Methylation of tetrakis(fod) europate NMR shift reagent by S-methyldibenzothiophenium ion

NMR and luminescence spectroscopy were combined to study S-methyldibenzothiophenium ion in the presence of the anionic shift reagent Eu(fod)₄^-, where fod is 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctane-3,5-dione. The NMR resonances of the methyl hydrogens are observed to shift from 3.4 pprn for the fluoroborate salt to 25.4 ppm for the Eu(fod)₄^- salt at 0.1 M concentration in chloroform-d. The thiophenium ion is observed to methylate Eu(fod)₄^- anion at 25°C resulting in the formation of Eu(fod)₃, CH₃fod and dibenzothiophene. The reaction is >95% complete in about 4 h. Evidence for the formation of dibenzothiophene and CH₃fod is revealed by ¹H NMR spectroscopy. The fod enolate anion is both C-and O-methylated in nearly equal amounts. C-Methylated fod exists almost entirely in the keto form with no evidence of enol formation, consistent with semiempirical AM1 calculations. Additional evidence supporting the methylation of Eu(fod)₄^- is observed in the luminescence spectrum of the Eu(III) ion. By monitoring changes in the luminescence spectrum as a function of time, the transformation of Eu(fod)₄^- into Eu(fod)₃ is observed. S-Methylbenzothiophenium ion also methylates Eu(fod)4~ but more rapidly than S-methyldibenzothiophenium cation.