TY - JOUR

T1 - Multifrequency Nuclear Magnetic Resonance as an Efficient Tool to Investigate Heterospin Complexes in Solutions

AU - Fishman, Natalya N.

AU - Lukzen, Nikita N.

AU - Ivanov, Konstantin L.

AU - Edeleva, Mariya V.

AU - Fokin, Sergey V.

AU - Romanenko, Galina V.

AU - Ovcharenko, Victor I.

N1 - Publisher Copyright: Copyright © 2020 American Chemical Society. Copyright: Copyright 2020 Elsevier B.V., All rights reserved.

PY - 2020/2/20

Y1 - 2020/2/20

N2 - We report a multifrequency nuclear magnetic resonance (NMR) study of heterospin complexes [Eu(SQ)3Ln], where SQ is 3,6-di(tert-butyl)-1,2-semiquinone, L is tetrahydrofuran (THF), pyridine (Py), or 2,2′-dipyridyl (Dipy), and n is the number of diamagnetic ligands. Multifrequency NMR experiments allowed us to determine the effective paramagnetic shifts of the ligands (L = THF or Py) and the chemical equilibrium constant for [Eu(SQ)3(THF)2]. In addition, we have found a strong magnetic field effect on the NMR line broadening, giving rise to very broad NMR lines at high magnetic fields. We attribute this effect to broadening under fast exchange conditions when the NMR spectrum represents a homogeneously broadened line with a width proportional to the square of the NMR frequency difference of the free and bound forms of L. Consequently, the line width strongly increases with the magnetic field. This broadening effect allows one to determine relevant kinetic parameters, i.e., the effective exchange time. The strong broadening effect allows one to exploit the [Eu(SQ)3(THF)2] complex as an efficient shift reagent, which not only shifts unwanted NMR signals but also broadens them, notably, in high-field NMR experiments. We have also found that [Eu(SQ)3Dipy] is a thermodynamically stable complex; hence, one can study [Eu(SQ)3Dipy] solutions without special precautions. We report an X-ray structure of the [Eu(SQ)3Dipy]·C6D6 crystals that have been grown directly in an NMR tube. This shows that multifrequency NMR investigations of heterospin compound solutions not only provide thermodynamic and kinetic data for heterospin species but also can be useful for the rational design of stable heterospin complexes and optimization of synthetic approaches.

AB - We report a multifrequency nuclear magnetic resonance (NMR) study of heterospin complexes [Eu(SQ)3Ln], where SQ is 3,6-di(tert-butyl)-1,2-semiquinone, L is tetrahydrofuran (THF), pyridine (Py), or 2,2′-dipyridyl (Dipy), and n is the number of diamagnetic ligands. Multifrequency NMR experiments allowed us to determine the effective paramagnetic shifts of the ligands (L = THF or Py) and the chemical equilibrium constant for [Eu(SQ)3(THF)2]. In addition, we have found a strong magnetic field effect on the NMR line broadening, giving rise to very broad NMR lines at high magnetic fields. We attribute this effect to broadening under fast exchange conditions when the NMR spectrum represents a homogeneously broadened line with a width proportional to the square of the NMR frequency difference of the free and bound forms of L. Consequently, the line width strongly increases with the magnetic field. This broadening effect allows one to determine relevant kinetic parameters, i.e., the effective exchange time. The strong broadening effect allows one to exploit the [Eu(SQ)3(THF)2] complex as an efficient shift reagent, which not only shifts unwanted NMR signals but also broadens them, notably, in high-field NMR experiments. We have also found that [Eu(SQ)3Dipy] is a thermodynamically stable complex; hence, one can study [Eu(SQ)3Dipy] solutions without special precautions. We report an X-ray structure of the [Eu(SQ)3Dipy]·C6D6 crystals that have been grown directly in an NMR tube. This shows that multifrequency NMR investigations of heterospin compound solutions not only provide thermodynamic and kinetic data for heterospin species but also can be useful for the rational design of stable heterospin complexes and optimization of synthetic approaches.

KW - LANTHANIDE SHIFT-REAGENTS

KW - PARAMAGNETIC NMR

KW - SPECTRA

KW - PERSPECTIVES

KW - DERIVATIVES

KW - OXIDATION

KW - RADICALS

UR - http://www.scopus.com/inward/record.url?scp=85080843313&partnerID=8YFLogxK

U2 - 10.1021/acs.jpca.9b11104

DO - 10.1021/acs.jpca.9b11104

M3 - Article

C2 - 31986040

AN - SCOPUS:85080843313

VL - 124

SP - 1343

EP - 1352

JO - Journal of Physical Chemistry A

JF - Journal of Physical Chemistry A

SN - 1089-5639

IS - 7

ER -