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.