Rigid Non-Carbocyclic Ancillary Ligands in Organothorium Chemistry

A new rigid, dianionic ligand, 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene {[XA₂]}, has been designed for use in the chemistry of the actinides. Pro-ligand H2[XA₂] (1) was synthesized by the Hartwig-Buchwald coupling of 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene with 2,6-diisopropylaniline. Stable alkali metal salts of the [XA₂] ligand, K₂(dme)₂[XA₂] (2) and Na₂[XA₂] (3), were accessible by deprotonation of H₂[XA₂] with KH or NaH in dme or toluene, respectively. The thermally unstable lithium salt of McConville's 2,6-bis(2,6diisopropylanilidomethyl)pyridine {Li₂[BDPP], 4} was isolated by deprotonation of pro-ligand H₂[BDPP] with nBuLi or LiCH₂SiMe₃ in hexanes at low temperature. Reaction of [ThCl₄(dme)₂] with Li₂[BDPP] or M₂(dme)_n[XA₂] (M = K, n = 2; M = Na, n = 0) resulted in the formation of pentagonal bipyramidal [LThCl₂(dme)] complexes {L = [BDPP], 5; [XA₂], 6}. Subsequent reaction of the dichloride complexes with LiCH₂SiMe₃ gave base-and salt-free dialkyl complexes [LTh(CH₂SiMe₃)₂] {L = [BDPP], 9; [XA₂], 10}, which are stable for days in solution at 90 and 70 °C, respectively. Reaction of 5 with LiNEt₂ or 10 with H₂NPh provided [(BDPP)Th(NEt₂)₂] (11) and [(XA₂)Th(NHPh)₂] (28), respectively. An alternative route to [(BDPP)ThCl₂(dme)] (5) and [LTh(CH₂SiMe₃)₂] (9 and 10) involved combination of two or four equivalents of LiCH₂SiMe₃ with [ThCl₄(dme)₂], followed by addition of H₂L. These reactions likely proceed by alkane elimination from dialkyl or tetraalkyl thorium intermediates. The solid-state structure of [(BDPP)Th(CH₂SiMe₃)₂] (9) suggests the presence of α-agostic C-H-Th interactions for both alkyl groups. In solution, 9 and 10 exhibit temperature-dependent ¹J_C-H coupling constants for ThCH₂, consistent with an equilibrium between products participating in aagostic C-H-Th bonding to a greater or lesser extent, with more agostic products favored at lower temperatures. Reaction of Li₂[BDPP] (4) with [(BDPP)ThCl₂(dme)] (5) at 0 °C, or the reaction of [(BDPP)Th(CH₂SiMe₃)₂] (9) with H₂[BDPP] at 100 °C resulted in the formation of extremely sterically encumbered [Th(BDPP)₂] (8), which adopts a highly distorted six-coordinate geometry with the four anilido groups arranged in an approximate tetrahedron around thorium. A his-ligand complex was not accessible with the [XA₂] ancillary ligand, presumably due to increased ligand rigidity. Addition of two equivalents of PhCH₂MgCl to [LThCl₂(dme)] yielded solvent-free [LTh(CH₂Ph)₂] {L = [XA₂] (12) and [BDPP] (13)]. The ¹J_C-H coupling constants in both complexes {120 and 139 Hz for 12; 127 and 138 Hz for 13} are indicative of η¹- and η²- or η³-coordinated benzyl ligands in solution; polyhapto benzyl coordination was also observed in the solid state. Reaction of [LThCl₂(dme)] with two equivalents of nBuLi provided highly soluble [LTh(nBu)₂] {L = [BDPP] (14), [XA₂] (15)]. These β-hydrogen-containing compounds are remarkably thermally stable, showing no sign of decomposition after days at 60 and 80 °C, respectively. Combination of [(BDPP)ThCl₂(dme)] (5) with three equivalents of MeLi yielded the thorium trimethyl 'ate' complex [(BDPP)ThMe₃{Li(dme)}] (16), which underwent thermal decomposition over 3 days at room temperature to produce the metalated complex [(BDPP*)Th(µ-Me)₂Li(dme)] (17) {BDPP* = 2,6-{NC₅H₃(CH₃NAr)(CH₂N {C₆H₃iPr(CMe₂)-2,6})}; Ar = 2,6-diisopropylphenyl; donor atoms in BDPP* are underlined}. Reaction of two equivalents of complex 16 with one equivalent of [(BDPP)ThCl₂(dme)] (5) yielded the dimethyl complex [(BDPP)ThMe₂] (18) which decomposes rapidly at room temperature to form a mixture of unidentified products. Labeling studies using ¹³CD₃ groups revealed that thermal decomposition of 16 and 18 occurs via a straightforward a-bond metathesis pathway. Reaction of [LThCl₂(dme)] with Grignard reagents {MeMgBr, L = [BDPP]; PhCH₂MgCl, L = [XA₂]} resulted, under certain conditions, in halide exhange and adduct formation as evidenced by the solid state structure of [{Th(BDPP)Br(µ-Br)₂Mg(µ-Me)(OEt₂)}₂] (19), or ancillary ligand transfer to magnesium to produce [(XA₂)Mg(dme)] (20). Complex 19 provides insight into the type of intermediates likely involved in undesired halide exchange reactions between d-or f-element halide complexes and Grignard reagents. Reaction of [(XA₂)Th(CH₂Ph)₂] (12) with either one or two equivalents of B(C₆F₅)₃ afforded the first non-carbocyclic actinide alkyl cation, [(XA₂)Th(CH₂Ph)][PhCH₂B(C₆F₅)₃] (21), and a rare example of an actinide dication, [(XA₂)Th][PhCH₂B(C₆F₅)₃]₂ (27). In both 21 and 27 the PhCH₂B(C₆F₅)₃⁻ anion is η⁶-coordinated to thorium. Reaction of neutral dialkyl complex [(XA₂)Th(CH₂SiMe₃)₂] (9) with [Ph₃C][B(C₆F₅)₄] in benzene or toluene at room temperature resulted in the formation of [(XA₂)Th(CH₂SiMe₃)(η⁶-arene)][B(C₆F₅)₄] (arene = C₆H₆, 22; arene = C₇H₈, 23). These complexes were characterized in solution by NMR spectroscopy (21, 22 and 23) and/or in the solid state by X-ray crystallography (22 and 27). In close analogy, [(XA₂)Th(CH₂Ph)₂] (12) reacted with [Ph₃C][B(C₆F₅)₄] in toluene at room temperature to form [(XA₂)Th(η²-CH₂Ph)(η⁶-C₇H₈)][B(C₆F₅)₄] (24). In contrast, related [(BDPP)Th(CH₂Ph)₂] (13) reacted with [Ph₃C][B(C₆F₅)₄] to precipitate a mixture of mononuclear and a dinuclear cations; the dinuclear cation was identified as [(BDPP)Th(η²-CH₂Ph)(µ-η¹:η⁶-CH₂Ph)Th(η¹-CH₂Ph)(BDPP)][B(C₆F₅)₄] (25) by X-ray crystallography. Complexes 22, 23, and 24 are rare examples of arene solvent-separated ion pairs, while complex 21 exists as a tight contact ion pair, and dinuclear 25 exhibits a unique benzyl ligand bridging mode. Cations 21-25 and 27 highlight a pronounced tendency for these systems to engage in arene π-coordination. Preliminary reaction studies with both neutral and cationic thorium complexes supported by the [BDPP] and [XA₂] ancillary ligands demonstrated significant activity for olefin polymerization and hydroamination catalysis. Reactions of 9 and 10 with 4 atm. of hydrogen also suggest that the [BDPP] and [XA₂] ligand frameworks may be suitable for the stabilization of thorium hydride complexes.

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