Expanding Thorium Hydride Chemistry Through Th²⁺, Including the Synthesis of a Mixed-Valent Th⁴⁺/Th³⁺ Hydride Complex

Abstract

The reactivity of the recently discovered Th(2+) complex [K(18-crown-6)(THF)2][Cp″3Th], 1 [Cp'' = C5H3(SiMe3)2-1,3], with hydrogen reagents has been investigated and found to provide syntheses of new classes of thorium hydride compounds. Complex 1 reacts with [Et3NH][BPh4] to form the terminal Th(4+) hydride complex Cp″3ThH, 2, a reaction that formally involves a net two-electron reduction. Complex 1 also reacts in the solid state and in solution with H2 to form a mixed-valent bimetallic product, [K(18-crown-6)(Et2O)][Cp″2ThH2]2, 3, which was analyzed by X-ray crystallography, electron paramagnetic resonance and optical spectroscopy, and density functional theory. The existence of 3, which formally contains Th(3+) and Th(4+), suggested that KC8 could reduce [(C5Me5)2ThH2]2. In the presence of 18-crown-6, this reaction forms an analogous mixed-valent product formulated as [K(18-crown-6)(THF)][(C5Me5)2ThH2]2, 4. A similar complex with (C5Me4H)(1-) ligands was not obtained, but reaction of (C5Me4H)3Th with H2 in the presence of KC8 and 2.2.2-cryptand at -45 °C produced two monometallic hydride products, namely, (C5Me4H)3ThH, 5, and [K(2.2.2-cryptand)]{(C5Me4H)2[η(1):η(5)-C5Me3H(CH2)]ThH]}, 6. Complex 6 contains a metalated tetramethylcyclopentadienyl dianion, [C5Me3H(CH2)](2-), that binds in a tuck-in mode.

Chemical structure and bonding in a thorium(iii)–aluminum heterobimetallic complex

“…The crystal structure of the Th terminal hydride complex 3 is of note as there are few crystallographically characterized examples of complexes containing this moiety. 43 , 64 – 68 Most importantly, the solid-state structures of 2 and 3 provide confirmation as to the binding mode of the H 3 AlC(SiMe 3 ) 3 ligand, which was found in both structures to bind to the Th center through all three hydrides of the alanate. All bridging and terminal hydride atoms in the structures of 2 and 3 were explicitly located and refined freely.…”

“…The 1 H NMR spectrum of 3 also contained an additional resonance at 15.11 ppm attributed to the terminal Th– H hydride, which is in the range reported for other known examples. 43 , 56 , 64 – 67…”

“… 54 Evans and coworkers reported the first examples of Th( iii ) bimetallics containing a Th( iii )–μ-H–Th( iv ) moiety; however, evidence for Th–Th interactions was not obtained from analysis of the solid structure or using Density functional theory (DFT) calculations. 43 Building on other synthetic routes to Th hydride complexes, 55 – 60 we turned to aluminum hydrides to install a Lewis acidic metal in close proximity to the Th center in order to promote a Th→Al interaction. Analogously, Lu and coworkers have utilized TM→Al (TM = Fe, Co, Ni) interactions to enable new chemistry in electron rich transition metal systems.…”

New vistas in the molecular chemistry of thorium: low oxidation state complexes

“…The formation of [Th(Cp′′) 3 (H)] implies that a 2eprocess has occurred but the concomitant formation of 4 is likely due to a 1eprocess; as such the overall mechanism of this reaction is unclear. In the same report, Evans and co-workers showed that 9a is capable of activating dihydrogen, leading to the formation Scheme 15) 32. This result is particularly remarkable as the Th(III) analogue 4 does not react with dihydrogen under these conditions.…”

“…cyclooctatetraene, yielding the heteroleptic Th(IV) complex [Th(Cp′′) 2 (C 8 H 8 )] and [K(18-crown-6)][Cp′′] (Scheme 14). 26 More recently, 9a was shown to react with [Et 3 NH][KBPh 4 ] to afford the Th(IV) hydride complex [Th(Cp′′) 3 (H)] and 4, which wereidentified by single crystal XRD and 1 H NMR spectroscopy (Scheme 15) 32. The formation of [Th(Cp′′) 3 (H)] implies that a 2eprocess has occurred but the concomitant formation of 4 is likely due to a 1eprocess; as such the overall mechanism of this reaction is unclear.…”