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.


I. XRD data
Table S3.Average M-B bond distances (Å) and B-P-B angles (°) from the XRD structures of the dimers.
Table S4.The TPSS-D3 geometry parameters and Gibbs free energy calculation with two different basis sets.In the mixed basis set def2-SV(P) is used for light atoms while def2-TZVP is used for Nd.In all calculations, def-TZVP was used on U. A single point calculation was performed on the geometry resulting from the mixed basis set with the larger basis set.The ΔG with the single-point corrected energy is reported in parentheses.assuming a concentration of 1 M for all species and benzene as the solvent.S12.Chelating Nd-B distances (Å) in the hypothetical tetramer Nd-i Pr-4 optimized with TPSS-D3/def2-SV(P) (def2-TZVP for Nd).Atom labels correspond to those in Figure S32.

Figure S1 .
Figure S1.Molecular structure of U(H3BP i Pr2BH3)3 (U-i Pr) with thermal ellipsoids at 35% probability.Hydrogen atoms attached to carbon were omitted from the figure.

Figure S2 .
Figure S2.Molecular structure of Nd(H3BP i Pr2BH3)3 (Nd-i Pr) with thermal ellipsoids at 35% probability.Hydrogen atoms attached to carbon were omitted from the figure.

Figure S3 .
Figure S3.Molecular structure of Tb(H3BP i Pr2BH3)3 (Tb-i Pr) with thermal ellipsoids at 35% probability.Hydrogen atoms attached to carbon were omitted from the figure.

Figure S4 .
Figure S4.Molecular structure of U(H3BPMe2BH3)3 (U-Me) with thermal ellipsoids at 35% probability.Hydrogen atoms attached to carbon were omitted from the figure.

Figure S5 .
Figure S5.Molecular structure of Nd(H3BPEt2BH3)3(THF)3 (Nd-Et-THF) with thermal ellipsoids at 35% probability.Hydrogen atoms attached to carbon and a disordered carbon on THF were omitted from the figure.

i Pr Nd-i Pr Sm-i Pr Tb-i Pr Er-i Pr
2 ] 1/2 for all reflections.

Table S6 .
The relative energy in kcal/mol structures optimized in DFT from lowest conformer from CREST and X-ray structure.

Table S7 .
The thermochemical data at 298.15 K for the TPSS-D3 electronic energies, enthalpies, entropies, and free energies for reaction Dimer → 2 Monomer.Free energies have been computed