120 Inorganic Chemistry, Vol. 14, No. I , 1975
0. T. Beachley, Jr., and €3. Washburn Contribution from the Department of Chemistry, State Universit) of New York at Buffalo, Buffalo, New York 14214
0. T. BEACHLEY, Jr.* and B. WASHBURN Received June 4 , 1974
AIC40360-t-
A series of B-monosubstituted amine-boranes and borazines have been compared according to their chemical reactivity and 1H nmr spectral properties. No previous direct comparisons between these classes of compounds have been made. Mercury(l1) halides have been observed to be more reactive toward H3B-N(CW3)3 than H3B3N3H3. This decreased reactivity of H3B3N3H3 is considered to be related to R bonding in the borazine ring which leads to a decreased bydridic character of the hydrogen bound to boron and partial positive charge on boron compared to H3R-N(CW3)3. Steric effects would have enhanced the reactivity of H3B3N3H3 compared to €13B.N(CH3)3. The comparison of the "i nmr data for HzXB*N(CW3)3,HzXB*N(CH3)zH,H2XB3N3(CH3)3 and H2XB3N3H3 (X = C1, Br, CH3: CN) supports the hypothesis that a resonance interaction between a substituent and the borazine ring is responsible for the changes in the relative chemical shifts of the ortho and para protons in H2XB3N3H1. Field effects might be important in accounting for the substituent effects on the C H resonance in H2XB3N3(CH3)3.
The elucidation of the nature of bonding in the borazine ring through the investigation of the effects of substituents on the chemical and spectral properties of unsymmetrically substituted borazines is of current interest. The data for Bmonosubstituted borazines (H2XB3N3H3) are consistent with the hypothesis that the n electrons are delocalized, a t least partially, and substituents interact with the n system by means of a resonance effect. Chemical reactivity studies2,3 have clearly demonstrated that a single substituent on one boron of the borazine ring in a H2XB3N3H3 derivative has a definite influence on the reactivity of the substituents bound to the other boron atoms. Correlations between the 1H chemical shift of the para protons of B-monosubstituted borazines435 with 11B chemical shift for the boron atom to which the substituent is bound are also consistent with n-electron delocalization. However, the nmr data could also be interpreted in terms of anisotropic effects and through-space field effects. In this paper, we report the preparation and chemical and spectral properties of a series of B-monosubstituted amineboranes. These types of compounds have been studied in order to understand more fully the differences between the chemical and spectral properties of borazines and other boron-nitrogen compounds. There has been no previous comparison between these classes of compounds. Amine-boranes are coordinately saturated. Both boron and nitrogen are four-coordinate. Since n bonding is unlikely, resonance interactions will be prevented and changes in the * H nmr data will result from field effects. A comparative study of the effects of boron substituents in H2XB*N(CH3)2H, HzXBvN(CH3)3, H2XB3N3H3, and H2XB3K3(CH3)3 requires a variety of types of substituents. The substituents were selected on the basis of the availability of spectral data for the B-monosubstituted borazine. Very little data on B-monosubstituted amine-boranes were previously available. Experimental Section All compounds described in this investigation were manipulated in a vacuum system or a purified nitrogen atmosphere. The solvents and reagents were purified by conventional means. Preparation of AXB.N(CH3)3 and HSB.N(CH3)2H (X = Cl, Br). The compounds H2ClB-N(CH3)3, H2BrB*N(CH3)3, H2CIB.N(CH3)2H, and H2BrB*N(CH3)zH were prepared by means of the reaction between H3B.N(CH3)3 or H3B*N(CH3)2Hwith HgCh or HgBrz in diethyl ether at room temperature. The trimethylamine-borane adducts were isolated, after solvent removal, by sublimation at 2.5' to a 0' cold finger. The average yields (based on the quantity of H3B.N(CH3)3 used) of H2ClB*N(CH3)3. nip 84-85' (lit.6 mp 8 5 ' ) and H2BrB.N(CH3)3, mp 68-69' (lit.6 mp 67'), were 87 and 76%, respectively. The compounds were further identified by elemental analysis and their spectral properties.
In the case of the synthetic procedure for H,XB.N(CH3)2I-I (X = C1, Br), the reaction mixture was filtered, after hydrogen evolution
was complete. The diethyl ether was removed from the filtrate by vacuum distillation to leave a colorless liquid. The product H2ClB*N(CI-I3)2Hwas then purified by a vacuum distillation at 59' using a short-path still. The average yield of H2ClB.N(CH3)2€I, fp 16' (lit.6 i s 0 ) ,was 65% based on the quantity of H ~ B < N ( C H ~ ) Z H used. The compound was further identified by elemental analysis and its spectral properties. For H2BrB*N(CH3)2H, a viscous, thermally unstable liquid was obtained after filtration and removal. of ether. Upon standing, H2 and R2fI5N(CH3)2 were evolved at room temperature. Reaction at 0" and keeping the product cool failed to yield a sufficiently pure product which could be adequately characterized. Preparation of H2(@ 2(C&)B*N(CI13)2H. The compound H2(CH3 ly prepared by means of the reaction between LiB(CI-I3)H3 and N(CHj)3HCI. The derivative, H ~ ( C H ~ ) B . N ( C H ~ ) was Z H ,obtained from H2(CH3)B.N(CH3)3 by means of an amine exchange reaction. A variety of other potential preparative routes to H2(CH3)B*N(CH3)3 and H2(CH3)B*N(CH3)2Hwere attempted but they did not give the desired compounds. In a typical reaction, 0.981 g (27.4 mmol) LiB(CH3)H37 was allowed to react with 2.57 g (27.0 mmol) of N(CH3)3*HClin diethyl ether. When hydrogen evolution was complete (about 1 hr), the volatile components were removed and separated by means of a fractional vacuum distillation using -78 and -196' traps. The product H2(CH3)BeN(CH3)3, which was stopped by the -78' trap, had a melting point of 0' and a vapor pressure of 3 mm at 20' (lit.8 mp OX', vp 3 mm (20')). The average yield of IH2(CH3)B*N(CH3)3 was 2496 based on the quantity of LiB(CH3)1-I3 used. The compound H2(CH3)B
