A Novel Synthetic Route for the Preparation of Ammonium
Indian Journal of Chemistry Vol. 27A, September 1988,pp. 759-763
A Novel Synthetic Route for the Preparation of Ammonium & Alkali Metal Tetrafluoroborates & Alkyl Substituted Ammonium Tetrafluoroborates us• ing Pyridinium Tetrafluoroborate as the Precursor
K SYED MOHAMEDt & D K PADMA* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012 Received 19 August 1987; revised and accepted 18 December 1987
Ammonium and alkali metal tetrafIuoroborates have been prepared by the cation exchange reaction of pyridi• nium tetrafIuoroborate with the corresponding hydroxides/halides. The reaction of pyridinium tetrafIuoroborate with primary, secondary and tertiary alkyl arnines at room temperature gives rise to mono-, di- and tri-alkylammo• nium tetrafIuoroborates, respectively. The yields are good and the samples are of high purity. The products have been characterised by elemental analysis, IR and PMR spectroscopy. The spectral data for most of the compounds are reported for the first time.
Tetrafluoroborates salts are widely used both in The infrared spectra of the compounds were research and industry. Some of the applications recorded both in nujol mull and KBr using a Per• are in electrolytic polishing of ammonium I, metal kin Elmer 599 spectrophotometer, IH (TMS finishing operationsz, bright dipping solutions for standard) and 19F (CFCl3 standard) NMR spectra Sn-Pb alloys in printed circuits and other electri• were recorded on Varian T60 and Varian Ff 80A cal components3, catalyst in organic synthesis4.s, (74-84 MHz) NMR spectrometers using DzO or high temperature fluxes, battery components and DMSO-d6 as solvents. I3CNMR spectra were re• boriding of steel surfaces8• The availability of corded on a Brucker WH-270 MHz F-T NMR, pure salts is of decided advantage for the applic• X-ray powder diffraction patterns were recorded ations mentioned above. However, the methods with a Phillips 1050/70 X-ray diffractometer us• so far reported for their preparation are not satis• ing Cu Ka radiation. factory as regards purity of products, simplicity of methods employed and yields. We report here a Preparation of ammonium tetrafluoroborate simple, one-step procedure for the synthesis of ti• To a solution of pyridinium tetrafluoroborate, tle compounds at room temperature, employing (5 g) was added a very slight excess of ammonium pyridinium tetrafluoroborate for exchange reac• hydroxide (1:1) solution dropwise. Pyridine' was tions. found to be liberated and was extracted with chloroform thrice. The aqueous solution was fil• Materials and Methods tered and evaporated slowly on a water-bath at All the reagents employed in the present study 60°-70°C when a solid was obtained, it was dried were of LR/ AR grade. in vacuo over phosphorus pentoxide, yield 92%. The precursor, pyridinium tetrafluoroborate (CsHsNHBF4) was synthesised by the reaction of Preparation of alkali metal (Na, K, Rb and Cs) t~ pyridinium poly(hydrogen fluoride) with boric ox• trafluoroborates ide or boric acid9. Recrystallized sample~, m.p. To a solution of pyridinium tetrafluoroborate (5 210°C, when treated with ammonium and alkali g) was added calculated amount of sodium hy• metal hydroxide!chlorides underwent cation ex• droxide solution (30 mI, 1.0 N), potassium chlo• change and formed the corresponding ammonium ride (2.5 g in 10 mI of water), rubidium bromide and alkali metal tetrafluoroborates. This prepara• (2.0 g in 5 mI of water) or cesium bromide (1.5 g tive mode has been extended to the preparation in 5 mI of water). Pyridine was found to be liber• of alkyl substituted ammonium tetrafluoroborates ated in the process. In the case of sodium tetraflu• in good yields and high purity. oroborate the pyridine was extracted with chloro• form and the solution concentrated. The yield was t Present address: Department of Chemistry, Gov!. Arts Col• 94%. In the other cases, the precipitate formed lege, Krishnagiri, Tarnilnadu. was filtered, washed' with alcohol-water mixture
759 INDIAN 1. CHEM., VOL. 27/\.. SEPTEMBER ]9~~
and dried in a hot air oven ( - 105°e). The yield Preparation of monoalkly ammonium tetrafluo• was 96%, 96% and 90% for the potassium, rubi• roborates dium and cesium salts respectively. Pyridinium tetrafluoroborate (5 g) dissolved in The compounds have been characterised by 15 ml of water was kept in ice-cooled water. To ,- chemical analysis (Table 1) and X-ray powder dif• this solution was added 2.6 ml of propylamine fraction pattern (Table 2). The BFi and the boron dropwise with stirring. Pyridine was found to be contents of all the compounds were estimated liberated which was extracted with chloroform. gravimetrically using nitron acetate as precipit• The aqueous solution was concentrated to dry• ant10 and titrimetrically by the method of Tatarskii ness on a water-bath maintained at 50-60°C and et apl. the solid so obtained dried over P401O' The yield of n-propylammonium tetrafluoroborate was 91%. Six other monoalkylammonium tetrafluorobo• Table Boron11080-13001075-10251100-10401090-10004.906.199.728.55Found-68.3139.5517.8650.16Analytical82.01v.j(F2:assignmentsv,(F2):(ealc.),v4(F2):1100-1000vJ(F,:vJF2):6(B-F)6(N-H)v4(F2):6(B-F)v,(F2):v(N-H)v4(F2):now,v3(F2):765770530b,YSb,YS6(13765w,m,v(B%v(B6(13and-v(B6(131305v(B720740517wF)IRfrequeneies--b,YS--InfraredF)--F)F)wSwF)wF) ProbableSpectra data of Am- rates were prepared in a similar manner employ• CsBF4RbBF4NaBF4KBF4 BF4-(82.82)NH4BF/ (39.51)(79.05)1295(68.99)(em-I)(50.40)5271430525530m,s,mS,s 518518516(4.92)(9.84)(8.58)(6.27)SSm v3(F2):530 moniums, 520v(B-s3330F)andsAlkali Metal Tetrafluoroborates ing methyl amine (25/30%), ethyl amine (40%), MBF4 isopropyl amine, n-butyl amine, t-butyl amine, benzyl amine and cyclohexyl amine. All the monoalkylammonium tetrafluoroborates were found to be hygroscopic and were preserved over P40IO• The analytical results are given in Table 3.
Preparation of di- and tri-alkylammonium tetraf luoroborates Dimethyl-, diethyl- and trimethyl-ammonium te• trafluoroborates were prepared by adopting the same procedure as described for monoalkylam• monium tetrafluoroborates. For the other di- and tri-alkylammonium salts a modified procedure •• was adopted. Pyridinium tetrafluoroborate (5 g) was taken in a beaker and cooled in an ice-bath; to it was added di-n-propyl amine (4.5 ml) dropwise. The reaction was instantaneous and exothermic. For
I 'iliA3.82 3.82, 'I !'III"1' ,-"ii' ""It II' 7.447.]0 , 9] --I doh\dob,3.182.303.881.501.65,dhtdlildli(dlltH '1""'I.f'l;tl2.212.032.043.8810.5011.722.313.171.821.5]"8.649.378.729.5944.2853.6858.3945.9858.53I'~I';I72.2464.98ill Ii 2.202.022.03i:l3.992.003.413.742.141.81~ 989594939093902.532.352.812.803.982.753.071.833.73 2.272.522.334.822.743.183.071.82 2.273.052.522.334.832.893.37 2.393.042.522.322.882.83 2.392.473.553.36 2.463.553.35 dob,diu RbBF4*CsBF4(10.54)KBF4(11.78)NaBF4Nitrogen(46.47)(53.98)(44.56)(59.13)(7'-P;')(8.70)(9.53)53.09(8.71)(7.49)(73.06)(65.36)CoHsCH2NH3BF4(M.P.,OC)BF"Compoundn-C3H7NH3BF4Yield(%)Found~'I'lilestimation:'f' (calc.),""'~""/'1"j1'"'1CoH1\NH3BF4%( ]34)""''''''1'calc.(=I126)I'13.36%:n-C4H9NH3BF4CH3NH3BF4!Nitrogen(150)(100)t-C4H9NH3BF4i-C3H7NH3BF4found = 13.25%.( 174)Mono-alkylammoniumCzHsNH3BF4Table 3 - Preparation(146)(180)(194)Tetrafluoroborateand Analyses of(RNH}BF4) 2.183.112.301.66 2.162.28 2.16 2.15 2.24 2.23 5.754.532.342.873.672.932.563.883.58 5.704.502.842.333.652.892.533.~43.55 3.4]3.391.562.321.842.012.852.15 3.391.562.311.832.84 2.083.492.333.273.40 2.083.502.32doh~3.263.40 2.403.852.81~.533.63 Ammonium2.383.843.523.62 2.954.654.082.274.21 2.964.664.08and2.274.20 Alkali Metal Tetrafluoroboratesprepared (in A) 760 NH4BF4 --- Table 2 - X-ray Powder Diffraction Pattern Data of !"~I'!I"Id'i MOHAMED & PADMA: PREPARATION OF TETRAFLUOROBORATES about 30 min, the solution was stirred and main• 10.629.508.586.384.908.006.287.327.3746.0240.2549.2445.8459.0354.0930.2832.9365.03 909492939184889085 tained at 0-5°C. The liberated pyridine was ex• Table 4 - Analytical(10.54)\40,04)(49.65)(53.98)(8.70(45.97)(30.48)(40.04)(45.98)(59.13)(8.01)(4.91)(7.41)(9.54)(6.45)(7.40)(65.36)BFiFound(M.P.,°C)Yield(%)Data(calc.),of%Di- and Tri-alkylammoni- (C6HsCHz)zNHzBF4(n-C4H9hNHzBF4(n-C3H7(i-C4H9)zNHzBF4((CzHs)zNHzBF4O)C4HgNHzBF4(CzHshNHBF4(CH3)3NHBF4)zNHzBF4prepared (204)(235)(106)(186)(220)(141)(169 Nitrogenurn Tetrafluoroborates tracted with ether,(~H3hNHzBF4the precipitate filtered Compoundsoff on suction, washed well with ether till free from pyri• dine. The compound was dried over P40IO; yield 90%. The other salts were prepared in a similar man• ner, making use of calculated amounts of amines and the prepared salts were stored over P40IO• Analytical results are given in Table 4, Table 5 gives the 1H NMR data for the alkylan:jmonium tetrafluoroborates.
Results and Discussion
Ammonium and alkali metal tetrafluoroborates The analytical data and spectral data (Tables 1 and 2) indicate that the products are of high pur• ity.. The infrared spectra of.NH4BF4 and other alka• li metal fluoroborates (Table 1) show a very strong broad absorption in the region 1100-1000 cm - 1 which is characteristic of the tetrafluorobor• ate ion and a sharp doublet in the region 515-530 3160-3270 em-I. In trialkylammonium salts, the cm - I. The former strong band has been assigned v(N - H) and v(C - H) appear below 3000 em - I. to v3 (B - F stretching) and the latter doublet to Several other bands in the region 2800-2300 v4 (B - F bending) of the tetrahedral BF4 ion. The cm - 1 might be due to the amine salt combination splitting of v4 and the appearance of the other bendsl4. The other absorptions are due to the or• bands are attributed to the lowering of site ganic moieties present in the molecule. symmetry of the anionlZ. In addition to the above The IH NMR spectra (DMSO-~ solvent, TMS• absorptions, NH4BF4 shows two more peaks; one internal standard) (Table 5) of all thealkylammo• at 3330 cm-I and the other at 1430 cm-I which nium tetrafluoroborates identify the organic mo• are due to the ammonium ions. The X-ray pow• ieties. The N - H resonances were identified by der diffraction pattern results (Table 2) are in deuterium exchange. In some cases, the a-protons good agreement with the values reported in litera• were found to couple with the N - H protons and ture13 indicating the purity of the compounds. these couplings were found to be relieved on deu• terium exchange. Alkyl substituted ammonium tetrafluoroborates It is of inte·rest to mention that other methods Tables 3 and 4 indicate that yields are good so far reported do not give such high yields or and the products are of high purity. The PMR purity. The methods reported for the preparation spectral data are presented in Table 5. The IR da• of substituted (alkyl or aryl) ammonium tetraflu• ta (spectra available with the authors) for all the oroborates are: (i) decomposing the adducts of salts show a medium split band in the region 500• boron trifluoride and aminesl5, (ii) reaction be• 530 cm -I and a characteristic broad band in the tween ether solution or the amine with boron trif'" region 1150-1000 cm -I. These can be assigned luoride etheratel6, and (ill) by the reaction of te• (as with the alkali metal salts) to v3 (B - Fstretch• trafluoroboric acid (HBF4) and aminesI7-19• The ing) and v4 (B - F bending) frequencies of the tetrafluoroboric acid is foUIid to be always in eq• BFi ion. uilibrium with its hydrolysis products,' In the infrared spectra of alkylammonium te• BFi + HzO-[BF30H]- +HE Equilibrium con• trafluoroborates, a strong broad absorption in the stant is 2.8 x 10-3 at 2'5°C (ref. 20). The salts pre• region 3200-3800 cm-I is observed. This has pared by using this acid would, therefore, be asso• been assigned to the merging of the N - H .and ciated with these impurities. The present method C - H stretching vibrations, both of which occur has the advantage ofellininating the drawbacks of in this region. In dialkylammonium salts, the the earlier methods such as the presence of diffi• N - H stretching vibrations appear in the region cultily removable impurities, the .poor yields and
761 INDIAN J. CHEM., VOL. 27A, SEPTEMBER 1988
deuteration1.43 mIt NH2) deuteration " deuteration 1.33 s ( '-... CH)) Table 57.4-0.91sH(-CH3)(C6HsNMRC6HsCH2NH)BF43.3C2HsNH3BF47.562.63-2.927.237.26protons2.96i-C)H7NH)BF41.231.31.0t(CH))4.86(C2HshNH2BF42.832.8n-C4H9NH3BF47.71.66n-C3H7NH)BF4Data) mItdbt sb,mItq(t(tb(a-CH2)q((_::::.CH)(--((-NH))(a-CHz)(-((10-s(-----CHzof-NH2)except((~-CH2)(a-proton)CH))CH3)CH))NH3)-CH2)CH~)Mono-,ringNH))on-) C6HIIDi-7.234.53(CH)hNH2BF44.05.567.53bandb(s(-(--(-NH))(-NH2)NH)BF4NH3)Tri-alkylamrnoniumNH))NH2)ononon 7.3 b (- NH))-Tetrafluoroborates [[) )ppm)] deuteration4.54.21s (-( - NH))NH2) on CH) ) a-proton)deuteration5.167.16 bs (-NH))(- NH)) on 4.66 s ( - NH3) on (~anddeuterationy-CHJCH)4.01 s (- 5.47s(-NH))onCH2 -) 6.46s(-NH)) 2.57.08t-C4H9NH)BF4s (-CH))s ( -/CH)NH)) CH3NH3BF4
(O)C4H8 NH2BF4 CH 3.16 mIt (N::: 2) CHz
.•..CHz 1.58 mIt 2.0 mIt (- CHC) 3.8 mIt(O ..... ) (~and y-CH2) CH2 2.9 b (a-CH2• 2.9 t (a-CH2) 2.76 d 7.63s(-NHz-) coupled with NH2 6.73b(-NH2-) (-CH2-CH:::::) protons) 2.9 t (ondeuteration) 4.16 b (- NH2 -) NH absorption was 4.5 s ( - NHz - ) on 8.06 b ( - NH2) on deuteration not seen deuteration
4.16 s (- CHz -) 2.86 s ( - CH3) 1.3t( -CH)) 7.43 s (C6Hs -) NHwasnot 3.16 mIt( -CH2-) observed coupled with;- NH) NH2 absorption 3.16 q (- CH2) on was not seen clearly deuteration NH was not observed
s - singlet, q - quartet, d - doublet, mIt - multiplet, t - triplet, b - broad.
the necessity of employing elaborate experimental procedures. This novel procedure is simple, one• RNH)BF4- + CSHSN pot reaction, occurring at room temperature R2NH2BF4- + cSH3N (25°C). R3NHBF4- + CSHSN The pKa values of the alkyl amines (8.7-11.2) employed are higher than that of pyridine (5.17) and therefore they displace pyridine from pyridi• References mum tetrafluoroborate with great ease. The dis• 1 JumerJ F, Met Finish, 56 (1958) 44; 56 (1958) 60. placement reactions could be summarised in 2 Hudson R M, Butler J J & C J Warning, Met Finish, 74 terms of the followingscheme: (1976) 37.
762 MOHAMED & PADMA: PREPARATION OF TETRAFLUOROBORATES
3 Beckwith M & Hau G F, US Pat. 3888778 (March 13, 12 Cote G L & Thompson H N, Proc Roy Soc, 2l0A (1951) 1973). 217. 4 O'Conner D E & Wyness G R, US Pat. 3808245 (Apr. 13 X-ray powder diffraction file (Joint Committee on Powder 301974). Diffraction Standards, Pennsylvania), 19103 (1967), 5 Pawlenko S, US Pat. 334107 (Oct. 24,1967). 1-0335, 11-671, 1-0820 and 9-394, 6 Boika V, USSR Pat. 495178 (Dec. 15, 1975). 14 Hoard J L & Blair V, JAm chem Soc, 51 (1935) 1985. 7 Lauck H, Ger Pat. 2334660 (Jan 23,1975). 15 Harris J J & Rudner B, lnorg Chern, 8 (1969) 1258. 8 Yon Mathschka G, Kunstofftechnik, 11 (1972) 304. 9 Kalbandkeri R G, Syed Mohamed K, Padma D K & Va• 16 Muller Eugen & Huber-Emden Helmut, Ann Chern, 649 sudeva Murthy A R, Polyhedron, 4 (1985) 787. (1961)70. 10 Lucchesi C A & DeFord D D, Analyt Chern, 29 (1957) 17 Muller Eugen, BUber-Emden Helmut & Rundel Wolfgang, 1169. Ann Chern, 6323 (1959) 34. 11 Taterskii S V, Kornilova E N & Sheinina F A, Trudy 18 Harris J J, lnorg Chern, 5 (1966) 1627. Komissii, Anal Khim Acad Nauk SSSR, 3 (1951) 287; 19 Cook D, Can I Chern, 39 (1961) 2009. ChemAbsts 47 (1953) 2642h. 20 Ryss I G, Jgen Chern, (USSR), 16 (1946) 531.
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