The Influence of Phosphorus Substituents on the Structures and Solution Speciation of Trivalent Uranium and Lanthanide Phosphinodiboranates

Here, we report the mechanochemical synthesis and characterization of homoleptic uranium and lanthanide phosphinodiboranates with isopropyl and ethyl substituents attached to phosphorus. M­(H₃BPⁱPr₂BH₃)₃ complexes with M = U, Nd, Sm, Tb, and Er were prepared by ball milling UI₃(THF)₄, SmBr₃, or MI₃ with three equivalents of K­(H₃BPⁱPr₂BH₃). M­(H₃BPEt₂BH₃)₃ with M = U and Nd were prepared similarly using K­(H₃BPEt₂BH₃), and the complexes were purified by extraction and crystallization from Et₂O or CH₂Cl₂. Single-crystal XRD studies revealed that all five M­(H₃BPⁱPr₂BH₃)₃ crystallize as dimers, despite the significant differences in metal radii across the series. In contrast, Nd­(H₃BPEt₂BH₃)₃ with smaller ethyl substituents crystallized as a coordination polymer. Crystals of U­(H₃BPEt₂BH₃)₃ were not suitable for structural analysis, but crystals of U­(H₃BPMe₂BH₃)₃ isolated in low yield by solution methods were isostructural with Nd­(H₃BPEt₂BH₃)₃. ¹H and ¹¹B NMR studies in C₆D₆ revealed that all of the complexes form mixtures of monomer and oligomers when dissolved, and the extent of oligomerization was highly dependent on metal radius and phosphorus substituent size. A comprehensive analysis of all structurally characterized uranium and lanthanide phosphinodiboranate complexes reported to date, including those with larger Ph and ^tBu substituents, revealed that the degree of oligomerization in solution can be correlated to differences in B–P–B angles obtained from single-crystal XRD studies. Density functional theory calculations, which included structural optimizations in combination with conformational searches using tight binding methods, replicated the general experimental trends and revealed free energy differences that account for the different solution and solid-state structures. Collectively, these results reveal how steric changes to phosphorus substituents significantly removed from metal coordination sites can have a significant influence on solution speciation, deoligomerization energies, and the solid-state structure of homoleptic phosphinodiboranate complexes containing trivalent f-metals.