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)₃L_n], 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.

Problem of a Wide Variety of Products in the Cu(hfac)2−Nitroxide System

The stereochemically flexible Cu(hfac)(2) metal-ligand system when combined with polyfunctional nitroxides leads to a variety of solids with varying structure and composition. While investigating the products of Cu(hfac)(2) interaction with spin-labeled pyrazole 4,4,5,5-tetramethyl-2-(1-methyl-1H-pyrazol-4-yl)-imidazoline-3-oxide-1-oxyl, we have isolated a family of (12) heterospin compounds differing in structure and composition in the solid state. In synthetic systems, these compounds often cocrystallize and must be separated mechanically. It is also shown that minor variation of the structure of the solid heterospin complex can substantially change the magnetic properties of compounds.