Chelating Borohydrides for Lanthanides and Actinides: Structures, Mechanochemistry, and Case Studies with Phosphinodiboranates

In this Forum Article, we review the development of chelating borohydride ligands called aminodiboranates (H3BNR2BH3-) and phosphinodiboranates (H3BPR2BH3-) for the synthesis of trivalent f-element complexes. The advantages and history of using mechanochemistry to prepare molecular borohydride complexes are described along with new results demonstrating the mechanochemical synthesis of M-2 ((H3BPBu2BH3)-Bu-t)(6), where M = U, Nd, Tb, Er, and Lu (1-5). Multinuclear NMR, IR, and single-crystal X-ray diffraction data are reported for 1-5 alongside complementary density functional theory calculations to reveal differences in their structure and reactivity with and without tetrahydrofuran. The results demonstrate how mechanochemistry can be used to access f-element complexes with chelating borohydrides in improved and reproducible yields, which is an important step toward investigating the properties of lanthanide and actinide phosphinodiboranate complexes with different phosphorus substituents. The relevance of these results is contextualized by a discussion of structural factors known to influence the volatility of f-element borohydrides and applications that require the development of volatile f-element complexes.