Replacement of the nitrogen of [1-N2B10H9]− by amines or nitriles, a route to hydrophobic monoanions

Polyhedron 18 (1999) 931–939

Replacement of the nitrogen of [1-N 2 B 10 H 9 ] 2 by amines or nitriles, a route to hydrophobic monoanions ¨ b , Bernard Bonnetot a , Henri Mongeot a , * Daoud Naoufal a , Bohumir Gruner a

´ Laboratoire des Multimateriaux et Interfaces, UMR no 5615, Universite´ Claude Bernard Lyon I, 43 Boulevard du 11 Novembre 1918, ´ , France F-69622 Villeurbanne Cedex b Institute of Inorganic Chemistry Academy of Sciences of the Czech Republic, 25068 Rez near Prague, Czech Republic Received 2 June 1998; accepted 23 September 1998

Abstract Starting from closo-[B 10 H 10 ] 22 hydrophobic monoanions [R 1 R 2 R 3 N–B 10 H 9 ] 2 hR5H, C 6 H 5 CH 2 , C 6 H 5 , CH 3 , C 18 H 37 (CH 3 ) 2 j could be obtained by a multistep process in which the displacement of nitrogen from [1-N 2 B 10 H 9 ] 2 by amines was the key step. Attempts at direct synthesis employing bulky tertiary amines were unsuccessful: no reaction occurred at 1208C and at 1508C [1-N 2 B 10 H 9 ] 2 decomposed to [B 20 H 18 ] 22 . PdhP(C 6 H 5 ) 3 j 2 Cl 2 used as a catalyst produced a favourable effect, but the [R 1 R 2 R 3 N–B 10 H 9 ] 2 ions were present in too low concentration to be isolated from the reaction mixtures. A more suitable route to monoanions carrying three bulky organic groups attached to the amino nitrogen consisted in preparing amino derivatives from the appropriate primary or secondary amines and reacting these intermediate products with alkylhalides in alkaline aqueous propanol solution. The displacement of N 2 by nitriles produced [1-RCNB 10 H 9 ] 2 monoanions hR5CH 3 , (C 6 H 5 ) 2 CHj which proved to be thermally stable, but were easily hydrolysed to [1-RCONH 2 B 10 H 9 ] 2 monoanions.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Hydroborates; Hydrophobic anions; Diazonium salts; Amine derivatives; Nitrile derivatives

The closo-decahydrodecaborate (22) anion [B 1 0 H 1 0 ] 22 has considerable thermal and hydrolytic stability and its quasiaromatic behaviour has been shown [1,2]. The chemistry of [B 10 H 10 ] 22 and other closo-hydroborate anions was initially developed in the sixties and it has again become extensively studied recently [3]. Our interest in [B 10 H 10 ] 22 and [B 12 H 12 ] 22 chemistry is currently focused on the preparation of hydrophobic monoanions, susceptible to application to the extraction of heavy metal cations from aqueous solution, using the liquid–liquid extraction methods. Such monoanions, starting from [B 12 H 12 ] 22 have been previously obtained [4] and we describe here the preparation of monoanions containing a ten boron atom cage. The cage in neutral carboranes, e.g. 1,10-C 2 B 8 H 10 is known to repel water molecules from its surface. The 2 minus charge of [B 10 H 10 ] 22 produces high electrostatic interactions which compete with the hydrophobic behaviour rendering the dianion soluble in aqueous phase. For this reason the overall negative charge had to be reduced to one unit. The hydrophobicity of the anion had

*Corresponding author. 0277-5387 / 99 / $ – see front matter PII: S0277-5387( 98 )00354-4



to be further enhanced by attaching bulky amino groups to the cage. The method envisaged for this preparation was based on the displacement of nitrogen from [1-N 2 B 10 H 9 ] 2 (1) by bulky amines or nitriles. Compound 1 was prepared according to published methods [5–9], although we attempted to improve the process in order to obtain the tetramethylammonium salt in better yield. Only two examples of displacement of nitrogen from 1 by bases leading to monoanions have been described. They concern the pyridine [8] derivative [1-C 5 H 5 NB 10 H 9 ] 2 (2) and the thioamide complex anion [1-h(CH 3 ) 2 N5CHSj–B 10 H 9 ] 2 prepared from N,N-dimethylthioformamide [10] and 1. More generally, considering also the molecular derivatives prepared from 1,10-(N 2 ) 2 B 10 H 8 , it was stated that the 22 diazo derivatives of [B 10 H 10 ] are capable of reacting easily with almost any nucleophile to give a large number of derivatives [8,11]. We have investigated the reaction of 1 with amines over the temperature range 120–1508C. The decomposition reaction of pure 1 was also studied in order to explain the presence of [B 20 H 18 ] 22 as main product when the reaction with tertiary amines proceeded only sparingly. Owing to the higher reactivity of 1 toward

1999 Elsevier Science Ltd. All rights reserved.

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D. Naoufal et al. / Polyhedron 18 (1999) 931 – 939

primary or secondary amines, encumbered tertiary amine derivatives could be conveniently obtained in a two step process: in the first step I was reacted with primary or secondary amines and in the second step the amino groups of the products obtained were alkylated. The reaction of 1 with nitriles was investigated with the aim of preparing the expected [1-RCNB 10 H 9 ] 2 monoanions and also to obtain information on the behaviour of acetonitrile used as a solvent when 1 was prepared.

1. Experimental

1.3. Exchange of cations Most anions were synthesized as tetramethylammonium salts and their conversion to acid was carried out by methods similar to those used for B 12 derivatives [4].

1.3.1. Method A Aqueous solutions of salt 6 were passed on strong acid Duolite C 20 H resin charged with protons, and solutions of 5 in water / acetonitrile (30%) mixtures were similarly treated. The free acid solutions were concentrated and in some instances the potassium salts were prepared by neutralization with aqueous KOH.

1.1. Physical measurements Boron ( 11 B) NMR spectra were obtained at 96.29 MHz on a Bruker WF-300 spectrometer and were externally referenced to Et 2 O.BF 3 (positive values downfield), and 1 H spectra were recorded at 300 MHz on the same instrument. IR spectra were recorded on a Nicolet Magna 550 FT spectrometer using KBr pressed discs. Mass Spectrometry measurements were performed in the Mass Spectrometry Laboratory, Central Analytical Service of the CNRS, Solaize (France), on a hybrid Mass Spectrometer ZAB-2-SEQ (Micromass) by SIMS technique with caesium beams. The samples were dissolved in acetonitrile and mixed with a matrix of thioglycerol or nitrobenzyl alcohol. Masses were measured with an accuracy of 0.1 mass unit. A salt such as hN(CH 3 ) 4 j 2 [B 10 H 10 ] formed from a dianion and monovalent cations was detected as the monoanion N(CH 3 ) 4 [B 10 H 10 ] 2 . All the [amine–B 10 H 9 ] 2 and [nitrile–B 10 H 9 ] 2 monoanions were easily characterized by this method but the characterization of hN(CH 3 ) 4 j 2 [B 20 H 18 ] required additional measurements by means of the Electrospray (ES) method on a VG-Platform Micromass spectrometer. Samples were introduced in the spectrometer as acetonitrile solutions. A B70 Setaram apparatus was used for TGA measurements.

1.2. Chromatography TLC was performed on laboratory made DEAE cellulose [4,17] (and references therein) with a 2 M solution of NH 4 NO 3 as eluent. The spots were detected by a solution of PdCl 2 (0.5% in 5% HCl). The purity of products was examined by ion-exchange HPLC using a Zorbax Sax analytical column (containing an ammonium functionalized silane chemically bound to silica). Chromatographic parameters: mobile phase 0.1 M NaClO 4 in a 40% acetonitrile aqueous solution, flow-rate 0.5–1 cm 3 min 21 , UV detection at 210 and 254 nm. For the separation of products at the preparative scale, silicagel 70–230 mesh (Aldrich) was used to fill either glass columns or stainless steel columns. The stainless steel columns were connected to a preparative instrument equipped with a conductivity detector.

1.3.2. Method B The tetramethylammonium salts of 2, 5, 6, 8 and 9 were converted to the corresponding conjugate acids as follows: the finely powdered tetramethylammonium salt was overlayered in a separation funnel by ether (50 cm 3 ) and then treated with 3 M HCl (5330 cm 3 ). The undissolved powder that remained at the interface of the aqueous and the organic layer was always kept with the organic layer. The combined aqueous solutions were extracted once by additional diethyl ether (30 cm 3 ) and the ether extracts were combined, poured into water and ether was evaporated to give an aqueous solution of the acid. 1.4. Syntheses and characterization 1.4.1. Materials All solvents were reagent grade (SDS, France) and were distilled from an appropriate drying agent under argon [12]. Solid amines (Aldrich) were purchased from commercial sources and used as received. Liquid amines were purified by distillation prior to use. Ammonium decahydrodecaborate (Katchem, Prague) was dried in vacuum at 1208C. The materials 2,4,6 tribromophenyldiazonium tetrafluoroborate [13] and DEAF cellulose [4] were prepared by published methods. Unless otherwise indicated, 1 was always reacted with amines and nitriles as a tetramethylammonium salt. Potassium salts and potassium hydroxide were used for alkylation reactions (see below preparation of compounds 8 and 9). 1.5. N( CH3 )4 [1 -N2 B10 H9 ] (1) Potassium hydroxide (5.72 g, 0.102 mol) dissolved in 20 cm 3 of water was added to an aqueous solution (100 cm 3 ) of (NH 4 ) 2 [B 10 H 10 ] (7.86 g, 0.051 mol). Concentration of the solution under reduced pressure in a rotary evaporator gave a crystalline solid which was subsequently dried for 8 h at 808C (Pressure: 1.3 Pa). The solid was dissolved in 600 cm 3 of acetonitrile in an argon atmosphere and the solution was cooled down to 2358C. A solution of 21.8 g (0.053 mol) of 2,4,6-tribromophenyl diazonium tetra-

D. Naoufal et al. / Polyhedron 18 (1999) 931 – 939

fluoroborate dissolved in 200 cm 3 of anhydrous acetonitrile was added dropwise over 1 h, then the mixture was maintained for 30 min at this temperature. The solution was allowed to warm to room temperature, 2 g of dried sodium propionate were added and the solution was stirred overnight. TLC analyses were performed and three spots could be seen, one violet at the start (undecomposed [B 10 H 9 –NH–N–C 6 H 2 Br 3 ] 2 and organic impurities), the most important due to 1 with Rf50.45 and a smaller spot with Rf50.35 (impurity). The reaction mixture was evaporated to a volume of 200 cm 3 and filtered to remove the insoluble violet impurities. The solid was washed twice 3 with 50 cm of acetonitrile. The acetonitrile fractions were combined, evaporated to dryness and treated with 100 cm 3 of water. The aqueous slurry was filtered to remove an additional quantity of organic impurities, the solid was washed twice with 50 cm 3 of water and crude 1 was precipitated from the combined water extracts by an excess of N(CH 3 ) 4 Cl. This solid was washed 3 times with 30 cm 3 of cold ethanol (08C) and three times with 30 cm 3 of (C 2 H 5 ) 2 O. After repeated recrystallization from hot C 2 H 5 OH 3.25 g of pure product were obtained. All the C 2 H 5 OH and (C 2 H 5 ) 2 O washing solutions and C 2 H 5 OH mother liquors were combined and evaporated to dryness. The solid was chromatographed on a silica gel column (2532 cm), eluting with CH 2 Cl 2 –CH 3 CN mixtures (the V/ V ratio varied from 9 / 1 to 9 / 2 during the preparative separation). Four coloured bands were observed and 1.4 g of product were recovered from the third one giving 0.85 g of pure 1 after recrystallization (the total yield was 36.7%). 11 B NMR data were in agreement with Ref. [8]. The thermal stability of 1 has been studied by TGA on 50 mg samples, the temperature was raised at a 2 K min 21 heating rate. The decomposition reaction started at 150– 1608C under an air atmosphere or an argon atmosphere. In order to obtain sufficient amounts of product for analysis 0.2 g samples were also pyrolysed under air or argon up to 2008C at a 6 K min 21 heating rate. The residual solid contained hN(CH 3 ) 4 j 2 [B 20 H 18 ] (90%), residual 1 (5%) and [B 12 H 12 ] 22 characterized by Electrospray mass spectrometry (m /z5216, calculated for N(CH 3 ) 4 [B 12 H 12 ] 2 : 215.6) and 11 B NMR d 11 B5214.83 (literature 215.63 [14–16]). Pure [B 20 H 18 ] 22 was isolated by chromatography on a silicagel column, eluent CH 3 CN (30%)– 22 CH 2 Cl 2 (70%), 1st fraction 1, second fraction [B 20 H 18 ] , 22 22 third fraction mixture of [B 20 H 18 ] and [B 12 H 12 ] . 22 Consistently with literature data [17], [B 20 H 18 ] proved to be a hydrophobic anion and was detected by TLC at Rf50. 11 B NMR data were in agreement with [18]: d 11 B (CH 3 CN)530.55 (d, JB – H 5148, 2B), 15.97 (s, 2B), 26.84 (d, JB – H 5142, 2B), 212.35 (d, JB – H 5146, 4B), 215.86 (d, JB – H 5140, 4B), 219.29 (d, JB – H 5135, 4B), 225.53 (d, JB – H 5148, 2B). Electrospray mass spectrometry: m /z5 308.5 for the most intense peak, calculated 308 for N(CH 3 ) 4 [B 20 H 18 ] 2 .

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1.6. N( CH3 )4 [1 -C5 H5 NB10 H9 ] (2) Employing the procedure described by Leyden and Hawthorne [8], a solution obtained by dissolving 0.5 g of 1 in 13 cm 3 of pyridine was refluxed for 3 h, forming a yellow precipitate. The precipitate was separated by filtration and washed with acetone and diethyl ether giving 0.44 g of product. TLC revealed the presence of the yellow product 2 (Rf50.45) and traces of an impurity (Rf50.90), very soluble in water. The 11 B NMR data corresponding to 2 (Table 1) were found to be slightly different from those previously published [8], but this difference remained unexplained. FAB 2 : maximum of the group of peaks m /z5196 (calculated 196).

1.7. Reaction of 1 with C18 H37 ( CH3 )2 N A suspension of 1 (0.5 g) in dimethyloctadecylamine (20 g) was heated for 8 h at 1208C under stirring. After cooling down, 50 cm 3 of hexane were added to the reaction mixture and the solid was collected by filtration and washed with hexane. TLC and NMR analyses showed that no reaction had occurred. When the reaction mixture was heated at 1408C under the same conditions, only degradation products were formed. No catalytic effect was observed when Cul (0.04 g) was added to the starting materials. Starting from 0.5 g of hC 18 H 37 (CH 3 ) 2 NHj[1N 2 B 10 H 9 ] which completely dissolved in 30 cm 3 of the amine upon heating, only decomposition products were detected after maintaining the reaction mixture for 48 h at 1058C. Attempts to use various solvents were also unsuccessful because these solvents (tert-butyl-methyl ketone, nitroethane, butanol) reacted with 1. The formation of N(CH 3 ) 4 [1-C 18 H 37 (CH 3 ) 2 NB 10 H 9 ] only occurred when PdhP(C 6 H 5 ) 3 j 2 Cl 2 (0.1 g), acting as catalyst, was added to the suspension of 1 in N,N-dimethyloctadecylamine and the reaction mixture was maintained at 1208C overnight. [C 18 H 37 (CH 3 ) 2 NB 10 H 9 ] 2 was unambiguously characterized by the FAB 2 technique owing to the presence of a B 10 group of peaks with a maximum at m /z5413.6. Due to the presence of impurities ([B 10 H 10 ] 22 and other closo hydrodecaborate derivatives) in high concentration, the NMR data could not be determined.

1.8. Reaction of N( CH3 )4 [1 -N2 B10 H9 ] with tribenzylamine No reaction occurred when 0.5 g of 1 and 40 cm 3 of tribenzylamine were stirred for 5 h at 1208C, but an almost complete decomposition of 1 to hN(CH 3 ) 4 j 2 [B 20 H 18 ] was observed after 15 h at 1508C.

1.9. N( CH3 )4 [1 -C6 H5 NH2 B10 H9 ] (3) A suspension of 1 g (4.57 mmol) of 1 in aniline (30

D. Naoufal et al. / Polyhedron 18 (1999) 931 – 939

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Table 1 11 B NMR data for M[B 10 H 9 L] (c) measured in CH 3 CN at 96.25 MHz Compound

Anion formula

B 1 apical (singlet)

B 10 apical (doublet) JB – H (Hertz)

Equatorials (doublets) JB – H (Hertz)

1

[1-N 2 B 10 H 9 ] 2 (a)

2

[1-C 5 H 5 NB 10 H 9 ] 2 ( b)

3

[1-C 6 H 5 NH 2 B 10 H 9 ] 2

4

[1-(C 6 H 5 )CH 3 NHB 10 H 9 ] 2

5

[1-(C 6 H 5 ) 2 NHB 10 H 9 ] 2

6

[1-(C 6 H 5 CH 2 )NH 2 B 10 H 9 ] 2

7

[1-(C 6 H 5 CH 2 ) 2 NHB 10 H 9 ] 2

8 9

[1-(C 6 H 5 CH 2 ) 2 (C 6 H 5 )NB 10 H 9 ] 2 [1-(C 6 H 5 CH 2 )(C 6 H 5 ) 2 NB 10 H 9 ] 2

212.84 – 6.36 – 3.70 – 2.77 – 3.61 – 9.62 – 9.98 – 1.97 1.88 – 22.39 – 5.01 – 23.61 10.22 –

22.00 (149) 2.18 (142) 3.70 – 3.93 (167) 3.61 – 20.60 – 20.27 – 4.68 4.90 (157) 5.93 (153) 20.30 (130) 7.89 27.16 –

216.59 (139) 225.47 (123) 225.86 (149) 225.89 (149) 225.86 (138) 226.16 (146) 226.10 (157) 225.26 225.52 (141) 224.43 (136) 228.60 (165) 223.58 226.76 –

10

[1-H 3 CNB 10 H 9 ] 2

11

[1-H 3 CCONH 2 B 10 H 9 ] 2

12 13

[1-(C 6 H 5 ) 2 CH 2 CNB 10 H 9 ] 2 [1-(C 6 H 5 ) 2 CH 2 CONH 2 B 10 H 9 ] 2

224.33 (140) 227.61 (187) 227.64 (151) 227.60 (146) 227.85 (129) 227.85 (129) 227.82 (143) 227.00 227.29 (146) 229.09 (136) 230.43 (153) 225.80 224.48 –

(a) Chemical shifts are in agreement with those reported in reference [8]. (b) NMR data reported in reference [8] were: 10.5 (B 1 ), 22.5 (B 10 , JB – H 5120 Hz), 231.2 (B 2 –B 9 ) (c) M5N(CH 3 ) 4 except for 5 (M5Na).

cm 3 ) was maintained for 15 h at 1208C. The reaction mixture became dark brown probably due to the formation of charge transfer complexes. After cooling down, aniline was distilled off under vacuum and toluene (50 cm 3 ) was added to the residue. A solid, (1.5 g) was recovered by filtration and was washed with toluene before drying under vacuum at room temperature. Three spots were observed by TLC analysis of the crude product: Rf50.38 (3), 0.64 (impurity) and 0 (impurity). The product was purified on 0.2 g fractions on a preparative silicagel column by means of the preparative HPLC instrument using a 1 / 1 CH 2 Cl 2 – CH 3 CN mixture as eluent. Pure 3 (0.84 g) was recovered from the third fraction with a 65% yield. Capacity factor measured by HPLC for 3, k9512.57. FAB 2 : m /z5209.3, calculated 210.

1.10. N( CH3 )4 [1 -C6 H5 ( CH3 )NHB10 H9 ] (4) The reaction was performed under the same conditions as with aniline except that after reacting the products, the amine excess was not distilled off, but was dissolved in toluene and the solid was recovered by filtration. The spot corresponding to 4 was observed by TLC with a Rf50.24

value. The most intense peak was observed by FAB 2 spectroscopy for m /z5223.3 (calculated 224).

1.11. Reaction of 1 with dimethylaniline No reaction was observed when the reaction mixture was heated for 48 h at 1208C and only decomposition products formed upon heating at 1408C.

1.12. N( CH3 )4 [1 -( C6 H5 )2 NHB10 H9 ] (5) A suspension of 1 g of 1 in diphenylamine (20 g) was heated for 15 h at 1258C and the solid was recovered as described for 4. Three spots were observed by TLC (Rf5 0.074, 0.233 and 0.326), the most intense (Rf50.074) corresponded to 5. A sample of 1.56 g of crude product were obtained and converted to the conjugate acid by method B and precipitated as a tetramethylammonium salt by addition of a N(CH 3 ) 4 Cl aqueous solution. The product containing impurities in low concentration was purified by preparative HPLC using a 1 / 1 CH 3 CN–CH 2 Cl 2 mixture as eluent (flow-rate 10 cm 3 min 21 ). It was isolated from the fourth fraction which was pale violet coloured and was characterized by FAB 2 spectroscopy (m /z5285.2 for the

D. Naoufal et al. / Polyhedron 18 (1999) 931 – 939

more intense peak, calculated 285.2). Capacity factor measured by HPLC, k953.74.

1.13. N( CH3 )4 [1 -( C6 H5 CH2 )NH2 B10 H9 ] (6) A suspension of 2 g of 1 in benzylamine (40 cm 3 ) was heated overnight at 1208C. TLC analysis of one drop of the reaction mixture revealed the presence of 3 spots (Rf50, 0.092 for impurities and 0.349 for 6). The benzylamine excess was distilled off under vacuum and a waxy solid was collected. After recrystallization from hot C 2 H 5 OH, 1.36 g (50% yield) of pure 6 were recovered (TLC analysis, Rf50.48, FAB 2 spectroscopy m /z5223.3, calculated 223.21). The monoanion was converted into its conjugate acid following method A, and the aqueous acidic solution was found to be stable for several days. The capacity factor measured by HPLC was k9517.

1.14. N( CH3 )4 [1 -( C6 H5 CH2 )2 NHB10 H9 ] (7) A suspension of 1 g of 1 in dibenzylamine (20 cm 3 ) was heated overnight. Three spots were observed by TLC of the amine solution (Rf50, 0.382 and 0.572), the amine excess was dissolved in hexane and the solid was separated by filtration and washed with hexane. Recrystallization of the solid from hot ethanol gave 1.1 g of pure product (62% 2 yield, Rf50.382), the FAB spectrum displayed a group of peaks with a maximum at m /z5314.3 (calculated 314.3). Due to the instability of the conjugate acid of 7 in aqueous media, only decomposition products (mainly boric 1 acid) formed when the replacement of N(CH 3 ) 1 4 by H was attempted either by method A or B. The capacity factor measured by HPLC was k9516.75

1.15. N( CH3 )4 [1 -( C6 H5 CH2 )2 ( C6 H5 )NB10 H9 ] (8) Starting from 3 (1.3 g, 4.57 mmol), an aqueous solution of H 3 O[1-C 6 H 5 NH 2 B 10 H 9 ] was obtained by method A. KOH (2 g) was added to the solution, water was evaporated and the solid obtained was dried and then finely ground. This solid was dissolved together with 5 g of KOH in 100 cm 3 of a hot 1-propanol (60%)–water (40%) mixture, then 2 cm 3 of benzyl bromide dissolved in 20 cm 3 of 1-propanol were added and the violet coloured reaction mixture was refluxed for 15 h. A very dark spot corresponding to 8 was observed by TLC at Rf50, but a very pale spot was also present (Rf50.32, K[1(C 6 H 5 CH 2 )(C 6 H 5 )NHB 10 H 9 ]). After addition of 1 cm 3 of benzyl bromide dissolved in 10 cm 3 of 1-propanol, the reaction mixture was allowed to stand at room temperature for 2 days, after which time the reaction was complete (the spot at Rf50.32 had disappeared). The solvent was evaporated, 50 cm 3 of water and 45 cm 3 of 6 M HCl were added and the mixture was stirred giving slurry. The

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organic impurities were extracted with toluene (20 cm 3 3 4) and 8 was extracted in the acidic form with diethyl ether (40 cm 3 33) as described above (method B) and then precipitated by N(CH 3 ) 4 Cl to give 0.3 g of 8. An additional amount of 8 was obtained by extracting the slurry with 30 cm 3 of acetonitrile. Two g of N(CH 3 ) 4 Cl dissolved in 50 cm of water were added to the acetonitrile solution and acetonitrile was evaporated while a precipitate (pure 8) formed. This solid (0.27 g) was recovered by filtration and 8 was obtained with a 27% total yield. The FAB 2 spectrum displayed a group of peaks with a maximum at m /z5388 (calculated 390). The capacity factor measured by HPLC was k9519.79.

1.16. N( CH3 )4 [1 -( C6 H5 CH2 )( C6 H5 )2 NB10 H9 ] (9) Starting from 5 (1 g), 9 was prepared following the procedure described for obtaining 8 except that the slurry was not extracted with acetonitrile. A total of 0.35 g of pure 9 was obtained (28% yield). FAB 2 maximum at m /z5375.3, calculated: 376.

1.17. N( CH3 )4 [1 -( CH3 CN)B10 H9 ] (10) A 0.2 g sample of 1 was refluxed in 40 cm 3 of CH 3 CN for 6 h, 2 spots Rf50.4 (10), Rf50.45 (1) were observed by TLC and the 11 B NMR of the mixture showed that 50% of 1 had been converted to 10. The reaction was complete after refluxing the mixture for 36 h. Compound 10 was quantitatively obtained (FAB 2 maximum at m /z5158, calculated: 158). Compound 10 was readily converted to [1-CH 3 CONH 2 B 10 H 9 ] 2 (11) in hot water (FAB 2 maximum of the massif m /z5175, calculated: 175). A mixture of 10 (0.3 g) and diphenylamine (5 g) was heated for 15 h at 2128C. Diphenylamine was dissolved in toluene, the solid was filtered and was washed with toluene (2325 cm 3 ) and then with diethylether (20 cm 3 ). After drying, ca 0.3 g of undecomposed 10 was recovered.

1.18. N( CH3 )4 [1 -(( C6 H5 )2 CHCN)B10 H9 ] (12) A mixture of 1 (0.5 g) and diphenylacetonitrile was stirred at 1308C for 15 h. After cooling down the solid products thus obtained were treated with ethanol (50 cm 3 ) which dissolved diphenylacetonitrile. The residual solid was washed with ethanol (25 cm 3 32) and with diethylether (20 cm 3 ) and was collected to be dried under vacuum (TLC: Rf50, FAB 2 maximum at m /z5310, calculated: 310). When dissolved in hot water, 12 was quantitatively converted to N(CH 3 ) 4 [1-((C 6 H 5 ) 2 CHCONH 2 )B 10 H 9 ] (13) (TLC: Rf50, FAB 2 maximum at m /z5327, calculated: 328).

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D. Naoufal et al. / Polyhedron 18 (1999) 931 – 939

2. Results and discussion The different routes to hydrophobic anions that we have investigated starting from 1 are shown in Fig. 1. Various attempts were made to improve the synthesis of 1 according to the method of Hawthorne et al. [5–9]. Compound 1 was obtained in a very pure form, but with yields not exceeding 40% partly because acetonitrile used as a solvent led to the formation of the undesired side product

10. Acetonitrile solutions of 1 could be stored for several days at room temperature without noticeable decomposition, it may be inferred that acetonitrile reacts with intermediates species to give 10 during the formation of 1. Attempts to use (NH 4 ) 2 [B 10 H 10 ] instead of K 2 [B 10 H 10 ] 2 were unsuccessful: the aryldiazonium salt of [B 10 H 10 ] did not significantly decompose to 1 in the presence of NH 41 when sodium propionate was added to the reaction medium. Due to its acidic character NH 1 4 may oppose the

Fig. 1. Synthesis of [1-LB 10 H 9 ] 2 from [1-N 2 B 10 H 9 ] 2 .

D. Naoufal et al. / Polyhedron 18 (1999) 931 – 939

buffer effect produced by sodium propionate, but the decomposition reaction of 1 has been reported to take place [9] with salts formed from Et 3 NH 1 , a slightly weaker acid than NH 41 . Other factors such as the solubility of the salts in acetonitrile may be more important. When the nitrogen atom was not encumbered by bulky substituents, amines reacted with 1 at 120–1308C giving the [amine–B 10 H 9 ] monoanions 2–7. Amines were used in excess in order to serve as a solvent, because all the other suitable solvents tested reacted with the starting materials. When the reaction proceeded with difficulty because of steric hindrance, a complex mixture of products was obtained at 1508C and the decomposition reaction of 1 prevailed 2[1 2 N 2 B 10 H 9 ] 2 → [B 20 H 18 ] 22 1 2N 2 The same decomposition reaction giving hN(CH 3 ) 4 j 2 [B 20 H 18 ] as main product occurred when pure 1 was heated at 1508C in an argon atmosphere. [B 20 H 18 ] 22 is well known and was prepared by aqueous ferric [19] or ceric [18,20] ion oxidation or electrochemical coupling [21,22] of [B 10 H 10 ] 22 . Upon heating 1, the formation of [B 20 H 18 ] 22 was also accompanied by its slow degradation, giving the very stable [B 12 H 12 ] 22 and other unidentified products. The formation of [B 12 H 12 ] 22 can be attributed to polyhedral rearrangements and such rearrangements have been observed at temperatures as low as 150–1608C [23,24]. Palladium catalysts are effective in reactions of diazonium salts with alkenes [25], carbon monoxide [26] and organotin compounds [27]. The addition of PdhP(C 6 H 5 ) 3 j 2 Cl 2 to a mixture of 1 with N,N-dimethyloctadecylamine allowed the [amine–B 10 H 9 ] 2 monoanion to form but in poor yield. The nature of the substituents attached to the amino nitrogen proved to have considerable effects on the extraction properties of the anions. Long aliphatic chains tend to give detergent-like properties causing problems in separation of aqueous and organic phases in the course of extraction. More suitable products were obtained when the substituents were aromatic groups. Compounds 8 and 9 were obtained by alkylation of 3 or 5 using an excess of benzyl bromide. As observed for the B 12 series [4], the ammonium groups of [amine–B 10 H 9 ] 2 anions are weaker nucleophiles than the initial free amines. Nevertheless the alkylation reaction could be carried out by refluxing in propanol–water mixtures in alkaline media, but the reaction time had to exceed 36 h. To be potentially interesting either for its conversion to a more hydrophobic anion or for extraction applications, an anion had to be stable in a strong acid medium. Unexpected behaviour was observed in the benzyl derivative series, 6 proving to be stable in acidic medium but the disubstituted anion 7 decomposed to boric acid. In contrast under the same conditions both the monosubstituted 3 and disubstituted 5 derivatives were

937

stable. Finally we could obtain pure samples containing 8 or 9 in which the proximity of phenyl rings to the ten boron atom cage produces a stabilizing effect. The trisubstitution enhances the hydrophobic character of these anions and according to preliminary tests, it should be worth examining their properties as extractors of caesium from aqueous solutions. The behaviour of 1 in the presence of nitriles was consistent with the properties of inner diazonium salts described in the literature because of the absence of any steric effect. The reaction affords a facile route to [1RCNB 10 H 9 ] 2 monanions whereas their stereoisomers carrying the nitrile substituents in an equatorial position can be obtained by reaction of nitriles with [B 10 H 10 ] 22 in the presence of strong acid [28]. The IR spectra of 10 and 21 21 12 showed absorptions at 2218 cm and 2255 cm respectively, these bands being assigned to the CN stretching vibration because no absorption was observed in the same region with the related [1-amine–B 10 H 9 ]2 and [1RCONH 2 B 10 H 9 ] 2 monoanions. The frequencies of these n CN vibrations are close to those observed with the free nitriles, showing that the association of the nitrile with the B 10 cage has not affected the triple bond. Compound 10 did not decompose when heated for 5 h at 2108C as a solution in diphenylamine indicating that the product is thermally stable and does not easily exchange the nitrile group for an amine. In contrast, addition of water to the triple bond occurred readily at temperatures above 908C to give [1-RCONH 2 B 10 H 9 ] 2 monoanions. The 1 H NMR spectra of all derivatives containing one or two benzyl substituents (Table 2) showed that the CH ]2 resonance of the benzyl groups was split into two peaks of equivalent intensity. A splitting into several peaks was also mentioned for their B 12 analogues [4] and suggest that for steric reasons, the two protons of the methylene groups are located in different environments. The 11 B NMR spectrum of [B 10 H 10 ] 22 displays two doublets at d 50.89 and 230.86 ppm in a 1 / 4 ratio [16] corresponding to the apical and the equatorial atoms respectively. As shown in Table 1, the coordination of the ten boron atom cage of the initial [B 10 H 10 ] 22 anion to the amino, nitrilo or amido nitrogen had no large effect on the 11 B NMR resonance frequencies of the various atoms: the equatorial atoms gave rise to two doublets about 2 ppm apart between 226 and 230 ppm; the singlet due to the substituted apical boron atom was observed between 23.6 ppm and 110 ppm and the doublet due to the unsubstituted apical boron appeared between 27 ppm and 18 ppm. In contrast, a strong perturbation was observed for 1 since the two doublets due to equatorial atoms were 8 ppm distant and were considerably shifted compared to [B 10 H 10 ] 22 , the resonances due the apical boron atoms (observed at 212.84 ppm and 122 ppm) were also considerably shifted. The perturbation of the 11 B spectra observed for 1 can be paralleled to the frequency of the main B–H absorption (Table 3) which

D. Naoufal et al. / Polyhedron 18 (1999) 931 – 939

938

Table 2 1 H NMR data (a) for M[B 10 H 9 L] ( b) measured in CD 3 CN at 300 MHz Compound

Formula

CH(arom) ] (multiplet)

1

[1-N 2 B 10 H 9 ] 2

–

2

[1-C 5 H 5 NB 10 H9] 2

3

[1-C 6 H 5 NH 2 B 10 H 9 ] 2

4

[1-(C 6 H 5 )CH 3 NHB 10 H 9 ] 2

5

[1-(C 6 H 5 ) 2 NHB 10 H 9 ] 2

6

[1-(C 6 H 5 CH 2 )NH 2 B 10 H 9 ] 2

7

[1-(C 6 H 5 CH 2 ) 2 NHB 10 H 9 ] 2

8

[1-(C 6 H 5 CH 2 ) 2 (C 6 H 5 )NB 10 H 9 ] 2

9

[1-(C 6 H 5 CH 2 )(C 6 H 5 ) 2 NB 10 H 9 ] 2

10

[1-H 3 CNB 10 H 9 ] 2

7.5–9 (5) 7.0–7.5 (5) 7.1–7.5 (5) 7.1–7.5 (10) 6.7–8.2 (5) 6.8–7.8 (10) 6.3–7.8 (15) 6.5–8.5 (15) –

11

[1-H 3 CCONH 2 B 10 H 9 ] 2

–

CH2 (2] peaks)

CH3 ] (singlet)

NCH3 ] (singlet)

NH ] (singlet)

–

–

–

–

–

–

–

–

3.12 (3) –

3.09 (12) 3.09 (12) 3.06 (12) 3.06 (12) –

– 4.06 (1) 4.22 (2) 4.6 (2) 3.9 (1)

5.11 (1) 4.30 (2) 5.0 (2) 4.0 (1)

– – – –

–

2.80 (3) 2.36 (3)

–

– 2.78 (2) 2.73 (1) –

3.08 (12) 3.06 (12) 3.08 (12) 3.01 (12) 3.10 (12) 3.09 (12)

– – – – – –

(a) For all compounds the hydroborate cage gave rise to broad BH signals spreading between 2.5 and 22 ppm with a total area corresponding to 9 BH. ] ] (b) M5N(CH 3 ) 4 except for 5 (M5Na).

Table 3 IR data for M[B 10 H 9 L] Compound ( b)

Formula

nN – H (cm 21 ) (br)(a)

nB – H (cm 21 ) (s)

ncage (cm 21 ) (m)

1 2 3 4 5 6 7 8 9 10 11 12 13

N(CH 3 ) 4 [1-N 2 B 10 H 9 ] (c) N(CH 3 ) 4 [1-C 5 H 5 B 10 H 9 ] (d ) N(CH 3 ) 4 [1-C 6 H 5 NH 2 B 10 H 9 ] N(CH 3 ) 4 [1-(C 6 H 5 )CH 3 NHB 10 H 9 ] Na[1-(C 6 H 5 ) 2 NHB 10 H 9 ] N(CH 3 ) 4 [1-(C 6 H 5 CH 2 )NH 2 B 10 H 9 ] (e) N(CH 3 ) 4 [1-(C 6 H 5 CH 2 ) 2 NHB 10 H 9 ] (e) N(CH 3 ) 4 [1-(C 6 H 5 CH 2 ) 2 (C 6 H 5 )NB 10 H 9 ] ( f ) N(CH 3 ) 4 [1-(C 6 H 5 CH 2 )(C 6 H 5 ) 2 NB 10 H 9 ] ( g ) N(CH 3 ) 4 [1-H 3 CNB 10 H 9 ] ( h) N(CH 3 ) 4 [1-H 3 CCONH 2 B 10 H 9 ] ( i ) N(CH 3 ) 4 [1-(C 6 H 5 ) 2 CH 2 CNB 10 H 9 ] ( j ) N(CH 3 ) 4 [1-(C 6 H 5 ) 2 CH 2 CONH 2 B 10 H 9 ] ( k)

– – 3373 – 3360 3375 – – – – 3333, 3429 – 3385, 3504

2510 2461 2467 2463 2466 2461 2475 2467 2472 2475 2474 2468 2473

953, 949, 950, 1005, 954, 948, 955, 955, 955, 947, 953, 1000, 1000,

1000 1004 1026 1023 1028 1010 1011 1006 1006 1015 1014 1031 1025

(a) br5broad, s5strong, m5medium, w5weak. 21 (b) for all compounds: n C–H hN(CH 3 ) 1 (m), n C–C (aromatic)51498 cm 21 (m). 4 j53029 cm (c) n N;N52242 cm 21 (s); (d) n C5N51540 cm 21 (m); (e) n C–N51288 cm 21 (w); (f) n C–C (phenyl ring)51485 cm 21 (s), n C–C (benzyl ring)51454 cm 21 (m); (g) n C–C (phenyl ring)51489 cm 21 (s), n C–C (benzyl ring)51455 cm 21 (w), n C–H (–CH 2 )52926, 2961 cm 21 (w); (h) n C–H (–CH 3 )52923, 2959 cm 21 N (w); n C;N52218 cm 21 (w); (i) n C–H (–CH 3 )52923, 2958 cm 21 (w) n C5051675 cm 21 (s), d N–H51586 cm 21 (m); (j) V C–H (–CH 2 )52857, 2929 cm 21 (w), n C;N52255 (w); (h) n C–H (–CH 2 )52857, 2929 cm 21 (w), n C5051622 cm 21 (s).

was found to be about 40 cm 21 higher for 1 than for the other monoanions or for [B 10 H 10 ] 22 . The frequency increase could be due to an electron transfer from the cage to the diazo group [29,30], producing a perturbation of the electron distribution of the B 10 cage.

Acknowledgements Partial support of this work by the European Community in the framework of the Project CIPA-CT93-0133 of the PECO program is highly appreciated. The authors would

D. Naoufal et al. / Polyhedron 18 (1999) 931 – 939

like to express their thanks to Dr R. Suffolk for having improved the style of this publication.

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