10.1002/anie.201808352
Angewandte Chemie International Edition
by treating a boron halide or an appropriate alkylboron halide with silver isocyanate in an
inert media, such as anhydrous benzene or toluene.[10,11] To overcome the disadvantages of
air-sensitivity, it is crucial to find a convenient and practical method to prepare and handle
boron isocyanates. Herein, we report an in situ synthesis and application of boron isocyanates
which makes it practical to handle these extremely high air and moisture-sensitive
compounds much more safely and conveniently. The resulting boron tri(isocyanate) was
found to be an effective reagent for the production of boron nanoparticles.
Carbon-based isocyanates can be prepared by the reactions of alkyl/aryl halides with alkali
cyanates (NaOCN, KOCN).[10] Therefore, we first investigated this reaction potential for the
synthesis of boron tri(isocyanate) (B(NCO)3) using an alkali metal cyanate (NaOCN, KOCN)
and boron trichloride (Equation 1). Since boron trichloride forms corrosive hydrogen chloride
upon exposure to moisture or alcohol, a toluene solution of boron trichloride (1.0 M in
toluene) was used in the procedure. Because BCl3 has also been reported to cleave C-O bonds
in ethers,[12,13] a relatively inert solvent, such as benzene or toluene, along with somewhat
polar solvent, acetonitrile (CH3CN), were used in the reaction. Unfortunately, the product
yields were extremely low, only a trace of B(NCO)3 was obtained from NaOCN and KOCN
salts as observed in the 11B-NMR spectra. When performed with silver isocyanate, the
reaction produced boron tri(isocyanate) in 85.4% yield based on ICP analysis (Equation 2).
…….. (Eq. 1)
…………………... (Eq. 2)
In the reaction between Ag(NCO) and BCl3 in benzene solution at 60-80C for 2 h, three
boron products, Cl2B(NCO), ClB(NCO)2 and B(NCO)3, were observed in the 11B-NMR
11
spectra. The B-NMR spectra showed chemical shifts at 46.4, 33.6, 22.9 and 13.7ppm for
1
BCl3, Cl2B(NCO), ClB(NCO)2 and B(NCO)3, respectively. The H and 13C-NMR spectra
displayed the typical toluene chemical shifts only. After heating at 100-105C for 1h,
followed by further heating at 115-125C for 5h, the reaction yielded an off-white product
mixture (Equation 2, see Figure S-1(a) in the Supporting Information). The B(NCO)3 was the
11
sole boron product in this reaction as evident in its B-NMR spectra in C6D6 solution (see
Figure 1 (a)). The product is essentially pure based on its NMR spectral data and, therefore, it
could be utilized in toluene solution. However, the isolation of highly air and moisture-
sensitive B(NCO)3 from the product mixture demands an extremely high vacuum of less than
0.1 Torr. [8,9]
The same reaction was also carried out in acetonitrile (CH3CN) as a solvent. As the reaction
temperature was raised gradually from room temperature to 125C, the reaction mixture
turned yellow (see Figure S-1 (b) in the Supporting Information). Consequently, four peaks
2
This article is protected by copyright. All rights reserved.