Quantifying the Influence of Covalent Metal‐Ligand Bonding on Differing Reactivity of Trivalent Uranium and Lanthanide Complexes

Abstract

Qualitative differences in the reactivity of trivalent lanthanide and actinide complexes have long been attributed to differences in covalent metal‐ligand bonding, but there are few examples where thermodynamic aspects of this relationship have been quantified, especially with U3+ and in the absence of competing variables. Here we report a series of dimeric phosphinodiboranate complexes with trivalent f‐metals that show how shorter‐than‐expected U−B distances indicative of increased covalency give rise to measurable differences in solution deoligomerization reactivity when compared to isostructural complexes with similarly sized lanthanides. These results, which are in excellent agreement with supporting DFT and QTAIM calculations, afford rare experimental evidence concerning the measured effect of variations in metal‐ligand covalency on the reactivity of trivalent uranium and lanthanide complexes.

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

“…In contrast, the longer M–B distances are consistent with κ 2 -BH 3 for i Pr-PDB complexes with smaller Er and Sm, but these shorten significantly for complexes with larger U and Nd consistent with a transition from κ 2 -BH 3 to κ 3 -BH 3 (Figure a). Averaging the four chelating M–B distances for each complex and plotting them against the ionic radius also reveals an excellent linear correlation ( R 2 = 0.998; Figure a) . Plots of the individual chelating M–B distances show how these distances adjust to accommodate a change in the size of the metal (Figure b).…”

“…However, there are distinct structural differences due to the changes in alkyl substituent. For reference, M- t Bu complexes maintain relatively symmetric bridging t Bu-PDB ligands with similar M–B distances to both metals . They also alter the denticity of their chelating ligands from κ 2 -BH 3 to κ 1 -BH 3 to accommodate smaller metals.…”

“…Previous variable-temperature NMR studies of M- t Bu complexes in C 6 D 6 revealed measurable differences in the dimer/monomer equilibrium for U 3+ as compared with similarly sized lanthanide ions like La 3+ and Ce 3+ . Thermodynamic values obtained from Van’t Hoff plots revealed ∼1 kcal/mol increase in the Δ H and Δ G for M = U, which was attributed to increased covalency in the U–H–B bonds . Unfortunately, NMR data collected on U- i Pr and Nd- i Pr revealed that i Pr-PDB complexes are not as amenable to teasing out such small differences in the thermodynamic values (though DFT calculations were used to quantify these values; see below).…”

“…1 H and 11 B NMR studies have shown that dimeric PDB complexes break up into metal-dependent mixtures of monomers and dimers (or higher order oligomers) when dissolved in aromatic solvents. M- t Bu complexes with the largest metal ions such as U 3+ , Ce 3+ , and Nd 3+ appear to exist primarily as dimers in solution while smaller metal ions like Tb 3+ , Er 3+ , and Lu 3+ exist primarily as monomers . M-Ph complexes with U 3+ and the larger lanthanide ions Ce 3+ , Pr 3+ , and Nd 3+ also appear to exist primarily as dimers when dissolved.…”

“…16 In addition to facilitating more routine access to pure t Bu-PDB complexes, mechanochemical methods also allowed access to M-(H 3 BPPh 2 BH 3 ) 3 (M-Ph) complexes for the first time for comparative structural and spectroscopic analysis. 20 Almost all the M-t Bu and M-Ph complexes reported to date form dimeric structures in the solid state, 16,20,21 which is unusual given the significant differences in ionic radii ranging from La 3+ (1.032 Å) and U 3+ (1.025 Å) to Lu 3+ (0.861 Å). 22 The only exceptions we have observed thus far are Ph-PDB complexes with U 3+ and Ce 3+ ; Ce-Ph is polymeric in the solid state, whereas both crystals of dimeric and polymeric U-Ph have been isolated.…”