Radical anions and lanthanide complexes as NMR shift reagents

Abstract

The negative ions of triphenylene and coronene give striking spectral simplifications in the NM R spectra of polyglycoldimethylethers (glymes) by virtue of reversible complexation, leading to isotropic Fermi contact shifts. In this aspect they resemble the lanthanide shift reagents, such as tris(dipivaIomethanato)-europimn and -praseodymium. The NM R spectra of glymes complexed with both types of shift re agents have been measured. Similarities but also difFerences were found, as manifested by measuring partially deuterated glymes. Information is obtained on the structure of the complexes and on the mechanisms of spin transfer from the radical center to the glymes.

Eu(fod)3 binding on the 1H‐NMR spectra of bis(2′‐ethylbenzoate)ethylene glycol podands

“…The changes of chemical shifts are in a linear relation with the increases of Eu 3+ /podand ratios, indicating the possibility of several degrees of podand interactions . This may be originated from sufficiently large Eu 3+ ionic radius and vacant 5d, 6s, and 6p orbitals, which could accommodate the extensive bindings .…”

“…The effects of Eu 3+ tris (β‐diketone) compounds on 1 H‐NMR spectra of organic molecules are quite well known and have been used for plenty of molecules . This application have covered the open chain bis (2′‐ethylbenzoate)ethylene glycol podands.…”

“…Lanthanide‐induced shifts mainly involves with two mechanisms: (i) contact shifts, which arise from the delocalization of the unpaired electron‐spin through bonds to the nuclei affected; and (ii) pseudocontact shifts of the magnetic moment of paramagnetic cation that generated the secondary field, although both effects may involve depending on lanthanides interacting with Lewis bases to bind …”

“…Although there is no angular dependence, the magnetic role of Eu 3+ is open to discussions. Probably magnetic moment arises through a second‐order paramagnetism, which is large for Eu 3+ , and this effect adducts to produce the remarkable down field shift …”

“…This study aims to determine complexation between podand and Eu(fod) 3 using 1 H‐NMR spectroscopy. We restrict the present study and results to the 1:1 complexes observing the secondary field effect, which dominates podand ester/Eu(fod) 3 complexes and the induced shifts is proportional with 1/r 3 of the distance regarding the role of cationic radii …”