Rare earth dialkyl cations and monoalkyl dications supported by a rigid neutral pincer ligand: synthesis and ethylene polymerization

Abstract

A palladium-catalyzed coupling reaction between 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene and 2 equiv. of 1,3-diisopropylimidazolin-2-imine afforded the rigid neutral 2,7-di-tert-butyl-4,5-bis(1,3-diisopropylimidazolin-2-imino)-9,9-dimethylxanthene (XII₂) pincer ligand. Reaction of XII₂ with YCl₃(THF)_3.5 provided [(XII₂)YCl₃] (1). However, compound 1 failed to react cleanly with 3 equiv. of LiCH₂SiMe₃, and the reaction of XII₂ with [Y(CH₂SiMe₃)₃(THF)₂] afforded a complex mixture of products. To access group 3 alkyl complexes without the intermediacy of [(XII₂)M(CH₂SiMe₃)₃], the XII₂ ligand was protonated using [H(OEt₂)₂][B(C₆F₅)₄] to form [H(XII₂)][B(C₆F₅)₄], and subsequent reaction with [M(CH₂SiMe₃)₃(THF)₂] (M = Y, Sc) directly afforded the cationic scandium and yttrium dialkyl complexes [(XII₂)M(CH₂SiMe₃)₂][B(C₆F₅)₄] {M = Y (2) and Sc (3)}. Reaction of 3 with B(C₆F₅)₃ in C₆D₅Br afforded dicationic [(XII₂)Sc(CH₂SiMe₂CH₂SiMe₃)][MeB(C₆F₅)₃][B(C₆F₅)₄] (4) featuring a CH₂SiMe₂CH₂SiMe₃ ligand, formed as a result of methyl anion abstraction from silicon, with concomitant migration of the neighbouring CH₂SiMe₃ group from scandium to silicon. The MeB(C₆F₅)₃ anion in 4 forms a contact ion pair. By contrast, reaction of 1 with [CPh₃][B(C₆F₅)₃] in C₆D₅Br/toluene or o-C₆H₄F₂/toluene afforded dicationic [(XII₂)Sc(CH₂SiMe₃)(η^x-toluene)_n][B(C₆F₅)₄]₂ (5). Compounds 2–4 showed negligible ethylene polymerization activity, whereas 5 is highly active (up to 870 kg mol⁻¹ h⁻¹ atm⁻¹ in o-C₆H₄F₂/toluene under 1 atm of ethylene at room temperature).

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Introduction

Homogeneous rare earth (and transition metal) catalysts for olefin insertion polymerization are primarily monocationic alkyl species, generated by electrophilic abstraction of an alkyl group from a neutral polyalkyl pre-catalyst (which may itself be generated in situ) in arene or alkane solvent.^1–3 Dicationic alkyl complexes, particularly those which are monometallic and can provide access to a vacant coordination site, offer new catalyst design possibilities and may afford particularly high polymerization activities due to enhanced electrophilicity of the metal centre. However, such complexes have rarely been described or utilized in catalysis (vide infra).

A handful of monometallic rare earth alkyl dications which are stabilized through coordination of THF and/or a crown ether have been reported: [MR(THF)_x]²⁺ (x = 4–6; R = Me,^4,5 CH₂Ar,⁶ or CH₂SiMe₃ ^4,7), [M(CH₂SiMe₃)(THF)_x(12-crown-4)]²⁺ (x = 2 or 3),⁷ [MR(12-crown-4)₂]²⁺ (R = Me or CH₂SiMe₃),^7–9 and [MMe(L)(THF)_x]²⁺ (L = 15-crown-5 or 18-crown-6; x = 1 or 2),^8,9 where M is a rare earth element.^10–12 These complexes were generated in THF, and have not been reported to be substantially active ethylene or α-olefin polymerization catalysts, presumably due to a high degree of coordinative saturation. However, highly active ethylene polymerization catalysts were accessed by treatment of toluene solutions of [M(CH₂SiMe₃)₃(THF)₂] (M = Tm, Er, Y, Ho, Dy, Tb) with 5 equiv. of [HNMe₂Ph][B(C₆F₅)₄] in the presence of excess Al(CH₂SiMe₃)₃ or AlⁱBu₃; one equivalent of [HNMe₂Ph][B(C₆F₅)₄] was insufficient to achieve high polymerization activities, suggesting that the active species is a coordinatively unsaturated alkyl dication. A similarly active ethylene polymerization catalyst was accessed via the reaction of [Y(CH₂SiMe₃)₂(thf)₄][Al(CH₂SiMe₃)₄] with 5 equiv. of [HNMe₂Ph][B(C₆F₅)₄] in toluene in the presence of AlⁱBu₃. However, neither of the catalytically active species generated in toluene were spectroscopically or crystallographically characterized due to low solubility.⁴

Rare earth alkyl dications which are free from oxygen-donor solvent or crown ether ligands are shown in Fig. 1. The scandium alkyl dications (A–B) were generated in situ via reactions of a neutral trialkyl precursor with two equiv. of [CPh₃][B(C₆F₅)₄] in chlorobenzene or dichloromethane (for A),^13,14 or bromobenzene or toluene (for B),¹⁵ and were assigned on the basis of solution NMR studies, and because two equivalents of [CPh₃][B(C₆F₅)₄] were required to generate an active α-olefin polymerization catalyst.¹⁶ Neodymium complex C can be viewed as an alkyl dication with tight ion pairing of the two HB(C₆F₅)₃⁻ anions. This compound was characterized in solution and by X-ray crystallography, and was found to be a highly active catalyst for butadiene polymerization.¹⁷

Fig. 1 Rare earth alkyl dications (S = arene solvent) free from oxygen-donor solvent or crown ether ligands: the scandium complexes (A–B) were characterized in solution, whereas neodymium complex C was characterized in solution and the solid state.

Herein we report the synthesis of a rigid neutral NON-donor pincer ligand, which has enabled the synthesis of scandium and yttrium dialkyl monocations, as well as the monoalkyl dications [(XII₂)Sc(CH₂SiMe₂CH₂SiMe₃)][MeB(C₆F₅)₃][B(C₆F₅)₄] and [(XII₂)Sc(CH₂SiMe₃)(η^x-toluene)_n][B(C₆F₅)₄]₂. These dications were characterized by solution NMR spectroscopy, and the latter was found to be highly active for ethylene polymerization.

Results and discussion

Palladium-catalyzed coupling of 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene with 2 equiv. of 1,3-diisopropylimidazolin-2-imine afforded the neutral XII₂ ligand (2,7-di-tert-butyl-4,5-bis(1,3-diisopropylimidazolin-2-imino)-9,9-dimethylxanthene; Scheme 1), in which the ether donor of the xanthene backbone is flanked by imidazolin-2-imine donors. Two resonance structures for the XII₂ ligand are depicted in Scheme 1; the zwitterionic resonance structure is responsible for the high donor ability of the imidazolin-2-imine groups, and also provides a means for the imidazole rings to lie perpendicular to the plane of the xanthene backbone.

Scheme 1 Synthesis and resonance structures of the neutral XII₂ pincer ligand.

The XII₂ ligand is a neutral analogue of our recently reported AII₂ monoanion (4,5-bis(1,3-diisopropylimidazolin-2-imino)-2,7,9,9-tetramethylacridanide; Fig. 2).¹⁸ It is also a close steric analogue of the dianionic XA₂ ligand (4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene; Fig. 2), which we reported in 2007.¹⁹ This dianionic pincer ligand, and relatives with alternative aryl substituents on nitrogen, have been employed for the synthesis of a broad range of highly reactive early transition metal,²⁰ rare earth,^21,22 actinide,^19,23–30 and main group^31–45 species.

Fig. 2 Monoanionic AII₂ and dianionic XA₂ pincer ligands which are structurally related to the neutral XII₂ ligand in this work.

Reaction of XII₂ with YCl₃(THF)_3.5 cleanly generated [(XII₂)YCl₃] (1; Scheme 2), which was isolated in 82% yield, and gave rise to ¹H and ¹³C NMR spectra corresponding to the expected C_2v symmetry. However, compound 1 failed to react cleanly with 3 equiv. of LiCH₂SiMe₃ to afford [(XII₂)Y(CH₂SiMe₃)₃]. Furthermore, the reaction of XII₂ with [Y(CH₂SiMe₃)₃(THF)₂] afforded a complex mixture of products. Based on these results, we reasoned that [(XII₂)Y(CH₂SiMe₃)₃] may be unstable as a consequence of steric crowding. A related, albeit less-pronounced, situation was previously observed in the uranium chemistry of the dianionic XA₂ ligand, which shares a similar steric profile to XII₂: [(XA₂)U(CH₂SiMe₃)₂] is thermally stable in solution at room temperature, whereas [Li(THF)_x][(XA₂)U(CH₂SiMe₃)₃] decomposed over several days to afford SiMe₄ and unidentified paramagnetic products.²⁶

Scheme 2 Synthesis of complexes 1–3 (Im = 1,3-diisopropylimidazol-2-ylidene).

To bypass potentially unstable [(XII₂)Y(CH₂SiMe₃)₃], an alternative ligand attachment strategy was pursued (Scheme 2). Reaction of the XII₂ ligand with [H(Et₂O)₂][B(C₆F₅)₄] in fluorobenzene, followed by the addition of hexanes provided [H(XII₂)][B(C₆F₅)₄]·0.5 hexane as a white solid in 86% yield, and subsequent reaction with [M(CH₂SiMe₃)₃(THF)₂] (M = Y or Sc) afforded [(XII₂)M(CH₂SiMe₃)₂][B(C₆F₅)₄] [M = Y (2) or Sc (3)] in greater than 90% yield. The room temperature ¹H and ¹³C NMR spectra of 2 are consistent with apparent C_2v symmetry, with the MCH₂ signal at −0.77 ppm (¹J_C,H 102 Hz) in the ¹H NMR spectrum and 37.2 ppm (d, ¹J_C,Y 41 Hz) in the ¹³C{¹H} NMR spectrum. The ¹H and ¹³C NMR spectra of scandium complex 3 are similar, but with broadened signals from atoms located outside of the plane of the ligand backbone, indicative of a fluxional process involving migration of the alkyl groups between coordination sites within and above/below the plane of the ligand backbone (with concomitant flexing of the ligand backbone). Consistent with this explanation, a sharper ScCH₂¹H NMR signal was observed at 48 °C (−0.33 ppm), whereas upon cooling to −33 °C, this signal decoalesced to afford two broad peaks at 0.22 and −0.87 ppm. A similar fluxional process is presumably operative for 2, but would be expected to be more rapid due to reduced steric hindrance around the larger metal ion.

X-ray quality crystals of 2·2hexane were grown from flurobenzene/hexanes at −29 °C (Fig. 3). The XII₂ ligand is κ³NON-coordinated, with a ligand backbone bend (the angle between the planes of the two aryl rings of the ligand backbone) of 38° (cf. 29° in [(AII₂)Y(CH₂SiMe₃)₂],¹⁸ and 25° in [(XA₂)Y(CH₂SiMe₃)(THF)]²¹).⁴⁶ One alkyl ligand is located above the plane of the XII₂ ligand backbone, whereas the other is located approximately in the plane, affording a distorted square pyramidal geometry.

Fig. 3 Front (a) and side (b) views of the cationic portion of the X-ray crystal structure of [(XII₂)Y(CH₂SiMe₃)₂][B(C₆F₅)₄]·2hexane (2·2hexane). Ellipsoids are set to 30% probability, and hydrogen atoms are omitted for clarity. Both CMe₃ groups and one SiMe₃ group (attached to C(42)) are disordered, and only one orientation is shown. In view (b), all atoms of the XII₂ ligand, except for O, N(1) and N(2) are shown in wireframe. Selected bond lengths (Å) and angles (°): Y–N(1) 2.336(4), Y–N(2) 2.330(3), Y–O 2.425(3), Y–C(42) 2.393(5), Y–C(46) 2.385(5), N(1)–C(24) 1.367(5), N(2)–C(33) 1.378(5), C(27)⋯C(36) 4.80, C(30)⋯C(39) 8.43, N(1)–Y–N(2) 122.1(1), O–Y–C(42) 145.5(2), O–Y–C(46) 107.4(2), C(42)–Y–C(46) 107.0(2), Y–C(42)–Si(1) 126.3(4), Y–C(42)–Si(1A) 122.8(7), Y–C(46)–Si(2) 119.6(2).

The flanking imidazole rings are oriented approximately perpendicular to the plane of the ligand backbone (with angles of 80° and 87° between the plane of each imidazole ring and the adjacent aryl ring). Furthermore, the imine N(1)–C(24) and N(2)–C(33) distances {1.367(5) and 1.378(5) Å} are significantly longer than those in free imidazolin-2-imines such as (NHC)=N{C₆H₄(OMe)-p} (1.308(2) Å; NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene)⁴⁷ and [{κ²-2,6-C₅H₃N(CH₂N={NHC})₂}FeCl₂] (1.294(3) Å; NHC = 1,3-di-tert-butylimidazol-2-ylidene).⁴⁸ These observations are consistent with a substantial contribution from the zwitterionic resonance form of the imidazolin-2-imine donors.

Cationic 2 is qualitatively isostructural with neutral [(AII₂)Y(CH₂SiMe₃)₂].¹⁸ However, the Y–C distances of 2.385(5) and 2.393(5) Å in 2 are considerably shorter than those in [(AII₂)Y(CH₂SiMe₃)₂] (2.434(2) and 2.482(2) Å). Similarly, the Y–N(1) and Y–N(2) distances of 2.330(3) Å and 2.336(4) Å in 2 are notably shorter than the Y–N_{imidazolin-2-imine} distances in [(AII₂)Y(CH₂SiMe₃)₂] (2.394(2), 2.421(2) Å).⁴⁹ These bond metrics are indicative of a significantly more electron deficient yttrium center in cationic 2 than in neutral [(AII₂)Y(CH₂SiMe₃)₂].

X-ray quality crystals of 3·2PhF were also obtained from fluorobenzene/hexanes at −29 °C (Fig. S44†). The structure of 3 is analogous to that of 2, but with M–C and M–N bonds which are shorter by 0.14–0.18 Å, primarily reflecting the lower ionic radius of Sc^III (0.745 Å) versus Y^III (0.90 Å).⁵⁰ The ligand backbone bend angle is slightly more obtuse (41°) than that in 2, and the N(1)–C(24) and N(2)–C(33) distances of 1.369(4) Å (for both) are equal within error to those in 2.

Reactions of compound 2 with B(C₆F₅)₃ or [CPh₃][B(C₆F₅)₄] in bromobenzene or toluene/bromobenzene afforded mixtures of unidentified products accompanied by SiMe₄. By contrast, the reaction of 3 with B(C₆F₅)₃ in bromobenzene (Scheme 3) cleanly produced a new C_s symmetric product with a ¹H NMR spectrum featuring SiMe_x signals at −0.12 and −0.31 ppm, integrating to 9H and 6H respectively, accompanied by two CH₂ signals (2H each) at 0.85 and −0.66 ppm, and a broad singlet (3H) at 1.51 ppm (Fig. 4). These data point to the formation of dicationic [(XII₂)Sc(CH₂SiMe₂CH₂SiMe₃)][MeB(C₆F₅)₃][B(C₆F₅)₄] (4), resulting from methyl anion abstraction from silicon, with concomitant migration of the neighbouring CH₂SiMe₃ group from scandium to silicon.⁵¹

Scheme 3 Synthesis of dications 4 and 5 (Im = 1,3-diisopropylimidazol-2-ylidene). In the structures of 4 and 5, the alkyl groups may be coordinated within or above the plane of the ligand backbone, and only one possibility is shown. The hapticity of toluene coordination in 5 is unknown (toluene coordination is proposed because attempted syntheses of 5 in the absence of toluene did not afford a stable product).

Fig. 4 Alkyl region of the ¹H NMR spectra of (a) [(XII₂)Sc(CH₂SiMe₃)₂][B(C₆F₅)₄] (3), (b) [(XII₂)Sc(CH₂SiMe₂CH₂SiMe₃)][MeB(C₆F₅)₃][B(C₆F₅)₄] (4) generated in situ, and (c) [(XII₂)Sc(CH₂SiMe₃)(η^x-toluene)_n][B(C₆F₅)₄]₂ (5) generated in situ in the presence of 5 equiv. of toluene. All spectra are in C₆D₅Br. The spectra of 3 and 5 are at 298 K, whereas the spectrum of 4 is at 252 K.

The ¹H–¹³C and ¹H–²⁹Si HMBC NMR spectra provide further support the formation of a CH₂SiMe₂CH₂SiMe₃ ligand. For example, in the ¹H–¹³C HMBC (Fig. S24†), the γ-CH₂ carbon signal couples to the proton signals of both the SiMe₂ and SiMe₃ groups, and the SiMe₂ proton signal couples to both the α-CH₂ and γ-CH₂ carbon signals. In the ¹H–²⁹Si HMBC (Fig. S27†), the α-CH₂ and SiMe₂ protons couple to just one silicon center (SiMe₂), the SiMe₃ protons couple to a different silicon center (SiMe₃), and the γ-CH₂ group appears to couple to both silicon centers, although this is harder to distinguish due to overlapping cross-peaks.

The ¹⁹F and ¹¹B NMR spectra are also consistent with the proposed structure of 4. For example, the ¹¹B NMR spectrum of 4 features two sharp signals corresponding to the B(C₆F₅)₄ (δ −16.2 ppm) and MeB(C₆F₅)₃ (δ −15.0 ppm) anions. The latter chemical shift is comparable to that in previously reported scandium contact ion pairs involving a MeB(C₆F₅)₃ anion.^51–53 The ¹⁹F NMR spectrum shows the expected set of three signals for a free B(C₆F₅)₄ anion, accompanied by another set of signals for the MeB(C₆F₅)₃ anion {δ −133.27 (o-F), −157.57 (p-F), −162.09 (m-F)}. The large difference in the chemical shift between the meta and para fluorine signals {Δδ_m,p} of the MeB(C₆F₅)₃ anion (4.52 ppm) is indicative of a contact ion pair where the borate interacts with the metal centre via the methyl group.⁵⁴ Contact ion pairing is also indicated by the high frequency chemical shift of the BMe methyl group in the ¹H NMR spectrum (1.51 ppm), relative to that of the free anion (1.13 ppm).⁵⁴

Analogous reactivity (leading to a monocation rather than a dication) has previously been reported.^51,55,56 Piers et al. described the reaction of [(nacnac)Sc(CH₂SiMe₃)₂] {nacnac = ArNC(^tBu)CHC(^tBu)NAr; Ar = 2,6-C₆H₃ⁱPr₂} with B(C₆F₅)₃ to afford the contact ion pair [(nacnac)Sc(CH₂SiMe₂CH₂SiMe₃)][MeB(C₆F₅)₃],⁵¹ and Gordon et al. reported the reaction of [(κ³-L)Lu(CH₂SiMe₃)₂] {L = NC₅H₃(CMe=NAr)(CMe₂NAr)-2,6; Ar = 2,6-C₆H₃ⁱPr₂} with B(C₆F₅)₃ in the presence of THF to form the solvent-separated ion pair [(κ³-L)Lu(CH₂SiMe₂CH₂SiMe₃)(THF)][MeB(C₆F₅)₃].⁵⁵ Notably, the ¹H NMR chemical shifts for the CH₂SiMe₂CH₂SiMe₃ ligand and MeB(C₆F₅)₃⁻ anion in 4 are very similar to those in [(nacnac)Sc(CH₂SiMe₂CH₂SiMe₃)][MeB(C₆F₅)₃]: 0.02 (SiMe₃), −0.20 (SiMe₂), 1.16 (α-CH₂), −0.73 (γ-CH₂) and 1.51 (BMe) ppm.⁵¹

The reaction of 3 with a slight excess of [CPh₃][B(C₆F₅)₄] in bromobenzene was also investigated, and with rapid stirring in the presence of 5 equiv. of toluene, the dicationic monoalkyl complex [(XII₂)Sc(CH₂SiMe₃)(η^x-toluene)_n][B(C₆F₅)₄]₂ (5; Scheme 3) was generated over the course of 2 hours. Compound 5 gave rise to a ScCH₂ signal at 0.70 ppm (2H) in the ¹H NMR spectrum (Fig. 4), and 63.9 ppm in the ¹³C NMR spectrum. The expected ³J_C,H correlations were observed between the ScCH₂ and SiMe₃ groups in the ¹H–¹³C HMBC NMR spectrum, and ¹¹B and ¹⁹F NMR spectra confirmed that the B(C₆F₅)₄ anions are intact, ruling out C₆F₅ transfer from boron to scandium. Compound 5 exhibits apparent C_2v symmetry in solution (between 25 and −33 °C; measurements at lower temperature were not possible due to the melting point of bromobenzene, insolubility of 5 in toluene, and decomposition of 5 in dichloromethane), indicating that the alkyl group migrates rapidly between positions above and below the plane of the ligand backbone, or less likely, is located within the plane of the ligand backbone.

Dicationic 5 is poorly soluble, so the reaction afforded a bright orange solution accompanied by a dark orange oil which precipitated over the course of the 2 hour reaction period. The formation of 5 was accompanied by previously described byproducts of trimethylsilylmethyl group abstraction by the trityl cation: Ph₃CH as well as a product attributed to “Ph₃CCH₂SiMe₃” (one of two typically observed isomers).‡^,19,57

Bromobenzene solutions of 5, in the presence of five equivalents of toluene, were stable for hours at room temperature, with a substantial amount of the dication remaining after two days. However, in the absence of toluene, the 1 : 1 reaction of [CPh₃][B(C₆F₅)₄] with 3 in C₆D₅Br resulted in multiple unidentified products, as well as SiMe₄, indicative of decomposition. This strongly suggests that compound 5 is stabilized by toluene π-coordination (i.e. n = 1). Efforts to obtain crystals of 5 were unsuccessful, and ¹H NMR spectra at −33 °C did not show separate signals for free and coordinated toluene (consistent with the apparent C_2v symmetry of 5 at this temperature). However, arene π-coordination has frequently been observed for sterically similar monoalkyl monocations of the XA₂ ligand, such as [(XA₂)ZrMe(η⁶-toluene)]⁺,²⁰ [(XA₂)Th(CH₂SiMe₃)(η⁶-benzene)]⁺, [(XA₂)Th(CH₂Ph)(η⁶-toluene)]⁺,²⁹ [(XA₂)U(CH₂SiMe₃)(η⁶-benzene)]⁺, and [(XA₂)U(CH₂SiMe₃)(η³-C₆H₅R)]⁺ (R = Me or F).³⁰

Dicationic 4 (a contact ion pair) showed negligible ethylene polymerization activity in fluorobenzene under 1 atm of ethylene at room temperature. By contrast, dicationic 5 proved to be a highly³ active catalyst for ethylene polymerization: exposure of a 0.2 mM solution of 5 in toluene/o-difluorobenzene to 1 atm of ethylene at room temperature (with a water bath around the flask),§ followed by quenching with acidified methanol after 3 minutes, afforded a polymerization activity of 870 kg mol⁻¹ h⁻¹ atm⁻¹ (Table 1). A similar activity of 740 kg mol⁻¹ h⁻¹ was obtained when the reaction was quenched after 2 minutes. However, a lower activity of 570 kg mol⁻¹ h⁻¹ atm⁻¹ was achieved after 5 minutes, perhaps as a result of ineffective stirring due to precipitation of substantial amounts of polymer, catalyst entrapment within the precipitated polymer, and/or catalyst decomposition arising from the exothermic nature of the reaction. Attempted polymerization in neat o-difluorobenzene afforded (with quenching after 3 minutes) a lower polymerization activity of 170 kg mol⁻¹ h⁻¹ atm⁻¹, presumably due to significant catalyst decomposition in the absence of toluene.

Table 1 Ethylene polymerization data for catalyst 5 (0.2 mM concentration)^a under 1 atm of ethylene at room temperature

Solvent	Polymerization time (min)	Yield (g)	Activity (kg mol^−1 h^−1 atm^−1)	M w (g mol^−1)	M w/Mn
a The catalyst solution was generated in situ by stirring 15 mg of [(XII2)Sc(CH2SiMe3)2][B(C6F5)4] (3) with 1 equiv. of [CPh3][B(C6F5)4] in 3 mL of solvent (either a 1 : 2 mixture of toluene and o-C6H4F2, or neat o-C6H4F2) for 2 hours, followed by the addition of an additional 40 mL of solvent (either a 3 : 1 mixture of toluene and o-C6H4F2, or neat o-C6H4F2). b Values from GPC are relative to polyethylene standards. Additional data (Mp, Mz, Mv and Mp) is provided in the ESI.
Toluene/o-C6H4F2	2	0.239	741	65 700	1.40
Toluene/o-C6H4F2	3	0.420	868	111 880	1.41
Toluene/o-C6H4F2	5	0.456	565	202 100	2.06
o-C6H4F2	3	0.135	168	170 550	1.29

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The polydispersity of the polyethylene produced in these experiments was fairly narrow, with values of 1.40 and 1.41 when the reaction was carried out in toluene/o-difluorobenzene and quenched after 2 or 3 minutes, respectively (Table 1), indicative of single-site catalysis. The weight average molecular weight (M_w) of the polyethylene also increased with increasing reaction time; from 66 kg mol⁻¹ when the reaction was quenched after 2 minutes, to 112 kg mol⁻¹ after 3 minutes, and 202 kg mol⁻¹ after 5 minutes, (although after 5 minutes, a higher polydispersity of 2.06 was observed).

The polymerization activity of 5 is comparable to that of [Y(CH₂SiMe₃)₃(THF)₂] activated with 5 equiv. of [HNMe₂Ph][B(C₆F₅)₄] in toluene to generate a dication in situ {activities between 272 and 1840 kg mol⁻¹ h⁻¹ atm⁻¹ were obtained under 5 atm of ethylene in the presence of 200 equiv. of MAO or AlR₃; R = ⁱBu or CH₂SiMe₃}, although it is notable that under comparable conditions, the analogous reaction with [Sc(CH₂SiMe₃)₃(THF)₂] exhibited negligible polymerization activity.⁴

Summary and conclusions

Palladium-catalysed coupling afforded the neutral XII₂ pincer ligand, 2,7-di-tert-butyl-4,5-bis(1,3-diisopropylimidazolin-2-imino)-9,9-dimethylxanthene (XII₂), and reaction with YCl₃(THF)_3.5 provided [(XII₂)YCl₃] (1). However, compound 1 failed to react cleanly with 3 equiv. of LiCH₂SiMe₃, and the reaction of XII₂ with [Y(CH₂SiMe₃)₃(THF)₂] afforded a complex mixture of products. To access group 3 alkyl complexes without the intermediacy of [(XII₂)Y(CH₂SiMe₃)₃], the XII₂ ligand was protonated to form [H(XII₂)][B(C₆F₅)₄], and subsequent reaction with [M(CH₂SiMe₃)₃(THF)₂] (M = Y, Sc) directly afforded the cationic dialkyl complexes [(XII₂)M(CH₂SiMe₃)₂][B(C₆F₅)₄] {M = Y (2) and Sc (3)}. Cations 2 and 3 feature substantially shorter M–C and M–N bond distances than in the isostructural neutral AII₂ complexes, [(AII₂)M(CH₂SiMe₃)₂] (M = Y or Sc),¹⁸ indicative of a much more electrophilic metal centre in 2 and 3.

Compound 2 did not react cleanly with B(C₆F₅)₃ or [CPh₃][B(C₆F₅)₄]. By contrast, reaction of 3 with B(C₆F₅)₃ in C₆D₅Br afforded the dicationic contact ion pair [(XII₂)Sc(CH₂SiMe₂CH₂SiMe₃)][MeB(C₆F₅)₃][B(C₆F₅)₄] (4), which features a CH₂SiMe₂CH₂SiMe₃ ligand, presumably resulting from methyl anion abstraction from silicon with concomitant migration of the neighbouring CH₂SiMe₃ group from scandium to silicon. This type of reactivity is uncommon, but has previously been observed in the synthesis of a scandium and a lutetium alkyl monocation. Alternatively, reaction of 3 with [CPh₃][B(C₆F₅)₃] in C₆F₅Br/toluene or o-C₆H₄F₂/toluene afforded dicationic [(XII₂)Sc(CH₂SiMe₃)(η^x-toluene)_n][B(C₆F₅)₄]₂ (5). Compounds 4 and 5 are extremely rare examples of monometallic rare earth alkyl dications, especially those free from ether solvent or crown-ether coordination. The reactivity of 3 with CPh₃⁺ also contrasts that of [(AII₂)Y(CH₂SiMe₃)₂] with CPh₃⁺ which generated [(AII₂-CH₂SiMe₃)Y(CH₂SiMe₃)₂][B(C₆F₅)₄] (AII₂-CH₂SiMe₃ is a neutral tridentate ligand with a central amine donor flanked by imidazolin-2-imine groups) in approx. 20% yield, accompanied by HCPh₃, an unidentified paramagnetic product, and a colourless precipitate, likely via a pathway involving initial oxidation of the acridanide ligand backbone.¹⁸

Compounds 2–4 showed negligible ethylene polymerization activity. By contrast, dicationic 5 is a highly active catalyst (up to 870 kg mol⁻¹ h⁻¹ atm⁻¹ in o-C₆H₄F₂/toluene under 1 atm of ethylene at room temperature). This illustrates the utility of highly electrophilic dicationic alkyl species for olefin polymerization, and highlights the ability of the neutral XII₂ pincer ligand to serve as a robust ancillary ligand, capable of stabilizing extremely reactive organometallic species. Ligand rigidity can be anticipated to be particularly important in the case of neutral ligands, to prevent dissociation of one or more neutral donor. The XII₂ ligand is sterically analogous to our dianionic XA₂ pincer ligand (4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene), which has been employed by our group and others for the synthesis of highly reactive organometallic complexes of the early transition metals and f-elements, as well as a range of reactive main group species.

Experimental

General experimental

An argon-filled M-Braun UNIlab glovebox equipped with a −29 °C freezer was employed for the manipulation and storage of all air sensitive compounds, and reactions were performed on a double manifold vacuum line (with all glass–glass connections, rather than connections via hose tubing) using standard techniques. A Fisher Scientific Ultrasonic FS-30 bath was used to sonicate reaction mixtures where indicated. The scandium and yttrium compounds reported in this article are very air and moisture sensitive, and the vacuum line operated at <5 mTorr. Argon (99.999%) and ethylene (99.999%) were purchased from Praxair, and both gases were further purified by passage through an Oxisorb-W scrubber from Matheson Gas Products.

Hexanes and THF were initially dried and distilled at atmospheric pressure from sodium/benzophenone, while toluene was dried and distilled at atmospheric pressure from sodium. These solvents were then stored over an appropriate drying agent (toluene, THF = Na/Ph₂CO; hexanes = Na/Ph₂CO/tetraglyme). Fluorobenzene, 1,2-difluorobenzene, C₆D₅Br, and CD₂Cl₂ were dried over 4 Å molecular sieves, and C₆D₆ was dried over sodium/benzophenone; these solvents were degassed via three freeze–pump–thaw cycles. All solvents were introduced to reactions or solvent storage flasks via room temperature vacuum transfer with condensation at −78 °C (except for C₆D₅Br which was distilled at elevated temperature in vacuo).

4,5-Dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene,¹⁹ [Y(CH₂SiMe₃)₃(THF)₂],⁵⁸ [Sc(CH₂SiMe₃)₃(THF)₂],⁵⁹ B(C₆F₅)₃,^60,61 and 1,3-diisopropylimidazolin-2-imine⁶² were synthesized using literature procedures. [H(Et₂O)₂][B(C₆F₅)₄] was prepared using a modification of the literature synthesis:⁶³ a 2.0 M solution of HCl in OEt₂ (100 mL) was added via canula to a −78 °C solution of K[B(C₆F₅)₄] (10.0 g) in OEt₂ (30 mL); the reaction was stirred for 4 h at −30 °C before removal of the cold bath, filtration to remove KCl, and evaporation of the filtrate to dryness; the resulting white solid was washed with 10 mL of OEt₂ and then dried in vacuo to afford 6.5–8.0 g of the product.

LiCH₂SiMe₃ (1.0 M in pentane), 1,3-diisopropylimidazolium chloride, HCl (2.0 M in OEt₂), anhydrous YCl_3, anhydrous ScCl₃, Pd(OAc)₂, NaO^tBu, and DPEPhos were purchased from Sigma Aldrich. [CPh₃][B(C₆F₅)₄] was purchased from Alfa Aesar. K[B(C₆F₅)₄] was purchased from Boulder Scientific. YCl₃(THF)_3.5 and ScCl₃(THF)₃ were obtained by refluxing anhydrous MCl₃ (M = Y, Sc) in dry THF for 24 hours, followed by removal of solvent under vacuum. Deuterated solvents were purchased from Cambridge Isotope Laboratories.

Combustion elemental analyses were performed by Midwest Microlab, LLC in Indianapolis, Indiana. NMR spectroscopy [¹H, ¹³C{¹H}, DEPT-Q, COSY, HSQC, ¹H–¹³C and ¹H–²⁹Si HMBC, ¹⁹F, ¹¹B, ²⁹Si] was performed on a Bruker AV-600 or AV-500 Spectrometer. All ¹H NMR and ¹³C NMR spectra were referenced relative to SiMe₄ using the resonance of the deuterated solvent (¹³C NMR) or protio impurity in the deuterated solvent (¹H NMR); in ¹H NMR, 7.16 ppm for C₆D₆, 5.32 ppm for CD₂Cl₂, and 7.30, 7.02 and 6.94 ppm for C₆D₅Br; in ¹³C NMR, 128.06 ppm for C₆D₆, 54.00 ppm for CD₂Cl₂, and 130.90, 129.41, 126.24 and 122.17 ppm for C₆D₅Br. All NMR spectra were obtained at 298 K unless otherwise specified.

X-ray crystallographic analyses were performed on crystals coated in Paratone oil and mounted on a Bruker SMART APEX II diffractometer with a 3 kW sealed-tube Mo generator and APEX II CCD detector in the McMaster Analytical X-Ray (MAX) Diffraction Facility. A semi-empirical absorption correction was applied using redundant and symmetry related data. Raw data was processed using XPREP (as part of the APEX v2.2.0 software), and solved by intrinsic (SHELXT)⁶⁴ methods. Structures were completed by difference Fourier synthesis and refined with full-matrix least-squares procedures based on F². In all cases, non-hydrogen atoms were refined anisotropically and hydrogen atoms were generated in ideal positions and then updated with each cycle of refinement. Refinement was performed with SHELXL⁶⁵ in Olex2.⁶⁶ Two equivalents of disordered solvent (C₆H₅F or C₆H₁₄) were masked in the structures of 2 and 3 using the BYPASS method within Olex2.⁶⁷ Definitive identification of the masked solvent as either C₆H₅F or C₆H₁₄ was not possible given the identical number of electrons in both solvents. For 2, two equivalents of C₆H₁₄ were masked, while for 3, two equivalents of C₆H₅F were masked, because the difference map for the un-masked data contained maxima (in the void between molecular units) that roughly corresponded to a 6-membered ring.

Polyethylene molecular weights and molecular weight distributions were obtained using a Polymer Labs (now an Agilent company) PL-220 gel permeation chromatograph with 1,2,4-trichlorobenzene as the solvent at a flow rate of 1 mL min⁻¹ and at 145 °C. 2,6-Di-tert-butyl-4-methylphenol (BHT) at a concentration of 0.5 g L⁻¹ was used as a stabilizer in the solvent. An injection volume of 400 μL was used with a nominal polymer concentration of 1 g L⁻¹. Dissolution of the stabilized sample was carried out by heating at 150 °C for 5 h with occasional agitation. Three Waters HT-6E columns (7.8 × 300 mm) were used and calibrated with a broad linear polyethylene standard (Phillips Marlex® BHB 5003) whose molecular weight had previously been determined.

2,7-Di-tert-butyl-4,5-bis(1,3-diisopropylimidazolin-2-imino)-9,9-dimethylxanthene (XII₂)

A mixture of Pd(OAc)₂ (46.7 mg, 0.208 mmol), DPEPhos (156 mg, 0.29 mmol), NaO^tBu (479 mg, 2.49 mmol) was dissolved in approximately 20 mL of toluene in a 100 mL bomb, and stirred at room temperature for 10 minutes in the glovebox. A solution of 1,3-diisopropylimidazolin-2-imine (720 mg, 4.3 mmol) and 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene (1.00 g; 2.08 mmol) in approximately 30 mL of toluene was added to the reaction mixture. The reaction mixture was sealed in the 100 mL bomb and placed in a room temperature oil bath which was heated slowly (over several hours) to 95 °C, and maintained at this temperature for 48 h {note: placing the flask directly into a 95 °C oil bath, or cooling and re-heating before the 48 h at 95 °C had elapsed, generated undesired byproducts}. The reaction solution was passed through a pad of Celite and volatiles were removed under reduced pressure to afford a dark brown oil. Under air, the product was extracted using approximately 60 mL of CH₂Cl₂ and was washed with 60 mL of water. The resulting aqueous layer was additionally extracted with 30 mL of CH₂Cl₂ and the organic layers were combined. The organic layer was dried over MgSO₄ and gravity filtered. Volatiles were removed from the filtrate under reduced pressure to afford a sticky dark brown solid. This solid was dissolved in approximately 50 mL of hexanes and was passed through a pad of Celite, yielding a dark orange filtrate. The filtrate was concentrated to approximately 10 mL and was placed at −18 °C overnight. The resulting beige powder was dried for 12 h at 80 °C under reduced pressure to yield 609 mg of XII₂ (45% yield). ¹H NMR (C₆D₆, 600 MHz, 298 K): δ 7.13 (s. 4H, CH^1,8 & CH^3,6), 5.92 (s, 4H, N-CH), 4.45 (sept, 4H, ³J_H,H 7 Hz, CHMe₂), 1.82 (s, 6H, CMe₂), 1.38 (s, 18H, CMe₃) 1.00 (d, 24H, ³J_H,H 7 Hz) ppm. ¹³C{¹H} NMR (C₆D₆, 151 MHz, 298 K): δ 144.64 (s, NCN), 144.47 (s, C^2,7), 142.77 (s, C^4,5), 140.09 (s, C^11,12) 130.46 (s, C^10,13), 118.00 (s, CH^1,8), 112.02 (CH^3,6), 107.73 (s, N-CH), 45.26 (s, CHMe₂), 35.57 (s, C⁹). 34.58 (s, CMe₃), 32.03 (s, CMe₂ & CMe₃), 21.78 (s, CHMe₂) ppm. C₄₁H₆₀N₆O₁ (652.95 g mol⁻¹): calcd C 75.42, H 9.26, N 12.87%; found. C 75.63, H 9.47, N 12.51%.

[(XII₂)YCl₃] (1)

A solution of XII₂ (100 mg, 0.153 mmol) in 5 mL of THF was added dropwise to a solution of [YCl₃(THF)_3.5] (69 mg, 0.153 mmol) in THF (5 mL) at room temperature and the reaction was stirred for 1 hour. Approximately 40 mL of hexanes was added to the reaction, resulting in the precipitation of a white solid. The solution was decanted, and the white solid was further dried under vacuum for 1 hour. The solid was then washed with hexanes (3 × ∼5 mL) and dried under vacuum for 2 hours, yielding 1 as an air-sensitive white solid (106 mg, 82% yield). ¹H NMR (CD₂Cl₂, 600 MHz, 298 K): δ 7.07 (s, 4H, N-CH), 6.81 (s, 2H, CH^1,8), 5.65 (s, 2H, CH^3,6), 5.08 (sept, 4H, ³J_H,H 7 Hz, CHMe₂), 1.73 (s, 6H, CMe₂), 1.54 (d, 12H, ³J_H,H 7 Hz, CHMe₂), 1.29 (d, 12H, ³J_H,H 7 Hz, CHMe₂), 1.20 (s, 18H, CMe₃) ppm. ¹³C{¹H} NMR (CD₂Cl₂, 151 MHz, 298 K): δ 148.39 (s, NCN), 147.12 (s, C^2,7), 141.66 (s, C^4,5), 137.89 (C^11,12), 129.23(s, C^10,13), 114.79 (s, N-CH), 112.94 (s, CH^1,8), 109.72 (s, CH^3,6), 48.81 (s, CHMe₂), 34.75 (s, CMe₃), 34.38 (s, C⁹), 34.04 (s, CMe₂), 31.50 (s, CMe₃), 23.63, 23.33 (2 × s, CHMe₂) ppm. C₄₁H₆₀N₆O₁Cl₃Y (848.22 g mol⁻¹): calcd C 58.06, H 7.13, N 9.91%; found. C 58.33, H 7.16, N 9.43%.

[H(XII₂)][B(C₆F₅)₄]·0.5hexane

A solution of [H(Et₂O)₂][B(C₆F₅)₄] (127 mg, 0.153 mmol) in approximately 10 mL of C₆H₅F was added dropwise to a solution of XII₂ (1) (100 mg, 0.153 mmol) in C₆H₅F (5 mL) at room temperature. The reaction was stirred for 1 hour and 40 mL of hexanes was added, which resulted in precipitation of a white solid. The solution was decanted, and the solid was dried under vacuum. The solid was further washed with hexanes (3 × ∼5 mL) and then dried under vacuum for 4 hours, yielding [H(XII₂)][B(C₆F₅)₄]·0.5 hexane as a white solid (182 mg, 86% yield). ¹H NMR (CD₂Cl₂, 600 MHz, 298 K): δ 7.03 (s, 2H, CH^1,8), 6.83 (br s, 4H, N-CH), 6.40 (br s, 2H, CH^3,6), 5.85 (br s, 1H, NH), 4.38 (sept, 4H, ³J_H,H 7 Hz, CHMe₂), 1.67 (s, 6H, CMe₂), 1.29 (d, ³J_H,H 7 Hz, 24H, CHMe₂), 1.26 (s, 18H, CMe₃) ppm. ¹³C{¹H} NMR (CD₂Cl₂, 151 MHz, 298 K): δ 146.76 (s, C^2,7), 142.3 (br s, NCN), 138.82 (s, C^11,12), 130.71 (s, C^10,13), 124.5 (br s, C^4,5), 115.62 (s, CH^1,8), 113.38 (br s, CH^3,6 & NCH), 35.52 (s, C⁹), 35.02 (s, CMe₃), 32.61 (s, CMe₂), 31.73 (s, CMe₃), 22.40 (s, CHMe₂) ppm. C₆₈H₆₈N₆O₁F₂₀B₁ (1376.08 g mol⁻¹): calcd C 59.35, H 4.98, N 6.11%; found. C 59.75, H 5.48, N 5.55%.

[(XII₂)Y(CH₂SiMe₃)₂][B(C₆F₅)₄] (2)

A solution of [H(XII₂)][B(C₆F₅)₄]·0.5 hexane (300 mg, 0.218 mmol) in 10 mL of C₆H₅F was added dropwise to a solution of [Y(CH₂SiMe₃)₃(THF)₂] (108 mg, 0.218 mmol) in C₆H₅F (5 mL) at room temperature and the reaction was stirred for 1 hour. Approximately 40 mL of hexanes was added to the reaction resulting in the precipitation of a white solid. The solution was decanted, and the white solid was dried under vacuum for 1 hour. The solid was washed with hexanes (3 × ∼5 mL) and dried under vacuum for 2 hours, yielding 2 as a highly air-sensitive white solid (323 mg, 93% yield). X-ray quality crystals of 2 were grown by layering hexanes on top of a solution of 2 in C₆H₅F and cooling to −29 °C. ¹H NMR (C₆D₅Br, 600 MHz, 298 K): δ 7.01 (s, 2H, CH^1,8), 6.72 (s, 4H, N-CH), 5.64 (s, 2H, CH^3,6), 4.63 (sept, ³J_H,H 7 Hz, 4H, CHMe₂), 1.68 (s, 6H, CMe₂), 1.32 (d, ³J_H,H 7 Hz, 12H, CHMe₂), 1.16 (s, 18H, CMe₃), 1.10 (d, ³J_H,H 7 Hz, 12H, CHMe₂), −0.19 (s, 18H, SiMe₃), −0.77 (appt s, 2H, YCH₂) ppm. ¹³C{¹H} NMR (C₆D₅Br, 151 MHz, 298 K): δ 148.49 (s, C^2,7), 147.47 (s, NCN), 140.49 (s, C^4,5), 138.01 (C^11,12), 130.23 (s, C^10,13), 114.68 (s, N-CH), 112.90 (s, CH^1,8), 109.28 (s, CH^3,6), 48.49 (s, CHMe₂), 37.69 (d, ³J_H,H 42 Hz, YCH₂), 34.57 (s, C⁹), 34.49 (s, CMe₃), 32.14 (s, CMe₂), 31.19 (s, CMe₃), 23.06, 21.92 (2 × s, CHMe₂), 3.46 (s, SiMe₃) ppm. C₇₃H₈₂N₆O₁F₂₀B₁Si₂Y (1595.32 g mol⁻¹): calcd C 54.96, H 5.18, N 5.27%; found. C 54.63, H 5.23, N 4.91%.

[(XII₂)Sc(CH₂SiMe₃)₂][B(C₆F₅)₄] (3)

A solution of [H(XII₂)][B(C₆F₅)₄]·0.5 hexane (300 mg, 0.218 mmol) in 10 mL of C₆H₅F was added dropwise to a solution of [Sc(CH₂SiMe₃)₃(THF)₂] (98 mg, 0.218 mmol) in C₆H₅F (5 mL) at room temperature and the reaction was stirred for 1 hour. Approximately 40 mL of hexanes was added to the reaction resulting in the precipitation of a white solid. The solution was decanted, and the white solid was dried under vacuum for 1 hour. The solid was further washed with hexanes (3 × ∼5 mL) and dried under vacuum for 2 hours, yielding 3 as a highly air-sensitive white solid (308 mg, 91% yield). X-ray quality crystals of 3 were grown by layering hexanes on top of a solution of 3 in C₆H₅F and cooling to −29 °C. ¹H NMR (C₆D₅Br, 600 MHz, 298 K): 6.94 (s, 2H, CH^1,8), 6.76 (s, 4H, N-CH), 5.62 (s, 2H, CH^3,6), 4.69 (br s, 4H, CHMe₂), 1.66 (s, 6H, CMe₂), 1.32 (d, 12H, ³J_H,H 6 Hz, CHMe₂), 1.16 (s, 18H, CMe₃), 1.10 (d, ³J_H,H 6 Hz, 12H, CHMe₂), −0.25 (s, 18H, SiMe₃), −0.35 (br s, 4H, ScCH₂) ppm. ¹³C{¹H} NMR (C₆D₅Br, 151 MHz, 298 K): 148.44 (s, NCN), 147.90 (s, C^2,7), 140.74 (s, C^4,5), 137.72 (C^11,12), 129.69 (s, C^10,13), 114.66 (s, N-CH), 112.14 (s, CH^1,8), 108.32 (s, CH^3,6), 48.34 (s, CHMe₂), 34.57 (s, C⁹), 34.46 (s, CMe₃), 31.10 (s, CMe₃ & CMe₂), 23.21, 21.54 (2 × s, CHMe₂), 2.87 (s, CH₂SiMe₃) ppm. Note: the ¹³C NMR ScCH₂ signal was not located from the ¹H–¹³C HSQC NMR spectrum, but may be a very broad peak at 43.9 ppm. C₇₃H₈₂N₆O₁F₂₀B₁Si₂Sc (1551.37 g mol⁻¹): calcd C 56.52, H 5.33, N 5.42%; found. C 56.21, H 5.46, N 5.08%.

In situ synthesis of [(XII₂)Sc(CH₂SiMe₂CH₂SiMe₃)][MeB(C₆F₅)₃][B(C₆F₅)₄] (4)

A solution of B(C₆F₅)₃ (9 mg, 0.0176 mmol) in 0.4 mL of C₆D₅Br was added dropwise to a solution of [(XII₂)Sc(CH₂SiMe₃)₂][B(C₆F₅)₄] (3) (25 mg, 0.0161 mmol) in C₆D₅Br (0.4 mL) at room temperature and the reaction was stirred for 2 hours. ¹H NMR (C₆D₅Br, 500 MHz, 252 K): δ 7.09 (s, 2H, C^1,8), 6.79, 6.77 (2 × br s, 2H, N-CH), 5.46 (s, 2H, C^3,6), 4.91 (sept, 2H, ³J_H,H 7 Hz, CHMe₂), 4.27 (sept, 2H, ³J_H,H 7 Hz, CHMe₂), 1.72 (s, 6H, CMe₂), 1.51 (br s, 3H, MeB(C₆F₅)₃), 1.26 (d, 6H, ³J_H,H 7 Hz, CHMe₂), 1.17 (d, 6H, ³J_H,H 7 Hz, CHMe₂), 1.15 (s, 18H, CMe₃), 0.99 (d, 6H, ³J_H,H 7 Hz, CHMe₂), 0.85 (s, 2H, ScCH₂), 0.82 (d, 6H, ³J_H,H 7 Hz, CHMe₂), −0.12 (s, 9H, SiMe₃), −0.31 (s, 6H, SiMe₂), −0.66 (s, 2H, CH₂SiMe₃) ppm. ¹³C{¹H} NMR (C₆D₅Br, 126 MHz, 252 K): δ 149.89 (s, C^2,7), 144.73 (s, NCN), 139.42 (s, C^4,5), 136.61 (s, C^11,12), 129.2 (s, C^10,13), 115.98, 115.38 (2 × s, N-CH), 114.87 (s, CH^1,8), 109.05 (s, CH^3,6), 65.7 (s, ScCH₂), 49.31, 48.47 (2 × s, CHMe₂), 34.59 (s, CMe₃ & CMe₂), 34.30 (s, C⁹), 30.85 (s, CMe₃), 31.4 (s, BMe), 30.02 (s, CMe₂), 23.32, 22.61, 21.59, 21.12 (4 × s, CHMe₂), 6.60 (s, CH₂SiMe₃), 2.46 (s, CH₂SiMe₂), 1.22 (s, CH₂SiMe₃) ppm. Note: the C,^10,13 ScCH₂ and BMe¹³C NMR signals were located via¹H–¹³C HSQC or HMBC NMR. ¹¹B NMR (C₆D₅Br, 161 MHz, 252 K): −15.02 (s, MeB(C₆F₅)₃), −16.20 (s, B(C₆F₅)₄) ppm. ¹⁹F NMR (C₆D₅Br, 471 MHz, 252 K): −131.76 (br s, o-B(C₆F₅)₄), −133.27 (d, ³J_F,F 22 Hz, o-MeB(C₆F₅)₃), −157.57 (br s, p-MeB(C₆F₅)₃), −161.76 (br s, p-B(C₆F₅)₄), −162.09 (br s, m-MeB(C₆F₅)₃), −165.58 (br s, m-B(C₆F₅)₄) ppm.

In situ synthesis of [(XII₂)Sc(CH₂SiMe₃){η^x-toluene}][B(C₆F₅)₄]₂ (5)

A solution of [CPh₃][B(C₆F₅)₄] (15.5 mg, 0.0168 mmol) in 0.4 mL of C₆D₅Br was added dropwise to a solution of [(XII₂)Sc(CH₂SiMe₃)₂][B(C₆F₅)₄] (3) (20 mg, 0.0129 mmol) and 5 equivalents of toluene (11.9 mg) in C₆D₅Br (0.4 mL) at room temperature. The reaction was stirred vigorously for 2 hours. ¹H NMR (C₆D₅Br, 600 MHz, 298 K): δ 7.11 (s, 2H, CH^1,8), 6.87 (s, 4H, N-CH), 5.63 (s, 2H, CH^3,6), 4.38 (br s, 4H, CHMe₂), 1.65 (s, 6H, CMe₂), 1.16 (s, 18H, CMe₃), 1.08 (appt t, ³J_H,H 6 Hz, 24 H, CHMe₂), 0.70 (s, 2H, ScCH₂), −0.22 (s, 9H, SiMe₃) ppm. ¹³C{¹H} NMR (C₆D₅Br, 151 MHz, 298 K): δ 151.03 (s, C^2,7), 143.93 (s, NCN), 137.90 (s, C^4,5), 130.9 (s, C^10,13), 116.08 (s, N-CH), 115.10 (s, CH^1,8), 108.92 (s, CH^3,6), 63.9 (s, CH₂SiMe₃), 49.27 (s, CHMe₂), 35.05 (s, C⁹), 34.73 (s, CMe₃), 30.86 (s, CMe₃), 30.4 (s, CMe₂), 22.72, 21.39 (2 × s, CHMe₂), 1.68 (s, SiMe₃) ppm. Note: the C,^10,13 ScCH₂ and CMe₂¹³C NMR signals were located by ¹H–¹³C HSQC or HMBC NMR. ¹¹B NMR (C₆D₅Br, 161 MHz, 298 K): δ −16.21 (s, B(C₆F₅)₄) ppm. ¹⁹F NMR (C₆D₅Br, 471 MHz, 298 K): δ −131.50 (br s, o-B(C₆F₅)₄), −161.67 (br s, p-B(C₆F₅)₄), −165.44 (br s, m-B(C₆F₅)₄) ppm.

Ethylene polymerization

In the glovebox, 15 mg (9.7 μmol) of [(XII₂)Sc(CH₂SiMe₃)₂][B(C₆F₅)₄] (3) was dissolved in approximately 1 mL of o-C₆H₄F₂ and 1 mL of toluene. To this was added a solution of 9 mg (9.8 μmol) of [CPh₃][B(C₆F₅)₄] in 1 mL of o-C₆H₄F₂, and the solution was stirred vigorously for 2 hours at room temperature. The solution was diluted with 10 mL of o-C₆H₄F₂ and 30 mL of toluene, and the flask was affixed to a vacuum line. A room temperature water bath was placed around the flask, and with rapid stirring, the solution was briefly evacuated before placing the flask under dynamic ethylene (1 atm), causing an exothermic reaction, with the onset of polyethylene precipitation after approx. 1 minute. After 2, 3 or 5 minutes, the solution was opened to air and approximately 5 mL of acidified methanol (10 vol% 12 M HCl_(aq.); 90 vol% MeOH) was added. The resulting solid was filtered, washed with methanol and acetone, and then dried at 50 °C under vacuum to afford dry polyethylene. This method was also carried out using the same masses and approximate solution volumes, but replacing toluene with o-C₆H₄F₂.

Data availability

All relevant data is in the ESI† or the CCDC.

Author contributions

DJHE conceptualized the project. AV performed the synthesis, characterization and polymerization experiments. JSP carried out the solution and refinement of X-ray data. All authors contributed to writing the manuscript and have approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

D. J. H. E. thanks NSERC of Canada for a Discovery Grant. We are also grateful to Dr Carlos A. Cruz and Chevron Phillips Chemical Company for GPC analysis of polymer samples, and to Dr Jim Britten of the McMaster Analytical X-ray Diffraction Facility for advice and support with X-ray diffraction.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: NMR spectra, the X-ray structure of compound 3, and polyethylene GPC data. CCDC 2247081 (2) and 2247082 (3). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc01830j

‡ Two isomers of “Ph₃CCH₂SiMe₃” are frequently observed, giving rise to ¹H NMR signals at 2.36 and 2.06 ppm (CH₂), and −0.01 and −0.26 ppm (SiMe₃) in C₆D₅Br; see ref. 19 and 57. In the reaction to form 5, only one of these isomers was formed (2.36, −0.02 ppm; relative integration 2 : 9). By contrast, in the reaction to form B in Fig. 1 (see ref. 15), only the other isomer (2.05, −0.26 ppm) was formed.

§ The polymerization reactions were exothermic, causing an increase in the solution temperature, despite the room temp. water bath around the flask.

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