26 Chemistry of B-Fluorinated Borazines HASSO BEYER
and
HARRY JENNE
Duke University, Durham, N. C. J O H N B. HYNES Hynes Chemical Research Corp., Durham, N. C. KURT NIEDENZU
Downloaded by COLUMBIA UNIV on June 2, 2013 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch026
Duke University and U. S. Army Research Office, Durham, N. C.
The preparation and properties of some B -fluorinated borazines are reported. Features of their infrared spectra, some general con siderations with respect to the diagnosis of a borazine structure by infrared spectroscopy, and possible correlations of borazines to other pseudoaromatic systems are discussed. Τ he aromatic character of borazine and its derivatives is still a • controversial subject. It is reasonable to assume, however, that substitution at the boron atom with electron-attracting groups will i n crease the participation of the free electron pair of the nitrogen in the B - N linkage and hence should increase the aromaticity of the bora zine molecule. Although a large number of borazines of the general formula (-BR-NR -) are known (15), the variety of substituted borazines is limited. For instance, borazines with N-substituents other than H, alkyl, or aryl are not known; of 5-halogenated compounds, the B fluorinated borazines were virtually unknown until recently (10) and only one B- iodinated derivative has been mentioned in the literature (13)Thus, further synthetic studies are necessary if an expanded knowledge of the borazine system is to be realized. One approach to this problem is presented by the present investigation of 5-fluorinated borazines. Preparation and General Properties of B-Fluorinated Borazines T
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Several B-fluorinated borazines have now been prepared by transhalogenation of B-trichloroborazines with various fluorinating agents, such as metal fluorides, antimony trifluoride, and mixtures of antimony halides. So far, the most convenient method of preparing B-trifluoroborazines has been to transhalogenate the chloro compounds with titanium tetrafluoride. One mole of B-trichloroborazine and 0.3 mole of titanium tetra fluoride are powdered separately, mixed, covered with a small amount of T i F powder, and slowly heated until the reaction is self-sustaining. Upon completion of the reaction (normally about 10 minutes), volatile materials are distilled off under reduced pressure (ca. 70 mm.). This material is redistilled at room temperature at approximately 4
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In Boron-Nitrogen Chemistry; Niedenzu, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
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BEYER ET AL
B-Fluorinated
Borazines
267
1.0-mm. pressure to remove titanium tetrachloride and the residual fluorinated borazine is purified by vacuum distillation, recrystallization, or sublimation. Using this method, the following compounds were obtained: [ -BF-NH-]
, m.p. (subl.) 122°C.
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[ - B F - N C H - ] , m.p. 90.5°C. 3
3
[ -BF-NC H -] 2
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, b.p. 26°C. (3 mm.)
[ - B F - N ( n C H ) - ] , b.p. 59°C. (3 mm.) 3
7
3
[ - B F - N ( n C H ) - ] , b.p. 88-90°C. (3 mm.)
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The structures were established by elemental analysis, molecular weight determination, infrared data, and a NMR investigation (11). Although IMluorinated borazines hydrolyze easily, their reaction with water or dilute aqueous acid is not as rapid nor as vigorous as the comparable reaction of their chlorinated counterparts. On the other hand, contact of £-trifluoroborazines with alcohols produces a nearly explosive reaction which results in complete decomposition of the borazine ring. Two mixed £-fluoro-chloroborazines — B F C1N H (v. p. 7 mm., 31°) and Bg F C 1 N H (m.p. 49-51°) — have been prepared. Both of these borazines can be obtained by reaction of Btrichloroborazine with slightly more than the stoichiometric requirement of antimony trifluoride in the manner described above. The yield of the difluoro derivative is increased considerably if, after the t r i chloroborazine is mixed with the SbF , some SbCLj is added to the mixture to start the reaction. To obtain reasonable yields of the desired products, careful temperature control is an absolute necessity. As a rule, the reaction vessel is cooled in a dry ice-methanol bath immediately after ignition of the reaction mixture and cooling is maintained until the vigorous reaction has almost subsided. The mixed halogenated borazines are extremely sensitive to hydrolysis and, on heating, they readily disproportionate to B-trichloroand #-trifluoroborazine. Therefore it is not surprising that, on refluxing a mixture of #-trichloroborazine with £-trifluoroborazine, no reaction occurs but the starting materials are almost quantitatively recovered. Preliminary experiments indicate that the chlorine of the mixed fluoro-chloroborazines is more reactive than the fluorine. Work with the two compounds, however, is severely hampered by their consistent tendency to disproportionate. In addition, only B F C 1 N H has been isolated as a pure material, whereas the B F C 1 N H still appears to be contaminated with small amounts of impurities. However, once these difficulties are overcome, the mixed halogenated borazines offer a new route to a variety of substituted borazines, such as inorganic analogs of aniline, styrene, and phenol. 3
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Infrared
Spectra of B-Fluorinated
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Borazines
The infrared spectra of 5-fluorinated borazines were recorded in the vapor phase, as liquid films and as mulls in Nujol and K e l - F o n In Boron-Nitrogen Chemistry; Niedenzu, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
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A D V A N C E S I N C H E M I S T R Y SERIES
Perkin-Elmer Model 21 and Beckman IR 8 spectrophotometers, using sodium chloride optics. Trifluoroborazine. — The strongest absorption of the B-trifluoroborazine was recorded at 1510 cm." with a shoulder at 1528 cm." and this absorption has consequently been assigned as due to B - N stretch, the shoulder indicating a B isotope effect. Two doublets in the 730-cm." region can be assigned to out-of-plane vibrations of the B - N ring. Generally, the B - N stretching frequency in the 1350- to 1500-cm." region has been used to identify the borazine structure. However, many other compounds containing normal covalent B - N bonds also absorb in this same region. A recent survey of more than 100 variously substituted borazines made in this laboratory demonstrated that the presence of a borazine ring structure in a given B - N compound can be unambiguously determined by the simultaneous existence of the B - N stretch and an out-of-plane vibration in the 700-cm." region. An absorption at 1401 c m : is probably also related to a B - N stretching mode, since an analogous band is also found in i M r i c h l o r o borazine. The NH stretching frequency was recorded near 3500 cm." A band of medium intensity at 922 cm." is probably indicative of a N-H in-plane bend, coupled perhaps with B - N ring bending if the parent borazine, Bg N H , is used for comparison (17). The only remaining absorption of medium-to-strong intensity, recorded at 1136 c m ? , was consequently assigned as due mainly to a B - F vibration. ^-Monochlorodifluoroborazine.-The NH frequency of this compound was recorded at 3460 cm." The B - N mode was evidenced by a relatively broad strong peak at 1513 to 1507 cm." with a shoulder at 1530 cmT , and a band corresponding to the 1401-cm." absorption o f B trifluoroborazine was evidenced at 1406 cm." An additional band of about equal intensity was recorded at 1482 cm." The B - F frequency was recorded at 1339 cm." A doublet (1042 and 1038 cm." ) which was not observed in the trifluoro derivative can reasonably be assigned as B - C l stretch, unless another doublet (of weaker intensity) at 827 to 823 cmT is related to the latter mode. Other features of the spectra of the trifluoro- and the difluoromonochloroborazine are almost identical, except that, in the 730-cm." region of the latter, two additional absorption peaks could be observed. Only a relatively weak vapor phase spectrum was obtained of B-monofluorodichloroborazine, the general features of which are in agreement with those discussed above. General Features of Spectra of B-Fluorinated Borazines.-The B - N stretching frequencies of B-fluorinated borazines are observed near 1500 cmT , wheareas the corresponding modes of the chlorinated products are observed near 1430 cmT One might assume that such occurrence illustrates an increase in the B - N bond order. However, coupling of B - F and B - N modes may be present in B-fluoroborazines, resulting in higher than normal B - N ring frequencies (and lower than normal B - F frequencies) without necessarily having stronger B - N bonds. The major controversy on the assignment of absorptions seems to reside, however, in the assignment of the B - F frequency. This is illustrated by values for the B - F vibration reported for (CHs^ B F at 1260 cmT (3) and for the etherate of C F B F at 1115 cmT (14). 1
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In Boron-Nitrogen Chemistry; Niedenzu, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
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Borazines
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However, comparison of modes related mainly to B-halogen stretching of B-trifluoroborazines with those of their chlorinated counterparts seems to substantiate the assignment of the B - F frequency, as illus trated in Table I. Table T. B-Halogen Stretch of Borazines ρ B-F, (-BX-NH-)
vB-Cl,
1
1136
3
(-BX-NCH -)
1032
Cm. (16) (16)
1170
975
3
1150
1025
(4)
