The Curious Case of [AnH(NR2)3] (An = Th, U; R = SiMe3): Two Monomeric Actinide Hydrides Revisited

The reaction of AnCl₄(DME)_x (An = Th, x = 2; An = U, x = 0) with 4 equiv of NaNR₂ (R = SiMe₃) in THF at 65 °C results in the formation of [An{N(R)(SiMe₂CH₂)}(NR₂)₂] (An = U, 1; An = Th, 2), and not the reported monomeric actinide hydrides, [AnH(NR₂)₃], as expected. Both complexes 1 and 2 were characterized by X-ray crystallography. Surprisingly, their unit cell parameters are remarkably close to those reported for [AnH(NR₂)₃], suggesting that the original crystals of [AnH(NR₂)₃] were, in fact, [An{N(R)(SiMe₂CH₂)}(NR₂)₂], but were misidentified. Reduction of 1 with 1.1 equiv of KC₈ in THF, in the presence of 1 equiv of 2.2.2-cryptand, results in the formation of [K(2.2.2-cryptand)][U{N(R)(SiMe₂CH₂)}(NR₂)₂] (3) in good yield. Likewise, the reaction of 1 with 1 equiv of bis(diisopropylamino)cyclopropenylidene (BAC) results in the formation of the BAC adduct, [(BAC)U{N(R)(SiMe₂CH₂)}(NR₂)₂] (4), in moderate yield. Finally, the addition of H₂ (10 bar) to 2 in C₆D₆ at room temperature results in the formation of the targeted monomeric hydride, [ThH(NR₂)₃], in 32% yield, according to integrations against an internal standard. However, removal of the H₂ atmosphere results in rapid reformation of 2. In contrast, the addition of H₂ (10 bar) to 1 in C₆D₆ at room temperature results in no apparent reaction.