Multifrequency Nuclear Magnetic Resonance as an Efficient Tool To Investigate Heterospin Complexes in Solutions

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.