11B NMR studies on some uranium(IV) poly(pyrazol-1-yl) borate complexes

Abstract

Although no full X-ray structural data are available yet for the actinide complexes of the poly(pyrazol-1-yl)borates, a good deal of information has been obtained from their electronic and 1H NMR spectra. In this work results from 11B NMR spectroscopy will be presented and a correlation with the above data is attempted. The compounds studied were of the type UCl x[H nBL 4−n 4−x where L = pyrazole or 3,5-dimethylpyrazole. In spite of the broad lines observed in these compounds, spectra are simple, and in some cases the large chemical shifts of some paramagnetic species seem to be quite helpful. The spectra of the compounds UCl 2[HB(3,5Me 2Pz) 3[ 2 and UCl 2[H 2B(3,5Me 2Pz) 2] 2 in tetrahydrofuran display two lines, one with a small isotropic shift and another with a large isotropic shift. Titrations od UCl 4 in tetrahydrofuran with the corresponding potassium salts were followed simultaneously by 11B NMR spectroscopy, electronic absorption spectroscopy and by conductivity measurements. In one of the experiments an intermediate species was found as shown by the 11B NMR spectrum. The first 11B NMR variable temperature measurements of the poly(pyrazol-1-yl)borates were made with the second compound dissolved in toluene and showed a Curie Weiss behaviour for one paramagnetic species. In contrast in the above two compounds which show to 11B NMR distinct lines, all the other analogous compounds studied, display only a single line. Results from T 1 measurements obtained by the inversion recovery method will also be presented for the ligands and for some uranium species. Two of the authors (N.M. and A.P.M.) are grateful to Prof. R.D. Fischer who stimulated work on Boron NMR spectroscopy in the Department of Chemistry in Sacavém.