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

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posted on 08.06.2015, 00:00 by Benedict M. Gardner, William Lewis, Alexander J. Blake, Stephen T. Liddle
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.