Characterization of Tetrachloroaminoborane

52 J. G. HAASNOOT ■ CHARACTERIZATION OF TETRACHLOROAMINOBORANE

J . G . H a a s n o o t a n d W. L . G r o e n e v e l d Department of Inorganic Chemistry, Gorlaeus Laboratory, State University, Leiden, The Netherlands

(Z. Naturforsch. 29 b, 52-54 [1974]; eingegangen am 12. September 1973)

Tetrachloroaminoborane, irB NMR spectroscopy

Tetrachloroaminoborane (C12BNC12) has been prepared from nitrogen trichloride and boron trichloride. Its irB NMR shift with respect to boron trifluoride-diethylether is -—34.7 ppm. The infrared frequency of the B-N stretching vibration appears to be 1312 cm -1.

Boron trichloride and nitrogen trichloride react diethylether as external reference. The mass spectra in tetrachloromethane forming hexachloroborazine were recorded on a AEI MS 902 mass spectrometer with an all glass heated inlet system operating at as a final product1. about 100 °C. 3BC13 + 3NC13 -> (C1BNC1)3 + 6 Cl4 (1) 11B N M R spectra It appeared that the outcome of this reaction is Fig. 1 shows the nB NMR spectra of a mixture sensitive to a certain excess of boron trichloride. of nitrogen trichloride and boron trichloride in It seemed probable that one or more other boron- tetrachloromethane at several stages of the reac nitrogen-chlorine compounds might be present as tion (1). intermediates. As can be observed from Figs la and lb a rapid In order to investigate this, nB NMR spectros formation of a compound producing signal A copy has been applied. (—34.7 ppm) occurs. A second intermediate, related to signal B Experimental (—39.8 ppm), is produced more slowly and in a Solutions of nitrogen trichloride were prepared small amount. Simultaneously hexachloroborazine according to the method of N o y e s 2 using only is produced (—32.8 ppm). See Figs. lc and Id. tetrachloromethane as solvent. The concentrations were determined by means of jodo metric titration The absorption which shows up intensively at and if necessary adjusted to about 1 Mol/1. Boron —46.6 ppm in all figures is of boron trichloride which trichloride and tetrachloromethane wrere commercial was present in about 100% excess. products used without further purification. At temperatures from —25 °C to -f 25 °C no The reaction mixtures were prepared by conden sation of boron trichloride in the reaction flask at NMR signal in the range from 0 to —20 ppm could —20 °C, adding the required amount of nitrogen be observed. In this range adducts of the tpye trichloride solution followed by warming up to R 3NBC13 usually absorb3. room temperature. Absorption A was assigned to tetrachloroamino Infrared spectra were recorded on a Beckman borane IR-10 spectrophotometer using pure liquid between cesium bromide windows. The Raman spectra Cl Cl were obtained with a Cary 81 instrument furnished \ / with a helium-neon laser (6458 Ä). Nuclear magnetic B—N resonance measurements were performed on a / \ Varian HA 100 apparatus using the sideband Cl Cl technique for calibration with boron trifluoride- on the basis of the observed UB chemical shift (—34.7 ppm). This shift may be compared with Requests for reprints should be sent to J. G. H a a s - those of triehloro-N-ethylaminoborane4 and di- n o o t , Gorlaeus Laboratory, Coordination Chemistry, P. O. Box 75, Leiden, The Netherlands. chloro-N-diethylaminoborane. Table I lists these J. G. HAASNOOT • CHARACTERIZATION OF TETRACHLOROAMINOBORANE 53

chemical shifts together with the nB-N infrared stretching frequencies of the corresponding com pounds.

Table I. X1B NMR shift (Ö) an d X1B-N stretching frequency ( v) of three aminoboranes. (5 in ppm relative to boron trifluoride-diethylether, v in c m '1.

(5 V C12B-NC12 — 34.7 1312 C12B-NC1C2H 5 — 32.4 1410 C12B-N(C 2H 5)2 — 30.5 1505

Fig. 1. X1B NMR spectra of a mixture of boron tri chloride and nitrogen trichloride in tetrachloromethane at 20 °C after a reaction time of 5 minutes (Fig. a), 20 minutes (Fig. b), 2 hours (Fig. c) and 3 days (Fig. d). Two intermediate products are formed, related to signals A and B. Abscissa in ppm relative to boron trifluoride-diethvlether.

As can be seen from Table I, the effect on chemi cal shift as well as on stretching frequency of sub stituting two ethyl groups on nitrogen by chlorine, is about twice as large as that of substituting only one of the ethyl groups. This implies that the shift is proportional to the stretching frequency. Because of its more negative “ B shift (—39.8 ppm), the compound related to signal B might have a composition with a less shielded boron. Penta- chlorodiborylamine, C1 2B-NC1-BC12, will meet this requirement, though this assignment is speculative.

Isolation of tetrachloroaminoborane In order to confirm the formula C1 2BNC12 for the compound related to signal A we attempted to isolate this substance. Distillation of tetrachloromethane from the reaction mixture under reduced pressure at room temperature affords only hexachloroborazine as a residue. These failures are probably caused by the unstable nature of tetrachloroaminoborane. How ever, when an equimolar mixture of boron trichlo ride and nitrogen trichloride in tetrachloromethane was kept at room temperature for about 30 min and subsequently evacuated at —20 °C to — 10 °C, thereby removing tetrachloromethane, a yellow liquid remained as a residue. This liquid can be distilled at — 10 °C at a pressure of about 1 mm. During this distillation some decomposition is observed. The analysis of this liquid yields 6.1% boron and 82.8% chlorine. Calculated for BNCL one finds 54 J. G. HAASNOOT • CHARACTERIZATION OF TETRACHLOROAMINOBORANE resp. 6.5% and 85.1%. The recorded mass spectrum IR and Raman spectrum showed the peak patterns of BNC14+ and BNC13+. The infrared spectrum of tetrachloroaminoborane Diluted with tetrachloromethane the liquid gave an has been recorded from 250 to 2000 cm’1, the nB NMR spectrum with an absorption at —34.7 Raman spectrum from 150 to 1000 cm'1. ppm. This indicates that BNC1 4 is indeed responsible A unique assignment could not be made. How for signal A. However, both mass spectrum and ever the similarity between tetrachloroamino NMR spectrum indicated the presence of small borane and tetrachloroethene favours an assign amounts of boron trichloride and hexachlorobora- ment of the stretching vibrations similar to that of zine, illustrating the slow decomposition of the tetrachloroethene5. A consistent fit of the spectra substance. with the more recent assignment of M a n n et al.6 In an attempt to isolate the compound of signal seems not possible. In Table III our assignment of B, we kept tetrachloroaminoborane and excess tetrachloroaminoborane and the referred assign boron trichloride in tetrachloromethane for 24 hours ments of tetrachloroethene are listed. at — 6 °C. Excess boron trichloride, chlorine, solvent and tetrachloroaminoborane were distilled Table III. Assignment of the stretching vibrations of off at —10 °C. The residue was a colourless liquid tetrachloroaminoborane and tetrachloroethene. together with a white solid. This mixture decom posed with a violent explosion slightly above room BN C lt C2C145 C2C146 temperature. No further attempts to isolate this 1350 (10B) 1571 1571 compound were made. B-N stretch (A,) 1312 (>>B) 1026 (10B) 913 908 B-Cl stretch (B x) Physical properties of tetrachloroaminoborane 986 (UB) B-Cl stretch (Ax) 446 447 447 Some physical properties of tetrachloroamino N-Cl stretch (B x) 735 782 777 borane have been determined. They are summarized N-Cl stretch (A,) 541 512 1000 in Table II along with those of the isoelectronic tetrachloroethene. W am i ng Tetrachloroaminoborane resembles nitrogen tri Table II. Physical properties of tetrachloroamino chloride in its unpredictable and explosive decom borane and tetrachloroethene. positions. Even dilute solutions can not be trusted, especially when in contact with compounds con BNC14 C2C14 taining more or less active hydrogen. It is strongly advised to work behind a safety screen and to take Molecular weight 166.65 165.83 D ensity d^° [g • cm*3] 1.533 1.623 adequate safety precautions. Refract ivity?f2° 1.4743 1.5044 Part of this research was performed in the labora Molecular refr. R ji [cm 3] 30.6 30.3 M elting point [°C] 21 to — 20 __22 tory of Prof. Dr. H. N ö th of the University of Boiling point [°C] ca. 130 (extr.) 121 Munich, W. Germany. We are indebted to him for liis friendly cooperation and valuable suggestions.

1 J . G . H a a s n o o t a n d W . L. G r o e n e v e l d , Inorg. 4 Unpublished results. nuclear Chem. Letters 3, 597 [1967]. 5 P. T o r k i n g t o n , Trans. Faraday Soc. 1949, 445. 2 W . A. N o y e s , J. Amor. chem. Soc. 50, 2902 [1928]. 6 D. E . M a n n , J . H . M e a l , and E. K . P l y l e r , J. 3 H . V a h r e n k a m p , Dissertation, Munich 1967. chem. Physics 24, 1018 [1956].