaction medium. The use of the tert-buthylbenzoate ligand gave unexpected results, very likely due to the steric hindrance caused by the voluminous Bu groups. The EPR spectra of each type of compound give some peculiar features that allow its identification. Attempts to fit these spectra have been made in order to determine the ZFS parameters, D and E, of the Mn ion (for mononuclear and dinuclear systems) or of the ground state (for trinuclear systems). For trinuclear systems, the single-ion ZFS parameters estimated from those of the ground state provided a good simulation of the EPR spectra of these compounds. The EPR signals observed in each case have been rationalised according to the energy level distribution and the plausible population in the excited states. In some particular situations, the sign of D could be determined from the fit of the EPR spectra of the antiferromagnetic dinuclear compounds, the source of the difference between the spectra lying in the second excited state.

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“…Thus, even minor errors in calculating either of the SS or SOC contributions can significantly affect the accuracy of the final results. 13 Efforts to employ density functional theory (DFT) for the accurate prediction of fine structure parameters are also complicated by the delicate balance between exchange and correlation contributions. Be that as it may, any demonstration that theoretical calculations are capable of yielding even qualitative insights into zero-field splittings would have wide application in the study of Mn-dependent enzymes and likely facilitate the interpretation of EPR spectra for multicenter systems, such as OxDC.…”

Section: Introductionmentioning

confidence: 99%

Assigning the EPR Fine Structure Parameters of the Mn(II) Centers in Bacillus subtilis Oxalate Decarboxylase by Site-Directed Mutagenesis and DFT/MM Calculations

et al. 2014

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Oxalate decarboxylase (OxDC) catalyzes the Mn-dependent conversion of the oxalate monoanion into CO2 and formate. EPR-based strategies for investigating the catalytic mechanism of decarboxylation are complicated by the difficulty of assigning the signals associated with the two Mn(II) centers located in the N- and C-terminal cupin domains of the enzyme. We now report a mutational strategy that has established the assignment of EPR fine structure parameters to each of these Mn(II) centers at pH 8.5. These experimental findings are also used to assess the performance of a multistep strategy for calculating the zero-field splitting parameters of protein-bound Mn(II) ions. Despite the known sensitivity of calculated D and E values to the computational approach, we demonstrate that good estimates of these parameters can be obtained using cluster models taken from carefully optimized DFT/MM structures. Overall, our results provide new insights into the strengths and limitations of theoretical methods for understanding electronic properties of protein-bound Mn(II) ions, thereby setting the stage for future EPR studies on the electronic properties of the Mn(II) centers in OxDC and site-specific variants.

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Investigation of the Zero‐Field Splitting in Six‐ and Seven‐Coordinate Mononuclear Mn^II Complexes with N/O‐Based Ligands by Combining EPR Spectroscopy and Quantum Chemistry

Abstract: International audienc

Cited by 31 publications

References 43 publications

Understanding the Effect of the Electron Spin Relaxation on the Relaxivities of Mn(II) Complexes with Triazacyclononane Derivatives

Understanding the Effect of the Electron Spin Relaxation on the Relaxivities of Mn(II) Complexes with Triazacyclononane Derivatives

Determination of ZFS parameters from the EPR spectra of mono-, di- and trinuclear Mn^IIcomplexes: impact of magnetic coupling

Assigning the EPR Fine Structure Parameters of the Mn(II) Centers in Bacillus subtilis Oxalate Decarboxylase by Site-Directed Mutagenesis and DFT/MM Calculations

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Investigation of the Zero‐Field Splitting in Six‐ and Seven‐Coordinate Mononuclear MnII Complexes with N/O‐Based Ligands by Combining EPR Spectroscopy and Quantum Chemistry

Abstract: International audienc

Cited by 31 publications

References 43 publications

Understanding the Effect of the Electron Spin Relaxation on the Relaxivities of Mn(II) Complexes with Triazacyclononane Derivatives

Understanding the Effect of the Electron Spin Relaxation on the Relaxivities of Mn(II) Complexes with Triazacyclononane Derivatives

Determination of ZFS parameters from the EPR spectra of mono-, di- and trinuclear MnIIcomplexes: impact of magnetic coupling

Assigning the EPR Fine Structure Parameters of the Mn(II) Centers in Bacillus subtilis Oxalate Decarboxylase by Site-Directed Mutagenesis and DFT/MM Calculations

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Investigation of the Zero‐Field Splitting in Six‐ and Seven‐Coordinate Mononuclear Mn^II Complexes with N/O‐Based Ligands by Combining EPR Spectroscopy and Quantum Chemistry

Determination of ZFS parameters from the EPR spectra of mono-, di- and trinuclear Mn^IIcomplexes: impact of magnetic coupling