Thorium Triamidoamine Complexes: Synthesis of an Unusual Dinuclear Tuck-in–Tuck-over Thorium Metallacycle Featuring the Longest Known Thorium−σ-Alkyl Bond

We
report the synthesis and characterization of a range of thorium­(IV)–halide,
−amide, and −alkyl complexes supported by a sterically
demanding triamidoamine ligand. Reaction of [ThCl4(THF)3.5] with [Li3(TrenDMBS)­(THF)3] (TrenDMBS = N­(CH2CH2NSiMe2But)3) gave [Th­(TrenDMBS)­(Cl)­(THF)]
(1), which was converted to [Th­(TrenDMBS)­(I)]
(2), with concomitant elimination of Me3SiCl,
by treatment with Me3SiI. Treatment of 2 with
[LiNEt2] afforded [Th­(TrenDMBS)­(NEt2)] (3), or treatment with [PhCH2K] produced
the dimeric tuck-in–tuck-over alkyl complex [Th­{N­(CH2CH2NSiMe2But)2(CH2CH2NSiMeBut-μ-CH2)}]2 (4), which features the longest reported Th−σ-alkyl
bond distance of 2.875(9) Å. Reaction of 4 with
[Et3NH]­[BPh4] to give the putative separated
ion pair complex [Th­(TrenDMBS)]­[BPh4] resulted
in an intractable product mixture. In order to furnish a greater understanding
of the latter reaction, the solvent-separated ion pair complex [U­(TrenDMBS)­(NCMe)2]­[BPh4] (5)
was prepared from the monomeric uranium analogue of 4, [U­{N­(CH2CH2NSiMe2But)2(CH2CH2NSiMeBut-μ-CH2)}] (I), to provide a related model complex and
to examine potential routes to useful TrenDMBS–actinide
precursors. The nascent reactivity of 4 is suggested
by the isolation of a small quantity of the oxo-insertion product
[Th­{N­(CH2CH2NSiMe2But)2(CH2CH2NSiMeButCH2-μ-O)}]2 (6) when 4 is
stored in diethyl ether. The complexes have been variously characterized
by single-crystal X-ray diffraction, NMR spectroscopy, FTIR spectroscopy,
Evans method solution magnetic moment determination, and CHN elemental
analyses.