Synthesis, structure and reactivity of 1-(α-C,α′-halo-o-xylyl)-2-trialkylsilyl-1,2-dicarbaboranes

Abstract

Treating Li[^tBuMe₂Si-1,2-C₂B₁₀H₁₀] with excess α,α′-dihalo-o-xylenes, 1,2-C₆H₄(CH₂X)₂ (X = Br, Cl), generates only 1-(α-C,α′-halo-o-xylyl)-2-(tert-butyldimethylsilyl)-1,2-dicarba-closo-dodecaboranes, 1-{o-(XCH₂C₆H₄CH₂)}-2-^tBuMe₂Si-1,2-C₂B₁₀H₁₀ (X = Cl 1a, Br 1b). The structures of both 1a and 1b were determined by single crystal X-ray diffraction. Reaction of either 1a or 1b with ⁿBuLi or MeLi affords the substituted ethane, (2-^tBuMe₂Si-1,2-C₂B₁₀H₁₀-1-o-CH₂C₆H₄)₂C₂H₄2 whereas reaction with ^tBuHNLi affords the substituted ethene (2-^tBuMe₂Si-1,2-C₂B₁₀H₁₀-1-o-CH₂C₆H₄)₂C₂H₂3; both structures were confirmed by X-ray diffraction. Cleavage of the carborane–silicon bond in 1a or 1b by Bu₄NF gives dihydronaphthocarborane, 4, which has been structurally characterised.

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Introduction

Functionalised cyclopentadienyl ligands have made a significant impact on the chemistry of the early transition metals in recent years through the high activity of constrained geometry cyclopentadienyl amide ligands in alkene polymerisation.¹ Pendant arm donor ligand cyclopentadienyls have also been of significant interest in the chemistry of main group and other transition metals.^2,3 Given the relationship between cyclopentadienyl ligands and the nido-C₂B₉H₁₁ ligand,⁴ we⁵ and others^6,7 have been exploring the synthesis of ligands containing nido-carboranes attached to cyclopentadienyl,⁸ amine and other donor atoms through carbon chains of various types and lengths. The development of bio-compatible amine-functionalised carboranes for a variety of other purposes, including use in boron neutron capture therapy (BNCT) remains an active area of research.⁹

We⁵ have recently reported the synthesis of closo- and nido-carboranylamines via modifications to known procedures, and were interested to explore other syntheses. Thus the displacement of a tosyl or halide group by nucleophilic azide ion¹⁰ and subsequent reduction to the amine¹¹ is a well-established procedure.¹² It has previously been reported however that this reaction fails for (halomethyl)carboranes, XCH₂C₂B₁₀H₁₁, X = Cl, Br, and only proceeds for the iodo derivatives.¹³ Similar failures have been reported for a variety of reactions by other groups and were rationalised by the electronic influence of the carborane.¹⁴

The reactions of carborane nucleophiles with α,α′-dihalo-o-xylenes are well-established. The di-lithiation of o-carborane gives Li₂C₂B₁₀H₁₀, which reacts with α,α′-dibromo-o-xylene to give dihydronaphthocarborane, a precursor to naphthocarborane.¹⁵ Mono-lithiation of o-carborane is not always a clean reaction, but the same effect is achieved by mono-silylation,^16–18 prior to metallation and Hawthorne reports that the reaction of two equivalents of Li(^tBuMe₂SiC₂B₁₀H₁₀) with α,α′-dihalo-o-xylenes gives substitution of both halides by the large carborane nucleophile (Scheme 1).¹⁷

Scheme 1 Product of the 2 ∶ 1 reaction reported by Hawthorne.

We decided to explore the reaction of one equivalent of Li(^tBuMe₂SiC₂B₁₀H₁₀) with α,α′-dihalo-o-xylenes as a route to the synthesis of carborane–xylyl–amine ligands since we reasoned that carborane–xylyl–halide intermediates would be amenable to nucleophilic displacement. Here we report that these compounds also exhibit inactivity to nucleophilic substitution that in retrospect may be exploited to synthetic advantage to allow access to novel molecular architectures.

Results and discussion

The reaction of one equivalent of Li(^tBuMe₂SiC₂B₁₀H₁₀) with an excess of α,α′-dihalo-o-xylenes results in mono-substitution and affords 1-(α-C,α′-halo-o-xylyl)-2-(tert-butyldimethylsilyl)-1,2-dicarba-closo-dodecaboranes, 2-^tBuMe₂Si-1-{o-(XCH₂C₆H₄CH₂)}-1,2-C₂B₁₀H₁₀ (X = Cl, 1a; Br; 1b) in excellent yield (Scheme 2). In keeping with the α,α′-dihalo-o-xylenes from which they are derived these compounds are potent irritants and lachrymators and as such skin contact should be avoided. Spectroscopic data for the Cl (1a) and Br (1b) derivatives are similar, so that only the chloro-derivative will be discussed. Considering the ¹H NMR data, the ^tBuMe₂Si substituent is clearly observed δ 0.46 (s, 6H, SiMe), 1.15 (s, 9H, Bu), and both methylene linkages are chemically distinct δ 3.70 (s, 2H, CH₂), 4.66 (s, 2H, CH₂). Due to the chemical inequivalence of the two methylene moieties the C₆H₄ unit is a complex multiplet between δ 7.18 and 7.39. The ¹¹B NMR spectrum confirms the retention of the closo-C₂B₁₀ framework. Retention of the silyl protective group lends useful solubility and crystallinity which greatly aids separation and purification. Single crystal X-ray diffraction analysis for both 1a and 1b serves to further confirm the structural features with all parameters well within the expected range for compounds of this type. Fig. 1 shows the molecular structure of 1a, that of 1b is essentially identical. Selected bond lengths and angles appear in Table 1 and reveal C(1)–C(2) distances of 1.699(2) Å for 1a and 1.697(4) Å for 1b, which, although long for C–C single bonds, are within the normal range for polyhedral closo-carboranes, where the carbon atoms have large coordination numbers, here six. The other metric parameters are unremarkable.

Table 1 Selected bond lengths (Å) and angles (°) for the structurally characterised compounds, where X represents the substituent on C(18). Atoms labelled with the suffix “A” are generated by the symmetry operation (1 − x, −y, −z)

	1a	1b	2	3	4
	X = Cl(1)	X = Br(1)	X = C(18A)	X = C(18A)	X = C(2)
C(1)–C(2)	1.699(2)	1.697(4)	1.694(2)	1.702(2)	1.649(2)
C(2)–Si(1)	1.9652(14)	1.961(3)	1.960(2)	1.9581(13)
C(1)–C(11)	1.547(2)	1.540(4)	1.549(2)	1.542(2)	1.519(2)
C(11)–C(12)	1.521(2)	1.526(4)	1.517(2)	1.518(2)	1.518(2)
C(18)–X	1.825(2)	1.981(3)	1.549(3)	1.334(2)	1.528(2)
C(1)–C(2)–Si(1)	123.36(9)	123.6(2)	124.91(9)	124.01(8)
C(18)–C(2)–C(1)					117.41(11)
C(2)–C(1)–C(11)	117.06(11)	116.8(2)	116.51(12)	117.63(9)	117.71(11)
C(1)–C(11)–C(12)	114.49(11)	114.8(2)	113.75(12)	114.47(10)	114.97(12)
C(17)–C(18)–X	110.14(10)	109.4(2)	111.3(2)	125.7(2)	114.96(12)

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Fig. 1 A view of the molecular structure of 1a showing 50% probability ellipsoids, hydrogen atoms as arbitrary sized spheres.

Scheme 2 Reactions discussed in this work. Reagents and conditions: (i) ⁿBuLi, THF, room temp; (ii) ^tBuHNLi, THF, −100 °C; (iii) ⁿBu₄NF, THF, −78 °C.

In keeping with previous studies, we find these compounds are either inert to or are decomposed by reagents typically used to effect conversion of a halo group to an amine. Reagents investigated included NaN₃ and hexamethylenetetraamine (Delépine reaction).¹⁹ No product of the desired formulation was obtained, as might be expected under such harsh conditions given that o-carborane is readily decapitated by amines.²⁰ Thus, whilst compounds 1a and 1b are potential precursors to functionalised closo- and nido-carboranes, they are not suitable for the synthesis of such carboranes carrying basic, or other, functions which are capable of deboronating 1a and 1b in competition with nucleophilic substitution of the chloride or bromide.

Given these limitations it is of interest to establish what transformations are possible for such systems. Carbarods²¹ derived from carboranes are attracting attention as potential one-dimensional conductors and some of these materials exhibit liquid crystal behaviour.²² For these reasons it was of interest to establish whether the halomethylene unit would undergo coupling reactions, as such a methodology could be applied to the rational stepwise construction of oligomeric materials.²³ Reaction of either 1a or 1b with ⁿBuLi or MeLi results in a Wurtz type coupling reaction to afford the substituted ethane (2-^tBuMe₂Si-1,2-C₂B₁₀H₁₀-1-o-CH₂C₆H₄)₂C₂H₄ (2) in reasonable yield as a colourless highly crystalline air-stable solid. The formulation is entirely consistent with spectroscopic and analytical data and is firmly established by a single crystal X-ray diffraction study. Considering ¹H NMR data, a large change in chemical shift is observed for the methylene unit following loss of the electronegative halogen substituent and coupling to form the ethane (δ 4.66 for CH₂Cl in 1a, δ 2.96 for CH₂CH₂ in 2). In both ¹H and ¹³C NMR data other characteristic resonances of both the aryl and BuMe₂Si units remain largely unaffected by the transformation. The ¹¹B NMR spectrum confirms retention of the closo-carborane fragment. The molecular structure determined by the single crystal X-ray study is shown in Fig. 2 and the structural parameters are reported in Table 1. The molecule sits on a crystallographic inversion centre at the mid-point of the C(18)–C(18A) bond, the length of which, 1.549(3) Å, is entirely consistent with a C–C single bond. The hybridisation of C(18) is sp³ as evidenced by the C(17)–C(18)–C(18A) angle of 111.3(2)°. Other distances fall within normal ranges.

Fig. 2 Molecular structure of alkane 2 showing 50% probability ellipsoids, hydrogen atoms as arbitrary sized spheres. Atoms labelled with the suffix “A” are generated by the symmetry operation (1 − x, −y, −z).

In the light of the alkyl lithium-promoted coupling reaction, we sought to discover if alternative bridge functionality could be generated. Treatment of either 1a or 1b with ^tBuHNLi results in bis-dehydrohalogenation to afford the alkene dimer trans-[2-^tBuMe₂Si-1,2-C₂B₁₀H₁₀-1-o-CH₂C₆H₄]₂C₂H₂3 in high yield as a colourless highly crystalline solid. Such a reaction is of particular synthetic utility as it provides a potential route to conjugated materials. Considering ¹H NMR data for 3, the most notable feature is the resonance for the alkenic protons observed at δ 7.20. This assignment was further confirmed by both ¹H–¹H COSY and ¹H–¹³C correlation experiments which also served to identify the alkenic carbons (δ 132.3). The product was further characterised by a structural study, the results of which appear in Fig. 3 with selected bond lengths and angles in Table 1. The molecule again lies across a crystallographic inversion centre at the mid-point of the C(18)–C(18A) bond, the length of which, 1.334(2) Å, is consistent with a C=C double bond. The C(17)–C(18)–C(18A) angle of 125.7(2)° is consistent with sp² hybridisation for C(18), and the trans- or E-conformation of the double bond is confirmed by the requirement for a molecular inversion centre.

Fig. 3 Molecular structure of alkene 3 showing 50% probability ellipsoids, hydrogen atoms as arbitrary sized spheres. Atoms labelled with the suffix “A” are generated by the symmetry operation (1 − x, −y, −z).

Careful removal of the SiMe₂^tBu group from 1a and 1b using Bu₄NF at low temperature generates nascent (Bu₄F)[1-{o-(XCH₂C₆H₄CH₂)}-1,2-C₂B₁₀H₁₀], which has a nucleophilic site at the carborane 2-carbon atom, which can attack the free benzyl halide. Intramolecular attack results in cyclization to generate dihydronaphthocarborane, whilst it is also possible to envisage intermolecular attack and combinations of inter- and intra-molecular attack leading to polymers and cyclic oligomers respectively. High temperatures need to be avoided for this reaction, since wet Bu₄NF is a potent reagent for the deboronation of closo-carboranes under these conditions.²⁴ The spectroscopic properties of the single product of this reaction, 4, are identical to those previously reported for dihydronaphthocarborane,¹⁵ although NMR cannot uniquely discriminate between this and cyclic dimers or trimers of the same unit. These oligomers are expected to fragment readily under mass-spectrometry conditions, so that this technique is also not able to uniquely confirm the product formula. For this reason, the molecular structure of 4 was determined by X-ray diffraction and confirms that the product is indeed dihydronaphthocarborane; the structure is shown in Fig. 4 with selected bond lengths and angles in Table 1. The structure can be compared with that of “dihydrobenzocarborane”.²⁵ The eight carbon atoms of the xylyl ring in 4 are essentially planar, with a maximum deviation from the least-square plane of 0.009 Å for C(14). One notable feature is a significant dishing of the molecule, as apparently required by the presence of sp³ carbon atoms at C(11) and C(18), so that the C(1)–C(11)–C(12)–C(17)–C(18)–C(2) ring in 4 is far from planar, the largest deviation from a least-squares plane is 0.18 Å, whilst the analogous ring in dihydrobenzocarborane is planar, with a largest deviation of 0.033 Å.

Fig. 4 Molecular structure of dihydronaphthocarborane 4 showing 50% probability ellipsoids, hydrogen atoms as arbitrary sized spheres.

In conclusion, we have demonstrated that (halo-o-xylyl)-o-carboranes are not suitable precursors to amine functionalised carborane ligands analogous to constrained geometry cyclopentadienyl ligands. Coupling reactions between the halo-o-xylyl units provide a means of preparing precursors to the rational stepwise construction of oligomeric carborane materials.

Experimental

All manipulations of air- and moisture-sensitive compounds were performed on a conventional vacuum/nitrogen line using standard Schlenk and cannula techniques or in a nitrogen filled glove box. When required, solvents were dried by prolonged reflux over the appropriate drying agent, prior to distillation and deoxygenation by freeze–pump–thaw processes where appropriate. NMR solvents were vacuum-distilled from suitable drying agents and stored under a dry nitrogen atmosphere. Elemental analysis was performed by the micro-analytical service within this department on an Exeter Instruments analyser. NMR spectra were recorded on the following instruments: Varian Unity-300 (¹H, ¹¹B, ¹³C), Varian 500 (¹H, ¹³C, HETCOR), ¹H and ¹¹B NMR were recorded on the Unity-300 unless otherwise stated. All chemical shifts are reported in δ (ppm) and coupling constants in Hz. ¹H NMR spectra were referenced to residual protio impurity in the solvent (CHCl₃, 7.26 ppm). ¹³C NMR spectra were referenced to the solvent resonance (CDCl₃, 77.0 ppm). ¹¹B NMR were referenced externally to Et₂O·BF₃δ = 0.0 ppm. Except where otherwise indicated, all spectra were recorded in CDCl₃ at ambient temperature.

CAUTION: α,α′-dihalo-o-xylenes and compounds 1a and 1b are potent irritants and lachrymators and as such skin contact should be avoided.

Syntheses

1-(-α-C,α′-Chloro-o-xylyl)-2-(tert-butyldimethylsilyl)-1,2-dicarba-closo-dodecaborane 1a. A solution of Li[^tBuMe₂Si-1,2-C₂B₁₀H₁₀] (5.00 g, 18.9 mmol) in 2 ∶ 1 benzene–Et₂O (50 ml) was cooled to 0 °C and treated dropwise with α,α′-dichloro-o-xylene (7.00 g, 40 mmol) as a 2 ∶ 1 benzene–Et₂O solution (50 ml). Following complete addition the mixture was allowed to warm to room temperature, and then brought to reflux for 12 h. The resulting pale yellow solution was evaporated to dryness in vacuo and the residue triturated with ethanol (ca. 30 ml). The mixture was cooled to −10 °C overnight to afford a colourless microcrystalline solid which was isolated by filtration, washed with an aliquot of chilled methanol (2 ml) and dried in vacuo. Yield 7.04 g, 94% (note: unreacted starting materials sublime in vacuo). (Found C 48.5; H 8.4; C₁₆H₃₃B₁₀ClSi requires C 48.4; H 8.4%); δ_H 0.46 (s, 6H, SiCH₃), 1.15 (s, 9H, Bu), 3.70 (s, 2H, CH₂), 4.66 (s, 2H, CH₂), 7.18–7.39 (m, 4H, C₆H₄); δ_C 136.3, 134.6, 132.8, 130.8, 128.9, 128.8 (Ar), 80.6, 73.7 (cage C), 44.6 (CH₂), 39.3 (CH₂), 27.7 (CCH₃), 20.6 (CCH₃), −2.2 (SiCH₃); δ_B −1.3 (1B), −3.4 (1B), −7.8 (2B), −9.4 (3B), −10.4 (3B).

1-(α-C,α′-Bromo-o-xylyl)-2-(tert-butyldimethylsilyl)-1,2-dicarba-closo-dodecaborane 1b. An identical procedure was employed for the synthesis of 1b from Li[^tBuMe₂Si-1,2-C₂B₁₀H₁₀] (5.00 g, 18.9 mmol) and α,α′-dibromo-o-xylene (10.6 g, 40 mmol). Yield 7.21 g, 86%. (Found C 43.0; H 7.4; C₁₆H₃₃B₁₀BrSi requires C 43.5; H 7.5%); δ_H 0.51 (s, 6H, SiCH₃), 1.20 (s, 9H, Bu), 3.73 (s, 2H, CH₂), 4.61 (s, 2H, CH₂), 7.18–7.39 (m, 4H, C₆H₄); δ_C 136.6, 134.6, 132.9, 130.9, 128.9, 128.8 (Ar), 80.4, 73.7 (cage C), 39.6 (CH₂), 31.9 (CH₂), 27.9, 27.6 (CCH₃), 20.7 (CCH₃), −1.9, −2.3 (SiCH₃); δ_B −1.0 (1B), −5.6 (1B), −9.9 (3B), −11.8 (5B).

Alkane 2. A THF solution (25 ml) of 1a (0.10 g, 0.25 mmol) was treated dropwise with a slight excess of ⁿBuLi at room temperature and left to stir for 12 h. Volatiles were removed in vacuo and the residue triturated with ethanol (ca. 30 ml) to afford a bright white solid which was dried in vacuo. Yield 0.065 g, 72%. Crystalline samples were obtained by recrystallisation from CH₂Cl₂–ethanol solutions. Comparable yields were obtained using 1b. (Found C 51.1; H 8.8; C₃₂H₆₆B₂₀Si₂·0.5CH₂Cl₂ requires C 51.0; H 8.8%); δ_H 0.42 (s, 6H, SiCH₃), 1.15 (s, 9H, Bu), 2.96 (s, 2H, CH₂), 3.38 (s, 2H, CH₂), 7.05–7.20 (m, 4H, C₆H₄); δ_C 139.9, 133.8, 132.0, 129.7, 128.4, 126.2 (Ar), 81.4, 74.0 (cage C), 39.5 (CH₂), 34.4 (CH₂), 27.9, 27.6 (CCH₃), 20.6 (CCH₃), −1.9, −2.3 (SiCH₃); δ_B −0.8 (1B), −5.5 (1B), −9.8 (2B), −11.4 (3B), −12.4 (3B).

Alkene 3. A THF solution (25 ml) of 1a (0.10 g, 0.25 mmol) was treated dropwise with two equivalents of ^tBuHNLi (0.04 g, 0.5 mmol) as a THF solution (2 ml) at −100 °C (liquid N₂–toluene) and allowed to warm slowly to room temperature and left to stir for 12 h. The reaction was quenched by addition of propan-2-ol (1 ml), volatiles removed in vacuo and the residue triturated with ethanol (ca. 30 ml) to afford a bright white solid which was dried in vacuo. Yield 0.078 g, 86%. Crystalline samples were obtained by recrystallisation from CH₂Cl₂–ethanol solutions. Comparable yields were obtained using 1b. (Found C 53.0; H 9.0; C₃₂H₆₄B₂₀Si₂ requires C 53.3; H 8.9%); δ_H 0.42 (s, 6H, SiCH₃), 1.11 (s, 9H, Bu), 3.65 (s, 2H, CH₂), 7.20 (s, 2H, CH), 7.22–7.62 (4H, C₆H₄); δ_C 137.2, 133.8, 129.2, 128.7, 127.8, 126.3 (Ar), 132.3 (CH), 81.0, 73.9 (cage C), 40.0 (CH₂), 27.7 (CCH₃), 20.6 (CCH₃), −2.1 (SiCH₃); δ_B 1.2 (1B), −3.5 (1B), −7.8 (3B), −9.6 (5B, vbr).

Dihydronaphthocarborane, 4. A stirred solution of 1a (0.10 g, 0.25 mmol) in THF (25 ml) was cooled to −78 °C and treated slowly and dropwise with a solution of ⁿBu₄NF (0.065 g, 0.25 mmol) in THF (10 ml) and maintained at this temperature for 1 h. After slowly warming to room temperature, the volatiles were removed under reduced pressure and the residue was triturated with methanol (5 ml). The resulting solids were isolated by filtration and purified by recrystallisation from CH₂Cl₂–methanol to afford colourless crystals of pure 4. Yield 0.044 g, 71%. Spectroscopic properties were identical to those reported by Matteson et al.¹⁵

X-Ray crystallography

Single crystal X-ray diffraction experiments were carried out with a SMART 1K CCD area detector, using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The reflection intensities for 1b were corrected by means of a ψ-scan, the reflections for the other compounds were not corrected for absorption. The structures were solved by direct methods and refined by full-matrix least squares against F² of all data, using SHELXTL programs.²⁶ Crystal data and experimental details are listed in Table 2.

CCDC reference numbers 168850–168854.

See http://www.rsc.org/suppdata/dt/b1/b107276e/ for crystallographic data in CIF or other electronic format.

Table 2 Crystal data and structure refinement for compounds described in this paper

Compound	1a	1b	2	3	4
Empirical formula	C16H33B10ClSi	C16H33B10BrSi	C32H66B20Si2	C32H64B20Si2	C10H18B10
Formula weight	397.06	441.52	723.23	721.21	246.34
Temperature/K	120(2)	120(2)	100(2)	120(2)	103(2)
Crystal system	Triclinic	Triclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P1̄	P1̄	P21/n	P21/n	P21/c
a/Å	7.466(2)	7.4927(2)	9.329(1)	10.731(2)	7.215(1)
α/°	107.79(3)	72.0640(10)
b/Å	10.056(2)	10.0475(3)	13.502(2)	16.232(2)	20.620(3)
β/°	91.10(3)	83.1420(10)	104.505(5)	110.64(2)	90.48(1)
c/Å	16.135(3)	16.2819(5)	18.097(2)	13.294(3)	9.268(1)
γ/°	99.24(3)	80.7340(10)
Volume/Å^3	1135.5(4)	1147.78(6)	2206.8(5)	2167.0(7)	1378.8(2)
Z	2	2	2	2	4
μ/mm^−1	0.222	1.844	0.105	0.107	0.056
Reflections measured	14262	8297	24049	22108	11211
Unique reflections	6087	5791	5471	4965	3165
R int	0.0985	0.0391	0.0538	0.0339	0.0493
Reflections I > 2σ(I)	5057	4153	4187	4135	2294
R[F^2 > 2σ(F^2)]	0.0458	0.0462	0.0453	0.0358	0.0459
wR(F^2), all data	0.1321	0.1031	0.1209	0.0950	0.1225

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Acknowledgements

We acknowledge the award of an EPSRC Senior Research Fellowship (J. A. K. H.), and support from the ERDF Centre for 21st Century Materials at the University of Durham (J. M. M.).

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Footnote

† Electronic supplementary information (ESI) available: rotatable 3-D crystal structure diagrams in CHIME format. See http://www.rsc.org/suppdata/dt/b1/b107276e/

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