Tandem mass spectrometric analysis of rare earth(III) complexes: evaluation of the relative strength of their Lewis acidity

Five series of rare earth(III) complexes coordinated with hexamethylphosphoramide, triethylphosphine oxide, trimethyl phosphate, N,N′-dimethylpropyleneurea, or dimethyl sulfoxide were subjected to tandem mass spectrometric analysis. The relative strength of the Lewis acidity of respective rare earth(III) species was evaluated using the peak intensity ratios of the product ions formed from the precursor ions (e.g. [M(OTf)2(hmpa)2]+). The exceptionally strong Lewis acidity of scandium(III) and ytterbium(III) complexes was clearly indicated by this tandem MS analysis. The analysis also showed that the Lewis acidity of ytterbium(III) is stronger than that of lutetium(III) although the ionic radius of Yb3+ is larger than that of Lu3+. The gadolinium break and the tetrad effect were observed in the Lewis acidity of the series of the lanthanide(III) complexes.