Oxidative Addition to Group 1 (K, Rb, Cs) Alumanyl Anions as a Route to o-Carboranyl (hydrido)aluminates

Abstract

The kinetic stability provided by the sterically demanding {SiN^Dipp}²⁻ dianion (SiN^Dipp = {CH₂SiMe₂NDipp}₂; Dipp = 2,6-i-Pr₂C₆H₃) is intrinsic to the isolation of not only the group 1 alumanyl reagents ([{SiN^Dipp}AlM]₂; M = K, Rb, Cs) but also facilitates the completely selective oxidative addition of a C-H bond of 1,2-C₂B₁₀H₁₂ to the aluminium centre. In each case, the resultant compounds comprise a four-coordinate o-carboranyl (hydrido)aluminate anion, [(SiN^Dipp)Al(H)(1,2-C₂B₁₀H₁₁)]⁻, in which the carboranyl cage is bonded to aluminium by an Al-C σ bond. Although the anions further assemble as extended network structures based on Al-H∙∙∙M, B-H∙∙∙M, and C-H∙∙∙M interactions, each structure is unique due to the significant variation in M⁺ ionic radius as group 1 is descended. The potassium derivative crystallises as a one-dimensional polymer, its rubidium analogue is a dimer due to the polyhapto-sequestration of a molecule of benzene solvent within the alkali metal coordination sphere, and the caesium species is a two-dimensional assembly of hexameric aggregates.

1. Introduction

The experimental realisation and structural rationalisation of charge-neutral carborane icosahedra and anionic carboranyl cage structures have proved influential in both the practical and theoretical development of main group element chemistry [1,2,3,4,5]. Initial access to C-bonded organometallic derivatives of carboranes was attempted from the reaction of C-lithiated ortho, meta, or para-carborane with compounds with metal–halogen bonds [6]. Despite long-standing interest, however, structurally characterised species are represented by a surprising paucity of examples, albeit the resultant carboranyl derivatives having been observed to coordinate metals through one [7], two [8], or three [9] B–H agostic interactions, as well as direct C–M [10,11,12,13,14,15,16,17,18] and B–M [19] sigma bonding.

With specific reference to the chemistry of aluminium, although several derivatives in which the nido-1,2-B₉C₂H₁₁²⁻ (dicarbollyl) dianion acts as a η⁵-bonded ligand toward the heavier group, 13 elements have been described [20,21,22,23,24,25], and two derivatives utilising a N,C-chelating 2-dimethylaminomethyl-o-carboranyl, [2-(Me₂NCH₂)C₂B₁₀H₁₀]AlX₂ (X = Br, CH₃), have been reported [26], C-aluminated examples of the unfunctionalised [closo-1,2-C₂B₁₀H₁₁]⁻ monoanion appear to have evaded definitive structural characterisation. A likely explanation for this lacuna is provided by Bregadze and co-workers’ report that reactions of aluminium, gallium, indium, and thallium trialkyls or i-Bu₂AlH with carboranes in the presence of TMEDA, HMPA, or DME predominantly result in degradation of the carborane cage [27]. These latter issues are reminiscent of various attempts to access well-defined C-bonded carboranyl derivatives of the alkaline earth elements, where crystallographically authenticated examples of carboranyl compounds comprising a group 2 atom were, for over 30 years, restricted to the C-methylated species, [Mg(2-Me-1,2-C₂B₁₀H₁₀)₂∙2C₄H₈O₂] [28]. In this latter regard, and in common with a multitude of advances in main group element chemistry, we have very recently demonstrated that the kinetic stability imparted by the β-diketiminate (BDI = HC{(Me)CNDipp}₂; Dipp = 2,6-i-Pr₂C₆H₃) spectator monoanion to the magnesium and calcium species, I and II, facilitates relatively straightforward access to the C-metallated derivatives of the parent o-carborane, 1,2-C₂B₁₀H₁₂ (Figure 1a) [29].

During the last five years, we have reported on the analogous ability of the {SiN^Dipp}^2- dianion (SiN^Dipp = {CH₂SiMe₂NDipp}₂) to provide the requisite kinetic stability to prevent the disproportionation of the low oxidation state group 2 and group 13 species, III and IV^M (Figure 1b/c) [30,31,32,33,34]. The reactivity of III and IV^M arises from the reducing potential of the constituent magnesium (I) and aluminium (I) centres [35,36], and we have, for example, very recently observed that the C-H bonds of terminal alkynes are prone to their ready oxidative addition at the low-valence aluminium centres of compounds IV^M to yield the corresponding group 1 (M = K, Rb, Cs) alkynyl (hydrido)aluminates (Figure 1c) [37]. The comparable pKa values of the C-H bonds of 1,2-C₂B₁₀H₁₂ and terminal acetylenes (~23) [38,39] suggested that an analogous treatment of IV^M with ortho-carborane may similarly provide Al(I) oxidative addition and access to the parent o-carboranyl group 13 aluminate complexes. In this contribution, therefore, we demonstrate that the kinetic stability leveraged by the {SiN^Dipp}²⁻ dianion in conjunction with the highly reducing nature of IV^M provide ready access to the C-Al bonded alkali metal o-carboranyl (hydrido)aluminates, [{SiN^Dipp}Al(H)(1,2-C₂B₁₀H₁₁)M] [M = K (1); Rb (2); Cs (3)].

2. Results and Discussion

The addition of two molar equivalents of ortho-carborane to benzene solutions of the dimeric derivatives, IV^M (M = K, Rb, Cs), resulted in the decolourisation of the characteristic yellow solutions of the alumanyl anions, either within 10 min (K) or instantaneously (Rb and Cs) at room temperature, to provide compounds 1–3, respectively (Scheme 1). In all three cases and indicative of the complete consumption of the group 1 alumanyl starting reagents, analysis by ¹H NMR spectroscopy revealed the disappearance of the {SiN^Dipp} iso-propyl methine signals at δ 3.97 (IV^K) [31], 4.04 (IV^Rb), and 4.12 ppm (IV^Cs). Although these spectra recorded in situ were contaminated by excess ortho-carborane, which was clearly apparent from a residual C-H methine signal at δ 2.07 ppm, the observed data, while not unambiguously diagnostic, were consistent with the formation of three compounds which presented a high degree of commonality in solution. The observation of a single multiplet at ca. δ 4.1 ppm, corresponding to four protons by relative integration in all three solutions, was consistent with the maintenance of the C₂-symmetric {SiN^Dipp} chelate about the aluminium centres. Although no Al-H resonances could be identified due to the quadrupolar nature of the aluminium nucleus (²⁷Al, 100%, I = 5/2), a broadened but diagnostic (1H) singlet could in each case be identified as arising from the remaining C-H methine environment of the o-carboranyl (1,2-C₂B₁₀H₁₁) anion. The frequencies of these latter signals evidenced a marginal deshielding with increasing alkali metal atomic weight (1, δ_H 3.35; 2, 3.33; 3, 3.30 ppm), a trend that was replicated in the ¹³C NMR chemical shifts in the residual o-carboranyl methine signals (1, δ_C 63.6; 2, 63.3; 3, 63.0 ppm), which were readily identified in the corresponding HSQC experiments. The unambiguous assignment of ¹¹B chemical shifts in even underivatised 1,2-C₂B₁₀H₁₂ can require careful consideration of residual quadrupolar couplings [40]. Although we were unable to obtain ¹¹B NMR spectra that were uncontaminated by 1,2-C₂B₁₀H₁₂ for compounds 1 and 2, all three derivatives provided identical ¹¹B{¹H} NMR spectra comprising four broad signals between δ¹¹_B −0.5 and −11 ppm and consistent with a lowering of the symmetry of the carborane cage. In totality, therefore, these solution-state data infer that the integrity of the carboranyl cage structure in each case has been maintained subsequent to its alumination at a single C-H vertex.

Compounds 1–3 were isolated in good yields (70–80%) as colourless crystalline solids, suitable for X-ray diffraction analysis, which confirmed the veracity of the solution-based experiments. Although the asymmetric unit of the potassium derivative (1) comprises a single potassium aluminate ion pair and a non-interacting molecule of occluded benzene, the gross structure propagates as one-dimensional polymers parallel to the crystallographic c-axis (Figure 2). The aluminium-bound hydride (H1) was located and refined without restraints and interacts with a potassium cation, which is further encapsulated by close contacts to the B2-H and B3-H units of the aluminated carborane cage and a combination of intra- and intermolecular polyhapto interactions with the Dipp substituents of the chelating {SiN^Dipp}^2- dianions. Although there is no direct precedent for the unsupported Al-C σ bond of 1 [Al1-C31 2.0804(14) Å], this bond length is closely comparable to the dimethylaluminium derivative, [2-(Me₂NCH₂)C₂B₁₀H₁₀]AlMe₂ [2.010(6) Å] [26], but is shorter than the range of Al-C interactions [typically ca. 2.25 Å] [20,21,22,23,24,25] reported for several aluminium η⁵-dicarbollylide derivatives. The Al-C bond of 1 is also only marginally elongated in comparison to the Al-C (sp) bond lengths typically observed in similarly four-coordinate terminal aluminium acetylides [ca. 1.91–1.98 Å] [41,42,43,44,45,46,47].

In contrast to the extended structure of 1, the asymmetric unit of its rubidium analogue (2) comprises half of a dimer, the remainder of which is generated via crystallographic inversion symmetry (Figure 3). The rubidium cations are again enveloped by a combination of Al-H, B-H, and C-H interactions to both the hydridoaluminate and carboranyl units, the latter of which also bridge via B-H∙∙∙Rb close contacts between the two dimer halves. The rubidium coordination environment is completed by its η⁶ engagement with a single {SiN^Dipp} N-aryl substituent [range of Rb-C distances: 3.2500(17)–3.5116(16) Å], while polymer propagation analogous to that observed for 1 is prevented by a further polyhapto interaction between both rubidium centres of each dimer and a molecule of benzene solvent [range of Rb-C distances: 3.337(9)–3.563(5) Å]. Although the Al1-C31 bond length in 2 [2.1018(16) Å] is slightly elongated in comparison to that of 1, we suggest that this observation, along with some minor variations across the comparable C-B and B-B bond lengths in the carboranyl cages, are more a reflection of the additional B-H∙∙∙Rb bridging interactions leveraged by the dimeric structure of 2 than any significant perturbation to the electronic structures of the two compounds through the replacement of K⁺ by Rb⁺.

A further impact of the increasing radius of the group 1 cation [effective six-coordinate radii: K⁺ 1.38; Rb⁺ 1.52; Cs⁺ 1.67 Å] [48] was made evident from the single crystal X-ray analysis of compound 3. Although complicated by various issues with disorder across both the {SiN^Dipp} ligands and two molecules of occluded benzene solvent, the structure was identified as a further polymeric array assembled by a complex series of Al-H∙∙∙Cs, B-H∙∙∙Cs, and C-H∙∙∙Cs interactions and η⁶ engagements between caesium and a Dipp substituent of each {SiN^Dipp} ligand (Figure 4a). While the asymmetric unit thus comprises three unique caesium o-carboranyl (hydrido)aluminates, the gross structure assembles as a sequence of hexacyclic arrays, which further propagate via peripheral B-H∙∙∙Cs interactions such that the structure is best viewed as an infinite two-dimensional polymer (Figure 4b). The three unique Al-C bond lengths within compound 3 [Al1-C31 2.127(3); Al2-C63 2.129(3); Al3-C95 2.118(3) Å] are self-consistent but are marginally elongated in comparison to the relevant metrics identified in compounds 1 and 2. It has been established that the C-C bonds within such o-carboranyl clusters can be notably longer than that observed in the unperturbed o-carborane (ca. 1.62 Å). While the C31-C32 bonds of all three of the current derivatives are thus comparably elongated [1, 1.6530(18); 2, 1.730(2); 3, 1.713 Å], these values lie at the low end of the range of C-C distances observed within related derivatives, which can even surpass 1.9 Å [49,50].

3. Materials and Methods

3.1. General Considerations

Except stated otherwise, all the experiments were conducted using the standard Schlenk line and/or glovebox techniques under an inert atmosphere of argon. NMR spectra were recorded with an Agilent ProPulse spectrometer (¹H at 400 MHz, ¹¹B at 128 MHz, and ¹³C at 101 MHz). The spectra are referenced relative to residual protio solvent resonances or an external standard. Elemental analyses were performed at Elemental Microanalysis Ltd., Okehampton, Devon, UK. Solvents were dried by passage through a commercially available solvent purification system and stored under argon in ampoules over 4 Å molecular sieves. Benzene-d₆ and THF-d₈ were purchased from Merck and dried over a potassium mirror before distilling and storage over molecular sieves. [{SiN^Dipp}AlK]₂ (IV^K) [30], [{SiN^Dipp}AlRb]₂ (IV^Rb), and [{SiN^Dipp}AlCs]₂ (IV^Cs) [32] were prepared according to reported procedures. Other chemicals were purchased from Merck and used without further purification.

3.2. Syntheses of New Complexes

Synthesis of [{SiN^Dipp}Al(H)(o-C₂B₁₀H₁₁)]K (1): In a J Youngs NMR tube, C₆D₆ (ca. 0.5 mL) was added to [{SiN^Dipp}AlK]₂ (IV^K, 28.0 mg, 0.025 mmol), and ortho-carborane (o-C₂H₁₂B₁₀, 7.2 mg, 0.05 mmol) was then added to the bright yellow solution. A gradual decolourisation of the benzene solution from bright yellow to colourless was observed within 10 min, whereupon analysis by ¹H NMR spectroscopy indicated complete consumption of the alumanyl starting material. The colourless solution was then transferred into a vial and layered with n-hexane (0.5 mL). Maintenance of the vial at room temperature for 3 days provided crystals of 1 suitable for X-ray diffraction analysis. Colourless crystals were collected and washed with n-hexane (2 × 0.3 mL) before the removal of all volatiles in vacuo to produce 1 as a colourless powder. Yield 27 mg, 77%. No meaningful elemental analysis results were obtained even after multiple attempts. Attempts to dissolve compound 1 in CDCl₃ or d₈-THF resulted in apparent decomposition. NMR data assigned from the in situ recorded spectra: ¹H NMR (400 MHz, 298 K, Benzene-d₆) δ 7.02–6.57 (m, 6H, ArH), 4.15 (sept, br, J = 6.8 Hz, 4H, CHMe₂), 3.51–1.81 (m, 11H, C₂H₁₁B₁₀, where CH can be assigned to a sharp singlet at δ_H = 3.35, 1H ), 1.50 (d, J = 6.8 Hz, 6H, CHMe₂), 1.34 (d, J = 6.8 Hz, 12H, CHMe₂), 1.32–1.27 (m, 6H, CHMe₂), 1.16–1.05 (m, 4H, SiCH₂), 0.76–0.37 (br, 6H, SiMe₂), 0.34–0.15 (br, 6H, SiMe₂). ¹H resonance correlated to AlH was not observed. n-hexane and grease impurities were observed. ¹¹B{¹H} NMR (128 MHz, 298 K, Benzene-d₆) δ −0.5, −3.3, −9.0, −10.9 (spectrum contaminated by o-carborane signals at −2.2, −7.6, −13.6, and −14.9 ppm). ¹³C{¹H} NMR (101 MHz, 298 K, benzene-d₆) δ 148.1 (i-C₆H₃), 147.9 (o-C₆H₃), 125.0 (m-C₆H₃), 122.3 (p-C₆H₃), 63.6 (AlCHB₁₀H₁₁), 27.9 (CHMe2), 27.9 (CHMe₂), 25.5 (CHMe₂), 14.4 (SiCH_2, *overlapping with hexane peak), 3.5 (SiMe₂), 0.7 (SiMe₂).

Synthesis of [{SiN^Dipp}Al(H)(o-C₂B₁₀H₁₁)]Rb (2): In a J Youngs NMR tube, C₆D₆ (ca. 0.5 mL) was added to [{SiN^Dipp}AlRb]₂ (IV^Rb, 30.5 mg, 0.025 mmol), and ortho-carborane (o-C₂H₁₂B₁₀, 7.2 mg, 0.05 mmol) was then added to the bright yellow solution. An instantaneous decolourisation of the benzene solution was observed, whereupon analysis by ¹H NMR spectroscopy indicated complete conversion of the alumanyl starting material. The colourless solution was then transferred into a vial and layered with n-hexane (0.5 mL). Maintenance of the vial at room temperature for 3 days provided single crystals suitable for X-ray diffraction analysis. The colourless crystals were collected and washed with n-hexane (2 × 0.3 mL) before the removal of all volatiles in vacuo to produce 2 as a colourless powder. Yield 26 mg, 69%. Anal. calcd. for C₃₈H₇₄AlB₂N₂RbSi₂ (2.C₆H₁₄): C, 60.92%; H, 9.96%; N, 3.74%. Found: 61.15%, H, 10.03%; N, 3.37%. Attempts to dissolve compound 2 in CDCl₃ or d₈-THF resulted in apparent decomposition. NMR data assigned from the in situ recorded spectra: ¹H NMR (400 MHz, 298 K, Benzene-d₆) δ 7.08–6.63 (m, 6H, ArH), 4.11 (sept, J = 6.6 Hz, 4H, CHMe₂), 3.33 (s, 1H, CH of C₂H₁₁B₁₀), 3.05–1.83 (m, 10H, BH of C₂H₁₁B₁₀), 1.50 (d, J = 6.6 Hz, 9H, CHMe₂), 1.31 (d, J = 6.6 Hz, 12H, CHMe₂), 1.29–1.25 (m, 3H, CHMe₂), 1.15–1.00 (m, 4H, SiCH₂), 0.75–0.35 (br, 6H, SiMe₂), 0.34–0.37 (br, 6H, SiMe₂). ¹H resonance correlated to AlH was not observed. n-hexane and grease impurities were observed. ¹¹B{¹H} NMR (128 MHz, 298 K, Benzene-d₆) δ −0.9, −3.3, −9.0, −11.1 (spectrum contaminated by o-carborane signals at −2.2, −7.6, −13.6, and −14.9 ppm). ¹³C{¹H} NMR (101 MHz, 298 K, benzene-d₆) δ 148.2 (i-C₆H₃), 148.0 (o-C₆H₃), 124.8 (m-C₆H₃), 122.8 (p-C₆H₃), 63.3 (AlCHB₁₀H₁₁), 27.9 (CHMe₂), 27.9 (CHMe₂), 25.6 (CHMe₂), 14.4 (SiCH_2, *overlapping with hexane peak), 4.5 (SiMe₂), 1.4 (SiMe₂).

Synthesis of [{SiN^Dipp}Al(H)(o-C₂B₁₀H₁₁)]Cs (3): In a J Youngs NMR tube, C₆D₆ (ca. 0.5 mL) was added to [{SiN^Dipp}AlCs]₂ (IV^Cs, 32.8 mg, 0.025 mmol), and ortho-carborane (o-C₂H₁₂B₁₀, 7.2 mg, 0.05 mmol) was then added to the bright yellow solution. An instantaneous decolourisation of the benzene solution was observed, and 3 precipitated directly as colourless crystals suitable for X-ray diffraction analysis. The pale pink supernatant was then carefully decanted, and the colourless crystalline solids were washed with n-hexane (2 × 0.3 mL) before the removal of all volatiles in vacuo to produce 3 as a colourless powder. Yield 32 mg, 80% (potentially overestimated due to an amount of [(CH₂SiMe₂)₂NDipp] [30] always present throughout various purification attempts and elemental analysis was, therefore, not attempted). Attempts to dissolve compound 3 in CDCl₃ or d₈-THF resulted in apparent decomposition. ¹H NMR (400 MHz, 298 K, benzene-d₆) δ 6.99–6.60 (m, 6H, ArH), 4.14 (sept, J = 6.7 Hz, 4H, CHMe₂), 3.35 (s, 1H, HC₂H₁₀B₁₀), 1.52 (d, J = 6.7 Hz, 12H, CHMe₂), 1.34 (d_app, J = 6.7 Hz, 16H, CHMe₂ and SiCH₂, overlapping peaks), 0.80 (s, 2H, SiMe₂?), 0.78_–0.34 (br, 4H, SiMe₂), 0.27–ca. −0.25 (br, 6H, SiMe₂). ¹¹B{¹H} NMR (128 MHz, 298 K, benzene-d₆) δ −0.2, −3.3, −7.6, −11.0.¹³C{¹H} NMR (101 MHz, 298 K, Benzene-d₆) δ 147.9 (i-C₆H₃), 147.7 (o-C₆H₃), 124.7 (m-C₆H₃), 123.7 (p-C₆H₃), 63.0 (CH of), 28.0 (CHMe₂), 27.9 (CHMe₂), 26.2 (CHMe₂), 14.4 (SiCH₂), 4.7 (SiMe₂), 2.1 (SiMe₂). * [(CH₂SiMe₂)₂NDipp] impurity observed.

4. Conclusions

The kinetic stability provided by the sterically demanding {SiN^Dipp}²⁻ dianion facilitates the selective oxidative addition of a C-H bond of 1,2-C₂B₁₀H₁₂ to the aluminium centre of the group 1 alumanyls (IV^M; M = K, Rb, Cs). In each case, these reactions provide a four-coordinate o-carboranyl(hydrido)aluminate anion, [(SiN^Dipp)Al(H)(1,2-C₂B₁₀H₁₁)]⁻, in which the carboranyl cage is bonded to aluminium by an unsupported Al-C σ bond. All three compounds further assemble as extended network structures based on Al-H∙∙∙M, B-H∙∙∙M, and C-H∙∙∙M interactions. Each structure is unique due to the significant variation in M⁺ ionic radius as group 1 is descended.