Synthesis, Crystal Structure, and Reactivity of the Strong Methylating Agent Me₂B₁₂Cl₁₂

Abstract

Give Me strength: Methylation of the easily accessible weakly coordinating dianion [B12Cl12]2− affords Me2B12Cl12 (see picture). This neutral compound is a stronger methylating agent than commonly used methyl triflate, and it even methylates benzene. The synthesis, crystal structure, and reactivity of Me2B12Cl12 in solution and the gas phase are discussed. The efficiency of methylating reagents strongly depends on the weakly coordinating properties of the anion. The introduction of carborane anions [CHB11R5X6]− (R=Me, Cl; X=Cl, Br) and the synthesis of the methylating agents Me(CHB11Me5X6) (X=Cl, Br) by Reed was a recent breakthrough.1 The replacement of triflate anions by the more weakly coordinating carborane anions [CHB11R5X6]− (R=Me, Cl; X=Cl, Br) significantly increased the methylating power.2 Me(CHB11Me5X6) (X=Cl, Br) methylates benzene and converts alkanes into the corresponding alkyl cations with concomitant formation of methane.1 Very recently, perhalogenated dodecaborate cluster anions [B12X12]2− (X=F,3 Cl4) came to attention as weakly coordinating dianions. Improved syntheses for [B12F12]2− 3b and [B12Cl12]2− 4a have been developed and make these dianions available on a large scale. They have been applied to stabilize unusual dications4b and the first diprotic superacid H2B12Cl12.4c These anions are thus of great interest as weakly coordinating dianions for methylating agents and stabilization of the resulting cations. M[AsF6] is only sparingly soluble in liquid SO2 and was partly removed by filtration. Subsequent removal of all volatiles results in a bulk material with the composition M[MeB12Cl12], which corresponds to a mixture of the two very soluble compounds M2[B12Cl12] and Me2B12Cl12. Colorless crystals of Me2B12Cl12 were obtained by slow evaporation of the solvent and fractional crystallization. The structure of neutral Me2B12Cl12 could be determined by single-crystal X-ray diffraction (Figure 1) and is the first structurally characterized example of a carborane or dodecaborate anion bound to a methyl cation.6 Neutral dodecaborate clusters are rare; other examples are 1,12-B12H10(CO)2 and 1,12-B12H10(CO2H2)2.7 Part of the X-ray crystal structure of Me2B12Cl12 including selected bond lengths [pm] and angles [°]. Thermal ellipsoids are set at 50 % probability. In solid Me2B12Cl12 two methyl groups are connected to the anion in the 1- and 12-positions similar to the coordination of two [Et3Si]+ cations in (Et3Si)2B12Cl12.4c, 8 In contrast to the rather long silicon–chlorine bond in solid (Et3Si)2B12Cl12 (231.75(8) pm versus 208.63(9) pm in Me3SiCl),9 the carbon–chlorine bond in Me2B12Cl12 (C1Cl1 182.1(4) pm, 180.6 pm calculated at the PBE0/def2-TZVPP level for comparison) is only slightly longer than the carbon–chlorine single bond in methyl chloride (180.5 pm (solid state),10a 178.2 pm (gas phase)10b). Dimethylchloronium [Me-Cl-Me]+ has been reported,11 and very recently experimental structural data was obtained.12 The CCl bond length in [Me-Cl-Me]+ (calcd (PBE0/def2-TZVPP) 180.5 pm; exp. 181.0(2) pm12) is in good agreement with the experimentally observed bond length in Me2B12Cl12. Only very recently, the crystal structure of the disilylated chloronium cation [Me3Si-Cl-SiMe3]+ was determined;13 the silicon–chlorine bond (223.8(5) pm)13 is significantly longer than the silicon–chlorine bond in the corresponding neutral Me3SiCl (208.63(9) pm).9 The carbon–chlorine distances in Me2B12Cl12 and [Me-Cl-Me]+ are thus extremely short and should be best described as covalent carbon–chlorine single bonds. The differences between the compounds having [H3C]+ and [Me3Si]+ groups can be attributed to the intrinsically better stabilization of [Me3Si]+ compared to [H3C]+. Correspondingly, the chlorine–bond Cl1B1 (187.1(4) pm) is almost 10 pm longer than the typical boron–chlorine bond (178.9 pm) in free [B12Cl12]2− 4a and is also longer than in the silylium compounds (R3Si)2B12Cl12 (R=Et, iPr; av. 184.1 pm),4c, 8 thus indicating a very strong carbon–chlorine and a weak chlorine–boron bond in Me2B12Cl12. The bonding situation should thus be described by equal contributions of mesomeric structures A and B (Scheme 1). This view is supported by calculated NPA charges (H3C +0.18, Cl1 +0.46, av. Cl −0.08, B1 −0.09, av. B −0.07) for [MeB12Cl12]− at the PBE0/def2-TZVPP level, which show only a very small positive charge on the methyl group but a significant positive charge on the bridging chlorine atom and the remaining negative charge of about −1.5 total on the anion. Thus, significant positive charge transfer from the methyl cation to the anion and mainly the bridging chlorine atom occurs. Mesomeric structures for [MeB12Cl12]−. In contrast, in sulfur dioxide solution, only one methyl group is bound to the [B12Cl12]2− anion regardless of the temperature, and a strongly bound [MeB12Cl12]− anion is formed. The 11B NMR spectrum has three signals in a 1:5:6 pattern (Figure 2). The cross-peaks in the 11B COSY NMR spectrum show that these signals arise from the same cluster. The chemical shift for the boron atom connected to the (Me⋅⋅⋅Cl)+ unit is shifted downfield by about 3.0 ppm compared to free [B12Cl12]2−. 11B,11B COSY spectrum (double quantum-filtered) of [MeB12Cl12]− in Na[MeB12Cl12] in SO2 solution at RT (calibrated on [B12Cl12]2−) and the calculated structure of [MeB12Cl12]−. The second methyl cation is solvated by sulfur dioxide. Computed energetics for the binding of one and two methyl cations to [B12Cl12]2− in SO2 solution confirm these experimental findings (see the Supporting Information). Binding of one methyl cation to the anion in solution is feasible, whilst the second methyl cation remains solvated, and is thus much more reactive. For comparison, the single methyl cation in Me(CHB11Me5Br6) is bound to the [CHB11Me5Br6]− anion in SO2 solution, as shown by NMR spectroscopy.1b Therefore, it can be concluded that [MeB12Cl12]− is a more weakly coordinating anion than the carborane [CHB11Me5Br6]− with the same total charge of −1. Despite the strong coordination of one methyl cation to the [B12Cl12]2− anion in solution, and a structure that is similar to covalent MeCl bound to [B12Cl11]− in the solid state, methyl chloride elimination has not been observed under normal laboratory conditions. However, in the gas phase, the situation is different, as shown by mass spectrometric analysis. Isolation and fragmentation (Figure 3) of ions with m/z 629 from the isotopic distribution of the anionic ion pair [(NMe4)(B12Cl12)]− showed the loss of NMe3 and formation of the methylated anion [MeB12Cl12]− (m/z 570), which subsequently eliminated methyl chloride. However, the expected remnant, a [B12Cl11]− anion with a naked boron vertex, was not observed. Instead, the very reactive [B12Cl11]− anion immediately adsorbed H2O and N2 from the residual gases within the mass spectrometer, giving the experimentally observed [B12Cl11(OH2)]− (m/z 538) and [B12Cl11(N2)]− (m/z 548) anions. This situation is reminiscent of the reactivity of "CB11Me11" in solution, which instantaneously reacts with nucleophiles.14 All the described gas-phase reactions in the mass spectrometer were verified by numerous experiments.15 Fragmentation of ions with m/z 629 from the isotopic distribution of the anionic ion pair [(NMe4)(B12Cl12)]−. Me2B12Cl12 was applied in methylating reactions to investigate its reactivity (Scheme 2). The reaction with dimethyl sulfide gave [SMe3]2[B12Cl12] as expected, which could be structurally characterized (see the Supporting Information). Both methyl cations in Me2B12Cl12 react with benzene in a similar fashion to the carborane reagents Me(CHB11Me5X6) (X=Cl, Br) to give protonated toluene (shown by NMR spectroscopy; see the Supporting Information). These reactions show the potential of Me2B12Cl12 for methylating reactions. At the same time, the very weakly coordinating dianion [B12Cl12]2− is introduced to stabilize the cations formed. Reactions of Me2B12Cl12. Air- and moisture-sensitive solid reagents were manipulated using vacuum and Schlenk techniques or in a glove box with an atmosphere of dry argon (H2O and O2 <1 ppm). The reactions using liquid sulfur dioxide as solvent were carried out in H-shaped glass vessels with J. Young Teflon-in-glass valves and an incorporated G4 fine frit. The liquid sulfur dioxide solvent was dried over CaH2 and distilled prior to use. M2[B12Cl12] was prepared by the reaction of [NEt3H]2[B12Cl12] with two equivalents of MOH in aqueous solution.4a This product was thoroughly dried at 180 °C at 10−3 mbar for several days and subsequently treated with SOCl2 in SO2. Methyl fluoride16 and arsenic pentafluoride17 were prepared by published procedures. Full experimental details and spectra are given in the Supporting Information. Me2B12Cl12: Li2[B12Cl12] (0.41 g, 0.72 mmol) was charged into a H-shaped Schlenk vessel equipped with a G4 frit. SO2 (10 mL) was condensed onto the solid at −196 °C and the solution was stirred at ambient temperature for 0.5 h. This side of the vessel was then closed and MeF (0.05 g, 1.55 mmol, 2.2 equiv), AsF5 (0.32 g, 1.86 mmol, 2.6 equiv), and SO2 (10 mL) were condensed at −196 °C into the other side of the vessel. This mixture was allowed to warm up to −70 °C and was then stirred at this temperature for 0.5 h. Thereafter the reaction mixture was poured to the solution of Li2[B12Cl12] in SO2 at the same temperature. The reaction mixture was stirred for an additional 2.5 h at −70 °C. The solution turned orange in color and a white solid (Li[AsF6]) precipitated. The solution was separated from the solid by filtration through the frit and all volatiles were subsequently removed, yielding a mixture of Li2[B12Cl12] and Me2B12Cl12 (equivalent to a bulk composition of Li[MeB12Cl12]; 0.35 g, 0.60 mmol, 83 %) as a pale-brown solid. 1H NMR (SO2; calibrated on MeCl (δ=3.01 ppm), 298 K): δ=4.56 ppm (s, [MeB12Cl12]−). 7Li NMR (SO2; not referenced, 298 K): δ=−0.1 ppm (Li+). 11B NMR (SO2; calibrated on [B12Cl12]2− (δ=−12.7 ppm), 298 K): δ=−13.5 (br, 5+1 B), −11.8 (br, 5 B), −9.6 ppm (br, 1 B). 13C NMR (SO2; calibrated on MeCl (δ=25.6 ppm), 298 K): δ=45.8 ppm ([MeB12Cl12]−). IR: =3067 (vw), 1407 (w), 1332 (w), 1027 (vs), 709 (m), 594 (m), 532 cm−1 (vs). Raman: =2961 (10), 304 (100), 127 cm−1 (80). Fractional crystallization from liquid sulfur dioxide afforded colorless crystals of Me2B12Cl12 suitable for X-ray diffraction. Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.