Pdylredra Vol. 6, No. 9, PP. 1737-1740.1983 Printed in Great Britain

02??-5387/87 $3.or+.oo 0 1987 Pergamon Journals Ltd

~LOG~NA~O~ OF 4,~DICA~A-A~C~O- NONABORANE(13),4,5-C2&H,3

TOMAS JELINEK, BOHUMIL &lBR,* FRANTISEK MARES, JAROMIR PLl@EK and STANISLAV HEfiMNEK

Institute of Inorganic Chemistry, Czechoslovak Academy of Sciences, 250 68 8ei near Prague, Czechoslovakia

(Received 10 February 1987 ; accepted 6 March 1987)

Abstract-The AlX,-catalyzed (X = Cl, Br, and I) halogenation of arachno-4,5-C,B,H,, with anhydrous hydrogen halides produces a series of 6-substituted derivatives, 6-X-4,5- C2B7H12. The same compounds along with 6,8-Iz-4,5-&B,H,, are obtained in non-cata- lyzed reactions with elemental halogens. The electrophile-induced nucleophilic substitution concept (EINS) of the substitution with hydrogen halides is suggested. The constitution of all compounds isolated was unambiguously determined via ‘H, 13C, ’ 'B, and two-dimen- sional (2-D) “B-liB NMR spectra.

The arochno-4,5-CzB7H13 carborane (1) (Fig. 1) was first reported by Rietz and Schaeffer ’ in 1973 ; however, it has been only recently that a high-yield synthesis of this compound has been developed in our laboratory. *s3 Nevertheless, carborane (1) has been for almost 15 years taken as “‘nido-4,5- G&Hi I” or, alternatively, as “nido-2,6-C2B7H1 ,” according to the latest trend in nomenclature.4 Just recently, its structure has been unambiguously rein- terpreted in our laboratory’ on the basis of multi- nuclear NMR measurements. This paper reports on halogenation reactions of (1) with hydrogen halides and elemental halogens.

EXPERIMENTAL

‘H (200 MHz), 13C (50.31 MHz), and “B (64.18 MHz) pulse Fourier transform NMR spectra were recorded in deute~ochlorofo~ on a Varian XL- 200 spectrometer, and data manipulation utilized standard Varian software. Chemical shifts are given in 6 [ppm, referenced to TMS (‘H and 13C) and BF3 * OEt, (’ ‘B) ; positive shifts downfield]. Two- dimensional (2-D) “B-‘lB NMR spectra were produced on all samples via procedures described elsewhere.6 Unit resolution mass spectra in the NI mode (chemical ionization of negative ions) were obtained on a GC/MS MAT 44s spectrometer.

*Author to whom correspondence should be addressed.

a Cl-l

0 BH

Fig. 1. Simplified structure and numbering system of aruchno-4,5-C,B,H,, (1).

TLC was performed on Siiufol sheets (silica gel on alu~ni~ foil ; detection by iodine vapours fol- lowed by AgN03 spray) in 1: 2 benzene-hexane.

Chemicals and syntheses

Carborane (1) was prepared by the previously reported method.” Hydrogen bromide was pre- pared in the reaction of bromine with refluxing tetraline and hydrogen iodide by adding dropwise concentrated hydroiodic acid to excess P401 o ; both products were condensed at -78°C and then gen- erated by slow evaporation. Benzene and hexane were distilled with sodium metal prior to use and other commercially available chemicals were reagent grade and were used as purchased. Except where otherwise indicated, all syntheses and stan- dard isolation procedures were conducted in an inert atmosphere or in vacua.

1737

1738 T. JELINEK et al.

Table 1. Signal assignments in the ‘H and ’ 3C NMR spectra of halogenated derivatives of arachno- 4,5-C,B,H,,

‘H NMR (6) 13C NMR (6)’

Substituent C(5)H exo-C(4)H endo-C(4)H PH C(5) C(4)

6-Cl (2a) 2.83 1.86 1.72 -1.28 6-Br (2b) 2.94 1.85 1.68 -1.26 29.34(184) 8.94( 165) 6-I (2~) 3.02 1.81 1.44 - 1.24 - -

6,8-I* (W 3.29 1.98 1.65 -1.27 29.01(163) 9.32(161)

‘Singlets of relative intensities 1 : 1 : 1 : 2. b Doublets and triplets of equal intensities, J(CH) in parentheses(in Hz).

Arachno-6-Cl-4,5-C,B,Hr2 (2a). (a) Dry HCl was slowly introduced for 2 h at ambient temperature to a stirred solution of (1) (1.1 g; 0.01 mol) in benzene (60 cm3) in the presence of AlCl, (0.13 g ; 1 mmol). After the hydrogen evolution had ceased (cu. 3 h), the mixture was filtered, the filtrate was reduced in volume to cu. 20 cm3, and hexane (40 cm3) was carefully added onto the surface of the solution. The two-layer mixture was left standing overnight to separate white crystals which were iso- lated by filtration, washed with cold hexane and vacuum-dried to give 1.4 g (95%) of IIb ; mass spectrum, high mass m/z 149, corresponding to [12C2”B71H1137C1]- (P-l, CzB7C1 pattern). For NMR spectra see Tables l-4.

(b) Gaseous chlorine was slowly passed through a solution of (1) (1.1 g ; 0.01 mol) in hexane (60 cm’) under stirring at - 30°C until carborane (1) had disappeared from the solution (checked by TLC, RF 0.4). The separated white crystals were isolated by filtration, washed with cold hexane and vacuum-dried to afford 0.7 g (47%) of (2a).

Arachno-6-Br-4,5-C,B,H 1 2 (2b). (a) Dry HBr was slowly passed through a stirred mixture of A1Br3 (0.27 g ; 1 mmol) and a solution of (1) (1.1 g ; 0.01 mol) in benzene (60 cm3) at ambient tem- perature until carborane (1) had disappeared from

the mixture (cu. 3 h). Analogous work-up as in the preceding experiment (a) led to the isolation of 1.6 g (83%) of (2b) ; high mass m/z 193, corresponding to [‘*C2”B,‘H 1I 8’Br]- (P-l; &B,Br pattern). For NMR data see Tables l-4.

When the AlBr, catalyst was replaced by AlC13 (1 : 1 molar ratio of the latter and carborane (1)) and the reaction was conducted exactly in the way described above, 1.4 g of cu. 4 : 1 mixture of (2a) and (2b) was obtained, as assessed by ’ 'B NMR by comparing the signal intensities of the B(6) res- onances of both compounds at 6, - 16.47 and - 22.87, respectively.

(b) A solution of (1) (1.1 g ; 0.01 mol) in benzene (30 cm’) was treated with a solution of bromine (0.6 cm3; 0.02 mol) in benzene (20 cm3) under stirring at 5°C. After the colour of bromine disappeared, the solution was reduced in volume to cu. 20 cm3 and filtered. Further work-up as in the preceding experiment with Cl2 (b) produced 0.7 g (36%) of (2h).

Aruchno-6-I-4,5-C,B,H,, (2~). (a) Dry HI was slowly introduced into a stirred mixture of AlI (0.41 g ; 1 mmol) and a solution of (1) (1.1 g ; 0.01 mol) in benzene (60 cm’) at ambient temperature until the hydrogen evolution had ceased (cu. 3 h). Further work-up as in the preceding experiment (a)

Table 2. Signal assignments in the ’ ‘B NMR spectra” of halogenated derivatives of arachno-4,5-c,B,H, 3

Substituent B(7)b B(9) B(2) B(1) B(8) B(6) B(3)

6-Cl (2a) 12.471156 2.75/161’ -2.91/165 -3.41/168 - 5.92/17gb - 16.471147 -56.21/162 6-Br (2b) 11.54/163 3&r/164/22 -2.93/182 -3.25/157 - 5.791176’ -22.87/151 -56.041157 6-I (2~) 10.95/145 4.221162131 -2.96/166d -2.96/166d - 5.24/177b -36.61/151 -55.471159

6,8-I* (2d) 12.99/156 5.721147’ -5.30/171 -3.401156 -18.09/O/48’ -36.121147 -51.781162

“G,/J(BH)/J(B+H), assignments based on 2-D spectra, all signals in proton coupled spectra are doublets. “Signals are broadened, but J(B-pH) is not measurable. “Doublets of the monosubstituted BH, group. dOverlapping doublets. e Singlet.

Halogenation of 4,5-dicarba-arachno-nonaborane(l3),4,5-C*B,H , 3 1739

Table 3. Cross-peaks indicated in the 2-D “B-“B NMR spectra of halogenated derivatives of aruchno-

4,5-CzB,H’,

Substituent Cross peaks”

6-Cl (2a)

6-Br (2b)

6-I (2c)

“Atoms giving cross peaks with the observed atom (on diagonal) are listed in brackets with right superscipts indicating relative intensities of the off-diagonal inter- actions (s-strong, m-medium, w-weak, O-zero inter- action). Observed atoms (off brackets) are listed upfield.

‘The cross peaks cannot be unambiguously defined due to a high degree of overlap.

resulted in the isolation of 1.1 g (80%) of (2e) ; mass spectrum, high mass m/z 239, corresponding to [‘2Cz”B,1H,, 1271]- (P-l, C2B71 pattern). For NMR data see Tables l-4.

(b) A solution of (1) (1.1 g ; 0.0 1 mol) in benzene (30 cm3) was refluxed with iodine (2.5 g; 0.02 mol) for 48 h. The mixture was then shortly evacuated, shaken with solid Na2S03 to remove excess iodine, filtered and reduced in volume to cu. 20 cm3. Analogous work-up as in the experiment with Cl2 (b) led to the isolation of 1.1 g (60%) of (2c).

Arachno-6,8-I,-4,5-C2B7HI 1 (2d). Carborane (1) (0.6 g ; 5 mmol) was added to solid iodine (1.3 g ; 0.01 mol). After the initial exothermic reaction had ceased, the reaction was finished by heating at 90°C for 1 h. The decolourized melt was dissolved in benzene (20 cm3), the solution was filtered and the filtrate was reduced in volume to CLI. 10 cm3. Hexane (30 cm’) was added onto the surface and the two- layer mixture was left standing overnight to separate pale yellow crystals which were washed with cold hexane and vacuum-dried to give 1.1 g (60%) of (2d) ; mass spectrum, high mass m/z 238, cor- responding to [*2C2*‘B7’H,01271]- (P-l-HI, CPB71 pattern). For NMR dam see Tables l-4.

RESULTS AND DISCUSSION

The AlX,-catalyzed reactions o( = Cl, Br, and I) of (1) with anhydrous hydrogen halides in benzene at ambient temperature give rise to a series of mono- halogenated compounds of the general formula 6- X-4,5-C2B7H1, [(2a), X = Cl ; (2b), X = Br, and (2c), X = I, for numbering see Fig. l] with the evol- ution of one mole of hydrogen. It is to be noted that these reactions do not proceed without catalysis at any measurable rate. In contrast, there is evidence for direct involvement of A1X3 in the halogenation process. When the reaction of (1) with hydrogen

Table 4. ’ ‘B NMR shift changes (A&)” for halogenated derivatives of aruchno-4,5-&B,H, 3

Substituent B(1) B(2) B(3) B(6) B(7) B(8) B(9) A&,* (kJ

6-Cl (2a) 1.14 1.64 0.47 13.74 2.87 0.58 -1.12 0.93(1.77) 6-Br (2b) 1.30 1.62 0.64 7.34 1.94 0.71 -0.47 0.96(0.75) 6-I (2e) 1.59 1.59 1.21 -6.40 1.35 1.26 0.35 1.23(0.20)

6,8-I, (W 1.15 -0.75 4.90 - 5.93 3.39 -11.59 1.85 2.10(4.65)

a G&substituted) - S,(parent compound’). i=n

‘Mean shift for unsubstituted atoms defined as AZ = l/n c 86, (n-number of unsubstituted i= I

atoms). i-n

‘Variance of the shift from the mean value defined as k, = l/n- 1 c (A&-A&*. i= I

T. JELINEK et al.

1) ii) [AlX,]----

Scheme 1. Electrophile-induced nucleophilic substitution of the cage BH2 group of (1) by hydrogen halides; (i) H+ attack, -H, : (ii) X- attack, -AlX3.

bromide is carried out in the presence of an equi- molar amount of AlC13, cu. 4: 1 mixture of 6-Cl and 6-Br derivatives of (1) is obtained, as assessed from “B NMR. The observed parallel substitution on the cage B(6)Hz group by Cl and Br is consistent with a proton attack by H+[AlX,]- at the B(6) exo-hydrogen to remove the H- anion and evolve dihydrogen. The vacant exohedral molecular orbital thus formed is subsequently filled with X- (from AlX;) to recover AlX3 and form exohedral B(6)H-X bond as in Scheme 1.

In view of this approach, the substitution process outlined above can be regarded as an electrophile- induced nucleophilic substitution reaction (EINS). This EINS concept, characteristic by hydride removal on the attack by the electrophilic particle with a subsequent attack by the nucleophilic particle, can be applied more generally to other “electrophilic” reactions of boron hydrides. The difference between these reactions and electrophilic FriedelCrafts substitutions on organic aromatic substrates’ obviously consists both in the hydridic character of the BH bond and in the electron deficient nature of the boron atom under attack.

The same series of compounds (2a-c) is also obtained in noncatalyzed reactions of (1) with elemental halogens (Cl,, - 30°C; Br,, 5”C, and I*, reflux) in hexane or benzene. Spontaneous solid- phase reaction of (1) with iodine, finished by heating at 90°C results in the formation of 6,8-I,-4,5- CzB7HI L (2d). The observed relative ease of the noncatalyzed halogenation seems to consist of a strong hydridic character of the cage B(6) H2 group.

NMR spectra of compounds (2a-d) (Tables 1 and 2) exhibit general features of those published for parent 4,5-CzB7H1 3. 5 ‘H NMR spectra show 1 : 1 : 1 : 2 patterns of singlets due to three CH res- onances [C(5)H, C(4)H,,,, and C(4)H,,d,] and two coincidentally overlapping bridge proton signals. 13C NMR spectra of (2b) and (2d), exhibiting one doublet and one triplet of equal intensities, clearly evidence the presence of one CH and one CHI groups. The relevant ’ 'B NMR data of (2a-d) unambiguously indicate the presence of seven non- equivalent boron environments with three borons coupled to hydrogen bridges. The most interesting feature is the presence of the doublet of the mono- halogenated B(6)Hr group.

Additional insight was given by 2-D ’ ‘B-l 'B measurements on (2a-d). In the scheme presented in Table 3, all adjacent borons give rise to observed cross peaks expected in 2-D spectra for the geometry depicted in Fig. 1 except for those between the B(l)-B(9), B(7)-B(8) and B(2)-B(3) nuclei which are not observable in a few cases.

Table 4 reflects the influence of the B(6) halogen substitution on the ‘lB NMR shifts in terms of A& A& and k2 values. The trends seen in this table for substituted borons are similar to those observed with other substituted borane systems, e.g. B , 0H 1 4,8 2,4-C2B,H7,’ and 6,9-C2BsHr4.” Taking unsub- stituted boron atoms of (2a-d) into account, a reg- ular trend of increase in AZ and a decrease in k2 values is seen on going through the 6-C& 6-Br, and 6-I series; similar trends were observed with all monohalogenated derivatives of B , oHL 4, * but not with 1-haloderivatives of 6,9-C2BsH r4. ‘O To draw more general conclusions, more derivatives sub- stituted on other cage atoms should be isolated and characterized.

AcknowledgementsThe authors wish to thank Drs I. Koruna and M. Ryska of the Research Institute for Phar- macy and Biochemistry, Prague, for mass spectral measurements.

REFERENCES

1. R. R. Rietz and R. Schaeffer, J. Am. Chem. Sot.

2.

3.

4.

5.

6.

7.

8.

9.

10.

1973,95, 6254. J. PleSek, B. Stibr and S. Heimhnek, Chem. Znd. (London) 1980,626. B. Stibr, J. PleSek and S. Hei;manek, Inorg. Synth. 1983,22,237. J. B. Casey, W. J. Evans and W. H. Powell, Znorg. Chem. 1983,22,2236. S. Hekminek, T. Jelinek, J. PleSek, B. Stibr and J. Fusek, J. Chem. Sot. Chem. Commun. in press. S. Hekminek, J. Fusek, B. Stibr, J. Plebek and T. Jelinek, Polyhedron 1986,5, 1303. E. S. Gould, Mechanism and Structure in Organic Chemistry. Rinehart and Winston, New York (19>9). R. F. Sprecher, B. E. Aufderheide, G. W. Luther III and J. C. Carter, J. Am. Chem. Sot. 1974,96,4404. N. G. Bradford, T. Onak, T. Banuleos, F. Gomes and E. W. DiStefano, Znorg. Chem. 1985, 24,409l. B. Stibr, Z. Janousek, J. PleSek, T. Jelinek and S. HeimBnek, Collec. Czech. Chem. Commun., in press.