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.

Barium phosphidoboranes and related calcium complexes

The attempted synthesis of [{Carb}BaPPh2] (1) showed this barium-phosphide and its thf adducts, 1·thf and 1·(thf)2, to be unstable in solution. Our strategy to circumvent the fragility of these compounds involved the use of phosphinoboranes HPPh2·BH3 and HPPh2·B(C6F5)3 instead of HPPh2. This allowed for the synthesis of [{Carb}Ae{PPh2·BH3}] (Ae = Ba, 2; Ca, 3), [{Carb}Ca{(H3B)2PPh2}·(thf)] (4), [{Carb}Ba{PPh2·B(C6F5)3}] (5), [{Carb}Ba{O(B(C6F5)3)CH2CH2CH2CH2PPh2}·thf] (6), [Ba{O(B(C6F5)3)CH2CH2CH2CH2PPh2}2·(thf)1.5] (7) and [Ba{PPh2·B(C6F5)3}2·(thp)2] (8) that were characterised by multinuclear NMR spectroscopy (thp = tetrahydropyran). The molecular structures of 4, 6 and 8 were validated by X-ray diffraction crystallography, which revealed the presence of Ba⋯F stabilizing interactions (ca. 9 kcal mol-1) in the fluorine-containing compounds. Compounds 6 and 7 were obtained upon ring-opening of thf by their respective precursors, 5 and the in situ prepared [Ba{PPh2·B(C6F5)3}2]n. By contrast, thp does not undergo ring-opening under the same conditions but affords clean formation of 8. DFT analysis did not highlight any specific weakness of the Ba-P bond in 1·(thf)2. The instability of this compound is instead thought to stem from the high energy of its HOMO, which contains the non-conjugated P lone pair and features significant nucleophilic reactivity.

Phosphido-borane-supported stannates

The reactions between SnCl₂ and three equivalents of the alkali metal phosphido-borane complexes [R₂P(BH₃)]M yield the corresponding tris(phosphido-borane)stannate complexes [L_nM{R₂P(BH₃)}₃Sn] [R₂ = iPr₂, L_nM = (THF)₃Li (2Li), (Et₂O)Na (2Na), (Et₂O)K (2K); R₂ = Ph₂, L_nM = (THF)Li (3Li), (THF)(Et₂O)Na (3Na), (THF)(Et₂O)K (3K); R₂ = iPrPh, L_nM = (THF)₄Li (4Li)]. In each case X-ray crystallography reveals an anion consisting of a trigonal pyramidal tin centre coordinated by the P atoms of the phosphido-borane ligands. These tris(phosphido-borane)stannate anions coordinate to the alkali metal cations via their BH₃ hydrogen atoms in a variety of modes to give monomers, dimers, and polymers, depending on the alkali metal and the substituents at the phosphorus centres. In contrast, reactions between SnCl₂ and three equivalents of [tBu₂P(BH₃)]M (M = Li, Na) gave the known hydride [M{tBu₂P(BH₃)}₂SnH], according to multinuclear NMR spectroscopy.

Mechanochemical synthesis and structural analysis of trivalent lanthanide and uranium diphenylphosphinodiboranates

Phosphinodiboranates (H₃BPR₂BH₃^-) are a class of borohydrides that have merited a reputation as weakly coordinating anions, which is attributed in part to the dearth of coordination complexes known with transition metals, lanthanides, and actinides. We recently reported how K(H₃BP^tBu₂BH₃) exhibits sluggish salt elimination reactivity with f-metal halides in organic solvents such as Et₂O and THF. Here we report how this reactivity appears to be further attenuated in solution when the ^tBu groups attached to phosphorus are exchanged for R = Ph or H, and we describe how mechanochemistry was used to overcome limited solution reactivity with K(H₃BPPh₂BH₃). Grinding three equivalents of K(H₃BPPh₂BH₃) with UI₃(THF)₄ or LnI₃ (Ln = Ce, Pr, Nd) allowed homoleptic complexes with the empirical formulas U(H₃BPPh₂BH₃)₃ (1), Ce(H₃BPPh₂BH₃)₃ (2), Pr(H₃BPPh₂BH₃)₃ (3), and Nd(H₃BPPh₂BH₃)₃ (4) to be prepared and subsequently crystallized in good yields (50-80%). Single-crystal XRD studies revealed that all four complexes exist as dimers or coordination polymers in the solid-state, whereas ¹H and ¹¹B NMR spectra showed that they exist as a mixture of monomers and dimers in solution. Treating 4 with THF breaks up the dimer to yield the monomeric complex Nd(H₃BPPh₂BH₃)₃(THF)₃ (4-THF). XRD studies revealed that 4-THF has one chelating and two dangling H₃BPPh₂BH₃^- ligands bound to the metal to accommodate binding of THF. In contrast to the results with K(H₃BPPh₂BH₃), attempting the same mechanochemical reactions with Na(H₃BPH₂BH₃) containing the simplest phosphinodiboranate were unsuccessful; only the partial metathesis product U(H₃BPH₂BH₃)I₂(THF)₃ (5) was isolated in poor yields. Despite these limitations, our results offer new examples showing how mechanochemistry can be used to rapidly synthesize molecular coordination complexes that are otherwise difficult to prepare using more traditional solution methods.